1. Membrane Transport
Plant Physiology
UNI

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

3. 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

4. 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

5. 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

6. 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

7. 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

8. 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

9. 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

10. 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

11. 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

12. 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

13. 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

14. 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