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Solute Transport
Physiology and Pharmacology
| Question | Answer |
|---|---|
| Membrane Structure | The cell membrane is the boundary between the cytoplasm and interstitial fluid compromised of a phospholipid bilayer in with embedded proteins. It is selectively permeable and instrumental in establishing the asymmetry of ions and non-electrolytes |
| Name three different ways of transfer across the membrane | Using vesicles Through the lipid bilayer Using proteins |
| How can vesicles transport substances | Endocytosis- receptor binds to a target, membrane buds in to internalise the receptor and the signal. The receptor must be processed and returned to the membrane Exocytosis- Vesicles containing molecules fuse to the membrane and release the chemical. |
| What is membrane recycling | Maintaining the normal cell membrane despite vesicles being added and removed in bulk transport processes. |
| Selective permeability of the membrane | Without proteins, the membrane would only be permeable to small, non-polar and uncharged solutes. Proteins allow larger, charged or polar substances e.g. glucose to enter the cell |
| Fick's law | J = D x A x (change in C/change in x) J = diffusion flux D = diffusion coefficient A = surface area C = concentration x = membrane thickness |
| Solubility diffusion | These solutes dissolve in the lipid bilayer (partitioning) and diffuse across the membrane (repartitioning). {solute} in lipid = {solute} in water x lipid solubility coefficient The solute must readily diffuse into and out of the bilayer |
| Electrodiffusion | Diffusion that depends on the electrical gradient, determined by the potential difference. This only applies to ions Positive ions will diffuse towards a negative electrode Negative ions will diffuse towards a positive electrode |
| Osmosis | A form of diffusion relating to water movement Tonicity = osmotic force exerted by a solution. Normal body osmotic potential is 300 mOsm/l Water diffuses from regions of high water (hypotonic) to regions of low water (hypertonic) |
| Structure and Function of aquaporins | Increase water permeability of the membrane They have an hourglass shape The middle has a narrow point which water moves through side on |
| Channel proteins | Aqueous filled pores that allow passage of ions and some organic osmolytes across the membrane by passive diffusion These can help protect cells - when cells swell channels can open to allow molecules out with water following by osmosis Often selective |
| What are some channels gated by | Change in membrane potential -voltage Extracellular binding of a ligand to the channel or associated receptor - G protein coupled Intracellular binding of a messenger in response to binding of an external receptor Membrane deformation- stretch mediated |
| Different types of channel proteins | Voltage gated - Na+ channels Extracellular ligand gated - nicotinic ACh receptor Intracellular ligand gated - cGMP regulated K+ channels Stress activated - TRP channels |
| How to G protein coupled receptors work | They can activate subunits directly - subunits rearranged and bind to other ion channels to open them Activate enzymes that generate a cascade of intracellular messengers that act on membrane proteins etc to open an ion channel |
| Example of different types of channel working - Knee jerk reflex | Pulling on the muscle detected by stretch mediated channels. That open and let cations into the nerve, depolarising it. The action potential is conducted by voltage gated Na channels. This causes acetylcholine released to bind to ligand operated receptors |
| Gap Junctions | These allow intercellular connection, formed by two aligned connexons (with six connexins) in cell membranes of adjacent cells. Two pores form a patent, non-selective channel for electrical and chemical cell-to-cell signalling e.g. in the heart |
| How do carrier proteins work | Carriers bind the solutes they transfer and undergo a conformational change (changing which side is open) Transport is slower, more sensitive to temperature and has a Vmax/Km) Passive or active |
| What is active transport | Accumulation of a substance above the level that would be predicted at electrochemical equilibrium (even if momentarily) is evidence of active transport e.g. pH should be 6.5 due to negative potential and passive equilibration, but AT of H+ leads to 7.1 |
| Passive carriers | These mediate facilitated diffusion. Only for carrier mediated transport. This is passive where solutes move down their gradients to reach equilibrium. This balances electrochemical gradients E.g. GLUT and AE1 (used in chloride shift) |
| Active carriers | These accumulate solutes above the level predicted by passive equilibration. Conformational changes of the carrier us ATP hydrolysis. Primary active transporters are ATPases e.g. Na+/K+ ATPase and H+/K+ ATPase. ATP hydrolysis phosphorylates the protein |
| Mechanism of the Na+/K+ pump | Na+ binds to pump, which is phosphorylated via ATP hydrolysis. This triggers a conformational change releasing the 3 Na+ out of the cell. K+ then binds and the phosphate is released, causing the pump to return to its original shape and release the 2 K+ in |
| Secondary Active carriers | Use gradients made by primary systems to energise transport of other solutes. This is normally an inward Na+ gradient. Ca2+ is never used. Solute binding triggers a conformational change and translocation of 2 solutes across the membrane. |
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