Membrane Structure and Function: Study Notes for General Biology

Membrane Structure and Function

Learning Objectives

• Explain the fluid mosaic model and describe the components of the cell membrane.

• Discuss how membrane structure results in selective permeability.

• Use examples to demonstrate the processes of diffusion, osmosis, and facilitated diffusion.

• Describe the process of active transport.

Fluid Mosaic Model of Membrane Structure

Overview

The fluid mosaic model describes the structure of cell membranes as a mosaic of diverse protein molecules embedded in or attached to a fluid bilayer of phospholipids. This model explains both the flexibility and the selective permeability of biological membranes.

• Phospholipids are amphipathic, meaning they have both hydrophilic (water-loving) heads and hydrophobic (water-fearing) tails.

• The hydrophilic heads face outward toward water, while the hydrophobic tails are shielded inside the bilayer.

• Membrane proteins are also often amphipathic and can move within the membrane.

• The membrane is fluid, allowing proteins and lipids to move laterally.

Example: The difference between a phospholipid and a fat molecule is that phospholipids have two fatty acid tails and a phosphate group, while fats (triglycerides) have three fatty acid tails.

Phospholipid Properties

• Unsaturated hydrocarbon tails (with kinks) prevent packing, increasing membrane fluidity.

• Saturated hydrocarbon tails pack together, increasing membrane viscosity.

Types of Membrane Proteins

• Integral proteins: Penetrate the hydrophobic interior of the lipid bilayer. Most are transmembrane proteins that span the entire membrane.

• Peripheral proteins: Loosely bound to the membrane surface or to integral proteins; do not penetrate the bilayer.

Major Functions of Membrane Proteins

• Transport: Move substances across the membrane (channels, carriers, pumps).

• Enzymatic activity: Catalyze reactions at the membrane surface.

• Signal transduction: Receive and transmit signals from outside to inside the cell.

• Cell recognition: Glycoproteins serve as identification tags for cell-cell recognition.

• Intercellular joining: Proteins help cells adhere to each other (e.g., tight junctions).

• Attachment to cytoskeleton and extracellular matrix (ECM): Maintain cell shape and stabilize protein location.

Membrane Carbohydrates

• Involved in cell-cell recognition.

• Usually short, branched chains attached to lipids (glycolipids) or proteins (glycoproteins).

• Human blood types (A, B, AB, O) are determined by variations in glycoprotein carbohydrates on red blood cells.

Selective Permeability of Membranes

Overview

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

• Nonpolar molecules (hydrophobic, e.g., hydrocarbons, CO2, O2, lipids) can pass through freely.

• Polar molecules (hydrophilic, e.g., sugars, ions) require transport proteins to cross the membrane.

Transport Across Membranes

Passive Transport

Passive transport is the movement of substances across the membrane without energy input from the cell.

• Diffusion: Movement of molecules from high to low concentration, down their concentration gradient.

• Osmosis: Diffusion of water across a selectively permeable membrane from low solute (high free water) to high solute (low free water) concentration.

• Tonicity: The ability of a solution to cause a cell to gain or lose water.

• Isotonic solution: No net movement of water.

• Hypertonic solution: Cell loses water, shrivels.

• Hypotonic solution: Cell gains water, swells, and may burst.

Facilitated Diffusion

Facilitated diffusion is passive transport aided by transport proteins.

• Channel proteins: Provide corridors for specific molecules or ions to cross.

• Carrier proteins: Change shape to move substances across the membrane.

• Transport proteins are specific for the substances they move.

Active Transport

Active transport moves substances against their concentration gradients, requiring energy (usually from ATP).

• Carrier proteins are involved in active transport.

• Sodium-potassium pump: Maintains high K+ and low Na+ inside animal cells by pumping Na+ out and K+ in.

Equation:

$\text{Rate of active transport} = k[\text{Carrier Protein}][\text{ATP}]$

Example: Glucose transporters use active transport to move glucose into cells against its concentration gradient.

Bulk Transport: Exocytosis and Endocytosis

Overview

Large molecules (proteins, polysaccharides) cross the membrane via bulk transport, packaged in vesicles.

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

• Endocytosis: The plasma membrane engulfs material to form a vesicle inside the cell.

• Types of endocytosis:

  • Phagocytosis: "Cell eating" of large particles.

  • Pinocytosis: "Cell drinking" of fluids and dissolved substances.

  • Receptor-mediated endocytosis: Specific uptake of molecules via receptor proteins.

Example: Pancreatic cells secrete insulin via exocytosis; immune cells engulf bacteria via phagocytosis.

Table: Major Types of Membrane Proteins and Their Functions

Type	Function	Example
Transport Protein	Move substances across membrane	Ion channels, glucose transporters
Enzyme	Catalyze reactions at membrane	ATPase, digestive enzymes
Receptor	Signal transduction	Hormone receptors
Cell-identity Marker	Cell recognition	Glycoproteins (blood type antigens)
Intercellular Joining	Cell adhesion	Tight junction proteins
Attachment Protein	Bind cytoskeleton/ECM	Integrins

Additional info: Some details and examples have been expanded for clarity and completeness, based on standard biology textbook content.