CBSE Class 9 Science The Fundamental Unit of Life Assignment Set A

KEY CONCEPTS

1. All the living organisms are made up of fundamental unit of life called” cell”.

2. The cell is a Latin word for “a little room”.

3. The scientist Robert Hooke saw a little room in the cork (the bark of a tree) resembled the structure of a honeycomb. The use of the word “Cell” to describe these units is used till this day in Biology as” Cell Biology”.

4. The Compound Microscope consist eye piece,objective lens and condenser to observe a cell after putting a drop of Safranin (for plant cell) and methylene blue (for animal cell).

5. The scientist Leeuwenhoek saw free living cells in the pond water for the first time. ( father of microbiology)

6. The scientist Robert Brown discovered the nucleus in the cell.

7. The cell theory states that all the plants and animals are composed of cells, it was proposed by Schleiden and Schwann.

8. The cell theory was further expanded by Virchow by suggesting that “all cells arise from the pre-existing cells”.

9. The cells differ in size, shape, structure (Please refer to Fig. 5.2/5.3: Onion peel/Various cells in Human body, NCERT Book Page-57/58):Types of cells: Onion cells, Smooth muscle cell, Blood cells, Bone cell, Fat cell, Nerve cell, Ovum, Sperm etc. Each kind of cell performs specific function.

10. A single cell may constitute a whole organism as in Amoeba, Chlamydomonas, Paramecium and Bacteria; these are called as unicellular organisms. Whereas in multi-cellular organisms (Human beings) division of labor is seen.

11. The feature in almost every cell is same: Plasma membrane, nucleus and cytoplasm.

12. Plasma membrane: It is the outermost covering of the cell.

- It is called as selective permeable membrane (because it prevents 
movement of some materials).
- It helps in diffusion and osmosis
- Diffusion: movement of substance from high concentration to low concentration.
Eg; exchange of carbon dioxide or oxygen with external environment.osmosis: it is the passage of water from the region of high water concentration to a region of low water concentration through a selective permeable membrane.

a) The cell gains water, if the medium surrounding the cell has a higher water concentration (Hypotonic solution) than the cell.

b) The cell maintains the same water concentration as the cell (Isotonic solution),water crosses the cell membrane in both directions.

c)The cell loses water, if the medium has lower water concentration (Hypertonic solution) than the cell.

Note - The cell drinking is endosmosis

- omission of water is called ex-osmosis.

13. The cell engulfs food is called endocytosis and ejects solid is called exocytosis. 
Amoeba acquires food through endocytosis and excretion of solid is called exocytosis.

14. The cell wall is a rigid outer covering composed of cellulose. It provides structural strength to plant cells. When a living cell loses water, there is shrinkage of contents of a cell away from the cell wall. This phenomenon is called as plasmolysis. The cell walls permit the cells of plants, fungi and bacteria to withstand very dilute (Hypotonic) external media without bursting.

15. The Nucleus: It is a dark colored, spherical or oval, dot-like structure near the center of a cell called Nucleus. The nucleus plays a central role in cellular activities/reproduction. The chromatin material gets organized into chromosomes. The chromosomes contain information for inheritance of features from parents to next generations in the form of DNA( Deoxyribo Nucleic Acid ) and protein molecules. The functional segments of DNA are called genes.

16. In some organisms like Bacteria nucleus is not covered by nuclear membrane. Hence it is called as prokaryote. (Pro= primitive; karyote = karyon = nucleus.) The organisms with cells having a nuclear membrane are called eukaryotes.

17. Differences between prokaryotes and eukaryotes( Please refer to Fig. 5.4: Prokaryotic cell NCERT Book Page-62)

18. Cell organelles: Every cell has fluid matrix (other than nucleus) is called cytoplasm.
The nucleus and cytoplasm is together called as protoplasm. The protoplasm term was coined by Purkinje. It has important cell organelles: Endoplasmic Reticulum (ER), Golgi apparatus, Lysosomes, Mitochondria, Plastids, and vacuoles.

19. Endoplasmic Reticulum (ER): It is a large network of membrane –bound tubules and vesicles.

• There are two types of Endoplasmic Reticulum
• Rough Endoplasmic Reticulum (RER) (It looks rough because Ribosomes are
  attached to its surface. They are the sites of protein synthesis).
• Smooth Endoplasmic Reticulum (SER) (It looks smooth because Ribosomes
  are not attached to its surface. They are the sites of fat molecules synthesis).

