Cell Membranes & Transport

Introduction

Cell membranes are essential structures that define the boundaries of cells and organelles, regulate the movement of substances, and facilitate communication. This study guide covers the structure, composition, and transport mechanisms associated with biological membranes.

Biological Membranes: Structure and Fluidity

General Structure of Membranes

• Biological membranes are primarily composed of lipids, proteins, and carbohydrates.

• The fluid mosaic model describes membranes as a dynamic bilayer of phospholipids in which proteins "float."

• Phospholipids form a continuous bilayer, creating a "lake" for proteins to move within.

Phospholipids and Bilayer Formation

• Phospholipids have two regions:

  • Hydrophilic (polar) heads associate with water molecules.

  • Hydrophobic (nonpolar) tails avoid water and interact with each other.

• The bilayer forms as fatty acid tails face inward, away from water, while heads face outward toward aqueous environments.

• Phospholipids may differ in fatty acid chain length, degree of saturation, and types of polar groups present.

Role of Cholesterol

• Cholesterol is a key component of animal cell membranes.

• Hydroxyl groups of cholesterol interact with polar heads of phospholipids.

• Cholesterol modulates membrane fluidity; other steroids may function as hormones.

Membrane Fluidity

• Fatty acids influence membrane fluidity:

  • Short, unsaturated chains increase fluidity.

  • Long, saturated chains decrease fluidity.

• Temperature affects fluidity: higher temperatures increase, lower temperatures decrease fluidity.

Membrane Proteins: Types and Functions

Integral and Peripheral Proteins

• All membranes contain proteins; the protein-to-lipid ratio varies.

• Peripheral membrane proteins lack hydrophobic regions and are not embedded in the bilayer.

• Integral membrane proteins are at least partly embedded in the bilayer.

Anchored and Transmembrane Proteins

• Anchored proteins have hydrophobic lipid components anchoring them in the bilayer.

• Proteins are asymmetrically distributed on membrane surfaces.

• Transmembrane proteins span the bilayer and may have domains on both sides.

Protein Mobility and Restriction

• Some proteins move freely within the bilayer; others are restricted.

• Cell fusion experiments demonstrate protein migration.

• Internal cell structures (e.g., cytoskeleton) can restrict protein movement.

Membrane Carbohydrates and Cell Communication

Types of Membrane Carbohydrates

• Carbohydrates are located on the outer cell membrane and aid in communication.

• Glycolipids: carbohydrates covalently bonded to lipids.

• Glycoproteins: one or more oligosaccharides covalently bonded to proteins.

• Proteoglycans: proteins with many and longer carbohydrate chains.

Cell Adhesion

• Cells adhere to each other via interactions between surface carbohydrates and proteins.

Membrane Dynamics

• Membranes are dynamic, constantly forming, fusing, and breaking down.

• Despite similar appearance, chemical differences exist among membranes of different cells.

Transport Across Membranes

Selective Permeability

• Biological membranes allow some substances to pass while restricting others.

Types of Transport

• Passive transport: does not require metabolic energy; substances move down their concentration gradient.

• Active transport: requires metabolic energy; substances move against their concentration gradient.

Passive Transport Mechanisms

• Simple diffusion: movement through the phospholipid bilayer.

• Facilitated diffusion: movement through channel or carrier proteins.

Diffusion

• Random movement of molecules toward equilibrium.

• Movement from regions of higher to lower concentration.

Factors Affecting Diffusion Rate

• Diameter of molecules: smaller molecules diffuse faster.

• Temperature: higher temperatures increase diffusion rate.

• Concentration gradient: greater gradients increase diffusion rate.

Osmosis and Osmotic Pressure

• Osmosis: diffusion of water across membranes through special channels.

• Depends on water molecule concentration and solute concentration gradient.

• Higher solute concentration = lower water concentration.

Osmotic Pressure Equation

• Osmotic pressure () is the pressure required to prevent water flow by osmosis:

• = total solute concentration

• = gas constant

• = absolute temperature

Comparing Solutions: Tonicity

• Hypertonic solution: higher solute concentration.

• Isotonic solution: equal solute concentrations.

• Hypotonic solution: lower solute concentration.

Effects of Osmosis on Cells

• Osmosis can cause cells to swell, shrink, or remain unchanged depending on the surrounding solution's tonicity.

• Water moves toward the solution with higher solute concentration.

Summary Table: Membrane Components and Functions

Key Equations

• Osmotic Pressure:

• Diffusion: No specific equation, but rate depends on diameter, temperature, and concentration gradient.

Examples and Applications

• Red blood cells in hypotonic solutions swell and may burst; in hypertonic solutions, they shrink.

• Cell fusion experiments demonstrate protein mobility within membranes.

• Transport proteins facilitate movement of ions and molecules across membranes, essential for nerve impulses and muscle contraction.