SPI Biology · Plant Biology
Nutrition and Respiration in Plants
A green plant makes its own food from carbon dioxide and water using sunlight, takes in gases through its stomata, and like every living thing respires to release energy from that food.
Why this matters
Six questions in the bank, four EASY and two MODERATE: the share of sunlight plants use, which events of photosynthesis are wrongly stated, the black paper experiment, guard cells, the crop that holds Rhizobium, and the seeds used to show respiration. The two MODERATE ones test the details of photosynthesis, so learn its three events exactly.
Concept 1 of 3: Photosynthesis: raw materials, events and experiments
A leaf is a small food factory. It takes in carbon dioxide from the air and water from the roots, and chlorophyll traps sunlight to join them into glucose, giving out oxygen. Each school experiment takes away one need (light, carbon dioxide or chlorophyll) and shows that no starch forms without it.
Definition
- Equation: carbon dioxide + water, with sunlight and chlorophyll, gives glucose + oxygen.
- Three events: (1) chlorophyll absorbs light energy; (2) light energy is changed to chemical energy and water is split into hydrogen and oxygen; (3) carbon dioxide is reduced to carbohydrate.
- The light phase makes the energy carriers ATP and NADPH; the dark phase uses them to make sugar.
- The energy change is light (radiant) to chemical, never the other way.
- Green plants store only about 1% of the sunlight that falls on them.
- Glucose is stored as starch, which turns blue black with iodine. A plant is first de-starched by keeping it in the dark.
| Experiment | What is taken away | Result | It shows |
|---|---|---|---|
| Part of a leaf covered with black paper | Light, from that part | No starch where covered | Light is necessary Air still reaches the covered part, so the black paper tests light only. |
| Variegated leaf (green and white patches) | Chlorophyll, from the white part | Starch only in green parts | Chlorophyll is necessary |
| Plant in a jar with potassium hydroxide | Carbon dioxide (KOH absorbs it) | No starch | Carbon dioxide is necessary |
In each experiment the plant is de-starched first, and the leaf is tested with iodine.
Practice this conceptself-check · 5 quick reps
The same idea in a practice question:
Practice questionEasy
Example 1 · Plant Biology · Nutrition and Respiration in Plants
A student covered a leaf from a de - starched plant with a black paper strip and kept it in the garden outside his house in fresh air. In the evening, he tested the covered portion of the leaf for the presence of starch. By doing so the student was trying to show that:
Putting ATP and NADPH in the dark phase
ATP and NADPH are made in the light phase. The dark phase uses them to reduce carbon dioxide to sugar.
10% for the sunlight plants use
Plants store only about 1% of the sunlight falling on them. The 10% figure belongs to food chains: the energy passed from one level to the next.
Reading the black paper test as a carbon dioxide test
The paper blocks light but not air, so it tests light alone. Carbon dioxide is tested by removing it with potassium hydroxide.
Concept 2 of 3: Stomata and root nodules
A leaf breathes through tiny pores, the stomata, each guarded by two bean shaped guard cells that open and close it. Nitrogen is the one raw material a plant cannot take from the air by itself; some plants keep bacteria in their roots that do it for them.
Definition
- Stomata: tiny pores, mostly on the underside of leaves. Gases go in and out; water vapour is lost (transpiration).
- Guard cells: a pair of kidney (bean) shaped cells around each stoma. They are found only in plants.
- Rhizobium: a bacterium that lives in the root nodules of legumes (pea, gram, bean, groundnut) and fixes nitrogen from the air for the plant.
- Cereals such as wheat, rice and barley are grasses, not legumes, and have no root nodules.
| Plant | Group | Root nodules with Rhizobium |
|---|---|---|
| Pea | Legume | Yes |
| Gram | Legume | Yes |
| Groundnut | Legume | Yes |
| Wheat | Cereal (grass) | No |
| Rice | Cereal (grass) | No |
Legumes enrich the soil with nitrogen, so farmers grow them between cereal crops.
Practice this conceptself-check · 4 quick reps
The same idea in a practice question:
Practice questionEasy
Example 2 · Plant Biology · Nutrition and Respiration in Plants
Amisha observed a slide and saw a pair of guard cells in it. She is observing slide of:
Picking a cereal for Rhizobium
Wheat, rice and barley are grasses and have no root nodules. Rhizobium lives with legumes such as pea, gram and bean.
Concept 3 of 3: Respiration in plants and seeds
Plants respire all the time, day and night, just as animals do. Photosynthesis makes food; respiration breaks it down to release energy and gives out carbon dioxide. A germinating seed is growing fast, so it respires fast and gives out a lot of carbon dioxide.
Definition
- Plants respire day and night; they photosynthesise only in light.
- Respiration takes in oxygen and gives out carbon dioxide; photosynthesis does the opposite.
- To show that seeds respire, use germinating (sprouting) seeds: they respire actively.
- Dry seeds respire very slowly, and boiled seeds are dead and do not respire at all.
- The carbon dioxide is shown by potassium hydroxide absorbing it (a liquid rises in a bent tube) or by lime water turning milky.
| Seeds | Alive | Respiration |
|---|---|---|
| Germinating | Yes | Fast, gives out much carbon dioxide |
| Dry | Yes, but resting | Very slow |
| Boiled | No | None Boiled seeds are used only as the control: they show nothing happens without life. |
Practice this conceptself-check · 3 quick reps
The same idea in a practice question:
Practice questionEasy
Example 3 · Plant Biology · Nutrition and Respiration in Plants
Seeds taken in the conical flask during respiration experiment must be
Plants respire only at night
Plants respire all the time. At night it is simply the only one of the two processes going on, because photosynthesis needs light.
Summary — formulas & gotchas at a glance
A revision cheat-sheet for the formulas and gotchas above. Click any concept name to jump back to its full explanation.
Reference tables (3)
Photosynthesis: raw materials, events and experiments3 rows
| Experiment | What is taken away | Result | It shows |
|---|---|---|---|
| Part of a leaf covered with black paper | Light, from that part | No starch where covered | Light is necessary Air still reaches the covered part, so the black paper tests light only. |
| Variegated leaf (green and white patches) | Chlorophyll, from the white part | Starch only in green parts | Chlorophyll is necessary |
| Plant in a jar with potassium hydroxide | Carbon dioxide (KOH absorbs it) | No starch | Carbon dioxide is necessary |
In each experiment the plant is de-starched first, and the leaf is tested with iodine.
Stomata and root nodules5 rows
| Plant | Group | Root nodules with Rhizobium |
|---|---|---|
| Pea | Legume | Yes |
| Gram | Legume | Yes |
| Groundnut | Legume | Yes |
| Wheat | Cereal (grass) | No |
| Rice | Cereal (grass) | No |
Legumes enrich the soil with nitrogen, so farmers grow them between cereal crops.
Respiration in plants and seeds3 rows
| Seeds | Alive | Respiration |
|---|---|---|
| Germinating | Yes | Fast, gives out much carbon dioxide |
| Dry | Yes, but resting | Very slow |
| Boiled | No | None Boiled seeds are used only as the control: they show nothing happens without life. |
Watch out for (5)
- Putting ATP and NADPH in the dark phase→ Photosynthesis: raw materials, events and experiments
- 10% for the sunlight plants use→ Photosynthesis: raw materials, events and experiments
- Reading the black paper test as a carbon dioxide test→ Photosynthesis: raw materials, events and experiments
- Picking a cereal for Rhizobium→ Stomata and root nodules
- Plants respire only at night→ Respiration in plants and seeds
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