Plasma Membrane Structure and Function

Overview of the Plasma Membrane

The plasma membrane is a selectively permeable barrier found in all cells, separating the internal environment from the external surroundings. Its major functions include protection, communication, and regulation of transport.

• Composition: Primarily composed of a phospholipid bilayer with embedded proteins, carbohydrates, and cholesterol.

• Fluid Mosaic Model: Describes the membrane as a dynamic structure with proteins and lipids moving laterally within the bilayer.

• Amphipathic Molecule: Phospholipids have both hydrophilic (water-loving) heads and hydrophobic (water-fearing) tails.

Membrane Fluidity

Membrane fluidity is influenced by temperature, cholesterol, and the saturation of fatty acid tails.

• Temperature: Higher temperatures increase fluidity; lower temperatures decrease it.

• Cholesterol: Acts as a fluidity buffer, preventing extremes in fluidity.

• Saturated vs. Unsaturated Fatty Acids: Unsaturated tails (with double bonds) increase fluidity; saturated tails decrease it.

Membrane Proteins and Carbohydrates

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

• Peripheral Proteins: Attached to the membrane surface; provide structural support.

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

• Receptor Proteins: Receive and transmit signals.

• Glycoproteins & Glycolipids: Carbohydrates attached to proteins/lipids; important for cell recognition and signaling.

Selective Permeability and Transport

The plasma membrane allows some molecules to cross while blocking others.

• Small, nonpolar molecules (e.g., O2, CO2) cross easily.

• Large or charged molecules require transport proteins.

• Passive Transport: Movement without energy input (diffusion, osmosis, facilitated diffusion).

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

Osmosis and Tonicity

Osmosis is the diffusion of water across a selectively permeable membrane.

• Hypertonic Solution: Higher solute concentration outside; water leaves cell.

• Hypotonic Solution: Lower solute concentration outside; water enters cell.

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

• Effects on Cells:

  • Animal Cells: Lyse (burst) in hypotonic, shrivel in hypertonic, normal in isotonic.

  • Plant Cells: Turgid (firm) in hypotonic, flaccid in isotonic, plasmolyzed in hypertonic.

• Aquaporin: Specialized protein channel for rapid water transport.

Bulk Transport

• Endocytosis: Cell takes in large particles by engulfing them.

• Exocytosis: Cell expels materials by fusing vesicles with the membrane.

Cell Signaling

Cells communicate via signaling pathways involving ligands and receptors.

• Signal Transduction Pathway: Series of steps converting a signal to a cellular response.

• Types of Signaling:

  • Paracrine: Local signaling to nearby cells.

  • Synaptic: Neurotransmitter release at synapses.

  • Endocrine: Hormones travel through bloodstream to distant cells.

  • Direct Contact: Cells communicate via membrane-bound molecules.

• Three Stages of Cell Signaling: Reception, transduction, response.

• Ligand: Molecule that binds to a receptor to initiate signaling.

• Responses: Can be changes in gene expression or cellular activity.

Metabolism and Enzyme Function

Metabolism Overview

Metabolism is the sum of all chemical reactions in a cell, organized into metabolic pathways.

• Catabolic Pathways: Break down molecules, releasing energy (e.g., cellular respiration).

• Anabolic Pathways: Build molecules, requiring energy (e.g., protein synthesis).

Energy Concepts

• Kinetic Energy: Energy of motion.

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

• Chemical Energy: Potential energy in chemical bonds.

Thermodynamics

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

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

• Entropy: Measure of disorder; increases with energy transformations.

Free Energy and Reactions

• Free Energy (G): Energy available to do work.

• Exergonic Reaction: Releases energy; spontaneous;

• Endergonic Reaction: Requires energy; nonspontaneous;

• Energy Coupling: Using exergonic reactions to drive endergonic ones.

ATP: Structure and Function

• ATP (Adenosine Triphosphate): Main energy currency of the cell.

• Hydrolysis of ATP: Releases energy by breaking a phosphate bond.

• Phosphorylation: Addition of a phosphate group to a molecule.

• ATP Regeneration:

Enzymes and Catalysis

• Enzyme: Protein catalyst that speeds up reactions by lowering activation energy.

• Activation Energy: Energy required to start a reaction.

• Substrate: Reactant molecule acted upon by an enzyme.

• Active Site: Region on enzyme where substrate binds.

• Specificity: Enzymes are specific to their substrates.

