Membrane Structure and Function

Overview of the Plasma Membrane

The plasma membrane is a fundamental structure that separates the living cell from its external environment. It plays a critical role in controlling the movement of substances into and out of the cell, maintaining the internal environment necessary for life.

• Selective permeability: The plasma membrane allows some substances to cross more easily than others, enabling the cell to regulate its internal composition.

• Cell-environment interaction: The ability of the cell to discriminate in its chemical exchanges with the environment is essential for survival and function.

Membrane Structure

The structure of biological membranes is based on a phospholipid bilayer with embedded proteins, forming a dynamic and functional barrier.

• Phospholipids: Each phospholipid molecule has a hydrophilic (water-attracting) head and two hydrophobic (water-repelling) tails. In water, phospholipids arrange themselves into a bilayer, with hydrophobic tails facing inward and hydrophilic heads facing outward toward the water.

• Fluid mosaic model: Proposed by S. J. Singer and G. Nicolson in 1972, this model describes the membrane as a mosaic of proteins floating in or on the fluid lipid bilayer. The proteins and lipids can move laterally within the layer, contributing to membrane fluidity.

• Historical models: Earlier, in 1935, Davson and Danielli suggested a model with proteins coating both sides of the bilayer, but this was later revised due to evidence of proteins with hydrophobic regions embedded within the membrane.

• Freeze-fracture technique: This method splits the membrane along the middle of the bilayer, revealing the mosaic arrangement of proteins.

Diagram: The phospholipid bilayer consists of hydrophilic heads facing the aqueous environment and hydrophobic tails sandwiched inside, forming a barrier to most water-soluble substances.

Membrane Fluidity

Membrane molecules are held together by hydrophobic interactions, which are weaker than covalent bonds, allowing for lateral movement of lipids and proteins within the layer.

• Lateral movement: Phospholipids and some proteins can move side-to-side within the membrane, but rarely flip-flop between layers.

• Factors affecting fluidity:

  • Membranes rich in unsaturated fatty acids are more fluid than those with saturated fatty acids, because unsaturated tails prevent tight packing.

  • Cholesterol acts as a fluidity buffer, restraining movement at high temperatures and preventing tight packing at low temperatures.

• Adaptations: Organisms can adjust membrane lipid composition in response to temperature changes (e.g., cold-adapted fish have more unsaturated fatty acids).

Membrane Proteins and Their Functions

Proteins embedded in the membrane determine most of its specific functions. There are two main types:

• Integral proteins: Penetrate the hydrophobic core of the bilayer; many are transmembrane proteins.

• Peripheral proteins: Loosely bound to the membrane surface.

Major functions of membrane proteins:

• Transport: Move specific solutes across the membrane.

• Enzymatic activity: Catalyze metabolic reactions.

• Signal transduction: Relay messages from outside to inside the cell.

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

• Intercellular joining: Connect adjacent cells.

• Attachment: Anchor the membrane to the cytoskeleton and extracellular matrix.

Cell-Cell Recognition

Cells recognize each other by binding to surface molecules, often carbohydrates, on the plasma membrane. This process is crucial for tissue formation and immune response.

• Glycolipids and glycoproteins: Carbohydrates covalently bonded to lipids or proteins serve as identification tags.

• Biological significance: Important in development, immune response, and organ transplant compatibility.

Synthesis and Sidedness of Membranes

Membranes have distinct inside and outside faces, with different lipid and protein compositions. This asymmetry is established during membrane synthesis in the endoplasmic reticulum (ER) and Golgi apparatus.

Selective Permeability of the Membrane

The plasma membrane is selectively permeable, allowing some substances to cross more easily than others.

• Hydrophobic molecules: Nonpolar molecules (e.g., hydrocarbons, O2, CO2) can dissolve in the lipid bilayer and pass through rapidly.

• Hydrophilic molecules: Polar molecules (e.g., sugars, ions) do not cross the membrane easily and require transport proteins.

Membrane Transport Proteins

Transport proteins facilitate the movement of hydrophilic substances across the membrane.

• Channel proteins: Provide hydrophilic tunnels for specific molecules or ions (e.g., aquaporins for water).

• Carrier proteins: Bind to molecules and change shape to shuttle them across the membrane.

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

Diffusion and Passive Transport

Diffusion is the movement of molecules from an area of higher concentration to an area of lower concentration (down their concentration gradient). No energy is required for this process.

• Dynamic equilibrium: When the concentration of molecules is equal on both sides of the membrane, there is no net movement.

• Passive transport: Diffusion of a substance across a biological membrane without energy input.

Osmosis and Water Balance

Osmosis is the diffusion of water across a selectively permeable membrane. Water moves from areas of lower solute concentration to areas of higher solute concentration until equilibrium is reached.

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

• Isotonic solution: Solute concentration is the same inside and outside the cell; no net water movement.

• Hypertonic solution: Solute concentration is higher outside the cell; cell loses water.

• Hypotonic solution: Solute concentration is lower outside the cell; cell gains water.

Effects on cells:

• Animal cells: Swell and may burst in hypotonic solutions (lysed), remain normal in isotonic, and shrink in hypertonic (shriveled).

• Plant cells: Become turgid (firm) in hypotonic, flaccid in isotonic, and undergo plasmolysis in hypertonic solutions.

Facilitated Diffusion

Facilitated diffusion is the passive movement of molecules across the membrane via transport proteins. It does not require energy.

• Channel proteins: Provide corridors for specific molecules or ions (e.g., aquaporins for water, ion channels for ions).

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

• Transport proteins are highly specific for the substances they move.

Active Transport

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

• Performed by specific proteins: These proteins enable cells to maintain internal concentrations of small molecules that differ from those in the environment.

• Sodium-potassium pump: A key example in animal cells, moving Na+ out and K+ in against their gradients.

Equation for sodium-potassium pump:

Ion Pumps and Membrane Potential

Membrane potential is the voltage difference across a membrane, created by differences in ion distribution. It is crucial for processes such as nerve impulse transmission.

• Electrochemical gradient: The combined effect of the ion's concentration gradient and the membrane potential.

• Electrogenic pumps: Transport proteins that generate voltage across a membrane (e.g., sodium-potassium pump in animals, proton pump in plants, fungi, and bacteria).

Bulk Transport: Exocytosis and Endocytosis

Large molecules, such as polysaccharides and proteins, cross the membrane in bulk via vesicles. This process requires energy.

• Exocytosis: Transport vesicles fuse with the plasma membrane, releasing their contents outside the cell. Used for secretion of hormones, neurotransmitters, and cell wall materials.

• Endocytosis: The cell takes in macromolecules by forming vesicles from the plasma membrane. Types include:

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

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

  • Receptor-mediated endocytosis: Specific molecules bind to receptors, triggering vesicle formation.

Summary Table: Effects of Tonicity on Animal and Plant Cells

Key Terms

• Phospholipid bilayer

• Fluid mosaic model

• Selective permeability

• Integral and peripheral proteins

• Osmosis

• Tonicity

• Facilitated diffusion

• Active transport

• Electrochemical gradient

• Exocytosis and endocytosis