Membrane transport 3-10-23.docx

Describe Membrane permeability. Gases e.g. N2, O2 and CO2 can easily diffuse through membrane (exchange of gases in alveoli) Hydrophobic molecules can also diffuse through. Small polar molecules can go through membrane to some extent but not very efficient. Diffusion of water require channel call...

Describe Membrane permeability. Gases e.g. N2, O2 and CO2 can easily diffuse through membrane (exchange of gases in alveoli) Hydrophobic molecules can also diffuse through. Small polar molecules can go through membrane to some extent but not very efficient. Diffusion of water require channel called aquaporins which facilitate diffusion of water. Anything charged and large cannot pass through the membrane. Describe Membrane transporters and channels. Transporters and channels provide selectivity and facilitate transport- very specific and allow certain molecules to pass. Molecules that pass from one side to another are turned into solutes. Channels- forms tiny hydrophilic pores across membrane so substances can pass via diffusion. Integral membrane protein Highly selective Opens to both side of membrane at same time. Gated therefore someone triggers it to open. Transporters- shift small organic molecules or small inorganic ions from one side to another by changing shape. Has binding sites for solutes. Confirmational change Only opens to one side of the membrane, binds to solute and opens on other side of membrane to release the solute. Each type of membrane has its own set of transports and channels therefore determines exactly which solutes can pass in and out of cell or organelle. A difference in concertation gradient represents chemical potential caused by barrier, process is UNIDIRECTIONAL. Concentration gradient across membrane represents potential energy because it drives diffusion. Passive vs active transport Passive- occurs in biological systems as transport is down the concertation gradient and chemical potential drives the diffusion. Simple diffusion, channel mediated and transported mediated are also passive transports as it goes down the concentration gradient. Active- against concentration gradient therefore require energy. Transporter mediated transport can be either active transport or passive its against concentration gradient. Active transporters can be P-type pumps (uses protein gradient), F-type proton pumps (generates energy and not use energy) or ABC transporter pumps (usually uses 2 ATP molecules)- all of these are primary ATP driven pumps because ATP is used as energy source (except of F-type pump that generates ATP instead) Secondary coupled transporters also transport molecules against concentration but won’t use ATP but will use previously established gradients to drive the transport. One molecule goes down the concentration gradient and this energy is used to transport another molecule against the concertation gradient. Example of F-type pump is inside mitochondria (ATP synthase)- Protons move down the proton gradient but as proton travel down, it generates ATP. Hydrolysis of phosphodiester bonds in ATP yields energy Binding or hydrolysis of ATP powers transport against the concentration gradient e.g. removal of toxins by ABC transporter from low concentration to high concentration Transport processes Uniport- only transports 1 single molecule. Symport -transports both molecules same direction Antiport -transports molecules in different directions Both antiport and symport are secondary active transport as ATP is used to generate concentration gradient of 1 molecule that is transported down the concentration gradient. Using the energy established from the concentration gradient, another molecule can be co-transported against the concentration gradient. e.g. of symport- Active transport of glucose against concentration gradient. High sodium concentration causes glucose to also co-transport with it as the transporter has 2 binding sites, 1 for glucose and other for Na+. e.g. of antiport- Electrochemical gradient of Na+ is used to rapidly expel Ca2+ from the cytosol. Active transport of calcium Binding of both solutes is cooperative in co-transporters as it cannot work with only 1 molecule binding as the binding of 1 enhances the binding of other (confirmational change) Transport of ions across membrane When ion crosses a membrane, it induces membrane potential. Charge flows from one side of membrane to another. Concentration of ions provide chemical potential; the charge of ions provides electrical potential- flow of ions across membrane happens until both potentials are equal Combination of the electrical and chemical potentials results in the electrochemical potential. Membrane potential= difference in voltage on both sides of membrane. Driving forces for transport. Electrochemical gradient Much bigger forces when voltage and concentration gradient work in same direction. E.g. if positive ions are transported from a positive side to a negative side of membrane, the positive ions will be more attracted to the negative side. Driving force for ions is the combination of electrical and chemical potentials= electrochemical potential. Electrical potential can be calculated using Nernst equation. Only the temperature, charge of ion and concentrations are needed. How are ion gradients established and maintained? Active transport is used to establish these gradients. E.g. Na+ and K+ pump, pumps 3 Na+ out and 2K+ in and uses 1 ATP per cycle. Gated ion channels Conducts ions rapidly. Passive transport therefore no energy required. Used to manipulate membrane potential. High specificity Number of control mechanisms The channels can open and close depending on voltage differences across membrane or depending on what ligand binds to it (either binds to extracellular part or intercellular part) and lastly can depend on mechanical or tension forces which can cause them to open or close. Control of membrane potential Influx of Na+ when Na+ channels are open. Charge goes from -70mV to +40mV therefore depolarise (same thing with Ca2+ channels) When K+ channels open, K+ efflux therefore membrane potential goes back down towards -70mV which causes hyperpolarisation. 15-30% of membrane proteins encode for transmembrane proteins (integral proteins) All transmembrane proteins are multipass meaning it passes the membrane at least once. 30-70% of total ATP is used by pumps eg sodium and potassium pump to establish and maintain gradients.