IMAT Biology · Cell Structure and Membranes

Diffusion, Osmosis and Active Transport

Substances cross membranes down their gradient for free, through the bilayer or through proteins, or against it using carrier pumps and ATP.

All Cell Structure and Membranes notes

Why this matters

The 2024 and 2026 ministry papers asked what carrier proteins and channel proteins do. The older papers asked which mechanisms need both a protein and ATP, and gave concentration data to judge which way each substance moves.

Concept 1 of 3: Simple and facilitated diffusion: channels and carriers

Molecules move about at random all the time. Where there are more of them, more wander away than wander in, so there is a net drift from high concentration to low until the two sides are equal. No energy from the cell is needed. Small non-polar molecules slip straight through the bilayer; ions and polar molecules need a protein doorway.

Definition

Diffusion is the net movement of particles from a region of higher concentration to one of lower concentration, down a concentration gradient. It is passive: no ATP.

Simple diffusion: straight through the bilayer. Small non-polar molecules: $\mathrm{O_2}$ , $\mathrm{CO_2}$ , steroids, fat-soluble vitamins.
Facilitated diffusion: through a membrane protein, still down the gradient. Channel proteins form water-filled pores that let specific ions or water (aquaporins) through; many can open and close (gated). Carrier proteins bind a specific molecule, such as glucose, and change shape to release it on the other side.
Carriers can be saturated: when all are busy, a higher concentration no longer speeds things up.
Rate rises with a steeper gradient, a larger surface, a thinner membrane and a higher temperature.
Each substance moves down its own gradient, whatever the others do.
At equilibrium molecules still cross in both directions, but at equal rates, so there is no net movement.

Mechanism	Example of what crosses	Membrane protein?	ATP?	Direction
Simple diffusion	$\mathrm{O_2}$ , $\mathrm{CO_2}$ , steroid hormones	No	No	Down the gradient
Facilitated diffusion by channel	$\mathrm{Na^+}$ , $\mathrm{K^+}$ , $\mathrm{Cl^-}$ , water through aquaporins	Yes, channel	No	Down the gradient
Facilitated diffusion by carrier	Glucose into red blood cells	Yes, carrier	No	Down the gradient
Active transport	$\mathrm{Na^+}$ out and $\mathrm{K^+}$ in by the pump	Yes, carrier (pump)	Yes	Against the gradient

Practice this conceptself-check · 4 quick reps

"Uses a carrier protein" does not mean "active"

Carrier proteins work in both facilitated diffusion and active transport. The difference is the direction and the energy: facilitated diffusion goes down the gradient without ATP, active transport goes up it using ATP. Only active transport needs both a protein and ATP.

Concept 2 of 3: Osmosis, osmolarity and tonicity

Water molecules diffuse too. Dissolved particles hold some water around them, so the side with more solute has fewer free water molecules. Water therefore moves across a membrane toward the side with more dissolved particles. What counts is the number of particles, not their kind, so a salt that splits into two ions counts twice.

Definition

Osmosis is the net movement of water across a partially permeable membrane, from a solution with a lower solute concentration to one with a higher solute concentration (from higher to lower water potential).

Osmolarity counts all dissolved particles: concentration times the number of particles each unit gives in solution ( $i = 1$ for glucose or sucrose, 2 for NaCl, 3 for $\mathrm{CaCl_2}$ ).
Human cells and plasma are about 0.3 osmol/L. Physiological saline, 0.9% NaCl (about 0.15 mol/L), matches it.
Tonicity compares a solution with a cell, for solutes that cannot cross the membrane. Hypotonic: lower than the cell, water enters. Isotonic: equal, no net movement. Hypertonic: higher, water leaves.
Animal cells in hypotonic solution swell and may burst (haemolysis for red blood cells); in hypertonic solution they shrink (crenation).
Plant cells in hypotonic solution become turgid and do not burst, because the cell wall resists. In hypertonic solution the membrane pulls away from the wall (plasmolysis).

Osmolarity of a solution

\text{osmolarity} = i \times c

$i$ number of particles one formula unit gives when dissolved
$c$ molar concentration, in mol/L

Worked example

Red blood cells (contents about 0.30 osmol/L) are placed in three solutions: 0.15 mol/L NaCl, 0.20 mol/L

\mathrm{CaCl_2}

and 0.10 mol/L sucrose. None of these solutes can cross the membrane. Predict what happens in each.

Practice this conceptself-check · 4 quick reps

Water moves toward the higher solute concentration

In osmosis water goes from the dilute solution to the concentrated one, which is the side with less free water. Saying water moves "from high concentration to low" is only right if you mean the concentration of water. IMAT options often reverse this.

