C1.3 Photosynthesis (IB SL + HL) - Learning Outcomes

Light energy is absorbed by pigment molecules → excites electrons → electron flow through photosystems → ATP (chemiosmosis) + NADPH are produced → these power carbon fixation in the Calvin cycle, converting inorganic CO₂ into organic triose phosphate.

Carbon dioxide + water → glucose + oxygen (with light and chlorophyll)

At Photosystem II, water undergoes photolysis: H₂O → 2H⁺ + 2e⁻ + ½O₂. The electrons replace those lost from PSII; oxygen diffuses out as a waste product.

Rf = (distance pigment moved) ÷ (distance solvent front moved). Different Rf values indicate differences in pigment solubility and polarity.

Each pigment has a unique absorption spectrum based on its molecular structure; chlorophyll absorbs red + blue strongly and reflects green, while accessory pigments absorb additional wavelengths.

Photosynthetic rate is highest where absorption is highest (red + blue). Plotting photosynthetic rate vs wavelength shows peaks aligning with pigment absorption.

Example: “Increasing CO₂ concentration will increase photosynthetic rate until rubisco becomes saturated.” Independent variable: CO₂ concentration / light intensity / temperature. Dependent variable: Photosynthetic rate (O₂ production / CO₂ uptake).

FACE exposes plants to elevated CO₂ outdoors → natural environment but difficult to control variables. Greenhouses control variables tightly → but less natural conditions.

Broader range of absorbed wavelengths → more efficient light harvesting → higher rate of photosynthesis, especially in variable environments.

Pigment molecules absorb photons → electrons jump to higher energy levels → transferred to primary electron acceptor in the reaction centre (chlorophyll a).

ETC pumps H⁺ into thylakoid lumen → forms proton gradient → H⁺ diffuses through ATP synthase → ATP formed from ADP + Pi. Electron source: water (PSII) or cyclic flow in PSI.

PSII absorbs light → P680 is excited → loses electrons → replaced by electrons from photolysis of water → oxygen is released as a byproduct.

Light excites electrons in PSI → electrons passed to ferredoxin → NADP⁺ reductase reduces NADP⁺ to NADPH using protons from the stroma.

Thylakoid membranes house PSII, PSI, ETC proteins, ATP synthase → allow compartmentalization → proton gradient forms in lumen → efficient ATP + NADPH production.

Rubisco catalyzes CO₂ + RuBP → unstable 6C → two 3-PGA molecules. It is the rate-limiting enzyme and most abundant protein on Earth.

3-PGA is phosphorylated by ATP → reduced by NADPH → forms TP (triose phosphate). TP is the first stable carbohydrate product.

ATP powers conversion of TP → RuBP, allowing the Calvin cycle to continue fixing CO₂.

TP synthesizes: * carbohydrates (starch, cellulose) * lipids (via glycerol + fatty acids) * amino acids (requires nitrogen + sulfur minerals)

LDR provide ATP + NADPH to power LIR; LIR regenerate NADP⁺ + ADP required for LDR. Both depend on environmental conditions: light, CO₂, temperature.