Forces acting across the membrane

Question 1

Explain the importance of the membrane in regards to the ECF and ICF.

Answer 1

The ECF and ICF have different chemical compositions (as shown below therefore the membrane is responsible for separating the two and maintain the difference in composition.

Question 2

What does and doesn’t have free movement through the membrane?

Answer 2

• There is no barrier to H2O movement.
• Ions pass freely across the capillary wall so exchange readily between plasma and ISF. They do not penetrate the cell membrane.

Question 3

What are the concentration of K+ and Na+ in the ECF and ICF?

Answer 3

• [K+] is high in ICF, low in ECF.

- [Na+] is low in ICF, high ECF.

Question 4

What are the 7 ‘important’ things about membranes?

Answer 4

1) Cell membrane is a selective barrier
2) Permeability can vary
3) Membranes are dynamic
4) Structure
5) Environment
6) Membranes are excellent insulators
7) Embedded with proteins

Question 5

Explain how the membrane acts as a selective barrier.

Answer 5

It is freely permeable to some substance e.g O2 and CO2 but the difference in composition between the ECF and ICF shows that permeability is selective and not universal.

Question 6

Explain how permeability can vary in the membrane.

Answer 6

May increase or decrease at different times, fundamentally important for various cell functions e.g transmission of the nervous impulse.

Question 7

Explain how membranes are dynamic.

Answer 7

Continually being formed and maintained or dismantled and metabolised according to the needs of the cell.

Question 8

Explain the general structure of the membrane,

Answer 8

Very think bi-layer of lipids. Major membrane lipids are phospholipids which are amphipathic in aq.

Question 9

Explain how the phospholipids are arranged in their environment in the membrane.

Answer 9

They arrange themselves so that the polar part is on the outside of the membrane and the fatty chains are always on the inside.

Question 10

Explain how membranes are flexible.

Answer 10

Membranes can be very flexible because of fatty acids in vivo, behave like an oil - can stretch although may rupture if over stretched.

Question 11

Explain how membranes are excellent insulators.

Answer 11

membranes are excellent insulators against the movement of electrical charge. This prevents the passage of electrons – important in maintaining electrical stability of cell.

Question 12

What are membranes embedded with?

Answer 12

Membranes are embedded with proteins and associated with carbohydrates.

Question 13

Describe the membrane carbohydrate.

Answer 13

• Small amounts linked to proteins and lipids as glycoproteins and glycolipids.
• They are ALL extracellular.
• They play important roles in cell to cell communication including self vs non-self recognition by the immune system.

Question 14

Define lipophobic.

Answer 14

Lipophobic means lipid repellent) = Hydrophilic.

Question 15

What are the 4 types of membrane proteins?

Answer 15

1) Receptors
2) Transporters
3) Enzymes
4) Peripheral membranes proteins

Question 16

Explain receptor proteins role in the membrane.

Answer 16

Integral to the membrane structure. Penetrate the membrane from ECF to ICF. Allow communication of an extracellular signal to the intracellular space to create cellular response.

Question 17

What are transporter proteins roles in the membrane?

Answer 17

transporter proteins allow movement of ions or molecules across the membrane.

Question 18

What are the two forms transporter proteins come in, in the membrane?

Answer 18

1) Carrier proteins

2) Channel proteins

Question 19

Briefly describe how carrier proteins work.

Answer 19

Carrier proteins do not create a continuous pore from ECF to ICF. Open to ECF and ICF but never at the same time. Typically move larger molecules than channels e.g glucose.

Question 20

Briefly describe how channel proteins work.

Answer 20

• Protein channels (typically gated) are what ions used to pass through the plasma membrane. The protein channels are IMPs that act as an aqueous route through the membrane for the diffusion of ions. Only allow the passage of mineral ions such as Na+ and K+, Cl-, Ca2+, H+ and H2O. Too small molecules such as glucose to go through. H2O passes through aquaporins, there is no barrier to water. Channels are typically gated.

Question 21

Explain the role of Enzymes in the membrane structure.

Answer 21

Membrane enzymes catalyse chemical reactions on the cell membrane. Can be external e.g found in the small intestine which break down nutrient into smaller units or internal such as those associated with converting signals carried from receptors into intracellular response.

Question 22

What are the 3 main roles of the peripheral membrane proteins?

