4. Plant Physiology

5/28/2018 4. Plant Physiology

1/23

LevetinMcMahon: Plants

and Society, Fifth Edition

II. Introduction to Plant

Life: Botanical Principles

4. Plant Physiology The McGrawHill

Companies, 2008

49

4Plant Physiology

CHAPTER OUTLINE

Plant Transport Systems 50Transpiration 51Absorption of Water from the Soil 51

A CLOSER LOOK 4.1 MineralNutrition and the Green Clean 52

Water Movement in Plants 53Translocation of Sugar 53

Metabolism 54

A CLOSER LOOK 4.2 Sugar andSlavery 55

Energy 57Redox Reactions 57Phosphorylation 57Enzymes 58

Photosynthesis 58Energy from the Sun 58Light-Absorbing Pigments 58Overview 58The Light Reactions 59The Calvin Cycle 62Variation to Carbon Fixation 63

Cellular Respiration 64Glycolysis 65The Krebs Cycle 65The Electron Transport System 68Aerobic vs. Anaerobic Respiration 69

Chapter Summary 70

Review Questions 70Further Reading 70

KEY CONCEPTS1. The movement of water in xylem is a

passive phenomenon dependent on thepull of transpiration and the cohesionof water molecules whereas thetranslocation of sugars in the phloemis best described by the Pressure FlowHypothesis.

2. Plants are dynamic metabolic systemswith hundreds of biochemical reactionsoccurring each second, which enableplants to live, grow, and respond totheir environment.

3. Life on Earth is dependent on theflow of energy from the sun, andphotosynthesis is the process duringwhich plants convert carbon dioxideand water into sugars using this solarenergy with oxygen as a by-product.

4. In cellular respiration, the chemical-bond energy in sugars is converted intoan energy-rich compound, ATP, whichcan then be used for other metabolicreactions.

C H A P T E R

Products of photosynthesis are translocated in the phloem and stored

in various plant organs. These pumpkins are excellent examples

of how energy from the sun is transformed into food by these processes.


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50 U N I T I I Introduction to Plant Life: Botanical Principles

Although plants lack mobility and appear static to thecasual observer, they are nonetheless active organ-

isms with many dynamic processes occurring within

each part of the plant. Materials are transported through

specialized conducting systems; energy is harnessed from

the sun; storage products are manufactured; stored foods are

broken down to yield chemical energy; and a multitude of

products are synthesized. Put simply, plants are bustling with

activity. This chapter will consider some of the major trans-

port and metabolic pathways in higher plants.

PLANT TRANSPORT SYSTEMSAs described in Chapter 3, there are two conducting, or

vascular, tissues in higher plants, the xylem and the phloem,each with component cell types. Water and mineral transport

in the xylem will be described first. Tracheids and vesselelements, which consist of only cell walls after the cyto-

plasm degenerates, are the actual conducting components in

xylem.

The source of water for land plants is the soil. Even

when the soil appears dry, there is often abundant soil mois-

ture below the surface. Roots of plants have ready access to

this soil water; leaves, however, are far removed from this

water source and are normally surrounded by the relatively

drier air. The basic challenge is moving water from the soil

up to the leaves across tremendous distances, sometimes up

to 100 meters (300 feet). This challenge is, in fact, met when

water moves through the xylem. There are three components

to this movement: transpiration from the leaves, the uptake

of water from the soil, and the conduction in the xylem(fig. 4.1).

Figure 4.1 Transpiration-Cohesion Theory of xylem transport. (a) As transpiration occurs in the leaf, it creates a cohesive pull on thewhole water column downward to the roots, where water is absorbed from the soil. (b) Vessel elements join to form a long vessel thatmay reach from the roots to the stem tip.

Xylem

Phloem

The tension createdby transpiration pullswater up into leaves.

Xylem

H2O

Phloem

Xylem

(a) Xylem transport

Water is absorbedby the roots.

Water is cohesive andforms a continuouscolumn in xylem.

H2O

Vesselelement

(b)


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C H A P T E R 4 Plant Physiology 51

TranspirationTranspiration, the loss of water vapor from leaves, is the

force behind the movement of water in xylem. This evapora-

tive water loss occurs mainly through the stomata (90%) and

to a lesser extent through the cuticle (10%). When stomata

are open, gas exchange occurs freely between the leaf and the

atmosphere. Water vapor and oxygen (from photosynthesis)

diffuse out of the leaf while carbon dioxide diffuses into the

leaf (fig. 4.2a). The amount of water vapor that is transpired

is astounding, with estimates of 2 liters (0.5 gallons) of water

per day for a single corn plant, 5 liters (1.3 gallons) for a sun-

flower, 200 liters (52 gallons) for a large maple tree, and 450

liters (117 gallons) for a date palm. Imagine the quantities of

water lost each day from the acres of corn and wheat planted

in the farm belt of the United States! Clearly, transpiration by

plants is a major force in the global cycling of water.

It is the action of the guard cells that regulates the rate

of water lost through transpiration and, at the same time,

regulates the rate of photosynthesis by controlling the CO2

uptake. Each stoma is surrounded by a pair of guard cells,

which have unevenly thickened walls. The walls of the

guard cells that border the stoma are thicker than the outer

walls. When guard cells become turgid they can only expandoutward owing to the radial orientation of cellulose fibrils;

this outward expansion of the guard cells opens the stomata.

Stomata are generally open during daylight and closed at

night. As long as the stomata are open, both transpiration and

photosynthesis occur, but when water loss exceeds uptake,

the guard cells lose turgor and close the stomata (fig. 4.2b).

(See A Closer Look 6.1The Influence of Hormones on

Plant Reproductive Cycles.) On hot, dry, windy days the

high rate of transpiration frequently causes the stomata to

close early, resulting in a near shutdown of photosynthesis

as well as transpiration. A fine balance must be struck in this

photosynthesis-transpiration dilemma to allow enough CO2

for photosynthesis while at the same time preventing exces-

sive water loss. Some plants have evolved an alternate path-way for CO

2uptake at night when rates of transpiration are

lower (see CAM Pathway later in this chapter). Other plants

have morphological or anatomical adaptations that reduce

rates of transpiration while keeping the stomata open. These

physiological and anatomical adaptations are most common

in xerophytes, plants occurring in arid environments.

The basis of transpiration is the diffusion of water mol-

ecules from an area of high concentration within the leaf to

an area of lower concentration in the atmosphere. Unless the

atmospheric relative humidity is 100%, the air is relatively

dry compared with the interior of a leaf, where the intercellu-

lar spaces are saturated with water vapor. As long as stomata

are open, a continuous stream of water vapor transpires from

the leaf, creating a pull on the water column that extends fromthe leaf through the plant to the soil.

Concept Quiz

Xerophytes are plants that are able to grow in arid environments.

Explain how the following adaptations of xerophytes would

reduce transpiration rates and enhance these plants survival in

arid regions:

Thick cuticle

Sunken stomata (stomata are found in cavities)

Leaf surface covered with dense mat of trichomes (hairs)(Hint: See Chapter 3.)

Absorption of Water from the SoilWater and dissolved minerals enter a plant through the root

hairs and can follow two paths, via either the symplastor the

apoplast. Water molecules can diffuse through the plasma

membrane into the cytoplasm of a root hair cell and continue

on this intracellular movement through the cytoplasm of cells

in the cortex. Recall that molecules will move from an area

of high concentration to one of low concentration. The water

molecules move from the dilute soil solution and enter the

Figure 4.2 Transpiration is the basic driving force behindwater movement in the xylem. (a) When stomata are open, bothtranspiration and photosynthesis occur as H

2O molecules diffuse

out of the leaves and CO2

molecules diffuse in. (b) When guardcells are turgid, stoma are open, and when guard cells are flaccid,stoma are closed.

Open Stoma

Closed Stoma

Chloroplast

Guard cells

Nucleus

(a)

(b)


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A CLOSER LOOK 4.1

Research has shown that certain minerals are requiredby plants for normal growth and development. These areincluded in the essential elements listed in Table 1.3. Thesoil is the source of these minerals, which are absorbed bythe plant with the soil water. Even nitrogen, which is a gas inits elemental state, is normally absorbed from the soil in theform of nitrate ions (NO

3). Some soils are notoriously defi-

cient in micronutrients and are therefore unable to supportmost plant life. Serpentine soils, for example, are deficient incalcium, and only plants able to tolerate the low levels of thismineral can survive. In modern agriculture, mineral depletionof soils is a major concern, since harvesting crops interruptsthe natural recycling of nutrients back to the soil.

