Cell membranes

1. Cell membranes Centrioles Mitochondrion Golgi apparatus Nucleus Name the following structures: 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. Phospholipid bilayer Cholesterol molecules Glycoproteins often receptors for hormones Glycolipids recognising “self” (blood groups) Embedded proteins extrinsic (in one layer) intrinsic (through both layers) Cell surface membrane - structure Components of cell membrane John Kryk cell membranes Quiz on functions of components of cell membrane

9. 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. Fluid – components are not fixed in place Mosaic – pattern of protein molecules if you looked down on the surface Cell surface membrane - structure 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?