PHOTOSYNTHESIS

                

PHOTOSYNTHESIS IN NATURE

- Plants and other autotrophs are the producers of the biosphere

- Chloroplasts are the sites of photosynthesis in plants

THE PATHWAYS OF PHOTOSYNTHESIS

- Evidence that chloroplasts split water molecules enabled researchers to track atoms through photosynthesis

- The light reactions and the Calvin cycle cooperate in converting light energy to the chemical energy of food: an overview

- The light reactions convert solar energy to the chemical energy of ATP and NADPH: a closer look

- The Calvin cycle uses ATP and NADPH to convert CO2 to sugar: a closer look

PHOTOSYNTHESIS IN NATURE

* Plants and other autotrophs are the producers of the biosphere

* Chloroplasts are the sites of photosynthesis in plants

Plants and other autotrophs are the producers of the biosphere

- Photosynthesis nourishes almost all of the living world directly or indirectly.

- An organism acquires the organic compounds it uses for energy and carbon skeletons by one of 2 major modes: autotrophic or heterotrophic nutrition

- The term autotrophic (from the Greek autos, self, and trophos, feed) may seem to contradict the principle that organisms are open systems, taking in resources from their environment

- For this reason, biologists refer to autotrophs as the producers of the biosphere (the global ecosystem).

- Plants are autotrophs; the only nutrients they require are CO2 from the air, and H2O and minerals from the soil.

- Specifically, plants are photo autotrophs, organisms that use light as a source of energy to synthesize organic substances. Photosynthesis also occurs in algae, including certain protists, and in some prokaryotes.

- Heterotrophs obtain their organic material by the second major mode of nutrition.

- Unable to make their own food, they live on compounds produced by other organisms; heterotrophs are the biosphere’s consumers.

- The most obvious form of this "other-feeding" (hetero means "other, different") occurs when an animal eats plants or other animals

Fig 2.19. Photoautotrophs. These organisms use light energy to drive the synthesis of organic molecules from carbon dioxide and (in most cases) water. They feed not only themselves, but the entire living world. (a) On land, plants are the predominant producers of food. Three major groups of land plants--mosses, ferns, and flowering plants--are represented in this scene. In oceans, ponds, lakes, and other aquatic environments, photosynthetic

organisms include (b) multicellular algae, such as this kelp; (c) some unicellular protists, such as Euglena; (d) the prokaryotes called cyanobacteria; and (e) other photosynthetic prokaryotes, such as

these purple sulfur bacteria (c, d, e: LMs).

(a) (b)

(c)

(d)

(e)

algae(Fern)

Protist Sulfur bacteriacyanobacteria

Chloroplasts are the sites of photosynthesis in plants

- The leaves are the major sites of photosynthesis in most plants (fig10-2).

- The color of the leaf is from chlorophyll, the green pigment located within the chloroplasts.

- Chlorophyll resides in the thylakoid membranes

- Chlorophyll absorbs the light energy and drives the synthesis of food molecules in the chloroplast.

- Chloroplasts are found mainly in the cells of the mesophyll, the tissue in the interior of the leaf.

- CO2 enters and O2 exits the leaf, by way of microscopic pores called stomata (singular, stoma; from the Greek, meaning "mouth").

- H2O absorbed by the roots is delivered to the leaves in veins.

- Leaves also use veins to export sugar to roots and other nonphotosynthetic parts of the plant.

Fig 2.20. Focusing in on the location of

photosynthesis in a plant. Leaves are the major organs of photosynthesis in plants. These

pictures take you into a leaf, then into a cell, and finally into a

chloroplast, the organelle where photosynthesis occurs. Gas

exchange between the leaf’s mesophyll tissue and the

atmosphere occurs through microscopic pores called stomata.

Chloroplasts, found mainly in the mesophyll, are bounded by two

membranes that enclose the stroma, a dense fluid. Membranes

of the thylakoid system separate the stroma from the thylakoid

space. Thylakoids are concentrated in stacks called

grana. (Middle right, LM; bottom right, TEM.)

