NCERT Class 9 Science Chapter 5 The Fundamental Unit of Life, Question Answer to each chapter is provided in the list so that you can easily browse throughout different chapters Class 9 Science Chapter 5 The Fundamental Unit of Life and select need one.
NCERT Class 9 Science Chapter 5 The Fundamental Unit of Life
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The Fundamental Unit of Life
Chapter – 5
GENERAL SCIENCE
TEXTUAL QUESTIONS AND ANSWERS
INTEX QUESTIONS AND ANSWERS
Textbook Page No. 59
1. Who discovered cells and how?
Ans. Robert Hooke, an English botanist, observed thin slices of cork from the bark of a tree under a microscope which he himself had designed. He was excited to see small spaces bound by firm walls just like a honey comb. He used the term “cell” first to denote the compartments or spaces. This discovery was made in 1665. This experiment was conducted at the “Royal Society of London”. He published his work in a book “Micrographia”.
2. Why is the cell called the structural and functional unit of life?
Ans. Cells are the structural and functional units of life because a cell is capable of independent existence and can carry out all the vital functions for living. A cell carries out functions like nutrition, respiration, excretion, transportation and reproduction; the way an individual organism does.
Textbook Page No. 61.
1. How do substances like CO₂ and water move in and out of the cell? Discuss.
Ans. CO₂ and water move in and out of cells by simple physiological processes such as diffusion and osmosis.
Diffusion refers to the movement of molecules from a zone of higher concentration to that of lower concentration across a permeable membrane until they reach equal concentration on either side.
Osmosis refers to the movement of solvent from a zone of higher concentration to that of lower concentration across a semipermeable membrane until they attain equilibrium concentration.
In case of CO₂, a cellular waste, its concentration is high inside the cell and low outside it. As a result, it moves out of the cell, governed by the process of diffusion.
Water moves obeying principles of diffusion process but across a semipermeable membrane depending on the difference in its concentration affected by the amount of substance dissolved in it on either side of the membrane. In other words, by osmosis, if the medium surrounding the cell has a higher water concentration than the cell, then water enters the cell, while if the medium has a lower water concentration than the cell, then water goes out of the cell which has comparatively more water. On the other hand, if the medium outside the cell has the same concentration of water as the cell, then no net movement of water takes place as the concentration is now in equilibrium.
A = high concentration of molecules
B = low concentration of molecules
A = high solute concentration low solvent/water concentration
B = low solute concentration high solvent/water concentration
High water concentration
Low water concentration
Equal water concentration as in cell
2. Why is the plasma membrane called a selectively permeable membrane?
Ans. The cell membrane is called selectively permeable as it only allows specific molecules to pass. Only specific molecules like water and gaseous molecules can pass through the cell membrane directly. It stops the flow of other molecules towards the two sides.
Textbook Page No. 63
1. Fill in the gaps in the following table illustrating differences between prokaryotic and eukaryotic cells.
Ans. The difference between prokaryotic and eukaryotic cells are:
| Prokaryotic cell | Eukaryotic cell |
| 1. Size: generally small (1-10 μm) 1 μm = 10⁻⁶m | 1. Size: generally large (5-100 μm) |
| 2. Nuclear region: poorly defined with no nuclear membrane and known as nucleoid | 2. Nuclear region: well-defined and surrounded by a nuclear membrane. |
| 3. Chromosome: single | 3. More than one chromosome |
| 4. Membrane-bound cell organelles absent | 4. Membrane-bound cell organelles like mitochondria, plastids, etc. are present |
Textbook Page No. 65
1. Can you name the two organelles we have studied that contain their own genetic material?
Ans. The two organelles that contain their own genetic material are:
(i) Mitochondria.
(ii) Plastid.
They have their own DNA and ribosomes.
2. If the organisation of a cell is destroyed due to some physical or chemical influence, what will happen?
Ans. Each cell has an organised membrane and organelles in specific ways which help it to acquire its structure and ability to function. Thus, destruction of the organisation of a cell by some physical or chemical influence will destroy its structures and make it unable to function leading to the death of the cell.
3. Why are lysosomes known as suicide bags?
Ans. Lysosomes are cell organelles containing powerful hydrolytic (digestive) enzymes. During disturbance in cellular metabolism, the cell is damaged and the lysosomes may burst. As a result, hydrolytic enzymes are released which not only digest foreign materials and worn out cell organelles, but also its own cell. Therefore, lysosomes are known as the ‘suicidal bags’ of a cell.
4. Where are proteins synthesised inside the cell?
Ans. Proteins are synthesised inside the ‘ribosomes’ which are called ‘protein factories.”
