L3 2025 Membrane Transport PDF

The Inner Life of the Cell Animation (youtube.com) Prof. Anna Amtmann SMB, CMVLS Membrane transport Not: the transport of membranes. Deal...

The Inner Life of the Cell Animation (youtube.com) Prof. Anna Amtmann SMB, CMVLS Membrane transport Not: the transport of membranes. Deals with the transport of molecules (in particular ions) across biological membranes. ‘Trans’- membrane transport, ion transport, nutrient transport  Why is membrane transport important? Necessity of life to protect metabolic reactions within the cell against the environment. Necessity of life to communicate and exchange materials between the cell and the environment. Transport proteins in the cell membrane allow for the ‘controlled’ interaction of the cell with the environment.  Membrane permeability High permeability for small hydrophobic molecules and gases (e.g. O2) Limited permeability for water. Very low permeability for ions (e.g. K+) and large solutes (e.g. glucose). Transport proteins create a hydrophilic passage. create a filter. provide possibility for energy coupling. provide possibility for regulation. ‘Facilitated diffusion’ Transport proteins create a hydrophilic pore Molecules diffuse through this pore Example: water channel (aquaporin)  Driving forces for solute transport 2 types of forces drive the movement of molecules across membranes: Chemical gradient = concentration gradient Electrical gradient = charge gradient Only relevant for charged molecules (ions). Cations: Anions: e.g. e.g. Proton H+ + Chloride Cl- + _ Potassium K+ - Nitrate NO3- Ammonium NH4 + Glutamate Histidine Spermidine Malate Pyruvate  The electrochemical gradient = the net driving force for the movement of a molecule resulting from the combination of the chemical and electrical gradient. + + + – – + + + + + – + – + + in – + out – + Case 1: Uncharged molecule. Case 2: Cation. + _ + _ _ – _ _ – _ + – + _ _ _ – _ _ _ – _ – + – + – Case 3:Anion: Direction of the net driving force depends on the relative sizes of chemical and electrical gradients.  Test your understanding + _ _ – _ + _ _ – _ _ – + Inside Outside The net driving force for the anion is directed inward because A) The chemical gradient is larger than the electrical gradient. B) The electrical gradient is larger than the chemical gradient. C) I haven’t got a clue!  Energy requirements of transport Energy Energy investment gain The electrochemical gradient determines the energy requirement of transport + + + – – + + + + + – + – + + in – + out – + Case 1: Uncharged molecule. Case 2: Cation. + _ + _ _ – _ _ – _ + – + _ _ _ – _ _ _ – _ – + – + – Case 3:Anion  Active and passive transport Active transport moves substances against the electrochemical gradient. It requires the input of energy. Passive transport moves substances down the electrochemical gradient. It requires no input of energy. ?! Note however that energy was invested to establish the electrochemical gradient. This means that active transport is a prerequisite for passive transport.  Active and passive transport Transport proteins for active transport:  Pumps  Co-transport systems Transport proteins for passive transport:  Channels  Carriers Pumps Sodium/potassium pump: Energy coupling ATPases: transport is coupled to the hydrolysis of ATP Pumps can also be driven by light energy e.g. bacteriorhodopsin Conformational change Examples: Sodium/potassium pump Proton pump Calcium pump Pumps  Pumps establish electrochemical gradients – EXTRACELLULAR + FLUID ATP – + H+ H+ Proton pump H+ – + H+ H+ – + CYTOPLASM These gradients can be used to drive the active transport of other molecules  Co-transport systems Co-transporters couple the downward movement of one ion (driver) to the uphill movement of another solute (substrate). Symport: Driver ion and substrate move in the same direction (‘piggyback’ principle)  Co-transport systems Co-transporters couple the downward movement of one ion (driver) to the uphill movement of another solute (substrate). Antiport: Driver ion and substrate move in the opposite direction (‘revolving door’ principle) Transport coupling is ubiquitous The functional linkage of primary pumps with co-transport systems - is inherent to all life forms - occurs in different cells and organelles  Test your understanding The salt bush (Atriplex) survives in saline environments because salt bladders on its leaves actively remove Na+. In a plant exposed to high salt, which transport system is needed to remove the toxic Na+ from the cells? A) A Na+/H+ symporter. B) A Na+ channel. C) A Na+/H+ antiporter.  Passive transport Passive transport moves substances down the electrochemical gradient. It requires no input of energy but relies on previously established electrochemical gradients Transport proteins for passive transport:  Channels  Carriers  Passive transport A channel provides an aquous pore for A carrier undergoes a conformational change that the passage of ions. exposes ion binding sites to different sides of the membrane. Both pathways facilitate the movement of solutes down their electrochemical gradients. Ion channels are not just pores They exert a tight control of passage. They are selective e.g. some K+ channels have a 100fold higher permeability for K+ than for Na+. They are ‘gated’ = they can open and close upon specific stimuli, e.g. voltage, chemical ligands (remember: action potential).  How can we measure ion channels? We can measure how single channel proteins gate using a method called patch clamp! current amplifier suction pipette open Channel current closed time  Test your understanding Which piece of equipment is NOT usually required for a patch clamp experiment? A) A suction pipette. B) An amplifier. C) A helmet.  Summary Ion transporters in the cellular membrane allow for the ‘controlled’ interaction with the environment. The electrochemical gradient is the combination of the chemical and the electrical gradient. Active transport moves substances against the electrochemical gradient. It requires the input of energy. Passive transport moves substances down the electrochemical gradient. It requires no input of energy. Pumps establish electrochemical gradients. These drive the active transport of other solutes through co-transport systems and the passive transport of solutes through channels and carriers. Channels exert control of passage through selectivity filters and ‘gating’ mechanisms.