Water potential

The **net movement of water** across a **partially permeable membrane**, from a **higher** water potential to a **lower** water potential.

A measure of **how freely water can move out** of a solution. **Pure water** has the highest water potential; adding solute lowers it.

The process in which **water molecules surround and separate** each dissolved **solute** particle, holding it in solution.

A membrane that lets **water** through but blocks (most of) the dissolved **solute** particles.

From a **higher** water potential (dilute) to a **lower** water potential (concentrated).

It **lowers** the water potential — the more concentrated the solution, the lower its water potential.

The **dilute** solution — it has fewer solutes and more free water, so a higher water potential.

A **partially permeable membrane** AND a **difference in water potential** (a solute-concentration gradient).

**Passive** — it needs no energy (no ATP); water moves down its own gradient.

There is **no net movement** of water — water still crosses both ways, but in equal amounts (isotonic).

Because **solvation** ties up water molecules around the solute particles, leaving fewer free to move.

The **more concentrated** side has the **lower** water potential, so water moves **into** it from the more dilute side.

The **total concentration of solute particles** in a solution — more solute means a higher osmolarity.

The net movement of **water** across a partially permeable membrane, from a **lower** osmolarity (dilute) toward a **higher** osmolarity (concentrated).

Toward the **higher osmolarity** — the more concentrated solution (the side with less water).

One with a **lower osmolarity** than the cell (more dilute). Water moves **into** the cell.

One with the **same osmolarity** as the cell. There is **no net movement** of water.

One with a **higher osmolarity** than the cell (more concentrated). Water moves **out** of the cell.

Water moves **in**, so the cell **gains water and swells** (and without a wall it may burst).

Water moves **out**, so the cell **loses water and shrinks**.

There is **no net movement** of water, so the cell **stays the same**.

A solution is only hypotonic / isotonic / hypertonic **compared to another** solution (or to the cell) — never on its own.

The solution was **hypotonic** — water moved into the tissue, so it gained mass.

The solution was **hypertonic** — water moved out of the tissue, so it lost mass.

The solution was **isotonic** — no net movement of water, so no change in mass.

Its **solute potential + pressure potential** (Ψ = Ψs + Ψp).

A measure of the tendency of water to leave a cell or solution by osmosis. Water moves from a **higher** to a **lower** water potential.

The part of the water potential caused by **dissolved solutes**. It is always **zero or negative** and lowers the water potential.

The part of the water potential caused by physical pressure — in a plant cell, the contents pushing on the **wall**. It **raises** the water potential.

They **lower** it (make it more negative), so they pull water into the cell.

It **raises** it (makes it less negative) as the cell fills and the wall pushes back.

From the **higher** (less negative) water potential to the **lower** (more negative) one.

Water **enters**, the pressure potential rises, and the cell becomes **turgid** (firm). The wall stops it bursting.

Water **leaves**, the cell goes **flaccid**, and with more loss the membrane pulls from the wall — it is **plasmolysed**.

Its rigid **cell wall** resists expansion, so water entry builds a **pressure potential** and the cell becomes turgid instead of bursting.

The firmness of a plant cell when it is full of water and pushing against its wall — the result of a **high pressure potential**.

The **external (surrounding) solution** that has drawn water out of the cell.

Only a **walled** cell can build a **pressure potential** as the wall pushes back; an animal cell has no wall, so no pressure term develops.

The net movement of **water** across a partially permeable membrane, from a **more dilute** solution to a **more concentrated** one.

They have **no cell wall** — only a flexible plasma membrane — so they can **burst** or **shrivel** as water moves in or out.

A solution that is **more dilute** than the inside of the cell (lower solute concentration); water moves **into** the cell.

A solution with the **same** solute concentration as the cell; there is **no net movement** of water.

A solution that is **more concentrated** than the inside of the cell (higher solute concentration); water moves **out** of the cell.

Water enters by osmosis, so the cell **swells and may burst** — this bursting is called **lysis** (haemolysis in red blood cells).

**No net movement** of water, so the cell **stays the same** shape and size.

Water leaves by osmosis, so the cell **shrinks and wrinkles** — this is called **crenation**.

The **bursting** of a cell when too much water enters it by osmosis (haemolysis if it is a red blood cell).

The **shrivelling / wrinkling** of an animal cell when water leaves it in a hypertonic solution.

Distilled (pure) water is strongly **hypotonic**, so water rushes in by osmosis and the cell **swells and bursts (lyses)**.

**Burst/swollen = hypotonic; unchanged = isotonic; shrunken/crenated = hypertonic** — read the cell's shape backwards.

The **control of water balance** in a cell or organism — keeping the internal water content steady.

A **contractile vacuole** collects the excess water that enters by osmosis and **pumps it back out** of the cell.

The **net movement of water** across a partially permeable membrane, from a **higher water potential (dilute)** to a **lower water potential (concentrated)**.

It is **fully permeable** (water passes through), but it **resists pressure** so the cell does not burst — making the cell turgid instead.

The **outward pressure** of the cell contents pushing against the cell wall when a plant cell has taken in water.

A plant cell that is **full of water and firm**, with the contents pressing hard against the wall (high turgor pressure).

A plant cell that has **lost water and is limp**, with little or no turgor pressure pushing on the wall.

When a plant cell loses so much water that the **cytoplasm and membrane pull away from the cell wall**.

Water **enters** by osmosis → the cell swells but the wall stops it bursting → it becomes **turgid**.

**No net water movement** → low turgor → the cell is **flaccid** (limp).

Water **leaves** by osmosis → the cell loses turgor and becomes **plasmolysed** → the tissue wilts.

The **external (bathing) solution** — the fully permeable wall lets it flow in.

The plant cell's **strong wall resists the pressure** (it becomes turgid). The animal cell has **no wall**, so it keeps swelling and bursts.

It is **isotonic** — the same concentration as the cell contents, so it estimates the cells' own **internal concentration**.

Towards the **more concentrated** solution (lower water potential). To plasmolyse a cell the outside must be the more concentrated one.