الأسبوع 11 · الدرس الأول «البناء الضوئي» (٢): التفاعلات اللاضوئية والمعادلة والنشا وعوامل سرعة البناء الضوئي والزراعة العمودية · 3 دروس
2
Where the food goes: starch stores
Science Prep 2 Term 1, Unit 3: pp. 70–74
GoalExplain how glucose is stored as starch and how a leaf test would show that light and chlorophyll are needed.
New words
starch · a long chain of glucose used for storage in plantsvariegated leaf · a leaf with green and white or red patchestuber · a swollen underground stem that stores food, like a potato
Explanation
A plant makes more glucose in the day than it needs at once. It turns the extra glucose into starch, which is easy to store. Starch is stored in leaves, roots and stems: potato tubers, sweet-potato roots and taro corms are full of it. A variegated leaf has green patches that contain chlorophyll and white patches that do not. If such a leaf is kept in light, and then tested with iodine, only the parts that were green turn dark blue. The blue shows starch, so this tells us that chlorophyll and light are needed to make starch. We only read about this test, since it uses heated leaves and chemicals that are not for children to do. Some textbooks say the leaf is boiled to “break the cell walls”: in fact heat kills the cells and softens the leaf. Using ethanol in a hot water bath is a fire risk, so we never do it. We can learn from a simple home experiment instead: cover a part of a leaf and compare after a week.
glucose made in the day→joined into starch→stored in leaves, roots, stems
Worked examples
A variegated leaf has a white edge and a green centre. After a bright day it is tested with iodine (a result we are told): the green centre turns dark blue, the white edge does not. The edge had no chlorophyll, so it made no starch.
Why is a potato rich in starch? The potato plant makes glucose in its leaves, turns it into starch, and stores it in the underground tubers for the next season.
Is oxygen a waste? No: it is a by-product, and animals and plants use it for respiration. The air we breathe depends on photosynthesis.
Class activity
Store hunt, with a grown-up. List ten plant foods from your kitchen (for example potato, rice, bean, apple, lettuce, onion, carrot, sweet potato, wheat grains, cucumber). Next to each, write which plant part it is: root, stem, leaf, seed or fruit. Underline the foods that are mainly a store of starch for the plant’s future use. Choose one leaf food and one tuber or root food, and say which you would expect to hold more stored starch. Do not test any food with iodine.
Practice
1
Why do plants turn extra glucose into starch?
Starch is a long chain that is easy to store and does not dissolve and leak away.
2
Name two plant parts where starch is stored.
For example potato tubers and sweet-potato roots (or leaves and stems).
3
In the variegated-leaf test, which parts turn dark blue with iodine, and what does this show?
The parts that were green and had light; chlorophyll and light are needed for starch.
4
Why is the ethanol test never done by children?
Ethanol is a fire risk when heated in a hot water bath.
