Image: Osmosis. The process plant roots use to take in water.

Plants use their roots to absorb (= take up) water and mineral ions from the soil. The water absorbed by plant roots is used for:

• Transporting dissolved mineral ions
• Keeping cells rigid (= stiff), so the plant stays upright
• Cooling the leaves (when the water evaporates from them)
• Photosynthesis

Root hair cells

The outside of roots is covered with root hair cells. The “root hair” is an extension of the cell and provides a larger surface area, so that water and mineral ions can be absorbed faster. In addition, root hair cells have very thin cell walls to improve the flow of water into the cells.

Diffusion and osmosis

Diffusion is the movement of particles from a place of high concentration to a place of low concentration. We say that the particles move along a concentration gradient.

Osmosis is the diffusion of water. Water particles move from a place with a high water concentration (lots of water) to a place with a low water concentration (less water). This happens across a semi-permeable membrane which lets small molecules like water pass, but keeps back larger molecules like sugar. The semi-permeable membrane acts like a sieve.

Root hair cells take in water from the soil using osmosis. The water moves from the soil with lots of water into the root with less water. Cell wall and cell membrane act as the semi-permeable membrane.

Active transport

Mineral ions are ionic compounds or salts that are found naturally in soil. Plants need these ions to build important substances like proteins.

The concentration of mineral ions inside the root is higher than outside. Mineral ions cannot diffuse from a lower concentration outside to a higher concentration inside the root. Therefore, the cell membrane needs to actively pump mineral ions inside. This process requires energy and is called active transport.

Questions

1. What is the water absorbed by plant roots used for?
2. What process do plants need carbon dioxide, water and sun light for?
3. Give two adaptations (= special features) that root hair cells have to let water enter the cells faster.
4. What is the difference between diffusion and osmosis?
5. How does water enter root hair cells? What is the name of the process?
6. What is a semi-permeable membrane?
7. What are mineral ions?
8. What is active transport?
9. Why do mineral ions not diffuse into root hair cells?