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

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Here we are providing assertion reason questions for class 9 science. This article covers assertion reason questions based on Class 9 Science Chapter 5 The Fundamental Unit of Life. To check the answer, click on ‘Answer’ given below each question. After clicking it will expand.

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

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

10 Questions · With Answer & Explanation

A-R

The Fundamental Unit of Life

Select the correct option (a, b, c, or d) for each statement pair

Ch 5 · Biology

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

a Both A and R are true, and R explains A

b Both A and R are true, but R does not explain A

c A is true, but R is false

d A is false, but R is true

Correct Answer (a) Both A and R are true, and R explains A ✅

Explanation

If a cell is placed in a hypotonic solution (which has a lower concentration of solute and a higher concentration of water compared to the cell sap), the water molecules move from the external solution into the cell sap via endosmosis, causing the cell to swell.

Assertion (A): The endoplasmic reticulum which lacks ribosomes is called smooth endoplasmic reticulum.
Reason (R): SER is mainly involved in protein synthesis.

a Both A and R are true, and R explains A

b Both A and R are true, but R does not explain A

c A is true, but R is false

d A is false, but R is true

Correct Answer (c) A is true, but R is false ✅

Explanation

Smooth Endoplasmic Reticulum (SER) possesses smooth membranes which do not bear ribosomes. The reason is false because SER is responsible for the synthesis of fats and lipids, not proteins. (Protein synthesis is the job of the Rough Endoplasmic Reticulum).

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

a Both A and R are true, and R explains A

b Both A and R are true, but R does not explain A

c A is true, but R is false

d A is false, but R is true

Correct Answer (c) A is true, but R is false ✅

Explanation

Both mitochondria and chloroplasts are semi-autonomous cell organelles because they possess their own DNA and arise from pre-existing organelles. However, the reason is false because they do not lack protein-synthesizing machinery; they have their own ribosomes (70S type) which help them synthesize some of their own proteins.

Assertion (A): Plasma membrane is selectively permeable.
Reason (R): Plasma membrane allows some molecules to pass through it more easily than others.

a Both A and R are true, and R explains A

b Both A and R are true, but R does not explain A

c A is true, but R is false

d A is false, but R is true

Correct Answer (a) Both A and R are true, and R explains A ✅

Explanation

Each cell is bound by an extremely delicate, thin, elastic, and living membrane called the plasma membrane. It is termed “selectively permeable” precisely because it regulates entry and exit, allowing some molecules to pass through more easily than others.

Assertion (A): Leucoplasts perform photosynthesis.
Reason (R): Chloroplasts store fats, starch and proteins.

a Both A and R are true, and R explains A

b Both A and R are true, but R does not explain A

c A is true, but R is false

d A is false, but R is true

Correct Answer (d) A is false, but R is true ✅

Explanation

The assertion is false because chloroplasts (not leucoplasts) contain chlorophyll and perform photosynthesis. Leucoplasts are actually the colorless storage plastids that store fats, starch, and proteins.

Assertion (A): Cell wall is a non-living part of the cell.
Reason (R): It offers protection, definite shape and support.

a Both A and R are true, and R explains A

b Both A and R are true, but R does not explain A

c A is true, but R is false

d A is false, but R is true

Correct Answer (b) Both A and R are true, but R does not explain A ✅

Explanation

Both statements are true. The cell wall is a non-living, rigid, and supportive covering outside the plasma membrane in plant cells, primarily composed of cellulose. It does offer protection and shape, but this function is not the reason it is non-living.

Assertion (A): A cell membrane shows fluid behaviour.
Reason (R): A membrane is a mosaic of lipids and proteins.

a Both A and R are true, and R explains A

b Both A and R are true, but R does not explain A

c A is true, but R is false

d A is false, but R is true

Correct Answer (a) Both A and R are true, and R explains A ✅

Explanation

A cell membrane shows fluid behavior as proved by the fluid-mosaic model (Singer and Nicolson, 1972). According to this model, the membrane is not a solid, uniform structure but a quasi-fluid mosaic of lipids and proteins, which explains its flexible and fluid nature.

Assertion (A): A plant cell bursts if placed in water.
Reason (R): High turgor pressure causes bursting of plant cells.

a Both A and R are true, and R explains A

b Both A and R are true, but R does not explain A

c A is true, but R is false

d A is false, but R is true

Correct Answer (d) A is false, but R is true ✅

Explanation

The assertion is false. Unlike animal cells, plant cells have a rigid cell wall. When placed in water, they swell and develop high turgor pressure, but the cell wall exerts an equal and opposite wall pressure that stops the entry of excess water, thus preventing the plant cell from bursting.

Assertion (A): Mitochondria are called ‘powerhouses’ of the cell.
Reason (R): Mitochondria produce cellular energy in the form of ATP.

a Both A and R are true, and R explains A

b Both A and R are true, but R does not explain A

c A is true, but R is false

d A is false, but R is true

Correct Answer (a) Both A and R are true, and R explains A ✅

Explanation

Mitochondria are the site of aerobic respiration. They are rightly called the powerhouses of the cell because they are the major centers for the release of energy, converting it into a usable form for the cell known as ATP (Adenosine Triphosphate).

Assertion (A): Plant cells have very large vacuoles.
Reason (R): In plant cells, vacuoles are full of cell sap.

a Both A and R are true, and R explains A

b Both A and R are true, but R does not explain A

c A is true, but R is false

d A is false, but R is true

Correct Answer (b) Both A and R are true, but R does not explain A ✅

Explanation

Both statements are true. Mature plant cells typically have a very large central vacuole that can occupy 50-90% of the cell volume, and this vacuole is indeed full of cell sap which helps maintain turgidity and rigidity. However, the fact that they are full of cell sap is a characteristic, not the evolutionary or physiological reason why they are large.

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