Cell Physiology

Question 1

Recite typical values for the volumes of plasma, extracellular fluid, and intracellular fluid

Answer 1

ECF = 13 L (Plasma volume accounts for 3 liters/13) + 5 for “3rd space”

ICF = 27 L

~45L total in body

Question 2

Na+ conc. in ICF, ECF. Membrane permeable to Na+?

Answer 2

ICF 14mM
ECF 140mM

(“functionally impermeable” - Na is pumped, steady state etc.)

Question 3

K+ conc. in ICF, ECF. Membrane permeable to K+?

Answer 3

ICF 145mM
ECF 5mM

Yes.

Question 4

Cl-

Answer 4

ICF 5mM
ECF 145mM

Yes.

Question 5

H20

Answer 5

ICF 55,000
ECF 55,000

Yes.

Question 6

Ca++ out

Answer 6

1 mM

Question 7

Ca++ in

Answer 7

.0001 mM

Question 8

H+ in

Answer 8

.0001 mM

Question 9

H+ out

Answer 9

.00004 mM

Question 10

Na+ out

Answer 10

140 mM

Question 11

K+ in

Answer 11

140 mM

Question 12

K+ out

Answer 12

4 mM

Question 13

HCO3-

Answer 13

24 mM

Question 14

Cl- out

Answer 14

115 mM

Question 15

Plasma mosM

Answer 15

300 mM

Question 16

Max urine m

Answer 16

1200 mM

Question 17

As we will see, the cytoplasm of nearly all cells is electrically____, compared to the ECF. The _____ create an electrical potential difference between the inside and the outside of the cell, and this membrane potential, which governs some vital cell processes, is wholly dependent on the integrity of the plasma membrane.

Answer 17

negative; few excess anions inside the cell

Question 18

Channels. Some charged/polar substances cross membranes by passing through channels, which behave as passive pores, or tunnels in the membrane. Channels have two especially important properties. What are they?

Answer 18

First, most are selective for particular ions. Second, some channels contain molecular gates.

Question 19

Types of channels (4)

Answer 19

1. voltage-gated channels (membrane potential)
2. mechanical stimulation (stretching of the
   membrane)
3. chemical (synaptic receptors for a neurotransmitter)
4. temperature (cutaneous thermal receptors)

Question 20

What are the BIG differences between a channel and a transporter? (2)

Answer 20

1. Transporters move big molecules

2. Channels are passive transport. If active transport is involved, it is a transporter.

Question 21

Channels work _____ than transporters.

Answer 21

Faster (much)

Question 22

By what three methods can a cell keep from swelling and bursting?

Answer 22

1. Membrane impermeable to water.
2. Cell wall
3. Maintain osmotic balance in and out

Question 23

What substance must move in and out of a cell in order for the volume to change? By what mechanism?

Answer 23

Water; osmosis

Question 24

What is the reflection coefficient and what does it mean?

Answer 24

The reflection coefficient (sigma) is a way to state whether a membrane is permeable to a solute. A coefficient of 1 means a membrane is perfectly impermeable, numbers less than 1 mean a membrane is increasingly permeable to said solute. A molecule of radioactive water would have a reflection coefficient of zero.

Question 25

What is molarity?

Answer 25

mol/L

Question 26

What is osmolarity?

Answer 26

Osmolarity is the total concentration of solute particles: for example, a 1 M solution of CaCl2 gives a 3 osM solution (3 solute particles/molecule dissolved).

The total concentration of solute particles, is the same. So levels of Na+ can be different in ECF/ICF, but the total concentration of all molecules (Na, K, Cl etc) must be balanced.

Question 27

What are equivalents?

Answer 27

Equivalents take into consideration the titration required to offset the charge of dissociated ions in solution.

Converting from mM to mEq is a two-step
process: first, for each ion, convert to mosM, and second, multiply by valence of the ion in question. In short, for each ion, multiply its osmolarity by valence to go from mM to mEq. (If the solute is uncharged, mEq = mM.)

Question 28

What is tonicity?

Answer 28

Tonicity is influenced only by solutes that cannot cross the membrane, as only these exert an effective osmotic pressure. Solutes able to freely cross the membrane do not affect tonicity because they will always be in equal concentrations on both sides of the membrane.

Any solution that makes a cell shrink is hypertonic; any solution that makes a cell swell is hypotonic.

Question 29

What is the osmolarity of 100mM K2SO4?

Answer 29

300 mosM (osmolar = osM; milliosmolar = mosM)

Question 30

100 mM NaCl:
Na+ = ___ mEq
Cl- = ___ mEq

Answer 30

100; 100

Question 31

100 mM K2SO4:
K+ = ___ mEq
SO4 = ___ mEq

Answer 31

200; 200

Question 32

100 mM CaCl2:
Ca++ = ___ mEq
Cl- = ___ mEq

Answer 32

200; 200

Question 33

Which is stronger, the osmotic or electric force?

