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2. Membrane Structure All biological membranes are composed mainly of lipid and protein molecules PHOSPHOLIPIDS – the most abundant CHOLESTEROL – responsible for stabilising the membrane GLYCOLIPIDS – found at the external surface of the membrane The three major types of lipids in cell membranes are: All of the lipids are described as being AMPHIPATHIC as they have a HYDROPHILIC (‘water-loving’) end and a HYDROPHOBIC (‘water-hating’) end to the molecule The PROTEINS within the membrane are largely concerned with the transport of molecules across the membrane

3. The phospholipid molecule has a polar phosphate – containing head group and two hydrophobic fatty acid tails The tails vary in length and may have one or more double bonds Each double bond creates a kink in the tail The differences in tail length and the presence of double bonds are important for influencing the FLUIDITY of the membrane Kink due to the presence of a double bond

4. The hydrophilic head consists of a phosphate and glycerol group Two non-polar hydrophobic tail groups are bonded to the hydrophilic head group

5. The FLUID MOSAIC MODEL envisages the membrane as a sea of phospholipids within which a mosaic of proteins float like icebergs The FLUID MOSAIC MODEL proposes a double layer of phospholipids with PROTEINS penetrating this layer to different extents The proteins are globular in nature and form a MOSAIC in the fluid-like lipid bilayer Extrinsic protein s ( are partially embedded in the bilayer) Lipid bilayer Intrinsic proteins (extend right across the bilayer)

6. The following two dimensional view of the cell membrane illustrates additional chemical components of the membrane Bimolecular phospholipid layer The lipid bilayer is asymmetrical in that certain protein and lipid molecules at the external surface contain carbohydrate chains as part of their structure Glycolipids and glycoproteins form part of the external structure of the membrane Glycolipids play a part in communication between cells and cell to cell recognition Many glycoproteins function as surface antigens enabling cells to distinguish self from ‘non-self’ Cholesterol molecules are positioned within the bilayer close to the fatty acid chains; these molecules partially immobilise these chains and help to stabilise the membrane glycolipid carbohydrate group glycoprotein Extrinsic protein Intrinsic protein cholesterol stabilising the membrane

7. Transport across Membranes The transfer of relatively small molecules across cell membranes may occur in a variety of ways The PASSIVE TRANSPORT of materials occurs in two ways: 1.SIMPLE DIFFUSION where molecules diffuse across the the lipid bilayer or through channel proteins in the direction of their concentration gradient and 2.FACILITATED DIFFUSION where protein carrier molecules within the membrane assist the passage of substances across the membrane in the direction of their concentration gradient Cellular energy is NOT required for passive methods of transport and relies largely on the random movement of molecules and ions The ACTIVE TRANSPORT of materials also involves carrier proteins assisting the molecules across the membrane. In this case molecules are transported against their concentration gradient and cellular energy is required for this to be achieved energy

8. Passive and active methods of transport are used by cells for the transfer of molecules and ions across membranes simple diffusion facilitated diffusion Passive Transport Active Transport

9. The rate of diffusion is affected by a variety of factors that include: Temperature Surface area Steepness of the concentration gradient Distance over which diffusion is taking place Three of these factors are expressed in FICK’S LAW, which states that: Rate of diffusion = surface area x steepness of concentration gradient thickness of membrane The larger the surface area The steeper the concentration gradient The thinner the membrane or diffusion barrier The faster is the rate of diffusion Surface Area and Simple Diffusion

10. Facilitated diffusion is a carrier-assisted transport mechanism in which molecules are transferred across membranes along their concentration gradients

11. Intrinsic globular proteins within the membrane function as carriers for the active transport of molecules across membranes Active transport is an energy – requiring transport system that is able to transfer material against a concentration gradient Many different ions are actively transported across membranes as is glucose when being absorbed from the gut into the blood stream Hydrolysis of ATP provides the energy for the protein carriers to change shape and transport the molecules across the membrane energy ATP ADP

12. Many of the substances that enter cells are too large to be transported through the bilayer or by transport proteins The bulk transfer of materials INTO the cell occurs by ENDOCYTOSIS The bulk transfer of materials OUT OF the cell occurs by EXOCYTOSIS The Bulk Transfer of Materials

13. Osmosis is a special kind of diffusion by which water molecules are transported across partially permeable membranes Partially permeable membrane with pores created by channel proteins solute molecules too large to pass through pores in the membrane

14. The term WATER POTENTIAL (  ) is used when describing the tendency of water molecules to move from one place to another To move requires energy; water potential is a measure of the free energy available for water molecules to move The free energy available for water molecules in PURE WATER to do work and move is GREATER than that for water molecules in a SOLUTION (water plus dissolved solutes) As pure water contains no dissolved solutes, the water potential of pure water is defined as ZERO – zero is the highest water potential possible The presence of dissolved solutes hinders the ability of water molecules to move The presence of any substances dissolved in water LOWERS THE WATER POTENTIAL THE WATER POTENTIAL OF A SOLUTION IS ALWAYS LESS THAN ZERO AND IS THEREFORE A NEGATIVE VALUE Water potential is measured in units called pascals and the definition of osmosis in terms of water potential is: Osmosis is the net movement of water molecules through a partially permeable membrane from a region of high water potential to a region of lower water potential i.e. movement DOWN a water potential gradient

15. In the situation shown below, two solutions with different water potential values are separated by a membrane Pure water has a water potential of ZERO The presence of solute molecules in this solution lowers the water potential, e.g. -4

16. This fully turgid plant cell has been placed in a hypotonic solution A hypotonic solution is one that is less concentrated than the protoplast of the plant cell and thus has a higher water potential than the cell The net movement of water by osmosis is into the cell and the protoplast swells and presses against the cell wall This plant cell has been placed in an isotonic solution An isotonic solution is one that has the same concentration as the protoplast of the plant cell and thus has the same water potential The cell displays incipient plasmolysis where the membrane is just beginning to pull away from the cell wall There is no net movement of water in this case and no pressure potential as the protoplast ceases to press against the cell wall This plant cell has been placed in a hypertonic solution A hypertonic solution is one that is more concentrated than the protoplast of the plant cell and thus has a lower water potential than the cell The net movement of water by osmosis is out of the cell and the protoplast shrinks The protoplast is pulled completely away from the cell wall and the cell is fully plasmolysed (flaccid)

17. Consider two adjacent plant cells (A and B) in a tissue where each cell possesses a different water potential CELL B has the higher water potential (less negative) and therefore water will move down the water potential gradient from Cell B to CELL A In which direction will water move as a consequence of osmosis?