Lecture 6: Carriers and Active Transport Flashcards
Remembering Diffusion: 10
- Diffusion can move molecules from high to low concentration, high to low chemical potential energy.
- Movement is down a concentration gradient as chemical potential energy is lower (like rolling down hill).
- At equilibrium there is no net movement because there is no potential energy difference to produce movement.
- (Also can say at equilibrium no net force.)
- Rate of diffusion depends on permeability of the
membrane. - PORES, CHANNELS AND CARRIERS CAN INCREASE PERMEABILITY AND MAKE DIFFUSION FASTER.
- Molecules diffuse from high to low concentration.
- High concentrations of molecules have chemical potential energy.
- ux = nRT * ln ([x]1- [x]2)
- Diffusion rate depends on permeability, surface area and concentration gradient
jx =P*([x]1−[x]2)
Pores and Channels:
Cell membranes and permeable to…
Purpose of pores and channels? - 3
- Cell membranes are permeable to lipid soluble molecules (O2).
PURPOSE:
* Pores and channels increase permeability.
- Pores and channels can be selective: aquaporin for water.
- Channels are gated pores allow control of permeability
What is the Rate of Transport:?
Limit?
What do Pore and channels do to the rate of transport?
What does carrier mediated transport show?
- Higher concentration difference higher rate of diffusion.
- In simple diffusion NO RATE LIMIT
- Pore and channels increase permeability but still simple diffusion.
- Carrier mediated transport shows saturation
**CARRIERS**
- what do they do?
- How is the rate of transport limited?
- Example?
- what type of diffusion is used?
- Energy is from?
- Carriers bind the solute and change conformation.
- Rate limited by speed of each carrier and total number
- Glucose transporter is an example
- Called facilitated diffusion
- Energy still from the concentration gradient
CARRIERS - the process of its diffusion.
- CARRIERS are CONDUITS that are GATED by 2 “DOORS” that are NEVER OPEN at the SAME TIME.
- The carrier is open to the outside
- X enters from outside and binds at a binding site.
- The outer gate closes and X becomes occluded, still attached to its binding site.
- The inner gate opens with X still bound.
- X exits and enters the inside of the cell.
- The outer gate closes, occluding an empty binding site. This cycle can also flow in reverse order.
*** Reversible reaction, can move both ways
Carriers Mediated Transport
- SATURATION
- CONCENTRATION; AFFINITY CONSTANT Km.
- Carrier mediated transport shows SATURATION has a
maximum transport rate Tmax or Jmax. - Concentration at which half the carriers are occupied is the AFFINITY CONSTANT Km.
- LOW Km is a high AFFINITY
What is Co-Transport?
Example?
- Some carriers facilitate the diffusion of more than one
molecule. - Co-transporters are carriers that move several molecules in a fixed ratio.
example:
* Na+-Glucose transporter moves both Na+ and glucose into cells in the intestine
Co-Transports:
SYMPORTS VS ANTIPORTS
EXAMPLES
- SYMPORTS move both molecules in theSAME DIRECTION
- Na+-glucose symport moves both Na+ and glucose into intestinal cells.
- ANTIPORTS move the molecules in OPPOSITE DIRECTIONS
- Na+-Ca2+ exchanger moves Na+ into smooth muscle cells and Ca2+ out of those cells
EXMPLAIN FACILITATED DIFFUSION: 6
1* Pores and channels can selectively increase permeability but do not show saturation.
2 * Carriers show saturation as the molecules must bind to the carrier and the proteins conformation changes.
3 * Each carrier protein has a maximum transport rate.
4* Facilitated diffusion is still powered by the energy in the concentration gradient, stops at equilibrium.
5 * But co-transport can link movement of molecules together.
6 * Can use diffusion of one molecule to move another.
Co-transport: Na+-glucose transporter.
- Na+-glucose transporter moves 1 glucose for 1 Na+
- Now glucose can diffuse down Na+ concentration
gradient.
What is Active Transport? (2)
- Active transport uses ATP to power movement.
- Substance can be pumped against concentration gradient.
Explain Pumps and Carriers: 7
1 * Facilitated diffusion and active transport (pumps) are
both called carrier mediated transport.
