NCERT Solutions of Science for Class 9 Lesson-5 “The Fundamental Unit Of Life”


Q.1- Who discovered cell and how?

Ans- Robert Hooke (1665) discovered cell for the first time. He examined a thin slice of cork under the microscope. He observed that cork resembled the structure of a honey comb consisting of many compartments and called these boxes ‘cells’.

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

Ans- Cells are arranged in proper and systematic order to form a body of living organisms. Thus cell serves as basic building structural units for more complex organisms. Cell is also functional unit of life because all the metabolic activities of like take place at cell level.


Q.1- How do substances like Carbon dioxide and water move in and out of cell? Discuss.

Ans- The substances like carbon dioxide and water move in and out of cell by the process of diffusion. There is spontaneous movement of substances from higher concentration to lower concentration. The movement of water across selectively permeable membrane is called osmosis.

Q.2- Why is plasma membrane called selectively permeable membrane?

Ans- Plasma membrane is called selectively permeable membrane because it permits the entry and exit of certain materials in and out of the cell.


Q.1- Fill in the gaps in the following table illustrating differences between prokaryotic and eukaryotic cells.


Q.1- Can you name two organalles we have studied that contain their own genetic material?

Ans- Chloroplasts, Mitochondria.

Q.2- If the organization of cell is destroyed due to some physical or chemical influence, what will happen?

Ans- Cell will stop working and it results in its death. The lysosomes will burst to release enzymes. These enzymes digest their own cell.

Q.3- Why lysosomes are called ‘suicidal bags’ ?

Ans- Lysosomes contain digestive enzymes. When a cell stops working or get damaged, the lysosomes carry out autolysis (self breakdown) to release enzymes. These enzymes digest their own cell. That’s why lysosomes are called suicidal bags.

Q.4- Where are proteins synthesised in the cell?

Ans- The proteins are synthesised in the ribosomes which are attached to the surface of RER ( rough endoplasmic reticulum). Ribosomes are also called protein factory of a cell.


Q.1- Make a comparison and write down ways in which plant cells are different from animal cells.

Q.2- How is prokaryotic cell different from a eukaryotic cell?

Q.3- What would happen if a plasma membrane ruptures or break down?

Ans- If plasma membrane burst, the materials will move freely in and out of the cell. Osmosis or diffusion will not take place and the cell will die.

Q.4- What would happen to the life of a cell if there was no Golgi apparatus?

Ans- The Golgi apparatus performs the function of storage, modification and packaging of materials synthesised in the cell. These functions will not be performed in the absence of Golgi apparatus. Golgi apparatus is also involved in the synthesis of lysosomes. If there was no Golgi apparatus, the lysosomes will not be synthesised.

Q.5- Which organelle is known as power house of the cell? Why?

Ans- Mitochondria is known as the power house of the cell. They are site for synthesis, storage and transport of energy in the form of ATP.

Q.6- How does an amoeba obtain its food?

Ans- Amoeba is a unicellular organism. It’s plasma membrane is flexible and it enables the cell to engulf food and other materials from external environment. This process is known as endocytosis.


Q.7- What is osmosis?

Ans- The movement of water molecules from the region of higher concentration to the region of lower concentration through a selectively permeable membrane is called osmosis.

Q.8- Carry out the following osmosis experiments.

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 potat. 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 cup A necessary for this experiment?

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

Ans- (i) The water gathers in the hollowed portion of potato cups B and C due the difference between the concentration of water in trough and water in the potato cups. Hence the process of osmosis takes place.

(ii) Potato cup A is necessary for the experiment because it acts as control.

(iii) Cup A does not contain hypertonic solution hence water does not gather in cup A.

In cup D, cells of potato cup being boiled are dead and no osmosis occurs.