Cell Composition & Volume Recognition

Question 1

1. Recite typical values for the volumes of extracellular fluid

Answer 1

• Extracellular Fluid (ECF): about 14 liters; 1/3 of total body fluid
• Intracellular fluid (ICF): about 28 liters;2/3 total body fluid)
• Plasma: 2.8 liters, 20% of ECF compartments.

Done. Third space = fluid in GI tract, urine, CSF, sweat, etc.
Of the ~45L of an adult body, 99% H2O, 0.5% Cl-, Na+, and K+. Other 0.5% = everything else.

Question 2

2. Describe the major differences in ionic composition between ECF and ICF.

Answer 2

ECF = interstitial fluid, lymph, plasma (mM)
Na+: 140
K+: 5
Cl-: 145
A(n-): 0
H2O: approx. 55,000

ICF = mitochondrial, nuclear, etc. (mM)
Na+: 14
K+: 145
Cl-: 5
A(n-): 126
H2O: approx. 55,000

*All are membrane permeable except for A(n-) and Na+ which is actively pumped
So inside of cell is slightly negative (holds on to K+). An- = big anions, which have an average net charge of -1.2.
Cells are actually permeable to Na, but it’s so actively pumped in the other direction that membranes are functionally impermeable.

Question 3

3. Describe the two most important functional properties of membranes, one conveyed by lipids, the other by channels and transporters.

Answer 3

1) Composed of lipids. 2) Full of holes.
Lipids are impermeable to charge and are electrically strong. This means they can keep opposite electric charges separated without collapsing. Excess anions inside cell give an electrical potential difference (aka membrane potential). Ability to withstand the imposed electric force is due to lipid composition.
Membrane also has lots of proteins that form channels and transporters (‘holes’). Channels are selective (ex: like for certain ions), and are generally gated (open and close under different conditions). Transporters are used for bigger molecules (ex: glucose) that require special binding sites so they can be ‘escorted’ across the membrane. They’re also used as pumps to create gradients (or pump things against their gradient). Channels are passive, transporters are active. If energy comes directly from metabolism (ex: ATP) = primary transport; from other sources = secondary active transport. Transporters are slower than channels.

Question 4

4. Recite the routes by which a given substance can traverse a membrane.

Answer 4

Channels and transporters. Or if they’re lipid soluble, can just go straight through.

Question 5

5. Determine which direction an uncharged substance will move across a membrane, given its concentrations on the two sides.

Answer 5

Substances move from areas of high to low concentration. Diffusion of water down a gradient (towards high solute concentration) = osmosis.

Question 6

6. Determine under a given set of conditions, whether a cell will swell or shrink.

Answer 6

Only water can cause a change in cell volume. If cell is in water with less solute than what’s inside, water moves into cell and it swells. If cell is in water that has more solute than what’s inside the cell, water leaves the cell and cell shrinks. Cells always burst in pure water.

Question 7

7. List the three mechanisms that different cells have evolved to keep from swelling and bursting.

Answer 7

1) Make cell membranes impermeable to water. Not a very good option because cells need to be able to grow (but can work in some specialized systems, like kidney cells where you need to retain water).
2) Brute force = build a cell wall to resist osmotic pressure. Plant, bacteria, and fungi have all evolved to do this  wall is permeable to water but resists the osmotic swelling force. BUT, this limits mobility and morphology of cell, and it takes a lot of uptake/energy to maintain cell wall.
3) Balance water concentrations osmotically  what animal cells do. Basically putting solute in the ECF to make water outside cells the same concentration as water inside cells. Solutes in and out don’t have to be the same.

Question 8

8. Describe which direction water will move across a semi‐permeable membrane, given the solute compositions of the fluids on either side of the membrane.

Answer 8

It depends on whether or not solutes can cross the membrane, and how fast they do so (reflection coefficients). If you have one solute that permeates membrane more quickly than the other solute, water follows that solute first. If you have 2 permeable solutes, one on either side of membrane, and they diffuse at same rate, then net flow of water is 0. If solutes can’t cross the membrane, water flows in the direction of greater solute concentration.

Question 9

9. Describe the difference between diffusion and osmosis.

Answer 9

Osmosis = movement of water across a semi-permeable membrane. Diffusion = movement of solutes from high concentration to low concentration.

Question 10

10. Describe which substance must move into or out of a cell in order for the cell’s volume to change.

Answer 10

Water is the only substance that can have an effect on cell volume.

Question 11

11. Describe the effect of having a membrane with different, non‐zero permeabilities (i.e., reflection coefficients less than 1) to different solutes.

Answer 11

Reflection coefficient = how permeable the cell membrane is to a substance. 0 = water; 1 = impermeable.
Some things pass into cells more quickly than others, and other things can’t pass at all.
If you have the same concentration of different solutes on each side of membrane, one of them will likely diffuse faster than the other  balance of solute will not be balanced throughout the diffusion process (so cell will either swell or shrink in the process as water follows the solute that diffuses faster).

Question 12

12. Define molarity, osmolarity, equivalents, and tonicity, and describe how to convert between them.

Answer 12

Molarity = measure of concentration = moles/liter; or grams/liter?
Osmolarity = sum of molarity of all the solute particles (ex: 1 M NaCl = 2Osm because 1 Na and 1 Cl). This pertains more to ions than to things that don’t dissolve apart, like glucose or sucrose.
-higher osmolarity outside = cell shrinks
-lower osmolarity outside, cell swells
-solutes that are permeable/can cross membrane do not change cell volume (i.e. glycerol – so we assume pure water outside?)
-Tonicity = measure of how a cell reacts to a given ECF solution.
-hypotonic = any solution that makes a cell swell
-hypertonic = any solution that makes a cell shrink
-isotonic = neither
-Equivalents = take the ions in the solute, break them down into an osmolarity, and multiply osmolarity by charge on the ion. Gives you a feel for how much of a given osmolarity is acidic or basic.