Week 1 · Unit 1 Aquatic Ecosystems, opener and introduction; 1-1 The Hydrosphere on Planet Earth: four spheres, the share of salt, fresh and frozen water, the water cycle, transpiration and excretion · 3 lessons
3
Transpiration and excretion: living things in the cycle
Integrated Science Sec 1 Term 1, Unit 1: pp. 3–7
GoalExplain transpiration and excretion as the living part of the water cycle, and say how water rises from roots to leaves.
New words
transpiration · loss of water vapour from the surfaces of a plant that are in the airstomata · microscopic openings in leaves and green stems through which vapour and gases passxylem · the plant tissue that carries water and minerals upward from the rootsexcretion · getting rid of the wastes made by metabolism
Explanation
Living things are part of the water cycle too. Plants release water vapour from surfaces in contact with air, mostly through the stomata of leaves and green stems; this is transpiration. It does two jobs. First, it cools the plant, because evaporating water takes heat away. Second, as vapour leaves the leaves, it tugs on the water in the xylem, so water and minerals are drawn up from the roots. Do not confuse transpiration with respiration: respiration releases energy from food inside cells. Animals and plants also excrete, which means getting rid of wastes of metabolism: carbon dioxide, water vapour and nitrogenous wastes such as ammonia, urea and uric acid. Respiration and excretion put water vapour and carbon dioxide into the air, and every breath you breathe out carries some water vapour.
water in the soilabsorbed→rootspulled up→xylem in the stem→leaftranspiration→stomata: vapour leaves
Worked examples
A plant is watered, and a loosely tied clear bag over some leaves shows drops of liquid after a sunny morning. The drops are condensed vapour that left the leaves through their stomata, so the bag shows transpiration. The roots took the water from the soil and the xylem carried it up.
What happens
Name
vapour leaves a leaf
transpiration
cells release energy from food
respiration
urea leaves the body
excretion
Choose the right word. A leaf loses water vapour through stomata: transpiration or respiration? Transpiration. Cells releasing energy from glucose: respiration. A kidney removing urea: excretion. So three different words describe three different jobs.
Why do plants in hot, dry places often have fewer or smaller leaves? Transpiration would carry away a lot of water. Smaller or fewer leaves mean fewer stomata open to the air, so less water is lost.
Class activity
Leaf bag, with a grown-up. Loosely tie a clear plastic bag over a few leaves of a house plant that stands in sunlight for a few hours (do not tie it tight). Describe what collects inside the bag. Then take the bag off. Write a cause-and-effect chain on cards: roots absorb water, xylem carries it up, stomata let vapour out, the leaf cools. Mix the cards and put them back in order.
Practice
1
Name the two jobs of transpiration.
It cools the plant, and it creates a pull that draws water and minerals up through the xylem.
2
Through which tiny openings does most transpiration happen, and where are they?
The stomata, found on leaves and young green stems.
3
Give two nitrogenous wastes excreted by living things.
Any two of ammonia, urea and uric acid.
4
A pupil says transpiration and respiration are the same. Correct the idea in one sentence.
Transpiration is water vapour leaving the plant; respiration is cells releasing energy from food.
