CBSE Important Questions Class 8 Science Chapter 2 – The Invisible Living World: Beyond Our Naked Eyes

Class 8 Science Chapter 2 The Invisible Living World: Beyond Our Naked Eyes is part of the new NCERT Curiosity textbook. It takes students into the world that is too small for our eyes to see. They learn how a microscope and stains help us look at cells, how plant cells and animal cells differ, what the main cell parts such as the nucleus, cytoplasm, cell wall, vacuole and mitochondria do, and how microorganisms affect our food.

These CBSE important questions for Class 8 Science Chapter 2 cover preparing an onion peel slide and the role of dyes, why refrigeration and pickling preserve food, vacuoles in plant and animal cells, an experiment comparing plant and animal cells, the shapes of red blood cells, muscle cells and nerve cells, naming cell parts, and why chloroplasts are found only in plants and algae. Every question has a clear step-by-step answer in simple English, so students can practise on their own, parents can check the answers, and teachers can use them for class tests.

CBSE Important Questions Class 8 Science Chapter 2 – The Invisible Living World: Beyond Our Naked Eyes

Q1. Sharad was preparing a slide to observe cells. He took a thin layer of onion peel and placed it on a drop of water on a glass slide. He quickly placed a coverslip on it and observed the slide under the microscope.
i. Will he be able to observe the cells under the microscope?
ii. What is the role of a dye?
iii. What precautions need to be taken while placing a coverslip?

Answer: i. No, Sharad will not be able to see the cells clearly, because he did not stain the peel.
ii. Dyes stain the different parts of the cell so that they become clearly visible under the microscope. Without a stain, the cell parts are colourless and hard to see.
iii. The coverslip should be lowered slowly with the help of a needle, so that no air bubbles get trapped underneath. Air bubbles look like dark circles and spoil the view.

Q2. Dr. Sharma, a food scientist, noticed that bread kept at room temperature (28 °C) with high humidity grew mould quickly, while bread stored in the refrigerator (4 °C) stayed fresh much longer. She also observed that traditional pickles and jams do not spoil easily even when stored at room temperature. Explain why refrigeration helps preserve food and how traditional methods like pickling prevent spoilage.

Answer: Refrigeration slows down the growth of microorganisms. Most harmful bacteria and fungi grow best at warm temperatures of about 25 to 30 °C, so cooling the food to 4 °C greatly slows their growth and reproduction.
Traditional methods like pickling use a lot of salt or sugar along with acids such as vinegar or lemon juice. Salt and sugar draw water out of the cells of the microbes, and the acid makes the surroundings too sour for most harmful bacteria to survive. That is why pickles and jams last a long time even without a refrigerator.

Q3. During a practical class, Meera and Arjun observed two slides, one of onion peel and one of human cheek cells, and noticed these differences:
I. The onion peel cells appeared rectangular and tightly packed like bricks in a wall.
II. The cheek cells appeared irregular in shape and more flexible.
III. Both had a nucleus and cytoplasm, but only the onion cells had a thick outer boundary.
IV. The onion cells had large empty-looking spaces, while the cheek cells had very small ones.
What are the large empty-looking spaces in onion cells called, and why are they much smaller in animal cells?

Answer: The large empty-looking spaces are called vacuoles.
In plant cells the vacuole is very large because it stores water and keeps the cell firm (turgid), which gives the plant its support and keeps it standing upright.
In animal cells the vacuoles are small, because animals can move about to find water and do not need a large store of water inside each cell to hold their shape. Their skeleton gives them support instead.

Q4. Design a simple experiment to observe and compare plant and animal cells using basic laboratory materials.

Answer: Aim: To observe the structural differences between plant cells (onion peel) and animal cells (cheek cells).
Materials: Microscope, glass slides, coverslips, onion, cotton swab, safranin stain, methylene blue stain, water, dropper, needle.
Method:
1. Peel off a thin layer from an onion, place it on a slide, add a drop of water, stain it with safranin and lower a coverslip with a needle.
2. Gently scrape the inside of the cheek with a clean cotton swab, spread it on another slide, add a drop of water, stain with methylene blue and put on a coverslip.
3. Observe both slides under the microscope, first at low and then at high magnification.
4. Draw labelled diagrams of both kinds of cells.
5. Record the shape, size and visible parts of each.
Observation: Onion cells look rectangular, with a clear cell wall, a large vacuole and a red-stained nucleus. Cheek cells look irregular and flexible, with small vacuoles and a blue-stained nucleus.
Conclusion: Plant cells have a cell wall and a large vacuole, while animal cells have neither; this is the main structural difference between them.

Q5. Compare plant cells and animal cells by explaining the differences in their structure and function.

Basis Plant cell Animal cell
Structure Has a rigid cell wall made of cellulose, one large central vacuole and chloroplasts. Has only a flexible cell membrane, small temporary vacuoles and no chloroplasts.
Function Makes its own food by photosynthesis using chloroplasts; the cell wall gives support so the plant can stand tall. Must take food from outside; the absence of a cell wall keeps the cell flexible, which helps in movement.

