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 membrane 2 - Others are receptor sites for hormones 3 - Transport using cell energy to pick up needed materials

Plasma membrane video Colouring sheet for Fluid Mosaic Model