Forces Acting Across Membranes

Question 1

Describe the basic structures of membranes

Answer 1

• cell membrane is made of phospholipid bilayer
• freely permeable to some substances, permeability is selective
• membranes provide binding sites for chemical recognition
• dynamic - constantly formed and maintained or dismantled and metabolised depending on needs of cell
• very flexible due to fatty acids
• insulators

Question 2

What are the two classes of membrane proteins?

Answer 2

• Integral

- Peripheral

Question 3

Describe integral membrane proteins

Answer 3

• cannot be removed without disrupting membrane
• amphipathic with the same orientation os the phospholipids
• may span the membrane

Question 4

Give four examples of how an integral membrane protein might work

Answer 4

As

• channels - through which ions can cross the membrane
• carriers - to transport substances across i.e. pumps
• enzymes - with binding sites at the surface
• receptors - recognition sites for chemicals

Question 5

Describe peripheral membrane proteins

Answer 5

• can be removed without major disruption of function
• not amphipathic
• contact IMPs on the intracellular side of the membrane and tend to have enzymatic function
• important for cell shape and motility

Question 6

In what cells do membranes have very little protein content (18%) as they are mainly composed of lipid to provide insulation against electrical signals?

Answer 6

Schwann cells in nerves

Question 7

What organelles are very active and have a membrane protein content of around 75%?

Answer 7

Mitochondria

Question 8

What is diffusion across membranes in the body?

Answer 8

Diffusion occurs between compartments in the body, from a high concentration to a low concentration, provided the barrier between the two is permeable to the diffusing substance

Question 9

List the factors which favour diffusion through the membrane

Answer 9

• a large surface area
• high permeability
• high concentration gradient

Question 10

To diffuse through the lipid bilayer, molecules need to be

Answer 10

• small
• uncharged
• hydrophobic (lipophilic)

Question 11

Give four molecules which can diffuse through the lipid bilayer

Answer 11

• O2
• N2
• CO2
• urea

Question 12

What are protein channels?

Answer 12

Trans-membranous IMPs that act as an aqueous route for the diffusion of ions

• H2O passes through aquaporins
• some are always open while others are gated

Question 13

What are the two types of gated channels?

Answer 13

• voltage gated

- ligand gated

Question 14

Describe voltage gated channels

Answer 14

• changes in electrical potential act on the charged regions of the channel proteins producing a change in the configuration in their shape
• this opens or closes the channel e.g. Na+ channels in nerve cells

Question 15

Describe ligand gated channels

Answer 15

• when a certain chemical binds to the channel protein it produces a change in the configuration and opens or closes the channel
• e.g. Acetylcholine receptors

Question 16

Define electrochemical gradients

Answer 16

• for ion diffusion, electrical and concentration gradients need to be considered
• separation of charges across most cell membranes
• so inside of cell carries a relative negative charge in respect to outside
• this membrane potential can affect the diffusion of ions across the membrane - electrochemical gradient

Question 17

What is carrier mediated transport?

Answer 17

• carrier mediated transport proteins have binding sites for substances which cannot diffuse across or cross cell membranes via channels
• they bind to solute and undergo a change in configuration which exposes the site on the other side of the membrane so solute can diffuse into the cell
• protein returns to normal shape

Question 18

What are the two types of carrier mediated transport systems?

Answer 18

• facilitated diffusion

- active transport

Question 19

What is facilitated diffusion?

Answer 19

• transport of solutes down their conc gradient

- needs no direct energy source

Question 20

What is active transport?

Answer 20

• requires energy to move substances against the gradient
• energy comes from ATP so these pumps are known as ATPases
• e.g. Na+/K+ ATPase is in all cells

Question 21

Define osmolarity

Answer 21

The measure of solute concentration - number of osmoles of solute per litre of solution

Question 22

Define osmolality

Answer 22

The measure of osmoles of solute per kilogram of solvent

Question 23

Tonicity is determined by the volume of the cells which depends on

Answer 23

the concentration of non-penetrating solutes on the two sides of the membrane

Question 24

If the ECF has a higher tonicity than the ICF, the solution is

Answer 24

hypertonic

- cell will shrink as water leaves via osmosis to compensate

Question 25

If the ECF has a lower tonicity than the ICF, the solution is

Answer 25

hypotonic

- cell will swell as water enters the cell

Question 26

What is an isosmotic solution?

Answer 26

A solution in which there is an equal number of both penetrating and non-penetrating solutes on either side of the cell membrane

Question 27

What is an isotonic solution?

Answer 27

One in which there is an equal number of non-penetrating solutes on either side of the cell membrane

Question 28

Describe the process of endocytosis

Answer 28

Invagination of the membrane to form a vesicle around the target substance

• it eventually separates from the membrane on the cytoplasmic side and migrates within the cell to its destination
• exocytosis is the reverse process