Metabolism, Cellular Respiration, and Photosynthesis

Chapter 8: An Introduction to Metabolism

8.1 — Metabolism and Energy Transformations

Metabolism is the sum of all chemical reactions in an organism. These reactions are organized into metabolic pathways, each step catalyzed by a specific enzyme.

• Catabolic pathways: Break down large molecules into smaller ones, releasing energy (e.g., cellular respiration).

• Anabolic pathways: Build larger molecules from smaller ones, requiring energy (e.g., protein synthesis).

• Kinetic energy: Energy of motion.

• Potential energy: Stored energy due to position or structure.

• Chemical energy: A form of potential energy stored in molecular bonds.

• Organisms as open systems: Exchange matter and energy with their environment (e.g., intake of food and oxygen, release of CO2 and heat).

Laws of Thermodynamics:

• First Law: Energy cannot be created or destroyed, only transformed.

• Second Law: Every energy transfer increases the entropy (disorder) of the universe.

8.2 — Free Energy and Spontaneous Reactions

Gibbs free energy (G) is the energy available to do work. The change in free energy determines whether a reaction is spontaneous.

• Equation:

• Exergonic reactions: Release free energy (), spontaneous.

• Endergonic reactions: Require input of free energy (), nonspontaneous.

• At equilibrium, .

• Spontaneous does not mean fast; high activation energy can slow reactions.

8.3 — ATP and Cellular Work

ATP (adenosine triphosphate) is the main energy currency of the cell, composed of adenine, ribose, and three phosphate groups.

• ATP hydrolysis:

• ATP powers cellular work by transferring a phosphate group (phosphorylation).

• Types of work powered by ATP:

  • Chemical: Building molecules

  • Transport: Active transport across membranes

  • Mechanical: Muscle contraction, movement of motor proteins

8.4 — Enzymes and Activation Energy

Enzymes are biological catalysts (usually proteins) that speed up reactions by lowering activation energy (Ea), without being consumed.

• Do not change or equilibrium.

• Substrate: Molecule acted on by enzyme.

• Active site: Region where substrate binds.

• Induced fit: Active site changes shape to fit substrate.

• Enzyme activity affected by temperature, pH, substrate concentration, and cofactors (coenzymes if organic).

• Enzyme inhibitors:

  • Competitive: Bind active site, compete with substrate.

  • Noncompetitive: Bind elsewhere, change enzyme shape.

8.5 — Regulation of Enzyme Activity

• Allosteric regulation: Regulatory molecule binds away from active site, stabilizing active or inactive form.

• Cooperativity: Binding of one substrate increases affinity for others (e.g., hemoglobin and oxygen).

• Feedback inhibition: End product inhibits an early enzyme in the pathway, preventing overproduction.

Chapter 9: Cellular Respiration and Fermentation

9.1 — Catabolic Pathways, Redox, and Electron Carriers

Cellular respiration breaks down organic molecules to produce ATP. The overall equation for aerobic respiration of glucose is:

• Oxidation: Loss of electrons

• Reduction: Gain of electrons

• NAD+: Electron carrier; reduced to NADH

• ATP production:

  • Substrate-level phosphorylation: Direct transfer of phosphate to ADP

  • Oxidative phosphorylation: ATP synthesis driven by proton gradient and electron transport chain (ETC)

9.2 — Glycolysis

Occurs in the cytosol; does not require oxygen. Breaks one glucose into two pyruvate molecules.

• ATP generated by substrate-level phosphorylation.

• NAD+ reduced to NADH.

9.3 — Pyruvate Oxidation and the Citric Acid Cycle

• Pyruvate oxidation: In mitochondrial matrix; pyruvate → acetyl-CoA, NADH, CO2.

• Citric acid cycle (Krebs cycle): Acetyl-CoA enters; produces CO2, NADH, FADH2, ATP (or GTP).

• Cycle turns twice per glucose.

• Major role: Generate NADH and FADH2 for the ETC.

9.4 — Oxidative Phosphorylation and Chemiosmosis

• Electron Transport Chain (ETC): Inner mitochondrial membrane; electrons from NADH and FADH2 pass through carriers, pumping H+ to create a gradient.

• Oxygen is the final electron acceptor, forming water.

• Chemiosmosis: H+ flows through ATP synthase, driving ATP production.

• Most ATP is produced by oxidative phosphorylation.

9.5 — Fermentation and Anaerobic Respiration

• Fermentation: Regenerates NAD+ so glycolysis can continue in the absence of oxygen; yields 2 ATP per glucose.

