Question

What is the function of guard cells?

Answer 1

Hint: The respiration process helps sustain life by enabling an organism to take up oxygen and eliminate carbon dioxide and assists in the oxidation of compounds to obtain energy. Both plants and animals conduct respiration processes, but plants lack specialized organs for gaseous exchange, unlike animals. In plants, stomata and lenticels perform this essential function.

Complete answer:
A stomatal apparatus contains stomatal opening, guard cells with or without surrounding subsidiary cells. Their distribution and shape vary widely among plants according to their physiology and environmental condition. Stomata (singular stoma) are defined as the tiny openings or pores present in the epidermis of plant leaves, young stems, and other organs. This microscopic aperture is bordered by a pair of parenchyma cells called guard cells.
In the case of photoactive stomata, during the daytime, the sucrose present in guard cells produces six-carbon sugars or hexoses, and hexoses are converted to form phosphoenolpyruvate. This three-carbon sugar is carboxylated to form oxaloacetic acid and then converted into malic acid. Malic acid dissociates to form malate ions and hydrogen ions. These hydrogen ions, along with potassium ions, are actively transported into the guard cells and result in increased osmotic concentration in guard cells compared to adjacent subsidiary cells. So, water moves into the guard cells, and it gets turgid. Due to turgor pressure, the outer cell wall bulges out, and the inner cell wall becomes crescent-shaped, and thus stoma opens.

Note:
Guard cells and surrounding subsidiary cells regulate the opening and closing of stomata. This stomatal regulation is achieved by mobilization of sucrose and reversible osmolarity change in the guard cells. Based on the shape of guard cells, the stomatal apparatus varies. The dicotyledonous leaves contain bean-shaped guard cells and their stomatal apparatus comparatively more on the lower epidermis. In contrast, monocotyledons have dumbbell-shaped guard cells that have nearly equal distribution in their isobilateral leaf.