Q6. A researcher studied different human cells and found that red blood cells have no nucleus, muscle cells are very long with many nuclei, and nerve cells have many branches with one large nucleus. Explain why these cells have such different structures and how their shapes help them do their jobs.

Answer: Each type of cell has a shape that suits its work.
• Red blood cells have no nucleus so there is more room for haemoglobin, the protein that carries oxygen, and so they stay flexible enough to squeeze through the narrowest blood vessels. Their flat disc shape gives a large surface area for taking in and giving out oxygen.
• Muscle cells are long so that they can shorten along their whole length and produce movement. They have many nuclei so they can make the large amount of protein that this hard work needs.
• Nerve cells have long branches so they can carry messages over long distances in the body, and their large nucleus makes the many proteins needed to maintain these long branches.

Q7. Are these statements true or false? Give reasons.
a) All cells have cell walls for protection.
b) The nucleus controls all activities in the cell.
c) Plant cells and animal cells have the same size vacuoles.

Answer: a) False. Only plant cells, bacterial cells and fungal cells have a cell wall. Animal cells have only a flexible cell membrane, which is what lets them change shape and allows animals to move.
b) True. The nucleus contains the DNA and acts as the control centre of the cell. It decides which genes are active and so controls growth, repair, reproduction and the making of proteins.
c) False. A plant cell has one large central vacuole that fills most of the cell and stores water. Animal cells have many small, temporary vacuoles used for jobs such as storing or removing waste.

Q8. Name the following:
a) The part of the cell that produces energy and is called the powerhouse of the cell
b) The large storage space in plant cells that helps maintain water pressure
c) The tough outer covering of plant cells that provides protection and support
d) The jelly-like substance inside cells where chemical reactions take place

Answer: a) Mitochondria. They break down glucose with oxygen to release energy as ATP for all the activities of the cell, which is why they are called the powerhouse.
b) Vacuole. This large water-filled space keeps the plant cell turgid and the plant firm and upright.
c) Cell wall. This rigid outer layer of cellulose protects the cell, supports it and keeps its shape.
d) Cytoplasm. This jelly-like material fills the cell, holds the organelles and is where many chemical reactions take place.

Q9. During a practical, students found that safranin worked best for onion cells while methylene blue was better for cheek cells. With iodine solution, the starch-containing parts of plant cells turned blue-black. Explain why different stains suit different cells and what makes iodine special for detecting starch.

Answer: Different stains work on different cells because each stain sticks to particular substances. Safranin is attracted to the cellulose of the plant cell wall and to the DNA in the nucleus, so it makes these parts stand out in red. Methylene blue sticks well to the proteins and DNA that are prominent in animal cells, so it suits cheek cells.
Iodine is special because it reacts chemically with starch to give a blue-black colour. This is a specific test for starch, so iodine shows exactly where starch is stored in a plant cell.

Q10. A student observed onion peel cells and human cheek cells under a microscope. The onion cells appeared rectangular with clear boundaries and looked very organised, while the human cells were irregular and seemed more flexible. Explain why plant and animal cells have different shapes and what causes these differences.

Answer: Plant cells have regular, rectangular shapes because a rigid cell wall of cellulose surrounds each cell like a strong frame. This wall holds the cell in a fixed shape and gives the plant the support it needs to stand upright.
Animal cells have no cell wall; they have only a flexible cell membrane, which can bend and change shape. This is why animal cells look irregular, and it is also what allows animals to move and change their body shape.

Q11. Read the Assertion (A) and Reason (R) and choose the correct option.
Assertion (A): Muscle cells show a long spindle-like (fusiform) shape suited to their contractile function.
Reason (R): This shape lets the cells interact efficiently and produce coordinated force.
(a) Both A and R are true, and R is the correct explanation of A.
(b) Both A and R are true, but R is not the correct explanation of A.
(c) A is true, but R is false.
(d) A is false, but R is true.

Answer: (a) Both A and R are true, and R is the correct explanation of A.
Muscle cells are long and spindle-shaped so that they can shorten along their whole length. This arrangement lets neighbouring cells pull together in the same direction, producing a smooth and coordinated force. So the Reason explains exactly why the shape is what it is.

Q12. A botanist examining photosynthetic organelles in plant cells would expect to find these specialised structures exclusively in which cellular type?
(a) All living cells universally
(b) Animal cells exclusively
(c) Plant and algal cells only
(d) Prokaryotic cells primarily

Answer: (c) Plant and algal cells only
The photosynthetic organelle is the chloroplast, which contains the green pigment chlorophyll. Chloroplasts are found only in plant cells and in algae, the organisms that make their own food by photosynthesis. Animal cells have none, and prokaryotes such as bacteria have no membrane-bound organelles at all.

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