Two ways of getting food
All organisms need food for energy and growth.
Autotrophic nutrition (auto = self, trophos = nourishment): the organism makes its own food from simple substances. Green plants are autotrophs.
Heterotrophic nutrition: the organism takes in food made by others. Animals, fungi and most bacteria are heterotrophs.
Photosynthesis: the food factory in the leaf
Leaves are the food factories of a plant. In them, green plants make food by photosynthesis (photo = light, synthesis = putting together).
The raw materials and conditions are:
Need | Where it comes from |
|---|---|
Carbon dioxide | taken from the air through tiny pores called stomata on the leaf surface |
Water | absorbed from the soil by the roots and carried up through the stem in pipe-like xylem vessels |
Sunlight | the energy source |
Chlorophyll | the green pigment in leaf cells, which captures sunlight |
The process can be summed up as:
carbon dioxide + water → (in sunlight, with chlorophyll) → carbohydrate + oxygen
The carbohydrate made first is a simple sugar, glucose, which is soon changed into starch and stored. The oxygen is released into the air through the stomata. Photosynthesis is therefore the source of almost all the oxygen we breathe and, directly or indirectly, of all the food on Earth.
Each stoma is surrounded by two guard cells that open and close it.
Testing a leaf for starch
Keep a potted plant in sunlight for a few hours.
Boil a leaf in water for a minute to soften it.
Place it in alcohol in a test tube, and heat the test tube in a water bath (never directly over a flame). The alcohol removes the green chlorophyll.
Wash the pale leaf and add a few drops of iodine solution.
The leaf turns blue-black, showing that starch was made. A leaf from a plant kept in the dark for two days does not turn blue-black, which shows that light is needed for photosynthesis.
Making proteins and fats
Plants also need nitrogen to make proteins. They cannot use nitrogen gas from the air directly. Certain bacteria in the soil convert it into soluble compounds that roots absorb. Farmers add fertilisers and manure rich in nitrogen to replenish the soil.
Leaves of other colours (red, brown, violet) also contain chlorophyll; the green is hidden by other pigments.
Other modes of nutrition in plants
Mode | How it works | Example |
|---|---|---|
Parasitic | takes food from a living host plant, harming it | Cuscuta (amarbel), a yellow, leafless climber that wraps around host plants |
Insectivorous | makes its own food but traps and digests insects to get extra nitrogen, as it grows in nitrogen-poor soil | pitcher plant, Venus flytrap |
Saprotrophic | takes in nutrients from dead and decaying matter, after digesting it outside the body | mushrooms, bread mould and other fungi |
Symbiotic | two organisms live together and both benefit | lichens (an alga makes food, a fungus gives shelter, water and minerals); Rhizobium bacteria in root nodules of pulses fix nitrogen for the plant and get food in return |
In the pitcher plant, the pitcher-shaped structure is a modified part of the leaf. Insects that fall in are trapped by hairs and digested by juices at the bottom.
Bread kept in a warm, moist place grows cotton-like patches of mould. These fungi grow from spores that are always present in the air.
Summary
Green plants are autotrophs; they make food by photosynthesis using carbon dioxide, water, sunlight and chlorophyll, releasing oxygen.
Iodine shows the presence of starch in a leaf.
Cuscuta is a parasite, the pitcher plant is insectivorous, fungi are saprotrophs, and lichens show symbiosis.