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Organelles
Biochem and medical genetics
| Question | Answer |
|---|---|
| Typical animal cell size and organelles | Diameter of around 15 micrometres Specialised cells can be smaller or have larger extensions Contain a nucleus, mitochondria, golgi, endoplasmic reticulum, (smooth ER and round ER) and a cell membrane |
| Composition of a mammalian cell | 70% water 18% protein 3% phospholipids and small metabolic molecules 2% lipids and polysaccharides 1.1% RNA 1% inorganic ions 0.25% DNA (high info density) |
| Functions of cell membranes | Barrier between inside and outside Selective permeability to allow metabolic processes Response to external stimuli and electrical excitability Energy conversion processes Signalling to the external environment - needed in multicellular animals |
| Evidence for membrane as a flexible, dynamic barrier - Heller, Schaefer, Schulten (stimulator of bilayer) | At low temperatures phospholipids form a crystal structure and are rigid At a medium temperature they form a gel. More mobile but maintain coherence At high temperature they form a fluid structure, same basic structure but more movement |
| Basic structure of a membrane | Phospholipid bilayer with a hydrophobic core. Transmembrane proteins sit through both layers. Peripheral proteins lay on one side. Channels e.g. oligomers have an interior different to the bilayer so can form a channel for charged/polar molecules. |
| Different phospholipid arrangement in membranes | Lipids of the membrane can self associate to form patches of distinct composition e.g. all phosphatidylcholine in one area. This can happen independently in the outer leaflet, inner leaflet or both. These are known as lipid rafts |
| Protein composition of lipid rafts | Varies as different proteins are selectively enriched into different rafts. Depends on protein domain interaction with lipids and can be regulated by protein modification. Composition is dynamic and can be regulated by the cell for uptake or signalling |
| How does changing proteins affect function of membranes | Proteins inform the phospholipid behaviour. There is communication between phospholipids and the proteins present. E.g. membrane curving. Proteins interact with linear membrane. Oligomerisation makes more proteins which changes the shape of the membrane |
| Common features of biological membranes | Sheet like structures a few molecules thick Form closed boundaries Contain proteins in a lipid bilayer Functions are mediated by proteins Non-covalent assemblies held together by co-operative non-covalent forces Highly fluid structure |
| Asymmetry of membranes | The two faces of the membrane differ in composition of: Proteins - absolute asymmetry Lipids - Relative asymmetry |
| Internal organelles | Contain membrane bound organelles. These mediate specific independent functions within the cell. Size and shape is highly variable and reflect functional requirements Cells 'know' how much and where each organelle is. |
| Examples of organelles | Nucleus - double membrane has diffusional continuity can pass large structures Mitochondria - bacterial like membrane and ribosomes ER, Golgi - secretory pathway Endosomes, phagosomes, lysosomes - uptake pathway Membrane less phase-separated droplets |
| Adding fluorescent tracers to show uptake pathways | Fluorescent tracers were added to a cells external environment to show its uptake pathway. A green dye was taken up by the plasma membrane, whilst a pale pink dye was taken up by the receptor system and concentrated in vesicles |
| Demonstration of temperature dependant morphology | At 37 degrees there are 3 aspects to a secretory pathway - golgi, ER and an intermediate. At 15 degrees membranes become more gell like, intemediate disappears. Transient intermediate disappears when transportation stops (membrane less permeable) |
| Demonstration of selectivity of uptake and transport | Mixed red and green fluorescent dyes added to cells. This was washed off to see what cells had taken up. The membrane bound structures only took up one colour dye, separating them. This shows that membranes selectively take up substances |
| Phase separation | A method of separating structures without a membrane When threshold is increased (temp, ionic strength etc) material is dispersed with no separation. When threshold is decreased material is condensed in some areas and phase separation occurs |
| Evidence for phase separation | Some segregated cells respond at a rate not compatible with transport through a membrane or using second messengers, so must be separated by another mechanism |
| Example of phase separation - RNA | Protein interactions drive formation of a pre-initiation intermediate. (soluble molecules form oligomers as a basis of phase separation). When RNA levels increase it is phase separated e.g in a stress granule, so when needed removed from separation |
| The cytoskeleton | Several complex networks of filaments extending through the cytoplasm. Organises intracellular space by providing a scaffold for organelle attachment. Orchestrates intracellular transport by providing guidance and motive power for vesicle movement. |
| Components of the cytoskeleton | Tubulin polymers form microtubules - linear polymers arranged in a circle Actin polymers form microfilaments - myosin motor proteins move organelles along these Intermediate filament polymers form intermediate filaments - structural roles |
| Dependence of vesicle movement on cytoskeleton | The network of microtubules has tubulovesicular compartments attached to it. These depend on the microtubules for shuttling through the cell |
| Dependence of organelle behaviour on cytoskeleton | With an intact microtubule network mitochondria are spread out through the cell. When the microtubules disassemble (e.g. due to disease) the mitochondria collapse back to the nucleus. |
| Transport and communication between organelles | Transport between organelles achieved by vesicular transport. This prevents loss of organelle content in the cytoplasm. Transport vesicles move between donor and acceptor compartment. Membrane composition regulates what is transported. |
| Secretory pathway as an example of vesicular transport | Multiple vesicles required to move proteins between organelles. Vesicles must collect proteins from correct compartment and move them to correct location. (Cargo selection and targeting). This acts as quality control, only correct proteins are selected |
| Machinery used to ensure fidelity of vesicle transport | Receptors on vesicles ensure that action mediated by other proteins allow vesicle formation and selective action. Machines used to make vesicles must have a method of knowing what is being transported - potentially these receptors? |
| Cystic fibrosis | A progressive disorder leading to lung damage. The airway becomes obstructed by a thick mucus. This is too thick to be moved so compromised gas exchange |
| What causes cystic fbrosis | Mutations in both copies of the CFTR gene. This forms an ATP dependant multiple membrane spanning anion channel. Loss of function prevents resorption of luminal Cl- into surrounding calls, preventing Na+ resorption and leading to local dehydration. |
| Common mutations in cystic fibrosis | Most common is delta F-508, a 3bp deletion resulting in loss of phenylalanine at position 508 in the protein. This protein folds poorly, so is removed by the quality control system in the ER and never reaches the plasma membrane. |
| Effect of temperature on CF | Some CFTR may reach the membrane. This functions best at low temps where reaction rate and ion movement is slower. But we cannot maintain a low temperature permenantly |
| New treatments for CF | Based on understanding of trafficking of CFTR through the cell. Potentiators - increase gating and conductance of CFTR present at the cell surface Correctors - increase number and function of CFTR Combination - increase number and enhance function |
| Example of a potentiator | Ivacaftor |
| Example of a corrector | Lumacaftor - improves folding in ER |
| Study into treatment for CF - Barry | A triple combination of two correctors (elexacaftor, tezacaftor) and a potentiator (ivacaftor) shows significant improvement in lung function over either class alone |
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