Chapter 7: Membrane Structure and Function – Study Notes

Membrane Structure and Function

Overview: Life at the Edge

The plasma membrane is a fundamental structure that separates the living cell from its external environment and regulates the movement of substances into and out of the cell. This selective barrier is essential for maintaining the internal conditions necessary for life.

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

• Cellular discrimination: The ability of the cell to regulate chemical exchanges with its environment is crucial for survival and function.

Cellular Membranes: Fluid Mosaics of Lipids and Proteins

Cellular membranes are primarily composed of lipids and proteins, arranged in a dynamic and flexible structure known as the fluid mosaic model.

• Phospholipids: The most abundant lipid in the plasma membrane. Their unique structure allows them to form a bilayer spontaneously.

• Amphipathic molecules: Phospholipids have both hydrophilic (water-attracting) heads and hydrophobic (water-repelling) tails.

• Bilayer formation: Hydrophilic heads face outward toward water, while hydrophobic tails are shielded inside, away from water.

• Fluid mosaic model: The membrane is a fluid bilayer of phospholipids with a mosaic of various proteins embedded or attached, allowing lateral movement of both lipids and proteins.

Example: The diagram shows phospholipids forming a bilayer, with hydrophilic heads facing water and hydrophobic tails facing inward.

The Fluidity of Membranes

Membranes are not static; their fluidity is essential for proper function, including the movement of proteins and lipids within the bilayer.

• Lateral movement: Phospholipids and some proteins can move sideways within the membrane.

• Experimental evidence: The Frye and Edidin (1970) experiment demonstrated that membrane proteins can move within the plane of the membrane by fusing mouse and human cells and observing the mixing of fluorescently labeled proteins.

• Temperature effects: Membrane fluidity decreases at lower temperatures as phospholipids pack more tightly and the membrane becomes more viscous.

• Fatty acid saturation: Membranes rich in unsaturated fatty acids are more fluid than those rich in saturated fatty acids.

• Cholesterol: Acts as a "fluidity buffer," reducing membrane fluidity at moderate temperatures and preventing tight packing at low temperatures.

Additional info: The degree of membrane fluidity affects processes such as membrane protein function, cell signaling, and the ability of cells to adapt to temperature changes.

Membrane Proteins and Their Functions

Proteins embedded in the membrane are responsible for most of its specific functions.

• Integral proteins: Penetrate the hydrophobic core of the membrane; many are transmembrane proteins that span the entire membrane.

• Peripheral proteins: Loosely bound to the surface of the membrane.

• Transmembrane proteins: Have hydrophobic regions that interact with the membrane's interior and hydrophilic regions that interact with the aqueous environment on either side.

Functions of membrane proteins include:

• Transport: Moving substances across the membrane.

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

• Signal transduction: Relaying signals from outside to inside the cell.

• Cell-cell recognition: Identifying other cells.

• Intercellular joining: Connecting adjacent cells.

• Attachment: Linking to the cytoskeleton and extracellular matrix.

The Role of Membrane Carbohydrates in Cell Recognition

Membrane carbohydrates are crucial for cell-cell recognition, allowing cells to distinguish themselves from others.

• Glycoproteins and glycolipids: Carbohydrates covalently bonded to proteins or lipids on the cell surface serve as identification tags.

• Immune system: Uses these tags to differentiate between self and foreign cells.

• Blood types: Determined by specific carbohydrates on the surface of red blood cells.

Example: The A, B, AB, and O blood types are defined by the presence of different sugars attached to cell-surface proteins.

Synthesis and Sidedness of Membranes

The asymmetric distribution of proteins, lipids, and carbohydrates in the plasma membrane is established during membrane synthesis in the endoplasmic reticulum (ER) and Golgi apparatus.

• Asymmetry: Each side of the membrane has a unique composition, which is functionally important.

Membrane Structure Results in Selective Permeability

Selective Permeability

The plasma membrane's structure allows it to regulate the passage of substances, maintaining the cell's internal environment.

• Permeable to: Small, nonpolar molecules (e.g., oxygen, carbon dioxide, steroids) can dissolve in the lipid bilayer and cross rapidly.

• Impermeable to: Large, polar molecules (e.g., sugars) and ions cannot cross the membrane easily without assistance.

Passive Transport: Diffusion Across a Membrane

Passive transport is the movement of substances across a membrane without the input of cellular energy. The most common form is diffusion.

• Diffusion: The net movement of molecules from an area of higher concentration to an area of lower concentration, down their concentration gradient.

• Concentration gradient: The difference in concentration of a substance across a space or membrane; represents potential energy for diffusion.

• Equilibrium: Reached when the concentration of the substance is equal on both sides of the membrane, and net movement stops.

• Independence: Each substance diffuses down its own concentration gradient, independent of other substances.

Equation for diffusion rate (Fick's Law):

$J = -D \frac{dC}{dx}$

• Where J is the rate of diffusion, D is the diffusion coefficient, and $\frac{dC}{dx}$ is the concentration gradient.

Additional info: Passive transport is essential for processes such as gas exchange, nutrient uptake, and waste removal in cells.