Bio 1000- Photosynthesis- The Final Part

PLANTS, PHOTORESPIRATION & PHOTOSYNTHESIS- HOW IS IT ALL RELATED?

Plants that survive in hot, dry climates have evolved to face the problems of both photorespiration and water loss. As suspected, the major photosynthetic organ of a plant is the leaf and because of its high surface area, it is easily expected that these regions would encounter high rates of water loss due to evaporation. However, leaves are in fact covered by a waxy cuticle that prevents water loss. To enable high rates of gas exchange with carbon dioxide in and oxygen out between the environment and the cells within the leaf, the surface of leaf contains small pores called the stomata.

Plants regulate the size of the stomata from fully closed to fully open to balance the gas exchange with the need to minimize water loss. However, plants that live in hot, dry climate face the dilemma of opening their stomata to allow for carbon dioxide to enter but keeping their stomata closed so water doesn't leave. Photorespiration increases as temperature increases.

SO HOW DO PLANTS DEAL WITH THIS DILEMMA?

Some plant species have evolved a mode of carbon fixation that minimizes photorespiration. Besides having the Calvin cycle, plants also have a second carbon fixation pathway called the C4 cycle.

C4 CYCLE-HOW IT WORKS?

General Process:

1. Carbon dioxide combines with a 3-carbon molecule, phosphoenolpyruvate (PEP) to produce 4-carbon oxaloacetate.
2. Oxaloacetate is reduced to malate by electrons transferred from NADPH
3. Malate then is transported to the site of the Calvin cycle in which the malate is oxidized to pyruvate, releasing carbon dioxide
4. To complete the cycle, pyruvate is converted back PEP in a reaction that consumes ATP

C3 VS. C4 PLANTS-WHAT'S THE DIFFERENCE?

C3 plants are those plants that possess the Calvin cycle where as C4 plants are those that possess the C4 pathway. The key difference between these two are really concerned around carboxylation reactions. In C4 cycle, the initial carboxylation reaction incorporates carbon dioxide into PEP which is catalyzed by the enzyme PEP carboxylase. Unlike Rubisco, PEP carboxylase has a greater affinity (stronger attraction/binding) for carbon dioxide and doesn't contain any oxygenase activity.

Note that the C4 cycle occurs in mesophyll cells and malate diffuses from mesophyll cells to bundle sheath cells in which Calvin cycle operates. Why photorespiration is unlikely to occur through the C4 pathway is because bundle sheath where the Calvin cycle operates, has extremely low concentrations of oxygen present and higher concentrations of carbon dioxide. Hence, oxygenase activity of Rubisco is inhibited and photorespiration is significantly reduced.

SO OKAY, WELL THE C4 CYCLE REDUCES PHOTORESPIRATION, WHY DON'T ALL PLANTS USE IT THEN?

Remember that though the C4 cycle does reduce photorespiration, it requires the use of ATP to regenerate PEP from pyruvate. In hot and dry climates, photorespiration can reduce the plant's efficiency by UP TO 50%! Using one molecule of ATP is nothing compared to losing 50% of its efficiency. Also, in hot climates, amount of light energy is quite high (well obviously), so the ATP cost is easily met by absorbing more light energy and increasing the output of light reactions.

CAM PLANTS-WHAT IS THAT?

CAM (craussulacean acid metabolism) plants are plants that run the Calvin and C4 cycles at different times. Often this distinction is found in plants that have thick and succulent leaves or stems. CAM plants often exist in regions that are hot and dry during the day and cool at night. Their leaves usually have a low surface-to-volume ratio and their stomata is reduced in number.

General Process:

1. The stomata is open only at night where they release oxygen that accumulates during the day from photosynthesis and allow carbon dioxide to enter the leaves
2. The entering carbon dioxide is fixed by the C4 pathway into malate which accumulates during the night and stored in large cell vacuoles.
3. As daylight becomes present, the stomata closes and malate diffuses from the cell vacuole into the cytosol where it is oxidized to pyruvate and carbon dioxide is released in high concentration
4. The high CO2 favors carboxylase activity of Rubisco and Calvin cycle proceeds
5. Pyruvate produced from malate breakdown accumulates during the day and as night falls, it enters the C4 cycle where it is converted back into malate

Well, congratulations, now you have learned the entire process of photosynthesis. Be sure to click on other pages to learn more about topics involved in Introductory Biology