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Flashcards in Forces acting across membranes Deck (23):
1

Describe the basic structure of cell membranes

- The cell membrane is made up of a phospholipid bilayer.
- Freely permeable to some substances, but permeability is selective.
- Permeability can vary (increase or decrease)
- Membranes provide binding sites for chemical recognition (e.g. Hormones)
- They are Dynamic (constantly formed and maintained, or dismantled and metabolised depending on needs of cell.
- Very flexible due to fatty acids (may rupture if over stretched)
- Insulators (hydrophobic hydrocarbon tails)

2

List the types and functions of membrane proteins

There are two classes of membrane proteins.

1) Integral Membrane Proteins - channels, carriers, enzymes, receptors

2) Peripheral Proteins
These can be removed without major disruption of function. They are not amphipathic. They contact IMP’s on the intracellular side of the membrane and tend to have enzymatic function. They are important for cell shape and motility (e.g. Dystrophin)

Membranes can vary in protein content.

3

Define what is meant by diffusion across membranes

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.

4

List the factors which favour diffusion through the membrane.

Diffusion is aided by :
o A large surface area
o High permeability
o High conc. Gradient

To diffuse through the lipid bilayer, molecules need to be:
o Small
o Uncharged
o Hydrophobic (Lipophilic)

e.g. O2 and N2 because they are small, uncharged and lipophilic. CO2 and Urea are small, uncharged and polar, but can still pass through.

5

Describe the different types of channel proteins

H2O passes through aquaporins, a family of water channels.
Some channels are always open, whereas others are “gated"

o Voltage Gated Channels
The changes in 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 nerve cells.

o Ligand Gated Channels
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.

6

Define electrochemical gradients

For ion diffusion, we need to consider the electrical gradient as well as the concentration gradient.
There is a separation 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.

7

Explain what is meant by carrier-mediated transport systems

Not all ions 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.

There are 2 types.
1) Facilitated Diffusion
Transport of solutes down their conc. Gradient. Needs no direct energy source.
2) Active Transport
Requires energy to move substances against the gradient. This energy comes from ATP, so these pumps are known as ATPases and are sometimes called “pumps”.

8

Define osmolarity, osmolality and tonicity, and their effects on cell volume

Osmolarity is the measure of solute concentration, or the number of osmoles of solute per litre of solution

Osmolality is the measure of osmoles of solute per kilogram of solvent.

The volume of the cell depends on the conc. of non-penetrating solutes on the 2 sides of the membrane. This determines the tonicity.

9

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

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.

10

Describe the processes of endocytosis and exocytosis.

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.

11

Describe the basic function of the plasma membrane

Separates compartments
Site of various enzymes, receptors - signalling
Responsible for maintaining difference in ECF and ICF
insulation - maintains electrical stability of the cell

12

What are the 4 general classes of membrane proteins?

Receptors (IMP)
Transporters (IMP) - channels and carrier proteins
Enzymes (IMP)
Peripheral proteins - do not span hydrophobic core

13

What are the 3 main functions of peripheral membrane proteins?

Maintain structure by anchoring membrane to cytoskeleton
Attach cells to ECM
Perform signalling functions within the cell e.g. G proteins

14

What is the major component of myelin?

Fat - why its a good insulator

15

What would you expect of cells with high membrane protein content?

Involved in energy transduction

16

Where are all membrane carbohydrates found?

Extracellular - used in self recognition

17

What is usually secreted in vesicles during exocytosis?

Water soluble molecules e.g. proteins for secretion. Fusion of vesicle with membrane allows the expulsion of molecules that could not pass through the membrane alone

18

What characteristics do molecule need to pass through the plasma membrane unaided?

Small
Lipophilic
Uncharged

19

Give an example of an electrogenic pump

Na+/K+ ATPase

20

What % of resting energy of the body is used by the Na+/K+ ATPase?

40%

21

Main difference between osmosis and diffusion

Osmosis - movement of WATER from high to low
Diffusion - movement of SOLUTE from high to low

22

Define osmotic pressure

Osmotic pressure is the pressure that would have to be applied to a pure solvent to prevent it from passing into a given solution by osmosis, often used to express the concentration of the solution.

23

What is normal human plasma osmolarity?

285mOsmol/L (primarily Na+ and Cl-)