Forces Acting Across Membrane 1-3

Question 1

Describe the basic structure of membranes.

Answer 1

Phospholipid bilayer - So has a outer head which is hydrophilic and an internal tail with is hydrophobic

Question 2

Describe the function of the cell membrane

Answer 2

Cell membrane is a selective barrier it also provides protection for a cell. as well as a fixed environment inside the cell, and that membrane has several different functions. One is to transport nutrients into the cell and also to transport toxic substances out of the cell.

Question 3

Define electrochemical gradients.

Answer 3

The electrochemical gradient determines the direction that ions will flow through an open ion channel and is a combination of two types of gradients: a concentration gradient and an electrical field gradient.

Question 4

List the different mechanisms of movement of particles across the membrane.

Answer 4

1. Diffusion
   
   • either:
     A. Passive diffusion directly through the lipid bilayer or;
     B. Facilitated diffusion via either:
     i) protein channels or;
     ii) protein transporters
2. Active Transport
3. Osmosis
4. Filtration

All of the above are influenced by electrochemical gradients.

Question 5

Describe the processes of endocytosis and exocytosis.

Answer 5

In endocytosis, there is invagination of the cell membrane to form a vesicle which eventually disintegrates on the cytoplasmic (inside) surface of the membrane, releasing contents which then migrate within the cell to their destination.

Exocytosis involves the reverse process. Many proteins manufactured in cells are released from those cells by the process of exocytosis.

Question 6

Define what is meant by diffusion across membranes.

Answer 6

If a molecule can pass freely through a cell membrane, it will cross the membrane by diffusion

Question 7

List the factors which favour diffusion through the lipid bilayer.

Answer 7

The concentration gradient - The greater the difference in concentration, the quicker the rate of diffusion.

The temperature - The higher the temperature, the more kinetic energy the particles will have, so they will move and mix more quickly.

The surface area of the cell membrane separating the different regions - The greater the surface area, the faster the rate of diffusion.

Question 8

Define the difference between passive diffusion and facilitated diffusion.

Answer 8

A. Passive diffusion – substance moves directly through the lipid bilayer
B. Facilitated diffusion – substance requires assistance from membrane proteins to cross the lipid bilayer. (requires energy)

Question 9

Describe the different types of membrane proteins and their functions.

Answer 9

Channel - These are transmembrane proteins that provide an aqueous route through the membrane for the simultaneous diffusion of water and ions. Only allow the passage of mineral ions such as Na+ and K+, Cl-, Ca2+ and H+ accompanied by H2O. Molecules such as glucose are too large to go through.

Mediated transport proteins - Molecules too big to move through channel proteins use carrier-mediated transport proteins or “transporters”. These proteins have binding sites for the solutes they transport such as glucose. When they bind the solute, the transporter undergoes a change in shape which exposes the binding site on the other side of the membrane. The solute moves away from the transporter and the transporter returns to its original configuration.

Question 10

Explain what is meant by carrier mediated transport systems (i.e. transporters) - this
covers both facilitated diffusion and active transport.

Answer 10

If electrochemical gradient favours direction of movement, movement occurs by diffusion and no energy input is required.

If the electrochemical gradient opposes movement then energy in the form of ATP is required to move the molecule against this gradient. In these cases the carrier protein also functions an enzyme (ATPase) as it hydrolyses ATP to release energy

Question 11

Explain what is meant by the term osmosis.

Answer 11

Net movement of H2O from regions of high H2O concentration to regions of low H2O concentration

Question 12

Describe the difference between osmosis and diffusion.

Answer 12

Water can move freely between cells and the ECF so that the body is in osmotic equilibrium. Not all solutes move freely.
ALL H2O movements in the body are passive, (via aquaporins, protein water channels).
H2O concentration is inversely related to the concentration of solute, ie the more solute particles there are in solution, the more they will displace H2O molecules lowering the concentration of H2O.

Question 13

Explain the difference between osmolarity and tonicity.

Answer 13

Osmolarity describes total number of particles in solution

Tonicity describes the number of non-penetrating particles in solution

In practice tonicity is much more important as it determines cell volume.

Question 14

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

Answer 14

Isotonic refers to a solution having the same solute concentration as in a cell or a body fluid. Isosmotic refers to the situation of two solutions having the same osmotic pressure.

Question 15

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

Answer 15

An osmosis concentration gradient drives the movement of water from the side of the membrane with a greater concentration of water to the side with a lower concentration of water.

Question 16

Understand the physiological significance of osmolarity vs tonicity

Answer 16

Normal human plasma has an osmolarity of 285 mosmol/l (often rounded up to 300) which is the same as within cells (has to be the same because there are no barriers to movement of H20). In the hospital, plasma samples may come back as mOsmol/l, giving information about osmolarity but not tonicity.

However, tonicity is much more functionally important as it determines cell volume. You need to know the composition of that osmolarity in order to predict tonicity and cell volume.
Only changes in tonicity cause changes in cell volume.
Na+ (and Cl-) are the major determinants of ECF tonicity.
Brain is most sensitive organ to changes in tonicity.

Question 17

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

Answer 17

Intravascular haemolysis (cells bursting) can kill.
Lysed (burst) cells introduce protein to ISF, increasing tonicity of ECF in an uncontrolled manner, making management very complicated.

NEVER transfuse a patient with pure water!

Difference is in vivo there is lots of NaCl in the ECF and this is isotonic, so when you add urea to an iso-tonic solution, the urea equilibrates across the membrane, but the resulting ECF remains iso-tonic, so no net movement of particles occurs. The ICF and ECF osmolarity has changed by the same amount of penetrating particle, but the non-penetrating particles remain unchanged so the ECF remains isotonic (despite now being hyper-osmotic).

This is critically relevant when administering fluids

FLUID MUST HAVE THE APPROPRIATE TONICITY
0.9% saline (150mM) is isotonic and routinely used although guidelines are changing.

Question 18

Describe the different types of membrane proteins and their functions.

Answer 18

Intergral - span the hydrophobic core of the lipid bi-layer

Peripheral - associated only with the phosphate head of the lipid bi-layer and do not penetrate the hydrophobic core