THE PATHWAYS OF PHOTOSYNTHESIS

* Evidence that chloroplasts split water molecules enabled researchers to track atoms through photosynthesis

* The light reactions and the Calvin cycle cooperate in converting light energy to the chemical energy of food: an overview

* The light reactions convert solar energy to the chemical energy of ATP and NADPH: a closer look

* The Calvin cycle uses ATP and NADPH to convert CO2 to sugar: a closer look

* Alternative mechanisms of carbon fixation have evolved in hot, arid climates

* Photosynthesis is the biosphere’s metabolic foundation: a review

Evidence that chloroplasts split water molecules enabled researchers to track atoms through photosynthesis

2.1.1. The splitting of water

Exp 1: CO2 + 2H2O CH2O + H2O + O2

Exp 2: CO2 + 2H2O CH2O + H2O + O2

The most important result of the shuffling of atoms during photosynthesis is the extraction of H2 from H2O and its incorporation into sugar.

The waste product of photosynthesis, O2, restores the atmospheric O2 consumed during cellular respiration. Fig10-3 shows the fates of all atoms in photosynthesis.

2.1.2. Photosynthesis as a redox process

- Photosynthesis, also a redox process, reverses the direction of electron flow. - H2O is split, and electrons are transferred along with H+ from the H2O to CO2, reducing it to sugar. - The electrons increase in potential energy as they move from H2O to sugar. - The required energy boost is provided by light

sugar

Fig 2.21. Tracking atoms through photosynthesis

Light energy

The light reactions and the Calvin cycle cooperate in converting light energy to the chemical energy of food:

an overview

- These two stages of photosynthesis are known as the light reactions (the photo part of

photosynthesis) and the Calvin cycle (the synthesis part) (fig10-4).

2.2.1. Light reaction

- The light reactions are processed in chlorophyll

- The light reactions are the steps of photosynthesis that convert solar energy to chemical

energy

- Light absorbed by chlorophyll drives a transfer of electrons and H2 from H2O to an acceptor called NADP+ (nicotinamide adenine dinucleotide phosphate), which temporarily stores the

energized electrons

- The light reactions also generate ATP by powering the addition of a phosphate group to ADP,

a process called photophosphorylation

- Thus, light energy is initially converted to chemical energy in the form of 2 compounds: NADPH and ATP

Fig 2.22. An overview of photosynthesis: cooperation of the light reactions and the Calvin cycle. In the chloroplast, the thylakoid membranes are the sites of the light reactions, whereas the Calvin cycle occurs in the

stroma. The light reactions use solar energy to make ATP and NADPH, which function as chemical energy and reducing power, respectively, in the Calvin cycle. The Calvin cycle

incorporates CO2 into organic molecules, which are converted to sugar. (Recall from Chapter 5 that most simple sugars have formulas that are some multiple of CH2O.)

chlorophyll

stroma

Calvin cycle

- The Calvin cycle is processed in stroma

- The cycle begins by incorporating CO2 from the air into organic molecules already present in the chloroplast known as carbon fixation.

- The Calvin cycle reduces the fixed carbon to carbohydrate by the addition of electrons

- The reducing power is provided by NADPH, which acquired energized electrons in the light reactions

- To convert CO2 to carbohydrate, the Calvin cycle also requires chemical energy in the form of

ATP, which is also generated by the light reactions.

- Thus, it is the Calvin cycle that makes sugar, with the help of the NADPH and ATP produced by the

light reactions

- The metabolic steps of the Calvin cycle are sometimes referred to as the dark reactions because none of the steps requires light directly.

- The Calvin cycle in most plants occurs during daylight, for only then can the light reactions regenerate the NADPH and ATP spent in the reduction of CO2 to sugar.

- Fig10-4 indicates, the thylakoids of the chloroplast are the sites of the light reactions, while the Calvin cycle occurs in the stroma. As molecules of NADP+ and ADP bump into the thylakoid membrane, they pick up electrons and phosphate, respectively, and then transfer their high-energy cargo to the Calvin cycle.

