Page No. 59
Q.1. Who discovered cells and how?
Ans: The cell was first discovered by Robert Hooke in 1665. He examined thin slices of cork under a self-made microscope and saw a multitude of tiny hollow spaces that he remarked looked like the walled compartments of a honeycomb. He termed these spaces as ‘cell’ meaning ‘small room’ in Latin.
Q.2. Why is the cell called the structural and functional unit of life?
Ans: All living organisms are made up of cells. This shows that the cell is the structural unit of life. Each living cell has the capacity to perform certain basic functions that are characteristics of all living forms. For example, phagocytic cells eat or kill unwanted or foreign particles inside the body (e.g., WBCs). Some cells, e.g., pancreatic cells, small intestinal cells, liver cells secrete enzymes and hormones. That is why cell is called the structural and functional unit of life.
Page No. 61
Q.1. How do substances like CO2 and water move in and out of the cell? Discuss.
Ans: Substances like CO2 move in and out of the cell by a process called diffusion from a region of high concentration to a region where its concentration is low. The water moves in and out of the cell by the process of osmosis.
Q.2. Why is the plasma membrane called a selectively permeable membrane?
Ans: The plasma membrane allows or permits the entry and exit of some materials in and out of the cell and prevents movement of some other materials through it. Hence, it is called a selectively permeable membrane.
Page No. 63
Q.1. Fill in the gaps in the following table illustrating differences between prokaryotic and eukaryotic cells.
|Prokaryotic cell||Eukaryotic cell|
|(i) Size: generally small (1-10 μm) 1μm = 10-6 m.|| (i) Size: generally large (5-100 μm)|
|(ii) Nuclear region: ______ and known ____||(ii) Nuclear region: well defined and surrounded by a nuclear membrane.|
|(iii) Chromosome: single||(iii) More than one chromosome.|
|(iv) Membrane-bound cell organelles absent.||(iv) ___________|
|(i) Size: generally small (1-10 μm). [1μm = 10-6 m]||(i) Size: generally large (5-100 μm)|
|(ii) Nuclear region: poorly defined due to the absence of nuclear envelop and known nucleoid .||(ii) Nuclear region: well defined and surrounded by a nuclear membrane.|
|(iii) Chromosome: single||(iii) More than one chromosome.|
|(iv) Membrane-bound cell organelles absent.||(iv) Membrane-bound cell organelles present.|
Page No. 65
Q.1. Can you name the two organelles we have studied that contain their own genetic material?
Ans: Two organelles that contain their own genetic material is Mitochondria and Plastids.
Q.2. If the organisation of a cell is destroyed due to some physical or chemical influence, what will happen?
Ans: Cell organelles are responsible for the organisation and proper functioning of a cell, as each of them performs some specific functions. Naturally, if any of these organelles are destroyed, the functions of the cell will be stopped and it may also result in the death of the cell.
Q.3. Why are lysosomes known as suicide bags?
Ans: Lysosomes are the cell organelles involved in the digestion of any foreign material that enters the cell as they contain digestive enzymes. In case anybody cell is dead or damaged, the lysosome bursts to release the digestive enzymes in order to digest its own cell. Thus, these are known as ‘suicide bags’.
Q.4. Where are proteins synthesized inside the cell?
Ans: Proteins are synthesised in ribosomes present in the cell and so they are also called as protein factories of the cell. These may be attached on the surface of rough endoplasmic reticulum or lie freely in the cell.
Page No. 67
Q.1. Make a comparison and write down ways in which plant cells are different from animal cells.
|Animal cells||Plant cells|
|(i) Animal cells are usually smaller in size.||(i) Plant cells are comparatively larger in size.|
|(ii) Enclosed by plasma membrane only. Cell wall is absent.||(ii) Plasma membrane of plant cells is surrounded by a thick cell wall.|
|(iii) Plastids are absent.||(iii) Plastids are present.|
|(iv) Cytoplasm consists largely of smaller vacuoles.||(iv) Central space of the cell is occupied by a large vacuole.|
|(v) Nucleus lies in the centre of the cell.||(v) Nucleus lies on one side of the cell.|
|(vi) Prominent and highly complex Golgi bodies present.||(vi) Golgi apparatus consists of several sub-units called dictyosomes.|
|(vii) Animal cells possess centrosome with two centrioles.||(vii) Plant cells lack centrioles.|
Q.2. How is a prokaryotic cell different from a eukaryotic cell?
