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 of the membrane, composed of amphipathic phospholipids with hydrophilic heads and hydrophobic tails.

• Membrane Proteins: Embedded within the bilayer, these proteins perform various functions (see below).

• Carbohydrates: Attached to proteins and lipids on the extracellular surface, important for cell recognition.

Membrane Proteins

Membrane proteins are essential for the diverse functions of the plasma membrane. They are classified as integral (spanning the membrane) or peripheral (attached to the surface).

• 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 are crucial for cell-cell recognition and communication.

• Glycolipids and Glycoproteins: Carbohydrates bound to lipids or proteins, respectively.

• Species Specificity: Carbohydrate patterns vary among species and cell types, aiding in immune response and tissue formation.

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.

Movement of Molecules

• Small Molecules: Can move across the membrane via diffusion or transport proteins.

• Large Molecules: Require vesicular transport (endocytosis/exocytosis).

Passive Transport

Passive transport does not require energy input from the cell. Substances move down their concentration gradient.

• Hydrophobic (nonpolar) molecules: Dissolve in the lipid bilayer and pass through rapidly.

• Hydrophilic (polar) molecules: Require transport proteins to cross the membrane.

Diffusion

Diffusion is the movement of molecules from an area of high concentration to low concentration until equilibrium is reached.

• Principle: Molecules move randomly, spreading out evenly in available space.

• Concentration Gradient: The difference in concentration across a space; substances diffuse down this gradient.

Osmosis

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

• Isotonic: Equal solute concentration inside and outside the cell; no net water movement.

• Hypotonic: Lower solute concentration outside; water enters the cell, which may swell or burst.

• Hypertonic: Higher solute concentration outside; water leaves the cell, causing it to shrink.

Facilitated Diffusion

Facilitated diffusion uses transport proteins to move polar molecules and ions across the membrane without energy input.

• Channel Proteins: Provide corridors for specific molecules or ions.

• Carrier Proteins: Bind to molecules and change shape to shuttle them across.

Active Transport

Active transport moves substances against their concentration gradient, requiring energy (usually from ATP) and carrier proteins.

• Pumps: Use ATP to maintain membrane potential (voltage difference across the membrane).

• Proton Pump: Moves hydrogen ions out of the cell, generating an electrochemical gradient.

• Co-transport (Coupled Transport): Uses the energy from one molecule moving down its gradient to drive another molecule against its gradient.

Bulk Transport

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

• Endocytosis: The cell engulfs macromolecules by forming vesicles from the plasma 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, providing energy and building blocks for cellular processes.

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

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

ATP: The Energy Currency

ATP (adenosine triphosphate) is the main energy source for cellular work. It consists of adenine, ribose, and three phosphate groups.

• Energy is released when ATP is hydrolyzed to ADP and inorganic phosphate.

Energy and Free Energy

• Kinetic Energy: Energy of motion.

• Potential Energy: Stored energy, such as in chemical bonds.

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

• Free Energy (G): The portion of a system's energy available to do work.

Spontaneous Reactions and Delta G

• In spontaneous reactions, free energy decreases ().

• Exergonic reactions: Release free energy, are spontaneous.

• Endergonic reactions: Absorb free energy, are not 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