Cellular Metabolism, Respiration, and Photosynthesis: Study Notes

Metabolism and Energy Pathways

Definition of Metabolism, Anabolic Pathway, and Catabolic Pathway

Metabolism encompasses all chemical reactions occurring within an organism, enabling life by managing material and energy resources. These reactions are organized into metabolic pathways, which can be classified as anabolic or catabolic.

• Metabolism: The totality of an organism’s chemical reactions; an emergent property of life arising from orderly molecular interactions.

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

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

Forms of Energy

Energy exists in various forms, each relevant to biological systems.

• Kinetic Energy: Energy associated with motion.

• Thermal Energy: Kinetic energy due to random movement of atoms or molecules.

• Potential Energy: Energy matter possesses due to its location or structure.

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

Laws of Thermodynamics and Their Relevance to Biology

Thermodynamics is the study of energy transformations. Biological systems obey these laws:

• Open vs. Closed Systems:

  • Open systems exchange energy and matter with surroundings (e.g., living cells).

  • Closed systems do not exchange energy or matter with surroundings.

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

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

Free Energy

Free energy is the portion of a system’s energy that can perform work when temperature and pressure are uniform.

• Definition: Energy that can do work under cellular conditions.

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

• Spontaneous processes decrease free energy and increase system stability.

Enzymes and Metabolic Regulation

Enzyme Structure and Function

Enzymes are biological catalysts that speed up chemical reactions without being consumed.

• Definition: A catalyst is a chemical agent that accelerates a reaction without being consumed; most enzymes are proteins.

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

• Substrate: The reactant an enzyme acts on; forms an enzyme-substrate complex.

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

• Enzyme specificity arises from the fit between the enzyme’s active site and the substrate’s shape.

Factors Affecting Enzyme Activity

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

• Cofactors: Non-protein helpers (inorganic or organic, e.g., vitamins).

• Inhibitors:

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

  • Non-competitive inhibitors: Bind elsewhere, changing enzyme shape and reducing effectiveness.

Enzyme Regulation

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

• Cooperativity: Substrate binding increases enzyme activity at other active sites.

• Feedback Inhibition: End product of a pathway inhibits an earlier step, preventing overproduction.

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 its phosphate bonds; hydrolysis releases energy for cellular work.

ATP Cycle and Energy Coupling

• Cells use exergonic reactions (e.g., ATP hydrolysis) to drive endergonic reactions via energy coupling.

• ATP is regenerated by adding a phosphate group to ADP, using energy from catabolic reactions.

Cellular Respiration

Overview and Types

• Fermentation: Partial degradation of sugars without O2.

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

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

Redox Reactions

• Chemical reactions that transfer electrons are called oxidation-reduction (redox) reactions.

• Oxidation: Loss of electrons; Reduction: Gain of electrons.

• During respiration, fuel is oxidized and O2 is reduced.

NAD+/NADH

• NAD+ acts as an electron carrier, accepting electrons during cellular respiration and becoming NADH.

• NADH stores energy used to synthesize ATP.

ATP Synthesis: Substrate-Level and Oxidative Phosphorylation

• Substrate-Level Phosphorylation: ATP formed directly in glycolysis and the citric acid cycle.

• Oxidative Phosphorylation: ATP formed via electron transport chain and chemiosmosis; accounts for most ATP produced.

Stages of Cellular Respiration

• Glycolysis: Breaks down glucose into two pyruvate molecules; produces 2 ATP and 2 NADH.

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

• Citric Acid Cycle: Completes breakdown of pyruvate to CO2; produces NADH and FADH2 for the electron transport chain.

• Electron Transport Chain (ETC): Series of proteins in the mitochondrial membrane transfer electrons, creating a proton gradient used to produce ATP.

Electron Transport Chain and Chemiosmosis

• Electrons from NADH and FADH2 pass through the ETC, releasing energy to pump protons (H+) across the membrane.

• This creates a proton-motive force; protons flow back through ATP synthase, driving ATP production (chemiosmosis).

• O2 is the final electron acceptor, forming H2O.

Fermentation

• Alcohol Fermentation: Pyruvate converted to ethanol in two steps (CO2 released, then ethanol formed); used by yeast in brewing and baking.

• Lactic Acid Fermentation: Pyruvate reduced by NADH to lactate; used by some fungi, bacteria, and muscle cells.

Photosynthesis

Heterotrophs vs. Autotrophs

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

• Heterotrophs: Obtain organic material by consuming other organisms.

Table: Comparison of Fermentation Types

Type	End Product	CO2 Released?	Organisms
Alcohol Fermentation	Ethanol	Yes	Yeast, some bacteria
Lactic Acid Fermentation	Lactate	No	Fungi, bacteria, muscle cells

Additional info: Some explanations and context have been expanded for clarity and completeness, including definitions, examples, and the table comparing fermentation types.