Membrane Structure and Function – Study Notes

Membrane Structure and Function

Introduction

The plasma membrane is a fundamental component of all cells, providing a selective barrier that regulates the movement of substances into and out of the cell. Understanding its structure and function is essential for comprehending many cellular processes.

Major Ways the Plasma Membrane Regulates Traffic

Overview of Membrane Transport

• Passive Transport: Movement of small molecules across the membrane without energy input, either by diffusion or with the help of transport proteins.

• Active Transport: Movement of small molecules against their concentration gradient, requiring both energy (usually from ATP) and a transport protein.

• Bulk Transport (Exocytosis and Endocytosis): Movement of large molecules into (endocytosis) or out of (exocytosis) the cell via vesicles.

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

Structure of Cellular Membranes

Phospholipids and Amphipathic Nature

• Phospholipids: The main component of membranes, consisting of hydrophilic (water-attracting) heads and hydrophobic (water-repelling) tails.

• Amphipathic: Molecules that have both hydrophilic and hydrophobic regions. Phospholipids arrange themselves into a bilayer, with hydrophobic tails facing inward and hydrophilic heads facing outward.

Fluid Mosaic Model

• The membrane is described as a "fluid mosaic" because it is a flexible layer made of lipid molecules interspersed with proteins.

• Proteins and lipids can move laterally within the layer, contributing to membrane fluidity.

Membrane Fluidity

• Saturated Fatty Acids: Make the membrane less fluid (more viscous) because their straight tails pack tightly together.

• Unsaturated Fatty Acids: Increase membrane fluidity due to kinks in their tails, preventing tight packing.

• Cholesterol: Acts as a "fluidity buffer" in animal cells, reducing fluidity at moderate temperatures and preventing solidification at low temperatures.

Example: At cold temperatures, membranes with more unsaturated fatty acids remain fluid, while those with more saturated fatty acids may become rigid.

Membrane Proteins and Their Functions

Types of Membrane Proteins

• Peripheral Proteins: Loosely bound to the membrane surface.

• Integral Proteins: Penetrate the hydrophobic core; those that span the membrane are called transmembrane proteins.

Functions of Membrane Proteins

• Transport of substances

• Enzymatic activity

• Signal transduction

• Cell-cell recognition

• Intercellular joining

• Attachment to the cytoskeleton and extracellular matrix

Role of Membrane Carbohydrates

• Carbohydrates are attached to proteins (glycoproteins) or lipids (glycolipids) on the extracellular surface of the membrane.

• They function as markers for cell recognition and play a role in immune response.

Selective Permeability of the Membrane

Permeability of the Lipid Bilayer

• Hydrophobic (nonpolar) molecules: Such as O2 and CO2, can dissolve in the lipid bilayer and pass through easily.

• Hydrophilic (polar) molecules: Such as ions and sugars, cannot pass through the hydrophobic core easily and require transport proteins.

Transport Proteins

• 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.

Passive Transport

Diffusion

• Movement of molecules from an area of higher concentration to an area of lower concentration (down their concentration gradient).

• No energy input is required.

$\text{Diffusion rate} \propto \text{concentration gradient}$

Osmosis

• Diffusion of water across a selectively permeable membrane.

• Water moves toward the side with higher solute concentration.

Effects of Tonicity on Cells

Solution Type	Animal Cell	Plant Cell
Hypotonic	Lysed (bursts)	Turgid (normal)
Isotonic	Normal	Flaccid
Hypertonic	Shriveled	Plasmolyzed

Facilitated Diffusion

• Passive transport aided by proteins (channel or carrier proteins).

• Does not require energy; moves substances down their concentration gradient.

Active Transport

Mechanism and Examples

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

• Performed by specific carrier proteins (e.g., sodium-potassium pump).

$\text{Na}^+/\text{K}^+\ \text{pump:} \ 3\ \text{Na}^+\ \text{out},\ 2\ \text{K}^+\ \text{in},\ \text{using ATP}$

Membrane Potential and Electrochemical Gradients

• Membrane potential is the voltage difference across a membrane, created by the unequal distribution of ions.

• Electrochemical gradients drive the movement of ions across membranes.

• Electrogenic pumps (e.g., sodium-potassium pump in animals, proton pump in plants) help maintain membrane potential.

Co-Transport (Coupled Transport)

• Active transport of one solute indirectly drives the transport of another solute.

• Example: In plants, the proton gradient created by the proton pump is used to co-transport sucrose into cells.

Bulk Transport: Exocytosis and Endocytosis

Exocytosis

• Vesicles fuse with the plasma membrane to release large molecules (e.g., proteins, polysaccharides) outside the cell.

• Example: Secretion of insulin by pancreatic cells.

Endocytosis

• Cell takes in macromolecules by forming vesicles from the plasma membrane.

• Types of endocytosis:

  • Phagocytosis: "Cell eating"; cell engulfs large particles.

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

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

Example: Uptake of cholesterol via LDL receptors in human cells.

Summary Table: Types of Membrane 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	Yes (ATP)	Varies	Exocytosis, Endocytosis