Membrane Structure and Function: Study Notes for Introduction to Chemistry

Membrane Structure and Function

Overview

This chapter introduces the structure and function of biological membranes, focusing on the fluid mosaic model, the roles of proteins, and key transport processes. Understanding membrane dynamics is essential for grasping cellular organization and chemical activity.

Fluid Mosaic Model of Cell Membranes

Definition and Components

• Fluid Mosaic Model: Describes the cell membrane as a dynamic structure composed of a phospholipid bilayer with embedded proteins, carbohydrates, and cholesterol.

• Phospholipid Bilayer: The fundamental structure of membranes, consisting of two layers of phospholipids.

• Proteins: Integral and peripheral proteins are distributed throughout the membrane, contributing to its function and flexibility.

• Cholesterol: Stabilizes membrane fluidity, especially in animal cells.

Key Features:

• Individual molecules (lipids and proteins) can move laterally within the layer, giving the membrane a semi-fluid nature.

• Some proteins span the entire membrane (integral/transmembrane), while others are loosely attached to the surface (peripheral).

Example: Glycoproteins in the membrane play roles in immune response and cell recognition.

Membrane Functions

Roles of the Cell Membrane

• Separation: Isolates the cell from its external environment, protecting internal components.

• Selective Permeability: Controls the passage of molecules, allowing only certain substances to cross.

• Compartmentalization: Partitions organelle functions in eukaryotes (e.g., mitochondria have membranous cristae).

• Organization of Chemical Reactions: Provides surfaces for enzyme activity and substrate organization.

Application: In frostbite, ice crystals can puncture cell membranes, leading to cell damage.

Additional info: Membranes also play a role in immune responses, as seen in T-cell activation pathways and the impact of viruses like Covid-19 on immune function.

Phospholipid Structure and Bilayer Formation

Phospholipids

• Structure: Each phospholipid has a polar (hydrophilic) "head" and two nonpolar (hydrophobic) "tails".

• Bilayer Formation: In water, phospholipids spontaneously arrange into a bilayer with heads facing outward toward water and tails inward, away from water.

• Barrier Function: The hydrophobic interior prevents the free passage of hydrophilic molecules.

Example: The bilayer structure is essential for maintaining the integrity and selective permeability of the cell membrane.

Membrane Proteins

Types and Functions

• Integral Proteins: Span the membrane and are amphipathic (contain both hydrophilic and hydrophobic regions).

• Peripheral Proteins: Loosely attached to the external or internal surface of the membrane.

• Glycoproteins: Proteins with carbohydrate chains; important for cell recognition and immune response.

Functions of Membrane Proteins:

• Identification tags/cell recognition

• Enzymatic activity

• Receptors for signaling molecules

• Cell junctions

• Transporters (channels and carriers)

Example: The CFTR protein is an integral membrane protein; defects in CFTR cause cystic fibrosis due to impaired ion transport.

Membrane Transport Mechanisms

Passive Transport

• Diffusion: Movement of molecules from high to low concentration; occurs spontaneously.

• Facilitated Diffusion: Protein channels or carriers help specific molecules cross the membrane down their concentration gradient; no energy required.

• Osmosis: Diffusion of water across a semi-permeable membrane.

Tonicity

• Isotonic: Equal solute concentration inside and outside the cell; no net water movement.

• Hypotonic: Lower solute concentration outside the cell; water enters the cell, which may swell.

• Hypertonic: Higher solute concentration outside the cell; water leaves the cell, which may shrink.

Example: Red blood cells in a hypotonic solution will swell and may burst (lyse); in a hypertonic solution, they will shrink (crenate).

Active Transport

• Definition: Movement of molecules against their concentration gradient, requiring energy (usually ATP).

• Mechanism: Transport proteins change shape via phosphorylation to move solutes.

• Sodium-Potassium Pump: Transports Na+ out and K+ into the cell, maintaining electrochemical gradients.

Types of Transporters:

• Uniport: One substance, one direction

• Symport: Two substances, same direction

• Antiport: Two substances, opposite directions

• Aquaporins: Specialized channels for rapid water transport

Electrogenic Pumps

• Definition: Transporters that generate voltage across membranes by moving ions.

• Example: The Na+-K+ pump creates membrane potential in nerve cells.

Bulk Transport: Endocytosis and Exocytosis

Mechanisms

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

• Endocytosis: The plasma membrane engulfs material to form vesicles inside the cell.

Types of Endocytosis:

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

• Pinocytosis: "Cell drinking"; uptake of fluids and small molecules.

• Receptor-Mediated Endocytosis: Specific uptake of molecules via receptor binding (e.g., cholesterol transport).

Summary Table: Membrane Transport Mechanisms

Transport Type	Energy Required	Direction	Example
Passive Diffusion	No	Down gradient	O2 diffusion
Facilitated Diffusion	No	Down gradient	Glucose via carrier protein
Active Transport	Yes (ATP)	Against gradient	Na+/K+ pump
Bulk Transport (Endocytosis/Exocytosis)	Yes	Variable	Phagocytosis, secretion of hormones

Additional info: These notes expand on the original slides by providing definitions, examples, and equations for key processes, ensuring a self-contained study guide for exam preparation.