Diffusion, osmosis & active transport

Substances move in and out of cells in three ways. The first two need no energy; the third does.

• Diffusion — movement of molecules from a region of high concentration to a region of low concentration. No energy needed.
• Osmosis — diffusion of water from a dilute solution to a more concentrated solution, through a selectively permeable membrane. No energy needed.
• Active transport (active uptake) — moves substances against the gradient (low → high), so it needs energy from respiration.

CCEA definition: osmosis is the diffusion of water from a dilute solution to a more concentrated solution through a selectively permeable membrane. Water moves only, and no energy is used.

Which way does water go?

Turgid & plasmolysed cells

Osmosis changes how firm a plant cell is. The cell wall stops the cell bursting by limiting how much water enters.

• In a dilute solution (e.g. pure water), water enters by osmosis. The cell swells and the membrane pushes on the wall — the cell is turgid.
• In a concentrated solution, water leaves by osmosis. The membrane pulls away from the wall — the cell is plasmolysed.
• Turgor gives plants support; wilting happens when cells lose water and become flaccid.

Red blood cells (no wall): if you place a red blood cell in pure water, so much water enters by osmosis that the cell bursts — this is cell lysis. Plant cells avoid this because their wall limits water entry.

What happens in strong sugar solution?

Which process is it?

Tap a statement, then tap the process it describes.

Root hairs, water & minerals

Root hair cells are specialised for absorbing water and minerals. Each cell has a long hair-like extension that greatly increases the surface area in contact with the soil water.

• Water enters the root hair by osmosis — the soil water is more dilute than the cell contents.
• Mineral ions (e.g. nitrates) are usually in low concentration in the soil, so they are taken in by active uptake — active transport that needs energy from respiration.
• Plants need nitrates to make proteins for growth; a shortage causes poor growth and yellowing leaves.

Why active transport? Minerals must move from a low concentration in the soil into a higher concentration in the root — against the gradient — so the cell must spend energy from respiration to pump them in.

How do minerals get in?

Xylem & phloem

Plants have two transport tissues, both running through the roots, stem and leaves:

• Xylem — carries water and dissolved minerals from the roots up to the leaves. Xylem vessels are dead, hollow tubes strengthened with lignin. Transport is one way (upwards).
• Phloem — carries dissolved sugars (made in the leaves) to the rest of the plant. Phloem is made of living cells and can transport both up and down.

Transpiration

CCEA defines transpiration as the evaporation of water from the mesophyll cells, followed by diffusion of water vapour through the airspaces and out of the stomata.

This loss of water from the leaves pulls the transpiration stream — a continuous column of water up the xylem from the roots.

The rate of transpiration increases when:

• Temperature rises — more evaporation.
• Wind speed rises — water vapour is carried away, keeping the diffusion gradient steep.
• Humidity falls — drier air means a steeper gradient out of the leaf.
• Surface area (or light intensity, which opens the stomata) increases.

Practical: a potometer (bubble potometer or the weighing method) measures the rate of water uptake, which is used to estimate the rate of transpiration under different conditions.

Your turn — rate of transpiration

Your turn — percentage change (osmosis)

Xylem or phloem?

Tap a statement on the left, then its matching tissue on the right.

The big ideas to know

You've covered CCEA Osmosis & plant transport. Press Finish to see your score.