L. 10 Lecture 10: Photosynthesis II (CO2 Fixation)

Question 1

LO

Answer 1

• Explain fixation of carbon dioxide in the Calvin (C3) cycle
• Compare carboxylation and oxygenation by Rubisco
• Describe the co-evolution of C4 photosynthesis with Earth’s atmosphere over time
• Describe the photosynthesis-transpiration compromise and how CAM photosynthesis breaks the nexus between photosynthesis and transpiration

Question 2

The photosynthesis REDOX reactions

Answer 2

Stage 1:
- Non-cyclic photophosphorylation (Light Reactions)

Stage 2:
- Calvin cycle (light independant)

Question 3

Light and Carbon reactions of photosynthesis

Answer 3

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Question 4

Rubisco

Answer 4

• Enzyme
• In stroma of chloroplast
• Catalyse first step in calvin cycle

Question 6

The Calvin (C3) cycle

Answer 6

• Cyclic pathway
• Light independant

1. 3x CO2 join cycle and are added to 3x 5C molecule by Rubisco.
2. 3x 6C molecules are created, then split into 6x 3C molecules.
3. the 3C molecules are reduced, ATP and NADPH are oxidized
4. A 3C molecule is taken from the cycle to create a 6C sugar. (Cycle occurs twice for sugar to be made)
5. ATP is oxidized into ADP +Pi, converting the 5x 3C molecules into 3x 5C molecules.
6. The 3x 5C molecules are then recycled in the cycle ready for CO2 to join.

Question 7

Rubisco Carboxylation & Oxygenation

Answer 7

• Rubisco can use carbon dioxide OR oxygen as a substrate.
• When Rubisco uses O2, the plant cannot sunthesize sugars
• O2 is problematic
• O2 causes Photorespiration (C2 Cycle)

Question 8

Photorespiration (C2 cycle)

Answer 8

• O2 binds to a 5C molecule
• 5CO molecule then splits into 3C and 2C molecules
• 3C molecule continues in cycle
• 2C molecule is toxic and is detoxified through photorespiration, producing CO2, losing a carbon.

Question 9

Solutions for Photorespiration

Answer 9

• C4 photosynthesis
• CAM Photosynthesis

Question 10

C4 Photosynthesis

Answer 10

• Rubisco is still used
• A series of metabolic adjustments exploiting PEPC to concentrate CO2 around Rubisco

Question 11

C4 Photosynthesis steps

Answer 11

1. There is a spacial seperation in biochemistry
2. CO2 diffuses into mesophyll cell
3. Converts into bicarbonate in aqueous liquid
4. Bicarbonate is fixed onto a 3C molecule to form a 4C molecule
5. 4C molecule moves down into bundle sheath cell.
6. 4C is decarboxylated in a mitochondria surrounding the chloroplast.
7. CO2 goes into chloroplast and 3C molecule is reused in C4 cycle.

Question 12

CAM photosynthesis

Answer 12

• Solves photorespiration and saves water
• When the gurad cells open to allow in CO2, H20 evaporates from the leaf
• Temporal seperation of biochemistry
• Stomata of plants open at night and close during day.

Question 13

CAM photosynthesis process

Answer 13

1. At night CO2 diffuses into leaf through stomata and converts into bicarbonate
2. Bicarbonate us fixed to a 3C molecule forming a 4C molecule similar to C4 process
3. The 4C molecule sits in a vacuole over night
4. During the day when the stomata are closed, 4C molecule leaves vacuole and is decarboxylated around rubisco to entre the calvin cycle.

Question 14

Co-evolution of Co2 fixation pathways with earths atmosphere

Answer 14

• Majority of plants today use C3 photosynthesis
• C3 is ‘ancient’
• C4 photosynthesis evolved as an adaptation to declining CO2 levels and increasing temperatures

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