AP Bio Expectations - Botany

7/27/2019 AP Bio Expectations - Botany

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Exam 7 Expectations: Plant Exam

1. Define mycorrhizae.Mycorrhizaemutualistic association of plant roots and fungus.

2. Define radicleRadiclean embryonic root of a plant.

3. Define critical period.Critical periodthe minimal or required amount of time a plant must spend in or out of

darkness continuously in order to flower, depending on whether it is a long-day or short-day

plant.

4. Describe long day plants.Long day plants are plants that flower (usually in late spring or early summer) only when

the light period is longer than a critical length (certain amount of hours). For example,spinach flowers when days are 14 hours or longer. Other examples are radishes, lettuce, irises

and many cereal varieties. Long-day plants are actually short-night plants, but the older

term was embedded already.

A long-day plant grown on photoperiods (the relative lengths of night and day; and the

environmental stimulus that plants use most often to detect the time of year) of long nights

that would not normally induce flowering will flower if the period of continuous darkness is

interrupted by a few minutes of light.

We distinguish long-day from short-day plants not by an absolute night length but by

whether the critical night length sets a maximum (long-day plants) or minimum (short-day

plants) number of hours of darkness required for flowering.

5. State the equation for water potential. = S+ P

Water potential = solute potential (or osmotic potential) + pressure potential

6. State the conditions when water potential is zero.It is zero when the cell is in pure water and turgid.

Adding solutes lowers water potential. If the positive pressure added orP turns out to be the

same as S, then the is zero, because eventually there is no net water movement.Water potential is zero when the cell is turgid, or very firm. Its at osmotic equilibrium with

its surroundings.

For example, if an initial flaccid cell was placed into an environment with pure water, when

= 0 MPa, that means because the cell contains solutes, it has a lower water potential than

the water. Its initial condition: cellular< environmental . So water enters the cell by

osmosis, causing it to become turgid. This tendency for water to enter is offset by the back


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are present. Water migrates laterally between neighboring cells

through pits.

Tracheids are long, thin, with tapered ends. Water moves through pits, where it does not have to

cross thick secondary walls. The secondary walls of tracheids are hardened with lignin so it

prevents collapse under the tensions of water transport and provides support.

Vessel elements are wider, shorter, thin walled, and less tapered than the tracheids. They are

aligned from end to end, forming long micropipes called vessels. The end walls of the vessel

elements have perforation plates that enable water to flow freely through vessels.

Phloem: the sugar-conducting cells of the phloem are

alive at functional maturity. In seedless vascular

plants and gymnosperms, sugars & other organic

nutrients are transported through long, narrow cells

called sieve cells. In the phloem of angiosperms,

these nutrients are transported through sieve tubes,which consist of chains of cells called sieve-tube

elements, or sieve-tube members. Sieve-tube elements

are alive but lack a nucleus, ribosomes, a distinct

vacuole, and cytoskeletal elements. This reduction in

cell contents enables nutrients to pass more easily

through the cell. The end walls of sieve-tube elements

are called sieve plates, which have pores that facilitate

the flow of fluid from cell to cell along the sieve tube.

Alongside each sieve-tube element is a nonconducting cell called companion cell, which is

connected to it by numerous channels, plasmodesmata. It has the nucleus and ribosomes and thecompanion serves as not only the cell itself but also adjacent sieve-tube element. In some plants,

companion cells in leaves help load sugars into the sieve-tube elements, which then transport

sugars to other parts of the plant.

15.List factors that might alter ethylene's effect on a plant.Ethylene plays a role in the speed of fruit ripening. You may be able to speed ripening by

storing green fruits in a paper bag, allowing ethylene to accumulate.

Ethylene is a gas, so the signal to ripen spreads from fruit to fruit once ethylene triggers

ripening and ripening triggers more ethylene production.

Circulating the air prevents ethylene from accumulating, and carbon dioxide inhibits

synthesis of new ethylene. It affects and lowers the concentration of ethylene. This is why

apples can be picked in autumn and can still be shipped to grocery stores the following

summer, if they are stored in bins flushed with carbon dioxide. The concentration of ethylene

is very important. In order to ripen fruit, low concentrations are used, because too high of

concentration will lead faster to rotting.


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Ethylenes effects on a plant not only depend on its concentration, but also depend on the site

of action within the plant, developmental (growth) stage of the plant, readiness of cell

membrane receptors for the ethylene, and ratios of other hormones.

