Membrane structure and function 2 Flashcards
Give an overview of membrane structure and function
• Membranes act as semi-permeable barriers.
• Transport across them may be passive or active.
• ATP production is dependent on the properties of membranes:
o Structure
o Transport
Diffusion
Active
• Intracellular compartmentalisation.
• Cell-cell recognition.
What are some membrane-bound organelles and give their function
Membrane Major Function Plasma membrane Barrier, transport, signal transduction Mitochondria Inner and outer membrane Energy transduction Barrier Endoplasmic reticulum Rough Smooth Translation protein processing Synthesis of complex lipids Golgi Post translational modification Processing for secretion Nuclear membranes Attachment of chromatin Lysosomes Hydrolytic enzymes Peroxisomes Fatty acid oxidation
Describe membrane permeability and transport
- The plasma membrane is semi-permeable, lipid soluble molecules e.g. N2, O2 (and small uncharged polar molecules like Urea, H2O, glycerol, CO2) can move either way through the membrane and the movement of these molecules is dependent on their concentration. This is called simple diffusion.
- Large uncharged polar molecules e.g. glucose and ions e.g. H+, Na+ cannot diffuse through by simple diffusion.
- Membrane transport can be passive (no energy required) or active (energy required) and it can be non-mediated (i.e. molecules just moves through, could be straight through the membrane or a channel, passive) OR it can be carrier mediated (could be passive or active).
- Co- Transport is when a transporter moves two molecules at the same time, this can be an antiporter (moving molecules in opposite directions) or a symporter (moving them in the same direction), this would be used to generally get one target molecule through using the concentration gradient of another.
Describe glucose transport
- Glucose Transport is carrier mediated, it cannot get through quick enough by simple diffusion. It can be passive (by facilitated diffusion) or active (by sodium symporter).
- If there was no carrier, glucose transport would be incredibly slow, the carrier mediated facilitated diffusion speeds up the rate of glucose transport greatly.
- There is a family of glucose transporters (GLUT transporters), these each have their own tissue locations and a Kt value (same as Km), which tells us the affinity of the transporter for glucose. The higher the affinity (lower Kt value) the more readily it’ll bind to glucose. Glucose transporters are very big molecules
- Membrane Transport Facilitated Diffusion: This is where conformational changes in the membrane protein result in transport, it is driven by the concentration gradient (so not active). This gradient is maintained by phosphorylation. Transport is bidirectional, it can go either way.
- In action = The transporter binds to glucose and it then undergoes a conformational change, this opens the inner portion to the cytoplasm and the glucose diffuses in.
- Because the transport of glucose is driven by the concentration gradient of glucose, the way the cell maintains this gradient is by the moment the glucose enters it gets phosphorylated.
- The kinetics of glucose transport is similar to enzyme kinetics, firstly the transporter is specific for the molecules they transport (i.e. D-glucose, not L-glucose).
- The interactions exhibit saturation type kinetics (i.e. begins to plateau) and they can be regulated.
- Importantly, glucose uptake can be increased by increasing the number of transporters on the cell surface, for example the GLUT4 in adipocytes and skeletal
Describe active transport
- An example of active transport would be Na+, the concentration of Na+ outside cells (143mM) is higher than inside the cells (14mM), but there is no free movement as they cannot get through the membrane.
- Like facilitative transport, active transport is selective and requires an integral membrane protein.
- Sodium is linked to antiport system with potassium, the protein that does this is a Na+/K+ ATPase. It requires energy in the form of ATP to work. It pumps 3 Na+ out for every 2K+ in, this is what establishes the concentration gradient between inside and outside.
- Foxglove contains the chemical digitalis, which is a cardio tonic steroid which inhibits the Na+/K+ pump.
- This increases the force of contraction of the heart, so is the treatment of choice for congestive heart disease. The reason it does this is because inhibition of the pump leads to increased intracellular Na+ which thus leads to slower removal of Ca2+ by the Na+/Ca2+ exchanger, resulting in increased ability of the muscle to contract.
- Sodium dependent glucose transporters are SGLUT-1 and 2. These are symporters which will only work if you have both Na+ and glucose bound, once both are bound both will be transported into the cell. This by itself is a passive process, the glucose gets phosphorylated while Na+ is pumped actively out by the Na+/K+ ATPase in order to maintain the Na+ concentration gradient. Hence, because of this, the SGLUT-1 and 2 would be said to be indirectly active processes because they rely on this concentration gradient brought about by an active process.
What is cellular asymmetry?
- Cellular Asymmetry allows transport of molecules, as can be seen in the picture below, the SGLUT-1 is a Na+ driven glucose symporter. Present on the gut lumen side of the enterocyte, this transports glucose from area of lower concentration to an area of higher concentration using the concentration gradient of Na+.
- On the other side of the cell (facing the extracellular fluid, the basal domain) is the GLUT -2 which transports glucose down its concentration gradient (from higher in the cell to lower in ECF). The Na+/K+ ATPase pump also works down here
What does rehydration therapy target?
• Targets the co-transport of glucose and sodium.
Give an example of a disease involved with transport
Cystic Fibrosis - mutation of chloride ion channel
What is the purpose of compartmentalisation and how are proteins targeted to different areas?
- Compartmentalisation separates reactions, enables the local environment to be regulated for example pH and also helps bring reactants together.
- Proteins get to their specific compartment/organelle by signal sequences which target them to specific organelles.
- You can target a protein to a different location by changing the signal sequence.
What are the two functions of cell receptors?
• Receptors will be present on membranes for hormones and neurotransmitters.
What are membrane carbohydrates involved with?
• Membrane carbohydrates are involved in cell recognition, such as blood antigens which are glycolipids, lymphocyte trafficking down blood vessel wall by selectins – requires a type of glycoprotein.
Give an example of how a protein is not targeted by there signal sequence
Not all proteins are targeted using their amino acid sequence, lysosomal enzymes (made in the Golgi) are targeted by the carbohydrate mannose-6-phosphate. Individuals that are deficient in phosphotransferase enzyme present in the Golgi cannot modify the mannose normally present on enzymes targeted for the lysosomes. Various diseases are caused by the inability of the cell to target the enzyme to the lysosome. Lysosomes are an acidic environment due to the proton pump that pumps H+ into the lysosome.
