Cellular Respiration and Photosynthesis

Oxidation-Reduction (Redox) Reactions

Redox reactions are chemical processes in which electrons are transferred between molecules, affecting their energy states.

• 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.

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

• Transformation: NAD+ + 2e- + 2H+ → NADH + H+

ATP Production in Glycolysis

Glycolysis is the first stage of cellular respiration, occurring in the cytoplasm and breaking down glucose into pyruvate.

• Reactants: Glucose, 2 NAD+, 2 ADP, 2 Pi

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

• ATP Formation: ATP is produced by substrate-level phosphorylation (direct transfer of a phosphate group to ADP).

• Equation:

Chemical Reactions for Cellular Respiration and Photosynthesis

• Cellular Respiration:

• Photosynthesis:

• ATP Yield: Cellular respiration produces up to 30-32 ATP per glucose molecule; glycolysis yields 2 ATP (net).

Electron Carriers in Redox Reactions

• NAD+ / NADH and FAD / FADH2 are key electron carriers.

• They shuttle electrons to the electron transport chain, where energy is used to produce ATP.

Stages of Aerobic Respiration (in order)

1. Glycolysis

2. Pyruvate Oxidation

3. Krebs Cycle (Citric Acid Cycle)

4. Electron Transport Chain and Oxidative Phosphorylation

Substrate-Level Phosphorylation

• Direct transfer of a phosphate group from a substrate to ADP, forming ATP.

• Occurs in glycolysis and the Krebs cycle.

• Accounts for a small percentage of total ATP produced (about 10%).

• Reactants: ADP and a phosphorylated substrate.

• Products: ATP and a dephosphorylated substrate.

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, then flow back into the matrix through ATP synthase.

• In photosynthesis, protons are pumped into the thylakoid space and flow back into the stroma via ATP synthase.

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

Krebs Cycle (Citric Acid Cycle)

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

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

• Cycle runs twice per glucose molecule.

Photosynthesis: Light and Dark Reactions

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

• Reactants: H2O, NADP+, ADP, light

• Products: O2, NADPH, ATP

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

• Reactants: CO2, ATP, NADPH

• Products: Glucose, ADP, NADP+

Structure of Chloroplasts

• Thylakoid Membrane: Site of light reactions; contains chlorophyll and electron transport chains.

• Stroma: Fluid 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.

The Cell Cycle and Cell Division

Stages of Mitosis and Interphase

The cell cycle consists of interphase (cell growth and DNA replication) and mitosis (nuclear division), followed by cytokinesis (cytoplasmic division).

• Interphase: Includes G0, G1, S, and G2 phases.

• G0 phase: Resting state; cells are not actively dividing.

• G1 phase: Cell growth and preparation for DNA synthesis.

• S phase: DNA replication; chromosomes are duplicated.

• G2 phase: Preparation for mitosis; further growth and organelle duplication.

Stages of Mitosis

1. Prophase: Chromatin condenses into visible chromosomes; spindle fibers form.

2. Metaphase: Chromosomes align at the cell's equator (metaphase plate).

3. Anaphase: Sister chromatids separate and move toward opposite poles.

4. Telophase: Nuclear envelopes reform around chromosomes at each pole; chromosomes decondense.

5. Cytokinesis: Division of the cytoplasm, resulting in two daughter cells.

Key Terms in Cell Division

• Haploid (n): Cells with one set of chromosomes (e.g., gametes).

• Diploid (2n): Cells with two sets of chromosomes (e.g., somatic cells).

• Sister Chromatids: Identical copies of a chromosome, connected by a centromere.

• Centromere: Region where sister chromatids are joined and where spindle fibers attach.

• Chromatin: DNA and protein complex that makes up chromosomes.

Asexual vs. Sexual Reproduction

• Asexual Reproduction: Offspring are genetically identical to the parent; involves mitosis.

• Sexual Reproduction: Offspring inherit a combination of DNA from two parents; involves meiosis and fertilization.

Cell Cycle Arrest (G0 Phase)

• Cells in G0 are not actively dividing; this is a non-reproductive stage.

Cytokinesis: Plants vs. Animals

• Animals: Cleavage furrow forms, pinching the cell in two.

• Plants: Cell plate forms, developing into a new cell wall between daughter cells.

Meiosis

Meiosis is a two-division process that reduces chromosome number by half, producing haploid gametes.

• Meiosis I: Homologous chromosomes separate; cells become haploid at the end of Meiosis I.

• Meiosis II: Sister chromatids separate.

• Crossing-over: Exchange of genetic material between homologous chromosomes during Prophase I; increases genetic diversity.

• Gamete Production: In humans, meiosis produces 1 ovum (egg) and 3 polar bodies in females, and 4 sperm cells in males.

Summary Table: Mitosis vs. Meiosis

Example: During human oogenesis, one primary oocyte produces one ovum and three polar bodies; during spermatogenesis, one primary spermatocyte produces four sperm cells.