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Chapter 5
The Fundamental Unit of Life
Class 9 – NCERT Science Solutions
Intext Questions 1
Question 1
Who discovered cells, and how?
Answer
Cells were discovered by Robert Hooke in 1665. He was examining a thin slice of cork with a self-designed microscope when he saw that the cork resembled the structure of a honeycomb consisting of many little compartments. Robert Hooke called these compartments as cells.
Question 2
Why is the cell called the structural and functional unit of life?
Answer
The cell is called the structural and functional unit of life because it is smallest part of the body which forms the structure of all living things and carries out all basic life processes like respiration, nutrition, excretion independently.
Intext Questions 2
Question 1
How do substances like CO2 and water move in and out of the cell? Discuss.
Answer
The cell membrane, is selectively permeable i.e., it allows some substance to pass through it while prevents other substances from crossing.
Movement of CO2 — CO2 moves in and out of the cell by diffusion. It accumulates in high concentrations inside the cell. In the cell’s external environment, the concentration of CO2 is low as compared to that inside the cell. Therefore, CO2 moves out of the cell, from a region of high concentration, to a region of low concentration by the process of diffusion.
Movement of Water — Water also moves in and out of the cell according to the concentration gradient by the process of osmosis. The movement of water across the plasma membrane is also affected by the amount of substance dissolved in water. Thus, osmosis is the net diffusion of water across a selectively permeable membrane toward a higher solute concentration.
Question 2
Why is the plasma membrane called a selectively permeable membrane?
Answer
The plasma membrane allows or permits the entry and exit of some materials in and out of the cell but also prevents movement of some other materials. Therefore, the cell membrane is called a selectively permeable membrane.
Intext Questions 3
Question 1
Fill in the gaps in the following table illustrating differences between prokaryotic and eukaryotic cells.
| Prokaryotic Cell | Eukaryotic Cell |
|---|---|
| 1. Size : generally small ( 1-10 μm) 1 μm = 10-6 m | 1. Size: generally large ( 5-100 μm) |
| 2. Nuclear region: …………… and known as …………… | 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. …………… |
Answer
| Prokaryotic Cell | Eukaryotic Cell |
|---|---|
| 1. Size : generally small ( 1-10 μm) 1 μm = 10-6 m | 1. Size: generally large ( 5-100 μm) |
| 2. Nuclear region: Not membrane bound 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 present |
Intext Questions 4
Question 1
Can you name the two organelles we have studied that contain their own genetic material?
Answer
Mitochondria and Plastids are the two organelles that contain their own genetic material.
Question 2
If the organisation of a cell is destroyed due to some physical or chemical influence, what will happen?
Answer
If the organisation of a cell is destroyed due to some physical or chemical influence, the cell may stop to function and finally die. A dead cell is digested by its own lysosomes.
Question 3
Why are lysosomes known as suicide bags?
Answer
Lysosomes contain powerful digestive enzymes capable of breaking down all organic material. During the disturbance in cellular metabolism or any damage to the cell, the lysosomes may burst and the enzymes digest their own cell. Therefore, lysosomes are known as suicide bags of a cell.
Question 4
Where are proteins synthesised inside the cell?
Answer
Ribosomes attached to the surface of rough endoplasmic reticulum (RER) are the sites of protein synthesis inside the cell.
Exercises
Question 1
Make a comparison and write down ways in which plant cells are different from animal cells.
Answer
Differences between plant and animal cells:
| Plant cell | Animal cell |
|---|---|
| It is larger in size. | It is usually smaller in size. |
| Plant cell has a definite cell wall. | Animal cell has no cell wall. |
| Cytoplasm not so dense and is less in amount. | Cytoplasm denser and more granular. It fills almost the entire cell. |
| Plastid are present. | Plastid are not present. |
| Vacuoles are very large. | Vacuoles are small sized. |
Question 2
How is a prokaryotic cell different from a eukaryotic cell?
Answer
| Prokaryotic cell | Eukaryotic cell |
|---|---|
| Cells contain primitive and undefined nucleus. Nuclear membrane is absent. | Have a well-defined nucleus along with a nuclear membrane |
| Membrane bound cell organelles are absent. | Membrane bound cell organelles are present. |
| Example – Blue-green algae. | Example – Humans |
Question 3
What would happen if the plasma membrane ruptures or breaks down?
Answer
In case the plasma membrane ruptures or breaks down, the protoplasm may leak out of the cell exposing the cell components to the external environment and eventually the cell will die.
Question 4
What would happen to the life of a cell if there was no Golgi apparatus?
Answer
The absence of Golgi apparatus will have the following consequences for the cell:
- It would severely impact the cell’s ability to modify, sort, and transport proteins and lipids, leading to disruptions in various cellular processes ultimately leading to cell death.
- Without Golgi apparatus, the lysosomes might not form properly, leading to cellular waste buildup and potential toxicity.
Question 5
Which organelle is known as the powerhouse of the cell? Why?
Answer
Mitochondria is known as powerhouse of the cell as it is responsible for the process of respiration releasing energy in the form of ATP.
Question 6
Where do the lipids and proteins constituting the cell membrane get synthesised?
Answer
The lipids and proteins constituting the cell membrane are synthesised by the smooth endoplasmic reticulum and rough endoplasmic reticulum respectively.
Question 7
How does an Amoeba obtain its food?
Answer
The flexibility of the cell membrane enables Amoeba to obtain its food through a process known as endocytosis. It uses pseudopodia, temporary extensions of its cell membrane, to surround and engulf its food. The engulfed particles are then enclosed in a food vacuole, where digestion takes place.
Question 8
What is osmosis?
Answer
Osmosis is the movement of water molecules from their region of higher concentration (dilute solution or with a lower solute concentration) to their region of lower concentration (concentrated solution or with a higher solute concentration) through a semi permeable membrane.
Question 9
Carry out the following osmosis experiment: Take four peeled potato halves and scoop 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 D. Keep these for two hours. Then observe the four potato cups and answer the following:
(i) Explain why water gathers in the hollowed portion of B and C.
(ii) Why is potato A necessary for this experiment?
(iii) Explain why water does not gather in the hollowed out portions of A and D.
Answer
(i) Potato cups B and C have sugar and salt respectively in them. The sugar and salt form a concentrated or hypertonic solution in the potato cups. This will cause osmosis to occur in which the water molecules will move from their higher concentration (outside) to their lower concentration (inside the cup) i.e., from hypotonic solution to hypertonic solution. Therefore, the level of water will increase in the potato cups B and C.
(ii) Potato A serves as a control for the experiment.
(iii) Water does not collect in cup A because cell sap and water become isotonic.
Water does not collect in cup D because the cells of potato died due to boiling. After boiling the cell membrane cannot function like a semi-permeable membrane and hence there will be no passage of water through it.
Question 10
Which type of cell division is required for growth and repair of body and which type is involved in formation of gametes?
Answer
Mitosis is the cell division required for growth and repair of body.
Meiosis is the cell division involved in formation of gametes (haploid cells).