• Alcohol fermentation: Pyruvate → ethanol + CO2 (e.g., yeast).

• Lactic acid fermentation: Pyruvate → lactate (e.g., muscle cells, some bacteria).

• Anaerobic respiration: Uses ETC with a final electron acceptor other than oxygen.

9.6 — Other Fuels and Regulation of Respiration

• Cells can use carbohydrates, fats, and proteins as fuel.

• Fats yield acetyl-CoA via beta oxidation; proteins enter after deamination.

• Respiration is regulated by energy needs; high ATP inhibits glycolysis (e.g., via phosphofructokinase).

Chapter 10: Photosynthesis

10.1 — Photosynthesis Feeds the Biosphere

• Autotrophs: Produce organic molecules from inorganic sources (e.g., plants, algae).

• Heterotrophs: Obtain organic molecules from other organisms (e.g., animals, fungi).

• Photoautotrophs: Use sunlight to drive synthesis of organic molecules.

10.2 — Chloroplasts and the Overall Process

• Photosynthesis occurs in chloroplasts, mainly in leaf mesophyll cells.

• Key structures: Stomata (gas exchange), stroma (Calvin cycle), thylakoids (light reactions), grana (stacks of thylakoids).

• Overall equation:

• Photosynthesis is an endergonic redox process; O2 released comes from water.

• Two stages:

  • Light reactions: In thylakoids; produce ATP, NADPH, O2.

  • Calvin cycle: In stroma; uses ATP and NADPH to fix CO2 into G3P.

10.3 — Light Reactions: Light, Pigments, and ATP Production

• Light: Electromagnetic energy; visible light (380–740 nm).

• Pigments: Absorb specific wavelengths (chlorophyll a, chlorophyll b, carotenoids).

• When chlorophyll absorbs light, electrons are excited and transferred to a primary electron acceptor in a photosystem.

• Photosystem II (P680): Functions first; splits water, releases O2.

• Photosystem I (P700): Functions second; produces NADPH.

• Linear electron flow: H2O → PSII → ETC → PSI → NADP+ → NADPH; produces ATP, NADPH, O2.

• Cyclic electron flow: Uses PSI only; produces ATP but not NADPH or O2.

• Chemiosmosis: H+ gradient in thylakoid space drives ATP synthase (photophosphorylation).

10.4 — The Calvin Cycle

• Uses ATP and NADPH to fix CO2 into G3P (glyceraldehyde-3-phosphate).

• Occurs in the stroma; does not directly require light but depends on light reactions.

• Three phases:

  1. Carbon fixation: CO2 + RuBP (via rubisco) → 3-PGA

  2. Reduction: ATP and NADPH convert 3-PGA to G3P

  3. Regeneration: Most G3P regenerates RuBP using ATP

• For every 3 CO2 fixed: 6 G3P formed, 1 net G3P output, 5 G3P used to regenerate RuBP, 9 ATP and 6 NADPH consumed.

• Two net G3P molecules are needed to make one glucose.

Connecting All Three Chapters

• Metabolism: Energy transformations, ATP, and enzyme regulation (Chapter 8)

• Cellular Respiration: Harvests energy from organic molecules to make ATP (Chapter 9)

• Photosynthesis: Uses light energy to build organic molecules (Chapter 10)

• Redox reactions: NADH (respiration) and NADPH (photosynthesis) carry electrons

• ATP: Powers cellular work, produced by respiration, and used in the Calvin cycle

Essential Concepts to Remember

1. Energy cannot be created or destroyed, only transformed.

2. Catabolic pathways break down molecules; anabolic pathways build molecules.

3. Exergonic reactions: ; endergonic reactions: .

4. Enzymes lower activation energy but do not change .

5. ATP couples energy-releasing reactions to energy-requiring work.

6. Oxidation is loss of electrons; reduction is gain of electrons.

7. Glycolysis: 2 pyruvate, 2 net ATP, 2 NADH per glucose.

8. Oxygen is the final electron acceptor in aerobic respiration.

9. Most ATP in aerobic respiration is made through oxidative phosphorylation.

10. Fermentation regenerates NAD+ so glycolysis can continue.

11. Light reactions produce ATP, NADPH, and O2.

12. Calvin cycle uses CO2, ATP, and NADPH to produce G3P.

13. Photosynthesis and respiration both involve electron transfer and energy transformations.

Study tip: For every pathway, ask: Where does it happen? What enters? What leaves? Why does the cell need it?