Problem 77 - Objective Biology for NEET

Question

Which of the following is true about guard cells? (a) Outer wall is thin (b) Inner wall (towards stomatal pore) is thick (c) Bean-shaped in dicots and dumb-bell-shaped in grasses (d) All of these

Step-by-Step Solution

Answer

The correct answer is (c). Guard cells are bean-shaped in dicots and dumb-bell-shaped in grasses.

Step 1: Understand a Guard cell

A guard cell is a specialized cell in the epidermis of leaves, stems and other organs that control the size of the stomata by changes in their shape. They help to regulate the rate of transpiration.

Step 2: Evaluate Option A

The statement that the outer wall of guard cells is thin is incorrect. The outer wall of guard cells is thicker compared to the inner wall in order to allow for changes in shape and opening/closing of the stomata.

Step 3: Evaluate Option B

The statement that the inner wall towards the stomatal pore is thick is incorrect. The inner wall of guard cells (towards the stomatal pore) is thinner, not thicker, that allows the cells to change shape effectively.

Step 4: Evaluate Option C

The statement that guard cells are bean-shaped in dicots and dumb-bell-shaped in grasses (monocots) is correct. This is the typical shape for guard cells in these groups of plants.

Step 5: Evaluate Option D

The statement that all of these are correct is false, since options A and B were both incorrect.

Key Concepts

Stomata

Stomata are tiny openings or pores found mainly on the underside of plant leaves, although they can also be present on stems and other plant organs. These pores are crucial for the plant as they facilitate gas exchange. Stomata allow carbon dioxide (\( CO_2 \)) to enter the leaf for photosynthesis and let oxygen escape. Additionally, they enable the release of water vapor, a process called transpiration. The opening and closing of each stoma are controlled by a pair of guard cells.

• Guard cells change shape to either open or close the stomata.
• When the guard cells swell with water, the stomata open, enabling greater exchange of gases and water vapor.
• When the guard cells lose water, they become flaccid and close the pore to conserve water.

Through these actions, guard cells play a critical role in a plant's ability to maintain homeostasis, thereby optimizing photosynthesis and limiting water loss.

Transpiration

Transpiration is the process by which moisture is carried through plants from roots to small pores on the underside of leaves, where it changes to vapor and is released into the atmosphere. This process is essential for maintaining nutrient uptake and cooling the plant.

• Transpiration helps in the upward movement of water and dissolved minerals from the roots to different parts of the plant.
• It also aids in the cooling of the plant through evaporative cooling, similar to how sweating cools the human body.

Guard cells have a direct influence on this process as they regulate the opening and closing of stomata. When the stomata are open, transpiration rates increase, allowing the plant to uptake more nutrients and maintain a steady flow of water. However, under dry or hot conditions, guard cells will close the stomata to reduce water loss.

Plant Anatomy

Plant anatomy deals with the study of the internal structure of plants. A key aspect of plant anatomy is understanding how various structures function together to maintain plant health and facilitate growth.

• The primary organs include roots, stems, and leaves, each with specialized cells and tissues.
• Leaves are composed of multiple layers, including the cuticle, epidermis, mesophyll, and vascular bundles.
• Special cells like guard cells are part of the epidermis and play a pivotal role in regulating gas exchange and water loss.

Guard cells, part of the complex plant anatomy, exhibit different shapes in different plant groups, like being bean-shaped in dicots and dumb-bell-shaped in grasses. Understanding these cellular structures is crucial in fields like agriculture and botany, as they help in developing strategies for enhancing crop productivity and resistance to environmental stresses.