Forces acting across membranes

Question 1

Describe the basic structure of membranes.

Answer 1

-Phospholipid bilayer
-Selectively permeable
-Membranes provide binding sites for chemical recognition(Hormones)
-They are dynamic(constantly formed and maintained or dismantled and metabolized depending on the needs of the cell)
- Flexible due to fatty acids however may rupture if stretched
Insulators (hydrophobic hydrocarbon tails)

Question 2

How many classes of the membrane proteins are there?

Answer 2

2 classes
Integral membrane protein
Peripheral proteins

Question 3

Define integral membrane protein

Answer 3

They cant be removed without disrupting the membrane. They are amphipathic with the same orientation as the phospholipids and may span the membrane.

Question 4

What types of integral membrane proteins are there?

Answer 4

There’s 4

i) Channels, through which ions can cross the membrane
ii) Carriers, to transport substances across membranes (e.g pumps)
iii) Enzymes, with binding sites at the surface
iv) Receptors, recognition sites for chemicals

Question 5

Define Peripheral proteins

Answer 5

These can be removed without major distruption of function. They arent amphipathic. They Contact IMPS on the intracellular side of the membrane and tend to have enzymatic function. They are important for cell shape and motility. (e.g Dystrophin)

Question 6

Membranes can vary in protein content. Give me 3 examples?

Answer 6

Schwann Cells in nerves have very little protein (18%) as they are mainly composed of lipid, to provide insulation against electrical signals.
Most other cells have greater activity and protein is around 50%
Mitochondria are incredibly active and have a protein content of around 75%

Question 7

Define what is meant by diffusion across membranes

Answer 7

In the body, diffusion occurs between compartments, from a high concentration to a low concentration, provided the barrier between the two is permeable to the diffusing substance.

Question 8

List the factors which favour diffusion through a membrane.

Answer 8

Diffusion is aided by:
- A large surface area
- High permeability
High conc. gradient

To diffuse through the lipid bilayer, molecules need to be:
- Small
Uncharged
Hydrophobic (Lipophilic)
for example, oxygen and nitrogen, are smal2l, uncharged, and lipophilic. CO2 and Urea are small, uncharged, and polar, but can still pass-through

Question 9

Describe the different general types of protein channels

Answer 9

Protein channels are trans-membranous IMPs that act as an aqueous route for the diffusion of ions
Water passes through aquaporins, a family of water channels.
Some channels are always open, whereas others are gated.

Question 10

Define Voltage-Gated channels

Answer 10

The changes in the electrical potential act on the charged regions of the channel proteins, producing a change in the configuration in their shape, opening or closing the channel e.g Na + channels in the nerve cell.

Question 11

Define ligand-gated channels

Answer 11

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 12

Define electrochemical gradients

Answer 12

For ion diffusion, we need to consider the electrical gradient as well as the concentration gradient.
There is a seperation of charge across most cell membranes, so that the inside of the cell carries a relative negative charge in respect to the outside. This membrane potential can affect the diffusion of ions across the membrane. There is thus, an electrochemical gradient.

Question 13

Explain what is meant by carrier-mediated transport systems

Answer 13

Not all ions can cross cell membranes via channels, nor can all uncharged molecules, such as glucose, diffuse across the membrane.

Carrier mediated transport proteins have binding sites for such substances. When they bind the solute, they undergo a change in configuration which exposes the site on the other side of the membrane. From here, the solute diffuses into the cell and the protein returns to its normal shape.

Question 14

What are the two types of carrier-mediated transport systems?

Answer 14

Facilitated Diffusion- Transport of solutes down their concentration gradient. Need no direct energy source

Active transport- require energy to move substances against the gradient. this energy comes from ATP, so these pumps are known as ATPases and are sometimes called “pumps”. e.g Na/K ATPase is in all cells. It extrudes 3 Na and takes in 2 K for each molecule of ATP hydrolysed. Since it produces a net positive charge out of the cell, it is an electrogenic pump. It uses about 40% of the resting energy of the body.

Question 15

Define the osmolarity and osmolality of a solution

Answer 15

Osmolarity is the measure of solute concentration.
- The number of osmoles of solute per litre of solution.
Osmolality is the measure of osmoles of solute per kilogram of solvent
e.g 1M of glucose has an osmolarity of 1 osmole/L
1M of NaCl has an osmolarity of 2 osmole/L
The volume of the cell depends on the conc. of non-penetrating solutes on the 2 sides of the membrane. This determines the tonicity.
If the ECF has a higher tonicity than the ICF, the solution is hypertonic and the cell will shrink as water leaves via osmosis to compensate.
If the ECF has a lower tonicity than the ICF, the solution is hypotonic, and the cell will swell as water enters the cell.

Question 16

• Explain the difference between an isosmotic solution and an isotonic solution.

Answer 16

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

An isotonic solution is one in which there is an equal number of non-penetrating solutes on either side of the cell membrane

Question 17

• Describe the processes of endocytosis and exocytosis.

Answer 17

In endocytosis, there is an 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.