Mineral deficiencies can often be detected by specificsymptoms such as chlorosis (loss of chlorophyll resulting inyellow or white leaf tissue), necrosis (isolated dead patches),anthocyanin formation (development of deep red pigmenta-tion of leaves or stem), stunted growth, and developmentof woody tissue in an herbaceous plant. Soils are mostcommonly deficient in nitrogen and phosphorus. Nitrogen-deficient plants exhibit many of the symptoms just described.Leaves develop chlorosis; stems are short and slender; and

anthocyanin discoloration occurs on stems, petioles, andlower leaf surfaces. Phosphorus-deficient plants are often

stunted, with leaves turning a characteristic dark green, oftenwith the accumulation of anthocyanin. Typically, older leavesare affected first as the phosphorus is mobilized to younggrowing tissue. Iron deficiency is characterized by chlorosisbetween veins in young leaves.

Much of the research on nutrient deficiencies is basedon growing plants hydroponically, using soil-less nutrientsolutions. This technique allows researchers to create solu-tions that selectively omit certain nutrients and then observethe resulting effects on the plants (box fig. 4.1). Hydroponicshas applications beyond basic research since it facilitates thegrowing of greenhouse vegetables during winter. Aeroponics,a technique in which plants are suspended and the rootsmisted with a nutrient solution, is another method for grow-ing plants in soil-less culture.

While mineral deficiencies can limit the growth of plants,an overabundance of certain minerals can be toxic and canalso limit growth. Saline soils, which have high concentrationsof sodium chloride and other salts, also limit plant growth, andresearch continues to focus on developing salt-tolerant vari-eties of agricultural crops. Research has focused on the toxiceffects of heavy metals such as lead, cadmium, mercury, and

aluminum; however, even copper and zinc, which are essentialelements, can become toxic in high concentrations. Although

Mineral Nutrition and the Green Clean

more concentrated cytoplasm of the root cells. The cytoplasm

of all cells is interconnected through plasmodesmata and is

referred to as the symplast. Thus, this pathway follows the

symplast from a root hair cell into the stele (fig. 4.3).

A second path is the diffusion of water through the cell

walls and intercellular spaces from the root hair through

the cortex (fig. 4.3). The intercellular spaces and the spaces

between the cellulose fibrils in the cell walls constitute the

apoplast of a plant; thus, the water molecules move unim-

peded through the apoplast until they reach the endodermis.The innermost layer of the cortex consists of a specialized

cylinder of cells known as the endodermis. The presence of

a Casparian strip on the walls of the endodermal cells regu-

lates the movement of water and minerals into the stele. The

Casparian strip is a layer of suberin (and in some instances

lignin as well) on the radial and transverse walls (top, bottom,

and sides) that prevents the apoplastic movement of water into

the stele (see Chapter 3, fig. 3.8c). The movement of water

through the selectively permeable plasma membrane is there-

fore directed to the tangential walls of the endodermis and into

the cytoplasm of these cells and, thus, the symplast. By forc-

ing the water and minerals through the symplastic pathway,

some control over the uptake of minerals is exerted. Some

Figure 4.3 Water and minerals can follow one of twopathways across the cortex into the vascular cylinder: (a) theapoplastic pathway, in which water diffuses through the cell wallsand intercellular spaces or (b) the symplastic pathway, in whichwater diffuses into the cytoplasm of a root hair cell and continuesmoving through the cytoplasm from one cell to the next. Inboth pathways, water must move through the symplast of theendodermal cells.

EpidermisCortex

Xylem

Root hair

Endodermis andCasparian strip

Symplasticpathway

Apoplasticpathway


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most plants cannot survive in these soils, certain plants havethe ability to tolerate high levels of these minerals.

Scientists have known for some time that certain plants,called hyperaccumulators, can concentrate minerals atlevels 100-fold or greater than normal. Certain minerals aremore likely to be hyperaccumulated than others. A surveyof known hyperaccumulators identified that 75% of themamassed nickel. Cobalt, copper, zinc, manganese, lead, andcadmium are other minerals of choice.

Hyperaccumulators run the gamut of the plant world.They may be herbs, shrubs, or trees. Many members of the

Brassicaceae or mustard family, Euphorbiaceae or spurge

family, Fabaceae or legume family, and Poaceae or grass familyare top hyperaccumulators. Many are found in tropical andsubtropical areas of the world where accumulation of high

concentrations of metals may afford some protection againstplant-eating insects and microbial pathogens.

Only recently have investigators considered using theseplants to clean up soil and waste sites that have beencontaminated by toxic levels of heavy metalsan environ-mentally friendly approach known as phytoremediation.A green clean scenario begins with the planting of hyperac-cumulating species in the target area, such as an abandonedmine or an irrigation pond contaminated by runoff. Toxicminerals would first be absorbed by roots but later trans-located to the stem and leaves. A harvest of the shootswould remove the toxic compounds off site to be ashed orcomposted to recover the metal for industrial uses. Afterseveral years of cultivation and harvest, the site would be

restored at a cost much lower than the price of excavationand reburial, the standard practice for remediation of con-taminated soils.

In field trials, alpine pennycress (Thlaspi caerulescens)removed zinc and cadmium from soils near the site ofa zinc smelter. Indian mustard (Brassica juncea) native toPakistan and India has been effective in reducing the level ofselenium salts by 50% in contaminated soils. Much interesthas focused on Indian mustard since it has also been shownto concentrate lead, chromium, cadmium, nickel, zinc, andcopper in the laboratory. The aquatic weed parrot feather(Myriophyllum aquaticum) shows promise in restoring con-taminated waterways. Research is ongoing as the searchcontinues for the plants best suited to purify polluted sites

quickly and cheaply.

Box Figure 4.1 The effects of mineral deficiencies are shownin these sunflower plants grown in hydroponic culture. The plants

grown in complete nutrient solution are shown on the right; thoseon the left are deficient in calcium.

minerals are prevented from entering the stele while others are

selectively absorbed by active transport. (See A Closer Look

4.1Mineral Nutrition and the Green Clean.) Once inside the

cytoplasm of the endodermal cells, water moves symplasti-

cally into the living cells of the pericycle, the outermost layer

of the stele. The water moves into the conducting cells of the

xylem, drawn by the pull of transpiration.

Water Movement in PlantsOnce water is in the xylem of the stele, its movement upward

in the plant is driven by the pull of transpiration as well as

certain properties of the water molecule itself, cohesion and

adhesion. Recall from Chapter 1 that the polarity of water

molecules creates hydrogen bonds between adjacent mole-

cules. These hydrogen bonds may form between water mole-

cules themselves (cohesion) or between water molecules and

the molecules in the walls of vessel elements and tracheids

(adhesion). The presence of these water molecules adhering

to the cell walls provides a continuous source of water that

can evaporate into the intercellular spaces of the leaf and

transpire through the stomata. The cohesive force is so strong

that any force or pull on one water molecule acts on all of

them as well, resulting in the bulk flow of water within the

plant. Bulk flow is usually defined as the movement of a fluid

because of pressure differences at two locations. In the xylem,

the pull of transpiration is the force causing the bulk flow. As

transpiration occurs in the leaf, it creates a cohesive pull on

the whole water column downward from the leaf through the

xylem to the root, where water uptake occurs to replace the

water lost through transpiration (fig. 4.1). This mechanism of

water movement in plants is known as the Transpiration-Cohesion Theory and has been used to explain rates of

water movement as high as 44 meters (145 feet) per hour in

angiosperm trees.

Translocation of SugarOrganic materials are translocated by the sieve tube members

of the phloem. In contrast to the xylem, where the conduct-

ing elements function when the cells are dead, the sieve

tube members of the phloem are living but highly special-

ized cells (see Chapter 3). While water movement in the

xylem is upward from the soil, phloem translocation moves

in the direction from source (supply area) to sink (area of

metabolism or storage). In late winter, the source may be


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cells and then move symplastically into sieve tube membersthrough plasmodesmata. This highly concentrated solution in

the sieve tube members causes water to enter by osmosis from

nearby xylem elements, resulting in a buildup of pressure.

When pressure starts to build in these cells, the solute-rich

phloem sap is pushed through the pores in the sieve plate

into the adjacent sieve tube member and so on down to the

sink. This movement of material en masse is known as mass

flow. At the sink, companion cells function in active phloem

unloading, which reduces the concentration of sugars and

allows water to diffuse out of these cells. Sugars unloaded

at the sink are taken up by nearby cells and either stored as

starch or metabolized. (See A Closer Look 4.2Sugar and

Slavery.) The loading and unloading of sugars by active

transport are energy-requiring steps.

