Module 2: Cell Membrane

Module 2: Cell Membrane Notes

Learning Objectives

• Describe the structure of cell membranes.

• List components and describe their basic functions.

• Describe the movement of molecules across a membrane.

• Describe passive transport and provide examples.

The Plasma Membrane as a Fluid Mosaic

• The plasma membrane, common to all cells, is often described as a fluid mosaic because of its dynamic, ever-changing composition, similar to the structures observed in a mosaic artwork.

Plasma Membrane Functions

• Separation: Separates the internal cytoplasm from the cell's external environment.

• Compartmentalization: Allows incompatible chemical reactions to occur simultaneously within the cell.

Components of the Plasma Membrane

1. Lipid Component (Phospholipid Bilayer):

   • Forms the fundamental structure of the membrane.

   • Structure:

     • Hydrophilic heads: Located at the surfaces, facing the aqueous environments (cytoplasm and extracellular fluid).

     • Hydrophobic tails: Make up the interior of the membrane, creating a nonpolar core.

2. Membrane Proteins:

   • Integral Proteins: Partially or wholly embedded (transmembrane) in the membrane; they can span the entire lipid bilayer.

   • Peripheral Proteins: Found on the inner membrane surface, typically associated with integral proteins or the lipid heads.

3. Cholesterol:

   • A lipid interspersed within the phospholipid bilayer.

   • Function: Affects the fluidity and stability of the membrane.

4. Carbohydrate Chains:

   • Found on the exterior surface of the cell, often attached to lipids or proteins.

   • Glycolipids: Carbohydrate chains attached to lipids.

   • Glycoproteins: Carbohydrate chains attached to proteins.

   • Function: Play roles in cell recognition and adhesion.

Types of Membrane Proteins and Their Functions

1. Channel Proteins:

   • Function: Allow specific molecules or ions to cross the plasma membrane freely through a channel or pore in the protein.

   • Example: A faulty chloride (Cl^{-}) channel is responsible for Cystic Fibrosis, an inherited disorder where thick mucus collects in airways and in pancreatic and liver ducts due to impaired chloride transport.

2. Carrier Proteins:

   • Function: Selectively interact with a specific molecule or ion to facilitate its passage across the plasma membrane by binding to it and changing shape.

   • Example: The GLUT carriers family is responsible for transferring glucose in and out of various cell types. The inability of some individuals to efficiently use energy for the sodium-potassium (Na^{+}-K^{+}) pump has been suggested as a factor in obesity.

3. Cell Recognition Proteins:

   • Function: Help the body recognize its own cells from foreign substances or cells (distinguish