Looking for accurate SEBA Class 9 Science Chapter 5 (The Fundamental Unit of Life) textual solutions for the 2026–27 academic year? Access complete step-by-step textbook exercise answers covering cell structure, plasma membrane, osmosis and diffusion, plant vs animal cells, prokaryotic vs eukaryotic cells, cell organelles (mitochondria, plastids, endoplasmic reticulum, Golgi apparatus), and cell division (mitosis and meiosis) to score top marks in your exams.
Blue question
Q1. Who discovered cells, and how?
Ans. Cells were discovered by Robert Hooke in 1665. He observed thin slices of cork under a primitive microscope and noticed small, honeycomb-like box compartments, which he named cells.
2. Why is the cell called the structural and functional unit of life?
Ans. The cell is called the structural and functional unit of life because all living organisms are composed of one or more cells, and all basic life processes—such as nutrition, respiration, excretion, and growth—are carried out inside individual cells.
3. How do substances like CO₂ and water move in and out of the cell? Discuss.
Ans. Substances move across the cellular boundary through two primary transport processes:
- Diffusion: Gases such as carbon dioxide (CO2) and oxygen (O2) move across the cell membrane from a region of higher concentration to a region of lower concentration.
- Osmosis: Water molecules move across the selectively permeable plasma membrane from a region of higher water concentration to a region of lower water concentration.
4. Why is the plasma membrane called a selectively permeable membrane?
Ans. The plasma membrane is called a selectively permeable membrane because it regulates cellular traffic by allowing only specific substances to enter and exit the cell while preventing the passage of other materials.
| Prokaryotic Cell | Eukaryotic Cell |
| Size: Generally small (1–10 µm) | Generally large (5–100 µm) |
| Not well-defined, lacks a nuclear membrane (known as nucleoid) | Well-defined and surrounded by a double nuclear membrane |
| Single, circular chromosome | More than one linear chromosome |
| Membrane-bound organelles are completely absent | Membrane-bound organelles are present |
5. Fill in the gaps in the following table illustrating differences between prokaryotic and eukaryotic cells.
Ans. The key structural differences between prokaryotic and eukaryotic cells are:
| Prokaryotic Cell | Eukaryotic Cell |
| Size is generally small (1-10 µm). | Size is generally large (5-100 µm). |
| Nuclear region is not well-defined, lacks a nuclear membrane, and is known as a nucleoid. | Nuclear region is well-defined and surrounded by a double nuclear membrane. |
| Possesses a single, circular chromosome. | Possesses more than one linear chromosome. |
| Membrane-bound cell organelles are completely absent. | Membrane-bound cell organelles are present. |
SEBA Class 9 Science Chapter 5 Solutions
6. Can you name the two organelles we have studied that contain their own genetic material?
Ans. The two cell organelles that contain their own DNA and ribosomes are mitochondria and plastids (specifically chloroplasts).
7. If the organisation of a cell is destroyed due to some physical or chemical influence, what will happen?
Ans. If the physical or structural organization of a cell is destroyed, its organelles will fail to perform vital functions such as respiration, protein synthesis, and waste disposal, ultimately leading to the death of the cell.
8. Why are lysosomes known as suicide bags?
Ans. Lysosomes are known as suicide bags because they contain powerful digestive enzymes; if the cell gets damaged or encounters metabolic disruption, the lysosomes may burst and release enzymes that digest their own host cell.
9. Where are proteins synthesised inside the cell?
Ans. Proteins are synthesised on ribosomes, which are either floating freely in the cytoplasm or attached to the surface of the rough endoplasmic reticulum (RER).
Class 9 Science Chapter 5 Question Answer SEBA 2026-27
Textbook exercise
Q1. Make a comparison and write ways in which plant cells are different from animal cells.