METABOLISMMetabolism is the sum total of all chemical reactions occur-

ring in living organisms. Metabolic reactions that synthesize

compounds are referred to as anabolic reactions and are

generally endergonic, requiring an input of energy. In con-

trast, catabolic reactions, which break down compounds, are

usually exergonic reactions, which release energy. Many of

these reactions also involve the conversion of energy from

one form to another.

an underground storage organ translocating sugars to apicalmeristems (the sink) in the branches of a tree. In summer,

the source may be photosynthetic leaves sending sugars for

storage to sinks such as roots or developing fruits (fig. 4.4).

In most plants, the primary material translocated in phloem

is sucrose in a watery solution that also may include small

amounts of amino acids, minerals, and other organic com-

pounds. Translocation in the phloem is quite rapid and has

been timed at speeds averaging 1 meter (3.3 feet) per hour.

The amount of material translocated is also quite impres-

sive. In a growing pumpkin, which reaches a size of 5.5 kg

(11 lbs) in 33 days, approximately 8 g (0.3 oz) of solution are

translocated each hour. Each fall at state and county fairs all

across the United States, prize-winning pumpkins routinely

weigh well over 400 kg (880 lb). The 2003 world recordholder was a 630 kg (1,385 lb) pumpkin grown by Steven

Deletas from Oregon. An even larger pumpkin (663 kg, or

1,458 lb) was grown in New Hampshire but was disqualified

because it was damaged. In the United States in 2003, there

were 63 pumpkins that weighed over 454 kg (1,000 lb).

The hypothesis currently accepted to explain transloca-

tion in the phloem is the Pressure Flow (or Mass Flow)

Hypothesis. This is a modified version of a hypothesis

first proposed by Ernst Mnch in 1926. According to this

hypothesis, there is a bulk flow of solutes from source to

sink (fig. 4.4). At the source, phloem loading takes place as

sugar molecules are first actively transported into companion

Figure 4.4 Translocation of sugar. (a) Products of photosynthesis move from source to sink. At the source, sugar molecules are loadedinto the phloem. As the sugar concentration increases, water moves in from adjacent xylem, pressure builds up, and the sugar solutionis forced through the plant to the sink, where sugar is unloaded from the phloem. (b) A physical model can be used to demonstrate thePressure Flow Hypothesis. The concentrated sugar solution on the left is comparable to the source, and the dilute solution on the rightis comparable to the sink. In bulb 1, water moves into the concentrated sugar solution from the surroundings, pressure builds up, and thesugar solution flows through the tube to bulb 2, where the sugars begin to accumulate, and the pressure causes the water to move out.Recall the discussion of osmosis and diffusion in Chapter 2.

Xylem

(a) (b)

Source

Sink

Sieve tubemember

Companioncell Flow of solution

Concentratedsugar

solution

Dilutesugar

solutionH2O

Selectivelypermeablemembranes

1 2

H2O H2O


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Sugar and Slavery

A CLOSER LOOK 4.2

Products of photosynthesis are typically transported togrowing fruits, storage organs, and other sinks throughoutthe plant. After being unloaded from the phloem, sugars areusually converted to starch or other storage carbohydrates.Although the disaccharide sucrose is the material translo-cated in the phloem of most plants, very few species storesignificant amounts of this sugar. Only two plants, sugarcaneand sugar beet, are commercially important sources ofsucrose, commonly known as table sugar. Sugarcane is themore important crop and, in terms of sheer tonnage, leadsthe global crop production list (see fig. 12.1).

Sugarcane is native to the islands of the South Pacific andhas been grown in India since antiquity. Small amounts ofsugarcane reached the ancient civilizations in the Near Eastand Mediterranean countries through Arab trading routes,but it was not grown in those regions until the seventh cen-tury. Even after cultivation was established, honey remainedthe principal sweetener in Europe until the fifteenth century.During the Middle Ages, sugar was an expensive luxury thatfound its greatest use in medicinal compounds to disguise thebitter taste of many herbal remedies. Early in the fifteenthcentury, sugar plantations were established on islands in the

eastern Atlantic: on the Canary Islands by Spain as well as onMadeira and the Azores by Portugal.Columbus introduced sugarcane to the Caribbean islands

on his second voyage in 1493. By 1509, sugarcane was har-vested in Santo Domingo and Hispaniola and soon spread toother islands. In fact, many Caribbean islands were eventu-ally denuded of native forests and planted with sugarcane.The Portuguese saw the opportunities in South America andstarted sugar plantations in Brazil in 1521. Although late toenter the West Indies, the British established colonies in theearly seventeenth century, and by the 1640s, sugar planta-tions were thriving on Barbados. The first sugarcane grownin the continental United States was in the French colony ofLouisiana in 1753.

The growing of sugarcane was responsible for the estab-lishment of slavery in the Americas, and early in the sixteenthcentury, sugar and the slave trade became interdependent.Decimation of the native populations led to the need fornew workers on the sugar plantations. The first suggestionto use African slaves was made in 1517. Within a short time,the importation of slaves was a reality in both the Spanishand Portuguese colonies, with the greatest number of slavesimported into Brazil. The introduction of African slaves tothese colonies was an outgrowth of the slave trade in Spainand Portugal that had begun in the 1440s. Initially, Spainexported the slaves, but by 1530 slaves were sent directlyfrom Africa to the Caribbean.

The sugar production in the Caribbean came at a timewhen supplies of honey in Europe were decreasing. TheCatholic monasteries were the traditional source of honey.Beehives were kept, principally, to produce beeswaxfor church candles. During the Protestant Reformation,Catholic monasteries were suppressed, and the sources ofhoney fell short of demand. Also, in the late seventeenthcentury, the introduction and growing popularity of coffee,tea, and cocoa in Europe accelerated the demand for sugar,since Europeans generally disliked the naturally bitter tasteof these beverages. Sugar became the most important com-modity traded in the world, and eventually England becamethe dominant force in this enterprise. The Triangular Tradewas the source of many fortunes. The first leg of the tri-angle was from England to West Africa, where trinkets,cloth, firearms, salt, and other commodities were barteredfor slaves. The second leg brought slaves to the CaribbeanIslands, where they were sold. The final leg of the journeycarried rum, molasses, and sugar back to England (boxfig. 4.2a). A second triangle became important in the mid-eighteenth century, linking the West Indies, New England,and West Africa. The use of slaves on sugar plantations

continued until the early nineteenth century, when theslave trade was abolished. It has been estimated that duringthis period 10 million to 20 million African slaves had beenbrought to the New World. Approximately 40% of theslaves brought to the New World went to the Caribbean

Box Figure 4.2a (a) The Triangular Trade.

Caribbean Sea

Atlantic

Ocean

West

Africa

West

Indies

Firearms,

etc.

South

America

North

America

Great

Britain

cloth,

salt,

Rum

,su

gar

Slaves


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Islands. During the seventeenth century, the British sugarplantations in these islands created one of the harshest sys-tems of slavery in history. It was physically crueler and moredemanding than slave conditions on the North Americancontinent. Although the life of a slave was never easy, it wasespecially arduous in the sugar plantations. Few survived thehard labor; consequently, the slave population had to beconstantly replenished.

Sugarcane, Saccharum officinarum, is a perennial memberof the grass family, Poaceae (box fig. 4.2b). Several species ofSaccharum are known to exist in the tropics, and it is believedthat S. officinarum originated as a hybrid of several species.

The species owes its importance to the sucrose stored inthe cells of the stem. Sugarcane, which uses the C4

Pathwayfor photosynthesis, is considered one of the most efficientconverters of solar energy into chemical energy. Canes areoften 5 to 6 meters (15 to 20 feet) tall, with individual stalksup to 10 cm (4 in) in diameter. Plants are grown from stemcuttings, with each segment containing three or four nodesand each node at least one bud. Segments are laid horizon-tally, bud upward, in shallow trenches. Roots soon developfrom the node. Generally 1218 months are needed beforethe canes can be harvested. On fertile land, subsequent cropsdevelop from the rhizome for 2 or 3 years before replant-ing is necessary. Sugarcane thrives in moist lowland tropicsand subtropics and today provides about 60% of the worlds

sugar supply.Canes generally contain 12% to 15% sucrose. After har-vesting, canes are crushed by heavy steel rollers to extractthe sugary juice. The fibrous residue (bagasse) can be usedto make fiberboard, paper, and other products, or used ascompost. Successive boilings concentrate the sucrose; impu-rities are usually precipitated by adding lime water (calciumoxide solution) and are removed by filtering. The solutionis then evaporated to form a syrup from which the sugaris crystallized. Centrifuges separate the thick brown liquidportion from the crystals. The liquid portion is molasses,which is used in foods or is fermented to make rum, ethylalcohol, or vinegar. The crystallized sugar (about 96%97%

pure sucrose) is further refined to free it from any additionalimpurities.

