9.1 Biology

9.1.9 State that guard cells can regulate transpiration by opening and closing stomata.

Guard cells can regulate transpiration by opening and closing stomata.

9.1.8 Explain how water is carried by the transpiration stream, including the structure of xylem vessels, transpiration pull, cohesion, adhesion and evaporation.

- Once water has been taken up by the roots it is pulled upward into the leaves where it then evaporates. - This flow of water from the roots to the leaves is called the transpiration stream. -This transpiration stream occurs in xylem vessels and the movement of water is passive. -Mature xylem vessels are long dead structures made up of cells arranged from end to end. - The cell walls between the adjacent xylem cells are broken down and the cytoplasmic content dies to form a continuous tube. The cells also lack a plasma membrane, which allows water to enter the vessels freely. In addition, they also contain pores in the outer cell walls that allow the movement of water out of the vessels and into the surrounding cells of leaves. - The outer cell walls contain thickenings that resemble spirals or rings impregnated with lignin, which makes the vessels strong and able to withstand low pressures. - Low pressure (suction) is created in the xylem vessels when water is pulled out of the transpiration stream via evaporation of water vapor from the spongy mesophyll cell walls in the leaves. - Heat from the environment is necessary as it provides the energy required for the evaporation of water. - The low pressure causes more water from the roots to be pulled upwards through the xylem tubes, this is called transpiration pull. - Transpiration pull works due to the cohesion of water molecules. - Hydrogen bonds form between the water molecules allowing the formation of columns of water, which are not easily broken by the low pressure. - In addition, adhesion also plays a role in maintaining transpiration pull. The water molecules adhere to the walls of the xylem vessels preventing the columns of water from breaking. - So to conclude, the structure of xylem vessels, transpiration pull, cohesion, adhesion and evaporation are all important in the carrying of water by the transpiration stream.

9.1.12 Outline four adaptations of xerophytes that help to reduce transpiration.

Any four of the following: - Small, thick leaves reduce water loss by decreasing the surface area of the leaves. - A reduced number of stomata decrease the number of openings through which water loss may occur. - Stomata are located in crypts or pits on the leaf surface. This causes higher humidity near the stomata. - A thickened, waxy cuticle reduces water loss by acting as an impenetrable barrier to water. - Hair-like cells on the leaf surface trap a layer of water vapor, thus maintaining a higher humidity near the stomata. - Many desert plants shed their leaves and/or become dormant in the driest months. - Cacti exist on water that the plant stores in fleshy, watery stems. Plants of this type are called succulents. This stored water is obtained in the rainy season. ¬ Xerophytes can use alternative photosynthetic processes. In Section 8.3 the most common form of photosynthesis was explained in detail. It is known as the C3 photosynthetic pathway. There are two alternative processes, called CAM photosynthesis and C4 photosynthesis. CAM plants close stomata during the day and incorporate carbon dioxide during the night. C4 plants have stomata that open during the day but take in carbon dioxide more rapidly than non-specialized plants.

9.1.11 Explain how the abiotic factors light, temperature, wind, humidity, soil water, and carbon dioxide affect the rate of transpiration in a typical terrestrial plant.

Light - The rate of transpiration is much greater when light is available as the stomata close in the dark. Speeds up transpiration by warming up with leaf and opening the stomata. Temperature - As temperature rises, so does the rate of transpiration. This is because heat is vital for the evaporation of water vapor from the cell walls of spongy mesophyll cells. A rise in temperature leads to an increase in the evaporation rate thereby increasing transpiration rate. Higher temperatures also increase the rate of diffusion between air spaces inside the leaf and the air outside. Finally, an increase in temperature causes a reduction in humidity in the air outside the leaf, which causes an increase in concentration gradient and therefore an increase in transpiration rate. Wind - Wind increases the transpiration rate by removing the humidity around the leaf produced by transpiration. Humidity - Water diffuses out of the leaf, down its concentration gradient, from a high concentration gradient inside the leaf to a lower concentration gradient in the air. The lower concentration gradient in the air is vital for transpiration. Humidity is the water vapor in the air; therefore a rise in humidity means a larger concentration of water vapor in the air and results in a decrease in transpiration rate. Soil water - If the intake of water at the roots does not keep up with transpiration, turgor loss occurs and the stomata close, and the transpiration rate decreases Carbon dioxide - High carbon dioxide levels in the air around the plant usually cause the guard cells to lose turgor and the stomata to close.

9.1.4 List ways in which mineral ions in the soil move to the root.

Mineral ions in the soil move to roots: - diffusion of mineral ions and mass flow of water in the soil that carries these ions - the action of fungal hyphae - active transport.

9.1.3 Outline how the root system provides a large surface area for mineral ion and water uptake by means of branching and root hairs.

Plant roots are very important for water and mineral ion absorption as well as the anchoring of the plant into the ground. Germination causes the embryonic root to break through the seed coat and start growing down into the soil. A whole root system then develops by the branching of this embryonic root into new roots, increasing the surface area for absorption. The branching of root hairs from these roots further increases the surface area.

9.1.6 State that terrestrial plants support themselves by means of thickened cellulose, cell turgor and lignified xylem.

Terrestrial plants support themselves by means of thickened cellulose, cell turgor and lignified xylem.

9.1.5 Explain the process of mineral ion absorption from the soil into roots by active transport.

The concentration of mineral ions inside the plant's roots is a lot higher than that found in the soil. Therefore, mineral ions have to be transported into the roots via active transport. Protein pumps exist in the plasma membranes of root cells. There are many types of these protein pumps for the absorption of many different mineral ions. Active transport requires ATP production by mitochondria (aerobic cell respiration, oxygen is needed) and therefore the root cells also contain many mitochondria. The branching of roots and the formation of root hairs increases the surface area for the absorption of mineral ions by active transport.

9.1.10 State that the plant hormone abscisic acid causes the closing of stomata.

The plant hormone abscisic acid causes the closing of stomata.

9.1.7 Define transpiration.

Transpiration is the loss of water vapor from the leaves and stems of plants.

9.1.2 Explain the relationship between the distribution of tissues in the leaf and the functions of these tissues.

Upper epidermis - Consists of a single layer of cells found on the upper surface of the leaf. A thick waxy cuticle covers it. The main function of the upper epidermis is water conservation. It prevents the loss of water from the upper surface where the light intensity and heat are the greatest. Palisade mesophyll - Consists of tightly packed cylindrical cells. This tissue contains many chloroplasts, as it is the main photosynthetic tissue. It is found on the upper half of the leaf (upper surface) where the light intensity is the greatest. Spongy mesophyll - Made up of loosely packed cells. This tissue is found in the lower half of the leaf (lower surface) and has few chloroplasts. It provides gas exchange (CO2 uptake and O2 release) and therefore needs to be close to the stomata found in the lower epidermis. Vascular tissue - Consists of xylem and phloem, which are found in the veins of the leaf. The veins in the leaf are positioned in the middle so that all the cells are in close contact with the vascular tissue. The xylem consists of xylem vessels (dead structure), which are long and tubular, and transports water into the leaf to replace the water that has been lost through transpiration. The phloem is made up of living cells with pores in between them. It transports the products of photosynthesis out of the leaf.