🧬 Biology · Topic 3 of 10
Start with the study notes on Nutrition in Plants (definitions, examples and a quick revision), then work through 60 practice questions, each with the correct answer and a clear explanation of why it is right. When you are ready, take this topic as an interactive quiz to test yourself.
Topic overview
Remember: Every food chain starts with a green plant, the producer.
Photosynthesis is the process by which green plants make food (glucose) from carbon dioxide and water, using sunlight and chlorophyll. Oxygen is given out.
Carbon dioxide + Water —(sunlight, chlorophyll)→ Glucose + Oxygen
The glucose made is stored as starch, and is carried to other parts of the plant by phloem.
Example: Water plants like Hydrilla give off bubbles in sunlight. The gas is oxygen.
The leaf is where most photosynthesis happens.
Remember: Leaves look green because chlorophyll reflects green light and absorbs red and blue light.
Boil the leaf in alcohol (in a water bath) to remove the chlorophyll, then add iodine. A blue-black colour shows that starch is present.
A parasite takes food from another living plant, the host, and harms it.
Example: Cuscuta (amarbel) is yellow because it has no chlorophyll. It winds round a host plant and sucks food from it.
Insectivorous plants trap and digest insects to get nitrogen, which their soil lacks. They still make food by photosynthesis.
Example: Pitcher plant (its pitcher is a modified leaf), Venus flytrap and sundew.
Saprotrophs feed on dead and decaying matter, then return its nutrients to the soil.
Example: Mushrooms, bread mould and many bacteria.
Symbiosis is when two organisms live together and both benefit.
Examples:
Plants take nitrogen, phosphorus and potassium from the soil, but they get carbon from carbon dioxide in the air. Nitrogen is needed to make proteins and chlorophyll.
Examples:
Remember: Plants respire at night too. By day photosynthesis is faster, so plants give out more oxygen than they use.
| Misconception | Correct understanding |
|---|---|
| Plants get their food from the soil. | Plants make food by photosynthesis; the soil gives only water and minerals. |
| Plants don't respire, they only photosynthesise. | Plants respire all the time, day and night. |
| Oxygen is needed for photosynthesis. | Oxygen is a product; carbon dioxide, water, light and chlorophyll are needed. |
| Insectivorous plants can't make food. | They photosynthesise; they eat insects only to get nitrogen. |
| Carbon for plants comes from the soil. | Carbon comes from carbon dioxide in the air. |
What are the raw materials of photosynthesis?
Carbon dioxide and water (with sunlight and chlorophyll).
What are the products of photosynthesis?
Glucose and oxygen.
Through what do gases enter and leave a leaf?
The stomata.
Why is a leaf boiled in alcohol before the starch test?
To remove chlorophyll so the blue-black colour can be seen.
Why is Cuscuta yellow?
It has no chlorophyll; it is a parasite.
Why does a pitcher plant catch insects?
To get nitrogen, which its soil lacks.
How does Rhizobium help a pea plant?
It fixes nitrogen from the air into a form the plant can use.
Do plants respire at night?
Yes, plants respire all the time.
Key takeaway: Green plants make their own food from carbon dioxide and water using sunlight and chlorophyll, and almost all life depends on that food and oxygen.
Notes written by Velnov for Class 6–8 students, following the topics of the NCERT science syllabus.
Q1. easy
The process by which green plants make their own food is called:
Answer: A — Photosynthesis
In photosynthesis, green plants use sunlight, carbon dioxide and water to make glucose and release oxygen. The word means 'putting together with light'.
Q2. easy
Organisms that make their own food are called:
Answer: A — Autotrophs
Green plants and some bacteria are autotrophs (auto = self, troph = nourishment). Animals and fungi, which depend on others for food, are heterotrophs.
Q3. medium
Organisms that depend on other organisms for their food are called:
Answer: A — Heterotrophs
All animals, fungi and most bacteria are heterotrophs. They cannot make food, so they eat plants or other animals.
Q4. medium
Which part of the plant is called the 'food factory' of the plant?
Answer: A — The leaf
Leaves contain chlorophyll and are exposed to sunlight, so most photosynthesis happens there. They are broad and flat to catch maximum light.
Q6. medium
Chlorophyll is present in cell structures called:
Answer: A — Chloroplasts
Chloroplasts are green plastids found mainly in leaf cells. Each one is packed with chlorophyll, making it the actual site of photosynthesis.
Q8. medium
The products of photosynthesis are:
Answer: A — Glucose (food) and oxygen
Photosynthesis makes glucose, which is stored as starch, and releases oxygen as a by-product - the oxygen on which almost all life depends.
Q10. medium
The cells that control the opening and closing of a stoma are called:
Answer: A — Guard cells
Guard cells swell to open the stoma and shrink to close it. Closing them at night or in dry weather prevents excessive water loss.
