Cellular Metabolism, Enzymes, Respiration, and Photosynthesis: Study Notes

Metabolism and Energy Pathways

Definition of Metabolism, Anabolic Pathway, and Catabolic Pathway

• Metabolism: The totality of an organism’s chemical reactions. It is an emergent property of life arising from orderly interactions between molecules.

• Metabolic Pathway: Begins with a specific molecule and ends with a product. Each step is catalyzed by a specific enzyme.

• Anabolic Pathways: Consume energy to build complex molecules from simpler ones (e.g., synthesis of protein from amino acids).

• Catabolic Pathways: Release energy by breaking down complex molecules into simpler compounds (e.g., cellular respiration).

Forms of Energy

• Kinetic Energy: Energy associated with motion.

• Thermal Energy: Kinetic energy associated with the random movement of atoms or molecules.

• Potential Energy: Energy that matter possesses because of its location or structure.

• Chemical Energy: Potential energy available for release in a chemical reaction.

Laws of Thermodynamics and Their Relevance to Biology

• Thermodynamics: The study of energy transformations.

• Open vs. Closed Systems:

  • Open system: Energy and matter can be transferred between the system and its surroundings.

  • Closed system: Energy and matter cannot be exchanged with surroundings.

• First Law of Thermodynamics: Energy can be transferred and transformed, but it cannot be created or destroyed.

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

Free Energy

• Definition: Energy that can do work when temperature and pressure are uniform, as in a living cell.

• The change in free energy ($\Delta G$) during a process is related to the change in enthalpy ($\Delta H$), change in entropy ($\Delta S$), and temperature in Kelvin (T): $\Delta G = \Delta H - T\Delta S$

• Spontaneous processes decrease free energy and increase the stability of a system.

Enzymes and Metabolic Regulation

Enzyme Structure and Function

• Enzyme: A macromolecule (usually a protein) that acts as a catalyst to speed up a chemical reaction without being consumed.

• Activation Energy: The initial energy needed to start a chemical reaction.

• Substrate: The reactant an enzyme acts on. The enzyme binds to its substrate, forming an enzyme-substrate complex.

• Enzymes are highly specific for their substrates.

3D Structure and Active Site

• Active Site: The region on the enzyme where the substrate binds.

• Induced fit: The active site changes shape to better fit the substrate.

Factors Affecting Enzyme Activity

• Environmental Conditions: Each enzyme has an optimal temperature and pH.

• Cofactors: Non-protein enzyme helpers (can be inorganic or organic; organic cofactors are called coenzymes).

• Inhibitors:

  • Competitive inhibitors: Bind to the active site, competing with the substrate.

  • Non-competitive inhibitors: Bind elsewhere, changing the enzyme’s shape and making the active site less effective.

Enzyme Regulation

• Allosteric Regulation: Regulatory molecules bind to a site other than the active site, affecting enzyme activity.

• Feedback Inhibition: The end product of a metabolic pathway inhibits an earlier step in the pathway.

ATP: The Energy Currency of the Cell

Structure and Function of ATP

• ATP (Adenosine Triphosphate): Composed of adenine, ribose, and three phosphate groups.

• ATP stores energy in the bonds between its phosphate groups.

• Hydrolysis of ATP releases energy that can be used to drive endergonic reactions.

ATP Cycle

• ATP is regenerated by the addition of a phosphate group to ADP.

• The ATP cycle couples exergonic and endergonic reactions in the cell.

Cellular Respiration

Overview and Types

• Fermentation: Partial degradation of sugars without O2.

• Aerobic Respiration: Consumes organic molecules and O2, producing ATP.

• Anaerobic Respiration: Similar to aerobic, but uses compounds other than O2 as final electron acceptors.

Redox Reactions

• Oxidation: Loss of electrons.

• Reduction: Gain of electrons.

• During cellular respiration, glucose is oxidized and O2 is reduced.

NAD+/NADH

• NAD+ acts as an electron carrier, accepting electrons during redox reactions.

• NADH stores energy used to synthesize ATP.

Stages of Cellular Respiration

• Glycolysis: Breaks down glucose into two molecules of pyruvate. Produces 2 ATP and 2 NADH.

• Pyruvate Oxidation: Pyruvate is converted to acetyl CoA, linking glycolysis to the citric acid cycle.

• Citric Acid Cycle: Completes the breakdown of pyruvate to CO2. Produces ATP, NADH, and FADH2.

• Electron Transport Chain (ETC): Electrons from NADH and FADH2 are transferred through protein complexes, creating a proton gradient used to produce ATP via oxidative phosphorylation.

ATP Production Table

Stage	ATP Produced (per glucose)
Glycolysis	2
Citric Acid Cycle	2
Oxidative Phosphorylation	~28
Total	~32

Fermentation

• Alcohol Fermentation: Pyruvate is converted to ethanol in two steps, releasing CO2. Used by yeast in brewing and baking.

• Lactic Acid Fermentation: Pyruvate is reduced by NADH to form lactate, with no release of CO2. Used by some fungi, bacteria, and muscle cells.

Photosynthesis

Heterotrophs vs. Autotrophs

• Heterotrophs: Obtain organic material from other organisms.

• Autotrophs: Sustain themselves without eating anything derived from other organisms. They are the producers of the biosphere, producing organic molecules from CO2 and other inorganic molecules.

Key Points

• Photosynthesis converts solar energy into chemical energy.

• Occurs in chloroplasts of plants, algae, and some bacteria.

Additional info: Some explanations and context have been expanded for clarity and completeness, including the ATP production table and the summary of photosynthesis.