17.4 Factors affecting photosynthesis

Question 1

define limiting factor

Answer 1

something required that, if in short supply can limit the rate at which photosynthetic reactions take place

Question 2

what are the limiting factors for photosynthesis

Answer 2

• CO2
• H2O
• light intensity
• availability of chlorophyll/ enzymes/ electron carriers

Question 3

importance of light in photosynthesis

Answer 3

• provides energy to power the LDS

- causes the stomata to open and increase transpiration rate which leads to the uptake of water

Question 4

describe the relationship between rate of photosynthesis and light intensity

Answer 4

• as light intensity increases, the rate of photosynthesis also increases until the point where it plateaus
• this is when another factor becomes the limiting factor

Question 5

what is the effect of reducing light intensity on the Calvin cycle

Answer 5

• reducing light intensity reduces rate of LDS
• less ATP and reduced NADP are produced
• needed to convert GP–> TP so GP increases and TP decreases
• RuBP is regenerated from TP so levels decrease in line with TP levels

Question 6

how does the Calvin cycle act to ensure more CO2 will diffuse into the leaf

Answer 6

provides a diffusion gradient by lowering CO2 levels inside the leaf compared to the atmosphere

Question 7

describe the effect of CO2 at different light intensities

Answer 7

• at low/high light intensities, CO2 acts as a limiting factor and increases rate of photosynthesis
• increasing CO2 increases rate up to a point where CO2 concentration in no longer a limiting factor

Question 8

when is the rate of photosynthesis the fastest

Answer 8

• high light intensity

- high CO2 levels

Question 9

describe what happens to the levels of RuBP, TP and GP when the CO2 conc falls below 0.01%

Answer 9

• GP levels decrease as there is not enough CO2 going into the cycle
• therefore TP levels also decrease
• RuBP starts to build up as it isn’t being used

Question 10

what happens at low temperatures (25-30) to the rate

Answer 10

the rate of photosynthesis increases as there is more kinetic energy

Question 11

what happens at temperatures above 30 to the rate

Answer 11

• growth rates may reduce due to photorespiration (O2 competes with CO2 for RuBisCO’s active site and reduces how much CO2 is accepted by RuBP)
• the reduction of CO2 accepted reduces quantity of GP and therefore TP
• initially causes accumulation of RuBP but once TP runs out, no more RuBP can regenerate

Question 12

what happens at temperatures above 45 to the rate of photosynthesis

Answer 12

• enzymes denature
• reduces conc of GP and TP and eventually RuBP
• rate starts to slow down

Question 13

what does the affect of a rise/fall in temperature have on relative levels in the Calvin cycle

Answer 13

• fall will reduce rate of reactions
• shouldn’t affect one step more than he other so the relative levels of compounds in the cycle should not change that much