Chapter 7: Membrane Structure and Function

Fluid Mosaic Model of Plasma Membranes

The fluid mosaic model describes the structure of plasma membranes as a dynamic combination of lipids, proteins, and carbohydrates. These components determine membrane fluidity and the ability of cells to interact with their environment.

• Phospholipids: Form a bilayer with hydrophilic heads facing outward and hydrophobic tails inward, providing the basic structure.

• Proteins: Embedded or attached to the bilayer, functioning in transport, signaling, and structural support.

• Carbohydrates: Attached to proteins or lipids on the extracellular surface, involved in cell recognition.

• Cholesterol: Interspersed within the bilayer, modulating fluidity and stability.

• Fluidity: Influenced by lipid composition, temperature, and cholesterol content.

Example: Red blood cell membranes contain specific glycoproteins that determine blood type.

Tonicity: Hypotonic, Hypertonic, and Isotonic Solutions

Tonicity refers to the relative concentration of solutes in two solutions separated by a semipermeable membrane, affecting water movement.

• Hypotonic: Lower solute concentration outside the cell; water enters the cell, possibly causing lysis.

• Hypertonic: Higher solute concentration outside the cell; water leaves the cell, causing shrinkage (crenation).

• Isotonic: Equal solute concentrations; no net water movement.

• Predicting Water Movement: Water moves from areas of low solute concentration to high solute concentration.

Example: Placing animal cells in pure water (hypotonic) causes them to swell and burst.

Diffusion Across Membranes

Diffusion is the passive movement of molecules from high to low concentration. Membrane permeability depends on molecule size, polarity, and charge.

• Simple Diffusion: Small, nonpolar molecules (e.g., O2, CO2) pass directly through the membrane.

• Facilitated Diffusion: Larger or polar molecules use transport proteins.

Example: Glucose enters cells via facilitated diffusion through GLUT transporters.

Electrochemical Gradients

Electrochemical gradients combine chemical (concentration) and electrical (charge) forces to drive ion movement across membranes.

• Sodium-Potassium Pump: Uses ATP to move 3 Na+ out and 2 K+ in, creating gradients.

• Proton Pump: Moves H+ ions, generating membrane potential.

Equation:

Bulk Transport: Endocytosis and Exocytosis

Bulk transport moves large particles or volumes across membranes via vesicles.

• Phagocytosis: "Cell eating"; uptake of large particles.

• Pinocytosis: "Cell drinking"; uptake of fluids and small molecules.

• Receptor-Mediated Endocytosis: Specific uptake via receptor binding.

Example: White blood cells engulf bacteria via phagocytosis.

Chapter 8: Metabolism and Enzymes

First and Second Laws of Thermodynamics

The laws of thermodynamics govern energy transformations in biological systems.

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

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

Equation:

Free Energy and Reaction Favorability

Free energy change (ΔG) determines whether a reaction is energetically favorable.

• Exergonic: ΔG < 0; energy released; spontaneous.

• Endergonic: ΔG > 0; energy required; non-spontaneous.

Example: Cellular respiration is exergonic; photosynthesis is endergonic.

Enzymes: Structure and Regulation

Enzymes are biological catalysts that speed up reactions by lowering activation energy.

• Active Site: Region where substrate binds and reaction occurs.

• Regulation: Inhibitors (competitive/noncompetitive) and activators modulate activity.

• Temperature/pH: Affect enzyme structure and function.

Example: Amylase catalyzes starch breakdown in saliva.

Chapter 9: Cellular Respiration

Structure and Function of Mitochondria

The mitochondrion is the site of cellular respiration, consisting of an outer membrane, inner membrane, and matrix.

• Three Stages: Glycolysis (cytoplasm), Krebs cycle (matrix), Electron transport chain (inner membrane).

Inputs and Outputs of Respiration Stages

Electron Pathway: Glucose to Oxygen

Electrons are transferred from glucose to NADH/FADH2, then through the electron transport chain to oxygen, forming water.

Chemiosmosis and ATP Synthesis

Chemiosmosis uses the proton gradient across the inner mitochondrial membrane to drive ATP synthesis via ATP synthase.

Equation:

Fermentation and Anaerobic Respiration

When oxygen is absent, cells use fermentation or anaerobic respiration to generate ATP.

• Alcohol Fermentation: Pyruvate → Ethanol + CO2

• Lactic Acid Fermentation: Pyruvate → Lactate

• Difference: Anaerobic respiration uses an electron transport chain with a final electron acceptor other than oxygen.

Chapter 10: Photosynthesis

Autotrophs vs. Heterotrophs

Autotrophs produce their own food via photosynthesis; heterotrophs consume other organisms for energy.

Structure of Chloroplasts

Chloroplasts contain an outer membrane, inner membrane, stroma, and thylakoid membranes where photosynthesis occurs.

Light Reactions and Calvin Cycle

Photosynthesis consists of light reactions (in thylakoid membranes) and the Calvin cycle (in stroma).

• Light Reactions: Convert light energy to chemical energy (ATP, NADPH); split water, release O2.

• Calvin Cycle: Uses ATP and NADPH to fix CO2 into glucose.

Photosystems and Electron Pathways

Photosystem II (PSII) and Photosystem I (PSI) work together to transfer electrons from water to NADP+ during the light reactions.

• Pathway: Water → PSII → Electron Transport Chain → PSI → NADP+

Adaptations: CAM and C4 Plants

CAM and C4 plants have adaptations to minimize water loss while maintaining photosynthesis.

• C4 Plants: Spatial separation of carbon fixation and Calvin cycle.

• CAM Plants: Temporal separation; fix CO2 at night, use it during the day.

Example: Cacti (CAM) and corn (C4) thrive in arid environments.

Additional info: Academic context and definitions have been expanded for clarity and completeness.