Fig 2.23. An overview of photosynthesis: cooperation of the light reactions and the Calvin cycle. In the chloroplast, the thylakoid membranes are the sites of the light reactions, whereas the Calvin cycle occurs in the

stroma. The light reactions use solar energy to make ATP and NADPH, which function as chemical energy and reducing power, respectively, in the Calvin cycle. The Calvin cycle

incorporates CO2 into organic molecules, which are converted to sugar. (Recall from Chapter 5 that most simple sugars have formulas that are some multiple of CH2O.)

chlorophyll

stroma

The light reactions convert solar energy to the chemical energy of ATP and NADPH: a closer look

- Chloroplasts are chemical factories powered by the sun. Their thylakoids transform light energy

into the chemical energy of ATP and NADPH

2.3.1. The nature of sunlight

- Light is a form of energy known as electromagnetic energy

- Electromagnetic waves are disturbances of electrical and magnetic fields

- The distance between the crests of electromagnetic waves is called the wavelength. Wavelengths range from less than a nanometer (for gamma rays) to more than a kilometer (for radio waves).

- This entire range of radiation is known as the electromagnetic spectrum (fig2.24). The segment most important to life is the narrow band that ranges from about 380 - 750 nm in wavelength.

- This radiation is known as visible light, because it is detected as various colors by the human eye.

- The model of light as waves explains many of light’s properties. It consists of photons. Photons has a fixed quantity of energy.

- The amount of energy is inversely related to the wavelength of the light; the shorter the wavelength, the greater the energy of each photon of that light. Thus, a photon of violet light packs

nearly twice as much energy as a photon of red light

Fig 2.24. The electromagnetic spectrum. Visible light and other forms of electromagnetic energy radiate through space as waves of various lengths. We perceive different wavelengths of visible light, which range from about

380 to 750 nm, as different colors. White light is a mixture of all wavelengths of visible light. A prism can sort white light into its component colors by bending light of different

wavelengths at different angles. Visible light drives photosynthesis

Photosynthetic pigments: The light receptors

- As light meets matter, it may be reflected, transmitted, or absorbed.

- Substances that absorb visible light are called pigments.

- Different pigments absorb light of different wavelengths, and the wavelengths that are absorbed disappear.

- If a pigment is illuminated with white light, the color we see is the color most reflected or transmitted by the pigment. (If a pigment absorbs all wavelengths, it appears black.)

- We see green when we look at a leaf because chlorophyll absorbs (red + blue light) while transmitting and reflecting green light (fig2.26).

- The ability of a pigment to absorb various wavelengths of light can be measured with an instrument called a spectrophotometer.

- This machine directs beams of light of different wavelengths through a solution of the pigment and measures the fraction of the light transmitted at each wavelength. A graph plotting a pigment’s light absorption (the fraction not transmitted or reflected) versus wavelength is called an absorption spectrum

Fig 2.25. Why leaves are green: interaction of light with chloroplasts. The pigment molecules of chloroplasts absorb blue and red light and reflect or transmit

green light. This is why leaves appear green. It turns out that blue and red are the colors of light most effective in photosynthesis

stroma

chlorophyll

Fig 2.26. Determining an absorption spectrum. A spectrophotometer measures the relative amounts of light of different wavelengths absorbed and transmitted by a pigment solution. Inside the spectrophotometer, white light is separated into colors

(wavelengths) by a prism. Then, one by one, the different colors of light are passed through the sample. The transmitted light strikes a photoelectric tube, which converts the light energy to electricity, and the

electrical current is measured by a galvanometer. Each time the wavelength of light is changed, the meter indicates the fraction of light transmitted through the sample (or, conversely, the fraction of light

absorbed). This figure shows the transmittance reading on the meter when (a) green light and then (b) blue light are passed through a chlorophyll solution

- The absorption spectra of chloroplast pigments provide clues to the relative effectiveness of different wavelengths for driving photosynthesis, since light can perform work in chloroplasts only if it is absorbed

Chlorophyll a

- Fig12.27a shows the absorption spectra of chlorophyll a and some other pigments in the chloroplast.

- If we look first at the absorption spectrum of chlorophyll a, it suggests that blue and red light work best for photosynthesis, while green is the least effective color.

- This is confirmed by an action spectrum for photosynthesis, which profiles the relative performance

of the different wavelengths

- The action spectrum for photosynthesis was first demonstrated in 1883 in an elegant experiment performed by the German botanist Thomas Engelmann, who used bacteria to measure rates of photosynthesis in filamentous algae.