Sugar beet, Beta vulgaris, formerly a member of the goose-foot family, Chenopodiaceae, now in the Amaranthaceae,is not closely related to sugarcane (box fig. 4.2c). It is actu-ally the same species as red beets, which are native to theMediterranean region and have been consumed since thetime of the ancient Romans. In the mid-eighteenth century, aGerman chemist, Andreas Marggraf, discovered that the rootscontained sugar that was chemically identical to that fromcane. The rise of the sugar beet industry can be tied to theEmperor Napoleon I. When a British naval blockade cut offthe sugar imports to France, Napoleon realized the value of a

domestic source of sugar and encouraged scientific researchon the sugar beet. After 1815, sugarcane imports wererestored, halting the developing sugar beet industry. By theearly twentieth century, the sugar beet industry was revivedin both Europe and North America, and today sugar beetsprovide close to 40% of the worlds supply of table sugar.

Sugar beet is a biennial plant, but it is harvested at theend of the first year, when the sucrose content is greatest.Selective breeding gradually raised the percentage of sugarin the root from 2% to approximately 20%. After harvesting,roots are shredded, steeped in hot water, and then pressedto extract the sucrose. Further processing is similar to sugar-cane processing and produces an identical final product.

Box Figure 4.2b (b) Sugarcane harvest near Luxor, Egypt.

Box Figure 4.2c (c) USDA geneticist checks growth anddisease resistance of a sugar beet variety.


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EnergyAll life processes are driven by energy, and consequently, a

cell or an organism deprived of an energy source will soon

die. Energy is defined by physicists as the ability to do work

and is governed by certain physical principles such as the

Laws of Thermodynamics.

The First Law of Thermodynamics states that energy

can neither be created nor destroyed, but it can be con-

verted from one form to another.

Among the forms of energy are radiant (light), thermal

(heat), chemical, mechanical (motion), and electrical. One

focus of this chapter is photosynthesis, the process that con-

verts radiant energy from the sun into the chemical energy of

a sugar molecule.

The Second Law of Thermodynamics states that in

any transfer of energy there is always a loss of useful

energy to the system, usually in the form of heat.

When gasoline is burned as fuel to drive an automobile

engine, chemical energy is converted into mechanical energy,

but the conversion is not very efficient. Some of the energy is

lost as heat to the surroundings.

All forms of energy can exist as either potential energy

or kinetic energy. Potential energy is stored energy that has

the capacity to do work; kinetic energy actually is doing work

or is energy in action. For example, a boulder at the top of a

hill has a tremendous amount of potential energy. If it rolls

down the hillside, the potential energy is changed into kinetic

energy, an exergonic process (fig. 4.5). To push it back up tothe top of the hill would be an endergonic process requiring

considerable input of energy. Transformations from potential

to kinetic and vice versa occur constantly in biological sys-

tems and are part of the underlying principles of both photo-

synthesis and respiration.

Redox ReactionsMany energy transformations in cells involve the transfer of

electrons or hydrogen atoms. When a molecule gains an elec-

tron or a hydrogen atom, the molecule is said to be reduced,

and the molecule that gives up the electron is said to be oxi-

dized. A molecule that has been reduced has gained energy;

likewise the oxidized molecule has lost energy. Oxidation

and reduction reactions are usually coupled (sometimes calledredox reactions); as one molecule is oxidized, the other is

simultaneously reduced.

AH2 B A BH2

(A-reduced) (B-oxidized) (A-oxidized) (B-reduced)

In many oxidation-reduction reactions, an intermediate

is used to transport electrons from one reactant to another.

One such electron intermediate is NAD (nicotinamide

adenine dinucleotide), which can exist in both oxidized

and reduced states (NAD oxidized form and NADH

H reduced form). Similarly, NADP (nicotinamide

adenine dinucleotide phosphate) and FAD (flavin adenine

dinucleotide) also can exist as NADP/NADPHH and

FAD/FADH2, respectively. NAD and FAD are common

electron carriers in respiration; NADP serves the same func-

tion in photosynthesis.

PhosphorylationOther energy transformations involve the transfer of a phos-

phate group. When a phosphate group is added to a molecule,

the resulting product is said to be phosphorylated and has a

higher energy level than the original molecule. These phos-

phorylated compounds may also lose the high-energy phos-

phate group and thereby release energy. The energy currency

of cells, ATP (adenosine triphosphate), is constantly recycled

in this way. When a phosphate group is removed from ATP,

ADP (adenosine diphosphate) is formed, and energy is

released in this exergonic reaction.

ATP ADP PO4 energy

Figure 4.5 Potential and kinetic energy. (a) The boulder at thetop of a hill has a tremendous amount of potential energy. (b) Ifthe boulder rolls down the hill, the potential energy is convertedto kinetic energy. To push the boulder back up to the top wouldrequire a considerable input of energy.

Kinetic energy

(b)

Potential energy

(a)


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Recreating ATP requires the addition of a phosphate groupto ADP (an endergonic reaction) with the appropriate input

of energy.

ADP PO4 energy ATP

EnzymesProteins that act as catalysts for chemical reactions in living

organisms are known as enzymes. Catalysts speed up the

rate of a chemical reaction without being used up or changed

during the reaction. The majority of chemical reactions in

living organisms require enzymes in order to occur at bio-

logical temperatures. Enzymes are highly specific for certain

reactants; the compound acted upon by the enzyme is known

as the substrate.The names of enzymes most commonly endin the suffix -ase, which is sometimes appended to the name

of the substrate or the type of reaction. Some enzymes func-

tion properly only in the presence of cofactorsor coenzymes.

Cofactors are inorganic, often metallic, ions such as Mg

and Mn, while organic molecules such as NAD, NADP,

and some vitamins are coenzymes. Both cofactors and coen-

zymes are loosely associated with enzymes; however, pros-

thetic groups are nonprotein molecules that are attached to

some enzymes and are necessary for enzyme action.

PHOTOSYNTHESIS

Photosynthesis is the process that transforms the vast energyof the sun into chemical energy and is the basis for most food

chains on Earth. The overwhelming majority of life depends

on the photosynthetic ability of green plants and algae, and

without these producers, life as is known today could not

survive.

Energy from the SunThe sun is basically a thermonuclear reactor producing tre-

mendous quantities of electromagnetic radiation, which

bathes Earth. Visible light is only a small portion of the elec-

tromagnetic spectrum, which includes radio waves, micro-

waves, infrared radiation, ultraviolet radiation, X rays, and

gamma rays (fig. 4.6). This radiant energy, or light, has

a dual nature consisting of both particles and waves. Theparticles are known as photons and have a fixed quantity of

energy. It is believed that the photons travel in waves and

thus display characteristic wavelengths. Wavelengths vary

from radio waves, which may be over 1 kilometer long to

gamma rays, which are a fraction of a nanometer (nm) long.

The energy content also varies and is inversely proportional

to the wavelengththat is, the longer the wavelength, the

lower the energy.

Approximately 40% of the radiant energy reaching Earth

is in the form of visible light. If visible light passes through

a prism, the component colors become apparent; these range

from red at one end of the visible band to blue-violet at

the other end. The wavelengths of visible light range from

380 nm (violet) to 760 nm (red) and are the wavelengthsmost important to living organisms (fig. 4.6). In fact, the

wavelengths within this range are the ones absorbed by the

chlorophylls and other photosynthetic pigments in green

plants and algae.

Light-Absorbing PigmentsWhen light strikes an object, the light can pass through the

object (be transmitted), be reflected from the surface, or be

absorbed. For light to be absorbed, pigments must be present.

Pigments absorb light selectively, with different pigments

absorbing different wavelengths and reflecting others. Each

pigment has a characteristic absorption spectrum, which

depicts the absorption at each wavelength. If all visible wave-lengths are absorbed, the object appears black; however, if all

wavelengths are reflected, the object appears white. Green

leaves appear green because these wavelengths are reflected.