16.List processes in plants that rely on proton gradients.Stomatal opening and closing

Cation uptake

Cotransport of an anion with H+Cotransport of a neutral solute with H

+

Acid growth hypothesis

Loading of sucrose into phloem

17.List reasons for angiosperms' evolutionary success.- Reduced gametophytes- Seeds enclosed in fruit- Fruits help dispersal of seeds- Pollinators, ex. Animals, wind, water- Highly efficient xylem

18.List the stages/events in alternation of generations.Plants and some species of algae exhibit a

second type of life cycle called alternation of

generations.

It includes both diploid and haploid stages that

are multicellular.

The multicellular stage is called thesporophyte.

Meiosis in the sporophyte produces haploid

cells called spores.

Unlike a gamete, a haploid spore doesnt fuse

with another cell but divides mitotically,


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generating a multicellular haploid stage called

the gametophyte.

Cells of the gametophyte give rise to gametes

by mitosis.

Fusion of two haploid gametes at fertilization

results in a diploid zygote, which develops into

the next sporophyte generation.

Thus, the sporophyte generation produces a

gametophyte as its offspring, and the

gametophyte generation produces the next

sporophyte generation.

19.Outline the transport of minerals from soil to leaves.Water and minerals are absorbed by root cells, transported through the endodermis, released

into the vessels and tracheids of the xylem, and carried to the tops of plants by the bulk flowdriven by transpiration. Review the transpiration-cohesion-tension mechanism.


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20.Outline the role of nitrogenase.Nitrogenase is the enzyme complex that catalyzes the entire, complex, multistep reaction

sequence (the conversion of N2 to NH3 or nitrogen fixation). The reactants and products aresummarized as follows:

N2 + 8 e-+ 8 H

++ 16 ATP 2 NH3 + H2 + 16 ADP + 16 Pi

Nitrogenase reduces N2 to NH3 by adding electrons and H+. Because the process of nitrogen

fixation requires eight ATP molecules for each NH3 synthesized, nitrogen-fixing bacteria

require a rich supply of carbohydrates from decaying material, root secretions, or (in the case

ofRhizobium) the vascular tissue of roots.

21.Outline apical dominance.Apical dominance is the concentration of growth at the tip of a plant shoot, where an apicalbud (terminal bud) partially inhibits axillary bud growth.

By concentrating resources on elongation, the evolutionary adaptation of apical dominance

increases the plants exposure to light. If an animal eats the end of the shoot, or if shading

results in the light being more intense to the side of a plant than directly above, axillary buds

break dormancy, meaning they start growing. A growing axillary bud gives rise to a lateral

shoot, complete with its own apical bud, leaves, and axillary buds. Removing the apical bud


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usually stimulates the growth of axillary buds, resulting in more lateral shoots. This is why

pruning trees and shrubs and pinching back houseplants will make them bushier.

22.Outline the activation of nitrogenase in a root nodule.Nitrogenase catalyzes the entire reaction sequence of nitrogen fixation, which reduces N2 to

NH3 by adding electrons and H+. The location of bacteroids inside living, nonphotosynthetic

cells is conducive to nitrogen fixation, which needs an anaerobic environment.

23.Outline the major events in flowering plant reproduction.


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24.Outline carpellate flowers.Carpellate flowers lack stamens. Seeds are developing from ovules while the ovary of the

flower is developing into a fruit, which protects the enclosed seeds and when mature, aids in

their dispersal by wind or animals. Fertilization triggers hormonal changes that cause the

ovary to begin its transformation into a fruit. If a flower has not been pollinated, fruit

typically does not develop, and the entire flower withers and falls away.


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Most fruits are derived from a single carpel or several fused carpels; they are called simple

fruits.

An aggregate fruit results from a single flower that has more than one separate carpel, each

forming a small fruit. These fruitlets are clustered together on a single receptacle.

A multiple fruit develops from many carpels of the many flowers that form an inflorescence,

a group of flowers tightly clustered together. The walls of many ovaries thicken and fuse

together to become incorporated into one fruit.

An accessory fruit develops largely from tissues other than the ovary.

25.Outline the following plant hormones: ABA, GA, IAA, gibbereric acid, salicylicacid, 2,4-D, ethylene, cytokinins

Hormone Location Function

Auxin (IAA:

indoleacetic acid)

Shoot apical meristems and

young leaves

Stimulates stem elongation (low conc

only); promotes lateral and

adventitious root formation; regulatesdevelopment of fruit; enhances apical

dominance; functions in phototropism

& gravitropism; promotes vascular

differentiation; retards leaf abscission.

Also used as herbicides

In cell elongation, it plays a role by

stimulating proton pumps, instigating

a loosening of cell wall fibers,

increasing vacuole size, and

permitting an increase in turgor

pressure.

