General Biology Study Guide: Enzymes, Cellular Energy, Photosynthesis, and Cellular Respiration

3.1 Enzymes

3.1.A: Role and Structure of Enzymes

Enzymes are biological catalysts that accelerate chemical reactions in living organisms by lowering the activation energy required. Their structure and function are crucial for the regulation of biological processes.

• Definition: Enzymes are proteins that speed up biochemical reactions without being consumed in the process.

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

• Regulation: The structure of an enzyme, especially the active site, determines its specificity and activity.

• Enzyme-Substrate Complex: The intermediate formed when an enzyme binds its substrate(s).

• Activation Energy: The minimum energy required to start a chemical reaction; enzymes lower this barrier.

Example: Amylase catalyzes the breakdown of starch into sugars in the human digestive system.

3.1.B: Enzyme Function and Reaction Rate

Enzymes affect the rate of biological reactions by providing an alternative reaction pathway with a lower activation energy.

• Factors Affecting Rate: Substrate concentration, enzyme concentration, temperature, and pH.

• Enzyme-Catalyzed vs. Uncatalyzed Reactions: Enzyme-catalyzed reactions proceed faster and under milder conditions than uncatalyzed reactions.

Example: Catalase rapidly decomposes hydrogen peroxide into water and oxygen, a reaction that would occur very slowly without the enzyme.

3.2 Environmental Impact on Enzyme Function

3.2.A: Effects of Environmental Changes

The activity of enzymes is sensitive to changes in environmental conditions such as temperature, pH, and the presence of inhibitors or activators.

• Denaturation: Extreme conditions can disrupt the three-dimensional structure of enzymes, leading to loss of function.

• Temperature: Increasing temperature generally increases reaction rate up to an optimum, after which the enzyme may denature.

• pH: Each enzyme has an optimal pH; deviations can reduce activity or denature the enzyme.

• Reversibility: Some denaturation processes are reversible, allowing the enzyme to regain activity.

Example: Pepsin works best in the acidic environment of the stomach, while trypsin functions in the alkaline environment of the small intestine.

3.2.B: Inhibitors and Enzyme Activity

Enzyme activity can be regulated by molecules that inhibit or enhance their function.

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

• Noncompetitive Inhibitors: Bind to another part of the enzyme, changing its shape and reducing activity.

• Allosteric Regulation: The binding of regulatory molecules at sites other than the active site can increase or decrease enzyme activity.

Example: Cyanide acts as a noncompetitive inhibitor of cytochrome c oxidase in the electron transport chain.

3.3 Cellular Energy

3.3.A: Energy in Living Organisms

All living systems require a constant input of energy to maintain order and support cellular processes.

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

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

• Exergonic Reactions: Release energy (e.g., cellular respiration).

• Endergonic Reactions: Require energy input (e.g., photosynthesis).

• Energy Coupling: The use of exergonic processes to drive endergonic ones, often via ATP.

Example: The hydrolysis of ATP releases energy that can be used to power cellular work.

3.3.B: Conservation of Core Metabolic Pathways

Core metabolic pathways, such as glycolysis and oxidative phosphorylation, are conserved across all domains of life (Archaea, Bacteria, Eukarya).

• Glycolysis: The breakdown of glucose to pyruvate, producing ATP and NADH.

• Oxidative Phosphorylation: The production of ATP using energy derived from the transfer of electrons in the electron transport chain.

Example: All organisms perform glycolysis, highlighting its evolutionary conservation.

3.4 Photosynthesis

3.4.A: Photosynthetic Processes and Chloroplast Structure

Photosynthesis is the process by which plants, algae, and some bacteria convert light energy into chemical energy stored in glucose. The chloroplast is the organelle where photosynthesis occurs.

• Light-Dependent Reactions: Occur in the thylakoid membranes; convert light energy to chemical energy (ATP and NADPH).

• Calvin Cycle (Light-Independent Reactions): Occurs in the stroma; uses ATP and NADPH to fix carbon dioxide into glucose.

• Chloroplast Structure: Includes the outer membrane, inner membrane, stroma, and thylakoid membranes (stacked into grana).

Example: In plants, the pigment chlorophyll absorbs light energy, initiating the process of photosynthesis.

3.4.B: Evolutionary Evidence and Adaptations

Scientific evidence supports the claim that photosynthetic pathways have evolved from prokaryotic cyanobacteria to eukaryotic chloroplasts.

• Endosymbiotic Theory: Suggests that chloroplasts originated from free-living cyanobacteria engulfed by ancestral eukaryotic cells.

• Adaptations: Different types of photosynthetic organisms have evolved to utilize various wavelengths of light and environmental conditions.

Example: The presence of similar DNA and double membranes in chloroplasts and cyanobacteria supports the endosymbiotic origin.

3.5 Cellular Respiration

3.5.A: Mitochondrial Structure and Function

Cellular respiration is the process by which cells extract energy from biological macromolecules to produce ATP. Mitochondria are the organelles where most of this process occurs in eukaryotes.

• Glycolysis: Occurs in the cytoplasm; breaks down glucose into pyruvate.

• Krebs Cycle (Citric Acid Cycle): Occurs in the mitochondrial matrix; processes pyruvate to produce electron carriers.

• Electron Transport Chain: Located in the inner mitochondrial membrane; uses electrons from NADH and FADH2 to generate ATP.

• ATP Synthase: Enzyme that synthesizes ATP as protons flow back into the mitochondrial matrix.

Example: Muscle cells use cellular respiration to generate ATP for contraction during exercise.

3.5.B: Energy Extraction from Macromolecules

Cells obtain energy from carbohydrates, fats, and proteins through a series of metabolic pathways that ultimately produce ATP.

• Carbohydrates: Broken down into glucose for glycolysis.

• Fats: Broken down into fatty acids and glycerol, which enter cellular respiration at various points.

• Proteins: Deaminated and their carbon skeletons enter the Krebs cycle.

Example: During fasting, the body metabolizes stored fats for energy via beta-oxidation and the Krebs cycle.

Summary Table: Key Processes and Their Locations

Process	Main Organelle/Location	Main Function
Glycolysis	Cytoplasm	Breakdown of glucose to pyruvate, producing ATP and NADH
Krebs Cycle	Mitochondrial Matrix	Oxidation of acetyl-CoA, producing NADH, FADH2, and CO2
Electron Transport Chain	Inner Mitochondrial Membrane	Production of ATP via oxidative phosphorylation
Light-Dependent Reactions	Thylakoid Membrane (Chloroplast)	Conversion of light energy to ATP and NADPH
Calvin Cycle	Stroma (Chloroplast)	Fixation of CO2 into glucose

Key Equations

• Photosynthesis:

$6CO_2 + 6H_2O + \text{light energy} \rightarrow C_6H_{12}O_6 + 6O_2$

• Cellular Respiration:

$C_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O + \text{ATP}$

• ATP Hydrolysis:

$ATP + H_2O \rightarrow ADP + P_i + \text{energy}$

Additional info:

• Some content was inferred and expanded for clarity and completeness, such as the detailed steps of photosynthesis and cellular respiration, and the summary table.

• Key terms and processes were defined and explained to ensure the notes are self-contained and suitable for exam preparation.