5B RUBISCO IN C3, C4, AND CAM PHOTOSYNTHESIS

Question 1

Rubisco overview

Answer 1

Rubisco is a key enzyme of the light-independent stage of photosynthesis. Its action varies: sometimes it binds to carbon dioxide and facilitates further reactions in the photosynthesis process, whilst other times it binds to oxygen and initiates a wasteful process called photorespiration.

Question 2

The light-dependent stage?

What is produced and Where

Answer 2

it is where light energy splits water molecules to produce ATP, NADPH, and oxygen. This stage occurs in the thylakoid membranes of the grana.

Question 3

The light-independent stage?

What is produced and Where

Answer 3

where carbon dioxide is converted into organic molecules using ATP and NADPH, and more water is produced. This stage occurs in the stroma, and is also known as the Calvin cycle, the dark stage, or the light-independent reactions.

Question 4

what is an enzyme?

Answer 4

an organic molecule, typically a protein, that catalyses (speeds up) specific reactions.

Question 5

What is rubisco?

Answer 5

a pivotal enzyme involved in initial carbon fixation during the light-independent stage of photosynthesis

Question 6

what is carbon fixation?

Answer 6

the process in living organisms where inorganic carbon, typically within carbon dioxide, is converted into organic compounds such as glucose. Carbon fixation is a central part of the light-independent stage of photosynthesis.

Question 7

inorganic

Answer 7

a compound that does not contain a carbon-hydrogen bond, e.g. carbon dioxide

Question 8

organic

carbon-hydrogen bond

Answer 8

a compound containing a carbon-hydrogen
bond, e.g. glucose

Question 9

summarized role of rubisco

Answer 9

the role of Rubisco can be summarized as binding CO2 and fixing the carbon into the organic 3-PGA, thus initiating the Calvin cycle.

Question 10

substrate

Answer 10

the reactant of a reaction catalysed by an enzyme

Question 11

photorespiration

Answer 11

a wasteful process in plants initiated by Rubisco that limits photosynthesis

Question 12

What is the substrate of the Calvin cycle pathway?

Answer 12

CO2 (carbon dioxide)

Question 13

What is the substrate of the photorespiration pathway

Answer 13

O2 (oxygen)

Question 14

stoma (pl. stomata) 

Answer 14

a small pore on a leaf’s surface that opens and closes to regulate gas exchange.

Question 15

affinity

Answer 15

the tendency of a molecule/atom to bind or react with another molecule/atom

Question 16

What are C3 plants?

Answer 16

C3 plants are plants with no evolved adaptation to minimize photorespiration.

Question 17

mesophyll cell

Answer 17

a plant cell type found in leaves that contain large amounts of chloroplasts

Question 18

bundle-sheath cell

Answer 18

a plant cell type that is the site of most of the Calvin cycle in C4 plants

Question 18

What are C4 plants?

Answer 18

C4 plants are plants that minimize photorespiration by separating initial carbon fixation and the remainder of the Calvin cycle over space.

Question 19

C4 plants

Answer 19

• The light-dependent stage of photosynthesis in C4 plants is still exactly the same as C3 plants.
• The carbon fixation is done in a different cell to the rest of the Calvin Cycle.
• C4 photosynthesis gets its name from the first four-carbon molecule produced in the initial carbon fixation.
• The rest of the Calvin cycle occurs in the bundle-sheath cells.

Question 19

C3 plants

Answer 19

C3 plants are approximately 85% of plants on Earth. As such, C3 plants undertake the ‘normal’ photosynthesis and these plants possess no adaptations to reduce photorespiration. C3 photosynthesis gets its name from the three-carbon (3-PGA) that the initial carbon fixation produces. Eg; of C3 plants include all trees, cereals such as wheat and rice, and the majority of nuts, fruits, and vegetables.

Question 20

What are CAM plants?

Answer 20

plants that minimise photorespiration by separating initial carbon fixation and the remainder of the Calvin cycle over time.

Question 21

CAM plants

Answer 21

• CAM stands for crassulacean acid metabolism.
• The light-dependent stage of photosynthesis in CAM plants is still identical to that of C3 and C4 plants, but the light-independent stage differs.
• rather than separating the initial carbon fixation and the remainder of the Calvin cycle spatially over two cells like in C4 plants, CAM plants separate the steps over time.
• At night, CAM plants open up their stomata to bring in CO2
• During the daytime, CAM plants do not open their stomata to prevent water loss.
• Water is also conserved in CAM plants as their stomata only open at night when it is typically cooler and more humid. Because of this, CAM plants are very prominent in very hot dry areas like deserts. CAM plants include almost all cacti, pineapples, vanilla, and orchids.