Cytokinins Synthesized in roots and

transported to other organs

Regulate cell division in shoots and

roots; modify apical dominance and

promote lateral bud growth; promote

movement of nutrients into sink

tissues; stimulate seed germination;

delay leaf senescence.

Has anti-aging effects

Gibberellins (GA) Meristems of apical buds

and roots, young leaves,

and developing seeds

Stimulate stem elongation, pollen

development, pollen tube growth, fruit

growth, and seed development and

germination; regulate sex

determination and the transition from

juvenile to adult phases.


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Breaks seed dormancy

Effects of stem elongation are

evidently shown with dwarf (mutant)

plants when they grow tall with GA.

Also supports growth of cereal

seedlings by stimulating synthesis of

digestive enzymes like -amylase that

mobilize stored nutrients.

Abscisic Acid (ABA) Almost all plant cells can

synthesize ABA

Inhibits growth; promotes stomatal

closure during drought stress;

promotes seed dormancy and inhibits

early germination; promotes leaf

senescence; promotes desiccation

tolerance.

Ethylene Can be produced by almostall parts of the plant

Promotes ripening of many fruits, leafabscission, triple response in seedlings

(inhibition of stem elongation,

promotion of lateral expansion, and

horizontal growth); enhances the rate

of senescence; promotes root and root

hair formation; promotes flowering in

pineapple family.

Salicyclic Acid

2,4-D 2,4-dichlorophenoxyacetic acid; a synthetic auxin used as a

herbicide. Monocots can rapidly inactivate synthetic auxins,

however, eudicots cannot and die from hormonal overdose.

Spraying cereal fields or turf with 2,4-D eliminates eudicot

(broadleaf) weeds.

Gibberellic Acid

26.Outline the pros and cons of fungicidal use.Pros: destroys fungi that can be harmful to plants

Cons: some fungicides are slightly toxic and the accumulation of the toxins can be harmful

to the soil, and the fungicides can runoff into local streams and rivers, causingeutrophication.

27.Outline active transport.Active transport is the pumping of a solute across a membrane against its electrochemical

gradient. It is called active because the cell must expend energy, usually in the form of

ATP, to transport a solute counter to the net direction in which the solute diffuses. In active


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transport in plant cells, the most important transport proteins are proton pumps, which use

energy from ATP to pump protons (H+) out of the cell. This movement results in an H

+

gradient, with a higher H+

concentration outside the cell than inside. The gradient across the

membrane is a form of potential energy, and the flow of the protons back into the cell can be

harnessed to do work.

28.Outline the importance of adhesion and cohesion in water transport.The transpirational pull on xylem sap is transmitted

all the way from the leaves to the root tips and even

into the soil solution. Cohesion and adhesion

facilitate this long-distance by bulk flow. The

cohesion of water due to hydrogen bonding makes it

possible to pull a column of xylem sap from above

without the water molecules separating. Water

molecules exiting the xylem in the leaf tug onadjacent water molecules, and then this pull is

relayed, molecule by molecule, down the entire

column of water in the xylem. And then meanwhile,

the strong adhesion of water molecules by hydrogen

bonds to the hydrophilic walls of xylem cells helps

offset the downward force of gravity.

29.Describe the adaptations associated with plants living in arid conditions.

C4plants are named because they preface the Calvin cycle with an alternate mode of carbon

fixation that forms a four-carbon compound as its first product. Some examples are


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sugarcane and corn. There are two distinct types of photosynthetic cells; bundle-sheath cell

and mesophyll cells. Bundle-sheath cells are arranged into tightly packed sheaths around the

veins of the leaf. Between the bundle sheath and the leaf surface are the more loosely

arranged mesophyll cells. The Calvin cycle is confined to the chloroplasts of the bundle-

sheath cells. However, the cycle is preceded by incorporation of CO2 into organic compounds

in the mesophyll cells.

1) The first step is carried out by PEP carboxylase, an enzyme in only mesophyll cells. Thisenzyme adds CO2 to phosphoenolpyruvate (PEP). PEP carboxylase has a much higher

affinity for CO2 than does rubisco and no affinity for O2. Therefore, PEP carboxylase can

fix carbon efficiently when rubisco cant when it is hot and dry and stomata are

partially closed, causing CO2 concentration in the leaf to fall and O2 to rise.

2) After the C4 plant fixes carbon from CO2, the mesophyll cells export their four-carbonproducts (for ex, malate) to bundle-sheath cells through plasmodesmata.

3) Within the bundle-sheath cells, the four-carbon compounds release CO2, which isreassimilated into organic material by rubisco and the Calvin cycle. The same reactionregenerates pyruvate, which is transported to mesophyll cells.