- Notice by comparing fig2.27a and 2.27b that the action spectrum for photosynthesis does not

exactly match the absorption spectrum of chlorophyll a.

Chlorophyll b

- Only chlorophyll a can participate directly in the light reactions, which convert solar energy to chemical energy

- But other pigments, chlorophyll b, in the thylakoid membrane can absorb light and transfer the

energy to chlorophyll a, which then initiates the light reactions.

Fig 2.27. Evidence that

chloroplast pigments

participate in photosynthesis:

absorption and action spectra for photosynthesis in

an alga.

Fig 2.28. Location and structure of chlorophyll molecules in plants. A plant’s chlorophyll molecules, along with accessory pigment molecules, are immersed in the

thylakoid membranes of chloroplasts, in association with protein (purple). Chlorophyll a, the pigment that participates directly in the light reactions of photosynthesis, has a "head," called a porphyrin ring, with a magnesium atom at its center. Attached to the porphyrin is a hydrocarbon tail, which interacts

with hydrophobic regions of proteins inside the thylakoid membrane. Chlorophyll b differs from chlorophyll a only in one of the functional groups bonded to the porphyrin.

2.3.3. Excitation of chlorophyll by light

What exactly happens when chlorophyll and other pigments absorb photons?

- Light supplies the photons

- Photons are absorbed by clusters of pigment molecules embedded in the thylakoid membrane.

- The energy of an absorbed photon is converted to the potential energy of an electron raised from the ground state to an excited state (UP-HILL).

- When pigments absorb light, their excited electrons drop back down to the ground-state orbital

in a billionth of a second, releasing their excess energy as heat (DOWN-HILL)

- The electron jumps to a state of greater energy, and as it falls back to ground state, a photon is

given off. This afterglow is called fluorescence.

- If a solution of chlorophyll isolated from chloroplasts is illuminated, it will fluoresce in the red part of the spectrum and also give off heat .

Fig 2.29. Location and structure of chlorophyll molecules in plants. A plant’s chlorophyll molecules, along with accessory pigment molecules, are immersed in the

thylakoid membranes of chloroplasts, in association with protein (purple). Chlorophyll a, the pigment that participates directly in the light reactions of photosynthesis, has a "head," called a porphyrin ring, with a magnesium atom at its center. Attached to the porphyrin is a hydrocarbon tail, which interacts

with hydrophobic regions of proteins inside the thylakoid membrane. Chlorophyll b differs from chlorophyll a only in one of the functional groups bonded to the porphyrin.

Fig 2.30. Excitation of isolated chlorophyll by light. (a) Absorption of a photon causes a transition of the chlorophyll molecule from its ground state to its excited state. The photon boosts an electron to an orbital where it has more potential energy. If the

illuminated molecule exists in isolation, its excited electron immediately drops back down to the ground-state orbital, and its excess energy is given off as heat and fluorescence (light).

(b) A chlorophyll solution excited with ultraviolet light will fluoresce with a red-orange glow.

Photosynthesis: Light-harvesting complex of the thylakoid membrane

- A photosystem has a light-gathering "antenna complex" consisting of a cluster of a few hundred chlorophyll a, chlorophyll b, and carotenoid molecules (fig2.31).

- Only chlorophyll a is located in the region of the photosystem called the reaction center, where the first light-driven chemical reaction of photosynthesis occurs.

- The reaction center with the chlorophyll a is the primary electron acceptor

- The thylakoid membrane is populated by 2 types of photosystems that cooperate in the light reactions of photosynthesis. They are called photosystem I and photosystem II.

- The reaction-center chlorophyll of photosystem I is known as P700 and the chlorophyll at the reaction center of photosystem II is called P680

- These two pigments, P700 and P680, are actually identical chlorophyll a molecules.

Fig 2.31. How a photosystem harvests light. Photosystems are the light-harvesting units of the thylakoid membrane. Each photosystem is a complex of proteins and other kinds of molecules and includes an antenna consisting of a few hundred pigment molecules. When a photon strikes a pigment molecule, the energy is passed from molecule to molecule until it reaches the reaction center. At the reaction center, an excited electron from the reaction-center

chlorophyll is captured by a specialized molecule called the primary electron acceptor

chlorophyll aChlorophyll bcarotenoid

Chlorophyll a

Noncyclic electron flow

How the two photosystems work together in using light energy to generate ATP and NADPH, the two main products of the light reactions ?