|(i) Size of the cell is generally small (1-10 μm).||(i) Size of the cell is generally large (5-100 μm).|
|(ii) Nuclear region is poorly defined due to the absence of nuclear membrane and known as nucleoid.||(ii) Nuclear region is well-defined and surrounded by a nuclear membrane.|
|(iii) It contains single chromosome.||(iii) It contains more than one chromosome.|
|(iv) Nucleolus is absent.||(iv) Nucleolus is present.|
|(v) Membrane bound cell organelles absent.||(v) Cell organelles such as mitochondria, plastids, endoplasmic reticulum, golgi apparatus, lyso- somes, peroxisomes, etc. are present.|
|(vi) Cell division takes place by fission or budding.||(vi) Cell division occurs by mitotic or meiotic cell division.|
|(vii) Centrioles absent.||(vii) Centrioles are present in animal cells.|
|(viii) Prokaryotic cells are found in bacteria, blue-green algae.||(viii) Eukaryotic cells are found in fungi, plant and animal cells.|
Q.3. What would happen if the plasma membrane ruptures or breaks down?
Ans: Plasma membrane is the selectively permeable membrane that surrounds the cell and allows the entry and exit of selected materials of the cell. If it ruptures, the contents of the cell will come in direct contact with the surrounding medium and not only unwanted material will be able to enter freely into the cell, but useful material will also find its way out of the cell easily. This will seriously disrupt the various metabolic activities of the cell and will result in its imminent death.
Q.4. What would happen to the life of a cell if there was no Golgi apparatus?
Ans: If there were no Golgi apparatus, the material synthesised by endoplasmic reticulum would not be carried to the various parts inside and outside of the cell. Also as the Golgi apparatus performs the function of storage and modification of the material synthesised in the cell, these materials would not be stored and modified further.
Moreover, there will be no production of lysosomes which will cause the accumulation of waste material, viz., worn out and dead cell organelles within the cell which will ultimately lead to cell death.
Q.5. Which organelle is known as the powerhouse of the cell? Why
Ans: Mitochondria are known as powerhouse of the cell because these are the sites of cellular respiration. They release energy in the form of ATP (Adenosine Triphosphate). This energy is then utilised by the organelles to carry out their basic functions.
Q.6. Where do the lipids and proteins constituting the cell membrane get synthesized?
Ans: Lipids are synthesised in smooth endoplasmic reticulum from where they are transported to the cell membrane.
Proteins are synthesised by the ribosomes in the cytoplasm. These are then transferred to Golgi complex and ER for further modifications from where they reach their destinations.
Q.7. How does an Amoeba obtain its food?
Ans: Amoeba takes in food using temporary finger-like extensions of the cell surface which fuse over the food particle forming a food-vacuole as shown in figure. Inside the food vacuole, complex substances are broken down into simpler ones which then diffuse into the cytoplasm. The remaining undigested material is moved to the surface of the cell and thrown out.
Q.8. What is osmosis?
Ans: It is the passage of solvent from a region of high concentration to a region of low concentration through a semipermeable membrane.
Q.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.
Ans: (i) Water gathers in the hollowed portion of cups B and C because of the process of endosmosis (moving in of the solvent). The potato wall acts as a semi-permeable membrane. As the cups B and C are filled with sugar and salt respectively and their outer part is in contact with the water, the concentration of water outside the cups is higher than inside the cups. So, water moves from its higher concentration towards the lower concentration, i.e., inside the cup.
(ii) Potato A acts as a control of the experiment. It is very necessary for comparing the results of the experiment. It shows that if the concentration of water is same on both sides, there will be no movement of water.
(iii) Water does not gather in the hollowed out portions of A as it does not contain hypertonic solution so there is no concentration difference and hence no movement of solvent. Water does not gather in the cup D as the cells of boiled potato are dead and the potato wall is no longer semi-permeable. Hence, no osmosis occurs.
Q.10. Which type of cell division is required for the growth and the repair of the body and which type is involved in formation of gametes?
Ans: For growth and repair mitotic division (mitosis) are involved as this type of division keeps the chromosome number constant.
For gamete formation, meiosis is involved as reduction of chromosome number is necessary for this case.