Transport in angiospermophytes
9.2.1 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 surface area is further increased by the branching of root hairs from these roots.
9.2.2 List ways in which mineral ions in the soil move to the root.
Mineral ions in the soil move to roots via fungal hyphae (mutualism), mass flow of water in the soil carrying ions and the diffusion of mineral ions.
9.2.3 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.
9.2.4 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.
9.2.5 Define transpiration.
Transpiration is the loss of water vapour from the leaves and stems of plants.
9.2.6 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 upwards 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 which allows the movement of water out of the vessels and into the surrounding cells of leaves. The outer cell walls contain thickenings which 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 vapour 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.
9.2.7 State that guard cells can regulate transpiration by opening and closing stomata.
Guard cells can regulate transpiration by opening and closing stomata.
9.2.8 State that the plant hormone abscisic acid causes the closing of stomata.
The plant hormone abscisic acid causes the closing of stomata.
9.2.9 Explain how the abiotic factors light, temperature, wind and humidity, affect the rate of transpiration in a typical terrestrial plant.
Four abiotic factors 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.
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 vapour in the air, therefore a rise in humidity means a larger concentration of water vapour in the air and results in a decrease in transpiration rate.
Temperature - As temperature rises, so does the rate of transpiration. This is because heat is vital for the evaporation of water vapour 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.
9.2.10 Outline four adaptations of xerophytes that help to reduce transpiration.
Any four of the following:
1) Reduced surface area of the plant - reduced leaves such as spines in cacti (modified leaves)
2) Thick waxy cuticle covering the epidermis
3) Reduced numbers of stomata
4) Water storage tissues in roots, leaves and stems
5) CAM physiology - Stomata open during the evening/night instead of during the day (when the temperature is at its highest) as the transpiration rate will be lower during cooler hours.
9.2.11 Outline the role of phloem in active translocation of sugars (sucrose) and amino acids from source (photosynthetic tissue and storage organs) to sink (fruits, seeds, roots).
Phloem tissue transports sugars and amino acids from sources which include photosynthetic tissue (leaves and stems) and storage organs, to sinks which include the fruits, seeds and roots of the plant. This transport is known as active translocation and requires energy.