Cell Membrane Structure

Plasma Membrane

The plasma membrane is the boundary that separates the living cell from its external environment. It is essential for maintaining cellular integrity and regulating the movement of substances into and out of the cell.

• Selective permeability: The membrane allows certain molecules to pass while restricting others.

• Hydrophobic interior: The core of the membrane is hydrophobic, preventing free passage of polar molecules.

Fluid Mosaic Model

The fluid mosaic model describes the structure of biological membranes as a dynamic arrangement of lipids, proteins, and carbohydrates. These components are not static but move laterally within the layer, contributing to membrane fluidity and function.

• Lipids (40-60%): Primarily phospholipids arranged in a bilayer.

• Proteins (30-50%): Embedded or attached to the lipid bilayer, serving various functions.

• Carbohydrates (5-10%): Often covalently bonded to lipids (glycolipids) or proteins (glycoproteins).

Membrane fluidity is influenced by:

• Saturated vs. unsaturated fatty acids: Unsaturated fatty acids increase fluidity; saturated fatty acids decrease it.

• Cholesterol: Modulates fluidity by preventing tight packing at low temperatures and restraining movement at high temperatures.

Adaptations: Organisms such as winter wheat and fish modify membrane composition to maintain fluidity under cold conditions.

Membrane Proteins

Proteins are crucial for membrane function and are classified based on their association with the lipid bilayer:

• Peripheral proteins: Bound to the surface of the membrane.

• Integral proteins: Embedded within the hydrophobic core; those spanning the membrane are called transmembrane proteins and are often amphipathic (having both hydrophobic and hydrophilic regions).

Functions of Membrane Proteins

Major Functions

Membrane proteins perform several essential functions:

• Transport: Facilitate movement of substances across the membrane.

• Enzymatic activity: Catalyze specific reactions at the membrane surface.

• Signal transduction: Relay signals from the external environment to the cell's interior.

• Cell-cell recognition: Allow cells to identify each other.

• Intercellular joining: Connect adjacent cells.

• Attachment to cytoskeleton and extracellular matrix: Maintain cell shape and stabilize membrane location.

Membrane Transport Mechanisms

Selective Permeability

The plasma membrane regulates molecular traffic based on:

• Polarity: Nonpolar molecules cross more easily than polar ones.

• Size: Small molecules pass more readily than large ones.

• Charge: Ions and charged molecules require specific transport proteins.

Passive Transport

Passive transport is the movement of substances across the membrane without energy input. It includes:

• Diffusion: Molecules move from areas of high concentration to low concentration, filling available space. Example: Sugar dissolving in coffee.

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

Key equation:

Tonicity and Water Balance

Tonicity refers to the ability of a surrounding solution to cause a cell to gain or lose water. The three main types are:

Facilitated Diffusion

Facilitated diffusion is a type of passive transport where transport proteins (channel or carrier proteins) help specific molecules cross the membrane without energy input.

• Channel proteins: Form pores for molecules (e.g., aquaporins for water).

• Carrier proteins: Change shape to move molecules (e.g., glucose transporter).

Active Transport

Active transport moves substances against their concentration gradients and requires energy, usually from ATP. It is performed by carrier proteins.

• Sodium-potassium pump: Moves Na+ out and K+ into the cell, maintaining electrochemical gradients.

Key equation:

Membrane Potential and Electrochemical Gradient

The membrane potential is the voltage difference across the membrane, resulting from the distribution of ions. The electrochemical gradient combines:

• Chemical force: Ion concentration gradient.

• Electrical force: Effect of membrane potential on ion movement.

Electrogenic pumps (e.g., sodium-potassium pump in animals, proton pump in plants, fungi, bacteria) generate membrane potential.

Cotransport

Cotransport occurs when the active transport of one solute indirectly drives the transport of another solute (e.g., sucrose transport coupled with proton movement).

Bulk Transport: Exocytosis and Endocytosis

Exocytosis

Exocytosis is the process by which cells export large molecules (such as proteins and polysaccharides) by vesicle fusion with the plasma membrane.

• Example: Secretion of hormones and neurotransmitters.

Endocytosis

Endocytosis is the process by which cells import large molecules or particles by engulfing them in vesicles. There are three main types:

• Phagocytosis: "Cellular eating"; cell engulfs large particles or cells.

• Pinocytosis: "Cellular drinking"; cell takes in extracellular fluid and dissolved solutes.

• Receptor-mediated endocytosis: Specific molecules are taken in after binding to receptors on the cell surface.

Summary Table: Membrane Transport Mechanisms

Key Terms and Definitions

• Phospholipid: Amphipathic molecule forming the basic structure of biological membranes.

• Glycolipid/Glycoprotein: Lipid or protein with covalently attached carbohydrate.

• Amphipathic: Having both hydrophobic and hydrophilic regions.

• Osmosis: Diffusion of water across a membrane.

• Tonicity: Effect of a solution on cell water balance.

• Electrochemical gradient: Combined effect of concentration and electrical gradients on ion movement.

Example: The sodium-potassium pump maintains high K+ and low Na+ inside animal cells, crucial for nerve impulse transmission.

Additional info: Some context and terminology were expanded for clarity and completeness, including definitions and summary tables for exam preparation.