Membrane Structure and Function

Overview

This section explores the structure and function of biological membranes, focusing on the physical and chemical principles that govern their behavior. Key topics include the laws of thermodynamics, the role of hydrophobic interactions, lipid composition, the fluid mosaic model, and mechanisms of transport across membranes.

Laws of Thermodynamics

First and Second Laws

• First Law of Thermodynamics: Energy can neither be created nor destroyed in a closed system; it can only be converted from one form to another.

• Second Law of Thermodynamics: In a closed system, the total disorder (entropy) will always increase over time unless energy is input to maintain order.

Entropy is a measure of the disorder of a system. Spontaneous reactions tend to increase entropy.

• Example: A tidy room (low entropy) becomes messy (high entropy) over time without effort (energy input).

Hydrophobic Interactions and Thermodynamics

Role in Biological Systems

• Hydrophobic interactions are non-covalent forces that arise because water molecules form ordered cages around nonpolar (hydrophobic) substances, decreasing entropy.

• When hydrophobic molecules cluster together, fewer water molecules are needed to form cages, increasing the entropy of the system.

• This effect is a driving force in the formation of biological membranes and protein folding.

• Example: Oil droplets in water coalesce to minimize the surface area exposed to water, increasing the entropy of the surrounding water molecules.

Lipids and Fatty Acids

Structure and Properties

• Lipids are a diverse group of hydrophobic molecules, including fats, oils, and membrane lipids.

• Fatty acids are carboxylic acids with long hydrocarbon chains (typically 12–18 carbons, always even-numbered in nature).

• Fatty acids can be saturated (no double bonds, straight chains) or unsaturated (one or more double bonds, causing kinks).

• Triglycerides (Triacylglycerols): Formed by esterification of three fatty acids to glycerol; serve as energy storage molecules.

Notation: Fatty acids are denoted as number of carbons:number of double bonds (e.g., palmitic acid is 16:0).

• Saturated fats are more likely to be solid at room temperature due to straight chains that pack tightly.

• Unsaturated fats (especially cis-unsaturated) are more likely to be liquid at room temperature due to kinks that prevent tight packing.

Cell Membrane Structure

Fluid Mosaic Model

• The fluid mosaic model describes the membrane as a dynamic structure with proteins floating in or on a fluid lipid bilayer.

• Phospholipids are amphipathic molecules with hydrophilic heads and hydrophobic tails, forming a bilayer in aqueous environments.

• Membranes also contain proteins and carbohydrates, which are not randomly distributed but often form functional groups.

Main benefits of membranes:

• Increase surface area for reactions

• Separate the cell or organelles from the environment

• Allow for specialization and compartmentalization

Membrane Fluidity

Factors Affecting Fluidity

• Membranes are held together by weak hydrophobic interactions and Van der Waals forces.

• Unsaturated fatty acid tails prevent tight packing, increasing fluidity.

• Saturated fatty acid tails pack tightly, making the membrane more viscous (less fluid).

• Cholesterol acts as a fluidity buffer in animal cell membranes, reducing fluidity at moderate temperatures and preventing solidification at low temperatures.

Membrane Proteins and Amino Acids

Types and Distribution

• Integral proteins span the membrane and often have hydrophobic amino acids in the membrane-spanning regions.

• Peripheral proteins are attached to the membrane surface and are more likely to have hydrophilic amino acids exposed to the aqueous environment.

• Membrane proteins serve various functions: transport, enzymatic activity, signal transduction, cell-cell recognition, intercellular joining, and attachment to the cytoskeleton or extracellular matrix.

Membrane Carbohydrates and Cell Recognition

Role in Cell-Cell Recognition

• Cells recognize each other by binding to surface molecules, often carbohydrates, on the plasma membrane.

• Carbohydrates may be attached to lipids (glycolipids) or proteins (glycoproteins).

• The diversity of surface carbohydrates enables cells to function as markers for cell recognition.

Transport Across the Membrane

Types of Transport

• Passive transport: Movement of substances across the membrane without energy input (includes simple diffusion, facilitated diffusion, and osmosis).

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

• Bulk transport: Large molecules (e.g., proteins, polysaccharides) are transported via vesicles in processes called endocytosis (into the cell) and exocytosis (out of the cell).

Selective Permeability

• The plasma membrane allows some substances to cross more easily than others.

• Nonpolar molecules (e.g., hydrocarbons, O2, CO2) dissolve in the lipid bilayer and cross easily.

• Polar molecules (e.g., ions, sugars, water) cross slowly or require transport proteins.

Facilitated Diffusion

• Transport proteins (channel and carrier proteins) speed the passive movement of molecules across the membrane.

• Each transport protein is specific for the substance it moves.

Active Transport and Cotransport

• Active transport uses energy to move solutes against their concentration gradients.

• Cotransport: The downhill diffusion of one solute is coupled to the uphill transport of another solute against its concentration gradient.

Osmosis

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

• Water moves from regions of lower solute concentration to regions of higher solute concentration until equilibrium is reached.

Water Balance and Tonicity

Effects on Cells

• Tonicity is the ability of a surrounding solution to cause a cell to gain or lose water.

• If the solution is hypertonic (higher solute concentration than the cell), water leaves the cell, causing it to shrink.

• If the solution is hypotonic (lower solute concentration than the cell), water enters the cell, causing it to swell.

• If the solution is isotonic, there is no net movement of water.

Summary Table: Types of Membrane Transport