Photosynthesis & Respiration

Photosynthesis and respiration are the two most important metabolic processes in biology. They are complementary - the products of one are the reactants of the other.

Photosynthesis (Anabolic)

Location: chloroplasts in plant cells, algae, and some bacteria
Equation: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂ (energy from light)
Summary: Light energy is converted to chemical energy stored in glucose. CO₂ is removed from the atmosphere - photosynthesis acts as a carbon sink. The glucose produced can be converted to sucrose (for transport in phloem) and complex carbohydrates (starch for storage, cellulose for cell walls).

Aerobic Respiration (Catabolic)

Location: cytoplasm (glycolysis) and mitochondria (Krebs cycle + ETC)
Equation: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + ATP (energy released)
Summary: Glucose is broken down to release energy as ATP, which powers all cellular activities.

Anaerobic Respiration

Respiration without oxygen. Much less efficient - produces only 2 ATP per glucose.
• In yeast: glucose → ethanol + CO₂ (used in brewing and baking)
• In animal muscles: glucose → lactic acid (causes muscle fatigue during intense exercise)

Factors Affecting Rate of Photosynthesis

Temperature: increases rate up to an optimum (~25–30°C), then enzymes denature.
Light intensity: increases rate until a plateau (another factor becomes limiting).
CO₂ concentration: increases rate until saturation. CO₂ is often the limiting factor in natural conditions.

Respiration - Two Stages

Stage 1: Glycolysis (in cytoplasm, no oxygen needed):

• 1 glucose (6C) → 2 pyruvate (3C)
• Net gain: 2 ATP + 2 NADH
• If no oxygen → fermentation

Fermentation (anaerobic pathway):

• In yeast: pyruvate → ethanol + CO₂ (used in brewing, baking, biofuel)
• In animal muscles: pyruvate → lactic acid (causes cramp during intense exercise)
• Conditions needed: sugars, microorganisms (yeast/bacteria), anaerobic environment, suitable temperature

Stage 2: Aerobic Respiration (in mitochondria, needs O₂):

• Pyruvate → Acetyl CoA (releases 1 CO₂ per pyruvate, produces NADH)
• Citric Acid Cycle (Krebs Cycle): Acetyl CoA enters the cycle → produces CO₂, NADH, FADH₂, and 1 ATP per turn
• Electron Transport Chain (ETC): NADH and FADH₂ donate electrons to protein complexes embedded in the inner mitochondrial membrane. Electrons transfer through complexes, creating a proton gradient. Protons flow back through ATP synthase, producing large amounts of ATP (up to ~34 ATP). This is called oxidative phosphorylation.

Photosynthesis - Two Stages

Light-Dependent Reactions (thylakoid membranes of chloroplasts):

• Non-cyclic pathway: A photon of light is absorbed by chlorophyll pigment molecules → energy passes between pigments until it reaches a reaction centre in chlorophyll
• Photolysis of water: The positively charged chlorophyll attracts electrons from water → H₂O → 2H⁺ + ½O₂ + 2e⁻ (oxygen released as by-product)
• Electron transfer through protein complexes produces ATP and NADPH (from NADP⁺)
• Cyclic pathway: Some electrons return to chlorophyll and transfer surplus energy to form additional ATP (no NADPH or O₂ produced)

Light-Independent Reactions / Calvin Cycle (stroma of chloroplasts):

• NADPH and ATP from the light reactions are used here
• CO₂ is fixed (incorporated into organic molecules) → protons and electrons from NADPH are transferred to CO₂
• Energy from ATP drives the conversion of CO₂ → C₆H₁₂O₆ (glucose)
• ADP and NADP⁺ return to the light-dependent reactions to be recharged

Transfer Molecules - Key Roles

ATP: the universal energy currency. Carries energy from where it's produced to where it's needed.
NAD⁺ / NADH: electron carrier in respiration. NAD⁺ picks up electrons (→ NADH) during glycolysis and Krebs cycle, delivers them to the ETC.
NADP⁺ / NADPH: electron carrier in photosynthesis. NADP⁺ picks up electrons (→ NADPH) during light reactions, delivers them to the Calvin Cycle.

ATP Synthase & Chemiosmosis (HL)

In both mitochondria and chloroplasts, electron transport chains pump protons (H⁺) across a membrane, creating a proton gradient (high H⁺ concentration on one side). Protons flow back down this gradient through the enzyme ATP synthase, which harnesses the flow to produce ATP. This process is called chemiosmosis.