Count particles, not moles

0.1 mol/L NaCl gives 0.2 osmol/L because each unit splits into two ions; 0.1 mol/L glucose gives only 0.1 osmol/L. Comparing solutions by molar concentration alone gives the wrong tonicity.

Concept 3 of 3: Active transport and the sodium potassium pump

Sometimes a cell needs more of a substance inside than outside, or less. Moving it uphill, against its gradient, takes energy, just as pumping water up a hill does. A carrier protein called a pump uses ATP to change shape and push the substance across. The most important pump keeps sodium out and potassium in.

Definition

Active transport moves a substance against its concentration gradient using a carrier protein (a pump) and energy.

Primary active transport uses ATP directly. The $\mathrm{Na^+/K^+}$ pump ( $\mathrm{Na^+/K^+}$ ATPase) moves 3 $\mathrm{Na^+}$ out and 2 $\mathrm{K^+}$ in for each ATP split. It keeps $\mathrm{Na^+}$ high outside and $\mathrm{K^+}$ high inside, and helps make the inside of the cell negative (about $-70\ \text{mV}$ in a resting neuron).
Secondary active transport (cotransport) uses the $\mathrm{Na^+}$ gradient the pump built. In the gut and kidney, $\mathrm{Na^+}$ flowing in drags glucose in with it, up glucose's gradient. No ATP is used at that carrier, but it stops if the pump stops.
Carriers moving one substance are uniports; two in the same direction, symports; two in opposite directions, antiports (the $\mathrm{Na^+/K^+}$ pump is an antiport).
Poisons that stop ATP production, such as cyanide, stop active transport but not diffusion.

One cycle of the sodium potassium pump

3\,\mathrm{Na^+_{in}} + 2\,\mathrm{K^+_{out}} + \mathrm{ATP} \rightarrow 3\,\mathrm{Na^+_{out}} + 2\,\mathrm{K^+_{in}} + \mathrm{ADP} + \mathrm{P_i}

$\mathrm{P_i}$ inorganic phosphate released when ATP is split

Worked example

One

\mathrm{Na^+/K^+}

pump completes 150 cycles per second. Each second, how many sodium ions does it move out, how many potassium ions in, how many ATP does it use, and what net charge does it move?

Practice this conceptself-check · 4 quick reps

Secondary active transport still depends on ATP

The sodium glucose cotransporter uses no ATP itself, but the sodium gradient it runs on is built by the ATP-driven pump. Stop the pump and glucose uptake stops too. It is active transport, not facilitated diffusion, because glucose moves against its gradient.

Summary — formulas & gotchas at a glance

A revision cheat-sheet for the formulas and gotchas above. Click any concept name to jump back to its full explanation.

Formulas (2)

Osmosis, osmolarity and tonicity
Osmolarity of a solution
$\text{osmolarity} = i \times c$
Active transport and the sodium potassium pump
One cycle of the sodium potassium pump
$3\,\mathrm{Na^+_{in}} + 2\,\mathrm{K^+_{out}} + \mathrm{ATP} \rightarrow 3\,\mathrm{Na^+_{out}} + 2\,\mathrm{K^+_{in}} + \mathrm{ADP} + \mathrm{P_i}$

Reference tables (1)

Simple and facilitated diffusion: channels and carriers4 rows

Mechanism	Example of what crosses	Membrane protein?	ATP?	Direction
Simple diffusion	$\mathrm{O_2}$ , $\mathrm{CO_2}$ , steroid hormones	No	No	Down the gradient
Facilitated diffusion by channel	$\mathrm{Na^+}$ , $\mathrm{K^+}$ , $\mathrm{Cl^-}$ , water through aquaporins	Yes, channel	No	Down the gradient
Facilitated diffusion by carrier	Glucose into red blood cells	Yes, carrier	No	Down the gradient
Active transport	$\mathrm{Na^+}$ out and $\mathrm{K^+}$ in by the pump	Yes, carrier (pump)	Yes	Against the gradient

Watch out for (4)

"Uses a carrier protein" does not mean "active"
→ Simple and facilitated diffusion: channels and carriers
Water moves toward the higher solute concentration
→ Osmosis, osmolarity and tonicity
Count particles, not moles
→ Osmosis, osmolarity and tonicity
Secondary active transport still depends on ATP
→ Active transport and the sodium potassium pump

Drill the past-year questions on this topic

Open the bank with this topic pre-filtered.

Open in the bank