There, ATP converts pyruvate to PEP, allowing the reaction cycle to continue; this ATP can be

thought of as the price of concentrating CO2 in the bundle-sheath cells. To generate this extra

ATP, bundle-sheath cells carry out cyclic electron flow. These cells contain PS I but no PS II, so

cyclic electron flow is their only photosynthetic mode of generating ATP.

In effect, the mesophyll cells of a C4 plant pump CO2 into the bundle sheath, keeping the CO2

concentration in the bundle-sheath cells high enough for rubisco to bind carbon dioxide rather

than oxygen. The cyclic series of reactions involving PEP carboxylase and the regeneration of

PEP can be thought of as a CO2-concentrating pump that is powered by ATP.

This way, C4 photosynthesis minimizes photorespiration and enhances sugar production. Thisadaptation is advantageous in hot regions with intense sunlight and when stomata partially close

during the day.

The mode of carbon fixation called CAM, crassulacean acid metabolism, is when CAM plants

open their stomata during the night and close them during the day, the reverse of how other

plants behave. Closing stomata during the day helps desert plants conserve water, but it also

prevents CO2 from entering the leaves so during the night, when their stomata are open, these

take up CO2 and incorporate it into a variety of organic acids.

The mesophyll cells of CAM plants store the organic acids they make during the night in their

vacuoles until morning, when the stomata close. During the day, when the light reactions can

supply ATP and NADPH for the Calvin cycle, CO2 is released from the organic acids made the

night before to become incorporated into sugar in the chloroplasts.


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The similarity between C4 and CAM pathways

is that carbon dioxide is first incorporated into

organic intermediates before it enters the Calvin

cycle. The difference is that in C4, the initial

steps of carbon fixation are separated

structurally from the Calvin cycle, whereas in

CAM plants, the two steps occur at separate

times but within the same cell. They both

eventually use the Calvin cycle to make sugarfrom carbon dioxide, like C3 plants.

30.Describe the movement of water through a plant (soil to atmosphere).Water can be moved through a plant either by root pressure (pushing xylem sap) or the

transpiration-cohesion-tension mechanism (pulling xylem sap)At night, when there is almost no transpiration, root cells continue pumping mineral ions into

the xylem of the stele. The resulting accumulation of minerals lowers the water potential

within the stele. Water flows in from the root cortex, generating root pressure, a push of

xylem sap. The root pressure sometimes causes more water to enter the leaves than is

transpired, resulting in guttation.

In bulk flow, the movement of fluid is driven by a water potential difference at opposite ends

of the xylem tissue. The water potential difference is created at the leaf end of the xylem by

the evaporation of water from leaf cells. Evaporation lowers the water potential at the air-

water interface, thereby generating the negation pressure (tension) that pulls water through

the xylem. Bulk flow is driven by differences in pressure potential, and not solute potential.

The plant uses no energy to use bulk flow.

31.Describe phototropism.Phototropism is the growth of a shoot toward light or away from it. Positive phototropism is

when the shoot grows toward light, and negative phototropism is when the shoot grows away


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from light. Plants which are grown in a partially dark environment will grow toward light

(positive).

In a crowded environment, phototropism directs growing seedlings toward the sunlight that

powers photosynthesis. It is learned from studies of grass seedlings, particularly oats. The

shoot of a sprouting grass seedling is enclosed in a sheath called the coleoptile, which grows

straight upward if the seedling is kept in the dark or if it is illuminated uniformly from all

sides. If the growing coleoptile is illuminated from one side, it grows toward the light. This

response results from a differential growth of cells on opposite sides of the coleoptile; the

cells on the darker side elongate fast than the cells on the brighter side.

Charles Darwin and his son Francis observed that a grass seedling could bend toward light

only if the tip of the coleoptile was present. If the tip (includes apical meristem) was

removed, the coleoptile did not curve. It was concluded that the tip was responsible for

sensing light. They also noted that the differential growth response that led to the curvature

of the coleoptile occurred some distance below the tip, so that means some signal was

transmitted downward from the tip to the elongating region of the coleoptile.Peter Boysen-Jensen demonstrated that the signal was a mobile chemical substance. He

separated the tip from the remainder of the coleoptile by a cube of gelatin, which prevented

cellular contact but allowed chemicals to pass. These seedlings responded normally, bending

toward light. But if the tip was experimentally separated from the lower coleoptile by an

impermeable barrier, no phototropic response occurred.

