Cellular Respiration and Photosynthesis

Oxidation-Reduction (Redox) Reactions

Redox reactions are fundamental to energy transfer in biological systems. They involve the transfer of electrons between molecules, affecting their potential energy.

• Oxidation: Loss of electrons from a molecule, atom, or ion.

• Reduction: Gain of electrons by a molecule, atom, or ion.

• Potential Energy: Electrons in chemical bonds have potential energy; when transferred, this energy can be harnessed for cellular work.

• Example: In cellular respiration, glucose is oxidized (loses electrons), and oxygen is reduced (gains electrons).

NAD+ and NADH: Electron Carriers

NAD+ (nicotinamide adenine dinucleotide) is a key electron carrier in redox reactions. It cycles between oxidized (NAD+) and reduced (NADH) forms.

• NAD+: Accepts electrons and a proton (H+), becoming NADH.

• NADH: Donates electrons to the electron transport chain, reverting to NAD+.

• Transformation:

• Role: NADH carries high-energy electrons to the electron transport chain for ATP production.

Glycolysis: ATP Production

Glycolysis is the first stage of cellular respiration, occurring in the cytoplasm. It breaks down glucose into pyruvate, generating ATP and NADH.

• Reactants: 1 glucose, 2 NAD+, 2 ADP, 2 Pi

• Products: 2 pyruvate, 2 NADH, 2 ATP (net), 2 H2O, 2 H+

• ATP Production: 4 ATP produced, 2 ATP used, net gain of 2 ATP (via substrate-level phosphorylation).

• Equation:

Chemical Reactions: Cellular Respiration and Photosynthesis

• Cellular Respiration (overall):

• Photosynthesis (overall):

• ATP Molecules:

  • Cellular respiration yields up to 30-32 ATP per glucose molecule (varies by cell type).

  • Glycolysis: net 2 ATP; Krebs cycle: 2 ATP; Oxidative phosphorylation: ~26-28 ATP.

Electron Carriers in Redox Reactions

Electron carriers shuttle electrons during cellular respiration and photosynthesis.

• NAD+/NADH and FAD/FADH2 are the main carriers in respiration.

• NADP+/NADPH is the main carrier in photosynthesis.

• They transfer electrons to the electron transport chain, enabling ATP synthesis.

Stages of Aerobic Respiration (in order)

1. Glycolysis

2. Pyruvate oxidation (formation of acetyl-CoA)

3. Krebs cycle (Citric Acid Cycle)

4. Electron Transport Chain and Oxidative Phosphorylation

Substrate-Level Phosphorylation

Substrate-level phosphorylation is the direct transfer of a phosphate group to ADP to form ATP, using an enzyme and a phosphorylated substrate.

• Occurs in: Glycolysis and Krebs cycle.

• Reactants: ADP and a phosphorylated intermediate.

• Products: ATP and a dephosphorylated product.

• ATP Yield: About 10% of total ATP in cellular respiration is formed this way; the rest is via oxidative phosphorylation.

• Equation (example):

Chemiosmosis

Chemiosmosis refers to the movement of ions (usually H+) across a semipermeable membrane, down their electrochemical gradient, to drive ATP synthesis.

• In Cellular Respiration: Protons are pumped from the mitochondrial matrix to the intermembrane space; they flow back into the matrix via ATP synthase, generating ATP.

• In Photosynthesis: Protons are pumped into the thylakoid space; they flow back into the stroma via ATP synthase, generating ATP.

• Direction of Proton Flow:

  • Respiration: Matrix → Intermembrane space → Matrix (via ATP synthase)

  • Photosynthesis: Stroma → Thylakoid space → Stroma (via ATP synthase)

Krebs Cycle (Citric Acid Cycle)

The Krebs cycle completes the oxidation of glucose derivatives, generating electron carriers and ATP.

• Reactants: Acetyl-CoA, 3 NAD+, 1 FAD, 1 ADP, 1 Pi, 2 H2O (per turn)

• Products: 2 CO2, 3 NADH, 1 FADH2, 1 ATP (or GTP), 3 H+, 1 CoA-SH (per turn)

• Cycle runs twice per glucose molecule.

Photosynthesis: Light and Dark Reactions

• Light Reactions (in thylakoid membranes):

  • Reactants: H2O, NADP+, ADP, light energy

  • Products: O2, NADPH, ATP

• Dark Reactions (Calvin Cycle, in stroma):

  • Reactants: CO2, NADPH, ATP

  • Products: G3P (sugar), NADP+, ADP, Pi

Chloroplast Structure

Chloroplasts are the site of photosynthesis in plant cells.

• Thylakoid Membrane: Contains chlorophyll and is the site of the light reactions.

• Stroma: Fluid-filled space surrounding thylakoids; site of the Calvin cycle (dark reactions).

Redox in Photosynthesis

• Water is oxidized (loses electrons) to O2.

• CO2 is reduced (gains electrons) to form glucose.

• Equation:

Summary Table: Key Features of Cellular Respiration and Photosynthesis