Answer 22

1) Maintain structure of cells by anchoring membrane to intracellular cytoskeleton. Dysfunction or loss can cause serious debility.
2) Attach cells to the extracellular matrix e.g collagen.
3) Perform signalling functions within cells e.g proteins

Question 23

What are cell membranes effectively impermeable to?

Answer 23

• Cell membranes are effectively impermeable to intracellular proteins and organic anions. These items cannot diffuse in any capacity so they stay inside the cell.

Question 24

Define diffusion.

Answer 24

Net movement of solute from regions of high solute concentration to regions of low solute concentration.

Question 25

Describe how protein content differers in different membranes (myelin, plasma membranes, membranes involved in energy transduction).

Answer 25

• Myelin: a membrane that serves as an insulator around myelinated nerve fibres has a low content of protein (18%), major component is lipid, very good insulator so ideal for function.
• Plasma membranes of most other cells have much greater activity and protein content is typically 50%.
• Membranes involved in energy transduction such as inner membrane of mitochondria, have highest protein content 75%.

Question 26

What 3 factors aid diffusion?

Answer 26

1) A large surface area
2) High permeability
3) High conc. gradient

Question 27

Describe how protein content differers in different membranes (myelin, plasma membranes, membranes involved in energy transduction).

Answer 27

• Myelin: a membrane that serves as an insulator around myelinated nerve fibres has a low content of protein (18%), major component is lipid, very good insulator so ideal for function.
• Plasma membranes of most other cells have much greater activity and protein content is typically 50%.
• Membranes involved in energy transduction such as inner membrane of mitochondria, have highest protein content 75%.

Question 28

What 3 factors aid diffusion?

Answer 28

1) A large surface area
2) High permeability
3) High conc. gradient

Question 29

Explain how voltage gated channels work.

Answer 29

Voltage gated channels open/close in response to alterations in membrane electrical potential (charge difference either side of the membrane). Found extensivley in muscle and nerve.

Question 30

Explain how Ligand gated channels work.

Answer 30

Open/close when they bind a chemical such as neurotransmitter or hormone to a receptor binding site on the channel protein.

Question 31

Explain how voltage gated channels work.

Answer 31

Voltage gated channels open/close in response to alterations in membrane electrical potential (charge difference either side of the membrane). Found extensivley in muscle and nerve.

Question 32

Explain how Ligand gated channels work.

Answer 32

Open/close when they bind a chemical such as neurotransmitter or hormone to a receptor binding site on the channel protein.

Question 33

What causes the conformation of the proteins in the membrane.

Answer 33

In both cases of gated channels electrical or chemical stimulus causes a conformational change in the configuration of the channel proteins causing the to open or close their channel as appropriate.

Question 34

What does diffusion depend on?

Answer 34

Direction of diffusion is dependent on the concentration gradient and (in cases of charged particles) electrical gradient – somtimes these forces co-op and sometimes appose. Net direction of movement depends on dominant force.

Question 35

What is the electrochemical gradient caused by?

Answer 35

Because the ion concentration in the ICF and ECF is different, therefore there is an ion concentration gradient. These are charged creating an electrochemical gradient which ultimately drives the direction of passive movement. – movement against the gradient requires energy.

Question 36

Explain the carrier mediated transport system.

Answer 36

Carrier-mediated transport proteins have binding sites for solutes such as glucose. When they bind to the solute, the carrier undergoes a change in shape which exposes the binding site on the other side of the membrane. The solute moves away and the carrier returns to its original configuration.
when the electrochemical gradient opposes movement – energy in the form of ATP needed – in these cases the carrier protein also functions an enzyme (ATPase) as it hydrolyses ATP to release energy

Question 37

Explain the difference in volume and solute conc. when membranes are permeable to both water and solute, and water but not solute.

Answer 37

• When the membrane is soluble to both H2O and solute there will be an equal concentration and equal volume either side of the membrane due to diffusion e.g with glucose and water – they will move in opposite directions to create equal conc. and volume.
• When membrane is permeable to H2O and not solute, there will be an equal concentration either side of the membrane but different volumes (when active pumps etc not present in theory).

Question 38

Define osmostic pressure.

Answer 38

• The pressure required to prevent water movement.

Question 39

Define osmolarity.

