Human Physiology - Chapter 5: The transport functions of the plasma membrane.

Question 1

What does the plasma membrane consist of?

Answer 1

a lipid bilayer which has many different proteins in it.

Question 2

The two types of membranes?

Answer 2

Natural membranes, and artifical lipid bilayers

Question 3

Both natural membranes and artifical lipid bilayers are permeable to what?

Answer 3

hydrophobic molecules such as gases and lipid-soluble molecules.

Question 4

Compared to lipid bilayers, natural membranes have…

Answer 4

… high permeability to water and to water-soluble molecules (hydrophilic or polar molecules).

Question 5

What are examples of water-soluble molecules?

Answer 5

glucose, ions such as sodium, potassium, and chloride.

Question 6

Why do natural membranes have high permeability to ions and polar molecules?

Answer 6

due to two types of proteins in their membranes: channel proteins and carrier proteins.

Question 7

What determines which direction uncharged molecules like carbon dioxide, oxygen, and urea travel across the plasma membrane?

Answer 7

determined by the concentration gradient

Question 8

What determines the direction that charged molecules and ions travel in?

Answer 8

an electrical potential of mammalian cells called membrane potential.

Question 9

What do membrane potentials influence

Answer 9

the diffusion of charged molecules and ions.

Question 10

Positively charged chemical species will tend to be attracted into…

Answer 10

…the cell

Question 11

Positively charged chemical species will tend to be attracted into the cell. While negative ones will…

Answer 11

… tend to be repelled.

Question 12

What three factors determine the directions in which ions and charged molecules move across the cell membrane?

Answer 12

1) the concentration gradient
2) the charge of the molecule or ion
3) the membrane potential.

Question 13

the factors combine to give rise to…

Answer 13

… the electrochemical gradient.

Question 14

how can the electrochemical gradient be calculated?

Answer 14

from the difference between the equilibrium potential for the ion in question and the membrane potential.

Question 15

how do molecules and ions diffuse across the plasma membrane?

Answer 15

diffuse across the plasma membrane down their electrochemical gradients.

Question 16

Molecules and ions diffuse across the plasma membrane down their electrochemical gradients. What is this called?

Answer 16

passive transport

Question 17

Polar materials such as ions and small organic molecules such as glucose and amino acids cross the plasma membrane much more..

Answer 17

…readily than they cross artificial lipid bilayers.

This is called facilitated diffusion.

Question 18

it is now clear that ions and polar molecules, inluding water, pass across the plasma membrane by…

Answer 18

… membrane proteins known as ion channels and carrier proteins.

Question 19

Cells can transport molecules and ions against their prevailing electrochemical gradients. This uphill transport requires a cell to expend metabolic energy either directly or indirectly and is called…

Answer 19

… active transport.

Question 20

While the direction of passive trnasport is determined by electrochemical gradient, the rate at which it can cross the plasma membrane is determined by…

Answer 20

… the number of properties of the channels and/or carrier proteins present.

Question 21

If ther are few channels or carrier proteins for a particular molecule or ion, the permeability of the membrane to that substance will be…

Answer 21

… low. The more channels or carriers that are available, the greater the permeability of the membrane to that substance.

Question 22

the relatively high permeability of natural membranes to water can be attributed to…

Answer 22

… the presence of specialised water channels known as aquaporins.

Question 23

What are aquaporins?

Answer 23

proteins that possess pores allowing water to pass from one side of the membrane to other according to the prevailing osmotic gradient.

Question 24

How many types of aquaporins are known in mammals?

Answer 24

9

Question 25

where are aquaporins commonly found in mammals?

Answer 25

tubules of the kidney and the secretory glands of the gut.

Question 26

In the collecting ducts of the kidney, aquaporins 2 and 3 play a central role in …

Answer 26

…regulating water reabsorption.

Question 27

What are ion channels?

Answer 27

specific membrane protein that have a pore which spans the membrane to provide a route for a particular ions to diffuse down its electrochemical gradient.

Question 28

what are ion channels responsible for?

Answer 28

the relatively high permeability of natural membranes to various ions such as sodium, potassium, and chloride.

Question 29

How can ion channels exist?

Answer 29

can exist in one of two main states: either open and allow the passage of the appropriate ion from one side of th emembrane to the other.

Or, they are closed, preventing such movements.

Question 30

Many ion channels have a very high capacity for…

Answer 30

…transport

Question 31

ions channels are able to discriminate…

Answer 31

… one type of ion from another so that they show selectivity with respect to the kind of ion they allow through their pore.d