Biology 3.5 The Cell-Surface Membrane

Known as plasma membrane, forms boundary between cell cytoplasm and environment.

Allows different conditions inside and outside cell and controls movement of substances in and out of cell.

Phospholipids

Phospholipids form a bilayer sheet because:

• One layer of phospholipids = hydrophilic heads pointing inwards (water inside cell cytoplasm)
• Other layer has hydrophilic heads pointing outwards (to water outside the cells)
• Hydrophobic tails of both layers point into centre of the membrane, protected from water on both sides.

Lipid soluble material moves through membrane via phospholipid portion. Function of membrane:

• allow lipid-soluble materials in and out.
• prevent water-soluble substances in and out.
• make membrane flexible.

NB: phospholipid has…