Module 5: The Working Cell and Cellular Respiration – Study Notes

Chapter 5: The Working Cell

Thermodynamics and Metabolism

The study of energy transformations in biological systems is governed by the laws of thermodynamics, which are fundamental to understanding metabolism.

• First Law of Thermodynamics: States that energy cannot be created or destroyed, only transformed from one form to another. In metabolism, cells convert chemical energy from nutrients into usable forms such as ATP.

• Second Law of Thermodynamics: States that every energy transfer increases the entropy (disorder) of the universe. Biological systems must constantly obtain and use energy to maintain order and support life.

• Entropy: A measure of disorder or randomness. Living organisms decrease local entropy by increasing the entropy of their surroundings.

Potential Energy and Metabolism

Potential energy is stored energy due to position or structure. In cells, chemical bonds in molecules like glucose store potential energy, which is released during metabolic reactions to power cellular processes.

Exergonic vs. Endergonic Reactions

• Exergonic reactions: Release energy; products have less free energy than reactants (e.g., cellular respiration).

• Endergonic reactions: Require an input of energy; products have more free energy than reactants (e.g., photosynthesis).

ATP: Structure, Function, and Cycle

Adenosine triphosphate (ATP) is the primary energy carrier in cells. It consists of adenine, ribose, and three phosphate groups. The hydrolysis of ATP releases energy used for cellular work.

• ATP Cycle: ATP is continuously regenerated from ADP and inorganic phosphate through cellular respiration.

Enzymes and Their Function

Enzymes are biological catalysts that speed up chemical reactions by lowering the activation energy barrier. They are essential for sustaining life by enabling metabolic reactions to occur rapidly and efficiently.

• Key Terms:

  • Substrate: The reactant on which an enzyme acts.

  • Active site: The region of the enzyme where the substrate binds.

  • Catalyst: A substance that increases the rate of a reaction without being consumed.

  • Activation energy: The energy required to initiate a chemical reaction.

  • Enzyme inhibitor: A molecule that decreases enzyme activity.

• Activation Energy Barrier: Prevents spontaneous breakdown of molecules; enzymes lower this barrier, allowing life-sustaining reactions to proceed at body temperature.

Enzyme Inhibition and Regulation

• Competitive inhibitors: Bind to the active site, blocking substrate access.

• Non-competitive inhibitors: Bind elsewhere on the enzyme, altering its shape and reducing activity.

• Feedback inhibition: The end product of a metabolic pathway inhibits an upstream enzyme, preventing overproduction and maintaining homeostasis.

Chapter 6: Cellular Respiration

Overview and Equation

Cellular respiration is the process by which cells extract energy from organic molecules to produce ATP. The overall goal is to convert biochemical energy from nutrients into ATP, releasing waste products.

• Overall equation: $\mathrm{C_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O + ATP}$

Redox Reactions: Oxidation and Reduction

• Redox reaction: Chemical reactions involving the transfer of electrons.

• Oxidation: Loss of electrons.

• Reduction: Gain of electrons.

• Energy intermediates: NAD+ and FAD are reduced to NADH and FADH2 during respiration, carrying high-energy electrons.

Stages of Cellular Respiration

1. Glycolysis: Occurs in the cytoplasm; breaks glucose into two pyruvate molecules. Inputs: glucose, 2 ATP, 2 NAD+. Outputs: 2 pyruvate, 4 ATP (net 2 ATP), 2 NADH.

2. Oxidation of Pyruvate: Occurs in the mitochondrial matrix; pyruvate is converted to acetyl-CoA. Inputs: 2 pyruvate, 2 NAD+. Outputs: 2 acetyl-CoA, 2 CO2, 2 NADH.

3. Citric Acid Cycle (Krebs Cycle): Occurs in the mitochondrial matrix; completes the breakdown of glucose. Inputs: 2 acetyl-CoA, 6 NAD+, 2 FAD, 2 ADP. Outputs: 4 CO2, 6 NADH, 2 FADH2, 2 ATP. It is a cycle because the starting molecule (oxaloacetate) is regenerated.

4. Oxidative Phosphorylation: Involves the electron transport chain and ATP synthase in the inner mitochondrial membrane. NADH and FADH2 donate electrons, which move through the chain to oxygen, forming water. The energy released pumps protons, creating a gradient used by ATP synthase to produce ATP.

Fermentation

When oxygen is scarce, cells use fermentation to regenerate NAD+ from NADH, allowing glycolysis to continue. The two major forms are:

• Lactic acid fermentation: Pyruvate is reduced to lactate (e.g., in muscle cells).

• Alcoholic fermentation: Pyruvate is converted to ethanol and CO2 (e.g., in yeast).

Alternative Fuels for Cellular Respiration

Cells can metabolize carbohydrates, fats, and proteins for energy:

• Carbohydrates: Broken down into glucose and enter glycolysis.

• Fats: Glycerol enters glycolysis; fatty acids undergo beta-oxidation to form acetyl-CoA.

• Proteins: Amino acids are deaminated and enter glycolysis or the citric acid cycle.