Organization in the Living World – NCERT solutions

Question 1. Who discovered cells, and how?

Answer.

Robert Hooke discovered cells. In 1665 under self-designed microscope, he examined a thin slice of cork. The cork resembled the structure of a honey comb consisting of many little boxes. These little boxes were termed cells by Hooke. Cell is a Latin word which means “a little room”.

Question 2. Why is the cell called the structural and functional unit of life?

Answer.

All living organisms are composed of cells. Each cell has several cell organelles which perform a specific function for the cell. Its an example of division of labour within the cell. In all organisms all cells have the same cell organelles. Thus cell has a basic structural organization and is the basic functional unit.

Question 3. How do substances like CO₂ and water move in and out of the cell? Discuss.

Answer.

Movement of CO₂: Substances like CO₂ and oxygen move in and out of the cell through plasma membrane by diffusion. When concentration of CO₂ is low outside the cell, CO₂ moves out of the cell by diffusion and vice versa.

Movement of water:

Movement of water is affected by amount of substance dissolved in water. Water moves from a region of high water concentration to a region of low water concentration by osmosis.

Question 4. Fill in the gaps in the following table illustrating differences between prokaryotic and eukaryotic cells.

Prokaryotic Cell 1. Size: generally small (1-10 m) 1rn = 10 -6 2. Nuclear region: ………………………. …………………………………………… and known as ………… 3. Chromosome: single 4. Membrance-bound cell organelles are absent Answer.	Eukaryotic Cell 1. Size: generally large (5-100m) 2. Nuclear region: well defined and surrounded by a nuclear membrane 3. More than one chromosome 4. ………………………………….. …………………………………. .……………………………….
Prokaryotic Cell	Eukaryotic Cell
1. Size: generally small (1-10m) 1rn = 10-6 m 2. Nuclear region: and known as … 3. Chromosome: single 4. Membrance-bound cell organelles are absent	1. Size: generally large (5-100m) 2. Nuclear region: well defined and surrounded by a nuclear membrane 3. More than one chromosome 4. Membrane bound cell organelles are present

 

Question 5. Can you name the two organelles we have studied that contain their own genetic material?

Answer.

Mitochondria and plastids have their own genetic material and ribosomes.

Question 6. If the organization of a cell is destroyed due to some physical or chemical influence, what will happen?

Answer.

In case of any physical or chemical injury to the cell, lysosomes become active. They burst and their digestive enzymes digest the whole cell. Ultimately the cell will die.

Question 7. Why are lysosomes known as suicide bags?

Answer.

Lysosomes are cell organelles which contain digestive enzymes capable of breaking down all organic materials. In case of any damage to the cell, lysosomes burst releasing the enzymes which digest the other cell organelles and finally the whole cell. So, they are called “suicide bags”.

Question 8. Where are proteins synthesized inside the cell?

Answer.

In a cell, ribosomes are the site of protein synthesis. Rough endoplasmic reticulum has ribosome particles attached to its surface. Ribosomes are also present in Mitochondria and Plastids, so these organelles. also have their own protein synthesis machinery . Thus site of protein synthesis in a cell are — RER, Mitochondria, Plastids and Nucleus.

 

Question 9. Make a comparison and write down ways in which plant cells are different from animal cells

Answer.

All living organisms are composed of cells which are the structural and functional units of life. Although the cells are similar in structure and function in all living organisms, plant cells are different from animals cells in some respects. A comparative study of plant and animal cell shows the presence of plasma membrane, cytoplasm with cell organelles like endoplasmic reticulum, golgi apparatus, lysosomes, vacuoles, mitochondria etc. Nucleus with nuclear membrane and chromatin material is also seen.

Major points of differences between plant and animal cells are following:

Plant cell	Animal cell
(i) A rigid and protective cell wall of cellulose present outside the plasma membrane. Cell wall provides a definite shape to the plant cell	Cell wall is a absent.
(ii) Cytoplasm has plastids which contain green coloured pigment called chlorophyll	Animal cell lack plastids.
(iii) Large sized vacuoles are present which provide rigidity and turgidity to the cell.	Small vacuoles are present.
(iv) Cenctriole I absent.	Centriole is present.

