CBSE Important Questions Class 12 Biology Chapter 12 – Ecosystem

Class 12 Biology Chapter 12 Ecosystem is the twelfth chapter of the NCERT Class 12 Biology textbook. It explains what an ecosystem is and its biotic (producers, consumers, decomposers) and abiotic components, productivity, decomposition, how energy flows from the sun through producers to consumers following Lindeman's 10 per cent law, food chains and food webs, trophic levels, and the pyramids of number, biomass and energy.

These CBSE important questions for Class 12 Biology Chapter 12 cover the law of conservation of energy, the 10 per cent law of energy transfer with a worked example, chlorophyll and chloroplasts, the raw materials and the energy conversion of photosynthesis, the role of stomata, and the source and sink in the translocation of food through phloem. The set has multiple-choice questions with answers and short explanations for quick revision before the board exam.

Q1. Energy in the universe is constant and is stated by the
(a) first law of thermodynamics.
(b) second law of thermodynamics.
(c) third law of thermodynamics.
(d) fourth law of thermodynamics.

Answer: (a) first law of thermodynamics.
The first law of thermodynamics, also called the law of conservation of energy, states that energy can neither be created nor destroyed; it can only be changed from one form to another. For example, in cellular respiration the chemical energy of glucose is converted into ATP, but the total energy in the universe remains constant.

Q2. Suppose 2000 J of solar energy is incident on green vegetation. According to Lindeman's 10% law, only 10% of energy is transferred from one trophic level to the next. Identify the energy available to each trophic level:

Flow of 2000 J solar energy through producer (A J), herbivore (B J) and carnivore (C J)

(a) A – 200 J, B – 20 J, C – 2 J
(b) A – 2000 J, B – 400 J, C – 40 J
(c) A – 2000 J, B – 20 J, C – 2 J
(d) A – 20 J, B – 2 J, C – 0.2 J

Answer: (d) A – 20 J, B – 2 J, C – 0.2 J
Solar energy incident = 2000 J. Green plants trap only about 1% of the incident solar energy, so the energy captured by the producers (A) = 1% of 2000 = 20 J. According to Lindeman's 10% law, only 10% of the energy of a trophic level passes to the next level. Energy available to herbivores (B) = 10% of 20 = 2 J, and energy available to carnivores (C) = 10% of 2 = 0.2 J.

Q3. Chlorophyll is present in the ________.
(a) mitochondrion
(b) chloroplast
(c) ribosome
(d) nucleus

Answer: (b) chloroplast
Chlorophyll is the green pigment that absorbs light energy for photosynthesis. Photosynthesis takes place in the chloroplasts of plant cells, where the chlorophyll molecules are present in the thylakoid membranes.

Q4. Stomata play the important function of
(a) exchange of gases
(b) absorption of water
(c) preparation of food
(d) protecting the leaf

Answer: (a) exchange of gases
Stomata are tiny pores on the epidermis of leaves, each guarded by two guard cells that open and close the pore. When the stomata are open, carbon dioxide enters the leaf for photosynthesis and oxygen goes out. Water vapour also escapes through them during transpiration. So the most important function of stomata is the exchange of gases.

Q5. The raw materials required for photosynthesis are
(a) carbohydrates and oxygen
(b) water and carbon dioxide
(c) water and oxygen
(d) vitamins and minerals

Answer: (b) water and carbon dioxide
In photosynthesis, green plants make carbohydrates from carbon dioxide and water in the presence of sunlight and chlorophyll: 6CO2 + 6H2O → C6H12O6 + 6O2. Water is absorbed by the roots from the soil, and carbon dioxide enters the leaves through the stomata.

Q6. When a plant prepares itself for facing harsh conditions, it translocates its nutrients from the leaves to the roots etc. for storage. Here the leaf is X and the roots are Y.
(a) X is the source, Y is the source
(b) X is a sink, Y is the source
(c) X is the source, Y is a sink
(d) X is a sink, Y is a sink

Answer: (c) X is the source, Y is a sink
In phloem translocation, a source is the organ where food is made or released, and a sink is the organ where food is used or stored. Mature leaves (X) make sugars by photosynthesis and export them, so the leaf is the source. The roots (Y) receive and store these sugars (as starch) to survive the harsh conditions, so the root is the sink.

Q7. Which of the following sentences is correct about translocation in plants?
(a) Xylem conducts water and minerals both in upward and downward direction.
(b) Phloem transports food in a downward direction.
(c) Xylem transports food in an upward direction.
(d) Phloem transports food both in an upward and downward direction.

Answer: (d) Phloem transports food both in an upward and downward direction.
Xylem carries water and minerals only upwards, from the roots to the leaves; it does not carry food. Phloem carries food (mainly sucrose) from the source to the sink, which may be above or below, so phloem transport is in both directions depending on where the source and sink are.

Q8. The primary function of photosynthesis is to convert
(a) light energy into chemical energy.
(b) chemical energy into light energy.
(c) chemical energy into mechanical energy.
(d) mechanical energy into electrical energy.

Answer: (a) light energy into chemical energy.
In photosynthesis, chlorophyll absorbs light energy from the sun. This energy is used to make glucose from carbon dioxide and water, and it is stored as chemical energy in the bonds of glucose. So photosynthesis converts light energy into chemical energy.

Q9. Green plants convert the energy of the sun into
(a) ionic energy
(b) hydrogen energy
(c) chemical energy
(d) hydrophobic energy

Answer: (c) chemical energy
Green plants carry out photosynthesis: 6CO2 + 6H2O → C6H12O6 + 6O2. During this process, the light energy of the sun is converted into chemical energy stored in glucose.

Q10. Chlorophyll is a ______ pigment.
(a) red
(b) blue
(c) green
(d) white

Answer: (c) green
Chlorophyll is the main photosynthetic pigment in the chloroplasts of green plants. It absorbs light mostly in the blue (about 430 nm) and red (about 680 nm) regions and reflects green light. Because green light is reflected, chlorophyll appears green.

Q11. Solar energy is utilised by the leaves with the help of
(a) chlorophyll
(b) chloroplast
(c) anthocyanin
(d) protoplast

Answer: (a) chlorophyll
Leaves capture solar energy to make food by photosynthesis. The green pigment that actually absorbs the solar energy is chlorophyll, which is located in the thylakoid membranes of the chloroplasts. So solar energy is utilised by the leaves with the help of chlorophyll.

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