1 Cell membrane (plasma membrane) n The plasma membrane can be thought of as a gatekeeper, allowing only specific substances in or out and passing messages from the external environment.

2 The cell membrane must perform several specific functions: n Isolate the cell cytoplasm from the environment. n Regulate the exchange of essential substances between the cytoplasm and the environment. n Communicate with other cells. n Identify the cell as belonging to a particular species and a particular member of that species.

3 Fluid mosaic model n A membrane when viewed from above, looks something like a lumpy, constantly shifting mosaic of tiles. n A bilayer of phospholipids forms a viscous fluid for the mosaic, while an assortment of proteins are the tiles, often sliding about within the phospholipid bilayer.

4 Structure of the cell membrane n Phospholipid bilayer » the polar end of the phospholipid interfaces with the watery environment surrounding the membranes » the nonpolar fatty acids are found in the interior of the bilayer sheet.

5 Structure of the cell membrane n Proteins function as: » receptors, channels, transporters and markers » some are integral; span entire membrane - f. ex. transport protein. » some are peripheral; at the borders - f. ex. receptor proteins for hormones.

6 Structure of the cell membrane n Composition of a typical membrane: ~ 50% lipid ~ 50% protein n Regulation of interactions 1. Passage of water. n Freely permeable to water. 2. Bulk passage into the cell. n Phagocytosis. Big gulps. 3. Selective transport of molecules. n Transports only some molecules.

7 Structure of the cell membrane n Regulation of interactions cont. 4. Reception of information. n Identify chemical messages. 5. Expression of cell identity. n Molecular name tags. 6. Physical connections with other cells. n In forming tissues, make special connections.

8 Solutions n Solvent – Water is the solvent, the most common substance n Solute – The molecules dissolved in the water, f.ex. sugars, amino acids and ions. n Solution – Solvent and solutes mixed together. Both the solvent and the solutes seek to move from the area of greater concentration to the area of less concentration.

9 Transport across membranes With reference to water and solutes a membrane is called: n permeable n non-permeable n partially-permeable n Semi-permeable n differentially permeable n Selectively permeable membrane Partially permeable membrane

10 Transport across membranes Passive transport - no energy used n Diffusion (simple diffusion) – Diffusion is the random movement of molecules from an area of high concentration to an area of low concentration. – Water, gasses like O 2, CO 2 and lipid- soluble molecules like ethyl alcohol and vitamin A easily diffuse across.

11 Transport across membranes n Osmosis (a special kind of diffusion) – Osmosis is the net movement of water molecules across a partially permeable membrane from a region of high water concentration to a region of low water concentration. – Osmosis will occur whenever two solutions containing different concentration of water molecules are separated by a partially permeable membrane.

12 Transport across membranes n The movement of water in osmosis can also be seen as the movement of water from a region of low solute concentration to a region of high solute concentration.

13 Transport across membranes In relation to cells the concentration of surrounding solutions can be: n Hypertonic if the concentration of solutes in the solution is higher than in the cell. – Water will move in or out? n Isotonic if the concentration of solutes in the solution is equal to that in the cell. – Water will move in or out? n Hypotonic if the concentration of solutes in the solution is lower than in the cell. – Water will move in or out?

14 Transport across membranes n Facilitated diffusion – Facilitated diffusion is the transport of molecules across a membrane by a carrier protein in the direction of lowest concentration. – Its is a boat with no oars, sail or engine - it can only work when the tide is in the right direction.

15 Transport across membranes Active transport - energy used – Active transport is the transport of a solute across a membrane to a region of higher concentration by the expenditure of energy. – It moves molecules against a concentration gradient. – The energy is provided by ATP or adenosine triphosphate ATP ADP + energy ATP ase

16 Transport across membranes n There are three main types – The sodium potassium pump (Na, K) n Most animal cells maintain a higher level of K+ in side the cell and lower Na+ level than on the outside. n The pump transports 3 Na+ out for every 2 K+ it moves in. – The proton pump (H+) n Most likely to be found in the membranes of mitochondria and chloroplasts. n Energy from metabolism or photosynthesis is used to produce ATP.

17 Transport across membranes – Coupled channels n In one channel a molecule is linked to another that is moving down a concentration gradient. n The other channel pumps out the carrier molecule and so keeps up the concentration gradient.

18 Transport across membranes n Endocytosis is when materials are surrounded by and taken into membrane lined vesicles. – Phagocytosis - cell eating n big parts or whole cells are taken in. – Pinocytosis - cell drinking n minute vacuoles (drops) are taken in n Exocytosis – The emptying of a membrane lined vesicle at the surface of the cell.

19 The relationship between the nucleus, rough endoplasmic reticulum (rER), Golgi complex and the cell surface – information about a protein leaves the nucleus through a nuclear pore – the protein is synthesised in the ribosomes on the rEr – after travelling through the rEr it is encapsulated in a vesicle – the vesicle fuses with a Golgi complex where the enzyme is further modified – at the ends or the cisternae it goes to a secretory vesicle – which carries it to the cell membrane, – where it fuses with the membrane and the enzyme is released outside the cell

20 Membrane proteins n Transport across membranes – channels, carriers, pumps n Antibody recognition sites – identification tags f.ex. immune cells recognise bacteria and mark them for destruction – blood groups, tissue groups n Hormone binding sites – act as a triggers that sets of a cellular response when hormones bind to them n Catalysis for biochemical reactions – act as enzymes, particularly on the inside of the cell membrane n Sites of electron carriers – the proton pump