Osmosis

Core definition.

"The net movement of water molecules from a dilute solution to a more concentrated solution, through a partially permeable membrane."

Extended definition (memorise this for full marks):

"The NET movement of water molecules from a region of HIGHER WATER POTENTIAL to a region of LOWER WATER POTENTIAL, through a PARTIALLY PERMEABLE MEMBRANE."

Why two definitions? "Dilute to concentrated" is intuitive but loses precision (e.g. comparing two solutions with different solutes is complicated). "Water potential" handles all cases.

Water potential ($\Psi$).

• Measures the tendency of water to leave a solution.
• Pure water has the HIGHEST water potential (set as 0).
• Adding solute LOWERS water potential (becomes more negative).
• Water moves DOWN the water potential gradient (from high to low).

Worked qualitative. Two solutions: A = 1% sugar; B = 5% sugar. Which has higher water potential?

• A (more dilute) has higher water potential (less negative).
• B (more concentrated) has lower water potential (more negative).
• Water moves from A to B.

Cambridge tip. At Extended level, use "water potential" — Cambridge mark schemes accept "from high water potential to low water potential" but not "from dilute to concentrated" for the higher-tier mark.

In pure water (or solution of higher water potential than the cell sap).

• Water enters the cell by osmosis (down the water potential gradient).
• Vacuole swells; cytoplasm pushes against cell wall.
• Cell becomes TURGID — firm and rigid.
• Cell wall is strong enough to PREVENT bursting.
• Plants need many turgid cells for support — wilted plants have lost turgor.

In a solution of equal water potential.

• No net movement of water.
• Cell stays the same — FLACCID is not quite right (used for limp cells, but generally just neutral).

In a solution of LOWER water potential than the cell sap (concentrated).

• Water leaves the cell by osmosis.
• Vacuole shrinks; cytoplasm pulls AWAY from the cell wall.
• Cell becomes PLASMOLYSED.
• Severe plasmolysis can damage the cell.

Why plants wilt. Without enough water uptake, vacuoles shrink → less turgor pressure → cells become flaccid → leaves and stems droop. Watering replaces water → cells swell → plant stiffens.

Worked qualitative. A wilted lettuce, soaked in water, becomes crisp again. Why? Water enters the plant cells by osmosis; cells become turgid; cell walls support the plant.

Animal cells have NO CELL WALL. They're at the mercy of osmosis.

In pure water (or HIGHER water potential than cell contents).

• Water enters the cell by osmosis.
• Cell SWELLS.
• May BURST (haemolysis for red blood cells; cell lysis for others).

In concentrated solution (LOWER water potential).

• Water leaves the cell by osmosis.
• Cell shrivels — CRENATED.

Why animal cells need to be in balanced fluid. Tissue fluid around cells must have water potential close to cell contents. The body controls this carefully (homeostasis):

• Drinking pure water dilutes blood → blood water potential rises → cells could burst.
• Eating very salty food concentrates blood → cells could shrivel.
• The KIDNEY adjusts blood water potential by varying urine concentration.

Worked qualitative. Why do red blood cells stored in saline solution last longer than in pure water?

• Pure water has higher water potential than blood → water flows IN → cells burst (haemolysis).
• Saline (0.9% salt) has water potential matching blood plasma → no net movement → cells survive.
• This is why intravenous drips use 0.9% saline ("isotonic"), not pure water.

1. Water uptake by roots.

• Soil water has higher water potential than root cell sap.
• Water moves IN to root hair cells by osmosis.
• Then moves cell-to-cell into the xylem (covered in Plant Transport notes).

2. Kidney function.

• Kidneys regulate blood water content.
• Tubules can concentrate or dilute urine to maintain stable blood water potential.

3. Food preservation by salting / sugaring.

• Salt or sugar drawn into food from the surface lowers water potential outside microbial cells.
• Microbes lose water by osmosis → can't grow.
• Used for preserving meat (jerky, ham), fruit (jams, candied fruit).

4. Why drinking sea water is dangerous.

• Sea water has very low water potential (high salt).
• After drinking it, the gut and blood water potential drop → cells lose water → dehydration accelerates.
• Body tries to excrete the salt → produces more urine than the water you drank → net dehydration.

Worked qualitative. Why are seedlings often watered with weak fertiliser instead of strong fertiliser? Strong fertiliser solution has very low water potential → roots could lose water by osmosis → seedlings dehydrate. Weak fertiliser is safer.