Frits Went extracted the chemical messenger by modifying experiments of Jensen. He

removed the coleoptile tip and placed a cube of agar, gelatinous material, on it. The agar

block that was centered on top of the coleoptile caused the stem to grow straight upward. But

when the block was placed off center, the coleoptile began to bend away from the side with

the agar block, as though growing toward light. Went concluded the block contained achemical produced in the tip, and that this chemical stimulated growth as it passed down the

coleoptile, and it curved toward light because of a higher concentration of the growth-

promoting chemical on the darker side of the coleoptile.

This chemical messenger is Auxin, or IAA, indoleacetic acid.

It does cause a growth increase on one side of the stem but it does not causes a decrease in

growth on the side of the stem exposed to light since there is no evidence. There is however,

asymmetrical distribution of certain substances that may act as growth inhibitors, and these

substances are more concentrated on the lighted side of a stem.

32.Describe germination.Germination depends on imbibition, the uptake of water due to the low water potential of the

dry seed. Imbibing water causes the seed to expand and rupture its coat and also triggers

metabolic changes in the embryo that enable it to resume growth. Following hydration,

enzymes begin digesting the storage materials of the endosperm or cotyledons, and the

nutrients are transferred to the growing regions of the embryo.


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The first organ that emerges from the germinating seed is the radicle, the embryonic root.

Next, the shoot tip breaks through the soil surface. In garden beans and other eudicots, a

hook forms in the hypocotyl, and growth pushes the hook above ground.

Stimulated by light, the hypocotyl straightens, raising the cotyledons and epicotyl.

Then the delicate shoot tip and bulky cotyledons are pulled upward rather than being pushed

tip-first through the abrasive soil.

The epicotyl now spreads it first foliage leaves, which are true leaves, as distinguished from

the cotyledons, or seed leaves.

The foliage leaves expand, become green, and begin making food by photosynthesis.

The cotyledons shrivel and fall away from the seedling, their food reserves having been

exhausted by the germinating embryo.

Some monocots, like maize and other grasses, have a different method for breaking ground

when germinating.

The coleoptile, the sheath enclosing and protecting the embryonic shoot, pushes upward

through the soil and into the air.The shoot tip then grows straight up through the tunnel provided by the tubular coleoptile

and eventually breaks through the coleoptiles tip.

33.Describe root hairs.In most plants, the absorption of water and minerals occurs primarily near the tips of roots,

where vast numbers of tiny root hairs increase the surface area of the root enormously. Root

hairs are short-lived and constantly replaced. A root hair is a thin, tubular extension of a root

epidermal cell. It should not be confused with a lateral root, which is a multicellular organ.

Despite their great surface area, root hairs, unlike lateral roots, contribute little to plant

anchorage. Their main function is absorption.All living plant cells absorb nutrients across their plasma membranes, but the cells near the

tips of roots are particularly important because most of the water and mineral absorption

occurs there. In this region, epidermal cells are permeable to water, and many are

differentiated into root hairs, modified cells that account for much of the absorption of water

by roots. The root hairs absorb the soil solution, which consists of water molecules and

dissolved mineral ions that are not bound tightly to soil particles.

34.Compare long day and short day plants.Long-day Plants (Short-night) Short-day Plants (Long-night)

Requires a light period longer than a critical

length in order to flower

Requires a light period shorter than a critical

length in order to flower

Generally flower in late spring or early

summer.

Generally flower in late summer, fall or

winter.

Ex. Spinach, radishes, lettuce, irises, many

cereal varieties

Ex. Chrysanthemums, poinsettias, some

soybean varieties, Maryland Mammoth


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(mutant variety of tobacco)

Red light is the most effective color in interrupting the nighttime portion of the photoperiod.

Action spectra and photoreversibility experiments show that phytochrome is the pigment that

detects the red light. For example, if a flash of red (R) light during the dark period is

followed by a flash of far-red (FR) light, then the plant detects no interruption of night

length. A flash of red (R) light interrupts and shortens the dark period, turning one long night

into two short nights. However, a subsequent flash of far-red (FR) light cancels the red

flashs effect.

35.Compare hypocotyls and epicotyls.Hypocotyl Epicotyl

In an angiosperm embryo, the embryonic

axis below the point of attachment of the

cotyledon(s) and above the radicle

(embryonic root).

In an angiosperm embryo, the embryonic

axis above the point of attachment of the

cotyledon(s) and below the first pair of

miniature leaves.


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36.Compare herbaceous and woody plants.Herbaceous Plant Woody Plant

The entire plant is mainly just the primary

plant body, or the results of only primary

growth.

Secondary growth, the growth in thickness

produced by lateral meristems, occurs in

stems and roots of woody plants, but rarely in

leaves.

All gymnosperms and many eudicots have

secondary growth, rare in monocots.

