Membrane Structure and Function II

Question 1

What are the major functions of the plasma membrane?

Answer 1

• barrier
• transport
• signal transduction

Question 2

What are the major functions of the inner and outer membranes of the mitochondria?

Answer 2

• INNER: energy transduction

- OUTER: barrier

Question 3

What are the major functions of the rough and smooth endoplasmic reticulum?

Answer 3

ROUGH:

• translation
• protein processing

SMOOTH:
- synthesis of complex lipids

Question 4

What is the major function of the Golgi Apparatus?

Answer 4

• post-translational modification

Question 5

What is the major function of the nuclear membranes?

Answer 5

• attachment of chromatin

Question 6

What is the major function of the lysosomal membrane?

Answer 6

• holding in hydrolytic enzymes

Question 7

What is the major function of the peroxisomal membrane?

Answer 7

• fatty acid oxidation

Question 8

What kinds of molecules can get through the membrane by simple diffusion?

Answer 8

• hydrophobic molecules, such as N2 or O2

- small, uncharged, polar molecules, such as H2O, CO2, urea and glycerol

Question 9

What kinds of molecules cannot get through the membrane by simple diffusion?

Answer 9

• large, uncharged, polar molecules, such as glucose

- ions, such as H+, Na+, K+, HCO3-, Ca2+, Cl-, Mg2+

Question 10

What are the different ways in which membrane transport occurs?

Answer 10

Membrane transport can be passive (no energy required) or active (energy required)
It can also be non-mediated (i.e. molecules just move through, always passive) or it can be carrier-mediated (could be passive or active).

Question 11

What are the two kinds of cotransport?

Answer 11

Cotransport 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.

Question 12

Why is glucose transport carrier-mediated?

Answer 12

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.

Question 13

Expand on GLUT transporters.

Answer 13

There is a family of glucose transporters called 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, meaning it’ll get saturated at low levels of glucose.

Question 14

List the five GLUT transporters, where they are found in the body and any additional comments.

Answer 14

GLUT1:

• found in all mammalian tissue
• responsible for basal glucose uptake

GLUT2:

• found in liver and pancreatic β cells
• in the pancreas, plays a role in regulation of insulin
• in the liver, removes excess glucose from the blood

GLUT3:

• found in all mammalian tissue
• responsible for basal glucose uptake

GLUT4:

• found in muscle and fat cells
• the amount in muscle plasma membranes increases with endurance training

GLUT5:

• found in the small intestine
• primarily a fructose transporter

Question 15

Describe glucose transporters in action.

Answer 15

The transporter binds to glucose and undergoes a conformational change. This opens the inner portion to the cytoplasm and the glucose diffuses in. The transporter goes back to its original shape.

Because the transport of glucose is driven by the concentration gradient of glucose, the cell maintains this gradient by phosphorylating the glucose after it passes the membrane.

Question 16

What are the kinetics of glucose transport?

Answer 16

They are similar to enzyme kinetics. They exhibit saturation-type kinetics (ie. starts to plateau) and can be regulated.
The kinetics are also specific to the molecules they transport.

Question 17

Glucose uptake can be increased by increasing the number of transporter. Illustrate this with the example of GLUT4 transporters in adipocytes and skeletal muscle, which are stimulated by the presence of insulin.

Answer 17

When there is no insulin present, the transporters are brought inside the cell via a vesicle (ie. away from the membrane). This stops the transporters from functioning.
When insulin is present, the vesicle with the transporters fuses with the membrane to express them on the membrane. This allows the transporters to function.

Question 18

Give an example of active transport.

Answer 18

An example of active transport would be with the movement of Na+. The concentration of Na+ outside cells (around 140mM) is higher than inside the cells (around 15mM), 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 an antiport system with potassium; the protein involved is a Na+/K+ ATPase. It requires energy in the form of ATP to work. It pumps 3 Na+ out for every 2K+ in, establishing the concentration gradient inside and out.

Question 19

How does foxglove affect the Na+/K+ pump?

Answer 19

Foxglove contains the chemical Digitalis. Digitalis is a cardiotonic steroid which inhibits the Na+/K+ pump. This increases the force of contraction of the heart, so it’s the treatment of choice for congestive heart disease.
The reason it does this is that the inhibition of the pump leads to increased intracellular Na+. This leads to slower removal of Ca2+ by the Na+/Ca2+ exchanger, resulting in an increased ability of the muscle to contract using the excess Ca2+.

Question 20

Describe the sodium-dependent glucose transporters and how they work.

Answer 20

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.

Question 21

Describe the positioning of transporters on the cells lining the gut

Answer 21

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.

Question 22

What is the point of compartmentalisation?

Answer 22

• separates reactions
• enables the local environment to be regulated for instant pH
• brings reactants together

Question 23

Proteins have signal sequences which target them to specific organelles. Give an example of a known signal sequence.

Answer 23

Manose-6-phosphate (carbohydrate group) is added to lysosomal proteins/enzymes in the Golgi, which targets them to the lysosome.
Membrane carbohydrates are involved in cell recognition.