S2: Membrane Structure and Function II Flashcards
Function of different types of membrane
Plasma Membrane —>
- Barrier
- Transport
- Signal transduction
Mitochondrial (inner and outer membrane) —>
- Energy transduction
- Barrier
Endoplasmic Reticulum (rough and smooth) —>
- Translation protein processing
- Synthesis of complex lipids
Golgi Body —>
- Post translational modification
- Processing for secretin
Nuclear membranes —>
- Attachment of chromatin
Lysosomes—>
- Hydrolytic enzymes
Peroxisomes—>
- Fatty acid oxidation
What is membrane function dependent on?
Membrane function is dependent on structure and the chemical composition of cell membrane.
Some are rich in lipid (eg. Myelin) whole others are rich in protein (eg. Inner mitochondrial membrane)
There are also varying amounts of carbohydrate in membrane
What type of molecules can diffuse through the membrane and what can’t?
The plasma membrane is semi permeable
Lipid soluble molecules (eg. N2, O2) and small uncharged polar molecules (eg. 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 eg. Glucose and ions like Na+, H+ cannot diffuse through by simple diffusion. Carrier proteins are required.
Explain how membrane transport can be: Passive Active Non mediated Carrier mediated
Passive - no energy required
Active - energy required
Non-mediated - molecules just move through membrane or channel - passive
Carrier mediated- passive or active
Define cotransport, antiporter and symporter
Co-transport is when a transporter moves two molecules at the same time. This is used generally to get one target molecule through using the concentration gradient of another
Antiporter - moving molecules in opposite directions
Symporter - moving them in the same direction
Why is glucose transport carrier mediated?
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 so the carrier mediated facilitated diffusion speeds up the rate of glucose transport greatly.
What is the family of glucose transporters?
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 large molecules. They have up to 12 transmembrane regions.
Describe
Membrane Transport Facilitated Diffusion:
Use an example
This is where conformational changes in the membrane protein result in transport, it is driven by the concentration gradient (so not active). Transport is bidirectional, it can go either way. The gradient is maintained by phosphorylation.
GLUT:
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 gradient needs to be maintained. The way the cell maintains this gradient is by the moment the glucose enters it gets phosphorylated so it is charged and cannot diffuse back out through the membrane.
Describe kinetics of glucose transport on rates of transport to [glucose ex ] graph
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.
Describe sodium active transport
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.
What does foxglove do?
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
Describe sodium dependent glucose transport
How is it an indirect AT process?
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.
How does cellular asymmetry allow transport of molecules with glucose as example?
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+.
Symmetry is maintained by tight junctions that prevent lateral diffusion of molecules.
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 is compartmentalisation?
Compartmentalisation uses a membrane and 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
How are lyosomal enzymes targeted and how is this different from most enzymes?
Not all proteins are targeted using their amino acid sequence, lysosomal enzymes (made in the golgi) are targeted by the carbohydrate mannose-6-phosphate.
