Osmosis as the movement of water across a partially permeable membrane, turgid, flaccid and plasmolysed plant cells, the roles of xylem and phloem, water uptake by root hair cells, and transpiration and the factors affecting it.

What this dot point is asking

CCEA Double Award wants osmosis defined and applied to plant cells, the difference between turgid, flaccid and plasmolysed cells, the roles of xylem and phloem, how root hair cells take up water, and transpiration with the factors that affect it. Osmosis is the key idea; everything else builds on it.

Osmosis

The membrane lets water through but not larger solute molecules, so only water moves. Osmosis is a special case of diffusion, but it only refers to water.

Turgid, flaccid and plasmolysed cells

When a plant cell is in a dilute solution (like pure water), water enters by osmosis and the cell becomes turgid - swollen and firm, with the contents pushing against the cell wall. Turgid cells support the plant.

When a plant cell is in a concentrated solution, water leaves by osmosis and the cell becomes flaccid - limp. If it loses a lot of water, the membrane and contents pull away from the cell wall and the cell is plasmolysed. A plant short of water wilts because its cells lose turgor.

Xylem, phloem and root hair cells

Plants have two transport tissues:

• Xylem carries water and minerals up from the roots to the leaves. It is made of dead, hollow cells strengthened with lignin.
• Phloem carries dissolved food (sugars) made in the leaves to the rest of the plant, in both directions. This is called translocation.

Root hair cells are adapted to take up water: each has a long thin extension that increases the surface area for absorbing water (by osmosis) and minerals from the soil.

Transpiration

Transpiration speeds up when conditions increase evaporation or the concentration gradient:

• Higher temperature - more evaporation.
• Lower humidity - drier air increases the gradient.
• More air movement (wind) - removes water vapour from around the stomata.
• Brighter light - opens the stomata.

Examples in context

Example 1. Why wilted lettuce crisps up in water. Limp lettuce leaves have flaccid cells. Placed in water, the cells take in water by osmosis and become turgid again, so the leaf becomes firm and crisp. This is osmosis restoring turgor in action.

Example 2. The potato osmosis practical. Cutting equal potato cylinders and leaving them in sugar solutions of different concentrations, then measuring the percentage change in mass, lets you find the concentration at which there is no net movement of water. This is a standard CCEA osmosis investigation.

Try this

Q1. Define osmosis. [2 marks]

• Cue. Movement of water from a dilute to a more concentrated solution across a partially permeable membrane.

Q2. Which tissue carries sugars around a plant? [1 mark]

• Cue. Phloem.