science: May 2016

Chapter-The Fundamental unit of life- Cell

Science (Bio) Class-9

1. Who discovered cells and how?

Ans- Robert Hooke discovered cells. He observed them while examining a thin slice of cork under his self made crude microscope as hollow compartments bounded by wall.

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

Ans- a. Cells are structural unit of life because all living organism are formed of one or more cells.

b. Cells are functional unit of life because each cell performs all the life activities like nutrition, respiration, excretion and multiplication.

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

Ans- a. Carbon dioxide moves across the cell membrane by diffusion. When CO₂ concentration in the cell increases in comparison to the CO₂ concentration outside the cell there is spontaneous movement of CO₂ from a region of higher concentration to a region of its low concentration.

b. Water molecules move through selectively permeable plasma membrane by osmosis. Dissolved substances present in water affect the movement of water across plasma membrane.

4. Why the plasma membrane is is called a selectively permeable membrane?

Ans-Plasma membrane is called selectively permeable membrane because it allows only selected substances to pass through it and at the same time prevents the movement of some other materials.

5. Differences between prokaryotic and eukaryotic cells.

s.no.	Prokaryotic cell	Eukaryotic cell
1. 2. 3. 4. 5. 6.	Size: Generally small(1-10m)=10^-6m Nuclear region: Poorly defined; due to absence of membrane the nuclear region is known as nucleoid. Chromosomes: Single Membrane – bound cell organelles absent. Nucleolus is absent. Cell division takes place by fission or budding (no mitosis).	1.Size: Generally large (5-100m) 2. Nuclear region: Well – defined and surrounded by a nuclear membrane. 3. Chromosomes: More than one. 4. Membrane – bound cell organelles, such as mitochondria, lysosomes, endoplasmic reticulum, Golgi complex, etc. are present. 5. Nucleolus is present. 6. Cell division occurs by mitotic or meiotic cell division.

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

Ans- a. Mitochondria b. Plastids.

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

Ans- A well organized cell maintains homeostasis, i.e., constant internal chemical composition. It is, therefore, able to perform basic functions such as respiration, obtaining nutrition, clearing of waste, forming new proteins, etc. If the organization of a cell is destroyed, it will not be able to maintain homeostasis and thus will not be able to perform above said basic functions and such a cell will ultimately die.

8. Why are lysosomes known as ‘suicide bags’?

Ans- Lysosomes are known as ‘suicide bags ‘of the cell because their digestive enzymes can digest any cell organelle or the whole cell when the situation so demands.

9. Where are proteins synthesized inside the cell?

Ans- The ribosome attached to endoplasmic reticulum are the site for protein synthesis. They are called protein factories of the cell.

Chapter End Exercises

1. Make a comparison and write down the ways in which plants cells are different from animal cells.

s.no.	Animal cell	Plant cell
1. 2. 3. 4. 5. 6.	Animal cells are generally small in size. Cell wall is absent. Except the protozoan Euglena, no animal cell possesses plastids. Vacuoles in animal cells are many, small and temporary. Animal cell have a single highly complex and prominent Golgi apparatus. Animal cells have centrosome and centrioles.	1. Plant cell are larger than animal cells. 2. The plasma membrane of plant cell is surrounded by a rigid cell wall of cellulose. 3. Plastids are present. 4. Most mature plant cells have a permanent and large central sap vacuole. 5. Plant cells have many simpler units of Golgi apparatus, called dictyosomes. 6. Plant cells lack centrosome and centrioles.

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

Ans- If the plasma membrane ruptures or breaks down, the cell will loose its constant internal environment due to uncontrolled movement of substances in and out of the cell. Hence, it will not perform its basic functions properly or may die.

3. What would happen to the life of a cell if there were no Golgi apparatus?

Ans- In the absence of Golgi apparatus, the following functions will not take place:

a) Lysosomes will not be formed and no intracellular digestion will take place:

b) Cell secretion, formation or repair of cell wall will not take place.

c) Formation of polysaccharides and other complex molecules will not occur.

d) The acrosome in sperm will not form.

4. Which organelle is known as the ‘powerhouse of the cell’ and why?

Ans-Mitochondrion is known as the powerhouse of the cell, because mitochondrion is the site of cellular respiration where energy in the form of ATP (adenosine triphosphate) is generated as a result of oxidation of food substances.

5. Where do the lipids and proteins constituting the cell membrane get synthesized?

Ans-Protein is synthesized in ribosomes of rough ER (RER) and lipids are synthesized over smooth ER (SER).

6. How does an Amoeba obtain its food?

Ans- Amoeba is unicellular animal. It acquires its food by the process of endocytosis. Plasma membrane of Amoeba is flexible with its help Amoeba engulfs food particles. The engulfed food particles pass into the body of organism as a phagosome. Phagosome combines with lysosomes to produce digestive or food vacuole digestion occurs in food vacuole. The digested food passes into surrounding cytoplasm. The undigested matter is thrown out of the cell.

7. What is osmosis?

Ans- Osmosis is diffusion of water from the region of its higher concentration (pure water or dilute solution) to the region of its lower concentration (strong solution) through a semi permeable membrane.

8. Carry out the following osmosis experiments:

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 into 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 2 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 hollow out portion of A and D?

Ans- i) When unboiled potato cups B and C were put into a trough containing water, the cells of potatoes gained water by endosmosis. When a teaspoon of sugar and salt were added later in the hollowed portion of B and C cups respectively, water movement occurred through the plasma membranes of the cells from within the cell of potato into the hollowed portion of both B and C cups because of exosmosis (i.e., exit of water molecules through permeable plasma membrane from within the cells of unboiled potatoes into the hollowed portions having hypertonic sugar and salt, respectively).

ii) Potato cup A is necessary in the experiments as a ‘control’ for providing comparison with situations created in potato cups B,C and D. it indicates that the potato cavity alone does not induce movement of water.

iii) Water does not gather in the hollowed portion of potato cup A because it does not possess highly osmotic concentration than the cells of potato.

Potato cup D is boiled potato cup. On boiling, potato cells die and the membranes of the potato cells lost their permeability. As a result, when teaspoon of sugar is added into the hollowed person of boiled potato cup D, water does not come out from within the potato cells into the hollowed portion.