2* Mostly carrier mediated transport means facilitated
diffusion but not always.
3* Facilitated diffusion uses carriers.
4 * Active transport uses pumps.
5 * Both carriers and pumps are called transporters
6 * Pumps are really ATPase enzymes that are also carriers.
7 * ATPase are enzymes that hydrolyse ATP to ADP and phosphate.
Energy for Active Transport:
What do they use?
The equation?
Equilibrium?
- Pumps use the chemical potential energy in ATP to move molecules up hill against chemical potential
- At equilibrium much more ADP than ATP
***. ATP —–> ADP + (PO4)-3
<–
* But not normally at equilibrium
- Living cells maintain much more ATP than ADP so chemical energy is stored in the reaction that is far from equilibrium.
** ATP—–> ADP + (PO4)-3
<–
Active Transport: POSPHORYLATION
CONFORMATIONAL CHANGE
- Phosphorylation is the addition of a phosphoryl (PO3) group to a molecule.
- In biological systems, this reaction is vital for the cellular storage and transfer of free energy using energy carrier molecules.
- Phosphorylation triggers conformational change.
- Conformational change alters binding affinity.
-Conformational change. (Science: cell biology) alteration in the shape usually the tertiary structure of a protein as a result of alteration in the environment ph, temperature, ionic strength) or the binding of a ligand (to a receptor) or binding of substrate (to an enzyme).
Understanding Na+ -K+ -ATPase
(3)
- Low intracellular Na+ and high K + is maintained by active transport.
- Na+-K+pump(Na+/K+-ATPase) used ATP to move Na+ and K+ against their concentration gradients.
- All cells have this pump.
Understanding Na+-K+- ATPase: The PROCESS
**5
- Pumps Na+ and K+ in sequence not together.
- Transports 3Na+ out for 2K+ in.
- And uses 1 ATP
- Phosphorylation changes binding affinity.
- Electrogenic net movement of charge
Secondary Active Transport VS Primary active transport.
- PRIMARY ACTIVE TRANSPORT uses ATP to power a pump
- Chemical gradients produced by a pump can be used to power movement of other substances
- Low intracellular Na+ from Na+-K+-ATPase
- Na+-Ca2+ ANTIPORT powers Ca2+ out using Na+ gradient
- SECONDARY ACTIVE TRANSPORT uses energy from ATP indirectly
Secondary Active Transport: glucose uptake by intestine; Na+
(6)
1 * Glucose uptake by intestine
2 * Primary active transport keeps cell Na+ low
3 * Na+ linked to Glucose entry on apical side (gut lumen).
4 * Na+ gradient powers glucose uptake into cell
5 * High Glucose in cell powers facilitated diffusion
6 * Carrier on basolateral (blood) side move glucose out
Secondary Active Transport: uptake of amino acids: 4
1 * Uptake of amino acids in gut also powered by Na+.
2 * Active transport of many other molecules is secondary.
3 * Not all active transport is secondary
4 * Primary active transport also for Ca2+, H+ etc
Pumping epithelia: the process
- Na+ enters across Apical membrane via channels, but is pumped out across basolateral membrane.
- The K+ pumped into cell recycles back out
- The lumen is negative compared with INTERSTITIUM
Equilibrium and Steady State = 7
1 * At equilibrium, no net energy use and no energy available.
2 * Our cells are NOT at equilibrium.
3 * Steady state is when nothing is changing.
4 * Living things are often in steady state
5 * Equilibrium is a special case of steady state (no flow is constantly zero)
6 * Keeping at a steady state away from equilibrium constantly uses energy
7 * Moving ATP towards equilibrium provides energy to move something else away from equilibrium
SUMMARY OF THIS LECTURE: 7
- Pores, channels and transporters increase permeability
- Transporters can be passive carriers or active pumps.
- Transporters show saturation.
- Primary active transport use an ATPase.
- Secondary active transport uses a co-transporter to move one thing down its concentration gradient and another up its gradient.
- Active transport keeps the contents of cells far from equilibrium.
- Isosmotic NaCl is isotonic because sodium is pumped out of cells
