THE PLASMA MEMBRANEStandard B-2.2

HOMEOSTASIS

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

Essential to the survival of a cell.

Maintaining homeostasis a primary job of the plasma membrane. (does this by being semipermeable)

PLASMA MEMBRANE

Defined as a flexible boundary between a cell and it’s environment.

ALL prokaryotic and eukaryotic cells have a plasma membrane to separate them from their watery environment.

The membrane allows nutrients into the cell and allows waste and other products to leave the cell.

PLASMA MEMBRANE

The plasma membrane has the property of selective permeability.

(also called semipermeable) This is how homeostasis is maintained.

Means: Allows some substances to pass through to the inside Does not allow some substances to pass though to the inside Doesn’t allow just anything to leave the cell either.

It is selective in what it lets enter and leave the cell

ANALOGIES FOR SELECTIVE PERMEABILITY

Fish net

Coffee filter

Flour sifterGold Mining too

STRUCTURE OF THE PLASMA MEMBRANE

Most of the molecules in the PM are phospholipids.

STRUCTURE OF THE PLASMA MEMBRANE PM is composed of a phospholipid

bilayer.

(2 layers of phospholipids are arranged tail-to-tail.)

THE PHOSPHOLIPID BILAYER

Each phospholipid is diagramed as a head with 2 tails.

PHOSPHOLIPIDS

Heads: made polar by the phosphate group Is attracted to water bc water is polar too Hydrophilic = water loving

Tails: Nonpolar Repelled by water Hydrophobic = water fearing

PHOSPHOLIPID BILAYER

The layers make a sandwich so that the fatty acid tails make up the inner most portion of the PM. Water loving heads are in contact with the

water inside of the cell and the watery outside of the cell.

Water fearing tails are farthest away from the water they hate.

Not touching the water on the inside or the outside of the cell.

OTHER COMPONENTS OF THE PM

Moving with and among the phospholipids are:

- cholesterol- carbohydrates- proteins

OTHER COMPONENTS OF THE PM

Cholesterol:-prevents the fatty-acid tails from sticking

together. - contributes to the fluidity of the pm. (A high cholesterol diet is not advised,

but cholesterol plays a critical role in membrane structure.)

OTHER COMPONENTS OF THE PM

Carbohydrates: Stick out from pm to define the cell’s

characteristics Helps cells identify chemical signals.(Carbs in membrane might help disease-

fighting cells recognize and attack a potentially harmful cell.)

OTHER COMPONENTS OF THE PM

Proteins:-2 types of proteins found in PM

1. peripheral proteins2. integral proteins

OTHER COMPONENTS OF THE PM

Peripheral proteins: proteins that do not protrude from the inside of the cell to the outside of the cell. Outer surface: called receptors and they

transmit signals from outside the cell to the inside of the cell

Inner surface: anchor the pm to the cells internal support structure, gives the cell it’s shape.

OTHER COMPONENTS OF THE PM

Integral proteins: span the entire membrane, creating tunnels through which certain substances can enter and leave the cell. Move needed substances in Move wastes out

Also called: Transport proteins

FLUID MOSAIC MODEL

The phospholipids in the bilayer create a “sea” in which other molecules can float, like apples floating in a barrel of water.

“sea” concept is the basis for the fluid mosaic model of the pm.

The phospholipids can move sideways w/i the membrane, just as the apples move around in the barrel.

FLUID MOSAIC MODEL

At the same time, other components in the membrane, like proteins, also move among the phospholipids.

Because there are different substances in the pm, a pattern, or mosaic is created on the surface.

The components of the pm are in constant motion, sliding past each other.

FLUID MOSAIC MODEL

Fluid: particles that can change position w/o

separating.

Mosaic: picture or pattern made of small pieces

of different colors and/or shapes.

REVIEW

What is the main function of the Plasma Membrane?

REVIEW

What name has been given to the sea-like movements of the plasma membrane?

REVIEW

What does hydrophobic mean?

REVIEW

Besides phospholipids, what is another structure you may see in a plasma membrane and what is that structures function within the membrane?

REVIEW

What does hydrophilic mean?

REVIEW

What do we call the 2 layers of phospholipids arranged tail to tail?