Activity 1.1
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What do we do with this peel?
We transfer a small piece of the peel onto a glass slide with a drop of water, stain it with safranin solution, cover it with a cover slip avoiding air bubbles, and observe it under a compound microscope.
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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. 1.2?
Yes, we observe rectangular, brick-like cells arranged side by side, each having a distinct cell wall, a prominent darkly stained nucleus, and cytoplasm, exactly as shown in Fig. 1.2.
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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?
We observe similar structures. The cells of the onion peel look identical regardless of the size of the onion bulb.
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Can we find out names of some more unicellular organisms?
Yes, some more examples of unicellular organisms are Euglena, Yeast, and Plasmodium.
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Every multi-cellular organism has come from a single cell. How?
Cells divide to produce cells of their own kind through the process of cell division. All cells thus come from pre-existing cells.
Activity 1.2
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(a) Do all cells look alike in terms of shape and size?
No, cells vary in shape and size depending on their location and the specific functions they perform.
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(b) Do all cells look alike in structure?
No, while they share common basic organelles, their overall structural details and specialized features vary according to their functions.
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(c) Could we find differences among cells from different parts of a plant body?
Yes, cells from different parts of a plant (such as leaf peel, root tip, and stem) show visible differences in shape, thickness of cell walls, and presence of specific organelles like chloroplasts.
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(d) What similarities could we find?
All cells possess basic structures including an outer boundary (cell wall and plasma membrane), cytoplasm, and a distinct nucleus.
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How does a living cell perform these basic functions?
A living cell performs basic functions through division of labour among its internal membrane-bound components known as cell organelles, each performing a specialized task.
Questions
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1. Who discovered cells, and how?
Robert Hooke discovered cells in 1665. While examining a thin slice of cork under a self-designed microscope, he observed that the cork resembled the structure of a honeycomb consisting of many little compartments, which he called 'cells'.
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2. Why is the cell called the structural and functional unit of life?
The cell is called the structural unit of life because all living organisms are made up of cells. It is called the functional unit of life because all essential life processes (such as respiration, excretion, nutrition, and protein synthesis) are carried out by individual cells.
Activity 1.3
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(a) Remove the shell of an egg by dissolving it in dilute hydrochloric acid. The shell is mostly calcium carbonate. A thin outer skin now encloses the egg. Put the egg in pure water and observe after 5 minutes. What do we observe?
The egg swells because water passes into it by endosmosis from the hypotonic medium.
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(b) Place a similar de-shelled egg in a concentrated salt solution and observe for 5 minutes. The egg shrinks. Why?
Water passes out of the egg into the salt solution by exosmosis because the external salt solution is hypertonic (more concentrated).
Activity 1.5
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Find out about electron microscopes from resources in the school library or through the internet. Discuss it with your teacher.
An electron microscope uses a beam of accelerated electrons instead of visible light to illuminate an object. It provides much higher magnification (up to 100,000 to 500,000 times) and resolving power than a compound light microscope, allowing us to observe the detailed internal structures of cells and organelles.
Questions
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1. How do substances like CO2 and water move in and out of the cell? Discuss.
CO2 moves in and out of the cell by diffusion, which is the spontaneous movement of molecules from a region of higher concentration to a region of lower concentration across the plasma membrane.
Water moves across the cell membrane by osmosis, which is the net diffusion 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. -
2. Why is the plasma membrane called a selectively permeable membrane?
The plasma membrane is called a selectively permeable membrane because it permits or allows the entry and exit of some selected substances while preventing the movement of other materials into and out of the cell.
Activity 1.6
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Mount the peel of a Rhoeo leaf in water on a slide and examine cells under the high power of a microscope. Note the small green granules, called chloroplasts. They contain a green substance called chlorophyll. Put a strong solution of sugar or salt on the mounted leaf on the slide. Wait for a minute and observe under microscope. What do we see?
We observe plasmolysis—the cell contents along with the plasma membrane shrink and pull away from the cell wall due to exosmosis.
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Now place some Rhoeo leaves in boiling water for a few minutes. This kills the cells. Then mount one leaf on a slide and observe it under a microscope. 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?
No, plasmolysis did not occur. Boiling kills the cells, damaging the selectively permeable nature of the cell membrane.
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What do we infer from this activity?
We infer that only living cells, and not dead cells, are able to absorb or lose water by osmosis.
Activity 1.7
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What do we observe? What is the shape of the cells we see? Draw it on the observation sheet. See P.No. 166, Fig. 1
Under the microscope, we observe irregularly shaped, polygonal cells with a thin boundary (cell membrane), granular cytoplasm, and a centrally placed, darkly stained nucleus. Cell walls are absent.
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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?
Yes, there was a darkly stained dot-like structure called the nucleus near the centre of cheek cells, and similar distinct nuclei were also observed in onion peel cells.
Questions
- 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: poorly defined due to the absence of a 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 present |
Questions
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1. Can you name the two organelles we have studied that contain their own genetic material?
Mitochondria and Plastids.
