Ac#ve Transport
Main idea of this lecture: How do molecules, big and small, get in OR out of a cell? 2 Main ways: Passive Transport (Does not require energy) Lecture 1 Ac=ve Transport (Requires energy) Lecture 2
Previously Passive transport - [Simple] diffusion - Osmosis - Facilitated Diffusion (ion channels, carrier proteins) Moving down the concentra#on gradient No energy required
Ac#ve Transport Molecules moving against their concentra#on gradient (from low to high concentra#on) - Ions (posi#vely charged elements), glucose (sugar) Requires transport proteins (ATPases) Primary ac#ve transport (directly uses ATP) Secondary ac#ve transport (uses energy stored in concentra#on gradient)
Primary (1 ) ac#ve transport
Primary ac#ve transport Directly converts energy stored in chemical bonds (ATP) into energy stored in a concentra#on gradient - Molecule binds - ATP reacts with transport protein - Transport changes conforma#on - Molecule is released to the other side
Important ATPases Proton Pumps - Transport protons out of cells, maintain an acidic environment - Stomach cells, lysosomes Calcium Pumps - Muscle contrac#on Sodium pumps (Na + /K ATPase)
Na + /K ATPase [Na + ] higher outside the cell (10mM inside, 100mM outside) [K + ] higher inside the cell (100mM inside, 10mM outside) The Na + /K + ATPase takes 3Na out of the cell and brings 2 K + into the cell Maintains an intracellular environment that has low Na + and high K + the opposite of the extracellular environment
The sodium potassium pump takes three (3) posi#ve charges out of the cell, and brings in two (2) posi#ve charges This leaves the cell with a net (overall) nega#ve charge [Money analogy]
video
Na/K ATPase cycle 1. 3 Na + bind on cytoplasmic side 2. ATP is hydrolyzed, transport protein changes conforma#on 3. 3 Na + are released to extracellular environment 4. 2 K + bind from extracellular environment 5. Carrier protein changes conforma#on, 2 K + are released into the cell
Neurons
Neurons: Ion channels AND Na/K ATPase Transmit electrical signals in your brain Res#ng poten#al of - 65mV 1. A s#mulus causes Na channels to open, Na floods into the cell (the cell becomes more +) 2. Na keeps flooding in un#l it reaches equilibrium (+58mV) 3. Na channels inac#vate, Na stops moving into the cell 4. The change in membrane poten#al causes an opening of voltage gated K channels 5. K moves out of the cell, the membrane poten#al reverses un#l K reaches equilibrium (- 90mV) 6. Change in membrane poten#al leads to a closing of K channels, return to res#ng state poten#al (- 65mV)
Video
Na + /K ATPase inhibi#on Ouabin - Toxin found in some African plants, used in arrows - Binds to the carrier protein and Inhibits the release of 3Na + to extracellular environment and the binding of 2 K + Tarantula toxin Binds to sensor paddles in K gated channels, preven#ng the opening of channels - Blocks nerve conduc#on
Calcium pumps Very similar to the Na + /K + ATPase 1. Carrier protein binds 2 Ca ions from cytoplasmic side of plasma membrane 2. ATP is hydrolyzed, Ca is released out of cell Muscle contrac#on: Ca floods down concentra#on gradient, elevated levels of Ca trigger muscle contrac#on
Secondary (2 ) ac#ve transport
Secondary ac#ve transport Transporter protein couples the movement of an ion down it s concentra#on gradient with another going up it s concentra#on gradient - - The energy stored in the electrochemical gradient of an ion is used to drive the transport of another solute against a gradient The energy needed to move a solute against a concentra=on gradient is provided by the concentra=on gradient of another ion
Secondary ac#ve transport: Symport The direc#on of transport is the same for both the driving ion and the driven ion Example: GLUT2 in the small intes#ne
GLUT2
GLUT2 In intes#nal cells - Glucose moves against it s concentra#on gradient, so energy is required 1. GLUT2 binds 2 Na ions and glucose on the lumen 2. The 2 Na ions and glucose are released 3. The Na that were brought in are taken out via the Na/K ATPase (2 K in)
How do we know glucose is coupled to Na movement? Double chamber experiment - Take a strip of intes#ne and place between two chambers LeD chamber Right chamber Movement? KCl Glucose None NaCl and KCl Glucose None KCl NaCl and glucose Yes; to the lel
Gatorade Entry of Na into the lumen of intes#nal cells is coupled to water Sodium (Na) moves into a cell, down it s concentra#on gradient, and drags water along with it
Secondary ac#ve transport: an#port The movement of one ion into a cell is coupled to the movement of another out of the cell The energy released from one ion moving down it s concentra#on gradient is used to move another up it s concentra#on gradient