Question

a. In sequence, name the structures and tissue systems through which a molecule of water passes from the time it is absorbed from the soil until it evaporates through a stoma. b. Explain how a guard cell regulates the opening and closing of stoma, describing the advantages and disadvantages of this regulatory system.

Answer 1

a.

Water is absorbed by roots from the soil and transported as a liquid to the leaves via xylem. In the leaves, small pores allow water to escape as a vapor. Of all the water absorbed by plants, less than 5% remains in the plant for growth.Xylem is the specialised tissue of vascular plants that transports water and nutrients from the plant–soil interface to stems and leaves, and provides mechanical support and storage. The water‐conducting function of xylem is one of the major distinguishing features of vascular plants.

The most distinctive xylem cells are the long tracheary elements that transport water. Tracheids and vessel elements are distinguished by their shape; vessel elements are shorter, and are connected together into long tubes that are called vessels. Xylem also contains two other cell types: parenchyma and fibers. It includes three different cell types: vessel elements and tracheids (both of which conduct water) and xylem parenchyma.

b.

The opening and closing of stomata are controlled by the guard cells. When water flows into the guard cells, they swell up and the curved surface causes the stomata to open. When the guard cells lose water, they shrink and become flaccid and straight thus closing the stomata.Guard cells are able to control how open or closed stomata are by changing shape. They are like an inflatable set of doors that make the opening between the two cells wider or narrower. The guard cells change shape depending on the amount of water and potassium ions present in the cells themselves.

Guard cells are located in the leaf epidermis and pairs of guard cells surround and form stomatal pores, which regulate CO₂ influx from the atmosphere into the leaves for photosynthetic carbon fixation. Stomatal guard cells also regulate water loss of plants via transpiration to the atmosphere.Guard cells are cells surrounding each stoma. They help to regulate the rate of transpiration by opening and closing the stomata.

The guard cells contain chloroplasts, so they can manufacture food by photosynthesis (The epidermal cells do not contain chloroplasts) Guard cells are the only epidermal cells that can make sugar. According to one theory, in sunlight the concentration of potassium ions (K+) increases in the guard cells.Between each pair of guard cells is a stoma (a pore) through which water and gases are exchanged. The opening and closing of these pores (collectively known as stomata) is made possible by the thickening and shrinking of guard cells on the epidermis.

Advatages

• Plants 'breathe' and 'sweat' through their stomata, which are regulated by guard cells.
• Guard cells have a very important job because by opening and closing stomata, they facilitate gas exchange for photosynthesis and help minimize water loss
• Guard cells are specialized plant cells in the epidermis of leaves, stems and other organs that are used to control gas exchange.
• They are produced in pairs with a gap between them that forms a stomatal pore.

Disadvatage.

• The downside to wide open stomata is increased water loss
• Excessive water loss through transpiration can result in plant injury or death.
• The water will be able to be stored for later use in the plant so carbon dioxide will also not be able to be released. This builds up carbon dioxide in the plant.

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