5.6.5: factors affecting photosynthesis

Question 1

What are some limiting factors of photosynthesis?

Answer 1

• raw materials
• CO2
• H2O
• Light intensity
• availability of chlorophyll, e- carriers, relevant enzymes.
• Temperature
• turgidity of cells.

Question 2

What is light important for plants?

Answer 2

• Provides energy to power 1st stage of photosynthesis and produce ATP and NADPH needed for the 2nd stage.
• Causes Stomata to open for gaseous exchange and transpiration.
• Transpiration → uptake of water, delivery to leaves.

Question 3

At constant favourable temp/ and CO2 conc. what is the limiting factor?

Answer 3

• Light intensity.
• Light intensity low→rate of photosynthesis is low.
• Light intensity increases→rate of photosynthesis increases.

Question 4

Why does the rate of photosynthesis not increase after a certain level of light intensity?

Answer 4

Another factor is limiting the process.

Question 5

What is the effect of reduced light levels on the relative concentrations of GP, TP and RuBP in the Calvin cycle?

Answer 5

• GP cannot be reduced to TP.
• TP levels fall, GP levels accumulate.
• If TP levels fall, RuBP cannot be regenerated.

Question 6

Why is CO2 not usually a limiting factor?

Answer 6

The levels of CO2 in the atmosphere and in aquatic habitats are high enough that CO2 is not usually a limiting factor.

Question 7

What are the effects of changing the CO2 concentration on the Calvin cycle?

Answer 7

If CO” conc. falls below 0,01%

1. RuBP cannot accept it, and it accumulates
2. GP cannot be made
3. TP cannot be made.

Question 8

Why is the Calvin cycle sensitive to temperature?

Answer 8

-It involves many enzyme-catalysed reactions.

Question 9

The effects of changing temperature on the Calvin cycle:

low to 25-30

Answer 9

If plants have enough H2O, CO2 and light intensity…

Temp increases→photosynthesis increases

Question 10

The effects of changing temperature on the Calvin cycle:

above 30

Answer 10

-Growth rates reduced due to photorespiration.

Question 11

what is photorespiration?

Answer 11

• O2 competes with CO2 for RuBisCO’s active site.
• This reduces the amount of CO2 being accepted by RuBP.
• Subsequently, reduces quantity of GP and therefore TP being produced.
• Initially, this causes RuBP accumulation.
• However, due to lack of TP, RuBP cannot be regenerated.

Question 12

The effects of changing temperature on the Calvin cycle:

Above 45

Answer 12

• Enzymes involved in photosynthesis denatured

- Reduces conc. GP and TP and eventually, RuBP as it cannot regenerate due to lack of TP.

Question 13

Why is it important that plants have sufficient water in the soil?

Answer 13

• Transpiration stream has a cooling effect on plants.
• Water passing up xylem to leaves keeps plant cells turgid so that they can function.
• Turgid guard cells keep stomata open for gaseous exchange.

Question 14

What is water stress?

Answer 14

When not enough water is available to the plant.

Question 15

If not enough water is available to the plant (water stress):

Answer 15

1. Roots not able to take up enough water to replace water lost via transpiration.
2. Cells lose water, become plasmolysed
3. Plant root produces abscisic acid, translocated to leaves, causes stomatal closure, (g) exchange reduced.
4. Tissues become flaccid and leaves wilt.
5. Rate of photosynthesis greatly reduced.