In higher plants, the major organ of photosynthesis is

the leaf, and the green chloroplasts within the mesophyll are

the actual sites of this process. The major photosynthetic

pigments are the green chlorophylls, which are located on

the thylakoid membranes of the chloroplasts (fig. 4.7).

Thylakoids can be found in stacks, known as grana, as well

as individually in the stroma, the enzyme-rich ground sub-

stance of the chloroplast. Chloroplasts may have 50 to 80

grana, each with about 10 to 30 thylakoids. The chlorophylls

can be located on the stroma thylakoids as well as in the

grana, and it is the abundance of these pigments that makesleaves appear green.

In green plants there are two forms of chlorophyll, a and

b, which differ slightly in chemical makeup. Most chloroplasts

have three times more chlorophyll a than b. The absorption

spectra of the chlorophylls show peak absorbances in the red

and blue-violet regions, with much of the yellow and green

light reflected (fig. 4.6). Other forms of chlorophyll and other

photosynthetic pigments occur in the algae (Chapter 22).

In addition to chlorophylls, chloroplasts also contain

carotenoids. These include the orange carotenes and yel-

low xanthophylls, which absorb light in the violet, blue, and

bluegreen regions of the spectrum. Carotenoids are called

accessory pigments, and the light energy absorbed by these

pigments is transferred to chlorophyll for photosynthesis.Although present in all leaves, carotenoids are normally

masked by the chlorophylls. Recall that these carotenoids

become apparent in autumn in temperate latitudes, when

chlorophyll degrades.

OverviewPhotosynthesis consists of two major phases, the light reac-

tions and the Calvin Cycle. The light reactions constitute

the photochemical phase of photosynthesis, during which

radiant energy is converted into chemical energy. During

the light reactions, water molecules are split, releasing oxy-

gen and providing electrons for the reduction of NADP to

NADPHH. The light reactions also provide the energy


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for the synthesis of ATP. The Calvin Cycle constitutes the

biochemical phase and involves the fixation and reduction of

CO2

to form sugars using the ATP and NADPH produced in

the light reactions (fig. 4.8).

The Light ReactionsThe light reactions are composed of two cooperating photo-

systems, Photosystems I and II, and take place on the thy-

lakoid membranes within the chloroplasts. Each photosystem

is a complex of several hundred chlorophyll and carotenoid

molecules (known as light-harvesting antennae) and associ-

ated membrane proteins. Countless units of these photosys-

tems are arrayed on the thylakoid membranes throughout the

chloroplast. When light strikes a pigment molecule in either

photosystem, the energy is funneled into a reaction center,

which consists of a chlorophyll a molecule bound to a mem-

brane protein (fig. 4.9). The reaction center for Photosystem I

is known as P700

, which indicates the wavelength of maximum

light absorption in the red region of the spectrum; the reaction

center for Photosystem II is P680

, again indicating the peak

absorbance. Associated with the photosystems are various

enzymes and coenzymes that function as electron carriers andare components of the thylakoid membranes.

When a photon of light strikes a pigment molecule in

the light-harvesting antennae of Photosystem I, the energy

is funneled to P700

(fig. 4.10). When P700

absorbs this energy,

an electron is excited and ejected, leaving P700

in an oxidized

state. The ejected electron is picked up by a primary elec-

tron acceptor, which then passes the electrons on to ferre-

doxin (Fd), another electron intermediate, and eventually to

NADP, reducing it to NADPH H, one of the products of

the light reactions.

Another photon of light absorbed by a chlorophyll mol-

ecule in Photosystem II will transfer its energy to the reaction

center P680

(fig. 4.10). When P680

absorbs this energy, an excited

Figure 4.6 Energy from the sun drives the process of photosynthesis. (a) Visible light is only a small portion of the electromagneticspectrum. (b) If visible light is passed through a prism, the component colors are apparent. The chlorophyll pigments in leaves absorb theblue-violet and orange-red portions of the spectrum. (c) Leaves reflect the green and yellow portions of the spectrum.

Absorption

Reflected

Absorbed

Transmitted

ElectromagneticSpectrum

Yellow Orange Red

Absorbed

Absorbed

Violet Blue Green

Gam

ma

rays

X

rays

Ult

rav

iole

tlight

Visible

light

Infra

red

light

Micr

owaves

Radi

ow

aves

Decreasing wavelength

High energy Low energy

Chlorophyllb

Chlorophylla

Transmittedand reflected

400 nm 500 nm 700 nm600 nm

(b)

(a)

(c)


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Figure 4.7 The major organ of photosynthesis is the leaf, andthe actual site of photosynthesis is the chloroplast. (a)(d) Leafcells with chloroplasts; (e)(f) internal structure of the chloroplast.Thylakoid sacs comprise the grana, sites of the light reactions. Thestroma contains the enzymes that carry out the Calvin Cycle.

(a)

Cuticle

Epidermis

Nucleus

Vacuole

Cellwall

Outer membrane

Inner membrane

Stroma

ThylakoidGranum

Chloroplast

Palisade cell

Spongycell

Stoma

Vascularbundle

(b)

(c)

(d)

(e)

Thylakoids in grana

Stroma(f)


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Figure 4.8 Photosynthesis consists of two major phases, the light reactions and the Calvin Cycle.

Light

Electrons

Chloroplast

H2O

O2

ATP

CO2

Sugar

P

NADP

ADP

Calvin

Cycle(in stroma)

NADPH

Light

Reaction(on

thylakoidmembranes)

Figure 4.9 Photosystem. Each photosystem is composed of several hundred chlorophyll and carotenoid molecules that make up light-harvesting antennae. When light strikes a pigment molecule, the energy is transferred and funneled into a reaction center.

Pigment molecule

Energy transfer

Reactioncenter

Light-harvestingantennaecomplex

Incomingphotons(light)

Light


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electron is ejected and passed on to another primary electron

acceptor, leaving P680

in an oxidized state. The electron lost by

P680

is replaced by an electron from water, in a reaction that

is not fully understood, and catalyzed by an enzyme on thethylakoid membrane that requires manganese atoms. In this

reaction, water molecules are split into oxygen and hydrogen;

the hydrogen is a source of both electrons and protons.

The primary electron acceptor passes the electron on to

a series of thylakoid membrane-bound electron carriers that

include plastoquinone (Pq),cytochrome complex,plasto-

cyanin (Pc), and others. The electron is eventually passed

to the oxidized P700

in Photosystem I. During the transfer

of electrons, ATP is synthesized as protons are passed from

the thylakoid lumen into the stroma by an ATP synthasein

the membrane. It is actually the passage of protons through

this enzyme that drives the production of ATP; however, the

mechanism for this reaction is still not completely under-

stood. This synthesis of ATP is known as photophosphory-lation, since the energy that drives the whole process is from

sunlight.

In the process just described, the two photosystems are

joined together by the one-way transfer of electrons from

Photosystem II to Photosystem I. Water is the ultimate source

of these electrons, continually replenishing electrons lost

from P680

. The photophosphorylation that occurs during this

process is referred to as noncyclic photophosphorylation

since the electron transfer is one-way, with the reduction of

NADP as the final step.

Photosystem I is also capable of functioning indepen-

dently, transferring electrons in a cyclic fashion. The elec-

trons, instead of being passed to NADP from ferredoxin, may

be passed to the cytochrome complex and then back to P700

(fig. 4.11). ATP, but not NADPH, may be generated during

this process, which is known as cyclic photophosphoryla-

tion, since the flow of electrons begins and ends with P700

.

As just described, when water is split, oxygen is released.The oxygen eventually diffuses out of the leaves into the

atmosphere and is Earths only constant supply of this gas.

The current 20% oxygen content in the atmosphere is the

result of 3.5 billion years of photosynthesis. The atmosphere

of early Earth did not contain this gas; oxygen began to accu-

mulate only after the evolution of the first photosynthetic

organisms, the cyanobacteria. Today, the vast majority of liv-

ing organisms depend on oxygen for cellular respiration and,

therefore, the energy that maintains life.

The overall light reactions proceed with breathtaking

speed as a constant flow of electrons moves from water to

NADPH, powered by the vast energy of the sun. The ATP

and NADPH that result from the light reactions are needed to

drive the biochemical reactions in the Calvin Cycle.

