5B: C4 and CAM photosynthesis

Question 1

How is RuBisCO important for the Calvin Cycle?

Answer 1

Controls the first reaction in the light independent stage

Question 2

Why is RuBisCO inefficient?

Answer 2

RuBisCO isn’t substrate specific and can bind to CO2 or O2. When this occurs, the Calvin Cycle is disrupted, preventing Glucose from being made.

Question 3

What is the name for when RuBisCO binds to O2

Answer 3

Photorespiration

Question 4

How is Photorespiration wasteful in plants?

Answer 4

It disrupts Photosynthesis because CO2 now has to compete with Oxygen to bind with RuBisCO, thus hindering its chances. Less photosynthesis consequently means less glucose is produced meaning less energy is produced along with the wasted energy used on photorespiration. Thus, photorespiration impairs the plant’s ability to grow and survive

Question 5

Why can photorespiration occur over photosynthesis?

Answer 5

1. Substrate Concentration. If there’s more O2 than CO2, RuBisCO will naturally have a higher chance to bind to it. When plants close their stomata to conserve water loss, the O2 produced in the LDS will also accumulate in the cell, leading to more photorespiration
2. Temperature. At low or normal temperature, RuBisCO has a higher affinity for CO2. At higher temperatures, RuBisCO has a higher affinity for O2, leading RuBisCO to bind to O2 more often at higher temperatures, hence increasing the amount of photorespiration.

Question 6

Where do C4 plants thrive in?

Answer 6

In HOT AND HUMID environments

Question 7

Where do CAM plants thrive?

Answer 7

In HOT AND DRY environments

Question 8

How are C4 and CAM different to C3 photosynthesis?

Answer 8

C4 and CAM separate Carbon fixation of the Calvin Cycle over space and time respectively

Question 9

How does C4 photosynthesis work?

Answer 9

• LDS is same as C3 plants
• Calvin Cycle occurs in neighbouring inner Bundle Sheath Cells (BSC)
• Adapted this way to ensure RuBisCO has a near exclusive supply of CO2
• CO2 enters the mesophyll and binds to PEP (via PEP Carboxylase) to form Malate.
• Malate is moved to BSC via plasmodesmata tubing,
• Once in BSC, malate breaks down into CO2, which enters the Calvin Cycle, and Pyruvate, which is transported back to the mesophyll and converted into PEP to be used again

Question 10

Advantages of C4 plants

Answer 10

• Through isolating RuBisCO in Bundle Sheath cells and funnelling CO2, the plant minimizes how much O2 can interact with the enzyme, thus minimizing photorespiration
• PEP Carboxylase only acts on CO2, ensuring a low O2 concentration in the BSC

Question 11

How does CAM photosynthesis work?

Answer 11

• LDS is same as C3
• DAY: stomata opens to take in CO2, CO2 is stored as Malate in Vacuoles
• NIGHT: stomata closes to prevent water loss from evaporation, stored Malate is reverted into CO2 to be used in the Calvin Cycle in the Mesophyll

Question 12

Advantages of CAM plants

Answer 12

• By closing their stomata during the day, CAM plants prevent water, a vital input for the LDS, from evaporating from the leaves

Question 13

Obligate vs Facultative CAM plants

Answer 13

Obligate:
* Can ONLY do CAM Photosynthesis

Facultative:
* Plants that can SWITCH BETWEEN CAM and C3 photosynthesis

(Facultative CAM plants have this adaptation because the weather might change to be rainy/cool, meaning that closing the plant’s stomata wouldn’t be beneficial anymore. Furthermore, C3 IS MUCH MORE EFFICIENT than C4 and CAM, making it better.)