Cell membrane plasma membrane n The plasma membrane
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. 1
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. 2
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. 3
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. 4
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. 5
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. 6
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. 7
Solutions n Solvent – n Solute – n Water is the solvent, the most common substance The molecules dissolved in the water, f. ex. sugars, amino acids and ions. 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. 8
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 9
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 lipidsoluble molecules like ethyl alcohol and vitamin A easily diffuse across. 10
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. 11
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. 12
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? 13
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. 14
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 15
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. 16
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. 17
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 n vacuoles (drops) are taken in Exocytosis – The emptying of a membrane lined vesicle at the surface of the cell. 18
The relationship between the nucleus, rough endoplasmic reticulum (r. ER), 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 r. Er after travelling through the r. Er 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 19
Membrane proteins n Transport across membranes – n Antibody recognition sites – – n act as a triggers that sets of a cellular response when hormones bind to them Catalysis for biochemical reactions – n identification tags f. ex. immune cells recognise bacteria and mark them for destruction blood groups, tissue groups Hormone binding sites – n channels, carriers, pumps act as enzymes, particularly on the inside of the cell membrane Sites of electron carriers – the proton pump 20
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