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2. If the organisation of a cell is destroyed due to some physical or chemical influence, what will happen?
The cell will not be able to perform its basic life functions like respiration, nutrition, synthesis of proteins, and excretion. Consequently, lysosomes may burst and digest their own cell, leading to cell death.
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3. Why are lysosomes known as suicide bags?
When the cell gets damaged or disturbed during cellular metabolism, lysosomes may burst, and their powerful digestive enzymes digest their own cell. Therefore, they are called 'suicide bags'.
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4. Where are proteins synthesised inside the cell?
Proteins are synthesised in the ribosomes (present freely in the cytoplasm or attached to the rough endoplasmic reticulum).
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Can you think as to why the chromosome number has reduced to half in daughter cells?
The chromosome number is reduced to half during meiosis so that when male and female gametes fuse during fertilisation, the original diploid number of chromosomes is restored in the zygote/offspring.
Exercises
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1. Make a comparison and write down ways in which plant cells are different from animal cells.
Plant cells and animal cells differ in the following ways:
| Plant Cell | Animal Cell |
|---|---|
| 1. Possesses a rigid outer cell wall made of cellulose. | 1. Cell wall is absent; bounded only by plasma membrane. |
| 2. Contains plastids (like chloroplasts for photosynthesis). | 2. Plastids are absent. |
| 3. Has a single, very large central vacuole. | 3. Possesses small, multiple vacuoles. |
| 4. Centrioles/centrosomes are generally absent. | 4. Centrioles are present near the nucleus. |
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2. How is a prokaryotic cell different from a eukaryotic cell?
The differences between prokaryotic and eukaryotic cells are:
| Prokaryotic Cell | Eukaryotic Cell |
|---|---|
| 1. Generally small in size (1–10 μm). | 1. Generally large in size (5–100 μm). |
| 2. Nuclear region is poorly defined (nucleoid), lacking a nuclear membrane. | 2. Nuclear region is well defined and surrounded by a nuclear membrane. |
| 3. Contains a single chromosome. | 3. Contains more than one chromosome. |
| 4. Membrane-bound organelles (mitochondria, plastids, ER, Golgi) are absent. | 4. Membrane-bound cell organelles are present. |
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3. What would happen if the plasma membrane ruptures or breaks down?
If the plasma membrane ruptures, the internal contents of the cell will spill into the external environment. The cell will lose its selective permeability, vital metabolic reactions will cease, and the cell will die.
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4. What would happen to the life of a cell if there was no Golgi apparatus?
Without the Golgi apparatus, packaging, dispatching, and modification of materials synthesised near the ER would not take place. Lysosome formation would not occur, causing waste materials to accumulate, and complex sugars would not be synthesised, disrupting cellular functions.
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5. Which organelle is known as the powerhouse of the cell? Why?
Mitochondria are known as the powerhouses of the cell. This is because they generate energy required for various cellular and metabolic activities in the form of ATP (Adenosine triphosphate) molecules.
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6. Where do the lipids and proteins constituting the cell membrane get synthesised?
Proteins are synthesised on the ribosomes of the Rough Endoplasmic Reticulum (RER), and lipids are synthesised by the Smooth Endoplasmic Reticulum (SER).
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7. How does an Amoeba obtain its food?
Amoeba obtains its food through endocytosis (phagocytosis). Due to the flexibility of its plasma membrane, Amoeba extends pseudopodia (false feet) around the food particle, engulfs it, and forms a food vacuole inside the cytoplasm.
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8. What is osmosis?
Osmosis is the net diffusion 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.
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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 the hollowed portion of cups B and C because the sugar/salt inside creates a hypertonic environment, causing water from the trough to enter the cavity through the living cells of the potato by endosmosis.
(ii) Potato cup A acts as a control experiment. It shows that water does not move across the potato tissue without a concentration gradient.
(iii) In potato cup A, water does not collect because there is no concentration difference (empty cup). In cup D, boiling destroys the cell membranes of the potato tissue, so osmosis cannot occur despite the presence of sugar. -
10. Which type of cell division is required for growth and repair of body and which type is involved in formation of gametes?
Mitosis is required for growth and repair of the body.
Meiosis is involved in the formation of gametes.
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11. Observe the given diagram
If it represents a plant cell, which of the following best describes it?Answer: A) 1- Cell wall, 2- Plasma membrane, 3- Nucleus, 4- Chromosome -
12. Observe the picture.
Which of the following statement is correct with respect to the above image?Answer: D) No relation between the size of the onion and the size of its cells - 13. Four students made the following statement regarding the number of chromosomes in the daughter cells formed by meiosis:
| Student | Statement |
|---|---|
| Vani | The daughter cells have half the number of chromosomes compared to the mother cell. |
| Rani | The daughter cells have double the number of chromosomes comapared to the mother cell. |
| Ramesh | The daughter cells have the same number of chromosomes as the mother cell. |
| Suresh | The number of chromosomes in the daughter cells depends on the type of cell. |
Answer: A) Vani
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