Forces acting across membranes 1 - 3

Question 1

Describe the basic structure of membranes.

Answer 1

Membranes are very thin double layers of sheets of lipids. (the lipid bilayer). The structure of membranes is sometimes referred to as the ‘fluid mosaic model’.
They are fluid in that they are flexible. This is due to fatty acids that can stretch.
They are mosaic in that they are composed of lots of different parts.
They are embedded with proteins.

Question 2

Describe the function of the cell membrane.

Answer 2

The cell membrane is a selective barrier, with varying permeability.
It is dynamic as it constantly remodelling itself to suit the cells needs.
The cell membrane allows the passive transport of water and some other molecules however the passage of ions and other uncharged polar molecules require other mechanisms when crossing the cell membrane.

Question 3

Define electrochemical gradients.

Answer 3

The electrochemical gradient is the result of the net effect of the concentration and electrical gradients of a cell membrane. Created by the differences in concentration and charge on both sides of the membrane.
** the electrochemical gradient drives the passive movement of substances into or out of the cell.

Question 4

Give a brief description of both kinds of membrane proteins and their functions.

Answer 4

Integral proteins span the hydrophobic core of the lipid bilayer.
Peripheral proteins are associated with only the phosphate head of the lipid bilayer.

Question 5

Describe receptor membrane proteins.

Answer 5

Receptor proteins are integral. They allow the communication of an extracellular signal from extracellular signalling molecules for example, neurotransmitters or hormones. This signal is received by the receptor and it initiates a response within the cell.

Question 6

Describe transporter (carrier mediated transport proteins) membrane proteins.

Answer 6

The transporter proteins are never fully open and must change conformation to open either to the extracellular or intracellular region of the cell.

Question 7

Describe channel membrane proteins.

Answer 7

Channel proteins create a water-filled pore, some are always open when some are gated.
Either voltage or ligand-gated.
Voltage-gated channel proteins open due to a change in electrochemical gradient.
Ligand-gated channel proteins open due to the binding of a ligand.

Question 8

Describe the enzyme membrane proteins.

Answer 8

Membrane enzymes catalyse chemical reactions on the cell membrane.
The enzymatic part may be on the external side e.g. those found in small intestine which break down nutrients into smaller parts, or on the internal side such as those associated with converting signals from a receptor into an intracellular response.

Question 9

Describe how structural membrane proteins maintain cell structure.

Answer 9

These structural proteins anchor the cell membrane to the intracellular cytoskeleton, to the extracellular matrix and/or to other cells. If there is any disfunction or loss of these cells this would lead to serious debility.

Question 10

Describe how membrane proteins are involved in communication.

Answer 10

This can be communication between or within cells.
An example could be glycoproteins*, which act as markers that tell the immune system whether a cell is our own or foreign.
Other types of these proteins are involved with communicating extracellular messages to the intracellular environment. e.g. receptor/enzyme proteins or G-proteins.

Question 11

Describe G-proteins.

Answer 11

G-proteins are peripheral proteins that relay a signal across the cell membrane from an integral receptor protein to an integral enzyme protein.

Question 12

Describe 3 different types of membrane and the importance of their protein content.

Answer 12

Myelin is a specialised type of membrane that serves as an insulator around myelinated nerve fibres. Myelin has a low protein content and its main component is lipid.
Plasma membranes have a 50% protein content and have a much greater activity than the above.
The membrane of mitochondria are composed of 75% protein and are very active.
*** the more function requires of the membrane the more protein in it.

Question 13

What is meant by ‘endo/exocytosis’.

Answer 13

Movement of macromolecules across membranes without disruption.

Question 14

Describe the process of endocytosis.

Answer 14

Endocytosis involves invagination of the cell membrane to form a vesicle.
The vesicle then disintegrates on the cytoplasmic surface of the membrane.
The contents of the vesicle are then released.
The contents then migrate within the cell to their destination.
**this is a common mechanism for terminating signals from extracellular ligands.

Question 15

Describe the process of endocytosis.

Answer 15

This is the reverse process of exocytosis.
Where molecules are assembled within the cell and packaged into vesicles that then fuse with the cell membrane, releasing them out of the cell.
Many proteins manufactured in the cell are released from cells by exocytosis.

Question 16

Describe what is meant by diffusion across membranes.

