Cell Membranes - The Basics

Membranes at the cell surface (plasma membrane)

• Plasma membranes control which substances enter and leave the cell. They're partially permeable - they allow some molecules through but not others. 
• Substances can move by diffusion, osmosis or active transport. They also allow recognition by other cells and cell communication.

Membranes within cells

• Membranes around organelles divide the cell into different compartments. This makes different functions more efficient. (Mitochondria - Respiration)
• The membranes of some organelles are folded, increasing their surface area and making chemical reactions more efficient (Mitochondria - Increased respiration)
• Membranes can form vesicles to transport substances between different areas of the cell. (Proteins - Protein Synthesis)
• Membranes within cells are also partially permeable. (RNA and DNA)

Membrane Structure

• Consist of phospholipids, proteins and carbohydrates.
• Fluid Mosaic Model : Phospholipid bilayer form a continuous, double layer.
• Fluid: because the phospholipids are constantly moving.
• Protein molecules are scattered through the bilayer like a mosaic.
• Glycoproteins: Protein with a polysaccharide (carbohydrate chain) attached.
  
• Glycolipids: Lipids with a polysaccharide chain attached. 
• Cholesterol molecules are also present. 

Membrane Components

Phospholipids

• Barrier to dissolved substances (Hydrophilic Head - Hydrophobic Tail)
• Arranged in a bilayer - heads face out to the water.
• The centre is hydrophobic so ions…