Question 10. How is a prokaryotic cell different from an eukaryotic cell?

Answer.

On the basis of presence or absence of a well organised nucleus, cells are of two types-Prokaryotic and Eukaryotic.

Differences between prokaryotic and eukaryotic cell are as follows:

 

Prokaryotic cell	Eukaryotic cell
(i) Lacks well organised nucleus surrounded by nuclear membrane. Such a nucleus is called nucleoid. (ii) Membrane-bound cell organelles is absent. (iii) Single chromosome present. (iv) Small in size	(i) Well organized nucleus surrounded by nuclear membrane is present, (ii) Membrane- bound cell organe­lles is present, (iii) More than one chromosome present. (iv) Large in size.

Question 11. What would happen if the plasma membrane ruptures or breaks down?

Answer.

Presence of plasma membrane is so crucial for a cell that we can’t even imagine the existence a cell without it. In case of any damage to the plasma membrane, the cellular metabolism get disturbed. Lysosomes become active and they burst. Their digestive enzymes digest the other cell organelles and finally the whole cell i.e., the cell suicides.

Question 12. What would happen to the life of a cell if there was no golgi apparatus?

Answer.

In the absence of golgi apparatus the transport of proteins and lipids synthesized near rough and smooth endoplasmic reticulum inside and outside the cell will stop. The essential materials synthesized inside the cell will not reach to the target where they are required. So the various activities carried out by different parts of the cell will be affected. Other functions of the cell like storage and packaging of products by vesicles and formation of complex sugars will also affected. Lysosomes would not be produced and bacteria could attack and destroy the cell as there will be no digestive enzymes of lysosomes to break them.

Question 13. Which organelle is known as the power house of the cell? Why?

Answer.

Mitochondria is also known as the power house of the cell because here energy rich compound ATP or adenosine triphosphate is produced which is required by body to carry out various metabolic activities.

Question 14. Where do the lipids and proteins constituting the cell membrane get synthesised?

Answer.

The lipids and proteins constituting the cell membrane get synthesised in endoplasmic reticulum present in cytoplasm. Rough endoplasmic reticulum containing ribosomes synthesise proteins while smooth endoplasmic reticulum synthesise lipids. The process of building cell membrane with those lipids and proteins is called ‘Membrane Biogenesis.

Question 15. How does an Amoeba obtain its food?

Answer.

The flexible nature of cell membrane helps unicellular Amoeba to engulf food by the process called endocytosis. Whenever, Amoeba comes in contact with a food particle, its cell membrane along with cytoplasm grows in that direction. Finally it surrounds the food and make it a part of its cytoplasm.

Fig.-Intake of food by Amoeba (Endocytosis)

Question 16. What is osmosis?

Answer.

The movement of water through a selectively permeable membrane from a region of high water concentration to a region of low water concentration is called osmosis. It is a kind of diffusion, eg. unicellular fresh water organisms and plant roots absorb water by osmosis. Entry of water in a cell is called endosmosis, movement of water outside the cell is called exosmosis.

Question 17. Carry out the following osmosis experiment. Take four peeled potato halves and scoops 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.

Answer (i).

In cups B and C the concentration of solution becomes more (due to sugar and salt) than that of the outer portion which remains in contact with water. So, there a endosmosis i.e., the water will move inside the hollow portion,

(ii) Why is potato A necessary for this experiment?

Answer (ii).

To show that when the concentration a same on both sides, there is no movement of water.

(iii) Explain why water does not gather in the hollowed out portions of A and D.

Answer (iii).

The water does not gather in cup A due to the same concentration of solution. The water does not gather in cup D bee; boiling kills the cells and dead cells are unable to absorb water by osmosis.

 

Question 18. Why is the plasma membrane called a selectively permeable membrane?

Answer.

Plasma membrane permits the entry and exit of some materials in and out of the cell. It also prevents movement of some other materials. There for, the plasma membrane is called selectively permeable membrane.