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