Answers to all "Let Us Enhance Our Learning" questions of Chapter 10, Life Processes in Plants (NCERT Class 7 Science, Curiosity, 2026-27): photosynthesis and respiration, the starch test, stomata, transport by xylem and phloem, and experiments on light, carbon dioxide and oxygen. All 10 questions are answered, with the key answer highlighted.
Photosynthesis: carbon dioxide + water → glucose + oxygen (in the presence of sunlight and chlorophyll). Respiration: glucose + oxygen → carbon dioxide + water + energy. Starch made from glucose turns blue-black with iodine.
Complete the table comparing photosynthesis and respiration: raw materials, products, word equation, importance.
Solution
Feature
Photosynthesis
Respiration
Raw materials
Carbon dioxide and water (with sunlight and chlorophyll)
Glucose and oxygen
Products
Glucose and oxygen
Carbon dioxide, water and energy
Word equation
Carbon dioxide + water sunlight, chlorophyll glucose + oxygen
Glucose + oxygen → carbon dioxide + water + energy
Importance
Makes food for the plant (and for all living things through food chains) and releases the oxygen we breathe
Releases energy from food for all the life activities of the plant, such as growth
Photosynthesis uses CO₂ and water to make glucose and oxygen (food and oxygen for all); respiration uses glucose and oxygen to release energy, CO₂ and water.
Imagine a situation where all the organisms that carry out photosynthesis on the earth have disappeared. What would be the impact on living organisms?
Solution
No food: green plants, algae and some bacteria are the producers; all animals, including humans, depend on them directly or indirectly. Without them, food chains would collapse and animals would starve.
No fresh oxygen: photosynthesis puts oxygen into the air; without it, the oxygen would slowly be used up and living things could not breathe.
More carbon dioxide: nothing would remove CO₂, so it would build up, making the Earth hotter.
Herbivores would die first, then carnivores; eventually almost all life would end.
All food chains would collapse (no producers), oxygen would no longer be replenished and carbon dioxide would build up, so most living things would die.
A potato slice shows the presence of starch with iodine solution. Where does the starch in potatoes come from? Where is the food synthesised in the plant, and how does it reach the potato?
Solution
The food is made in the leaves of the potato plant by photosynthesis, as glucose. The glucose is carried from the leaves through the phloem to the underground stem, the tuber (potato). There, the extra glucose is converted into starch and stored. That is why the potato turns blue-black with iodine.
Glucose made in the leaves by photosynthesis is carried by the phloem to the potato (a stem tuber), where it is stored as starch.
Does the broad and flat structure of leaves make plants more efficient for photosynthesis? Justify your answer.
Solution
Yes. A broad, flat leaf has a large surface area, so it can absorb more sunlight, which is needed for photosynthesis. It also has many stomata spread over the surface for taking in carbon dioxide, and because the leaf is thin, gases and light reach all its cells easily.
Yes: the large flat surface absorbs more sunlight and has more stomata for gas exchange, and its thinness lets light and gases reach all cells.
X is broken down using Y to release carbon dioxide, Z and energy: X + Y → carbon dioxide + Z + energy. X, Y and Z are three different components of the process. What do X, Y and Z stand for?
Solution
This is respiration: X = glucose, Y = oxygen, Z = water.
Krishna set up two potted plants of the same size, one in sunlight and the other in a dark room (Fig. 10.10). (i) What idea might she be testing? (ii) What are the visible differences in plants in both conditions? (iii) According to you, leaves of which plant confirm the iodine test for the presence of starch?
Solution
(i) Whether sunlight is necessary for photosynthesis (and for healthy growth).
(ii) The plant in sunlight is green, fresh and healthy; the plant in the dark turns yellowish (pale), weak and drooping.
(iii) Leaves of the plant kept in sunlight turn blue-black with iodine, confirming starch. The plant in the dark could not make food, so its leaves show little or no starch.
