Ch 7 - Cell Membranes & Transport

Cell Membranes: Structure and Function

The cell membrane is a dynamic structure that separates the interior of the cell from its external environment, maintaining homeostasis and regulating transport.

• Fluid-Mosaic Model: Describes the membrane as a flexible layer made of lipids and proteins. Components move laterally within the layer.

• Selective Permeability: Only certain molecules can pass through the membrane, allowing the cell to control its internal environment.

• Membrane Components:

  • Phospholipids: Amphipathic molecules with hydrophilic heads and hydrophobic tails, forming a bilayer.

  • Cholesterol: Stabilizes membrane fluidity, especially in animal cells.

  • Membrane Proteins:

    • Integral Proteins: Span the membrane, involved in transport and signaling.

    • Peripheral Proteins: Attached to the membrane surface, often involved in cell signaling or structure.

• Functions of Membrane Proteins:

  • Transport

  • Enzymatic activity

  • Signal transduction

  • Cell-cell recognition

  • Intercellular joining

  • Attachment to cytoskeleton and extracellular matrix

Synthesis of Membranes

Membranes are synthesized in the endoplasmic reticulum and modified in the Golgi apparatus before being transported to their final destinations.

Cellular Transport

Transport across membranes is essential for nutrient uptake, waste removal, and communication.

• Transport Proteins: Facilitate movement of substances across the membrane.

  • Channel Proteins: Form pores for specific molecules (e.g., aquaporins for water).

  • Carrier Proteins: Bind and transport substances by changing shape.

• Equilibrium: Achieved when concentrations are equal across the membrane.

• Concentration Gradient: Difference in concentration across a membrane drives passive transport.

Passive Transport

Movement of substances without energy input.

• Diffusion: Movement of molecules from high to low concentration.

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

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

  • Hypertonic: Higher solute outside; cell loses water.

  • Hypotonic: Lower solute outside; cell gains water.

Effects on Cells

• Plant Cells:

  • Flaccid: In isotonic solution; limp.

  • Turgid: In hypotonic solution; firm due to water uptake.

  • Plasmolysis: In hypertonic solution; membrane pulls away from wall.

• Animal Cells: May burst (lyse) in hypotonic, shrink (crenate) in hypertonic solutions.

Osmoregulation

Control of water balance; essential for survival in varying environments.

Facilitated Diffusion

Passive transport aided by proteins; no energy required.

Active Transport

Movement against concentration gradient; requires energy (usually ATP).

• Examples:

  • Sodium-potassium pump: Maintains electrochemical gradients.

  • Proton pump: Moves H+ ions across membranes.

Bulk Transport

• Endocytosis: Uptake of materials via vesicles.

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

  • Pinocytosis: "Cell drinking"; uptake of fluids.

  • Receptor-mediated endocytosis: Specific uptake using receptors.

• Exocytosis: Release of substances from cell via vesicles.

Ch 8 - Energy & Enzymes

Metabolism

Metabolism is the sum of all chemical reactions in a cell, divided into catabolism (breakdown) and anabolism (synthesis).

• Metabolic Pathways:

  • Linear

  • Branched

  • Cyclical

  • Inward Spiral

  • Outward Spiral

Thermodynamics in Biology

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

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

• Open vs. Closed Systems: Cells are open systems, exchanging energy and matter with surroundings.

• Entropy: Measure of disorder; increases in spontaneous processes.

Types of Energy

• Thermal

• Kinetic

• Radiant

• Electrical

• Mechanical

• Atomic

• Chemical

Gibbs Free Energy

Determines whether a reaction is spontaneous.

• Formula:

• Spontaneous Reactions: Occur without energy input; .

• Non-spontaneous Reactions: Require energy; .

Exergonic vs. Endergonic Reactions

• Exergonic: Release energy; .

• Endergonic: Consume energy; .

• Linking Reactions: Cells couple exergonic and endergonic reactions to drive cellular processes.

