Membrane Transport Plant Physiology UNI

Two requirements Route through the membrane Energy to move substances PP09 UNI Membrane Transport

Energy – 2 ways Passive Active Energy already there (energy difference) Don’t need to add Active “Uphill” movement (energetically) Need to add energy Direct or indirect addition of energy PP09 UNI Membrane Transport

Concentration isn’t everything What makes transport “uphill?” From lower energy to higher energy Source of the energy depends on the solute Uncharged solutes Charged solutes PP09 UNI Membrane Transport

Energy for uncharged solutes Example: sucrose Difference in chemical activity is what counts Proportional to concentration for solutes So we usually just refer to concentration Dots in diagrams = concentration for these PP09 UNI Membrane Transport

Energy for charged solutes Example: K+, Cl- Difference in electrochemical potential (ECP) is what counts Includes differences in chemical energy (concentration) Also includes electrical charge attraction Dots on diagrams = ECP, not concentration PP09 UNI Membrane Transport

Uphill means… From lower energy (ECP) to higher Includes both sources of energy Chemical (concentration-based) Electrical All cells use both to move solutes All cells are charged (across membrane) PP09 UNI Membrane Transport

Active Transport #1: Pumps Route: protein crossing membrane (uniport) Binds ATP Takes Pi off ADP drifts away Now can bind solute Protein changes shape Solute dropped off on other side of membrane Pi drops off Protein restored PP09 UNI Membrane Transport

AT #2: Cotransport Route: protein symport Carry two solutes together In the same direction Transport is needed “uphill” for one The other provides energy (“downhill” for this) PP09 UNI Membrane Transport

Cotransport: Energy membrane Energy from concentration (or ECP) difference will drive pink substance from top to bottom. Luckily there is another (green) substance here, whose energy (from concentration or ECP difference) will drive it from bottom to top. membrane But cell needs to move the pink substance from bottom to top. The pink “wants” to go down 2 units, but the green “wants” to go up 6 units (greater concentration difference for green). PP09 UNI Membrane Transport

Cotransport: Route A protein symport spans the membrane. The energy from the combined transport (pink = 2 down, and green = 6 up) gives a net force of 4 units pushing both substances through together. It can transport the green substance only if it also transports the pink substance. 4 units The symport will not operate unless both substrates are loaded. Then it will transport both of them in the same direction. PP09 UNI Membrane Transport

AT #3: Countertransport Route: protein antiport Carry two solutes together In opposite directions Transport is needed “uphill” for one The other provides energy (“downhill” for this) PP09 UNI Membrane Transport

Countertransport: Energy Energy from concentration (or ECP) difference will drive blue substance from bottom to top. Luckily there is another (green) substance here, whose energy (from concentration or ECP difference) will drive it from bottom to top. membrane But cell needs to move the blue substance from top to bottom. The blue “wants” to go up 2 units, and the green “wants” to go up 6 units (greater concentration difference for green), but the transporter can only trade them. It can’t move both in the same direction. PP09 UNI Membrane Transport

Countertransport: Route A protein antiport spans the membrane. The energy from the combined transport (blue = 2 up, and green = 6 up) gives a net force of 4 units pushing both substances through together. It can transport the green substance only if it also transports the blue substance in the opposite direction. 4 units Blue “wants” to go up 2 units, and green “wants” to go up 6 units (greater concentration difference for green). Green wins, and transports blue the other way at the same time. PP09 UNI Membrane Transport