7.2 – The Plasma Membrane

Maintaining a Balance

Cell Membrane

1. The plasma membrane is a SELECTIVELY (SEMI-) PERMEABLE membrane that allows nutrients and wastes to enter and exit the cell

a) Homeostasis: maintaining balance in the cell

The Plasma Membrane

1. Selective (SEMI-) Permeability – allows some molecules to pass through and keeps others out

a) Screen on a window – let fresh air in but keep bugs out

2. Water can freely enter and exit the cell.

3. Other particles such as sodium and calcium ions are only allowed in at certain times (selective permeability)

Function of Plasma Membrane

Structure of Plasma Membrane

1. The plasma membrane is composed of a PHOSPHOLIPID BILAYER (double layer)

2. A phospholipid molecule is a lipid which is composed of a glycerol, a phosphate, and two fatty acids

3. The two fatty acid tails of the phospholipid are hydrophobic or nonpolar (not attracted to water).

4. The head of the phospholipid molecule is hydrophilic or polar (attracted to water)

1. Polymer of lipids - triglycerides2. When a phosphate replaces a

fatty acid, a phospholipid is formed

3. Plasma membrane composed of phospholipid bilayer

Triglycerides

Polar Head (glycerol + phosphate)

Nonpolar Tail(fatty acids)

Phospholipid Bilayer

1. The two layers of phospholipid molecules make a sandwich with the fatty acid tails forming the interior of the membrane and the phospholipid heads facing the watery environments found inside and outside the cell

Fatty Acid

Fatty Acid

Phosphate

Phosphate

Inside the Cell

Outside the Cell

Outside of cell

Inside of cell

Unsaturated Fatty Acids

Saturated Fatty Acids

Fluid Mosaic Model

1. The model of the plasma membrane is called the FLUID MOSAIC MODEL

a) The model is FLUID because the phospholipids move around within the membrane

b) The model is MOSAIC because there are transport proteins, cholesterol, and other molecules mixed in with the phospholipids

Cholesterol and Transport Proteins1. Cholesterol is mixed into the plasma

membrane and helps keep the fatty acid tails from sticking together

2. Transport (carrier) proteins span across the plasma membrane and movelarger molecules through the membrane