01 / 22

Membrane Transport
& Osmoregulation

Master passive and active transport, understand osmotic pressure, and learn how cells maintain homeostasis across selectively permeable membranes.

MCAT Biological Foundations

The Selectively Permeable Membrane

The cell membrane is a phospholipid bilayer embedded with proteins. It acts as a gatekeeper: allowing some molecules to pass freely while blocking others. This selectivity is the foundation of all membrane transport.

Channel Extracellular Intracellular Blocked Phospholipid heads Tails

Two Fundamental Categories

Passive Transport
No energy (ATP) required. Moves down the concentration gradient. Includes simple diffusion, facilitated diffusion, and osmosis.
Active Transport
Requires energy (ATP or electrochemical gradient). Moves against the concentration gradient. Includes pumps, endocytosis, and exocytosis.

Passive Transport: No Energy Required

Simple Diffusion
Small nonpolar molecules move directly through the lipid bilayer. Example: \(\text{O}_2\), \(\text{CO}_2\), steroid hormones.
Facilitated Diffusion
Polar/charged solutes pass through channel or carrier proteins. Example: glucose via GLUT transporters, ions via ion channels.
Osmosis
Water moves through aquaporins or the bilayer from low solute (high water) to high solute (low water) concentration.

Active Transport: Energy Required

1
Primary active transport: Directly uses ATP hydrolysis. The \(\text{Na}^+/\text{K}^+\)-ATPase pumps 3 Na\(^+\) out and 2 K\(^+\) in per ATP molecule.
2
Secondary active transport: Uses the gradient created by primary transport. Symporters move two solutes in the same direction; antiporters move them in opposite directions.
3
Vesicular transport: Endocytosis (phagocytosis, pinocytosis, receptor-mediated) brings material in; exocytosis releases material out.

The \(\text{Na}^+/\text{K}^+\) ATPase Pump

Membrane Na⁺/K⁺ ATPase Extracellular Intracellular Na⁺ Na⁺ Na⁺ 3 Na⁺ out K⁺ K⁺ 2 K⁺ in ATP ADP + Pᵢ

Osmosis and Tonicity

Osmolarity measures total solute concentration in a solution (in osmoles/L). Tonicity describes how a solution affects cell volume: it depends only on non-penetrating solutes.

Hypotonic Cell swells (water enters) Isotonic No net change Hypertonic Cell shrinks (water exits) Solute < Cell Solute = Cell Solute > Cell

The Osmotic Pressure Equation

\(\Pi\)
Osmotic pressure (atm)
\(i\)
van 't Hoff factor (number of particles upon dissociation)
\(M\)
Molarity of solute (mol/L)
\(R\)
\(0.0821 \text{ L}\cdot\text{atm}/(\text{mol}\cdot\text{K})\)
\(T\)
Temperature (Kelvin)

Osmolarity vs Tonicity

Property Osmolarity Tonicity
Definition Total solute concentration (all solutes) Effect on cell volume (non-penetrating solutes only)
Considers All solutes, penetrating and non-penetrating Only non-penetrating solutes
Measurable? Yes: lab osmometer Must be determined experimentally by observing cell behavior
Example Urea solution: high osmolarity Urea solution: hypotonic (urea penetrates, so cell swells)

The Electrochemical Gradient

For charged solutes like ions, the driving force for transport has two components: the concentration gradient (chemical) and the electrical gradient (charge). Together, they form the electrochemical gradient.

\(\Delta\psi\)
Membrane potential (voltage across membrane)
\(z\)
Charge of the ion
\(F\)
Faraday's constant (\(96{,}485\text{ C/mol}\))

The Nernst Equation

The Nernst equation calculates the equilibrium potential for a single ion: the membrane voltage at which there is no net movement of that ion.

Solving Transport Problems

1
Identify the solute. Is it polar/nonpolar? Charged? Large/small? This determines if it can cross the bilayer alone.
2
Determine the gradient. Where is the solute more concentrated? Is there an electrical component (for ions)?
3
Determine transport direction. Is the solute moving down or against its electrochemical gradient?
4
Classify the mechanism. Down the gradient = passive. Against = active. Does it need a channel, carrier, pump, or vesicle?
5
Apply equations if needed. Use \(\Pi = iMRT\) for osmotic pressure or the Nernst equation for equilibrium potential.

