7.2 The Plasma Membrane

Objective:4(B) Investigate and explain cellular processes, including homeostasis, energy conversions, transport of molecules, and synthesis of new molecules.

Function of the Plasma Membrane

The process of maintaining balance in an organism’s internal environment is called homeostasis.

One of the structure responsible for homeostasis is the plasma membrane.

The Plasma membrane forms a thin, flexible boundary between a cell and its environment.

Plasma membranes have selectively permeability, meaning they allow some substances to pass through while keeping others out.

Control of how, when, and how much of various substances enter and leave a cell depends on the structure of the plasma membrane.

Structure of the Plasma Membrane

The plasma membrane is composed of a phospholipid bilayer – composed of phospholipid molecules arranged with polar heads facing the outside and nonpolar tails facing the inside.

The phospholipid bilayer is polar on its surface and nonpolar in the middle.

The plasma membrane separates the internal and external environment. Because water soluble substances will

not move easily through due to the nonpolar middle.

Other Components of the Plasma Membrane Proteins

Some are receptors that transmit signals to the inside of the cell.

Some serve as support structures for the membrane.

Some are transport proteins that move needed substances and wastes through the membrane.

Cholesterol Prevents the fatty acids tails of the

phospholipid bilayer from sticking together.

Contributes the fluidity of the membrane.

Carbohydrates Define the cell’s characteristics. Help cell identify chemical signals.

The fluid mosaic model describes the bilayer as a “sea” in which other components can float and move around.

Different substance in the plasma membrane create a mosaic pattern on the surface of the cell.