Unit: Cellular Structure & Function

All cells have a cellmembrane thatseparates the cellfrom its non-livingsurroundings.

It is a thin barrier;approximately 8 nm thick!

Problems with the cell membrane are involved in many diseases

Type 2 Diabetes, organ transplant rejection, Cholera, Cystic fibrosis, cancer

The cell membrane has several roles to play in a living cell.

For example:

• It helps thethe cell maintain homeostasis by regulating what enters & leavesleaves the cell (controls traffic in and out)

• It receives and produces signals to and from other cells.

• It identifies the cell as belonging to a particular organism and tissue

• It maintains connections between cells in organs and tissue

This fluidity is caused by phospholipids, which form the foundation of the plasma membrane.

Remember: Lipids and water REPEL one another

Cell membranes have a flexible, fluid like structure.

Phosphate group head Polar hydrophilic

Fatty acid tails (2)Nonpolar hydrophobic

Phosphate Head

Fatty acid tails

The nonpolar “tails” cluster

together trying to get as far

away from the water as they

can.

The hydrophilic “heads” are

attracted to the water

The resulting formation is referred to as a lipid bilayer

Outside the cell

Inside the cell

polarhydrophilicheads

nonpolarhydrophobictails

polarhydrophilicheads

The Lipid Bilayer is Semipermeable

The lipid bilayer is a barrier that can be described

as Selectively Permeable (A membrane that

allows only certain substances in the cells

environment to pass through it.)

Some things have no trouble crossing the

Membrane (Ex. small, nonpolar things like O2 &

CO2) ;others need help across (ex: glucose)

1) Proteins

Some proteins stick to the surface of the lipid

bilayer (peripheral proteins) and others penetrate

the hydrophobic core (integral proteins).

Amino acids, which make up proteins, may be

polar or nonpolar. How do integral proteins stay in

place?

Answer: The water found on either side of the membrane repels the non-polar part of the membrane protein. It is attracted to the interior of the lipid bilayer

The polar part of a membrane protein is attracted to the water on both sides of the membrane.

This duel attraction holds the protein in place within the lipid bilayer

They may be attached to

some proteins

(glycoproteins) or lipids

(glycolipids) at the

membranes surface.

Function- act as a

chemical identification

card which allows cells to

recognize & interact with

one another.

Because the proteins embedded in the lipid bilayer can

move around and “float” among the lipids, and because so

many different kinds of molecules make up the cell

membrane, scientists describe the cell membrane as a

“fluid mosaic.”

Extracellular fluid

Cholesterol

Cytoplasm

Glycolipid

Transmembraneproteins

Filaments ofcytoskeleton

Peripheralprotein

Glycoprotein

Phospholipids