Transport in Plants
ICSE Class 8 · Biology · Chapter 1
What this chapter covers
- Why a plant needs a transport system
- Diffusion
- Osmosis and the semipermeable membrane
- Root pressure
- Active transport
- Xylem and phloem: structure and function
- Transpiration
- Mineral nutrients
Key points
- Simple plants such as Chlamydomonas and Spirogyra move materials by diffusion; higher plants need a vascular system of xylem and phloem.
- Diffusion moves particles from high to low concentration without using the cell's energy.
- Osmosis is the movement of water through a semipermeable membrane from a dilute to a concentrated solution; the potato osmoscope demonstrates it.
- Root hairs absorb water by osmosis; root pressure pushes water a short way up the xylem.
- Active transport uses energy from respiration to move minerals from low to high concentration.
- Xylem (vessels and tracheids, mostly dead, lignified) carries water and minerals upwards and gives strength; phloem (sieve tubes and companion cells, living) carries food both ways.
- Transpiration is the loss of water vapour, mainly through stomata; it pulls water up, carries minerals and cools the leaves.
- Light, temperature, wind and dry air increase transpiration; humid air and dry soil reduce it.
- Macronutrients (N, P, K, Ca, Mg, S) are needed in large amounts and micronutrients (Fe, Mn, Zn, Cu, B, Mo, Cl) in tiny amounts.
- Shortage of a nutrient causes deficiency diseases such as chlorosis, stunted growth with purple leaves, and scorched leaf edges.
Questions answered
Why does Spirogyra not need a vascular system, while a mango tree does?
Every cell of Spirogyra touches the surrounding water and can exchange materials by diffusion. A mango tree is large, and most of its cells are far from soil and air, so it needs xylem and phloem to carry materials over long distances.
Define diffusion and give one example from a plant.
Diffusion is the movement of particles from a region of higher concentration to a region of lower concentration. Carbon dioxide diffusing into a leaf through its stomata is one example.
What is a semipermeable membrane? Name one found in a plant.
A semipermeable membrane lets water pass but stops dissolved substances such as sugar. The cell membrane of a plant cell is one.
In the potato osmoscope, why does the level of sugar solution rise? Why must the potato not be boiled?
Water moves by osmosis from the dish, through the living potato cells, into the stronger sugar solution. Boiling kills the cells, so their membranes no longer act as semipermeable membranes.
A balsam stem is cut near the soil and water rises in a tube fixed to the stump. What does this show?
It shows root pressure, the push developed by the roots that forces water up the xylem.
State two ways in which active transport differs from diffusion.
Active transport moves substances from low to high concentration, while diffusion moves them from high to low. Active transport needs energy from respiration, while diffusion needs none.
Give two differences between xylem and phloem.
Xylem carries water and minerals upwards only, while phloem carries food both up and down. Xylem is mostly dead tissue with lignified walls, while phloem is made of living cells.
What is transpiration? Give two ways in which it helps the plant.
Transpiration is the loss of water vapour from the aerial parts of a plant, mainly through the stomata. It pulls water and minerals up the xylem, and it cools the leaves.