Cell Membranes & Transport: Structure, Function, and Mechanisms

Cell Membranes & Transport

Overview

Cell membranes are essential structures that define the boundaries of cells and regulate the movement of substances in and out. Their unique composition and properties enable selective transport, communication, and dynamic changes in cell shape and function.

Biological Membranes: Structure and Fluidity

Constituents of Membranes

• Lipids, proteins, and carbohydrates are the main components of biological membranes.

• The fluid mosaic model describes membranes as a bilayer of phospholipids with proteins "floating" within it.

Phospholipids and Bilayer Formation

• Phospholipids are amphipathic molecules with hydrophilic (water-attracting) heads and hydrophobic (water-repelling) tails.

• Bilayers form when fatty acid tails associate with each other, and polar heads face the aqueous environment.

• 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.

• It interacts with polar heads of phospholipids and modulates membrane fluidity.

• Other steroids may function as hormones.

Membrane Fluidity

• Fatty acids influence membrane fluidity; short, unsaturated chains increase fluidity.

• Temperature also affects fluidity; lower temperatures decrease fluidity.

Proteins in Membranes

• All membranes contain proteins; the ratio of proteins to phospholipids varies.

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

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

• Transmembrane proteins span the bilayer and may have domains with different functions on each side.

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

• Proteins can move within the bilayer, but movement may be restricted by attachments to the cytoskeleton.

Carbohydrates in Membranes

• Carbohydrates are located on the outer cell membrane and play a role in cell communication.

• Glycolipids: carbohydrates covalently bonded to lipids.

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

• Proteoglycans: proteins with many and longer carbohydrate chains.

Membrane Dynamics

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

• Cells adhere to one another through interactions between cell surface carbohydrates and proteins.

• Despite similarities, chemical differences exist among membranes of different cell types.

Transport Across Membranes

Selective Permeability

Biological membranes allow some substances to pass while restricting others, maintaining cellular homeostasis.

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 of small, nonpolar, lipid-soluble molecules (e.g., O2, CO2) through the phospholipid bilayer.

• Facilitated diffusion: movement through channel proteins or carrier proteins.

Diffusion

• Random movement of molecules toward equilibrium.

• Speed of diffusion depends on:

  • Diameter of molecules (smaller molecules diffuse faster)

  • Temperature (higher temperatures increase diffusion rate)

  • Concentration gradient (greater gradient increases diffusion rate)

• Polar molecules do not pass through the hydrophobic interior of the membrane easily; amino acids, sugars, and ions require transport proteins.

Osmosis and Osmotic Pressure

• Osmosis: diffusion of water across membranes through special channels, depending on water concentration gradient.

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

$\Pi = cRT$ where $c$ = total solute concentration, $R$ = gas constant, $T$ = absolute temperature.

• Higher solute concentration means lower water concentration.

• If a membrane allows water but not solutes, water moves toward the solution with higher solute concentration.

Comparing Solutions: Tonicity

• Hypertonic: higher solute concentration.

• Isotonic: equal solute concentrations.

• Hypotonic: lower solute concentration.

Effects of Osmosis on Cells

Osmosis can cause cells to swell, shrink, or maintain their shape depending on the tonicity of the surrounding solution.

Summary Table: Membrane Components and Functions

Component	Structure	Function
Phospholipids	Amphipathic molecules with hydrophilic heads and hydrophobic tails	Form bilayer, provide barrier and fluidity
Cholesterol	Steroid with polar and nonpolar regions	Modulates membrane fluidity
Proteins	Peripheral, integral, transmembrane	Transport, signaling, structural support
Carbohydrates	Glycolipids, glycoproteins, proteoglycans	Cell recognition, communication

Key Equations

• Osmotic Pressure: $\Pi = cRT$

Additional info:

• Membrane proteins may be involved in cell signaling, transport, and enzymatic activity.

• Cell adhesion molecules are crucial for tissue formation and immune responses.