Problem 123 - Objective Biology for NEET

Question

Stomata in angiosperms open and close due to (a) Their genetic constitution (b) Effect of hormones (c) Changes of turgor pressure in guard cells (d) Pressure of gases inside the leaves

Step-by-Step Solution

Answer

The correct answer is (c) Changes of turgor pressure in guard cells. These changes occur due to the water status of the cell caused by osmosis, leading to opening or closing of stomata.

Step 1: Eliminating non-relevant options

Begin by removing the options that don't directly impact the mechanism of stomata opening or closing. Option (a) 'Their genetic constitution', while related to the existence of stomata, does not directly cause stomata to open or close. Therefore, this option can be eliminated.

Step 2: Determining the correct option

Option (b) 'Effect of hormones' and (d) 'Pressure of gases inside the leaves' do play a part, but they are not direct reasons for stomata opening or closing. Hormones and internal gases provide signals for stomata to react, but the actual action is due to the change in turgor pressure within guard cells. Hence, option (c) 'Changes of turgor pressure in guard cells' is the correct answer. This process is regulated by water status of the cell which is a result of osmosis and it causes the guard cells to either swell or deflate, thus opening or closing the stomata respectively.

Key Concepts

Turgor Pressure in Guard Cells

The stomata are tiny pores on the surface of plants that play a crucial role in gas exchange and transpiration. The opening and closing of these stomata are largely determined by the turgor pressure within the guard cells, which flank each stoma.

Turgor pressure is the pressure exerted by the fluid contents of a cell against its cell wall. In guard cells, this pressure can increase or decrease due to changes in the cell's water volume. When guard cells take up water, they swell, causing the stomatal pore to open. Conversely, when they lose water, they deflate and the pore closes. This process is pivotal for plants to regulate the exchange of gases, like carbon dioxide for photosynthesis and oxygen for respiration, as well as manage water loss through transpiration.

Osmosis plays a key role here. It allows water to move from an area of lower solute concentration (the surrounding cells) to a higher concentration (the guard cells), thus causing the changes in cell volume and turgor pressure. In simple terms, when guard cells are 'full' or turgid, the stomata open, and when they are 'empty' or flaccid, the stomata close.

Plant Hormones

Plant hormones are chemical substances produced in one part of a plant and transported to others, where they help to coordinate growth, development, and responses to the environment. Regarding the stomata, certain hormones, particularly abscisic acid (ABA), play a significant role.

During times of water stress, ABA levels in plants increase. ABA then signals the guard cells that it's time to reduce the loss of water by closing the stomata. This hormone essentially acts as a messenger, informing the plant of the need to conserve water. On the other hand, auxins and cytokinins can promote stomatal opening under certain conditions.

It's critical to recognize that while these hormones influence stomatal behavior, they do not directly enact the opening or closing. Instead, they alter the internal conditions of the guard cells, which then affects the turgor pressure and leads to stomatal movements. This is a delicate balance, where plants have to regulate the levels of different hormones to respond appropriately to their environment.

Osmosis in Plant Cells

Osmosis is the movement of water across a semi-permeable membrane from an area of low solute concentration to an area of high solute concentration. In plants, osmosis is essential for maintaining cell turgidity, nutrient uptake, and overall plant vigor.

In guard cells, the process of osmosis allows these cells to swell or shrink by regulating the flow of water in and out. When the concentration of solutes (mainly potassium ions) inside the guard cells increases, water flows inwards, and the cells swell with the ensuing turgor pressure, thereby opening the stomata. To close the stomata, the process is reversed: solute concentration decreases within the guard cells, causing water to flow out and the cells to become flaccid.

The regulation of solute concentration, and thus osmosis, is tightly intertwined with the action of plant hormones such as ABA, which modulates the guard cells' water permeability and solute concentration. By understanding osmosis, students can grasp the vital mechanisms that underpin many of plant cells' functions, not just in guard cells but throughout the plant system.