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Chapter 16Photosynthesis

Leaf Structure and Function& Factors necessary

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Leaves Function of leaves?

photosynthesis energy production CH2O production

gas exchange transpiration simple vs. compound

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Which cells appear to contain chloroplasts?

CollenchymaCollenchyma

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Summary of stages of photosynthesis

Summary of stages of photosynthesis

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Stomates Function of stomates?

blue-light wavelengths of daylight, detected by zeaxanthin (a carotenoid)

activate proton pumps in the guard cell membranes, which proceed to extrude protons from the cytoplasm of the cell;

this creates a "proton motive force" (an electrochemical gradient across the membrane) which;

opens voltage operated channels in the membrane, allowing positive K ions to flow passively into the cell, from the surrounding tissues.

Chloride ions also enter the cell.

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Water passively follows these ions into the guard cells, and as their tugidity increases so the stomatal pore opens, in the morning.

As the day progresses the osmotic role of potassium is supplanted by that of sucrose, which can be generated by several means, including starch hydrolysis and photosynthesis.

At the end of the day (by which time the potassium accumulation has dissipated) it seems it is the fall in the concentration of sucrose that initiates the loss of water and reduced turgor pressure, which causes closure of the stomatal pore.

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Regulating Stomatal Opening:-the potassium ion pump hypothesis

Guard cells flaccid

Stoma closed

K+

K+

K+

K+

K+K+

K+

K+

K+

K+K+

K+

K+ ions have the same concentration in guard cells and epidermal cells

Light activates K+ pumps which

actively transport K+ from the epidermal cells into the guard

cells

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Regulating Stomatal Opening:-the potassium ion pump hypothesis

K+K+

K+

K+

K+K+

K+

K+

K+

K+K+

K+

Increased concentration of KIncreased concentration of K++ in guard cellsin guard cells

Lowers the Lowers the (water potential) (water potential) in the guard cellsin the guard cells

Water moves in by osmosis, Water moves in by osmosis, down down gradient gradient

HH22OO

HH22OO

HH22OO

HH22OO

HH22OO

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Stoma open

Guard cells turgid

K+

K+

K+

K+

K+

K+

K+

K+

K+

K+

K+

K+

Water moves in by osmosis, Water moves in by osmosis, down down gradient gradient

HH22OO

HH22OO

HH22OOHH22OO

HH22OO

HH22OO

Factors affecting the rate of photosynthesis

The main factors are light intensity, carbon dioxide concentration and temperature, known as limiting factors.

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Factors affecting the rate of photosynthesis

As light intensity increases, the rate of the light-dependent reaction, and therefore photosynthesis generally, increases proportionately.

As light intensity is increased however, the rate of photosynthesis is eventually limited by some other factor.

Chlorophyll a is used in both Photosystems. The wavelength of light is also important. PSI absorbs energy most efficiently at 700 nm and PSII at 680 nm. Light with a high proportion of energy concentrated in these wavelengths will produce a high rate of photosynthesis.

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Factors affecting the rate of photosynthesis

An increase in CO2 increases the rate at which carbon is incorporated into carbohydrate in the light-independent reaction and so the rate of photosynthesis generally increases until limited by another factor.

Photosynthesis is dependent on temperature. It is a reaction catalysed by enzymes. As the enzymes approach their optimum temperatures the overall rate increases. Above the optimum temperature the rate begins to decrease until it stops.

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C3/C4 Plants? The photosynthetic reactions we have

been discussing related to C3 plants CO2 combines with RuBP to form a 6C

molecule that immediately splits into two 3C molecules

In some plants the first thing produced is a 4C molecule

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Avoiding photorespiration Rubisco catalyzes the CO2 combining

with RuBP

But, can also catalyze O2 combining with RuBP – photorespiration

RuBP is being wasted Photorespiration occurs more readily in

high temperature/high light conditions

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Avoiding photorespiration Tropical grasses evolved a method of

avoiding photorespiration Bundle sheath cells – keep RuBP and

rubisco arranged in vascular bundles away from high oxygen concentration areas

Mesophyll cells containing the enzyme PEP carboxylase absorb CO2

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Avoiding photorespiration CO2 + PEP (phosphoenolpyruvate)

yields oxaloacetate converted to malate Malate passed to bundle sheath cells

where the CO2 is removed

Light independent reactions then proceed normally

i.e. rubisco catalyzes RuBP + CO2

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CAM Plants (crassulacean acid metabolism)

CAM plants are adapted to life in arid conditions by conserving water.

During the night, the CAM plant's stomata are open, allowing CO2 to enter and be fixated as organic acids (CAM) that are stored in vacuoles. During the day the stomata are closed (thus preventing water loss), and the carbon is released to the Calvin cycle so that photosynthesis may take place.

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The carbon dioxide is fixed in the mesophyll cell's cytoplasm by a PEP reaction similar to that of C4 plants. But, unlike C4 plants, the resulting organic acids are stored in vacuoles for later use; that is, they are not immediately passed on to the Calvin cycle.

Of course, the latter cannot operate during night because the light reactions that provide it with ATP and NADPH cannot take place without light.

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During the day The carbon in the organic acids is freed

from the mesophyll cell's vacuoles and enters the chloroplast's stroma and, thus, into the Calvin cycle.

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The benefits of CAM The most important benefit to the plant

is the ability to leave most leaf stomata closed during the day.

Being able to keep stomata closed during the hottest and driest part of the day reduces the loss of water through evapotranspiration, allowing CAM plants to grow in environments that would otherwise be far too dry.

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C3 plants, for example, lose 97% of the water they uptake through the roots to transpiration - a high cost avoided by CAM plants.

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Any Questions??