Cell Membrane Structure and Function

Plasma Membrane Overview

The plasma membrane is a dynamic boundary that separates the living cell from its environment. It regulates the movement of substances in and out of the cell, maintaining homeostasis through selective permeability.

• Phospholipid Bilayer: The fundamental structure, composed of two layers of phospholipids with hydrophilic heads facing outward and hydrophobic tails inward.

• Membrane Proteins: Embedded or associated proteins that perform various functions.

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

Types of Membrane Proteins:

• Integral Proteins: Span the membrane (transmembrane proteins).

• Peripheral Proteins: Loosely attached to the membrane surface.

Membrane Protein Functions

Membrane proteins are essential for various cellular processes. Their six major functions include:

• Transport: Move substances across the membrane.

• Enzymatic Activity: Catalyze chemical reactions.

• Signal Transduction: Relay signals from outside to inside the cell.

• Cell-Cell Recognition: Allow cells to identify each other.

• Intercellular Joining: Connect adjacent cells.

• Attachment: Anchor the membrane to the cytoskeleton and extracellular matrix (ECM).

Membrane Carbohydrates

Carbohydrates on the cell surface play a key role in cell-cell recognition and communication.

• Cells distinguish neighboring cells by binding to glycoproteins and glycolipids.

• Carbohydrate composition varies among species and cell types.

Transport Across the Membrane

Selective Permeability

The plasma membrane allows some substances to cross more easily than others, maintaining the internal environment of the cell.

Types of Transport

• Passive Transport: Movement of substances without energy input.

• Active Transport: Movement of substances against their concentration gradient, requiring energy (usually ATP).

Passive Transport

• Diffusion: Movement of molecules from high to low concentration until equilibrium is reached.

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

• Facilitated Diffusion: Transport proteins help move polar molecules and ions across the membrane.

Key Terms in Osmosis

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

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

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

Table: Tonicity Effects on Animal Cells

Active Transport

Active transport moves molecules against their concentration gradient using energy, typically from ATP.

• Pumps: Proteins that use ATP to transport ions (e.g., sodium-potassium pump, proton pump).

• Electrogenic Pumps: Generate voltage across membranes (e.g., proton pump moves H+ ions).

• Co-transport: Uses the energy from one molecule moving down its gradient to move another molecule against its gradient.

Bulk Transport

• Exocytosis: Vesicles fuse with the membrane to release contents outside the cell.

• Endocytosis: Cell takes in macromolecules by forming vesicles from the membrane.

  • Phagocytosis: "Cell eating"; engulfs large particles.

  • Pinocytosis: "Cell drinking"; engulfs extracellular fluid.

  • Receptor-mediated endocytosis: Specific molecules are taken in after binding to receptors.

Cellular Metabolism and Energy

Metabolism Overview

Metabolism is the sum of all chemical reactions in a cell, divided into metabolic pathways that are catalyzed by enzymes.

• Catabolic Pathways: Break down complex molecules, releasing energy.

• Anabolic Pathways: Build complex molecules from simpler ones, consuming energy.

Energy in Cells

• Kinetic Energy: Energy of motion.

• Potential Energy: Stored energy (e.g., in chemical bonds).

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

Free Energy and Spontaneity

• Free Energy (G): The portion of a system's energy that can perform work.

• Spontaneous reactions decrease free energy ().

• Non-spontaneous reactions require energy input ().

Equation for Free Energy Change:

Types of Reactions

• Exergonic Reactions: Release free energy; spontaneous.

• Endergonic Reactions: Absorb free energy; non-spontaneous.

Chemical Reactions and Equilibrium

• Exergonic reactions in a closed system reach equilibrium and stop; cells remain in disequilibrium to sustain life.

Summary Table: Types of Membrane Transport

Key Definitions

• Selective Permeability: Property of membranes that allows some substances to pass more easily than others.

• Concentration Gradient: Difference in concentration of a substance across a space.

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

• Enzyme: Protein that catalyzes chemical reactions.