Q12. medium
Water and minerals are absorbed from the soil by:
Answer: A — Root hairs
Root hairs are fine outgrowths of root cells that greatly increase the surface area in contact with soil water, making absorption efficient.
Q14. medium
The food prepared in the leaves is transported to other parts of the plant through:
Answer: A — Phloem
Phloem carries dissolved food (mainly sugar) from the leaves to growing parts and storage organs such as roots, fruits and seeds.
Q16. medium
The presence of starch in a leaf is tested with:
Answer: A — Iodine solution, which turns the leaf blue-black
The leaf is boiled in alcohol to remove chlorophyll, then iodine is added. A blue-black colour shows that starch - and therefore photosynthesis - is present.
Q18. medium
A variegated leaf (partly green, partly white) tested for starch shows blue-black only in the green parts. This proves:
Answer: A — Chlorophyll is necessary for photosynthesis
Only the parts containing chlorophyll can trap light energy and make starch. The white parts, lacking chlorophyll, give no blue-black colour.
Q20. medium
Plants get carbon dioxide for photosynthesis from:
Answer: A — The air, through stomata in the leaves
Carbon dioxide makes up about 0.03-0.04% of air. It diffuses into the leaf through the stomata and reaches the photosynthesising cells.
Q22. medium
Plants that trap and digest insects are called:
Answer: A — Insectivorous plants
Pitcher plant, Venus flytrap and sundew are insectivorous. They are green and photosynthesise, but catch insects to obtain the nitrogen their soil lacks.
Q24. medium
In a pitcher plant, the pitcher is a modified:
Answer: A — Leaf
The leaf is modified into a deep pitcher with a lid. Insects attracted inside slip on the slippery walls and are digested by juices at the bottom.
Q26. medium
Cuscuta (amarbel) is yellow, not green, because it:
Answer: A — Has no chlorophyll and cannot make its own food
Without chlorophyll, Cuscuta cannot photosynthesise. It survives entirely by drawing nutrients from the host plant, which gradually weakens.
Q28. medium
Saprotrophs are important in nature because they:
Answer: A — Decompose dead matter and return nutrients to the soil
By breaking down dead plants and animals, saprotrophs clean up the environment and release nutrients back into the soil for plants to reuse.
Q30. medium
A relationship in which two organisms live together and BOTH benefit is called:
Answer: A — Symbiosis
In symbiosis, both partners gain. Lichens are a symbiosis of an alga and a fungus, and Rhizobium bacteria in legume roots are another example.
Q32. medium
Rhizobium bacteria live in the root nodules of leguminous plants and help by:
Answer: A — Fixing atmospheric nitrogen into a form plants can use
Plants cannot use nitrogen gas directly. Rhizobium converts it into soluble nitrogen compounds, and in return the plant supplies the bacteria with food and shelter.
Q34. medium
Why do farmers add manure and fertilisers to the soil?
Answer: A — To replace nutrients such as nitrogen that crops have used up
Repeated cropping removes minerals from the soil. Manure and fertilisers restore nitrogen, phosphorus and potassium so the next crop grows well.
Q36. medium
Do plants respire at night?
Answer: A — Yes - plants respire all the time, day and night
Respiration is continuous in all living cells. At night there is no photosynthesis, so plants only take in oxygen and release carbon dioxide.
Q38. medium
In a food chain, green plants are called:
Answer: A — Producers
Producers make their own food from sunlight. Herbivores that eat them are primary consumers, and the carnivores that eat those are secondary consumers.
Q40. medium
Nitrogen is essential for plants because it is needed to make:
Answer: A — Proteins and chlorophyll
Nitrogen is a component of amino acids, proteins and chlorophyll. A nitrogen-deficient plant grows poorly and its leaves turn yellow.
Q42. medium
The network of veins in a leaf serves to:
Answer: A — Carry water and minerals in, food out, and give the leaf support
Veins contain xylem and phloem, connecting every part of the leaf to the plant's transport system, and their branching framework holds the blade flat.
Q44. medium
Which colour of light is LEAST used by chlorophyll for photosynthesis?
Answer: A — Green
Chlorophyll absorbs red and blue light strongly but reflects green - which is exactly why leaves look green to us.
Q46. medium
Which of these is NOT required for photosynthesis?
Answer: A — Oxygen
Oxygen is a product of photosynthesis, not a requirement. The inputs are carbon dioxide, water, sunlight and chlorophyll.
Q48. medium
Which plant part is modified into a tendril, spine or storage organ but originally is a LEAF?
Answer: A — The pitcher of a pitcher plant and the spines of a cactus
Leaves can be modified for many purposes - spines in cactus to reduce water loss, pitchers in insectivorous plants to trap insects, and tendrils in peas for climbing.
