Topic 2.8 Tonicity and Osmoregulation: explain how concentration gradients of water and solutes affect the movement of water into and out of cells, and how organisms regulate their water balance.

What this topic is asking

The College Board (Topic 2.8) wants you to explain how solute and water gradients drive osmosis, to use tonicity (hypotonic, hypertonic, isotonic) to predict water movement, to apply the water-potential equation, and to describe how organisms osmoregulate. This is a quantitative topic.

Tonicity

Water moves by osmosis toward the higher solute concentration:

• Hypotonic surroundings: water enters the cell. Animal cells may swell and burst (lyse); plant cells become turgid.
• Hypertonic surroundings: water leaves the cell. Animal cells shrink (crenate); plant cells plasmolyse as the membrane pulls from the wall.
• Isotonic surroundings: no net movement; animal cells keep their shape.

Water potential

Pure water at standard pressure has $\psi = 0$. Adding solute makes $\psi_s$ negative, lowering water potential. Positive pressure (as in a turgid plant cell) raises it. The exam provides the formula and constants on the equations sheet.

Osmoregulation

Organisms control their water and solute balance:

• Contractile vacuoles in freshwater protists pump out the water that constantly enters from their hypotonic surroundings.
• Kidneys in animals adjust water and solute excretion to keep blood isotonic.
• Active transport of solutes sets up gradients that move water where it is needed (for example reabsorption in the kidney).

The cell wall is itself an osmoregulatory feature: it lets plant cells take in water and become turgid (which supports the plant) without bursting.

Water potential combines two influences. The solute potential ($\psi_s$) is always zero or negative, because adding solute lowers the free energy of water and reduces its tendency to move away. The pressure potential ($\psi_p$) can be positive (as in a turgid plant cell pushing against its wall) or negative (as in the tension that pulls water up the xylem during transpiration). Adding them gives the overall water potential, and comparing the water potential of two regions tells you which way water moves: always toward the lower (more negative) value, until the values are equal. This single framework explains turgor in plants, the uptake of water by roots, and what happens to any cell placed in a solution, which is why the College Board provides the equation and expects you to use it confidently.

Plant and animal cells respond differently

Because tonicity drives water across the membrane, the same external solution produces opposite-looking outcomes in walled and unwalled cells, and the College Board likes contrasting the two. An animal cell has only its plasma membrane, so it has no defense against net water movement: in a hypotonic solution it swells and can burst (lysis, called haemolysis in red blood cells); in a hypertonic solution it shrinks and crinkles (crenation). Animals must therefore keep their internal fluids close to isotonic, which is why blood osmolarity is tightly regulated.

A plant cell has a rigid cell wall outside the membrane. In a hypotonic solution water enters and the cell swells until the membrane presses on the wall; the wall pushes back, generating a positive pressure potential ($\psi_p$, turgor) that opposes further water entry, so the cell becomes firm and turgid rather than bursting. Turgor is what holds non-woody plants upright; losing it causes wilting. In a hypertonic solution water leaves, the membrane pulls away from the wall, and the cell plasmolyses. The wall is thus an osmoregulatory structure in its own right. Single-celled freshwater organisms, which constantly gain water from their hypotonic surroundings, instead bail water out using contractile vacuoles that fill and then pump water back across the membrane, an active, energy-using form of osmoregulation.

Try this

Q1. Identify what happens to a red blood cell placed in a hypertonic solution and explain. [2 points]

• Cue. Water leaves the cell by osmosis (toward the higher external solute concentration), so the cell shrinks (crenates).

Q2. Calculate the water potential of a cell with a solute potential of $-0.8$ bars and a pressure potential of $0.3$ bars. [1 point]

• Cue. $\psi = \psi_s + \psi_p = -0.8 + 0.3 = -0.5$ bars.