Unit 3: Cellular Energetics

The study of how cells accomplish energy transformation is called bioenergetics.

Thermodynamics

• First Law: Energy cannot be created or destroyed; it can only be transferred. Cells must harvest energy from somewhere.

• Second Law: Energy transfer leads to less organization; the universe tends toward disorder (entropy). To power cellular processes, energy input must exceed energy loss.

Types of Reactions

• Exergonic: Products have less energy than reactants.

• Endergonic: Reactions require an input of energy; products have more energy than reactants.

Enzymes

• Catalysts: Speed up reactions by lowering activation energy.

• Enzymes do not change the energy of the starting or ending points of reactions.

• Each enzyme is specific to one reaction (enzyme specificity) and binds substrates at the active site, forming an enzyme-substrate complex.

Induced-fit & Cofactors

• Enzymes change shape to fit the substrate (induced-fit).

• Cofactors: Help enzymes; can be organic (coenzymes, like vitamins) or inorganic (metal ions).

Factors Affecting Reaction Rates

• Reaction rates are influenced by temperature and pH.

• Concentration of substrates affects the speed; beyond saturation point, further substrate increase doesn't speed up the reaction.

Enzyme Regulation

• Enzymatic activity is controlled by conditions that influence enzyme shape (allosteric regulation).

• Competitive inhibition blocks active site; noncompetitive inhibition distorts enzyme shape.

Cellular Respiration

• Process: C6H12O6 + 6O2 → 6CO2 + 6H2O + ATP.

• Two types: Aerobic (in presence of oxygen) and Anaerobic (without oxygen).

Stages of Aerobic Respiration

1. Glycolysis: Splits glucose into 2 pyruvic acids, yielding 2 ATP and 2 NADH. Occurs in cytoplasm.

2. Formation of Acetyl-CoA: Converts pyruvic acid to acetyl-CoA, releasing CO2 and generating 2 NADH.

3. Krebs Cycle: Acetyl-CoA produces 1 ATP, 3 NADH, 1 FADH2 per turn. Occurs in mitochondrial matrix.

4. Oxidative Phosphorylation: Electrons from carriers (NADH, FADH2) pass through the electron transport chain, producing a proton gradient and generating ATP (via chemiosmosis).

Photosynthesis

• Process: 6CO2 + 6H2O → C6H12O6 + 6O2.

• Occurs in two stages: Light Reactions (convert light energy to ATP) and Calvin Cycle (uses ATP to fix carbon).

Summary of ATP Sources

• ATP is produced through cellular respiration (aerobic in autotrophs via photosynthesis; heterotrophs from food).