Answer 39

Osmolarity is the measure of solute concentration. - describes the total number of particles in solution.

Question 40

What is the cells volume determined by?

Answer 40

Tonicity. The conc. of non penetrating solutes on the 2 sides of the membrane.

Question 41

What are the non-penetrating solutes in ECF?

Answer 41

In ECF Na+ and Cl- act as non-penetrating solutes.

Question 42

Define tonicity.

Answer 42

Tonicity describes the number of non-penetrating particles in a solution. Isotonic solution has the same number of non-penetrating solute particles as ECF (plasma). Solutions with fewer non-penetrating solute particles are hypotonic. Solutions with greater number of non-penetrating solute particles are hypertonic

Question 43

What determines the tonicity of the ECF?

Answer 43

Na+ (and Cl-) are the major determines of ECF tonicity.

Question 44

Define tonicity.

Answer 44

Tonicity describes the number of non-penetrating particles in a solution. Isotonic solution has the same number of non-penetrating solute particles as ECF (plasma). Solutions with fewer non-penetrating solute particles are hypotonic. Solutions with greater number of non-penetrating solute particles are hypertonic

Question 45

What is the osmolarity of normal plasma?

Answer 45

285 mosmol/l (rounded up to 300)

Question 46

What effect does urea have on the osmotic movement of water?

Answer 46

none.

Question 47

Why is there no difference between ICF and ECF osmolarity?

Answer 47

• Because water will always move down any osmotic gradient (not ions)

Question 48

Where would 1 litre of isotonic saline that is transfused go in the body and why?

Answer 48

• The saline solution will remain in the ECF as the ions cannot cross the cell membrane and therefore hold the water in the ECF due to the osmotic effect. The Na+ and Cl – ions prevent the water moving into the cell. Most effective way to increase plasma volume quickly.

Question 49

Define Hypotonic.

Answer 49

Water is less concentrated than the cell the water surrounds. – causes cell to swell with water.

Question 50

Define Hypertonic.

Answer 50

Hypertonic solution: Water has higher solute concentration that that of the cell the water surrounds. – causes cell to shrink.

Question 51

What happens to a cell put in hypertonic solution?

Answer 51

• Cells in hypertonic solutions shrink because water leaves down a chemical gradient.

Question 52

State the appropriate tonicity of routinely used fluid.

Answer 52

• 0.9% saline (150mM) is isotonic and routinely used although guidelines are changing.

Question 53

What happens to a cell put in hypertonic solution?

Answer 53

• Cells in hypertonic solutions shrink because water leaves down a chemical gradient.

Question 54

Why does urea effect cell volume in vitro?

Answer 54

• In vitro, in an aq. solution, the urea is accumulating in a hypotonic solution (water), so once equmilibrated the solution is still hypotonic so water moves into the cell. Even a hyperosmotic aq. solution of urea will be hypotonic and cause cells to swell.

Question 55

Why does urea not effect cell volume in vivo?

Answer 55

• Difference in vivo there is lots of NaCl in the ECF and this is isotonic, so when you add urea to an isotonic solution, and it equilibrates across the membrane, the resulting ECF remains isotonic, so no net movement of particles occurs. The ICF and ECF osmolarity has changed by the same amount of penetrating particles but the non-penetrating particles remain uncharged so the ECF remains isotonic (despite now being hyperosmotic. - In effect urea makes no difference to movement of water across membranes.

Question 56

Why does urea effect cell volume in vitro?

Answer 56

• In vitro, in an aq. solution, the urea is accumulating in a hypotonic solution (water), so once equmilibrated the solution is still hypotonic so water moves into the cell. Even a hyperosmotic aq. solution of urea will be hypotonic and cause cells to swell.

Question 57

Describe the physiological significance of osmolarity vs tonicity.

Answer 57

• Tonicity is much more functionally important as it determines cell volume. Cells in hypotonic solution swell – because the water enters down a chemical gradient. Cells in hypertonic solution will shrink – because water leaves down a chemical gradient.

Question 58

Describe the process of endocytosis and exocytosis.

Answer 58

They are mechanisms for moving macromolecules across membranes without disrupting them. In endocytosis, there is invagination of the membrane to form a vesicle which eventually separates on the cytoplasmic (inside) surface of the membrane and migrates within the cell to its destination. Exocytosis involves the reverse process.