Physiology chapter 2-Assisted membrane transport

Question 1

What two different mechanisms can be used to accomplish selective transport?

Answer 1

Carrier-mediated transport and vesicular transport.

Question 2

What is carrier mediated transport used for?

Answer 2

For transfer of small, water-soluble substances across the membrane.

Question 3

What is vesicular transport used for?

Answer 3

For movement of large molecules and multi-molecular particles between the ECF and ICF.

Question 4

What are the basic steps of carrier-mediated transport?

Answer 4

Molecule to be transported binds to binding site, causes conformational change, site is exposed at other side of membrane and the bound molecule detaches from the carrier.

Question 5

What are the three important characteristics of carrier-mediated transport?

Answer 5

Specificity, saturation and competition.

Question 6

What are the different forms of carrier-mediated transport?

Answer 6

Facilitated and active transport.

Question 7

What is facilitated diffusion?

Answer 7

Doesn’t require energy and uses carrier to transfer a particular substance with its concentration gradient.

Question 8

What is active transport?

Answer 8

Involves the use of a carrier protein to transfer a specific substance across the membrane against its concentration gradient. This requires ATP. When phosphorylated, the binding site has greater affinity for the molecule on the lower concentration side.

Question 9

Describe the important characteristics of the sodium-potassium pump.

Answer 9

The carrier transports 3 sodium ions from the ICF for every 2 potassium ions in the ECF.
Splitting of the ATP through ATPase activity and the subsequent phosphorylation of the carrier on the intracellular side increases the carrier’s affinity for sodium and induces a change in carrier shape.
When exposed to the ECF, there is increased affinity for potassium which binds and allows it to return to its initial conformation.

Question 10

What are the three main roles of the sodium-potassium pump.

Answer 10

Establish sodium and potassium gradients across the plasma membrane of all cells.
Helps regulate cell volume.
The energy used to run the pump also indirectly serves for secondary active transport of glucose and amino acids across intestinal and kidney cells.

Question 11

Why does this occur in the kidney and intestinal cells? (Secondary active transport)

Answer 11

The intestinal cells transport amino acids and glucose against their concentration gradients from inside the intestinal lumen into the blood, concentrating the latter so they are not lost in the feces.
The kidney cells save these nutrients for the body by transporting them out of the fluid that is to become urine.

Question 12

What are co-transport carriers?

Answer 12

The luminal carriers in intestinal and kidney cells.

They have two binding sites, one for sodium and one for the nutrient molecule.

Question 13

Where are the co-transport carriers located?

Answer 13

In the basolateral membrane. This is the membrane at the base of the cell opposite the lumen and along the lateral edge of the cell below the tight junction.

Question 14

What are the basic steps for glucose to reach the blood?

Answer 14

Sodium is kept in high concentration in the lumen (ECF) due to the sodium-potassium pump.
The co-transport carrier has a high-affinity for sodium in the ECF, this binds first.
The binding of sodium increases the affinity for glucose.
When both are bound, a conformational change occurs and the carrier opens to the inside of the cell.
The concentration gradient forces sodium into the cell which lowers the affinity for glucose.
Glucose then undergoes facilitated diffusion through the basolateral membrane down its concentration gradient.
The glucose diffuses into the blood through the capillaries.

Question 15

Describe vesicular transport.

Answer 15

Requires energy expenditure. Can either be exocytosis or endocytosis.

Question 16

What are the three different types of endocytosis?

Answer 16

Pinocytosis, receptor-mediated endocytosis and phagocytosis.

Question 17

Briefly describe endocytosis.

Answer 17

The plasma membrane surrounds the substance to be ingested.
This then fuses over the surface.
A membrane enclosed vesicle pinches off the membrane so that the engulfed material is trapped within the cell.
Once inside, a vesicle either fuses with lysosomes or bypasses the later and has its contents released by exocytosis.

Question 18

Describe Pinocytosis.

Answer 18

First, the plasma membrane dips inward, forming a pouch that contains a small bit of ECF.
The endocytic pouch is formed as a result of membrane deforming coat proteins.
The linking of coat proteins causes the plasma membrane to dip inward.
The plasma membrane then seals at the surface of the pouch, trapping contents in an intracellular endocytic vesicle.
Dynamin severs the vesicle from the surface membrane.

Question 19

Describe receptor mediated endocytosis.

Answer 19

Highly selective.
Triggered by the binding of a specific molecule to a surface receptor.
The binding causes the plasma membrane at that site to sink in, then seal at the surface, trapping the ligand inside.

Question 20

Describe phagocytosis.

Answer 20

Large multi-molecular particles are internalized.
WBC are generally capable of phagocytosis.
Pseudopods completely surround the particle and trap it within an internalized vesicle.
A lysosome fuses with it.
The enzymes break the contents down into reusable material.

Question 21

Describe exocytosis.

Answer 21

A membrane enclosed vesicle formed within the cell fuses with the plasma membrane, then opens up and releases its contents to the exterior.
Materials packaged up for export by the ER and Golgi Complex are externalized by exocytosis.

Question 22

What are the two different purposes of exocytosis?

Answer 22

Provides a mechanism for secreting large polar molcules such as protein hormones and enzymes that are unable to cross the plasma membrane. (need specific signal for this)
Enables the cell to add specific components to the membrane such as carriers, channels, etc..

Question 23

What is the balance provided by exocytosis and endocytosis?

Answer 23

Portions of the membrane are constantly being restored, retrieved and generally recycled.