Activity 5.1 (Page 50)
Question: What do we observe as we look through the lens? Can we draw the structures that we are able to see through the microscope, on an observation sheet? Does it look like Fig. 5.2?
Answer: When we look through the lens of the microscope, we see small structures that look like little rooms or compartments. These are the cells of the onion peel. When we draw these structures, they look similar to Figure 5.2, showing many rectangular units arranged side-by-side. Each cell has a boundary called the cell wall and a dark dot inside called the nucleus.
More to know (Page 51)
Question: We can try preparing temporary mounts of peels of onions of different sizes. What do we observe? Do we see similar structures or different structures?
Answer: We observe that onion bulbs of different sizes have similar small structures when seen under a microscope. The size of the onion does not change the basic structure of its cells.
Question: What are these structures?
Answer: These small structures are the basic building units of the onion bulb. These structures are called cells.
Activity 5.2 (Page 51)
Question (a): Do all cells look alike in terms of shape and size?
Answer: No, all cells do not look alike in terms of shape and size. Different cells in an organism can have different shapes and sizes. For example, nerve cells are long and branched, while blood cells are round.
Question (b): Do all cells look alike in structure?
Answer: While all cells have some basic common components like a cell membrane, nucleus, and cytoplasm, they do not all look alike in structure. Their structure is related to the specific function they perform.
Question (c): Could we find differences among cells from different parts of a plant body?
Answer: Yes, we could find differences among cells from different parts of a plant body. For example, cells from the tip of a root will be different from the cells of a leaf peel.
Question (d): What similarities could we find?
Answer: We could find that all cells, no matter their shape or function, have the same basic components: a plasma membrane, a nucleus, and cytoplasm. All cells are the basic units of life.
In-text Questions (Page 52)
Question 1: Who discovered cells, and how?
Answer: Robert Hooke discovered cells in the year 1665. He observed a thin slice of cork using a self-designed microscope and saw that it was made of many small, honeycomb-like compartments. He called these boxes “cells”.
Question 2: Why is the cell called the structural and functional unit of life?
Answer: The cell is called the structural unit of life because all living organisms are made up of cells; they are the basic building blocks of the body. It is called the functional unit of life because each living cell has the capacity to perform the basic functions of life, such as obtaining nutrition, respiration, clearing waste, and making new proteins.
Activity 5.3 (Page 53)
Question: Put the de-shelled egg in pure water and observe after 5 minutes. What do we observe?
Answer: We observe that the egg swells up. This is because the water concentration outside the egg is higher than inside, so water enters the egg through the process of osmosis.
Question: Place a similar de-shelled egg in a concentrated salt solution and observe for 5 minutes. The egg shrinks. Why?
Answer: The egg shrinks because the water concentration inside the egg is higher than in the concentrated salt solution outside. Therefore, water moves out of the egg and into the salt solution through osmosis, causing the egg to shrink.
Activity 5.4 (Page 53)
Question: Put dried raisins or apricots in plain water and leave them for some time. Then place them into a concentrated solution of sugar or salt. What will you observe?
Answer: (a) When placed in plain water, each raisin or apricot gains water through osmosis and swells up. (b) However, when these swollen raisins or apricots are placed in a concentrated solution of sugar or salt, they lose water and consequently shrink.
In-text Questions (Page 53)
Question 1: How do substances like CO₂ and water move in and out of the cell? Discuss.
Answer: Substances like carbon dioxide (CO₂) move in and out of the cell by a process called diffusion. Diffusion is the movement of a substance from an area of high concentration to an area of low concentration. For example, when CO₂ builds up inside the cell (high concentration), it moves out to the environment where its concentration is low.
Water moves in and out of the cell through a special type of diffusion called osmosis. Osmosis is the movement of water molecules through a selectively permeable membrane (the cell membrane) from a region of high water concentration to a region of low water concentration.
Question 2: Why is the plasma membrane called a selectively permeable membrane?
Answer: The plasma membrane is called a selectively permeable membrane because it allows only some selected materials to enter and exit the cell. It prevents the movement of some other materials, thus controlling what goes in and what comes out.
Activity 5.6 (Page 54)
Question: Mount the peel of a Rhoeo leaf in water on a slide… Put a strong solution of sugar or salt on the mounted leaf on the slide. Wait for a minute and observe under a microscope. What do we see?
Answer: We see that the contents of the cell shrink and move away from the cell wall. This phenomenon is called plasmolysis. It happens because the cell loses water to the strong sugar or salt solution through osmosis.
Question: Now place some Rhoeo leaves in boiling water for a few minutes. This kills the cells… Put a strong solution of sugar or salt on the mounted leaf on the slide. Wait for a minute and observe it again. What do we find? Did plasmolysis occur now?
