Membrane Structure and Function (Chapter 7) – General Biology Study Notes

Membrane Structure and Function

Introduction to Cellular Membranes

The plasma membrane is a fundamental structure that defines the boundary of the cell, separating its internal environment from the external surroundings. It plays a crucial role in regulating the movement of substances into and out of the cell, maintaining homeostasis.

• Plasma membrane: The outer boundary of the cell, composed primarily of lipids and proteins.

• Selectively permeable: The membrane allows certain substances to pass while restricting others, enabling the cell to control its internal composition.

• Key functions: Protection, communication, transport, and structural support.

Fluid Mosaic Model of Membrane Structure

The fluid mosaic model describes the dynamic nature of the plasma membrane, where proteins float in or on a fluid lipid bilayer. This model explains both the flexibility and the selective permeability of the membrane.

• Phospholipid bilayer: The basic structure consists of two layers of phospholipids with hydrophilic (water-attracting) heads facing outward and hydrophobic (water-repelling) tails facing inward.

• Proteins: Embedded within the bilayer, proteins serve various functions such as transport, signaling, and structural support.

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

• Weak hydrophobic interactions: These interactions hold the membrane together but allow for flexibility and movement.

Types of Membrane Proteins

Membrane proteins are essential for the diverse functions of the plasma membrane. They are classified based on their association with the lipid bilayer.

• Peripheral proteins: Bound to the surface of the membrane; often involved in signaling or structural support.

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

• Functions of membrane proteins:

  • Transport of molecules

  • Enzymatic activity

  • Signal transduction

  • Cell-cell recognition

  • Intercellular joining

  • Attachment to the cytoskeleton and extracellular matrix (ECM)

Selective Permeability of the Membrane

The plasma membrane's structure results in selective permeability, allowing the cell to regulate its internal environment by controlling the passage of substances.

• Hydrophobic (nonpolar) molecules: Such as hydrocarbons, can dissolve in the lipid bilayer and pass through rapidly.

• Hydrophilic (polar) molecules and ions: Do not cross the membrane easily and require transport proteins.

• Transport proteins: Facilitate the movement of hydrophilic substances across the membrane.

Transport Mechanisms Across the Membrane

Cells utilize various mechanisms to move substances across the plasma membrane, depending on the nature of the substance and the direction of movement.

• Passive transport: Movement of substances down their concentration gradient without energy input. Includes simple diffusion and facilitated diffusion.

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

• Bulk transport: Movement of large molecules via vesicles, including exocytosis (out of the cell) and endocytosis (into the cell).

Passive Transport

• Diffusion: The tendency of molecules to spread out evenly into available space. Molecules move from high to low concentration.

• Facilitated diffusion: Passive transport aided by proteins, such as channel and carrier proteins.

• Osmosis: The diffusion of water across a selectively permeable membrane. Water moves from regions of lower solute concentration to higher solute concentration.

Equation for diffusion rate:

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

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

Effects of Osmosis on Cells

• Hypotonic solution: Lower solute concentration outside the cell; water enters the cell, which may cause it to swell or burst (lysed in animal cells, turgid in plant cells).

• Isotonic solution: Equal solute concentration; no net movement of water, cell remains normal.

• Hypertonic solution: Higher solute concentration outside; water leaves the cell, causing it to shrink (shriveled in animal cells, plasmolyzed in plant cells).

Facilitated Diffusion

• Channel proteins: Provide corridors for specific molecules or ions to cross the membrane.

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

• Gated channels: Open or close in response to stimuli, allowing regulated transport.

Active Transport

• Requires energy: Usually from ATP hydrolysis.

• Moves substances against their concentration gradients.

• Example: Sodium-potassium pump ($Na^+/K^+$ pump) maintains electrochemical gradients in animal cells.

Equation for ATP hydrolysis:

$ATP + H_2O \rightarrow ADP + P_i + \text{energy}$

Cotransport

• Coupled transport: A membrane protein simultaneously transports two substances, often using the gradient of one to drive the movement of another (e.g., sucrose-H+ cotransporter).

Bulk Transport: Exocytosis and Endocytosis

• Exocytosis: Large molecules are secreted when vesicles fuse with the plasma membrane.

• Endocytosis: Large molecules are taken in when the plasma membrane pinches inward, forming a vesicle.

• Types of endocytosis:

  • Phagocytosis: "Cell eating"; uptake of large particles.

  • Pinocytosis: "Cell drinking"; uptake of fluids and dissolved solutes.

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

Summary Table: Types of Membrane Transport

Transport Type	Energy Required?	Direction	Example
Simple Diffusion	No	Down gradient	O2, CO2
Facilitated Diffusion	No	Down gradient	Glucose via carrier protein
Active Transport	Yes (ATP)	Against gradient	Na+/K+ pump
Bulk Transport (Exocytosis/Endocytosis)	Yes	Bulk movement	Secretion of hormones, uptake of nutrients

Example: The sodium-potassium pump in animal cells uses ATP to move sodium ions out and potassium ions in, maintaining essential gradients for nerve impulse transmission.

Additional info: The notes above expand on fragmented points and diagrams, providing academic context and definitions for clarity and completeness.