The Cell Membrane

• ALL Cells of ALL Organisms have a:

• The cell membrane separates cells from the external environment.

• Analogy: a fence around a yard protects and acts as a boundary for whatever is inside.

Are fences designed to completely restrict access?

Function:

• The cell (plasma) membrane acts as a protective barrier that regulates which particles can enter and exit the cell.

• Because of this regulation it’s said to be SELECTIVELY PERMEABLE

(or semi-permeable)

Function cont.

• The membrane separates all cells from the external environment which allows cells to be chemically different from their environment.

• The cell membrane also works to keep the internal functions of the cell constant and to maintain cell “balance” (Homeostasis)

Looking through a microscope..

• The picture above is an electron micrograph of a cell membrane at approx. 240,000x magnification

• This picture shows the cell membrane at a much lower magnification. The lighter area to the right represents the extracellular space.

Basic structural component of the plasma membrane is the phospholipid

Structure - The phospholipid:• Head: phosphate group

(polar: hydrophilic) -like water and is soluble in water

• Two Tails: fatty acids (non-polar: hydrophobic) - hates water and is insoluble

in water

• The water-soluble ends face the outer environment and inner cell components

Cell Membrane Structure• It is composed of two layers of lipid material with protein

molecules interspersed

• This is called the phospholipid bilayer

The Fluid Mosaic Model

• Hydrophilic: “water loving”• Hydrophobic: “water hating”

So, how will the phospholipids behave in water?

Dynamic (fluid) structure NOT rigid!

• Can change shape to perform different functions!

• If it breaks easily fixed!– WHY DO YOU THINK THIS IS??

The membrane is not phospholipids alone!!

• There are several other components to the membrane that play vital parts in its function.

1. Cholesterol – (keep membrane fluid)

2. Glycoproteins - specialized sugars attached to a protein (used for cell recognition)

3. Proteins

4. Glycolipid- specialized sugar on some lipids

1. Cholesterol: Stabilizes the Membrane*Cholesterol is a steroid lipid*Flat shape: Inserted between phospholipids*Present in animal cell membranes*Absent in bacteria and most plants because cell walls

provide the stability

Cholesterol…• Amoeba: single-celled

animal (protozoa) that moves by extending “feet” of cytoplasm (pseudopodia)

• Red blood cell: carries oxygen: requires a regular shape to perform it’s function.

2. Glycoproteins: sugar attached to a protein which provides cellular ID

Example:• Immune system recognizes invaders by the

unique structure (the glycoprotein) on the cell membrane

– Ex. Transplanted organs are often rejected by recipients because of the sugars on the proteins.

– Ex. Type A red blood cell is different from a type B because of the sugar in the protein

2.Glycoproteins

Glycoproteins and blood type:

3. Proteins: “gatekeepers”, receptors and transport molecules

Proteins in the membrane can:1 - Act as a gatekeeper, opening and closing paths through the membrane2 - Others are receptor sites for hormones3 - Transport using cell energy to pick up needed materials

• Plasma membrane video• Plasma membrane video

• Colouring sheet for Fluid Mosaic Model