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

• Competitive Inhibition: Inhibitor binds to active site, blocking substrate.

• Noncompetitive Inhibition: Inhibitor binds elsewhere, changing enzyme shape.

• Allosteric Regulation: Regulation by binding at a site other than the active site.

• Denaturation: Loss of enzyme structure and function due to heat or pH.

• Feedback Inhibition: End product inhibits pathway to regulate activity.

Cellular Respiration and Fermentation

Overview of Cellular Respiration

Cellular respiration is a catabolic pathway that breaks down glucose to produce ATP. It occurs in three main stages: glycolysis, citric acid cycle, and oxidative phosphorylation.

• Fermentation: Anaerobic process producing ATP without oxygen.

• Redox Reaction: Transfer of electrons; oxidation (loss), reduction (gain).

• NADH: Electron carrier; stores energy for ATP production.

Stages of Cellular Respiration

• Glycolysis: Occurs in cytoplasm; splits glucose into pyruvate; produces ATP and NADH.

• Pyruvate Conversion: Pyruvate enters mitochondria, converted to Acetyl CoA.

• Citric Acid Cycle: Occurs in mitochondrial matrix; Acetyl CoA combines with oxaloacetate to form citrate; produces NADH, FADH2, ATP.

• Oxidative Phosphorylation: Occurs in cristae; electron transport chain and chemiosmosis produce most ATP.

Electron Transport Chain and Chemiosmosis

• Electron Transport Chain: Series of proteins in membrane; electrons from NADH/FADH2 passed to oxygen.

• ATP Synthase: Enzyme that uses proton gradient to make ATP.

• Chemiosmosis: Movement of protons across membrane to drive ATP synthesis.

• Role of Oxygen: Final electron acceptor; essential for aerobic respiration.

ATP Production

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

• Oxidative Phosphorylation: ATP produced via electron transport chain and chemiosmosis.

• ATP Yield: Glycolysis (2 ATP), Citric Acid Cycle (2 ATP), Electron Transport Chain (~26-28 ATP).

Fermentation

• Occurs when oxygen is absent.

• Major Products: Lactic acid or ethanol, plus ATP.

• Aerobic vs. Anaerobic: Aerobic uses oxygen; anaerobic does not.

• Pyruvate: Central intermediate; can enter respiration or fermentation pathways.

Regulation of Cellular Respiration

• ATP and NADH levels regulate pathway via feedback inhibition.

• Carbohydrates, fats, and proteins can all be used to produce ATP.

Photosynthesis

Types of Organisms

• Autotrophs: Produce their own food.

• Photoautotrophs: Use light energy (e.g., plants).

• Chemoautotrophs: Use chemical energy.

• Heterotrophs: Consume other organisms for energy.

Chloroplast Structure

• Chlorophyll: Main pigment for photosynthesis.

• Stomata: Openings for gas exchange.

• Mesophyll Cells: Contain chloroplasts.

• Stroma: Fluid inside chloroplast.

• Thylakoid Membranes: Site of light reactions.

• Grana: Stacks of thylakoids.

Photosynthesis Equation

Basic chemical equation:

Light Reactions

• Location: Thylakoid membranes.

• Inputs: Light, water, NADP+, ADP.

• Outputs: Oxygen, ATP, NADPH.

• Provides ATP and NADPH to Calvin Cycle.

Color and Pigments

• Chlorophyll absorbs blue and red light; reflects green.

• Absorption spectra: Shows wavelengths absorbed.

• Action spectra: Shows effectiveness of wavelengths for photosynthesis.

Photosystems

• Photosystem I and II: Involved in light reactions; transfer electrons and produce ATP/NADPH.

Calvin Cycle

• Location: Stroma.

• Inputs: CO2, ATP, NADPH.

• Outputs: Glucose, ADP, NADP+.

• Three Phases: Carbon fixation, reduction, regeneration.

• Key Enzyme: Rubisco.

Photorespiration and Plant Types

• Photorespiration: Rubisco binds O2 instead of CO2; wastes energy.

• C3 Plants: Standard pathway; susceptible to photorespiration.

• C4 Plants: Spatial separation; PEP carboxylase minimizes photorespiration.

• CAM Plants: Temporal separation; open stomata at night to minimize water loss.

• Affinity: Strength of enzyme binding to substrate.

Comparison of C3, C4, and CAM Plants

Spatial separation: C4 plants fix CO2 in different cells. Temporal separation: CAM plants fix CO2 at different times.