Answer: We find that plasmolysis did not occur now. Boiling the leaves killed the cells. This shows that only living cells are able to absorb or lose water by osmosis. Dead cells cannot do this.
Activity 5.7 (Page 54)
Question: What do we observe? What is the shape of the cells we see?
Answer: We observe cells from the inside of the cheek. The cells are generally flat and irregular in shape.
Question: Was there a darkly coloured, spherical or oval, dot-like structure near the centre of each cell? This structure is called nucleus. Were there similar structures in onion peel cells?
Answer: Yes, there was a darkly coloured, spherical or oval, dot-like structure near the centre of each cheek cell. This is the nucleus. Yes, similar structures (nuclei) were also seen in the onion peel cells.
In-text Question (Page 55)
Question 1: Fill in the gaps in the following table illustrating differences between prokaryotic and eukaryotic cells.
Answer:
In-text Questions (Page 58)
Question 1: Can you name the two organelles we have studied that contain their own genetic material?
Answer: The two organelles that contain their own genetic material (DNA) are mitochondria and plastids.
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, the cell will not be able to perform its basic functions like respiration, obtaining nutrition, or clearing waste. The lysosomes might burst and digest the cell itself. Ultimately, the cell will die.
Question 3: Why are lysosomes known as suicide bags?
Answer: Lysosomes are known as the ‘suicide bags’ of a cell because they contain powerful digestive enzymes. During a disturbance in cellular metabolism, for example, if the cell gets damaged, the lysosomes may burst and their enzymes will digest their own cell, causing the cell to die.
Question 4: Where are proteins synthesised inside the cell?
Answer: Proteins are synthesised in the ribosomes inside the cell.
Exercises (Page 59)
1. Make a comparison and write down ways in which plant cells are different from animal cells.
Answer:
2. How is a prokaryotic cell different from a eukaryotic cell?
Answer:
3. What would happen if the plasma membrane ruptures or breaks down?
Answer: If the plasma membrane ruptures or breaks down, the cell will lose its contents. The organised interior of the cell will mix with the outside environment. The cell will not be able to control the movement of substances in and out. This will lead to the death of the cell.
4. What would happen to the life of a cell if there was no Golgi apparatus?
Answer: If there was no Golgi apparatus, the life of a cell would be seriously affected. The functions of storing, modifying, and packaging materials synthesised in the cell (like proteins and lipids) would stop. The transport of these materials to various destinations inside and outside the cell would also be affected. The formation of lysosomes would not happen, leading to the accumulation of waste and worn-out organelles in the cell.
5. Which organelle is known as the powerhouse of the cell? Why?
Answer: The mitochondria is known as the powerhouse of the cell. This is because it releases the energy required for all the chemical activities of the cell. This energy is released in the form of ATP (Adenosine Triphosphate) molecules, which is the energy currency of the cell.
6. Where do the lipids and proteins constituting the cell membrane get synthesised?
Answer:
- Proteins are synthesised in the rough endoplasmic reticulum (RER).
- Lipids (fat molecules) are synthesised in the smooth endoplasmic reticulum (SER).
7. How does an Amoeba obtain its food?
Answer: An Amoeba obtains its food through a process called endocytosis. The cell membrane of the Amoeba is flexible. It extends to surround a food particle and engulfs it, forming a food vacuole inside the cell.
8. What is osmosis?
Answer: Osmosis is the movement of water molecules from a region of high water concentration to a region of low water concentration through a selectively permeable membrane.
9. Carry out the following osmosis experiment…
Question (i): Explain why water gathers in the hollowed portion of B and C.
Answer: Water gathers in the hollowed portions of potato cups B and C because of osmosis. The sugar in cup B and salt in cup C create a concentrated solution inside the potato cup. The cells of the potato act as a selectively permeable membrane. Since the water concentration inside the potato cups is lower than in the trough, water from the trough moves into the potato cups through osmosis, causing water to gather.
Question (ii): Why is potato A necessary for this experiment?
Answer: Potato cup A is necessary for this experiment because it acts as a control. A control is used for comparison. Since cup A is empty, it shows that water does not gather on its own. This proves that the gathering of water in cups B and C is due to the presence of sugar and salt, and not some other factor.
Question (iii): Explain why water does not gather in the hollowed-out portions of A and D.
Answer:
- Cup A: Water does not gather in cup A because there is no concentration difference between the inside of the cup and the water in the trough. Therefore, there is no large movement of water into the cup.
- Cup D: Water does not gather in cup D because the potato was boiled. Boiling destroys the cells and their selectively permeable membranes. Without the living, semi-permeable membranes, osmosis cannot occur.
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 type of cell division required for the growth and repair of the body.
- Meiosis is the type of cell division that is involved in the formation of gametes (reproductive cells).