(i) Whether sunlight is needed for photosynthesis (ii) the plant in sunlight stays green and healthy; the one in the dark turns pale and weak (iii) the plant in sunlight.
Vani believes that carbon dioxide is essential for photosynthesis. She kept potted plants (with sufficient water) under four conditions: (a) sunlight with carbon dioxide (b) sunlight without carbon dioxide (c) dark with carbon dioxide (d) dark without carbon dioxide. (i) In which plant(s) will starch be formed? (ii) In which will starch not be formed? (iii) In which will oxygen be generated? (iv) In which will oxygen not be generated?
Solution
Photosynthesis needs both sunlight and carbon dioxide.
(i) Starch is formed only in (a).
(ii) Starch is not formed in (b), (c) and (d).
(iii) Oxygen is generated only in (a).
(iv) Oxygen is not generated in (b), (c) and (d).
Only (a), which has both sunlight and carbon dioxide, forms starch and oxygen; (b), (c) and (d) do not.
Ananya took four test tubes three-fourths filled with water: A with a snail, B with a water plant, C with a snail and a plant, D with only water. She added a carbon dioxide indicator to all and recorded colour changes after 2–3 hours. What does she want to find out? How will she know if she is correct?
Solution
She wants to find out how animals and plants change the amount of carbon dioxide in water: that animals give out CO₂ by respiration, and plants (in light) take in CO₂ for photosynthesis, so plants and animals balance each other.
Expected results (in light):
A (snail): CO₂ increases; the indicator changes colour showing more CO₂.
B (plant): CO₂ decreases (used in photosynthesis); the colour changes the other way.
C (snail + plant): little change, because the CO₂ given out by the snail is used by the plant.
D (water only): no change; this is the control.
If the colours change in this way, her idea is correct.
She is testing how animals (respiration) add CO₂ and plants (photosynthesis) remove it; A shows more CO₂, B less, C almost no change and D (control) none.
Design an experiment to observe if water transportation in plants is quicker in warm or cold conditions.
Solution
Materials: two similar fresh white flowers or celery stalks with leaves (or balsam plants), two glasses, water, red ink or food colour, a thermometer, a warm place and a cold place (e.g. near a fridge or with ice around the glass).
Method:
Fill both glasses with equal amounts of coloured water.
Cut the stems at the bottom (slantwise, under water) and place one in each glass.
Keep one glass in a warm place (about 30–35 °C, e.g. in sunlight) and the other in a cold place (about 10 °C, e.g. a glass kept in a bowl of ice water).
Keep everything else the same: the same kind of plant, size, amount of water and colour, and the same light if possible.
Every 15 minutes, observe how far the colour has risen in the stem, petals or veins of the leaves.
Expected result: the colour rises faster in the warm conditions, because warmth increases evaporation of water from the leaves (transpiration), which pulls water up through the xylem faster.
Put similar stems in equal coloured water, one in a warm place and one in a cold place, keeping everything else the same, and see where the colour rises faster (expected: in the warm one).
Photosynthesis and respiration are essential to maintain balance in nature. Discuss.
Solution
Gas balance: in photosynthesis, plants take in carbon dioxide and give out oxygen; in respiration, all living things take in oxygen and give out carbon dioxide. The two processes together keep the levels of oxygen and carbon dioxide in the air nearly constant.
Food and energy: photosynthesis stores the Sun's energy in food (glucose); respiration releases this energy so that plants and animals can live, grow and move.
Cycle of matter: the products of one process (oxygen, glucose; carbon dioxide, water) are the raw materials of the other.
If either process stopped, or if one became much greater than the other (for example, burning fuels and cutting forests adds CO₂ faster than plants can remove it), this balance would be disturbed, leading to problems like global warming.
The two processes are opposite: photosynthesis uses CO₂ and gives O₂ and food, and respiration uses O₂ and food and gives CO₂ and energy, so together they keep gases and energy balanced in nature.