1. SER ; help in the functioning of enzymes and hormones to carryout biochemical activities.
2. SER detoxifies many poisons and drugs from the cell.

2. ER serves as channel for the transport of material between various regions of the cytoplasm and the nucleus.
3. Proteins and fat molecules produced by ER helps in membrane biogenesis.

20. Golgi apparatus: It was first described by a scientist Camillo Golgi. It is a system of membrane bound vesicles called cisterns. It functions include the storage,
modification and package of cell products. The complex sugars are made from simple sugars in the Golgi apparatus. It is also involved in the formation of lysosomes.

21. Lysosomes: They contain membrane-bound sacs with powerful digestive enzymes (enzymes are made by RER) to digest the worn-out cell organelles. When the cell gets damaged, lysosomes may burst and the enzymes digest their own cell, hence called as “Suicidal bags of a cell”. It is a waste disposal system of the cell.

22. Mitochondria: It is covered by a double membrane. Outer membrane is very porous and the inner membrane is deeply folded. These folds create a large surface area for ATP (Adenosine Triphosphate) molecule synthesis. ATP is the energy currency of a cell; hence the Mitochondria are called as Power House of a Cell. Mitochondria have their own DNA and Ribosomes; therefore they can make their own proteins.

23. Plastids: They are present only in plant cells. They are two types.

1. Chromoplasts (Colored Plastids: Chloroplasts – Green pigmented and useful in Photosynthesis and also contains various other pigments like yellow or orange)

2. Leucoplasts (White or colorless plastids; stores materials such as oils,proteins, fats etc. ) Plastids are also covered by a double membrane.
The matrix is called Stroma, seat for enzymatic actions. Plastids have their own DNA and Ribosomes; therefore they can make their own proteins.

24. Vacuoles: Storage sacs for solid or liquid contents. They are small in size in animals while plants have large, may occupy 50-90 % of the cell volume. Helps to provide turgidity and rigidity to the cell. Many substances like amino acids, sugars, organic acids and proteins are stored in vacuoles. In Amoeba food vacuole is specialized to play an important role.

25. Cell: It is the fundamental structural unit of living organisms, helps in respiration, obtaining nutrition and clearing waste material or forming a new protein.

Differences between Plant cell and Animal Cell

Question. Who discovered cells and how?
Answer: An English Botanist, Robert Hooke discovered cells. In 1665, he used self-designed microscope to observe cells in a cork slice.

Question. Why cell is called the structural and functional unit of life?
Answer: Cells are called the structural and functional unit of life because all the living organisms are made up of cells and also all the functions taking place inside the body of organisms are performed by cells.

Question. How do substances like CO2 and water move in and out of the cell? Discuss.
Answer: The substances like CO2and water move in and out of a cell by diffusion from the region of high concentration to low concentration.
When the concentration of CO2and water is higher in external environment than that inside the cell, CO2and water moves inside the cell. When the concentration outside the cell becomes low and it is high inside the cell, they move out.

Question. Why is the plasma membrane called a selectively permeable membrane?
Answer: Plasma membrane called a selectively permeable membrane because it regulates the movement of substances in and out of the cell. This means that the plasma membrane allows the entry of only some substances and prevents the movement of some other materials.

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

Answer

Question. Can you name the two organelles we have studied that contain their own genetic material?
Answer: Mitochondria and plastids

Question. If the organization of a cell is destroyed due to some physical or chemical influence, what will happen?
Answer: If the organization of a cell is destroyed due to some physical or chemical influence then cell will not be able to perform the basic functions like respiration, nutrition, excretion etc. This may stop all the life activities and may result in its death.

Question. Why are lysosomes known as suicide bags?
Answer: Lysosomes are called suicide bags because in case of disturbance of their cellular metabolism they digest their own cell by releasing own enzymes.

Question. Where are proteins synthesized inside the cell?
Answer: The proteins are synthesized in the Ribosome inside the cell.

EXERCISE

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

Question. How is a prokaryotic cell different from a eukaryotic cell?
Answer:

Question. What would happen if the plasma membrane ruptures or breaks down?
Answer: If the plasma membrane ruptures or breakdown then the cell will not be able to exchange material from it’s surrounding by diffusion or osmosis. Thereafter the protoplasmic material will be disappeared and the cell will die.

