Question

In majority of plants, guard cells are
a. Dumb-bell shaped
b. Reniform
c. Rounded
d. Polygonal

Answer 1

Guard cells are plant cells which are present on the epidermis of the leaves, stem, and other parts of the plant. They are called guard cells because they guard each stoma and function its opening and closing along with gas exchange. They also play an important role in photosynthesis.

Complete answer:
Guard cells are specialized bean shaped cells present on the epidermis of leaves, stems, and other parts of the plant. They are present in pairs surrounding each stoma. They play an important role in gaseous exchange across the plant. They help in opening and closing the stomata. When stomata are filled with water, it is open and the guard cells are turgid and when water level is low in stomata, it is closed, and guard cells become flaccid. Light also triggers the opening and closing of stomata. Guard cells affect the photosynthesis by allowing the entry of materials necessary.

Cuticle is relatively thicker on guard cells and is also permeable to water. Guard cells have a protein called phototropin which trigger responses such as phototropism, chloroplast movement, and leaf expansion. In developing guard cells, pectin and cellulose are concentrated in the plasmodesmata, it disappears as the guard cells mature. Many organelles are also found in guard cells, such as, endoplasmic reticulum, microtubules, lipids, nuclei, plastids, and mitochondria.

Guard cells are also affected by various factors such as, temperature, humidity, light, carbon dioxide and potassium ions.

Hence, the correct answer is option (a).

Note: Dumb-bell shaped guard cells are present in monocots. They are considered to be more advanced and more efficient physiologically. They are mostly present in grasses, wheat, and other monocots because it requires fewer solutes and water.