Cellular Work & ATP

• Chemical Work: Building molecules

• Transport Work: Moving substances across membranes

• Mechanical Work: Movement (e.g., muscle contraction)

• ATP: Main energy currency; hydrolysis releases kcal/mol.

Enzymes

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

• Models:

  • Lock and Key Model: Substrate fits exactly into active site.

  • Induced Fit Model: Active site changes shape to fit substrate.

• Active Site: Region where substrate binds.

• Substrate: Reactant acted upon by enzyme.

• Product: Resulting molecule after reaction.

Environmental Effects on Enzymes

• Temperature

• pH

• Cofactors: Non-protein helpers (e.g., metal ions, vitamins)

• Inhibitors: Molecules that decrease enzyme activity

Regulating Enzymes

• Allosteric Activation: Enhances activity by binding at a site other than active site.

• Allosteric Inhibition: Reduces activity by binding at a site other than active site.

• Feedback Inhibition: End product inhibits pathway.

• Enzyme Repression: Decreases enzyme synthesis.

• Enzyme Induction: Increases enzyme synthesis.

Ch 9 - Cellular Respiration

Overview of Cellular Respiration

Cellular respiration is the process by which cells extract energy from food molecules using oxygen, producing CO2 and H2O.

• General Equation:

Food + O2 → energy + CO2 + H2O

Key Steps

• Glycolysis

• Citric Acid Cycle (Krebs Cycle)

• Electron Transport System

• Fermentation (when O2 is absent)

Oxidation vs. Reduction

• Oxidation: Loss of electrons

• Reduction: Gain of electrons

• NADH: Electron carrier; stores energy for later use

Glycolysis

• Location: Cytoplasm

• Initial Molecule: Glucose

• Final Products: 2 ATP, 2 NADH, 2 pyruvate

Fermentation

Occurs when oxygen is absent; regenerates NAD+ for glycolysis.

• Purpose: Allows glycolysis to continue

• Products: Lactic acid (animals), ethanol and CO2 (yeast)

Transition Step

• Pyruvate to Acetyl CoA + CO2 + NADH

• Location: Mitochondrial matrix

Citric Acid Cycle (Krebs Cycle)

• Location: Mitochondrial matrix

• Initial Molecule: Acetyl-CoA

• Final Products (per turn): 2 CO2, 1 ATP, 3 NADH, 1 FADH2

• Total Results (per glucose): 4 ATP, 10 NADH, 2 FADH2

Electron Transport Pathway

• Location: Inner mitochondrial membrane

• Initial Molecules: NADH and FADH2 (source of electrons)

• NADH vs. FADH2: NADH yields more ATP per molecule

• Final Electron Acceptor: Oxygen

Chemiosmosis

• Location: Inner mitochondrial membrane

• Role of H+: Gradient drives ATP synthesis

• Role of ATP Synthase: Enzyme that synthesizes ATP as H+ flows through

Phosphorylation Types

• Substrate-level Phosphorylation: Direct transfer of phosphate to ADP

• Oxidative Phosphorylation: ATP synthesis via chemiosmosis

Energy Yield

• Theoretical Maximum: ~36-38 ATP per glucose

• Factors Affecting Yield: Leaky membranes, use of intermediates for biosynthesis

Alternate Fuel Sources

• Non-glucose Carbohydrates: Enter glycolysis or other pathways

• Proteins: Deaminated and enter as intermediates

• Lipids: Fatty acids enter via beta-oxidation

Oxygen Requirements

• Pathways Requiring Oxygen: Citric acid cycle, electron transport chain

• Pathways Not Requiring Oxygen: Glycolysis, fermentation

• Aerobic vs. Anaerobic: Aerobic yields more ATP; anaerobic yields less

Summary Table: Cellular Respiration Pathways

Example: During intense exercise, muscle cells switch to fermentation, producing lactic acid when oxygen is scarce.

Additional info: The summary table was inferred and expanded for clarity and completeness.

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