The Calvin CycleThe source of carbon used in the photosynthetic manufac-

ture of sugars is carbon dioxide from the atmosphere. This

gas makes up just a tiny fraction, approximately 0.035%, of

Earths atmosphere and enters the leaf by diffusing through

the stomata. The reactions that involve the fixation and reduc-

tion of CO2

to form sugars are known as the Calvin Cycle and

are sometimes referred to as the C3Pathway. These reactions

utilize the ATP and NADPH produced in the light reactions

but do not involve the direct participation of light and are

hence sometimes referred to as light-independent reactions,

or dark reactions. The Calvin Cycle takes place in the stroma

Figure 4.10 Two cooperating photosystems work together to transfer electrons from water to NADPH. The passage of electronsfrom Photosystem II to Photosystem I also drives the formation of ATP, a process known as noncyclic photophosphorylation.

Incoming e

e

Photosystem IPhotosystem II

1O2 2H

e e

2H2O

Reaction center Reaction center

Light Light

photons(light)


Cytochromecomplex

ATP

ADP

Fd

NADPH

Pq

Pc

Primaryacceptor

Primaryacceptor

NADP

reductase

NADP H

P680 P700


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of the chloroplasts, which contains the enzymes that catalyze

the many reactions in the cycle. This pathway was worked out

by Melvin Calvin, in association with Andrew Benson and

James Bassham, during the late 1940s and early 1950s. The

pathway is named in honor of Calvin, who received a Nobel

Prize for his work in 1961.The following discussion will be limited to the main

events of the Calvin Cycle, which are depicted in Figure 4.12.

The end product of this pathway is the synthesis of a six-

carbon sugar; this requires the input of carbon dioxide. Six

turns of the cycle are needed to incorporate six molecules

of CO2

into a single molecule of a six-carbon sugar. The

initial event is the fixation or addition of CO2to ribulose-1,

5-bisphosphate (RUBP), a five-carbon sugar with two

phosphate groups. This carboxylation reaction is cata-

lyzed by the enzyme ribulose bisphosphate carboxylase

(RUBISCO). In addition to its obvious importance to

photosynthesis, RUBISCO appears to be the most abundant

protein on Earth since it constitutes 12.5% to 25% of total

leaf protein. The product of the carboxylation is an unstablesix-carbon intermediate that immediately splits into two

molecules of a three-carbon compound, with one phosphate

group called phosphoglyceric acid (PGA), or phospho-

glycerate. Six turns of the cycle would yield 12 molecules

of PGA.

The 12 PGA molecules are converted into 12 molecules

of glyceraldehyde phosphate, or phosphoglyceraldehyde

(PGAL). This step requires the input of 12 NADPH H

and 12 ATP (both generated during the light reactions), which

supply the energy for this reaction. Ten of the 12 glyceral-

dehyde phosphate molecules are used to regenerate the six

molecules of RUBP in a complex series of interconversions

that require six more ATP and allow the cycle to continue.

Two molecules of glyceraldehyde phosphate are the net gain

from six turns of the Calvin Cycle; these are converted into

one molecule of fructose-1,6-bisphosphate, which is soon

converted to glucose. The glucose produced is never stored as

such but is converted into starch, sucrose, or a variety of other

products, thus completing the conversion of solar energy into

chemical energy.The complex steps of photosynthesis can be summarized

in the following simple equation, which considers only the

raw materials and end products of the process:

CHLOROPHYLL6CO

2 12H

2O sunlight C

6H

12O

6 6O

2 6H

2O

Concept Quiz

Photosynthesis consists of two major phases: the light reac-tions, in which light energy is converted into chemical energy,and the Calvin Cycle, in which the fixation and reduction of

carbon dioxide to form sugars take place.How is each phase dependent upon the other?

Variation to Carbon FixationMany plants utilize a variation of carbon fixation that consists

of a prefixation of CO2

before the Calvin Cycle. There are two

pathways in which this prefixation occurs, the C4

Pathway

and the CAM Pathway. The C4

pathway occurs in several

thousand species of tropical and subtropical plants, including

the economically important crops of corn, sugarcane, and sor-

ghum. This pathway, occurring in mesophyll cells, consists of

incorporating CO2 into organic acids, resulting in a four-carbon

Figure 4.11 Photosystem I can also function in a cyclic fashion. Instead of reducing NADP, electrons are passed back to P700

. Thisprocess results in the generation of ATP by cyclic photophosphorylation.

Photosystem I


Fd

ATP

Pq

P700

Pc

e

Light

Cytochromecomplex

Primaryacceptor

Reactioncenter


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compound and, hence, the name of the pathway. This com-

pound is soon broken down to release CO2

to the Calvin Cycle,

which occurs in cells surrounding the vascular bundle. The C4

pathway ensures a more efficient delivery of CO2

for fixation

and greater photosynthetic rates under conditions of high light

intensity, high temperature, and low CO2

concentrations.

These same steps are part of the CAM (Crassulacean

Acid Metabolism) pathway, which functions in a number

of cacti and succulents, plants of desert environments. This

pathway was initially described among members of the plant

family Crassulaceae. CAM plants are unusual in that theirstomata are closed during the daytime but open at night. Thus,

they fix CO2

during the nighttime hours, incorporating it into

four-carbon organic acids. During the daylight hours, these

compounds are broken down to release CO2

to continue on

into the Calvin Cycle. This alternate pathway allows carbon

fixation to occur at night when transpiration rates are low, an

obvious advantage in hot, dry desert environments.

CELLULAR RESPIRATIONAs discussed, photosynthesis converts solar energy into

chemical energy, stored in a variety of organic compounds.

Starch and sucrose are common storage compounds in

plants and as such are the energy reserves for the plants

themselves and the animals that feed on them. Ultimately

the survival of all organisms on Earth is dependent on the

release of this chemical energy through the catabolic pro-

cess of cellular respiration. All living organisms require

energy to maintain the processes of life. Even at the cel-

lular level, life is a highly dynamic system, requiring

continuous input of energy that is used in the processes of

growth, repair, transport, synthesis, motility, cell division,

and reproduction. Cellular respiration occurs continuously,

every hour of every day, in all living cells; the need forenergy is nonstop.

Cellular respiration is actually a step-by-step breakdown

of the chemical bonds in glucose, involving many enzymatic

reactions, and results in the release of usable energy in the

form of ATP. The overall process is the complete oxidation

of glucose, resulting in CO2

and H2O and the formation of

ATP:

C6H

12O

6 6O

2 6CO

2 6H

2O 36 ATP

This equation of cellular respiration is merely a summary of

a complex step-by-step process that has three major stages

or pathways: glycolysis, the Krebs Cycle, and the Electron

Transport System.

Figure 4.12 The Calvin Cycle. For every six molecules of CO2

that enter the cycle, one six-carbon sugar is produced. The ATP andNADPH required by this cycle are generated by the light reactions.

ADPNADPH

NADP

ADP

12

12

12

12

6

6

Carbon

Glucose and other sugars

ATP

6 CO2

6 RUBP

10 PGAL12 PGAL

12 PGA

2 PGAL

ATP

Manyintermediatereactions (andrearrangementsof carbon bonds)


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Glycolysis is a series of reactions that occur in thecytoplasm and result in the breakdown of glucose into two

molecules of a three-carbon compound. Along the way, NAD

is reduced and some ATPs are produced. The Krebs Cycle

continues the breakdown of the three-carbon compounds in

the matrix of the mitochondria and results in the release of

CO2. Additional ATP, NADH, and FADH

2are also gener-

ated during these steps. The final stage of respiration, the

Electron Transport System, occurs on the cristae, the inner

membrane of the mitochondria, and consists of a series of

redox reactions during which significant amounts of ATP

are synthesized. A comparison of photosynthesis and cellular

respiration is presented in Table 4.1.

GlycolysisThe word glycolysis means the splitting of sugar. It starts

with glucose, which arises from the breakdown of poly-

saccharides, most commonly either starch or glycogen (in

animals and fungi), or the conversion from other substances,

especially other sugars. The first few steps in glycolysis

actually add energy to the molecule in the form of phosphate

groups (fig. 4.13). These phosphorylations are at the expense

of two molecules of ATP. In addition, the glucose molecule

undergoes a rearrangement that converts it to fructose-1,

6-bisphosphate. These steps prime the molecule for the later

oxidation. The next step splits fructose-1, 6-bisphosphate

into glyceraldehyde phosphate and dihydroxyacetone

phosphate, but the latter is converted into a second mole-cule of glyceraldehyde phosphate. Both glyceraldehyde

phosphate molecules continue on in the glycolytic pathway

so that each of the remaining steps actually occurs twice.

Glyceraldehyde phosphate is phosphorylated and oxidized

in the next step, which also reduces NAD to NADH H.