The secondary plant body consists of the

tissues produced by the vascular cambium

and cork cambium. The vascular cambiumadds secondary xylem (wood) and secondary

phloem, increasing vascular flow and support

for shoot system. Cork cambium produces a

tough thick covering.

In the stem, the vascular cambium consists of

a continuous cylinder of undifferentiated

parenchyma cells, located outside the pith

and primary xylem and to the inside of the

cortex and primary phloem.

In the root, the vascular cambium forms to

the exterior of the primary xylem and interior

to the primary phloem and pericycle.

If you walk from the center of the stem that

has been through primary and secondary

growth, you will go through the pith, primary

xylem, secondary xylem, vascular cambium,

secondary phloem, primary phloem, and the

periderm (mainly cork cambia and cork).

The bark consists of all tissues exterior to thevascular cambium, mostly secondary phloem

and periderm.

37.Compare monocots and dicots.Monocot Dicot

Specie with one cotyledon (seed leaf) Specie with two cotyledons


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Organization of primary tissues in young stems:

Eudicots: Monocot:

38.Compare plant hormones and animal hormones.Plant Hormones Animal Hormones

39.Compare perfect and complete flowers.Perfect Flower Complete Flower

Has both stamen and carpel Has all four basic floral organs; sepal, petal,

stamen, and carpel

Fertile, but may be either complete or

incomplete (lacks one of the basic floral

organs)

40.Compare monoecious and dioecious plants.Monoecious Plant Dioecious Plant


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Has staminate and carpellate flowers at

separate locations but on the same individual

plant.

Plants cannot self-fertilize because different

individuals have either staminate flowers

(lacking carpels) or carpellate flowers

(lacking stamens). Its one of the

mechanisms that prevent self-fertilization

that contribute to genetic variety by ensuring

sperm and eggs come from different parents.

41.Compare the anatomy of roots and leaves.Roots Leaves

Consist of dermal, vascular, and ground

tissue

Consist of dermal, vascular, and ground

tissue

Does not have cuticle In leaves and most stem, the cuticle, a waxycoating on the epidermal surface, helps

prevent water loss.

42.Compare tap roots and fibrous roots.Tap Roots Fibrous Roots

A taproot system consists of one main

vertical root, the taproot, that develops from

an embryonic root. The taproot gives rise to

lateral roots, also called branch roots.

A fibrous root system is a mat of generally

thin roots spreading out below the soil

surface, with no root functioning as the main

one.

Most eudicots and gymnosperms have this. Most monocots and seedless vascular plants

have this.

In many angiosperms, the taproot stores

sugars and starches that the plant will

consume during flowering and fruit

production.

Because of this, root crops like carrots,turnips, and beets are harvested before they

flower.

The embryonic root dies and does not give

rise to a main root, so instead, many small

roots grow from the stem. They are

adventitious, describing a plant organ that

grows in an unusual location, such as rootsarising from stems or leaves. Each small root

forms its own lateral roots.

Taproot systems generally penetrate deeply

and are therefore well adapted to deep soils

where the groundwater is not close to the

surface.

They usually do not penetrate deeply and are

therefore best adapted to shallow soils or

regions where rainfall is light and does not

moisten the soil much below the surface


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layer.

Most grasses have shallow roots,

concentrated in the upper few centimeters of

the soil. Because these shallow roots hold the

topsoil in place, grass makes excellent

ground cover for preventing erosion.

43.Suggest advantages of sexual reproduction.Water can be moved through a plant either by root pressure (pushing xylem sap) or the

transpiration-cohesion-tension mechanism (pulling xylem sap)

At night, when there is almost no transpiration, root cells continue pumping mineral ions into

the xylem of the stele. The resulting accumulation of minerals lowers the water potential

within the stele. Water flows in from the root cortex, generating root pressure, a push of

xylem sap. The root pressure sometimes causes more water to enter the leaves than is

transpired, resulting in guttation.In bulk flow, the movement of fluid is driven by a water potential difference at opposite ends

of the xylem tissue. The water potential difference is created at the leaf end of the xylem by

the evaporation of water from leaf cells. Evaporation lowers the water potential at the air-

water interface, thereby generating the negation pressure (tension) that pulls water through

the xylem. Bulk flow is driven by differences in pressure potential, and not solute potential.

The plant uses no energy to use bulk flow.

44.Suggest how apical dominance might be stopped.Cytokinins, auxin (IAA), and other factors interact in the control of apical dominance, the

ability of the apical bud to suppress the development of axillary buds. The direct inhibition

hypothesis proposed that auxin and cytokinins act antagonistically in regulating axillary bud

growth. Auxin transported down the shoot from the apical bud directly inhibits axillary buds

from growing, causing a shoot to lengthen and not the lateral branches. Meanwhile,

cytokinins entering the shoot system from the roots counter auxin by telling the axillary buds

to grow.

