Maintaining a Balance

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

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)

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 - triglycerides 2.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)

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 Phosphate Inside the Cell Outside the Cell

Outside of cell Inside of cell

Unsaturated Fatty Acids Saturated Fatty Acids

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

1.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 move larger molecules through the membrane