THE FUNDAMENTAL UNIT OF LIFE - Q&A
Questions Page 51
1. Who discovered cells, and how?
Cells were discovered by Robert Hooke in 1665. He observed a thin slice of cork under a self-designed crude microscope. He noticed that the cork resembled the structure of a honeycomb consisting of many little compartments. He called these boxes "cells".
2. Why is the cell called the structural and functional unit of life?
1. Structural Unit: All living organisms are made up of cells. Just as bricks make up a building, cells make up the body of an organism.
2. Functional Unit: Each cell performs basic functions characteristic of the organism, such as nutrition, respiration, and clearing of waste material. The collective work of cells keeps the organism alive.
Questions Page 53
1. How do substances like CO2 and water move in and out of the cell? Discuss.
Carbon dioxide (CO2): Moves by diffusion. When CO2 accumulates in high concentration inside the cell (as cellular waste), it diffuses out to the region of lower concentration outside the cell across the cell membrane.
Water: Moves by osmosis. Water molecules move from a region of higher water concentration (dilute solution) to a region of lower water concentration (concentrated solution) through the semi-permeable plasma membrane.
2. Why is the plasma membrane called a selectively permeable membrane?
The plasma membrane allows the entry and exit of some materials in and out of the cell while preventing the movement of other materials. Because it selects which substances can pass through it, it is called a selectively permeable membrane.
Questions Page 55
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: undefined 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 |
Questions Page 57
1. Can you name the two organelles we have studied that contain their own genetic material?
The two organelles that contain their own DNA and ribosomes (genetic material) are:
1. Mitochondria
2. Plastids (found in plant cells)
2. If the organisation of a cell is destroyed due to some physical or chemical influence, what will happen?
If the organization of a cell is destroyed, the cell will not be able to perform its basic life functions (like respiration, nutrition, excretion, etc.). As a result, the cell will die. Lysosomes may burst and their enzymes will digest the dead cell (autolysis).
3. Why are lysosomes known as suicide bags?
Lysosomes contain powerful digestive enzymes capable of breaking down all organic material. When a cell gets damaged, lysosomes may burst, and the enzymes digest their own cell. Because they cause the destruction of their own cell, they are known as "suicide bags".
4. Where are proteins synthesised inside the cell?
Proteins are synthesised in the Ribosomes. Ribosomes can be found floating freely in the cytoplasm or attached to the surface of the Rough Endoplasmic Reticulum (RER).
Exercises
1. Make a comparison and write down ways in which plant cells are different from animal cells.
| Characteristic | Plant Cell | Animal Cell |
|---|---|---|
| Cell Wall | Present (made of cellulose) | Absent |
| Plastids | Present (e.g., Chloroplasts) | Absent |
| Vacuoles | Large, central permanent vacuole | Small, temporary vacuoles |
| Shape | Fixed, rigid shape | Irregular, flexible shape |
| Golgi Apparatus | Many simple units (dictyosomes) | Single complex unit |
2. How is a prokaryotic cell different from a eukaryotic cell?
| Feature | Prokaryotic Cell | Eukaryotic Cell |
|---|---|---|
| Nuclear Membrane | Absent (undefined nuclear region called Nucleoid) | Present (well-defined nucleus) |
| Organelles | Membrane-bound organelles (like mitochondria) absent | Membrane-bound organelles present |
| Chromosomes | Single chromosome | More than one chromosome |
| Size | Generally small (1-10 µm) | Generally large (5-100 µm) |
3. What would happen if the plasma membrane ruptures or breaks down?
If the plasma membrane ruptures:
1. The regulation of substances entering and exiting the cell will stop.
2. The cellular contents (protoplasm) will leak out.
3. The constant internal environment required for life functions will be lost.
4. The cell will eventually die.
4. What would happen to the life of a cell if there was no Golgi apparatus?
The Golgi apparatus is responsible for storing, modifying, and packaging products (like proteins and lipids) in vesicles. If it were absent:
1. Materials synthesized by the Endoplasmic Reticulum would not be packaged and transported to their targets.
2. Formation of lysosomes would stop (since Golgi forms them).
3. The cell would fail to secrete necessary substances, affecting its functioning and life.
5. Which organelle is known as the powerhouse of the cell? Why?
Mitochondria are known as the powerhouse of the cell.
Reason: They perform cellular respiration to release energy from food. This energy is stored in the form of ATP (Adenosine Triphosphate) molecules, which the cell uses for various chemical activities and mechanical work.
6. Where do the lipids and proteins constituting the cell membrane get synthesised?
1. Lipids: Synthesised by the Smooth Endoplasmic Reticulum (SER).
2. Proteins: Synthesised by the Rough Endoplasmic Reticulum (RER) (specifically by the ribosomes attached to it).
7. How does an Amoeba obtain its food?
Amoeba obtains its food through a process called Endocytosis.
1. Its flexible cell membrane forms finger-like projections called pseudopodia.
2. These surround the food particle and engulf it to form a food vacuole.
3. Digestive enzymes inside the vacuole break down the food.
8. What is osmosis?
Osmosis is the movement of water molecules from a region of higher water concentration (dilute solution) to a region of lower water concentration (concentrated solution) through a selectively permeable membrane.
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 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.
(i) Water gathers in B and C due to Osmosis (specifically endosmosis). The sugar/salt inside the cup creates a lower water concentration compared to the water in the trough. Water moves from the trough (high water concentration) through the potato cells into the cup (low water concentration).
(ii) Potato A acts as a Control. It helps compare the results and proves that water does not enter the cup simply on its own without the presence of a solute (sugar/salt) to trigger osmosis.
(iii) Water does not gather in:
- A: Because there is no solute (sugar/salt) to create a concentration difference for osmosis to occur.
- D: Because the potato is boiled. Boiling kills the cells and destroys the semi-permeable membrane. Osmosis requires living cells with a semi-permeable membrane to work.
10. Which type of cell division is required for growth and repair of body and which type is involved in formation of gametes?
1. Mitosis: Required for growth and repair of the body (produces identical cells).
2. Meiosis: Involved in the formation of gametes (produces reproductive cells with half the chromosomes).