Q50. medium
The word equation for photosynthesis is:
6 CO2 + 6 H2O --light/chlorophyll--> C6H12O6 + 6 O2Answer: A — Carbon dioxide + Water → (sunlight, chlorophyll) → Glucose + Oxygen
The reactants are carbon dioxide and water; sunlight and chlorophyll drive the reaction; and glucose and oxygen are the products. Option B describes respiration.
Q52. medium
The loss of water from a plant in the form of vapour through the stomata is called:
Answer: A — Transpiration
Transpiration cools the plant and helps pull water up from the roots. It happens mainly through the stomata, and increases in heat and wind.
Q54. medium
Which of these correctly describes the mode of nutrition of a mushroom?
Answer: A — Saprotrophic - it absorbs nutrients from dead and decaying matter
Mushrooms are fungi with no chlorophyll. They secrete enzymes onto dead organic matter, digest it externally and absorb the dissolved nutrients.
Q56. medium
Which overall statement about nutrition in plants is CORRECT?
Answer: A — Green plants make food by photosynthesis using CO2, water, sunlight and chlorophyll, and store it as starch
This sums up the topic: leaves are the food factories, chlorophyll traps light, stomata admit carbon dioxide, roots supply water and minerals, and the glucose produced is stored as starch and shared through the phloem.
Q57. medium
Why does a potted plant kept near a window bend towards the glass?
Answer: A — The shoot grows towards light so the leaves can photosynthesise better
This growth response to light is called phototropism. By turning its leaves towards the brightest source, the plant maximises the light available for photosynthesis.
Q59. medium
Which of these correctly compares photosynthesis and respiration in plants?
Answer: A — Photosynthesis makes food and needs light; respiration breaks food down for energy and happens all the time
Photosynthesis stores energy in glucose using light; respiration releases that energy in every living cell, day and night. They are opposite but complementary processes.
Q5. hard
The green pigment in leaves that captures sunlight is called:
Answer: A — Chlorophyll
Chlorophyll absorbs light energy and makes it available for the chemical reactions of photosynthesis. It is present in tiny green bodies called chloroplasts.
Q7. hard
The raw materials needed for photosynthesis are:
CO2 + H2O --sunlight/chlorophyll--> Glucose + O2Answer: A — Carbon dioxide and water, with sunlight and chlorophyll
Carbon dioxide comes from the air through the stomata, and water comes from the soil through the roots. Sunlight supplies the energy and chlorophyll captures it.
Q9. hard
The tiny pores on the surface of a leaf through which gases are exchanged are called:
Answer: A — Stomata
Each stoma is a pore guarded by two bean-shaped guard cells that open and close it. Carbon dioxide enters and oxygen and water vapour leave through them.
Q11. hard
Most stomata are present on the LOWER surface of a leaf because:
Answer: A — This reduces water loss, as the lower surface is shaded and cooler
The upper surface receives direct sun and is hotter, so stomata there would lose a lot of water. Placing them underneath conserves water while still allowing gas exchange.
Q13. hard
Water absorbed by the roots is carried to the leaves through:
Answer: A — Xylem vessels
Xylem forms continuous pipe-like vessels from root to leaf, carrying water and dissolved minerals upwards. Phloem carries food in both directions.
Q15. hard
The food made during photosynthesis is stored in plants mainly as:
Answer: A — Starch
Glucose is quickly converted to starch, an insoluble form that can be stored in leaves, stems, roots, fruits and seeds without upsetting the cell's water balance.
Q17. hard
In the starch test, why is the leaf first boiled in ALCOHOL?
Answer: A — To remove the green chlorophyll so the iodine colour can be seen clearly
Chlorophyll would mask the blue-black colour. Boiling in alcohol (in a water bath, since alcohol is inflammable) decolourises the leaf so the result is visible.
Q19. hard
If part of a leaf is covered with black paper and the plant is kept in sunlight, the covered part shows no starch. This proves:
Answer: A — Sunlight is necessary for photosynthesis
The covered portion received no light, so no photosynthesis occurred there and no starch was formed - direct evidence that sunlight is essential.
Q21. hard
Some plants like cactus have green STEMS that photosynthesise because:
Answer: A — Their leaves are reduced to spines to save water
Desert plants sacrifice broad leaves to conserve water. The thick green stem contains chlorophyll and takes over the job of making food.
Q23. hard
Insectivorous plants catch insects mainly to obtain:
Answer: A — Nitrogen, which is lacking in the soil where they grow
These plants grow in marshy, nitrogen-poor soils. Digesting insect protein supplies the nitrogen they need to make their own proteins.
Q25. hard
Plants that derive food from another living plant, harming it, are called:
Answer: A — Parasites
Cuscuta (amarbel) is a parasitic plant. It has no chlorophyll and uses special roots called haustoria to absorb food from the host plant it climbs.