Example 1: Osmotic Pressure

Problem: Calculate the osmotic pressure of a 0.15 M NaCl solution at \(37^\circ\text{C}\) (\(310\text{ K}\)).

1
Identify variables: \(M = 0.15\text{ M}\), \(T = 310\text{ K}\), \(R = 0.0821\text{ L}\cdot\text{atm}/(\text{mol}\cdot\text{K})\)
2
van 't Hoff factor: NaCl \(\rightarrow\) Na\(^+\) + Cl\(^-\), so \(i = 2\)
3
Calculate: \(\Pi = iMRT = (2)(0.15)(0.0821)(310) \approx 7.6\text{ atm}\)

Example 2: Nernst Equilibrium

Problem: Calculate the equilibrium potential for Na\(^+\) at \(37^\circ\text{C}\), given \([\text{Na}^+]_{\text{out}} = 145\text{ mM}\) and \([\text{Na}^+]_{\text{in}} = 12\text{ mM}\).

1
Simplified Nernst: \(E_{\text{Na}} = \dfrac{61.5\text{ mV}}{z}\log_{10}\!\left(\dfrac{[\text{Na}^+]_{\text{out}}}{[\text{Na}^+]_{\text{in}}}\right)\)
2
Plug in values: \(z = +1\), so \(E_{\text{Na}} = 61.5 \times \log_{10}\!\left(\dfrac{145}{12}\right)\)
3
Calculate: \(\log_{10}(12.08) \approx 1.08\), so \(E_{\text{Na}} \approx +66.4\text{ mV}\)

Example 3: Kidney Osmoregulation

Scenario: A patient drinks 2 liters of pure water. How does the kidney maintain osmotic balance?

1
Plasma osmolarity drops below 300 mOsm due to dilution of blood.
2
Osmoreceptors in the hypothalamus detect the decrease and reduce ADH (antidiuretic hormone) release from the posterior pituitary.
3
Less ADH means fewer aquaporin-2 channels inserted into collecting duct membranes, so less water is reabsorbed.
4
Result: Dilute, high-volume urine is produced, restoring plasma osmolarity to ~300 mOsm.

Common Mistakes

✕ Confusing osmolarity with tonicity
A solution with a penetrating solute (like urea) can be iso-osmolar but hypotonic. Always ask: does the solute cross the membrane?
✕ Forgetting the van 't Hoff factor
Ionic compounds dissociate: NaCl has \(i = 2\), CaCl\(_2\) has \(i = 3\). Forgetting \(i\) gives osmolarity that is too low.
✕ Thinking water moves toward water
Water moves toward higher solute concentration (lower water potential), not toward more water.

Transport Summary

Type Energy? Protein? Direction
Simple Diffusion No No Down gradient
Facilitated Diffusion No Yes Down gradient
Osmosis No Sometimes Down water potential
Primary Active ATP Yes (pump) Against gradient
Secondary Active Ion gradient Yes (co-transporter) Against gradient (coupled)
Endocytosis ATP No (vesicle) Into cell
Exocytosis ATP No (vesicle) Out of cell

Question 1 of 5

A red blood cell is placed in a 0.45% NaCl solution (which is half normal saline). What will happen to the cell?

Question 2 of 5

The \(\text{Na}^+/\text{K}^+\)-ATPase transports 3 Na\(^+\) out and 2 K\(^+\) in per ATP hydrolyzed. This pump is classified as:

Question 3 of 5

A solution of 300 mOsm urea is added to cells with an internal osmolarity of 300 mOsm. Urea freely permeates the membrane. The solution is:

Question 4 of 5

Using the simplified Nernst equation at \(37^\circ\text{C}\), calculate the equilibrium potential for Cl\(^-\) given \([\text{Cl}^-]_{\text{out}} = 105\text{ mM}\) and \([\text{Cl}^-]_{\text{in}} = 10\text{ mM}\). (\(z = -1\))

Question 5 of 5

ADH (antidiuretic hormone) increases the number of aquaporin-2 channels in the collecting duct. If a patient has a mutation that prevents ADH secretion, what would you expect?