Answer 33

Electric force ( 10^18 times stronger, to be precise)

Question 34

What three points are worth remembering regarding electrochemical gradients?

Answer 34

1. The passive movement of an ion across the membrane is governed by two forces: its concentration difference and the electrical potential difference across the membrane (the
   membrane potential). The two forces, combined, make an electrochemical gradient.
2. Membrane potentials are produced by only one thing, namely an imbalance in the number of cations and anions inside a cell.
3. The electric force is very much more powerful than the diffusional force produced by a concentration difference, which means that relatively few excess ions are needed to counter large concentration differences. In other words, as we will see, it is very safe and appropriate to consider that the concentration of chloride in the cell does not change, or more generally, bulk solutions are always electrically neutral.

Question 35

In the Nernst equation, What is E?

Answer 35

It is the electrical potential difference across the membrane that must exist if the ion is to be at equilibrium at the given concentrations.

[ Please note that we arbitrarily define E as the potential of the inside of the cell with respect to the outside. Thus, if E=
-40 mV, we say that the inside must be 40 mV negative to the outside in order for the ion in question to be at equilibrium.]

Question 36

Suppose [K+]i = 140 mM and [K+]o = 4 mM. Will EK be positive or negative?

Answer 36

Well, to keep potassium from diffusing down its concentration gradient out of the cell, we must make the inside of the cell negative to hold on the positively-charged potassium ions. Nernst tells us that EK = -92.6 mV.

Question 37

Equilibrium potentials are not real voltages - that’s why they’re written with an E (for “electromotive force”) not a V. The real voltage is called the _____.

Answer 37

membrane potential, Vm

Question 38

When Vm = E, what must be true?

Answer 38

The ion in question is distributed at its electrochemical equilibrium.

Question 39

A cell is permeable to Na+. Vm= -40 mV, ENa = 60mV

Is there a Na+ pump? In which direction does it pump Na+?

Answer 39

Answer: Na+is pumped out of the cell.

Question 40

What is the Nernst equation?

Answer 40

E (mV) = 60/z x log([Co]/[Ci])

Where z is the valence of the ion in question. DONT forget to put the proper CHARGE ON Z!

Question 41

To what are the differences in Vm in different types of living cells due? (ie RBC, glial, myocardial)

Answer 41

The answer is: relative permeability. A cell with many more K+channels than Na+ channels will have a membrane potential close to EK. Conversely, a cell with
relatively more Na+ channels will have a membrane
potential closer to ENa. Glial cells are nearly perfect
‘potassium electrodes’ (permeable only to K), while red
blood cells are about equally permeable to Na and K (Vm is close to 0).

Question 42

How to treat hyperkalemia?

Answer 42

C BIG K; give, in order: Calcium, Bicarb, Insulin + Glucose, Kayexalate. In actual practice, bicarbonate is used less often. (Dialysis works too)

Question 43

Bulk solutions are electrically_____.

Answer 43

neutral

Question 44

What is the best known example of facilitated diffusion? Does this mechanism expend ATP? How is the target molecule concentrated in the cell?

Answer 44

Glucose transporter uses no ATP, but will transfer glucose either in or out with no preference. Glucose, then, is immediately converted to G6P upon entry, making it ineligible for transport back out.

Question 45

How is glucose uptake regulated by insulin?

Answer 45

In the absence of glucose, the transporter is not even present in the plasma membrane; it is sequestered inside the cell, in the membrane of intracellular vesicles. Insulin triggers a biochemical cascade that causes the vesicle membranes to fuse with the surface membrane (exocytosis),
exposing the glucose transporter to the ECF. The transporter then gets busy and ‘carries’ glucose inside. When insulin subsides, the transporter molecules are reinternalized (endocytosis).

Question 46

What is a common source of energy for secondary transporters?

Answer 46

The downhill “leak” of Na into cells. (So if the Na/K pump goes down due to lack of ATP, most secondary transporters will also stop working.)

Question 47

When a secondary transporter generates a charge across a membrane, what is this called?

Answer 47

Electrogenic.

Question 48

What characterizes the Ca/Na pump? Which direction does each go? What is the term for that? Where is this of particular importance?

Answer 48

The Na/Ca exchanger’s (antiporter) main job is to pump calcium ions out of the cell. The inward leak of sodium ions
provides the energy source. Of particular importance in the heart.

Question 49

What are three transporters driven by the inward Na leakage? In which direction does each work?

Answer 49

Calcium (Ca out of the cell)
Hydrogen (H out of the cell)
Chloride (Cl into the cell)

Question 50

There are several clinical situations that suggest the presence of a system that will exchange K+ for H+, and vice versa. For example, infusing K+ causes ______, and infusing acid causes ______.

Answer 50

acidemia; hyperkalemia

*Such a transporter does not exist