Cell Membranes, Transport, and Cellular Respiration: Study Guide for General Biology

Chapter 6: Membrane Structure and Function

Types of Lipids

Lipids are a diverse group of hydrophobic molecules that play critical roles in cell structure and function. The main types include steroids, fats, and phospholipids.

• Steroids: Lipids characterized by a four-ring structure; examples include cholesterol.

• Fats: Composed of glycerol and fatty acids; function in energy storage.

• Phospholipids: Consist of a glycerol backbone, two fatty acid tails, and a phosphate group; major component of cell membranes.

Key Point: The physical properties of hydrocarbon chains depend on their saturation status (saturated vs. unsaturated fatty acids).

• Saturated fatty acids: No double bonds; straight chains; pack tightly; solid at room temperature.

• Unsaturated fatty acids: One or more double bonds; kinked chains; less tightly packed; liquid at room temperature.

Example: Compare the structure and function of a steroid (cholesterol), a fat (triglyceride), and a phospholipid.

Phospholipids and Membrane Permeability

Phospholipids form bilayers that are selectively permeable, allowing some substances to cross more easily than others.

• Selective permeability: Small, nonpolar molecules (e.g., O2, CO2) cross easily; large or charged molecules (e.g., ions, glucose) require transport proteins.

• Factors affecting permeability: Degree of saturation and length of hydrocarbon tails, cholesterol content, and temperature.

Example: Membrane permeability increases with shorter, unsaturated hydrocarbon tails and decreases with longer, saturated tails and higher cholesterol.

Diffusion and Osmosis

Diffusion and osmosis are passive processes that move substances across membranes without energy input.

• Diffusion: Net movement of molecules from high to low concentration.

• Osmosis: Diffusion of water across a selectively permeable membrane.

• Concentration gradient: Drives movement; substances move "down" their gradient.

Example: Water moves into a cell placed in a hypotonic solution and out of a cell in a hypertonic solution.

Passive and Active Transport

Transport proteins facilitate the movement of substances across membranes.

• Passive transport: Includes simple diffusion and facilitated diffusion; does not require energy.

• Active transport: Requires energy (usually ATP) to move substances against their concentration gradient.

• Co-transport: Coupled transport of two substances; e.g., Na+/glucose co-transporter.

Example: The Na+/K+ ATPase pump uses ATP to move Na+ out and K+ into the cell.

Table: Factors Affecting Membrane Permeability

Factor	Effect on Permeability
Temperature	Higher temperature increases permeability
Cholesterol	Decreases permeability
Length of hydrocarbon tails	Longer tails decrease permeability
Saturation of hydrocarbon tails	More saturated tails decrease permeability

Chapter 7: Cell Structure and Organization

Prokaryotic vs. Eukaryotic Cells

Cells are classified as prokaryotic or eukaryotic based on their structure and complexity.

• Prokaryotic cells: Lack membrane-bound organelles; include bacteria and archaea.

• Eukaryotic cells: Have membrane-bound organelles (nucleus, mitochondria, etc.); include plants, animals, fungi, and protists.

Example: Eukaryotic cells have compartmentalization, allowing specialized functions within organelles.

Cellular Compartmentalization

Compartmentalization increases cellular complexity and efficiency.

• Organelles: Specialized structures for specific functions (e.g., mitochondria for energy production).

• Cytoskeleton: Provides structural support and facilitates transport within the cell.

Chapter 8: Chemical Reactions and Enzymes

Energy in Chemical Reactions

Chemical reactions involve changes in energy, including potential and kinetic energy.

• Potential energy: Stored energy due to position or structure.

• Kinetic energy: Energy of motion.

• Activation energy: Minimum energy required to start a reaction.

Equation: Rate of a chemical reaction depends on temperature and concentration: $\text{Rate} = k[\text{Substrate}]$

Redox Reactions and ATP

Redox reactions involve the transfer of electrons and are central to cellular metabolism.

• Oxidation: Loss of electrons.

• Reduction: Gain of electrons.

• ATP: Main energy currency of the cell; produced during cellular respiration.

• NADH and FADH2: Electron carriers in metabolic pathways.

Enzymes and Metabolic Pathways

Enzymes are biological catalysts that speed up chemical reactions.

• Enzyme regulation: Includes competitive inhibition, allosteric regulation, and covalent modification (phosphorylation).

• Feedback inhibition: End product of a pathway inhibits an earlier step.

Example: Phosphofructokinase is inhibited by ATP in glycolysis.

Chapter 9: Cellular Respiration

Glucose Metabolism

Glucose is central to cellular metabolism and energy production.

• Glycolysis: Breakdown of glucose to pyruvate; occurs in cytoplasm.

• Krebs Cycle (Citric Acid Cycle): Oxidizes acetyl-CoA to CO2; produces NADH and FADH2.

• Electron Transport Chain (ETC): Uses electrons from NADH and FADH2 to produce ATP via oxidative phosphorylation.

Equation: $\text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2 \rightarrow 6\text{CO}_2 + 6\text{H}_2\text{O} + \text{ATP}$

Fermentation

Fermentation allows ATP production in the absence of oxygen.

• Lactic acid fermentation: Occurs in muscle cells; produces lactate.

• Alcohol fermentation: Occurs in yeast; produces ethanol and CO2.

Regulation and Inhibition

Cellular respiration is regulated by substrate availability, enzyme activity, and feedback mechanisms.

• Inhibitors: Can block components of the electron transport chain, affecting ATP production.

• Mutations: May disrupt proteins involved in respiration, leading to metabolic disorders.

Apply-Analyze-Evaluate Skills

• Predict movement of molecules and ions across membranes based on composition and concentration gradients.

• Compare and contrast prokaryotic and eukaryotic cell structures.

• Make predictions about enzyme activity and metabolic pathway rates based on substrate and product concentrations.

• Predict consequences of mutation or inhibitor disruption in cellular respiration, especially in the electron transport chain.

Modeling Membranes and Transport

Tips on Drawing Membranes

• Show organization of molecules in bilayer (phospholipids with hydrophilic heads and hydrophobic tails).

• Illustrate facilitated diffusion of solutes via carrier proteins (e.g., GLUT-1 for glucose).

Example: When there is a concentration gradient of glucose across a membrane containing GLUT-1, glucose moves from high to low concentration via facilitated diffusion.

Summary Table: Types of Membrane Transport

Type	Energy Required?	Direction	Example
Simple Diffusion	No	Down gradient	O2, CO2
Facilitated Diffusion	No	Down gradient	Glucose via GLUT-1
Active Transport	Yes (ATP)	Against gradient	Na+/K+ pump
Co-transport	Indirect (uses gradient)	Against one gradient, down another	Na+/glucose co-transporter

Additional info: Academic context and examples have been expanded for clarity and completeness. All equations are provided in LaTeX format as required.