Q27. hard
Organisms that feed on DEAD and decaying matter are called:
Answer: A — Saprotrophs
Fungi such as mushrooms and bread mould, and many bacteria, are saprotrophs. They secrete digestive juices onto the dead matter and absorb the dissolved food.
Q29. hard
Bread left in a warm, damp place develops fungus because:
Answer: A — Fungal spores in the air germinate and grow on the moist bread
Fungal spores are always present in the air. Given moisture, warmth and food, they germinate and grow as visible mould, digesting the bread saprotrophically.
Q31. hard
In a lichen, the alga and the fungus help each other because:
Answer: A — The alga makes food by photosynthesis and the fungus provides shelter, water and minerals
The green alga contributes photosynthesis; the fungus contributes a protective body that absorbs and retains water and minerals. Together they survive on bare rock.
Q33. hard
Which crops are grown to naturally enrich the soil with nitrogen?
Answer: A — Legumes such as gram, pea, moong and beans
Legumes host Rhizobium in their root nodules. Growing them in rotation replenishes soil nitrogen, reducing the need for chemical fertilisers.
Q35. hard
Which gas do plants take in during photosynthesis and give out during respiration?
Answer: A — They take in carbon dioxide for photosynthesis and give out carbon dioxide in respiration
Photosynthesis uses CO2 and releases O2; respiration uses O2 and releases CO2. During the day photosynthesis dominates, so plants are net oxygen producers.
Q37. hard
Photosynthesis is important for ALL life on Earth because it:
Answer: A — Produces the oxygen we breathe and the food at the base of every food chain
Every food chain begins with a green plant, and virtually all the oxygen in the atmosphere came from photosynthesis. Without it, most life would end.
Q39. hard
Which of these elements is NOT obtained by plants from the soil?
Answer: A — Carbon
Carbon comes from carbon dioxide in the air, not from the soil. Nitrogen, phosphorus, potassium and other minerals are absorbed as dissolved salts through the roots.
Q41. hard
Why are the leaves of many plants BROAD and FLAT?
Answer: A — To expose the maximum surface area to sunlight and air
A large, thin, flat blade catches plenty of light and lets carbon dioxide diffuse quickly to every cell - an ideal design for photosynthesis.
Q43. hard
Which experiment shows that CARBON DIOXIDE is necessary for photosynthesis?
Answer: A — Enclosing a leaf in a flask with potassium hydroxide, which absorbs CO2 - that leaf forms no starch
Potassium hydroxide absorbs the carbon dioxide inside the flask. Although the leaf gets light and has chlorophyll, no starch forms - proving CO2 is essential.
Q45. hard
Leaves appear GREEN because chlorophyll:
Answer: A — Reflects green light while absorbing other colours
We see the colour that is reflected. Since chlorophyll uses red and blue light and bounces green light back to our eyes, leaves look green.
Q47. hard
Algae in a pond appear green and can photosynthesise because they:
Answer: A — Contain chlorophyll
Algae are simple autotrophs containing chlorophyll. They photosynthesise like plants and are an important source of oxygen and food in water bodies.
Q49. hard
Why do plants growing in a dark room become pale and weak?
Answer: A — Without light they cannot photosynthesise, so no chlorophyll forms and no food is made
Light is needed both to make chlorophyll and to drive photosynthesis. Deprived of it, the plant becomes yellow, spindly and eventually dies once its stored food runs out.
Q51. hard
Where in the plant is the water for photosynthesis split into hydrogen and oxygen?
Answer: A — In the chloroplasts of the leaf cells, using light energy
Light energy trapped by chlorophyll splits water inside the chloroplast. The hydrogen goes on to help build glucose, and the oxygen is released through the stomata.
Q53. hard
Which of these plants is NOT an autotroph?
Answer: A — Cuscuta (amarbel)
Cuscuta lacks chlorophyll and lives as a parasite on other plants. Mango, grass and rose are all green autotrophs.
Q55. hard
Which statement about plant nutrition is CORRECT?
Answer: A — Green plants are autotrophs, but some plants are parasitic, insectivorous or symbiotic
Most plants photosynthesise, but Cuscuta is parasitic, the pitcher plant supplements its diet with insects, and lichens and legume-Rhizobium pairs live symbiotically.
Q58. hard
Aquatic plants such as Hydrilla release bubbles in sunlight. These bubbles are:
Answer: A — Oxygen given out during photosynthesis
This classic experiment demonstrates photosynthesis. The gas collected relights a glowing splint, confirming that it is oxygen produced by the submerged green plant.
Q60. hard
Why is a leaf's upper surface usually darker green than its lower surface?
Answer: A — The upper layer of cells contains more chloroplasts because it receives more sunlight
The palisade cells just below the upper surface are packed with chloroplasts to capture the light falling on the leaf, so that side looks deeper green.