If the apical bud, the primary source of auxin, is removed, the inhibition of axillary buds is

removed and the plant becomes bushier.

45.Discuss the regulation of guard cells.When guard cells take in water from neighboring cells by osmosis, they become more turgid.

The changes in turgor pressure results primarily from the reversible absorption and loss of

K+. Stomata open when guard cells actively accumulate K

+from nearby epidermal cells.

The flow of K+

across the plasma membrane of the guard cell is coupled to the generation of

a membrane potential by proton pumps, so stomatal opening correlates with active transport

of H+

out of the guard cell. The resulting voltage (membrane potential) drives K+

into the


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cell through membrane channels. The absorption of K+

causes the water potential to become

more negative within the guard cells, and the cells become more turgid as water enters by

osmosis. Losing K+

from guard cells to neighboring cells leads to loss of water and stomatal

closing.

See #16 for a picture.

46.Discuss the use of water potential calculations in botany.Water potential is important in regarding the movement of water (water moves from regions

of higher water potential to regions of lower water potential).

Water potential is also important in how it affects absorption and loss of water by a living

plant cell.

47.Explain the movement of sugar through phloem.The phloem transports the products of photosynthesis to other parts of the cell, specifically

the sinks. In contrast to the unidirectional transport of xylem sap from roots to leaves,

phloem sap moves from sites of sugar production to sites of sugar use or storage. Sugar must

be transported, or loaded, into sieve-tube elements before being exported to sugar sinks.

Sugar can move from mesophyll cells to sieve-tube elements via the symplast, passing

through plasmodesmamta. Or, it can move by symplastic and apoplastic pathways. Much of

it then moves into the apoplast and is accumulated by nearby sieve-tube elements, either

directly or through companion cells.


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Phloem sap moves through a sieve tube by bulk flow driven by positive pressure, known as

pressure flow. The building of pressure at the source end and reduction of that pressure at

the sink end cause water to flow from source to sink, carrying the sugar along.

48.Explain how various climatic conditions can affect the rate of transpiration.Transpiration is greatest on a day that is sunny, warm, dry, and windy because these

environmental factors increase evaporation as long as stomata are open. If transpiration

cannot pull sufficient water to the leaves, the shoot becomes slightly wilted as cells lose

turgor pressure. Even with closing stomata, plants still lose some water by evaporation.

One problem plants face when the temperature of the environment falls is a change in the

fluidity of cell membranes. Plants respond to cold stress by altering the lipid composition oftheir membranes. At freezing temperatures, the plant cells increase the amount of solutes in

the cytoplasm so even lower temperatures are needed to freeze the cell completely.

49.Explain nitrogen fixation.


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Nitrogen fixation is the conversion of atmospheric nitrogen (N2) to ammonia (NH3).

Biological nitrogen fixation is carried out by certain prokaryotes, some of which have

mutualistic relationships with plants.

Ammonifying bacteria, which are usually decomposers living in humus-rich soil, release

ammonia (NH3) by breaking down proteins and other organic compounds in humus.

Nitrogen-fixing bacteria convert gaseous nitrogen (N2) into NH3. The NH3 produced picks

up another H+

in the soil to form NH4+, which plants can absorb. Plants acquire nitrogen

mainly in the form of NO3-, and it is largely formed by nitrification, which consists of the

oxidation of NH3 to nitrite (NO2-), followed by oxidation of nitrite to nitrate (NO3

-).

Nitrifying bacteria mediate each step. After the root absorbs NO3-, a plant enzyme reduces it

back to NH4+. Most plant species export nitrogen from roots to shoots via the xylem as NO3

-

or organic compounds synthesized in the roots. Some soil nitrogen is lost when denitrifying

bacteria convert NO3-to N2, which diffuses into the atmosphere.

See #22 for a picture.

50.Explain primary growth in roots and shoots.Primary growth is growth in length produced by apical meristems. The results of this growth

are called the primary plant body. In herbaceous plants, it is usually the entire plant while in

woody plants, its only the youngest parts.

The tip of the root is covered by a root cap, which protects the delicate apical meristem as the

root pushes through the abrasive soil during primary growth. It also secretes a polysaccharide

slime that lubricates the soil around the tip of the plant. Growth occurs behind the tip in three

different zones; the zone of cell division, zone of elongation, and zone of differentiation.

