NCERT Class 10 Science — Chapter 1: Life Processes

mcqmedium

Assertion (A): The opening and closing of the stomatal pore is a function of the guard cells. Reason (R): The guard cells swell when water flows out of them, causing the stomatal pore to open.

Official previous-year question

Verified 1 Aug 2026.

Options

ABoth A and R are true and R is the correct explanation of A
BBoth A and R are true but R is NOT the correct explanation of A
CA is true but R is false
DA is false but R is true

Did you get this right?

Solution

Correct Option

The correct option is

A is true but R is false

Explanation

Stomata are minute pores found primarily on the epidermis of leaves. They facilitate gaseous exchange for photosynthesis and respiration, as well as transpiration. The opening and closing of these pores are regulated by specialized kidney-shaped or dumbbell-shaped cells known as guard cells.

Statement-wise Analysis

Assertion (A): Correct
The primary function of the guard cells is to regulate the opening and closing of the stomatal pore. By changing their shape, guard cells control the size of the aperture, thereby managing gas exchange and water loss.
Reason (R): Incorrect
The mechanism of stomatal opening relies on turgor pressure. When water flows into the guard cells (endosmosis), they swell and become turgid. Due to the specific orientation of microfibrils and the thicker inner walls, this swelling causes the guard cells to bow outward, pulling the stomatal pore open. Conversely, when water flows out of the guard cells (exosmosis), they lose turgidity, become flaccid, and the pore closes. The reason statement incorrectly suggests that guard cells swell when water flows out.

Key Takeaway

Stomatal opening occurs when guard cells become turgid due to the entry of water. Stomatal closing occurs when guard cells become flaccid due to the loss of water.

Your note

More from Chapter 1: Life Processes

Which enzyme, present in saliva and pancreatic juice, is primarily responsible for the breakdown of starch into simpler sugars?

easy

The accumulation of which compound in muscle cells leads to cramps during sudden physical activity or anaerobic respiration?

easy

Which hair-like structures lining the upper respiratory tract help in filtering air by trapping dust and particles?

easy

In the circulatory system of fishes, the heart is characteristically composed of how many chambers?

easy

With reference to maintenance functions in organisms, consider the following statements: 1. The transfer of energy source from outside to inside is referred to as respiration. 2. These processes require energy sourced from outside the body. 3. All organisms obtain energy directly from sunlight. Which of the statements given above is/are correct?

medium

Keep practising Chapter 1: Life Processes

Work through every question from this NCERT chapter.

Browse all NCERT Class 10 Science chapters

Correct Option

The correct option is

A is true but R is false

Explanation

Statement-wise Analysis

Assertion (A): Correct
The primary function of the guard cells is to regulate the opening and closing of the stomatal pore. By changing their shape, guard cells control the size of the aperture, thereby managing gas exchange and water loss.
Reason (R): Incorrect
The mechanism of stomatal opening relies on turgor pressure. When water flows into the guard cells (endosmosis), they swell and become turgid. Due to the specific orientation of microfibrils and the thicker inner walls, this swelling causes the guard cells to bow outward, pulling the stomatal pore open. Conversely, when water flows out of the guard cells (exosmosis), they lose turgidity, become flaccid, and the pore closes. The reason statement incorrectly suggests that guard cells swell when water flows out.

Key Takeaway

Stomatal opening occurs when guard cells become turgid due to the entry of water. Stomatal closing occurs when guard cells become flaccid due to the loss of water.