Chapter 7- Membrane Structure and Function

Chapter 7- Membrane Structure and Function

Introduction

The plasma membrane is a fundamental component of all living cells, serving as a selective barrier that regulates the movement of substances in and out of the cell. Its structure and function are essential for maintaining cellular homeostasis, communication, and energy transduction.

Membrane Structure

Phospholipid Bilayer

• Phospholipids are amphipathic molecules, meaning they have both hydrophilic (water-loving) heads and hydrophobic (water-fearing) tails.

• In aqueous environments, phospholipids spontaneously arrange into a bilayer, with hydrophobic tails facing inward and hydrophilic heads facing outward toward water.

• This bilayer forms the basic structure of the plasma membrane, providing fluidity and flexibility.

• Example: The image above shows a molecular model of a phospholipid bilayer with embedded proteins and other molecules.

Fluid Mosaic Model

• The Fluid Mosaic Model describes the plasma membrane as a mosaic of protein molecules bobbing in a fluid bilayer of phospholipids.

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

• Cholesterol molecules are interspersed within the bilayer, modulating fluidity and stability.

• Additional info: Membrane fluidity is temperature-dependent and influenced by the proportion of saturated vs. unsaturated fatty acid tails.

Membrane Proteins

Types of Membrane Proteins

• Integral proteins are embedded within the lipid bilayer and often span the entire membrane.

• Peripheral proteins are attached to the membrane surface, either to the lipid bilayer or to integral proteins.

• Different cell types have unique collections of membrane proteins, determining their specific functions.

Functions of Membrane Proteins

• Transport: Facilitate the movement of substances across the membrane (channels, carriers).

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

• Signal transduction: Transmit signals from the external environment to the cell interior.

• Cell-cell recognition: Allow cells to identify each other, important for immune response and tissue formation.

• Intercellular joining: Connect adjacent cells through junctions.

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

• Example: Glycoproteins on the cell surface determine blood types (e.g., ABO system).

Membrane Permeability

Selective Permeability

• The plasma membrane is selectively permeable, allowing some molecules to cross more easily than others.

• Nonpolar molecules (e.g., hydrocarbons, O2, CO2, lipids) can diffuse through the bilayer without assistance.

• Polar molecules (e.g., glucose, ions, water) require transport proteins to cross the membrane.

• The lipid bilayer acts as a gatekeeper, while proteins regulate specific transport processes.

Transport Across Membranes

Passive Transport

• Passive transport is the movement of substances across the membrane without energy input.

• Includes simple diffusion, facilitated diffusion, and osmosis.

• Simple diffusion: Molecules move from areas of higher concentration to lower concentration, down their concentration gradient.

• Facilitated diffusion: Transport proteins (channels, carriers) help polar or charged molecules move down their concentration gradient.

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

• Equation: $J = -D \frac{dC}{dx}$ (Fick's law of diffusion, where J is flux, D is diffusion coefficient, and dC/dx is concentration gradient)

Osmosis and Water Balance

• Water moves from areas of lower solute concentration to higher solute concentration.

• Animal cells in hypotonic solutions may lyse (burst), in isotonic solutions remain normal, and in hypertonic solutions shrivel.

• Plant cells become turgid (firm) in hypotonic solutions, flaccid in isotonic, and plasmolyzed in hypertonic solutions.

• Osmoregulation is the control of water balance to prevent excessive uptake or loss.

Active Transport

• Active transport requires energy (usually ATP) to move substances against their concentration gradient.

• Transport proteins (pumps) change shape to move specific molecules across the membrane.

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

• Equation: $ATP \rightarrow ADP + P_i$ (ATP hydrolysis provides energy for active transport)

Bulk Transport

• Large molecules (e.g., polysaccharides, proteins) are transported via vesicles in processes called endocytosis and exocytosis.

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

• Endocytosis: The membrane engulfs material to bring it into the cell. Includes phagocytosis (cell eating), pinocytosis (cell drinking), and receptor-mediated endocytosis (specific uptake).

• Example: White blood cells use phagocytosis to engulf pathogens.

Table: Comparison of Membrane Transport Mechanisms

Transport Type	Energy Required?	Direction Relative to Gradient	Example
Simple Diffusion	No	Down	O2 crossing membrane
Facilitated Diffusion	No	Down	Glucose via carrier protein
Osmosis	No	Down	Water via aquaporin
Active Transport	Yes (ATP)	Up	Na+/K+ pump
Bulk Transport (Endo/Exocytosis)	Yes	Variable	Secretion of hormones

Summary

• The plasma membrane's structure enables selective permeability, crucial for cellular function.

• Membrane proteins perform diverse roles, including transport, signaling, and recognition.

• Transport mechanisms include passive (diffusion, osmosis) and active (pumps, bulk transport) processes.

• Understanding membrane dynamics is essential for topics such as cell signaling, metabolism, and physiology.