Assertion and Reason Questions for Class 9 Science Chapter 5 The Fundamental Unit of Life

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Directions: In the following questions, a statement of assertion (A) is followed by a statement of reason (R). Mark the correct choice as:
(a) Both assertion (A) and reason (R) are true and reason (R) is the correct explanation of assertion (A).
(b) Both assertion (A) and reason (R) are true but reason (R) is not the correct explanation of assertion (A).
(c) Assertion (A) is true but reason (R) is false.
(d) Assertion (A) is false but reason (R) is true.

Q.1. Assertion : A cell swells up when present in a hypotonic solution.
Reason : More water molecules enter the cell than they leave.

Answer Answer: (a) Both assertion (A) and reason (R) are true and reason (R) is the correct explanation of assertion (A).
If a cell is placed in a hypo-tonic solution which has lower concentration of solute and higher concentration of water as compared to the concentration of cell sap (i.e., the solution inside the cell), the water molecules move from external solution into the cell sap and thus, the cell swells.

Q.2. Assertion : The endoplasmic reticulum which lacks ribosomes is called smooth endoplasmic reticulum
Reason : SER is mainly involved in protein synthesis.

Answer Answer: (c) Assertion (A) is true but reason (R) is false.
Smooth Endoplasmic Reticulum (SER) possesses smooth membranes which do not bear ribosomes. SER is responsible for synthesis of fats inside the cells of adipose tissue, formation of sphaerosomes, synthesis of glycogen as well as glycogenolysis (hydrolysis of glycogen) in liver cells, synthesis of sterols and steroid hormones as in the interstitial cells of testis and ovary and formation of visual pigments from vitamin A in retinal cells.

Q.3. Assertion : Mitochondria and chloroplasts are semiautonomous organelles.
Reason : They are formed by division of pre-existing organelles and contain DNA but lack protein synthesizing machinery.

Answer Answer: (c) Assertion (A) is true but reason (R) is false.
Both mitochondria and chloroplasts are double membrane bound, semi-autonomous cell organelles. Their structure and functions are partially controlled by nucleus of the cell and partially by themselves. Both possess their own DNA and arise from the pre-existing cells. 70S type of ribosome is present in both organelles which can help to translate the coded information contained in mRNA and protein synthesis.

Q.4. Assertion : Plasma membrane is selectively permeable.
Reason : Plasma membrane allows some molecules to pass through it more easily than others.

Answer Answer: (a) Both assertion (A) and reason (R) are true and reason (R) is the correct explanation of assertion (A).
Each cell is bound by an extremely delicate, thin, elastic, selectively permeable, living membrane called plasma membrane. It is selectively permeable as it allows some molecules to pass through more easily than others.

Q.5. Assertion : Leucoplasts perform photosynthesis.
Reason : Chloroplasts store fats, starch and proteins.

Answer Answer: (d) Assertion (A) is false but reason (R) is true.
Chloroplasts perform photosynthesis while leucoplasts are storage plastids.

Q.6. Assertion : Cell wall is a non-living part of the cell.
Reason : It offers protection, definite shape and support.

Answer Answer: (a) Both assertion (A) and reason (R) are true and reason (R) is the correct explanation of assertion (A).
Cell wall is a non-living part of the cell. It is an outer, rigid, protective, supportive and semi-transparent covering of plant cells only. The cell wall lies outside the plasma membrane. The cell wall is mainly composed of cellulose. It provides a definite shape to the cell. It protects plasma membrane and internal structures from the attack of pathogens and mechanical injury.

Q.7. Assertion : A cell membrane shows fluid behaviour.
Reason : A membrane is a mosaic of lipids and proteins.

Answer Answer: (a) Both assertion (A) and reason (R) are true and reason (R) is the correct explanation of assertion (A).
A cell membrane shows fluid behaviour. It was proved by fluid- mosaic model of a biomembrane by Singer and Nicolson in 1972. According to this model, the membrane does not have a uniform disposition of lipids and proteins but is a mosaic of the two. Further, the membrane is not solid but is quasi fluid.

Q.8. Assertion : A plant cell bursts if placed in water.
Reason : High turgor pressure causes bursting of plant cells.

Answer Answer: (d) Assertion (A) is false but reason (R) is true.
Plant cells have cell wall to counteract turgor pressure (T.P.) by exerting exactly equal and opposite wall pressure. Wall pressure stops entry of water into plant cells beyond a certain limit thus prevents their bursting.

Q.9. Assertion : Mitochondria are called ‘powerhouses’ of the cell.
Reason : Mitochondria produce cellular energy in the form of ATP.

Answer Answer: (a) Both assertion (A) and reason (R) are true and reason (R) is the correct explanation of assertion (A).
Mitochondria are cell organelles of aerobic eukaryotes. These are site of aerobic respiration, where Krebs cycle occurs in matrix, while ETS and oxidative phosphorylation enzymes are located in inner membrane. They are called powerhouses of cell because they produce energy in the form of ATP. They are the major centres of release of energy in the aerobic respiration.

Q.10. Assertion : Plant cells have very large vacuoles.
Reason : In plant cells, vacuoles are full of cell sap.

Answer Answer: (a) Both assertion (A) and reason (R) are true and reason (R) is the correct explanation of assertion (A).

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