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Flashcards in Cell Physiology Deck (50)

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

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

ICF = 27 L

~45L total in body


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

ICF 14mM
ECF 140mM

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


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

ICF 145mM




ECF 145mM




ICF 55,000
ECF 55,000



Ca++ out

1 mM


Ca++ in

.0001 mM


H+ in

.0001 mM


H+ out

.00004 mM


Na+ out

140 mM


K+ in

140 mM


K+ out

4 mM



24 mM


Cl- out

115 mM


Plasma mosM

300 mM


Max urine m

1200 mM


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.

negative; few excess anions inside the cell


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?

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


Types of channels (4)

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


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

1. Transporters move big molecules
2. Channels are passive transport. If active transport is involved, it is a transporter.


Channels work _____ than transporters.

Faster (much)


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

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


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

Water; osmosis


What is the reflection coefficient and what does it mean?

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.


What is molarity?



What is osmolarity?

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.


What are equivalents?

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.)


What is tonicity?

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.


What is the osmolarity of 100mM K2SO4?

300 mosM (osmolar = osM; milliosmolar = mosM)


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

100; 100


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

200; 200


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

200; 200


Which is stronger, the osmotic or electric force?

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


What three points are worth remembering regarding electrochemical gradients?

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.


In the Nernst equation, What is E?

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.]


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

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.


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 _____.

membrane potential, Vm


When Vm = E, what must be true?

The ion in question is distributed at its electrochemical equilibrium.


A cell is permeable to Na+. Vm= -40 mV, ENa = 60mV
Is there a Na+ pump? In which direction does it pump Na+?

Answer: Na+is pumped out of the cell.


What is the Nernst equation?

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!


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

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).


How to treat hyperkalemia?

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


Bulk solutions are electrically_____.



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

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.


How is glucose uptake regulated by insulin?

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).


What is a common source of energy for secondary transporters?

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.)


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



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

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.


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

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


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 ______.

acidemia; hyperkalemia

*Such a transporter does not exist