What are life processes?
A living body is always doing repair and maintenance work, even when it looks still. Cells break down and are rebuilt, and molecules keep moving. This needs energy, and energy comes from food.
The basic jobs that keep an organism alive are called life processes:
- Nutrition: taking in food (energy and raw material) from outside.
- Respiration: breaking food inside cells to release energy.
- Transportation: carrying food, oxygen and water to every cell.
- Excretion: removing harmful waste made by the body.
A one-celled organism does all this across its surface. A big many-celled body needs special organs for each job, because most of its cells are far from the outside.
Modes of nutrition: autotrophic and heterotrophic
Autotrophs (self-feeders) make their own food from simple inorganic things: carbon dioxide and water. Green plants and some bacteria do this. Heterotrophs (other-feeders) cannot; they eat food made by others, like animals and fungi.
Food made by autotrophs is the base of every food chain, so all life depends on them.
Photosynthesis: how plants make food
Photosynthesis is the process in which green plants use sunlight and chlorophyll to turn carbon dioxide and water into glucose, releasing oxygen.
6CO₂ + 12H₂O → (sunlight, chlorophyll) → C₆H₁₂O₆ + 6O₂ + 6H₂O (simple balanced form: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂)
Three events of photosynthesis
- Chlorophyll absorbs light energy.
- Light energy becomes chemical energy, and water is split into hydrogen and oxygen.
- Carbon dioxide is reduced (joined with hydrogen) to make carbohydrate.
These need not happen one right after another. Desert plants take in CO₂ at night (stomata open when it is cool) and store it; they use it in the day when light is available.
Where it happens
Mostly in leaf cells packed with chloroplasts. Glucose that is not used at once is stored as starch, the plant's food bank.
Stomata and the raw materials
Stomata are tiny pores, mostly on the lower surface of leaves. Each is guarded by two bean-shaped guard cells. When water flows into the guard cells they swell and the pore opens; when they lose water they shrink and the pore closes. Stomata let CO₂ in and O₂ out, and a lot of water vapour also escapes through them, so the plant closes them when it is short of water.
- CO₂: from air, through stomata.
- Water: from soil, through roots and xylem.
- Minerals (nitrogen, phosphorus, magnesium, iron): from soil. Nitrogen is taken as nitrates/nitrites to make proteins; magnesium is part of chlorophyll.
Experiments and board-exam focus
Starch test: keep a potted plant in the dark for 3 days (to use up starch), then in sunlight. Boil a leaf in alcohol to remove green colour, then add iodine. Parts that had light and chlorophyll turn blue-black (starch present).
Is chlorophyll needed? Use a variegated leaf (green and white patches). Only the green part turns blue-black.
Is CO₂ needed? Keep a plant in a closed jar with potassium hydroxide (which absorbs CO₂). Its leaf shows no starch.
Exam tip: questions on the equation, the three events, stomata opening/closing and the destarching experiment are asked every year, usually for 2–3 marks.
Key formulas and definitions
- 6CO₂ + 6H₂O → (sunlight, chlorophyll) → C₆H₁₂O₆ + 6O₂
- Life processes = nutrition + respiration + transportation + excretion
- Autotroph: makes own food; heterotroph: depends on others
- Chlorophyll: green pigment in chloroplasts that absorbs light
- Stoma (plural stomata): pore for gas exchange, opened/closed by guard cells
- Starch: stored form of glucose in plants; turns blue-black with iodine
Worked examples
1. Name the raw materials and products of photosynthesis.
Raw materials: carbon dioxide (from air) and water (from soil). Needed conditions: sunlight and chlorophyll. Products: glucose (stored as starch) and oxygen (released).
2. Why is a plant kept in the dark for 2–3 days before a starch test?
In the dark the plant cannot make food, so it uses up the starch already in its leaves. Now, if starch appears after the experiment, we know it was made during the experiment, not before.
3. A variegated croton leaf is tested with iodine after being in sunlight. What will you see and what does it prove?
Only the parts that were green turn blue-black; the white parts stay brownish. Starch was made only where chlorophyll was present, so chlorophyll is needed for photosynthesis.
4. Why do guard cells close stomata on a hot afternoon?
On a hot day the plant loses much water as vapour through stomata. When water runs short, guard cells lose water, become limp and close the pore. This saves water, though it also slows photosynthesis because less CO₂ enters.
5. Why are autotrophs said to be the base of all life on Earth?
They alone turn light energy into food energy. Herbivores eat plants, carnivores eat herbivores, and decomposers eat the dead of all. Remove autotrophs and no food energy enters the chain.
Common mistakes
- Saying plants do photosynthesis instead of respiration. Plants do BOTH: photosynthesis in green parts in light, respiration in all cells all the time.
- Thinking the oxygen released comes from CO₂. It comes from splitting water.
- Writing stomata are on the upper surface. In most leaves they are mainly on the lower surface.
- Forgetting to decolourise the leaf in alcohol (in a water bath) before adding iodine; the green colour hides the result.