The zone of cell division includes the root apical meristem and its derivatives. In this region,

new root cells are produced including the root cap. In the zone of elongation, root cellselongate, sometimes to more than ten times their original length. Cell elongation pushes the

tip farther into the soil while the root apical meristem keeps adding cells to the younger end

of the zone of elongation. Before the root cells finish lengthening, many just begin

specializing in structure and function. In the zone of differentiation, or zone of maturation,

cells complete their differentiation and become distinct cell types.


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The primary growth of a root produces its epidermis, ground tissue, and vascular tissue. Root

hairs account for the most of the water and mineral absorption from the soil. It enhances the

absorption process by increasing the surface area of epidermal cells. Water and minerals

absorbed from the soil must enter through the roots epidermis.

The stele is a vascular cylinder or a solid core of xylem and phloem. In most eudicot roots,

xylem has a starlike appearance and phloem occupies the indentations between the arms of

the xylem star. In monocot roots, the vascular tissue consists of a central core of

parenchyma cells which are surrounded by a ring of xylem and a ring of phloem. The centralregion is called a pith. This is different from the stem pith, which is a ground tissue.

The ground tissue of roots consists mostly of parenchyma cells. It fills the cortex, the region

between the vascular cylinder and epidermis. Cells within the ground tissue store

carbohydrates and their plasma membranes absorb water and minerals from the soil. The

innermost layer of the cortex is the endodermis, a cylinder one cell thick that forms the

boundary with the vascular cylinder. The endodermis is also a selective barrier that regulates

passage of substances from the soil into the vascular cylinder.

Lateral roots arise from the pericycle, the outermost cell layer in the vascular cylinder which

is adjacent to and just inside the endodermis. A lateral root pushes through the cortex and

epidermis until it emerges from the established root. A lateral root cannot originate near theroots surface because its vascular system must be continuous with the vascular cylinder at

the center of the established root.

A shoot apical meristem is a dome-shaped mass of dividing cells at the shoot tip. Leaves are

developed from leaf primordial, which are finger-like projections along the sides of the apical

meristem. Axillary buds develop from islands of meristematic cells left by the apical

meristem at the bases of the leaf primordial.

Within a bud, leaf primordia are spaced close together because the internodes are very short.

Most shoot elongation is due to the lengthening of internode cells below the shoot tip.

Some plants produce leaf cells in areas of meristematic tissue separated from the apical

meristem. These areas are intercalary meristems, which remain at the base of leaf blades and

stem internodes. An example is grass, and it is why grasses tolerate grazing because the

elevated part of the leaf blade can be removed without stopping growth.


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The epidermis covers stems as part of the continuous dermal tissue system. Vascular tissue

runs the length of a stem in vascular bundles. Unlike lateral roots which arise from vascular

tissue deep within a root and disrupt the vascular cambium, cortex, and epidermis as they

emerge, lateral shoots develop from axillary bud meristems on the stems surface anddisrupts no other tissues. The vascular bundles of the stem converge with the roots vascular

cylinder in a zone of transition located near the soil surface. In eudicots, the vascular tissue

consists of vascular bundles arranged in a ring. The xylem in each vascular bundle is adjacent

to the pith, and the phloem in each bundle is adjacent to the cortex. In monocots, the vascular

bundles are scattered throughout the ground tissue. In the stems of both eudicots and

monocots, the ground tissue consist mostly of parenchyma cells. Collenchyma beneath the

epidermis strengthen many stems and sclerenchyma provide support in those no longer

elongating parts of the stems.

The epidermal barrier is interrupted by stomata, which allow gas exchange between the

surrounding air and the photosynthetic cells inside the leaf. Stoma refers to the entirestomatal complex consisting of a pore flanked by two guard cells.

The ground tissue of a leaf, a region called the mesophyll, is between the upper and lower

epidermal layers. Mesophyll consists mainly of parenchyma cells specialized for

photosynthesis. The leaves of eudicots have two distinct areas: palisade mesophyll and

spongy mesophyll. Palisade mesophyll consists of one or more layers of elongated

parenchyma cells on the upper part of the leaf. Spongy mesophyll is below the palisade

mesophyll. The vascular tissue of each leaf is continuous with the vascular tissue of the stem.

Leaf traces are connections from vascular bundles in the stem, which pass through petioles

and into leaves. Veins are the leafs vascular bundles, which subdivide repeatedly and branch

throughout the mesophyll. Each vein is enclosed by a protective bundle sheath, consisting of

mainly one of more layers of parenchyma cells. Unlike stems and roots, leaves rarely

undergo secondary growth.

51.Explain secondary growth in woody plants.52.Analyze graphs.

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AP Bio Expectations - Botany