Ans. Plant cells and animal cells differ in several fundamental structural features:
| Plant Cell | Animal Cell |
| Possesses a rigid cell wall outside the cell membrane. | Lacks a cell wall entirely. |
| Contains chloroplasts for conducting photosynthesis. | Lacks chloroplasts and cannot perform photosynthesis. |
| Features a single, large central vacuole that takes up most space. | Features small, temporary vacuoles or none at all. |
| Typically larger in overall size. | Typically smaller in overall size. |
| Possesses a generally fixed, regular shape. | Possesses a flexible, irregular shape. |
2. How is a prokaryotic cell different from a eukaryotic cell?
Ans. Prokaryotic and eukaryotic cells show major differences in cellular architecture and complexity:
| Prokaryotic Cell | Eukaryotic Cell |
| Features an ill-defined nuclear region called a nucleoid that lacks a membrane. | Features a well-defined nucleus enclosed by a double nuclear membrane. |
| Lacks membrane-bound cell organelles. | Contains distinct, membrane-bound organelles. |
| Generally smaller in size (1-10 µm). | Generally larger in size (5-100 µm).).. |
| Possesses a single, circular chromosome. | Possesses more than one linear chromosome. |
| Examples include bacteria and blue-green algae. | Examples include plant, animal, and fungal cells. |
3. What would happen if the plasma membrane ruptures or breaks down?
Ans. If the plasma membrane ruptures or breaks down, the internal cellular contents will leak out into the surroundings and harmful external substances will freely enter the cell. Without a functional selective barrier, the cell loses control over its internal environment and will eventually die.
4. What would happen to the life of a cell if there was no Golgi apparatus?
Ans. If a cell lacks a Golgi apparatus, the proteins and lipids synthesised by the endoplasmic reticulum cannot be modified, packaged, or dispatched to their required target locations. Furthermore, the cell will be unable to form lysosomes, leading to an accumulation of cellular waste that will ultimately cause the cell to stop functioning and die.
5. Which organelle is known as the powerhouse of the cell? Why?
Ans. Mitochondria are known as the powerhouses of the cell because they generate usable cellular energy in the form of Adenosine Triphosphate (ATP) through cellular respiration.
The Fundamental Unit of Life Class 9 SEBA Textual Exercise Solutions
6. Where do the lipids and proteins constituting the cell membrane get synthesised?
Ans. The proteins required for the cell membrane are synthesised by the ribosomes attached to the Rough Endoplasmic Reticulum (RER), whereas the lipids are synthesised by the Smooth Endoplasmic Reticulum (SER). Together, these molecules are processed to build and repair the cell membrane through membrane biogenesis.
7. How does an Amoeba obtain its food?
Ans. An Amoeba obtains its food through the process of endocytosis (specifically phagocytosis), where it extends flexible finger-like projections called pseudopodia to engulf food particles, enclosing them within a temporary food vacuole for digestion.
8. What is osmosis?
Ans. Osmosis is the passive movement of water molecules from a region of higher water concentration to a region of lower water concentration across a selectively permeable membrane.
9. Osmosis Experiment with Potato Cups
(i) Why does water gather in the hollowed portion of B and C?
Ans. Water gathers in the hollowed portions of cups B and C because the presence of sugar in cup B and salt in cup C creates a higher solute concentration inside the cavity, causing water from the outer trough to move inward across the living potato cell membranes by osmosis.
(ii) Why is potato A necessary for this experiment?
Ans. Potato cup A is necessary because it serves as an experimental control, proving that water movement in cups B and C is specifically driven by the presence of a solute gradient rather than the potato cavity itself.
(iii) Why does water NOT gather in the hollowed portions of A and D?
Ans. Water does not gather in cup A because it contains no solute to create a concentration gradient, while in cup D, boiling the potato destroys the cell membranes, preventing osmosis from taking place even though sugar is present.
SEBA Class 9 Science Chapter 5 Textbook Answers
10. Which type of cell division is required for growth and repair of the body, and which type is involved in the formation of gametes?
Ans. Mitosis is the type of cell division required for body growth, tissue development, and cellular repair, whereas meiosis is the specialized cell division process involved in the formation of gametes (egg and sperm cells) for reproduction.
🔬 Updated Solutions Notice: This page features complete, step-by-step SEBA Class 9 Science Chapter 5 (The Fundamental Unit of Life) Textual Exercise Solutions updated for the 2026–27 academic session. All cell biology concepts, organelle function explanations, structural diagrams, and osmotic process answers strictly follow the latest revised Assam Board (SEBA) curriculum.
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