Membrane Structure and Function: Study Notes

Membrane Structure and Function

Overview of Membrane Function

The plasma membrane is a dynamic structure that regulates the movement of substances into and out of the cell, maintaining cellular homeostasis. It achieves this through selective permeability, allowing only certain molecules to cross while restricting others.

• Passive Transport: Small molecules move across the membrane without energy input, sometimes with the help of transport proteins.

• Active Transport: Small molecules are moved against their concentration gradients, requiring both energy (usually ATP) and transport proteins.

• Bulk Transport: Large molecules are transported via vesicles in processes called exocytosis (out of the cell) and endocytosis (into the cell).

• Example: Glucose can enter cells via facilitated diffusion (passive transport), while ions like Na+ and K+ are moved by active transport mechanisms.

Main Components of the Plasma Membrane

The plasma membrane is primarily composed of lipids, proteins, and carbohydrates, each contributing to its structure and function.

• Phospholipids: Form the basic bilayer structure; amphipathic molecules with hydrophilic heads and hydrophobic tails.

• Cholesterol: Interspersed within the bilayer, modulates membrane fluidity and stability.

• Proteins: Embedded or attached to the membrane, responsible for transport, signaling, and structural support.

• Carbohydrates: Attached to lipids (glycolipids) or proteins (glycoproteins), function in cell recognition and signaling.

Phospholipids: Structure and Properties

Phospholipids are the fundamental building blocks of the plasma membrane, forming a bilayer that serves as a barrier to most water-soluble substances.

• Amphipathic Nature: Each phospholipid has a hydrophilic (water-attracting) head and two hydrophobic (water-repelling) tails.

• Bilayer Arrangement: Hydrophobic tails face inward, shielded from water, while hydrophilic heads face outward toward the aqueous environment.

• Fluid Mosaic Model: The membrane is described as a mosaic of proteins floating in or on the fluid lipid bilayer.

• Example: The diagram shows two phospholipids with their hydrophilic heads exposed to water and hydrophobic tails sandwiched in the interior.

Cholesterol: Role in Membrane Fluidity

Cholesterol is a steroid molecule found within the plasma membrane, influencing its physical properties.

• Buffering Effect: Cholesterol stabilizes membrane fluidity, preventing it from becoming too rigid at low temperatures or too fluid at high temperatures.

• Temperature Adaptation: At warm temperatures, cholesterol restrains movement of phospholipids; at cool temperatures, it prevents tight packing.

• Example: Animal cell membranes contain cholesterol to maintain optimal fluidity across temperature changes.

Membrane Proteins: Types and Functions

Proteins embedded in the plasma membrane perform a variety of essential functions, contributing to the membrane's versatility.

• Integral Proteins: Span the membrane, often involved in transport and signaling.

• Peripheral Proteins: Attached to the membrane surface, provide structural support and cell signaling.

• Functions:

  • Transport: Move substances across the membrane (channels, carriers).

  • Enzymatic Activity: Catalyze reactions at the membrane surface.

  • Signal Transduction: Relay signals from outside to inside the cell.

  • Cell Recognition: Identify cells to each other via glycoproteins.

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

• Example: Aquaporins are channel proteins that facilitate water transport.

Transport Across the Membrane

Transport mechanisms are classified based on energy requirements and the nature of the substances being moved.

• Passive Transport: Includes simple diffusion, facilitated diffusion, and osmosis. No energy required.

• Active Transport: Requires energy (usually ATP) to move substances against their concentration gradients.

• Bulk Transport: Involves vesicle-mediated processes such as exocytosis and endocytosis for large molecules.

Passive Transport

• Simple Diffusion: Movement of nonpolar molecules (e.g., O2, CO2) directly through the lipid bilayer.

• Facilitated Diffusion: Movement of polar molecules and ions via transport proteins (channels or carriers).

• Osmosis: Diffusion of water across a selectively permeable membrane.

• Equation: $J = -D \frac{dC}{dx}$ Where J is the flux, D is the diffusion coefficient, and dC/dx is the concentration gradient.

Active Transport

• Requires Energy: Moves substances against their concentration gradients.

• Carrier Proteins: Use ATP to change shape and transport molecules.

• Sodium-Potassium Pump: Maintains electrochemical gradients in animal cells.

  • Equation: $3\,\text{Na}^+_{\text{out}} + 2\,\text{K}^+_{\text{in}} + \text{ATP} \rightarrow 3\,\text{Na}^+_{\text{in}} + 2\,\text{K}^+_{\text{out}} + \text{ADP} + \text{P}_i$

Bulk Transport

• Exocytosis: Vesicles fuse with the plasma membrane to release contents outside the cell.

• Endocytosis: Plasma membrane engulfs material, forming vesicles to bring substances into the cell.

• Types of Endocytosis:

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

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

  • Receptor-Mediated Endocytosis: Specific uptake of molecules via receptor proteins.

Summary Table: Membrane Transport Mechanisms

Transport Type	Energy Required?	Protein Involved?	Example
Simple Diffusion	No	No	O2, CO2
Facilitated Diffusion	No	Yes (channel/carrier)	Glucose, ions
Active Transport	Yes (ATP)	Yes (carrier)	Na+/K+ pump
Bulk Transport (Exocytosis/Endocytosis)	Yes (ATP)	Yes (vesicle)	Secretion of hormones, uptake of nutrients

Key Terms and Definitions

• Amphipathic: Molecules with both hydrophilic and hydrophobic regions.

• Selective Permeability: Property of membranes that allows some substances to cross more easily than others.

• Transport Protein: Membrane protein that assists in the movement of substances across the membrane.

• Electrochemical Gradient: Combined effect of concentration gradient and electrical charge difference across a membrane.

• Fluid Mosaic Model: Describes the plasma membrane as a mosaic of proteins floating in a fluid lipid bilayer.

Additional info: These notes expand on the provided slides and images, adding academic context and definitions for clarity and completeness.