The resulting organic acids, with two phosphate groups, give

up both phosphates in the remaining steps of glycolysis,

yielding two molecules of pyruvate (pyruvic acid), plus 4

ATP and 2 NADH H. Note that during steps 7 and 10

a total of 4 ATP are produced; however, two of those ATP

molecules are used during the initial steps, resulting in a netgain of only 2 ATP.

The Krebs CycleThe remainder of cellular respiration occurs in the mitochon-

dria of the cell (fig. 4.14). Recall from Chapter 2 that mito-

chondria are organelles with a double membrane. Although

the outer membrane is smooth, the inner membrane is

invaginated; these folds are referred to as cristae. The area

between the outer and inner membranes is referred to as the

intermembrane space; the enzyme-rich area enclosed by

the inner membrane is known as the matrix. The enzymes in

the matrix catalyze each step in the Krebs Cycle.

Once inside the mitochondrial matrix, each of the twopyruvate molecules from glycolysis undergoes several

changes before it enters the Krebs Cycle. The molecule

is oxidized and decarboxylated, losing a CO2, with the

remaining two-carbon compound joining to coenzyme A

to form a complex known as acetyl-CoA. During this step,

NAD is also reduced to NADH H. Acetyl-CoA enters

the Krebs Cycle by combining with a four-carbon organic

acid known as oxaloacetate (oxaloacetic acid) to form a

six-carbon compound known as citrate (citric acid). (The

Krebs Cycle, named in honor of Hans Krebs, who worked

out the steps in this pathway in 1937 and later received a

Nobel Prize for this work, is alternatively known as the

Citric Acid Cycle.)

The steps in the cycle consist of a series of reactionsduring which two more decarboxylations (going from a six-

carbon to a five-carbon and then to a four-carbon compound)

and several oxidations occur (fig. 4.15). During these steps,

three more molecules of NAD are reduced to NADH H,

a molecule of FAD is reduced to FADH2, and one molecule

of ATP is formed. At the end of these steps, the four-carbon

oxaloacetate is regenerated, allowing the cycle to begin

anew. For each molecule of pyruvate that entered the mito-

chondrion, three molecules of CO2

are released and 1 ATP,

4 NADH H, and 1 FADH2

are produced. Since two

Table 4.1Comparison of Photosynthesis and Cellular Respiration

Photosynthesis Cellular Respiration

Overall reaction 6CO212H

2O sunlight C

6H

12O

66O

26H

2O C

6H

12O

66O

26CO

26H

2O 36ATP

Reactants Carbon dioxide, water, sunlight Glucose, oxygen

Products Glucose Energy

By-products Oxygen Carbon dioxide water

Cellular location Chloroplasts Cytoplasm, mitochondria

Energetics Requires energy Releases energy as ATP

Chemical pathways Light reactions and Calvin Cycle Glycolysis, Krebs Cycle, and Electron Transport System

Summary Sugar synthesized using the energy of the sun Energy released from the breakdown of sugar


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Figure 4.13 Glycolysis.

1.

Glucose

Glucose6phosphate

Fructose6phosphate

Fructose1,6diphosphate

Glyceraldehyde3phosphate (G3P)

2.

3.

4. 5.

Dihydroxyacetone phosphate

13diphosphoglycerate (DPG)

6.

Occurs

twice

Carbon

3PGA

7.

2PGA

8.

PEP

9.

Pyruvate

10.

ADP

1. Phosphorylation of glucose by ATP

2-3. Rearrangement to fructose, followed by a second

ATP phosphorylation

4-5. The six-carbon molecule is split into 2 three-carbon molecules of G3P

6. Oxidation followed by phosphorylation produces

2 NADH molecules and gives 2 molecules ofDPG, each with one high-energy phosphate bond

10. Removal of high-energy phosphate by 2 ADPproduces 2 ATP molecules and gives 2 pyruvate

molecules

ADP

ADP

ATP

ADP

NADH

NAD

H2O

P

P

P

P P

P

P

P

P

P

P

P

ATP

ATP

ATP

7. Removal of high-energy phosphate by 2 ADPmolecules produces 2 ATP molecules and gives

2 molecules of 3 phosphoglyceric acid (PGA)

8-9. Phosphate is moved and removal of water gives2 phosphoenol pyruvate (PEP) molecules each

with a high-energy phosphate bond

ADP


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Figure 4.15 Krebs Cycle. Before the cycle begins, pyruvate is converted to acetyl-CoA with the loss of CO2. The two-carbon

acetyl group combines with oxaloacetate to form the six-carbon citrate. Two decarboxylations and several redox reactions regenerateoxaloacetate. For every pyruvate that enters the mitochondrion, 4 NADH, 1 FADH

2, and 1 ATP are produced, and 3 CO

2are released.

CoA

CoA

Carbon

CO2leaves cycle

Citrate

CO2leaves cycle

KrebsCycle

Pyruvate

(from glycolysis)

CO2removed

- Ketoglutarate

Succinate

Malate

Oxaloacetate

NADH

NAD

ATP ADP

NADH NAD

NADH

NAD

NADH

NAD

Acetyl-CoA

2 carbonsenter cycle

FADH2

FAD

Figure 4.14 Mitochondrial structure. The inner mitochondrial membrane has numerous infoldings known as cristae. The enzymes andcoenzymes of the Electron Transport System occur on these membranes. The matrix contains the enzymes that carry out the Krebs Cycle,(a) 85,000, TEM. (b) Cut-away diagram of mitochondrion to show internal organization.

Matrix

Cristae

(a)

Cristae

Matrix

Intermembranespace

Outer membrane

Inner membrane

(b)


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pyruvate molecules are formed from each glucose molecule,the cycle turns twice, resulting in 6 CO

2released and yield-

ing 2 ATP, 8 NADH H, and 2 FADH2

as energy-rich

products. At this point, the entire glucose molecule has been

totally degraded; a portion of its energy has been harvested

in these Krebs Cycle products as well as the 2 ATP and 2

NADH H produced in glycolysis.

The Electron Transport SystemThe third and final stage of cellular respiration, the Electron

Transport System, occurs on the inner membranes of the

mitochondria and involves a series of enzymes and coen-

zymes, including several iron-containing cytochromes that

are embedded in this layer and function as electron carri-ers. (This series of electron carriers is similar to the ones

described in the light reactions of photosynthesis.) During

this stage, electrons and hydrogen ions are passed from the

NADH H and FADH2

molecules formed in glycolysis

and the Krebs Cycle down a series of redox reactions and

are finally accepted by oxygen-forming water in the process

(fig. 4.16).

The Electron Transport System is a highly exergonic

process and is coupled to the formation of ATP. This method

of ATP synthesis is referred to as oxidative phosphoryla-

tion. When the electron flow begins from NADH produced

within the mitochondria, enough energy is available to pro-

duce three molecules of ATP from each NADH for a total of

24 ATP from the 8 NADH. Two molecules of ATP are alsosynthesized during the flow of electrons from each FADH

2

produced in the Krebs Cycle (4 ATP) and each NADH from

glycolysis (4 ATP). During the Electron Transport System

then, a total of 32 ATP are generated. This number is added

to the net yield of 2 ATP from glycolysis and the 2 ATP

produced in the Krebs Cycle, for a grand total of 36 ATP

for each glucose molecule that completes cellular respira-

tion (table 4.2). These ATP molecules are then transported

out of the mitochondria and are available for use within

the cell. It should be noted, however, that this production

of 36 ATP harnesses only a fraction, 39%, of the original

chemical energy of the glucose molecule; the remainder is

lost as heat. (Although this 39% efficiency seems low, it is

actually much higher than energy conversions in mechanicalsystems.)

The formation of ATP during the transport of electrons

is believed to occur by the same mechanism described for

photophosphorylation during photosynthesis. During the

transfer of electrons, protons pass from the intermembrane

space into the matrix through an ATPase in the membrane

(fig. 4.17). It is actually the passage of protons through this

ATP synthase enzyme that somehow drives the production

of ATP. This model for ATP synthesis is known as chemi-

osmosis and was first proposed by Peter Mitchell during

the early 1960s. Mitchell received a Nobel Prize for this

Figure 4.16 Electron Transport System. Electrons fromNADH and FADH

2are passed along electron-carrier molecules

(number 1 to 5), including several cytochromes, and finally areaccepted by oxygen. This process drives the formation of ATP bychemiosmosis.

NADHNAD

O212

H2O

FADH2

1

2

3

4

5

FAD

2H

theory, which applies to ATP synthesis in both respiration

and photosynthesis.