- Light drives the synthesis of NADPH and ATP by energizing the 2 photosystems embedded in the thylakoid membranes of chloroplasts.

- The key to this energy transformation is a flow of electrons through the photosystems and other molecular components built into the thylakoid membrane.

- During the light reactions of photosynthesis, there are 2 possible routes for electron flow: cyclic and noncyclic.

- Noncyclic electron flow, the predominant route

- The numbers in the text description correspond to the numbered steps in the figure.

- The energy changes of electrons as they flow through the light reactions are analogous to the cartoon in fig10-13

- The light reactions use solar power to generate ATP and NADPH, which provide chemical energy and reducing power, respectively, to the sugar-making reactions of the Calvin cycle.

Fig 2.32. How noncyclic electron flow during the light reactions generates ATP and NADPH.

The gold arrows trace the current of light-driven electrons from water to NADPH. Each photon of light excites a single electron, but the diagram tracks two electrons at a time,

the number of electrons required to reduce NADP+. The numbered steps are described in the text.

noncyclic electron flow

2e-2e-

Fig 2.33. A mechanical

analogy for the light reactions.

Cyclic electron flow

- Under certain conditions, photoexcited electrons take an alternative path called cyclic electron flow, which uses photosystem I but not photosystem II.

- You can see in fig2.34 that cyclic flow is a short circuit: The electrons cycle back from ferredoxin (Fd) to the cytochrome complex and from 3 continue on to the P700 chlorophyll. There is no production of NADPH and no release of oxygen.

- Cyclic flow does generate ATP. This is called cyclic photophosphorylation, to distinguish it from

noncyclic photophosphorylation.

What is the function of cyclic electron flow?

- Noncyclic electron flow produces ATP and NADPH in roughly equal quantities, but the Calvin cycle consumes more ATP than NADPH.

- Cyclic electron flow makes up the difference. The concentration of NADPH in the chloroplast may help regulate which pathway, cyclic versus noncyclic, electrons take through the light reactions.

- If the chloroplast runs low on ATP for the Calvin cycle, NADPH will begin to accumulate as the Calvin cycle slows down.

- The rise in NADPH may stimulate a temporary shift from noncyclic to cyclic electron flow until ATP supply catches up with demand

Fig 2.34. Cyclic electron flow. Photoexcited electrons from photosystem I are occasionally shunted back from ferredoxin (Fd) to chlorophyll via the cytochrome complex and plastocyanin (Pc). This electron shunt

supplements the supply of ATP but produces no NADPH. The "shadow" of noncyclic electron flow is included in the diagram for comparison with the cyclic route. The two ferredoxin

molecules shown in this diagram are actually one and the same--the final electron carrier in the electron transport chain of photosystem I .

Cyclic electron flow

plastocyanin

ferredoxin2e-

2.3.7. A comparision of chemiosmosis in chloroplst and mitochondria

The similar of 2 organelles

- Chloroplasts and mitochondria generate ATP by the same basic mechanism: chemiosmosis

- An electron transport chain assembled in a membrane pumps protons across the membrane as electrons are passed through a series of carriers that are progressively more electronegative.

- Potential energy stored in the form of an H+ gradient across a membrane.

- Built into the same membrane is an ATP synthase complex that couples the diffusion of H+ down their gradient to the phosphorylation of ADP.

- Some of the electron carriers are cytochromes (similar in chloroplasts and mitochondria).

- The ATP synthase complexes of the two organelles are also very much alike

- Chloroplasts do not need food to make ATP; their photosystems capture light energy and use it to drive electrons to the top of the transport chain

- Mitochondria transfer chemical energy from food molecules to ATP, whereas chloroplasts transform light energy into chemical energy

The difference between 2 organelles

- The spatial organization of chemiosmosis also differs in chloroplasts and mitochondria (fig2.35).