Answer 16

Diffusion is the process by which substances move from an area of high concentration to low concentration down their concentration gradient.
Molecules can diffuse passively i.e. straight through the lipid bilayer, or by facilitated diffusion where they diffuse via the proteins embedded in the membrane.

Question 17

List the factors which favour diffusion through the bilayer.

Answer 17

For passive diffusion;
Particles must be small, uncharged and lipophilic (this makes diffusion easier but is not a requirement).
Facilitated diffusion is used for larger molecules such as glucose.
Both kinds of diffusion rely on the electrochemical gradient of the membrane to diffuse, if the molecules was moving against its EC gradient, energy would be required.

Question 18

Define the difference between passive diffusion and facilitated diffusion.

Answer 18

In passive diffusion the particles pass directly through the lipid bilayer, e.g. O2 entering cells.
In facilitated diffusion, membrane proteins are used to facilitate which molecules, usually nutrients like glucose, are allowed entry to the cell.
Using channel or transporter proteins.

Question 19

Define the difference between facilitated diffusion and active transport.

Answer 19

Facilitated diffusion is the transport of molecules through membrane proteins such as channels or transporters. This does not require energy and is used when molecules are too big to diffuse directly through the bilayer.
Active transport on the other hand is used when energy is required to pump substances (usually ions) across the membrane, against their electrochemical gradient.
So the main difference really between both mechanisms of movement here is, the use of energy.

Question 20

Explain what is meant by carrier mediated transport systems.

Answer 20

In facilitated diffusion:
The carrier mediated transport protein allows diffusion for solutes that are perhaps too big for diffusion through the plasma membrane.
In active transport:
The carrier mediated transport protein acts as both a transporter and an enzyme, this is because active transport is only required when the solute is trying to cross the membrane against its electrochemical gradient, energy in the form of ATP is required to drive the movement of the solute. It hydrolyses this ATP to generate the energy for the transport.

Question 21

Explain what is meant by the term osmosis.

Answer 21

Osmosis is the net movement of water from an area of high concentration to low concentration, down its concentration gradient.

Question 22

Describe the difference between osmosis and diffusion.

Answer 22

Osmosis is the net movement of water …etc…
Whereas diffusion is the net movement of a solute from regions of high concentration to regions of low concentration.
Where we have osmosis, we may or may not have diffusion, where we have diffusion we will always have osmosis.

Question 23

Describe osmolarity and its units.

Answer 23

Osmolarity measures the concentration of biological solutions in units called OSMOLES, (more commonly MilliOsmoles).

Question 24

Explain the difference between osmolarity and tonicity.

Answer 24

Osmolarity measures the total number of particles in solution.
~ Tonicity measures the number of non-penetrating particles **in solution.
= in practice tonicity is much more important as it determines cell volume.

Question 25

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

Answer 25

An isosmotic solution has the same total number of particles in solution, as normal extracellular fluid (plasma).
An isotonic solution has the same number of non-penetrating particles as normal extracellular fluid (plasma).

Question 26

Apply the concept of chemical gradients to the movement of water.

Answer 26

Water has a concentration gradient, however is doesn’t have an electrical gradient as it doesn’t have a charge. Therefore, for solutes, (which do have electrochemical gradients), that cannot cross the membrane, whether that is because of their electrochemical gradient or their size = any change in their concentration results in an osmotic flux, causing a net movement of water in one direction or the other, which changes the cell volume.

Question 27

Outline how the principles of osmosis apply to prescribing fluids in the clinical setting.

Answer 27

In the hospital, plasma samples come back as mOsmoles/L, giving information about osmolarity but not tonicity. However tonicity is much more functionally important as it gives cell volume. We need to know the composition of that osmolarity in order to predict tonicity or volume.

Question 28

Describe the significance of osmolarity vs tonicity.

Answer 28

Tonicity = all non-penetrating molecules in solution.
Therefore, non-penetrating molecules + penetrating molecules (e.g. urea) = osmolarity.
We need osmolarity to determine tonicity and volume.

Question 29

Describe what can happen to cells if they are hypo/hypertonic.

Answer 29

Hypotonic cells swell (and could potentially burst) because water enters down its concentration gradient.
Hypertonic cells shrivel and shrink because water leaves down its concentration gradient.