Question. What would happen to the life of a cell if there was no Golgi apparatus?
Answer: Golgi apparatus has the function of storage modification and packaging of the products. If there is no Golgi apparatus then the packaging and transporting of materials synthesized by cell will not happen.

Question. Which organelle is known as the powerhouse of the cell? Why?
Answer: Mitochondria are known as the powerhouse of cells because energy required for various chemical activities needed for life is released by mitochondria in the form of ATP (Adenosine triphosphate) molecules.

Question. Where do the lipids and proteins constituting the cell membrane get synthesized?
Answer: Lipids are synthesized in Smooth endoplasmic reticulum (SER) and the proteins are synthesized in rough endoplasmic reticulum (RER).

Question. How does an Amoeba obtain its food?
Answer: 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.

Question. What is osmosis?
Answer: Osmosis is the process in which water molecules moves from the region of highconcentration to a region of low concentration through a semi permeable membrane.

Question. 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.
Answer: (i) Water gathers in the hollowed portions of set-up B and C because water enters the potato as a result of osmosis. Since the medium surrounding the cell has a higher water concentration than the cell, the water moves inside by osmosis. Hence, water gathers in the hollowed portions of the potato cup.
(ii) Potato A in the experiment acts as a control set-up. No water gathers in the hollowed portions of potato A.
(iii) Water does not gather in the hollowed portions of potato A because potato cup A is empty. It is a control set-up in the experiment.
Water is not able to enter potato D because the potato used here is boiled. Boiling denatures the proteins present in the cell membrane and thus, disrupts the cell membrane. For osmosis, a semipermeable membrane is required, which is disrupted in this case. Therefore, osmosis will not occur. Hence, water does not enter the boiled potato cup.

Exam Questions NCERT Class 9 Science Chapter 5 The Fundamental Unit of Life

Question. What is the function of ribosome?
Ans : Ribosome helps in protein synthesis.

Question. What is the function of cell wall and plasma membrane?
Ans : Cell Wall : Gives rigidity, shape and protection to plant cell.
Cell Membrane : Allows only selected materials to move in and out of the cell.

Question. Give the function of nuclear membrane.
Ans : The nuclear membrane present as outer covering of nucleus allows transfer of material inside and also out of the nucleus to cytoplasm.

Question. Write the functions of cell wall in plant cell.
Ans : (i) It provides rigidity and strength to the cell.
(ii) It gives a definite shape to the cell.
(iii) It withstands the osmotic pressure which is developed by cell contents.
(iv) It protects the inner cell organelles bounding the cell from outside.

Question. How many types of proteins are present in cell structure?
Ans : There are two types of protein molecules : intrinsic proteins, which completely covers the lipid bilayer and extrinsic proteins, which occur either on the outer surface or on the inner surface of the lipid membrane.

Question. What are vacuoles?
Ans : Vacuoles are fluid-filled structures surrounded by a membrane. The fluid in the vacuoles is called cell sap.
In animal cells, either they are absent or are very small in size.

Question. List any six functions of nucleus of a cell.
Ans :(i) Nucleus plays a central role in cellular reproduction.
(ii) It plays an important role in determining the way the cell will develop.
(iii) It also determines what form the cell will exhibit at maturity.
(iv) It contains chromosome thus inherits characters.
(v) It is the control centre of the cell.
(vi) It directs chemical activities of the cell.

Question. Name some organelles which are found only in animal cells and those which occur only in plant cells.
Ans : (i) Structures found only in animal cells : centrosomes, lysosome.
(ii) Structures found only in plant cells : cell wall, plastids and big vacuoles.

Question. (i) What will happen when eukaryotic cells are placed in hypotonic solution?
(ii) What will happen if eukaryotic cells are placed in hypertonic solution?
(iii) What will happen if eukaryotic cells are placed in isotonic solution?
Ans :
(i) When eukaryotic cells are placed in hypotonic solution, the water molecules will enter into the cell and the cell will swell up.
(ii) If eukaryotic cells are placed in hypertonic solution, the water molecules will come out of the cell and the cell will shrink.
(iii) If the eukaryotic cell is placed in isotonic solution, the amount of water molecule will remain the same; it will neither move out nor will go inside.
The cell will remain same sized.