Cellular Respiration

Overview of Aerobic Respiration

Aerobic respiration is a process by which cells convert glucose and oxygen into energy, carbon dioxide, and water. It is a central pathway for energy production in most eukaryotic cells.

• Formula for Aerobic Respiration:

• Oxidation vs. Reduction:

  • Oxidation: Loss of electrons (or hydrogen atoms); in respiration, glucose is oxidized.

  • Reduction: Gain of electrons (or hydrogen atoms); in respiration, oxygen is reduced.

• Stages of Aerobic Respiration:

  1. Glycolysis: Occurs in the cytoplasm; breaks glucose into two pyruvate molecules.

  2. Pyruvate Oxidation: Occurs in the mitochondrial matrix; converts pyruvate to acetyl-CoA.

  3. Citric Acid Cycle (Krebs Cycle): Occurs in the mitochondrial matrix; completes the breakdown of glucose.

  4. Oxidative Phosphorylation: Occurs in the inner mitochondrial membrane; includes the electron transport chain and chemiosmosis.

• Initial Main Organic Molecule: Glucose (C6H12O6)

• Products: Carbon dioxide, water, and ATP

ATP Production and Pathways

• Glycolysis: ATP is produced by substrate-level phosphorylation; occurs in aerobic, anaerobic, and fermentation pathways.

• ATP Yield:

  • Glycolysis: 2 ATP

  • Citric Acid Cycle: 2 ATP

  • Oxidative Phosphorylation: ~26-28 ATP

  • Total per glucose: ~30-32 ATP

• Calculating ATP: Multiply the ATP yield by the number of glucose molecules.

Oxidative Phosphorylation and Chemiosmosis

Oxidative phosphorylation is the final stage of aerobic respiration, where most ATP is produced. Chemiosmosis couples the electron transport chain to ATP synthesis.

• Electron Transport Chain (ETC): Transfers electrons from NADH and FADH2 to oxygen, creating a proton gradient.

• Chemiosmosis: The movement of protons (H+) down their gradient through ATP synthase drives ATP production.

• Equation for Chemiosmosis:

Anaerobic Respiration and Fermentation

• Anaerobic Respiration: Uses an electron transport chain, but a molecule other than oxygen (e.g., sulfate) is the final electron acceptor.

• Fermentation: Does not use an electron transport chain; regenerates NAD+ by transferring electrons to organic molecules.

• Types of Fermentation:

  • Lactic Acid Fermentation: Pyruvate is reduced to lactic acid.

  • Alcohol Fermentation: Pyruvate is converted to ethanol and CO2.

• Obligate Anaerobes: Cannot survive in the presence of oxygen.

• Facultative Anaerobes: Can survive with or without oxygen.

Regulation and Alternative Fuels

• Feedback Inhibition: Regulates aerobic respiration; high ATP levels inhibit key enzymes.

• Alternative Fuels: Other molecules (fats, proteins) can enter the pathway at different points.

Photosynthesis

Overview and Formula

Photosynthesis is the process by which autotrophs convert light energy into chemical energy, producing glucose and oxygen from carbon dioxide and water.

• Formula for Photosynthesis:

• Endergonic Process: Requires energy input.

• Location: Occurs in the chloroplasts of plant cells.

Autotrophs vs. Heterotrophs

• Autotrophs: Organisms that produce their own food (e.g., plants).

• Heterotrophs: Organisms that consume other organisms for food.

Light Reactions and Calvin Cycle

• Light Reactions: Occur in the thylakoid membranes; produce ATP, NADPH, and O2.

• Calvin Cycle: Occurs in the stroma; uses ATP and NADPH to convert CO2 into G3P (a sugar).

Visible Light and Pigments

• Visible Light Spectrum: 380–750 nm; shorter wavelengths have higher energy.

• Pigment Molecules:

  • Chlorophyll a

  • Chlorophyll b

  • Carotenoids

• Absorption and Reflection: Pigments absorb specific wavelengths; reflected wavelengths determine color.

Photosystems and Electron Flow

• Photosystem Parts: Reaction center, light-harvesting complexes, primary electron acceptor.

• Electron Path: Photon excites pigment, energy transferred to reaction center, electron passed to primary acceptor.

Chemiosmosis in Photosynthesis

• Electron Transport Chain: Generates a proton gradient across the thylakoid membrane.

• ATP Synthase: Uses the gradient to synthesize ATP.

C3, C4, and CAM Pathways

• C3 Pathway: Most common; CO2 combines with RuBP via rubisco; first product is 3-carbon 3-phosphoglycerate; produces G3P.

• Photorespiration: Rubisco binds O2 instead of CO2; wastes energy and reduces sugar production.

• C4 Pathway: CO2 is fixed into a 4-carbon compound; adaptation to hot, dry environments.

• CAM Pathway: CO2 is fixed at night; adaptation to arid environments.

Cell Division and the Cell Cycle

Importance of Cell Division

Cell division is essential for growth, repair, reproduction, and maintenance of organisms.

• Chromatin: DNA and protein complex; condenses to form chromosomes.

• Somatic Cells vs. Gametes:

  • Somatic Cells: Body cells; diploid (2 sets of chromosomes).

  • Gametes: Sex cells; haploid (1 set of chromosomes).

• Centromere: Region where sister chromatids are joined.

Interphase and Chromosome Replication

• Interphase: G1, S, and G2 phases; cell grows and replicates DNA.

• S Phase: DNA replication occurs.

• Majority of Cell Cycle: Spent in interphase.

Mitotic Spindle and Mitosis

• Mitotic Spindle: Structure made of microtubules; separates chromosomes during mitosis.

• Stages of Mitosis:

  1. Prophase: Chromatin condenses; spindle forms.

  2. Prometaphase: Nuclear envelope breaks down; spindle attaches to chromosomes.

  3. Metaphase: Chromosomes align at the metaphase plate.

  4. Anaphase: Sister chromatids separate; move to opposite poles.

  5. Telophase: Nuclear envelope reforms; chromosomes decondense.

• Cytokinesis: Division of cytoplasm; forms two daughter cells.

Binary Fission and Evolution of Mitosis

• Binary Fission: Prokaryotic cell division; simpler than mitosis.

• Mitosis Evolution: Evolved from binary fission.

Cell Cycle Regulation and Cancer

• Checkpoints: Control points in the cell cycle; ensure proper division.

• Cancer Cells: Uncontrolled division; ignore checkpoints.

• Malignant vs. Benign Tumors:

  • Benign: Non-invasive; do not spread.

  • Malignant: Invasive; can metastasize.

• Cancer Stages: Based on size, spread, and involvement of lymph nodes.

• Testing for Cancer: Imaging, biopsy, blood tests.

• Treatments: Surgery, radiation, chemotherapy.

• Chemotherapy Side Effects: Targets rapidly dividing cells; causes hair loss, nausea, fatigue.

• Reducing Cancer Risk: Healthy diet, exercise, avoiding tobacco and excessive sun exposure.

Summary Table: Cellular Respiration and Photosynthesis

Summary Table: Cell Division

Example: During aerobic respiration, a cell starting with 2 glucose molecules would produce approximately 60-64 ATP, depending on efficiency and cell type.

Example: In photosynthesis, the light reactions produce ATP and NADPH, which are then used in the Calvin Cycle to synthesize G3P, a precursor to glucose.

Example: Cancer cells often bypass cell cycle checkpoints, leading to uncontrolled growth and the formation of tumors.