Problem 12 - Biology Concepts and Applications

Question

Stomata open in response to light when a. ions flow into guard cell cytoplasm b. ions flow out of guard cell cytoplasm c. water evaporates out of guard cells

Step-by-Step Solution

Answer

Stomata open when ions flow into guard cell cytoplasm, making them turgid, resulting in stomatal opening.

Step 1: Understanding Stomata Function

Stomata are tiny openings on plant leaves that regulate gas exchange. The opening and closing of stomata are influenced by water and ion movement in the guard cells surrounding the stomata.

Step 2: Identifying Ion Movement

In response to light, specific ions, such as potassium ( K^+ ), move into the guard cells. This ion influx increases the osmotic pressure inside the cells.

Step 3: Water Movement into Guard Cells

The increased ion concentration lowers the water potential inside the guard cells, prompting water to move into these cells by osmosis, making them turgid.

Step 4: Effect on Stomatal Opening

The influx of water increases the turgor pressure in guard cells, causing them to swell. This swelling changes the shape of the guard cells in such a way that the stoma opens.

Key Concepts

Guard Cells

Guard cells are a pair of specialized cells that flank each stoma on plant leaves. They play a critical role in controlling the opening and closing of the stomatal pore. This process is essential for regulating gas exchange; specifically, allowing carbon dioxide to enter for photosynthesis and letting oxygen and water vapor exit.

The opening and closing mechanism is largely driven by changes in the shape and volume of the guard cells in response to environmental stimuli, like light. When guard cells take in water, they become turgid, and the stoma opens; when they lose water, they become flaccid, closing the stoma.

Ion Movement

Ion movement is a crucial first step in the process of stomatal opening. Potassium ions ( K^+ ) are particularly important. During daylight, in response to sunlight, these ions actively move into the guard cells. This happens through ion channels in the cell membrane.

As K^+ ions accumulate inside the guard cells, the internal osmotic potential of the cells changes significantly. This shift is essential for creating the environment needed for the next important process—osmosis. This ion movement is not just limited to potassium; other ions can participate, but potassium plays the key role in initiating the sequence of events that open stomata.

Osmosis

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 the context of guard cells, as potassium ions increase inside these cells, the water potential decreases. Consequently, water rushes into the guard cells from the surrounding tissues.

This influx of water occurs passively and is driven by the water potential gradient created by the active uptake of ions. The resulting water movement makes the guard cells swell, which is crucial for the stomatal opening.

Turgor Pressure

Turgor pressure is the pressure exerted by the cell contents against the cell wall. In guard cells, increased turgor pressure is a direct outcome of water influx due to osmosis. Imagine inflating a balloon; the guard cells swell up similarly as they fill with water.

This pressure is key to altering the shape of the guard cells. As turgor pressure rises, the guard cells become more curved and rigid, causing the stoma to open. Thus, without adequate turgor pressure, guard cells cannot achieve the shape needed to maintain open stomata. Managing this pressure effectively allows plants to control gas exchange critical for survival.