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# Rate of photosynthesis?

Denvis
Okay, so i'm doing a couple questions for my biology homework. I got stuck on this one question and it states.

List the factors that affect the rate of photosynthesis. In each case explain how the factor influences this process.

I know the factors which affect the rate of photosynthesis. They are light, water, co2 and temp. But how do they affect the rate of photosynthesis?

Afaceinthematrix
Temperature: When you increase temperature, the molecules in the cells will move at a faster rate due to kinetics. If the temperature gets too high, the rate of photosynthesis will drop because the plant's enzymes are denatured. They will therefore be more likely to join onto the enzymes and react.

Water: If the plant does not have enough water, the plant's stomata will shut and the plant will be deprived of carbon dioxide. Carbon dioxide has a direct affect on the rate of photosynthesis because CO2 is used in the photosynthesis reaction. As a matter of fact, H2O is also in the reaction. Just look at the Calvin Cycle....

3 CO2 + 9 ATP + 6 NADPH + 6 H+ ----> C3H6O3-phosphate + 9 ADP + 8 Pi + 6 NADP+ + 3 H2O
Arnie
Actually there's an optimum temperature for each enzyme, since too low temperatures are also not good.

For the rest, I'm not going to do your homework for you, but look at the light and the dark reactions and what they "need".
Gagnar The Unruly
Denvis,

This isn't a hard question if you just think it through. For light, do you expect photosynthetic rate to be higher or lower if it's bright outside? For water availability, should assimilation be higher if the plant is experiencing drought stress or if it is actively transpiring? I wouldn't want to give you answers but I do need to make a few corrections...

 Afaceinthematrix wrote: Temperature: When you increase temperature, the molecules in the cells will move at a faster rate due to kinetics. If the temperature gets too high, the rate of photosynthesis will drop because the plant's enzymes are denatured. They will therefore be more likely to join onto the enzymes and react.

That's an extreme case that only happens at very high temperatures, when excess light energy will probably already have broken the photosystems. More relevant is the fact that rubisco begins to preferentially fix O2 instead of CO2 at higher temperatures. For C3 plants this means that quantum yield takes a nose dive and growth rates stall. Also, respiration rates increase with high temperature faster than photosynthetic rates do, which means that net photosynthetic yield drops. Loss of membrane integrity at high temperature is also a major problem. Of course, at low temperatures rubisco is very slow and that also slows C assimilation rates.

 Quote: Water: If the plant does not have enough water, the plant's stomata will shut and the plant will be deprived of carbon dioxide. Carbon dioxide has a direct affect on the rate of photosynthesis because CO2 is used in the photosynthesis reaction. As a matter of fact, H2O is also in the reaction. Just look at the Calvin Cycle.... 3 CO2 + 9 ATP + 6 NADPH + 6 H+ ----> C3H6O3-phosphate + 9 ADP + 8 Pi + 6 NADP+ + 3 H2O

It's true that water is needed for photosynthesis, but about half of that water will be regenerated by respiration. Only about 1-3% of the water that plants take up is 'used up' in photosynthesis. The reason photosynthesis stops due to low water availability is 1) because stomata shut and CO2 stops entering the leaves, as you said, or 2) because cells lose turgor and the chloroplasts can no longer carry out photosynthesis.
Afaceinthematrix
Gagnar The Unruly wrote:
 Afaceinthematrix wrote: Temperature: When you increase temperature, the molecules in the cells will move at a faster rate due to kinetics. If the temperature gets too high, the rate of photosynthesis will drop because the plant's enzymes are denatured. They will therefore be more likely to join onto the enzymes and react.

That's an extreme case that only happens at very high temperatures, when excess light energy will probably already have broken the photosystems. More relevant is the fact that rubisco begins to preferentially fix O2 instead of CO2 at higher temperatures. For C3 plants this means that quantum yield takes a nose dive and growth rates stall. Also, respiration rates increase with high temperature faster than photosynthetic rates do, which means that net photosynthetic yield drops. Loss of membrane integrity at high temperature is also a major problem. Of course, at low temperatures rubisco is very slow and that also slows C assimilation rates.

Yeah, your explanation is more relevant. I was just pulling an answer out of my ass because I wanted to fill Denvis' mind with ideas but leave an answer that still requires research and studying.

Quote:
 Quote: Water: If the plant does not have enough water, the plant's stomata will shut and the plant will be deprived of carbon dioxide. Carbon dioxide has a direct affect on the rate of photosynthesis because CO2 is used in the photosynthesis reaction. As a matter of fact, H2O is also in the reaction. Just look at the Calvin Cycle.... 3 CO2 + 9 ATP + 6 NADPH + 6 H+ ----> C3H6O3-phosphate + 9 ADP + 8 Pi + 6 NADP+ + 3 H2O

It's true that water is needed for photosynthesis, but about half of that water will be regenerated by respiration. Only about 1-3% of the water that plants take up is 'used up' in photosynthesis. The reason photosynthesis stops due to low water availability is 1) because stomata shut and CO2 stops entering the leaves, as you said, or 2) because cells lose turgor and the chloroplasts can no longer carry out photosynthesis.

Ahh... I forgot about number 2. It's been a while since I've taken biology. I'm more into the physical sciences.