EXERCISE (Textbook Page No. 66-67)
1. Make a comparison and write down ways in which plant cells are different from animal cells.
Ans. The differences between plant cells and animal cells are mentioned below:
| SI No. Basis difference | Plant cells | Animals cells |
| (i) Outer covering | The outermost covering of plant cells is called the cell wall which is formed of cellulose. | The outermost covering of animal cells is called the plasma membrane. Animal cells lack the cell wall. |
| (ii) Size | They are larger than animal cells. | Comparatively smaller in size. |
| (iii) Centrosomes | Centrosomes are absent but polar caps are present. | Centrosomes are present with two centrioles. |
| (iv) Plastids | Plastids are present. | Plastids are absent except in euglena, a protozoan. |
| (v) Vacuoles | Central, large and prominent vacuoles are present and they are mostly permanent. | Vacuoles are either absent or very small in size and they are temporary. |
| (vi) Golgi complex | Golgi complex consists of many unconnected sub-units called dictyosomes. | Golgi complex present near the nucleus is highly complex and prominent. |
| (vii) Cytokinesis | Cytokinesis of cell division takes place by formation of cell plate from centre to outwards. | Cytokinesis takes place by formation of a groove from outwards to centre. |
2. How is a prokaryotic cell different from a eukaryotic cell?
Ans. The differences between a prokaryotic cell and a eukaryotic cell are mentioned below:
| SI No. Basis of difference | Prokaryotic Cell | Eukaryotic Cells |
| (i) Size | Has small-sized cell of about 1-10 mm. | Generally has large-sized cell of about 5-100 mm. |
| (ii) Nucleus | The nucleus is not enclosed by the nuclear membrane (and is called nucleoid). | True nucleus is present and is surrounded by a nuclear membrane. |
| (iii) Chromosome | Single chromosome is present. | More than one chromosome is present. |
| (iv) Nucleolus and mitotic apparatus | Nucleolus and mitotic apparatus are absent. | Nucleoli and mitotic apparatus are present. |
| (v) Cell organelles | Membrane-bound cell organelles are absent. | Membrane-bound cell organelles are present. |
| (vi) Filament | Lacks 9+2 axial filament structure. | Possesses 9 + 2 axial filament structure. |
| (vii) Cell division | Cell divides by asexual processes such as budding or fission (not by mitosis). | Cell divides by mitosis and meiosis processes. |
| (viii) Ribosomes | Has 70s ribosomes. | Has 80s ribosomes. |
3. What would happen if the plasma membrane ruptures or breaks down?
Ans. The plasma membrane encloses the cell contents and demarcates it from the external environment. It regulates the entry and exit of molecules by carefully selecting them. It also prevents the movement of some materials from the cell to the exterior. By these functions it maintains the internal functional and structural environment of the cell keeping it alive. Thus, if the plasma membrane ruptures or breaks down, the contents of the cell will come out and it will die.
4. What would happen to the life of a cell if there was no golgi apparatus?
Ans. In the absence of golgi apparatus, the cell would neither be able to secrete hormones, proteins, lipids, etc. nor synthesise enzymes and discharge them. So, the functions performed by these biomolecules would not have taken place. Lysosomes which are also produced by golgi bodies would not be available resulting in accumulation of foreign materials in the cell which would destroy it. Improper functional stability of the cell due to absence of the golgi apparatus may even lead to its death.
5. Which organelle is known as the powerhouse of the cell? Why?
Ans. Mitochondria are known as the powerhouse of cells. It is because the mitochondrion is the site of cellular respiration where energy in the form of ATP (Adenosine triphosphate) is generated as a result of oxidation of food constituents.
6. Where do the lipids and proteins constituting the cell membrane get synthesised?
Ans. The liquids and proteins constituting the cell membrane are synthesised by:
Lipids: by smooth endoplasmic reticulum (smooth SER).
Proteins: by rough endoplasmic reticulum (Rough RER)
7. How does an Amoeba obtain its food?
Ans. The food in amoeba is obtained by the process of endocytosis. Endocytosis is a cellular process where the substances are brought into the cell by a cell membrane surrounding the cell. These cell membranes then break off and form a vesicle surrounding the food material.
8. What is osmosis?
Ans.Osmosis is the passage of water molecules across a semi-permeable membrane from a solution with a high concentration to a solution with a lower concentration. It is a generalized process in which gases also participate.
9. Carry out the following osmosis experiment:
Take four peeled potato halves and scoos each one out to make potato cups. One of these potato cups should be made from a boiled potato. Put each potato cup in a trough containing water. Now,
(a) Keep cup A empty.
(b) Put one teaspoon sugar in cup B.
(c) Put one teaspoon salt in cup C.
(d) Put one teaspoon sugar in the boiled potato cup
Keep these for two hours. Then observes the four potato cups and answer the following:
(i) Explain why water gathers in the hollowed portion of B and C.
Ans. The concentration in the hollowed portion of the potato is high while solvent concentration is negligible. As a result, the higher concentration of water present outside the potato enters the hollowed portion due to osmosis across the semipermeable membrane of the potato.
(ii) Why is potato A necessary for this experiment?
Ans: It acts as a control to prove the process of osmosis, i.e. when a solution with low concentration of water is separated by semipermeable cell membrane from the region of higher concentration of water, water moves accordingly.
(iii) Explain why water does not gather in the hollowed out portions of A and D.
(iii) Cup A is not kept in any solution which does not allow osmosis to occur. Cup D, by being boiled has lost the quality of semi-permeability, i.e. it became inactive. As a result, it does not permit movement of water from the trough into the potato cup D.
