WK4 LEC10: Photosynthesis II (Light independent Calvin Cycle CO2 Fixation) 낱말 카드

Explain fixation of carbon dioxide in the Calvin (C3) cycle

Stage 1: (Non-cyclic Phosphorylation) -> Produces ATP and NADPH to use in Calvin Cycle. (Light dependent in thylakoids)

OXIDISE H2O

Stage 2: Calvin (C3) Cycle -> Fix CO2 to form carbohydrate glucose. (Light independent in lumen)

REDUCE CO2

1. RuBP (5C Precursor) + CO2 (1C input) -> Rubisco Catalyses to (6C molecule)

[via Carboxylation]

2. Unstable 6C splits to 2X stable 3C which is reduced by reducing agent NADPH using ATP.

[via reduction]

3. one 3C Molecule exits (3C output) to be processed into carbohydrate.

[via regeneration]

4. Remaining 3C molecule regenerated back into RuBP (5C precursor) using ATP.

Compare carboxylation and oxygenation by Rubisco

Carboxylation (C3 Cycle)

- RuBP + CO2 -> produces 2 3C molecules

Oxygenation (C2 Cycle)

- Positive for cell

- RuBP + O2 -> produces 3C molecule, toxic 2C molecule (detoxified to recover C via peroxisome and mitochondrion expending energy)

- Negative for cell

Describe the co-evolution of C4 photosynthesis with Earth's

atmosphere over time

C3 Photosynthesis Issues: Photorespiration --

(Rubisco is an old molecule - Oxygenation was not as great an issue in previous environments given the higher O2 atmosphere content at the time)

C4 Photosynthesis:

(Evolution of alternate mechanism to mitigate decreased efficiency due to Photorespiration sine O2 content in atmosphere increased alongside proliferation of plant life which produces it via photorespiration)

C4 Cycle: (Physical Separation)

1. Additional enzyme PEPcase captures CO2 with C3 to form a C4 Molecule in mesophyll cell

2. C4 molecule is transported to bundle sheath cell, where the CO2 is dumped around Rubisco. 3C is cycled back to capture more CO2 in mesophyll cell.

3. Rubisco is saturated due to the high concentration of CO2 around it, fixing primarily CO2 instead of O2.

Describe the photosynthesis-transpiration compromise and how CAM

photosynthesis breaks the nexus between photosynthesis and transpiration

Photosynthesis-Transpiration compromise:

- Allowing CO2 to diffuse in via stoma results in SIGNIFICANT water loss due to transpiration.

CAM (Physical Separation + Temporal Separation)

- Combines C4 Cycle with Night/Day stomate opening

Night CO2 Capture

1. Stoma open during night to capture CO2 and minimise transpiration.

2. CO2 converted to C4 via PEPase & stored in vacuole

Daytime Photosynthesis (C4 Cycle)

1. Stoma close during day

2. Photosynthesis occurs -- C4 is decarboxylated next to Rubisco to saturate it.

NOTE:

- photorespiration occurs more when hot

- transpiration more harmful for dry evironments