Cell Membranes - Structure PDF

# Cell Membranes - Structure ## 3.2.3 Transport Across Cell Membranes ### Content * The basic structure of all cell membranes, including cell-surface membranes and the membranes around the cell organelles of eukaryotes, is the * The arrangement and any movement of phospholipids, proteins, glyco...

# Cell Membranes - Structure ## 3.2.3 Transport Across Cell Membranes ### Content * The basic structure of all cell membranes, including cell-surface membranes and the membranes around the cell organelles of eukaryotes, is the * The arrangement and any movement of phospholipids, proteins, glycoproteins and glycolipids in the fluid-mosaic model of membrane structure. Cholesterol may also be present in cell membranes where it restricts the movement of other molecules making up the membrane. * Movement across membranes occurs by: * Simple diffusion (involving limitations imposed by the nature of the phospholipid bilayer) * Facilitated diffusion (involving the roles of carrier proteins and channel proteins) * Osmosis (explained in terms of water potential) * Active transport (involving the role of carrier proteins and the importance of the hydrolysis of ATP) * Co-transport (illustrated by the absorption of sodium ions and glucose by cells lining the mammalian ileum). * Cells may be adapted for rapid transport across their internal or external membranes by an increase in surface area of, or by an increase in the number of protein channels and carrier molecules in, their membranes. * Students should be able to: * Explain the adaptations of specialised cells in relation to the rate of transport across their internal external membranes. * Explain hows surface area, number of channel arrier proteins and differences in gradients of concentration or water potential affect the rate of movement across cell membranes. ## Required Practicals * **Required practical 3:** Production of a dilution series of a solute to produce a calibration curve with which to identify the water potential of plant tissue. * **Required practical 4:** Investigation into the effect of a named variable on the permeability of cell-surfac of cell-surface membranes. ## TEM view of the Plasma Membrane * The bi-layer cannot be seen with the light microscope * Through the electron microscope is appears as two dark tramlines around the cell. * This has revealed it to be around 7-10nm thick. ## The Phospholipid * The membrane is also known as a **PHOSPHOLIPID BILAYER** because it main component is phospholipids. * It is a small polar molecule (contains charged and non-charged regions). * It has a hydrophobic (water hating) fatty acid tail and a hydrophilic (water loving) head which contains a phosphate group. This end interacts easily with water. > If a load of phospholipids were dumped into a beaker of water how would they arrange themselves? Can you show this in a diagram? ## Controlling The Membrane * Controlling entry of substances to the cells from the environment (vice versa) * Recognition and signalling between cells. * Holding the components of metabolic pathways in place. ## The Fluid Mosaic Model * FLUID because the phospholipids can move around in it and * MOSAIC because it is studded with proteins like a mosaic. ## Key Components 1. **Phospholipids** 2. **Intrinsic proteins (span the entire bilayer)** (often channels or carriers for molecules/atoms that must move across the membrane. 3. **Extrinsic proteins (found on embedded on one side)** May be enzymes or co-enzymes with roles in processes such as respiration. 4. **Glyco-proteins** (sugar attached to protein) 5. **Glyco-lipids** (sugar attached to lipid) > **Roles in recognition and signalling** 6. **Cholesterol** - Role in variable fluidity/permeability ### Diagram of a cell membrane >(Image: A diagram of a cell membrane showing the different components of the membrane) * **A:** Phospholipids * **B:** Hydrophobic tail * **C:** Glycolipids * **D:** Cholesterol * **E:** Hydrohobic tails