Membrane Structure and Function II

Question 1

What are the functions of plasma membrane?

Answer 1

Barrier, transport, signal transduction

Question 2

What are the functions of mitochondria?

Answer 2

inner membrane - energy transduction

outer membrane - barrier

Question 3

What is the function of endoplasmic reticulum ?

Answer 3

rough- translation protein processing

smooth - synthesis of complex lipids.

Question 4

What are the functions of golgi membrane ?

Answer 4

post- translational modification

processing for secretion

Question 5

what are the functions of the nuclear membrane?

Answer 5

attachment of chromatin

Question 6

what are the function of lysosomes?

Answer 6

hydrolytic enzymes

Question 7

what are the function of peroxisomes?

Answer 7

fatty acid oxidation

Question 8

What is the role of myelin?

Answer 8

Myelin acts as an insulator and forms a sheet around the neurons and the component of that membrane is lipid fitting with this role as an insulator. the plasma membranes they function largely and have similar functions. the composition is slimmer. 50 50 split between proteins and lipids. the higher protein content is associated with the hepatocyte plasma membrane which fits with a more interactive role.

Question 9

Membrane permeability

Answer 9

• lipid soluble molecules can move either way through the membrane.
• movement of these molecules is dependent on their concentration.
• this is called simple diffusion

Question 10

membrane permeability

Answer 10

membrane will act as semi permeable barrier to molecules. the molecules that can readily cross the membrane are hydrophobic molecules e.g. nitrogen and oxygen or small uncharged polar molecules e.g. water, urea and glycerol.

some molecules wont readily pass these types of membranes large, uncharged or polar molecules e.g. glucose and ions.

in order to cross the membrane there has to be a transport mechanism.

Question 11

Membrane transport

Answer 11

• passive mechanisms- simple diffusion or simple diffusion through a protein.
• some require a protein/ ion channel. they move from a region of high concentration to a region of low conc.
• carrier mediated mechanisms- proteins in the membrane which acts to pass these molecules from one side to the other. Concentration gradients- movement of single molecules in one direction - uriporters.
  active mechanisms.

Question 12

Membrane transport - co transport

Answer 12

symporter - molecules travelling across the membrane in the same direction. Requires the presence of a and b transporter to work.

antiporter- movement of one molecule in one direction and another molecule in the opposite direction.

Question 13

Carrier mediated facilitative diffusion vs. simple diffusion

Answer 13

one benefit of carrier mediated diffusion is that it is quicker than simple diffusion. However the conc. of the solute is it increases the rate of which the transport increases begins to tail off and this will lead to saturation of the response. This is a significant difference between diffusion, simple diffusion and carrier mediated diffusion.

carrier mediated transport can be saturated but simple diffusion is unlimited.

Vmax- maximum rate at which transport can take place no matter how much more the solute conc. goes up.
Kt- as an affinity of the transporter for its substrate, the molecule that it will transport.

Question 14

How do carrier mediated transporters function?

Answer 14

• transport of glucose is selective- i.e. D glucose not L glucose
• Transport of glucose can be increased by increasing the number of transporters on the cell surface
• GLUT 4 transporters are increased on muscle and adipocytes by insulin.

Question 15

Active Transport

Answer 15

• Active Transport
• Requires energy in the form of ATP

Co-transporter - antiporter, as its transporting both sodium and potassium in opposite directions. it requires the consumption of energy

Transporters dont exist in isolation. The cell may have different types of transporters. Some of these transport mechanisms will work together.

Question 16

Sodium dependent glucose transporters SGLUT-1 and 2

Answer 16

The aim of this transporter is to increase the uptake of glucose from the gut into the bloodstream. It does this by transporting both glucose and sodium together. This transporter called SGLUT-1 and 2 will transport glucose when there is sodium present. The driving force for the movement of this glucose is not the conc. gradient of glucose but the sodium ions. The conc. of sodium ions in the cell is about 10x less than that on the outside of the cell. If the transport of glucose was to continue, then the sodium must be removed from the cell. This is done by the action of the sodium potassium ATPase. Sodium- glucose cotransporter is a passive transport. It does not require the consumption of energy. ATP hydrolysis has to continue.

Question 17

Cellular Asymmetry

Answer 17

In order for glucose to be taken up from the gut and released into the bloodstream the transporters that are involved are asymmetrically distributed. Glucose and sodium co transporter of the SGLUT 1 which will take up both glucose and sodium. The sodium will then be exchanged for potassium using sodium potassium ATPase. The location of this transporter is on the basal surface of the cell. Sodium is taken from the gut and transported into extracellular fluid. The conc. of glucose will start to rise and it will reach a point where the conc. is greater than the conc. of glucose in extracellular fluid. Glucose will be transported across the basal membrane by facilitated diffusion using transporter GLUT2 by conc. gradient.

Asymmetry is maintained as the plasma membrane of the adjacent cells comes together very tightly and forms tight junctions and they prevent the distribution of these transport proteins around the cell.

Question 18

Rehydration therapy

Answer 18

targets the co-transport of glucose and sodium.

• to combat the loss of water, loss of food during diarrhoea, therapy exploits the transport mechanisms. Co transport of sodium and glucose into the cells across the walls of the gut.
• Glucose and sodium are absorbed from the gut and the glucose can then be used as an energy source, the sodium will have the added effect of drawing in water and rehydrating the individual.

Question 19

Compartmentalization

Answer 19

• intracellular membranes create local environments and separate reactions.
• enables electrochemical gradients to be established
• brings enzymes and reactants together and/or keep them apart.

Question 20

Compartmentalization energy production

Answer 20

proteins in inner mitochondrial membrane is much greater than proteins in the outer mitochondrial membrane. Inner mitochondrial membrane generates ATP. There are lots of proteins present in the membrane which are important for a hydrogen ion electrochemical gradient and in the generation of ATP. Impermeability of inner mitochondrial membrane allows the gradient to form.

Question 21

Macromolecule degradation

Answer 21

lysosome- hydrolytic enzymes which have maximum activity at low ph below 4. cytoslic ph is 7.2. these enzymes would not be active at cytosolic ph . The purpose of the membrane is to allow the difference in ph to be established. The membranes of the lysosomal have proton pumps which will hydrogen ions into the lumen of the lysosome which requires the hydrolysis of atp. Activity of enzymes can be maximum as the ph is lowered.

Peroxisome- important for breakdown of long chain fatty acids, over 50 different enzymes which are present within the peroxisome. The environment is very different from the lysosome. Ph is similar to that of cytosol. Movement of hydrogen ions is uneven.

Question 22

How do proteins get to the correct compartment?

Answer 22

Proteins have signal sequences which target them to specific sequences

Question 23

A disease of misdirection

Answer 23

Add a carbohydrate signal to the protein.

Directing an enzyme to the lysosome. Organelle- proteins function at low ph. Under normal circumstances, protein will undergo post-translation modification (addition of a mannose signal) which will be phosphorylated. Phosphorylated mannose will direct a hydrolytic enzyme to lysosomes.

Question 24

Key facts about membranes

Answer 24

• membranes are important in establishing local environments.
• they help form gradients or separate metabolic processes
• failure of the membranes and or their associated proteins can lead to disease