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; transferring electrons to more electronegative atoms releases energy.

• NAD+ and NADH: NAD+ (nicotinamide adenine dinucleotide) is an electron carrier. It is reduced to NADH by accepting two electrons and one proton. The reaction is:

• Example: In glycolysis and the Krebs cycle, NAD+ is reduced to NADH as glucose is oxidized.

ATP Production in Glycolysis

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

• Substrate-Level Phosphorylation: Direct transfer of a phosphate group to ADP to form ATP, without the involvement of the electron transport chain.

• Percentage of ATP: About 10% of the total ATP generated in cellular respiration is produced by substrate-level phosphorylation.

Chemical Reactions for Cellular Respiration and Photosynthesis

• Cellular Respiration (Aerobic):

• Photosynthesis:

• ATP Molecules: Cellular respiration yields up to 30-32 ATP per glucose molecule. Photosynthesis uses ATP in the Calvin cycle (dark reactions) but does not produce net ATP for the cell.

Electron Carriers in Redox Reactions

Electron carriers transport electrons during cellular respiration and photosynthesis, facilitating energy transfer.

• NAD+/NADH: Accepts and donates electrons in glycolysis and the Krebs cycle.

• FAD/FADH2: Another electron carrier, reduced during the Krebs cycle.

• Electron Transport Chain (ETC): Series of protein complexes in the inner mitochondrial membrane (or thylakoid membrane in chloroplasts) that transfer electrons and pump protons.

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 Chemiosmosis

Chemiosmosis and ATP Synthesis

Chemiosmosis refers to the movement of ions (H+) across a membrane, generating ATP via ATP synthase.

• Cellular Respiration: Protons are pumped from the mitochondrial matrix to the intermembrane space. They flow back into the matrix through ATP synthase, driving ATP production.

• Photosynthesis: Protons are pumped into the thylakoid space. They flow back into the stroma through ATP synthase, generating ATP for the Calvin cycle.

• Direction of Proton Flow: Opposite in mitochondria (matrix to intermembrane space) and chloroplasts (stroma to thylakoid space).

Krebs Cycle (Citric Acid Cycle)

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

• Reactants: Acetyl-CoA, 3 NAD+, 1 FAD, 1 ADP, 1 Pi

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

• Cycle occurs twice per glucose molecule.

Photosynthesis: Light and Dark Reactions

• Light Reactions: Occur in the thylakoid membranes. Use light energy to split water, release O2, produce ATP and NADPH.

• Reactants: H2O, ADP, NADP+, light

• Products: O2, ATP, NADPH

• Dark Reactions (Calvin Cycle): Occur in the stroma. Use ATP and NADPH to fix CO2 into glucose.

• Reactants: CO2, ATP, NADPH

• Products: Glucose (or G3P), ADP, NADP+

Structure of the Chloroplast

• Thylakoid Membrane: Site of the light reactions; contains chlorophyll and electron transport chain components.

• 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 glucose.

• Overall: Light energy drives the transfer of electrons from water to CO2.

Summary Table: Key Features of Cellular Respiration and Photosynthesis

Additional info: The study guide references videos and textbook figures for further clarification. For a deeper understanding, consult your textbook's figures on chemiosmosis and the electron transport chain.