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Leaf structure, minerals and starch tests
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Leaves are organs built for photosynthesis. Plants also need mineral ions, and you can test leaves for starch.
How a leaf is adapted
Large surface area to absorb light. Thin, so gases diffuse a short distance.
Waxy cuticle reduces water loss. The upper epidermis is transparent to let light through.
Palisade mesophyll cells, near the top, are packed with chloroplasts.
Spongy mesophyll has air spaces for gas diffusion. Stomata, mainly on the lower surface, let carbon dioxide in.
Veins contain xylem (bringing water) and phloem (taking sugars away).
Waxy cuticle reduces water loss. The upper epidermis is transparent to let light through.
Palisade mesophyll cells, near the top, are packed with chloroplasts.
Spongy mesophyll has air spaces for gas diffusion. Stomata, mainly on the lower surface, let carbon dioxide in.
Veins contain xylem (bringing water) and phloem (taking sugars away).
Mineral ions
Plants absorb mineral ions from the soil through their roots.
Magnesium ions are needed to make chlorophyll. Without them, leaves turn yellow.
Nitrate ions are needed to make amino acids, and so proteins. Without them, growth is stunted.
Magnesium ions are needed to make chlorophyll. Without them, leaves turn yellow.
Nitrate ions are needed to make amino acids, and so proteins. Without them, growth is stunted.
Testing a leaf for starch
First destarch the plant by leaving it in the dark for about 48 hours.
To test a leaf: dip it in boiling water (kills cells), then warm it in ethanol in a water bath (removes chlorophyll), rinse, and add iodine. Blue-black means starch.
Variegated leaf: only the green parts turn blue-black, so chlorophyll is needed.
Foil strip: the covered part stays orange-brown, so light is needed.
Sodium hydroxide beside the plant absorbs carbon dioxide: no starch forms, so carbon dioxide is needed.
Oxygen given off by pondweed can be collected to show photosynthesis produces oxygen.
To test a leaf: dip it in boiling water (kills cells), then warm it in ethanol in a water bath (removes chlorophyll), rinse, and add iodine. Blue-black means starch.
Variegated leaf: only the green parts turn blue-black, so chlorophyll is needed.
Foil strip: the covered part stays orange-brown, so light is needed.
Sodium hydroxide beside the plant absorbs carbon dioxide: no starch forms, so carbon dioxide is needed.
Oxygen given off by pondweed can be collected to show photosynthesis produces oxygen.
Why is a leaf warmed in ethanol before testing it with iodine?
- Ethanol dissolves and removes the green chlorophyll.
- This makes the leaf pale, so a colour change with iodine is easy to see.
Answer: To remove chlorophyll so the colour change can be seen.
Leaves have a large surface area, are thin, have palisade cells full of chloroplasts, air spaces and stomata. Magnesium is needed for chlorophyll and nitrate for amino acids. Destarch the plant, then test leaves with iodine to show light, chlorophyll and carbon dioxide are needed.
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