Concept Quiz

All living organisms carry out cellular respiration. In plantsthe sugars that are broken down during cellular respirationwere produced by the plant during photosynthesis.

What is the immediate source of compounds broken down

during cellular respiration in humans? What is the ultimate

source of these compounds?


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Aerobic vs. Anaerobic RespirationThe complete oxidation of glucose requires the presence of

oxygen and is therefore known as aerobic respiration. As

indicated, it is the last step of respiration and involves the

direct participation of oxygen as the final electron acceptor.

Without oxygen, this last step cannot occur since no othercompound can serve as the ultimate electron acceptor. In fact,

both the Electron Transport System and the Krebs Cycle are

dependent on the availability of oxygen and cannot operate

in its absence.

Some organisms, however, have metabolic pathways

that allow respiration to proceed in the absence of oxygen.

This type of respiration is known as anaerobic respiration,

or fermentation, and is found in some yeast (a unicellular

fungus), in bacteria, and even in muscle tissue. The most

familiar example is alcoholic fermentation found in cer-

tain types of yeast and utilized in the production of beer

and wine (see Chapter 24). When oxygen is not available,

the yeast cells can switch to a pathway that can convert

pyruvic acid to ethanol and CO2. In the process, NADH isoxidized back to NAD, allowing this coenzyme to recycle

back to glycolysis. Recycling of the coenzyme allows gly-

colysis to continue and thus supply the energy needs of

the yeast, at least in a limited way. The only energy yield

from this alcoholic fermentation is the 2 ATP produced

during glycolysis (compared with 36 ATP during aerobic

respiration). The still energy-rich alcohol is merely a by-

product of the oxidation of NADH. If oxygen becomes

available, yeast can switch back to aerobic respiration

with its higher energy yield. Other anaerobic pathways

also exist in bacteria and muscle cells, in which the by-

products are different from alcohol but the yield of NAD

and ATP is similar.

Figure 4.17 Chemiosmosis in mitochondria and chloroplasts. In the mitochondria, protons (H) are translocated to the intermembranespace during the transfer of electrons down the Electron Transport System. The proton gradient drives ATP synthesis as protons movethrough the ATP synthase complex back to the matrix. In the chloroplast, protons are translocated into the thylakoid compartment. Asprotons move through the ATP synthase complex back to the stroma, ATP is synthesized.

Electrontransportchain

Mitochondrion Chloroplast

Membrane

ADP P

H

ATPsynthase

H

H

H

H HH

H

ATP

High H

concentration

Low H

concentration

Intermembranespace

Thylakoidcompartment

Matrix Stroma

Table 4.2 Tally of ATP Producedfrom the Breakdown of Glucose

During Cellular Respiration

Pathway Net ATP Yield*

Glycolysis

2 ATP 2 ATP

2 NADH 4 ATP

Acetyl-CoA formation (2 turns)

1 NADH 2 6 ATP

Krebs Cycle (2 turns)

1 ATP 2 2 ATP

3 NADH 2 18 ATP

1 FADH22 4 ATP

TOTAL 36 ATP

*Each NADH produced in the mitochondrion yields 3 ATP while each NADH

produced during glycolysis has a net yield of 2 ATP. Each FADH 2 also yields 2 ATP.


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CHAPTER SUMMARY1. Plants obtain water from the soil, moving it up within the

xylem through the entire plant. This movement of water is

a passive phenomenon dependent on the pull of transpira-

tion and the cohesion of water molecules. Minerals are

also obtained from the soil and transported in the xylem.

The translocation of sugars occurs in the phloem, moving

from source (photosynthetic leaves or storage organs)

to sink (growing organs or developing storage tissue)

through mass flow within sieve tube members.

2. Sucrose is the usual sugar transported in the phloem;

however, very few plants actually store this economically

valuable carbohydrate. Sugarcane and sugar beet are the

major sucrose-supplying crops. The early developmentof sugarcane plantations in North America greatly influ-

enced the course of history by introducing the slave trade

to the continent.

3. Plants are dynamic metabolic systems with hundreds of

reactions occurring each second to enable plants to live,

grow, and respond to their environment. All life processes

are driven by energy, with some metabolic reactions being

endergonic and others exergonic. Energy transformations

occur constantly in biological systems and are part of the

underlying principles of both photosynthesis and cellular

respiration.

4. Photosynthesis takes place in chloroplasts of green plants

and algae and results in the conversion of radiant energyinto chemical energy (linking the energy of the sun with

life on Earth). Using the raw materials carbon dioxide and

water, along with chlorophyll and sunlight, plants are able

to manufacture sugars. In the light reactions of photosyn-

thesis, energy from the sun is harnessed, forming mol-

ecules of ATP and NADPH. During this process, water

molecules are split, releasing oxygen to the atmosphere

as a by-product. During the Calvin Cycle, carbon dioxide

molecules are fixed and reduced to form sugars, using the

energy provided by the ATP and NADPH from the light

reactions.

5. Cellular respiration is the means by which stored energy

is made available for the energy requirements of the

cell. Through respiration, the energy of carbohydrates istransferred to ATP molecules, which are then available

for the energy needs of the cell. During aerobic respira-

tion, each molecule of glucose is completely oxidized

during the many reactions of glycolysis, the Krebs Cycle,

and the Electron Transport System, resulting in the for-

mation of 36 ATP molecules. In anaerobic respiration,

only two molecules of ATP are formed from each glu-

cose molecule.

REVIEW QUESTIONS1. Explain how water enters a root.

2. What is transpiration, and how does it affect water move-

ment in plants?

3. How are the properties of water important to the theory of

water movement in plants?

4. What are the advantages and disadvantages to having

stomata open during the daylight hours?

5. Explain how the Pressure Flow Hypothesis accounts for

translocation in the phloem.

6. How is light harnessed during the light reactions of pho-

tosynthesis, and what pigments are involved?

7. What is carbon fixation? How is carbon fixed during the

Calvin Cycle?

8. Why is glycolysis important to living organisms, and

where does it occur?

9. Describe mitochondria and the respiratory events that

occur in them.

10. Few plants can survive the saline soils of deserts or

coastal areas, where mineral salts such as sodium chloride

accumulate in extremely high concentrations. Why?

11. If a 500 kg (1,100 lb) pumpkin develops during a 5-month

growing season, determine how much photosynthate

(products of photosynthesis) is transported into the grow-

ing fruit each hour.

12. How are the processes of transpiration and photosynthesis

interrelated?

13. In what way is life on Earth dependent on the energy of

the sun?

FURTHER READING

Beardsley, Tim. 1998. Catching the Rays. Scientific American

278(3): 2526.

Brown, Kathryn S. 1995. The Green Clean.BioScience 45(9):

579582.

Caldwell, Mark. 1995. The Amazing All-Natural Light

Machine.Discover16(12): 8896.

Demmig-Adams, Barbara, and William W. Adams III. 2002.

Antioxidants in Photosynthesis and Human Nutrition.

Science 298: 21492153.

Dunn, Richard S. 1972. Sugar and Slaves. W.W. Norton,

New York, NY.

Govindjee, and William J. Coleman. 1990. How Plants Make

Oxygen. Scientific American 262(2): 5056.


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Hopkins, William G. and Norman P. A. Hner. 2004.Introduction to Plant Physiology, 3rd Edition. John Wiley

& Sons, New York.

Lee, David W. and Kevin S. Gould. 2002. Why Leaves Turn

Red.American Scientist90(6): 524531.

Mintz, Sidney W. 1991. Pleasure, Profit, and Satiation.

Pages 112129 in Seeds of Change: A Quincentennial

Commemoration, ed. Herman J. Viola and Carolyn

Margolis. Smithsonian Institution Press,Washington, DC.

Moffat, Anne Simon. 1995. Plants Providing Their Worth in

Toxic Metal Cleanup. Science 269: 302303.

Moore, Randy,W. Dennis Clark, and Darrell Vodopich.

1998. Botany: Plant Form and Function, 2nd Edition.

WCB/McGraw-Hill, Dubuque, IA.Seymour, Roger S. 1997. Plants That Warm Themselves.

Scientific American 276(3): 104109.

Taiz, Lincoln, and Eduardo Zeiger. 2006. Plant Physiology,4th Edition. Sinauer Associates, Sunderland, MA.

Turgeon, Robert. 2006. Phloem Loading: How Leaves Gain

their Independence.BioScience 56(1): 1524.

Williams, John B. 2002. Phytoremediation in Wetland

Ecosystems: Progress, Problems, and Potential. Critical

Reviews in Plant Sciences 21(6): 607635.

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