CH 4 APK

Chemical Driving Force (Concentration Gradient)

• Movement caused by differences in solute concentration between the intracellular fluid (ICF) and extracellular fluid (ECF)

• Solutes naturally move from an area of higher concentration to lower concentration.

Electrical Driving Force

Movement caused by the electrical charge (membrane potential) acting on charged particles (ions).

Governed by the rule that opposites attract and like repel. A positively charged ion (Na+ or K+) is attracted to the negatively charged interior of a resting cell (inward direction), while anions are repelled outward.

Sodium (Na+) is found in a higher concentration

outside the cell (in the ECF) compared to the inside of the cell (ICF), which is why its chemical driving force pushes it inward.

Potassium (K+) is found in a higher concentration

inside the cell compared to the extracellular fluid. cell keeps it inside

Membrane Potential (Vm)

Difference in electrical potential across the plasma membrane; sign of the membrane potential is the net charge inside a cell

If a cell has both a concentration gradient and an electrical charge that pull in two different directions, we need to evaluate the

Electrochemical driving force (who wins out to dictate the direction of movement?)

The equilibrium potential for a given ion is the

specific membrane potential at which the electrical driving force exactly balances/opposes the chemical driving force.

Electrochemical gradient = 0

Equilibrium = no net movement of the ion

Equilibrium Potential Value: Na+

Sodium (Na⁺) = +60 mV

• Concentration: Sodium is highly concentrated outside the cell.

• Natural Flow: If channels open, Na⁺ naturally rushes into the cell down its concentration gradient.

• Equilibrium: To stop this inward rush, the inside of the cell would need to become strongly positive (+60 mV) to electrically repel the positive Na⁺ ions.

Equilibrium Potential Value: K+

Potassium (K⁺) = -94 mV

• Concentration: Potassium is highly concentrated inside the cell.

• Natural Flow: If channels open, K⁺ naturally diffuses out of the cell down its concentration gradient.

• Equilibrium: As positive K⁺ leaves, the inside of the cell becomes negative. It would need to drop all the way to -94 mV to create an electrical pull strong enough to prevent any more K⁺ from leaving

Electrochemical gradient / Electrochemical driving force

The combined net force resulting from both the chemical (concentration) and electrical driving forces acting on an ion.

Determining Movement: To find the net direction of an ion, you must evaluate:

1. The charge of the ion.

2. The concentration gradient (chemical).

3. The resting membrane potential (typically $-70 \text{ mV}$ inside).

4. The equilibrium potential for that ion.

• Example ($K^+$ at rest): The chemical gradient pushes $K^+$ out (high to low), but the interior negative charge pulls $K^+$ in. At a resting membrane potential of $-70 \text{ mV}$ (which is closer to $K^+$'s equilibrium of $-94 \text{ mV}$), the outward chemical force slightly wins, resulting in a net outward electrochemical driving force for potassium

Excess ? inside the plasma membrane. There is more ? charge inside the cell relative to the outside of the cell.

anions, negative (-)