3a

Question 1

The plasma membrane resting potential

Answer 1

It is a separation of charge across a membrane
✓ Difference in the relative number/concentration of
cations (+ ions) and anions (- ions) in the intracellular
fluid (ICF) and extracellular fluid (ECF)
✓ Difference in permeability of key ions
✓ Therefore, ion movement (and hence ‘membrane
potential’) is influenced by both ion concentration and
membrane permeability
✓ Membrane potential is measured as the Volt or
millivolt (mV)

Question 2

The movement of ions across a membrane
are determined by

Answer 2

✓ Concentration gradient * Move from ‘high to low’ ✓ Electrical gradient * Opposites attract, similar charges repulse ✓ The combined effects of concentration and
electrical gradients = the electrochemical
gradient
✓ Membrane permeability (restrictions to
movement)

Question 3

In biological cells, there are many ions in

Answer 3

In biological cells, there are many ions in the ICF and ECF, but there are a greater concentration of
ions located in a thin layer along the outer and inner surface of a cell membrane.

Question 4

magnitude

Answer 4

The magnitude of the potential (mV) depends on the number of opposite NET charges that are separated

Question 5

Key Ions

Answer 5

sodium (Na+), potassium (K+) and negatively charged intracellular proteins (A-)
Na is most outside little inside and low permabilty
k is little outside most inside and has greater permeability when at rest
A- has zero permeability and is not outside only inside

Question 6

How do K+ and Na+ cross the plasma membrane?

Answer 6

Ions such as Na+ or K+ are water-soluble and therefore cannot diffuse across the lipid-rich plasma
membrane…
These ions can move passively through protein channels

Question 7

k+ scenerio

Answer 7

» The concentration gradient tends to
push K* out of the cell (Green
arrow)

• The outside of the cell becomes
  more positive as K* ions move out
  down their concentration gradient
• The membrane is impermeable to
  large proteins (A) => This makes
  the ICF more negatively charged
  than the ECF
• Eventually, K* efflux will begin to
  reverse the electrical gradient. This
  will tend to move K* back into the
  cell (blue arrow).
• An equilibrium is reached where
  there is no net movement of K*
  at this point it has a -90mV

Question 8

Na+ scenario

Answer 8

same as k+
na will move inside and make outside more negative and inside will start to repel away and go back outside.