1. Name the following structures:
Cell membranes
Name the following structures:
Centrioles
Mitochondrion
Golgi apparatus
Nucleus
Cell surface membrane (plasma membrane)
Nuclear envelope

2. Lesson objectives:
Outline the roles of membranes in cells and at the surface of cells.
State that the plasma (cell surface) membrane is a partially permeable barrier.

3. Roles of membranes: Cell surface membrane:
Controls movement of substances into and out of cell.
Recognition site for bodies immune system
Receptor site for hormones and neurotransmitters
May be folded – increasing surface area.
Helps cell attachment and tissue formation.
Other membranes:
Separate cell components from cytoplasm
Isolate enzymes that might damage the cell e.g. lysosomes

4. Basic structure - phospholipids
Phosphate ‘head’ = hydrophilic (water loving) Fatty acid ‘tails’ = hydrophobic (water hating)

5. Phospholipid

6. The phospholipid bilayer
Basic structural component of all biological membranes.
Hydrophobic layer formed from tails forms a barrier to many molecules and separates the cell contents from the outside world.
Membranes are approx 7-9nm thick.

7. Glycoprotein

8. Cell surface membrane - structure
Phospholipid bilayer
Cholesterol molecules
Glycoproteins
often receptors for hormones
Glycolipids
recognising “self” (blood groups)
Embedded proteins
extrinsic (in one layer)
intrinsic (through both layers)
Components of cell membrane
John Kryk cell membranes
Quiz on functions of
components of cell membrane

9. cholesterol glycoproteins proteins phospholipids
Stabilises phospholipid bilayer
Regulates fluidity of phospholipid bilayer
cholesterol
Receptors
Cell-cell recognition
(form hydrogen bonds with water outside stabilising membrane)
glycoproteins
Act as channels, pores and carriers for polar molecules
Receptors
proteins
Permeable to non-polar molecules
Permeable to small polar molecules
Impermeable to ions and larger polar molecules
phospholipids

10. Cell surface membrane - structure
Fluid – components are not fixed in place
Mosaic – pattern of protein molecules if you looked down on the surface
Fluid mosaic model
Videos showing fluidity
Of cell surface membrane
Inner Life of Cell (again)

11. Performing different roles
Other components are needed in order to make a functioning biological membrane.
Number and type of these components varies according to function of membrane.
This is part of differentiation.

12. Some examples of roles.....
Plasma membranes in growing regions – receptors to detect molecules which regulate growth.
Muscle cell membranes contain large number of channels that allow rapid uptake of glucose.
Internal membranes of chloroplasts contain chlorophyll and other molecules needed for photosynthesis.
Plasma membranes of leukocytes (WBC) contain special recognition proteins

13. Summary of the Main Features of the cell surface membrane
7nm wide (you need to learn this value!)
Based on phospholipid bilayer
Proteins float within this layer and can move freely from side to side
Some are tethered to the cytoskeleton if they need to be in a particular place
Some are embedded in the inner layer of the bilayer, some in the other layer
Some lipids and proteins have carbohydrate chains attached on the external surface of the membrane, giving glycolipids and glycoproteins
Within the bilayer are also molecules of cholesterol

14. Factors affecting fluidity of membrane
proportion of phospholipids that are unsaturated/saturated
temperature
proportion of cholesterol

15. Permeability
All membranes are permeable to water molecules – water can diffuse through the lipid bilayer.
Some membranes are 1000times more permeable – presence of aquaporins (protein channels which allow passage of water)
Cell membranes = partially permeable membranes as they are permeable to water and some solutes.

16. Questions to try.....
Why can phospholipid molecules in a bilayer move only in the plane of the bilayer?
Why do muscle cells need to be able to take up glucose rapidly?
Why do we describe cell membranes as partially permeable rather than semi-permeable?