4 Cell Membranes and Transport PDF

4 Cell Membranes and Transport 4.1 Fluid mosaic membranes at low temperatures, cholesterol increases the fluidity of the membrane preventing it from being Fluid mosaic model...

4 Cell Membranes and Transport 4.1 Fluid mosaic membranes at low temperatures, cholesterol increases the fluidity of the membrane preventing it from being Fluid mosaic model too rigid, this is because it prevents close packing ‘fluid’ refers to the movement of phospholipids while of phospholipid tails ‘mosaic’ refers to the scattered proteins (and at high temperatures, cholesterol decreases the glycoproteins) in the phospholipid bilayer fluidity of membrane and stabilises the cell 1) Phospholipids 2, 3) Glycolipids and glycoproteins Lipid and protein molecules on the outer surfaces of cell membrane have carbohydrate chains attached to them forming glycolipids and glycoproteins These carbohydrate chains projecting out like antennae: stabilise the membrane structure by forming hydrogen bonds with water molecules surrounding the cell glycocalyx – sugary cell coating formed by carbohydrate chains act as receptor molecules: Þ signalling receptors – recognise messenger molecules like hormones and neurotransmitters Þ endocytosis – bind to molecule to be engulfed by membrane Image: https://courses.lumenlearning.com/ act as cell markers/antigens allowing cell-cell phospholipids are arranged so that hydrophobic, non- recognition polar tails do not face water. Water is on both the intracellular and extracellular sides 4) Proteins therefore, tails point inwards, and hydrophilic heads face the aqueous medium proteins that are found can be present inside or Image: https://www.chegg.com/ embedded within the outside of the cell membrane membrane i.e., Membrane fluidity intracellular, and Membrane fluidity refers to the viscosity of the lipid extracellular bilayer of a cell membrane. may be found in inner extracellular peripheral Membrane fluidity is affected by: layer, outer layer or proteins – spanning the whole communication, 1) tail length – longer the tail, the less fluid the membrane (these are receptors, and membrane transmembrane recognition proteins 2) saturation of fatty acid – the more unsaturated they proteins) are, the more fluid the membrane. This is as helps in movement in intracellular peripheral unsaturated fatty acid tails are bent and fit together and out of cell proteins- structural more loosely support, attached to the cytoskeleton of the cell 3) cholesterol regulates the fluidity of membrane 1 www.alevel-notes.weebly.com Function of transmembrane proteins a) If the signalling molecules are hydrophobic act as gateways and can transform, helping in (e.g., steroid hormones such as oestrogen) facilitated diffusion and active transport they can diffuse directly across the cell membrane and bind to receptors in the cytoplasm or nucleus. b) If the signalling molecule is water-soluble 1) signal arrives at protein receptor in cell membrane 2) the receptor’s shape is complementary to the ligand 3) the signal brings about a change in the receptor’s shape 4) changing the shape of the receptor allows it to interact with the next component of the pathway so the message gets transmitted 5) binding triggers/stimulates reactions within the cell Image: https://o.quizlet.com/MZFY3U-L4j6xL86C4rWTOQ_b.jpg 6) cell signalling results in a response which may be intracellular or extracellular Channel proteins do not require energy transport substances through membrane passively, along their concentration gradient used for both active transport and facilitated diffusion Carrier proteins require energy go against the concentration gradient Image: https://croteaubio.wordpress.com/ take substances from outside and pumps it inside or vice versa 4.2 Movement of substances into used for active transport and out of cells Cell surface receptors a) Diffusion present in membranes and binds with particular > Net movement of molecules or ions from a region of substances higher concentration to a region of lower used for signalling, endocytosis, cell adhesion, cell concentration down a gradient, as the result of the markers random movement of particles. passive process Cell surface antigen molecules tend to reach an equilibrium situation acts as cell identifying markers Factors affecting diffusion each type of cell has its own antigen this enables cells to recognise other cells and behave as steepness of gradient increases, diffusion in an organised way increases as temperature increases, diffusion increases Cell signalling as surface area increases, diffusion increases cells detect signals with cell receptors, i.e., as distance increases, diffusion decreases glycoproteins and glycolipids, present on their smaller and non-polar molecules like fats diffuse membrane much easily across the cell surface membrane as the signalling molecule binds to the receptor as their they’re soluble in phospholipid tails shapes are complementary to each other this creates a chain of reactions in the cell, leading to b) Facilitated diffusion a response > Diffusion of a substance through transport proteins in a cell surface membrane. 2 www.alevel-notes.weebly.com  the proteins provide hydrophilic areas that allow the energy is used to make the channel/carrier proteins molecules or ions to pass through the membrane change shape, transferring molecules/ions across the which would otherwise be less permeable to them membrane in the process Channel proteins Sodium/Potassium pump allow charged substances, usually ions to diffuse can move to open or close the pore, like a gate controlling ion exchange Carrier proteins flip between 2 shapes, as a result, the binding site opens alternatively to each side FOR EVERY ATP MOLECULE USED 3Na+ - given out the cell Image: https://www.khanacademy.org/ 2K+ taken in the cell c) Osmosis e) Bulk transport > Net movement of water molecules from a region of > A type of active transport where large molecules are higher water potential to a region of lower water transported across the cell surface membrane, using potential through a partially permeable membrane as energy from ATP. a result of their random motion. Water potential > Tendency of water to move out of solution. water always moves down a water potential gradient, this happens until water potential is the same throughout the solution denoted by psi (Ѱ) water potential becomes negative if the solute concentration is very high RBC /ANIMAL PLANT CELLS CELLS plasmolysed, WATER LOSS crenated flaccid haemolysed / WATER GAIN turgid lysed Image: http://lifeofplant.blogspot.com/ 1) Endocytosis d) Active transport > Bulk movement of liquids (pinocytosis) or solids > Movement of molecules or ions through transport (phagocytosis) into a cell by the infolding of the cell proteins, across a cell membrane, against their membrane to form vesicles containing the substance. concentration gradient, using energy from ATP. achieved by carrier and channel proteins 2) Exocytosis these are specific to the type of molecule they’re transporting > Bulk movement of liquids or solids out of a cell by the fusion of vesicles containing the substance with the requires energy; supplied by ATP cell surface membrane. 3 www.alevel-notes.weebly.com