cell physiology 2 Flashcards
Tonicity
Effect of bathing solutions on cell volume.
Some terminology:
Isotonic solution: no change in cell volume
Hypotonic solution: cells swell
Hypertonic solution: cells shrink
* Thus 0.15 M NaCl is isotonic.
How do we get molecules and ions that cannot pass through the lipid bilayer across the cell membrane?
A: Specialised membrane proteins!!!!
Allow transportation through certain substances
Membrane proteins enable trans-membrane solute movement of four kinds…
- Simple diffusion (of ions, through channels)
- Facilitated diffusion (larger molecules)
- Primary active transport &
- Secondary active transport
(both requiring energy to move solutes against their concentration gradient)
* All display: specificity, saturation and competition
Specificity:
Specificity: Each carrier protein is specialised to transport one or, at most, a few closely related substances
Saturation:
Limited number of carriers in the membrane + limited number of binding sites for a particular substance, therefore can become ‘full’ - known as the transport maximum (Tmax)
Competition:
If closely related substances can use the same carrier they will compete for the use of that carrier
Carrier-mediated transport vs simple diffusion through phospholipid bilayer
- Channels are an easy way to go for ions
They come in many types, depending on:
Ion selectivity (e.g. for Na+,K+, Cl-, etc.)
Gating
…ungated: always open
…gated: voltage, ligand or mechanically-gated
Carrier-mediated transport: facilitated diffusion
Step 1: Transported solute binds weakly to a carrier protein (trans-membrane protein, but no pore)
Step 2: Binding of the solute molecules induces change in conformation of carrier protein
Step 3: Transported solute detaches from carrier protein in area of low concentration
Step 4: Carrier protein reverts to original shape
Why use active transport ?
What the cell does when the solute it needs to move in or out does not have a favourable concentration gradient
Carrier-mediated transport:
Primary active transport
• “uphill” movement, thus work to be done
…needs energy
• energy direct from ATP hydrolysis:
…primary active transport
• energy derived from existing concentration
gradient of another solute:
…secondary active transport ( Driven by active concentration gradient derived from primary active transport)
• both involve a protein carrier that binds one or more solutes, thus substrate specific & saturable
Primary active transport cycle
- Carrier protein splits ATP into ADP, plus phosphate. Phosphate group binds to carrier, increasing affirmity of its binding site for ion.
- Ion to be transported binds to carrier on low-concentration side
- In response to ion binding, carrier change conformation so that binding site is exposed to the opp. side of the membrane. Change in shape also reduces affinity of site for ion.
- Carrier releases ion to side of higher concentration. Phosphate group is also released
- when binding site is free, carrier reverts to its original shape
* Makes concentration system outside higher and inside lower
Primary active transport cycle 2
- The most important example of primary active transport is the Na+/K+ ‘pump’
- pumps in opp. directions
- Pumps out 3 sodium and pumps in 2 potassium ions
- They are in every cell in your body, maintain the low Na+ & high K+ concentration inside your cells
Secondary active transport
Similar steps as those in facilitated diffusion but ‘extra’ solute transported against concentration gradient
Example: Na+/glucose symport
Moves glucose from intestine into epithelial cells
Na+ concentration gradient used to drive glucose ‘uphill’
Secondary active transport process
Binding of Na+ on luminal side, where Na+ concentration is higher, increases affinity of SGLT for glucose. Therefore, glucose also binds to SGLT on luminal side, where glucose concentration is lower
> When both Na and Glucose are bound, SGLT changes shape, opening the cell interior
> SGLT releases Na+ to cell interior, where Na+ concentration is lower. Because affinity of SGLT for glucose decreases on release of Na+, SGLT also releases glucose to cell interior where glucose concentration is higher.
