composition of cells BETZ lectures

Question 1

Recite typical values for the volumes of plasma, ecf, icf

Answer 1

plasma (3 liters), extracellular fluid (ECF; about 13 liters plus
another 5 for the ‘third space’), and intracellular fluid (ICF; about 27 liters) compartments.

Question 2

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

Answer 2

In order to discuss some basic principles that illustrate how the differences between ICF and ECF arise, it will be helpful to make a simplified table (Table 1). We will lump some things together (HCO3- is put with Cl-; also HPO4= and SO4= are put with proteins, and this group is named A-n, where A stands for “big anions”, which have an average net charge n of about -1.2). We will also ignore other things (H+, Mg++, Ca++, organic acids).

Question 3

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

Answer 3

Lipids are ‘strong’, in an electric sense. That is, they can keep opposite electric charges separated, without collapsing. As we will see, the cytoplasm of nearly all cells is electrically negative, compared to the ECF, because cells contain a few more negative than positive ions. The excess anions 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. The ability of the membrane to withstand the imposed electric force (which has a whopping strength of about 100,000 volts/cm) is due to its lipid composition.
Impermeable to charged substances! This includes dipoles.
The second physiologically important point about membranes is that they are apparently shot full of holes. That is, many charged and polar molecules can cross membranes, no thanks to the lipids. Their ability to do so depends primarily on the presence of certain proteins inserted in the lipid membrane; these proteins have special abilities to transport particular substances between the ECF and the ICF. From a functional (i.e., our) standpoint, there are two kinds of proteins that mediate the transmembrane movements of charged substances: channels and transporters

Question 4

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

Answer 4

Channels, transporters, tunnels, proteins, etc.

Question 5

5 . Identify physical forces that can determine the gating properties of ion channels.

Answer 5

Some depend on the electric field (membrane potential) across the membrane. The gates in many such voltage-gated channels swing open when the membrane is depolarized (the cell is made less negative inside). Gates in other channels require mechanical stimulation (stretching of the membrane) to open (e.g., hair cells in the cochlea; touch receptors in the skin). Still others require the binding of a particular chemical (synaptic receptors for a neurotransmitter). Others open or close depending on temperature (cutaneous thermal receptors). Some gates in channels depend on more than one force, making for some pretty complicated behavior. For example, a channel gate may require the binding of a particular chemical substance and, simultaneously, a particular value of membrane potential in order to open up. Some channels contain no gates (e.g., water channels, also called aquaporins) , while others contain two gates, and both have to be open to allow ions to pass!