- The inner membrane of the mitochondrion pumps protons from the mitochondrial matrix out to the intermembrane space, which then serves as a reservoir of H+ that powers the ATP synthase

- The thylakoid membrane of the chloroplast pumps protons from the stroma into the thylakoid space, which functions as the H+ reservoir. The thylakoid membrane makes ATP as the H+ diffuse from the thylakoid space back to the stroma through ATP synthase complexes, whose catalytic knobs are on the stroma side of the membrane. Thus, ATP forms in the stroma, where it is used to help drive

sugar synthesis during the Calvin cycle.

- Fig2.36 shows a current model for the organization of the light-reaction "machinery" within the thylakoid membrane. Each of the molecules and molecular complexes in the figure is present in numerous copies in each thylakoid.

Notice that NADPH, like ATP, is produced on the side of the membrane facing the stroma, where the

Calvin cycle reactions take place.

Fig 2.35. Comparison of chemiosmosis in mitochondria and chloroplasts.In both kinds of organelles, electron transport chains pump protons (H+) across a membrane from a region of low H+ concentration (light brown in this diagram) to one of high H+ concentration (darker

brown). The protons then diffuse back across the membrane through ATP synthase, driving the synthesis of ATP. The diagram identifies the regions of high and low H+ concentration in the two

organelles

food

light

Fig 2.36. The light reactions and chemiosmosis: the organization of the thylakoid membrane. This diagram shows a current model for the

organization of the thylakoid membrane. The gold arrows track the noncyclic electron flow outlined in fig10-12. As electrons pass from carrier to carrier in redox reactions, hydrogen ions removed from the stroma are deposited in the thylakoid space, storing energy as a proton-motive force (H+ gradient). At least three steps in the light reactions contribute to the proton gradient: (1)   Water is split by photosystem II on the side of the membrane

facing the thylakoid space; (2) as plastoquinone (Pq), a mobile carrier, transfers electrons to the cytochrome complex, protons are translocated across the membrane; and (3) a

hydrogen ion is removed from the stroma when it is taken up by NADP+. The diffusion of H+ from the thylakoid space to the stroma (along the H+ concentration gradient) powers

the ATP synthase. These light-driven reactions store chemical energy in NADPH and ATP, which shuttle the energy to the sugar-producing Calvin cycle

The Calvin cycle uses ATP and NADPH to convert CO2 to sugar: a closer look

- Carbon enters the Calvin cycle in the form of CO2 and leaves in the form of sugar. The cycle spends ATP as an energy source and consumes NADPH as reducing power for adding high-

energy electrons to make the sugar

Fig10-17 divides the Calvin cycle into three phases :

Phase 1: Carbon fixation RuBP carboxylase

3 CO2 + 1 RuBP -------------------- 6 (3-phosphate glycerate )

Phase 2: Reduction 6 (3-phospho glycerate) + 6ATP 6 (1,3-biphosphate glycerate) + 6 NADPH 6 G3P (glycealdehyde-3-phosphate) + 6NAD+ + 6Pi 1 G3P is out put

Phase 3: Regeneration of CO2 acceptor (RuBP) 5 G3P + 3ATP 3 RuBP (ribulose biphosphate) + 3ADP

Net3 CO2 + 1 RuBP + 9 ATP + 6 NADH 1 G3PThe light reactions regenerate the ATP and NADPH

Fig 2.37. The Calvin cycle. This diagram tracks carbon atoms (gray balls) through the cycle. The three phases of the cycle

correspond to the phases discussed in the text. For every three molecules of CO2 that enter the cycle, the net output is one molecule of glyceraldehyde-3-phosphate (G3P), a three-carbon sugar. For each

G3P synthesized, the cycle spends nine molecules of ATP and six molecules of NADPH. The light reactions sustain the Calvin cycle by regenerating ATP and NADPH

regeneration

export

reductioncarboxylation

(6)

(6)

(6)

(1)

(5)

(3)

Photosynthesis is the biosphere’s metabolic foundation: a review

Fig 2.38. A review of photosynthesis. This diagram outlines the main reactants and products of the light reactions and the Calvin cycle as they occur in the chloroplasts of plant cells. The entire ordered operation depends on the structural integrity

of the chloroplast and its membranes. Enzymes in the chloroplast and cytosol convert glyceraldehyde-3-phosphate (G3P), the direct product of the Calvin cycle, into many other organic compounds