Chapter 7: Membrane Structure and Function – Study Notes

Membrane Structure and Function

Overview: Life at the Edge

The plasma membrane is a critical boundary that separates the living cell from its external environment and regulates the movement of substances into and out of the cell. This selective regulation is essential for maintaining cellular homeostasis and supporting life processes.

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

• Fundamental role: The ability to discriminate in chemical exchanges is vital for cell survival and function.

Cellular Membranes: Fluid Mosaics of Lipids and Proteins

Cellular membranes are dynamic structures composed primarily of lipids and proteins. The arrangement and movement of these molecules are described by the fluid mosaic model.

• Phospholipids: The most abundant lipid in the plasma membrane. Their molecular structure drives the formation of a bilayer.

• Amphipathic nature: 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 consists of a phospholipid bilayer with proteins embedded or attached, creating a mosaic of components that can move laterally within the layer.

Example: The diagram shows phospholipids arranged in a bilayer, with proteins interspersed throughout, illustrating the fluid mosaic model.

Membrane Fluidity

Membrane fluidity is crucial for proper membrane function, affecting the movement of lipids and proteins within the bilayer.

• Phospholipid movement: Lipids and proteins can move laterally within the membrane, contributing to its fluid nature.

• 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: Lower temperatures decrease fluidity, causing membranes to become more viscous and potentially solidify.

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

• Cholesterol: Cholesterol acts as a "fluidity buffer," reducing fluidity at moderate temperatures and preventing solidification at low temperatures.

Additional info: The degree of membrane fluidity is adapted in organisms living in different temperature environments. For example, bacteria in cold environments have more unsaturated fatty acids to maintain fluidity.

Membrane Proteins and Their Functions

Proteins embedded in the membrane are responsible for most of its specific functions. They are classified based on their association with the membrane.

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

• Peripheral proteins: Bound to the surface of the membrane, often attached to integral proteins or the cytoskeleton.

• Transmembrane proteins: Integral proteins that span the membrane, with hydrophobic regions interacting with fatty acid tails and hydrophilic regions exposed to aqueous environments.

Functions of membrane proteins:

• 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 interior.

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

• Intercellular joining: Connect adjacent cells.

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

Role of Membrane Carbohydrates in Cell Recognition

Membrane carbohydrates play a key role in cell-cell recognition, which is essential for immune function and tissue organization.

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

• Example: Blood type antigens (A, B, AB, O) are determined by specific sugars attached to cell-surface proteins on red blood cells.

Synthesis and Sidedness of Membranes

Membranes have an asymmetric distribution of proteins, lipids, and carbohydrates, established during their synthesis in the endoplasmic reticulum (ER) and Golgi apparatus.

• Asymmetry: The two sides of the membrane differ in composition and function.

• Membrane assembly: The ER and Golgi apparatus are responsible for building and modifying membrane components.

Membrane Structure Results in Selective Permeability

Selective Permeability

The plasma membrane's structure enables it to regulate the transport of molecules, allowing some to cross more easily than others.

• Nonpolar molecules: Such as steroids, carbon dioxide, and oxygen, can dissolve in the lipid bilayer and pass through rapidly.

• Polar molecules: Such as sugars, do not cross the membrane easily due to the hydrophobic core.

Passive Transport: Diffusion

Passive transport is the movement of substances across a membrane without energy investment by the cell. The most common form is diffusion.

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

• Concentration gradient: The difference in concentration across a space; molecules diffuse down their own gradient.

• Equilibrium: Reached when the net movement of molecules is equal in both directions.

• Independence: Each substance diffuses independently of others present.

Equation:

$J = -D \frac{dC}{dx}$ Where: J = flux (rate of diffusion) D = diffusion coefficient \frac{dC}{dx} = concentration gradient

Additional info: Passive transport is driven by the potential energy stored in the concentration gradient and does not require cellular energy (ATP).

Summary Table: Types of Membrane Proteins

Type	Location	Function
Integral	Embedded in the membrane; may span the bilayer	Transport, signal transduction, enzymatic activity
Peripheral	Attached to membrane surface	Cell signaling, structural support
Transmembrane	Span the entire membrane	Transport, cell recognition