Epidermal cells can differentiate into specific specialized structures
Qn. 1 Epidermal cells can differentiate into specific specialized structures. Name three different types of specialized epidermal cells and discus for each type, their role and adaptive importance of xerophytes.
The Epidermis cells in plants are usually made up of three specialized and differentiated cell types that include the pavement cells, the guard cells, and the trichomes. Out of the three epidermal cells, the pavement cells are the least specialized. In xerophytes, the pavement cells are tightly packed to protect the innermost cells from harm and prevent excessive water loss in leaves. Additionally, the pavement cells secrete the epicuticular wax. This thick layer of wax around the entrance of the stomata prevent xerophytes from desiccation by reducing the loss of water through transpiration.
The stomata guard cells are more specialized than the pavement cells. Two guard cells make up the stomata that allow for the intake of carbon dioxide, which is essential for respiration in plants. In xerophytes, the stomata guard cells are usually turgid during the night and less turgid during the day such that the stomata opens and closes respectively to minimize water loss via the stomata. Additionally, in some xerophytes, the stomata guard cells are located beneath the leaves to avoid direct sunlight hence conserving water.
Trichomes are tinny hair-like cells in the plant epidermis. In xerophytes, trichomes hairs protect the inner part of plant leaves from harm and also prevent it from being eaten by animals. In other cases, trichomes trap moisture to raise the humidity around the leaves of plants and prevent excessive water loss through transpiration.
Qn 2. Describe how the transpiration cohesion-tension mechanism can explain the water’s movement from the roots through the leaves to the atmosphere. Make sure you refer to the key physical properties and principles that explain the movement of water between cells and tissues.
Absorption of water in plants takes place in the roots through osmosis. The high concentration of minerals in the roots causes the movement of water from the surrounding low concentration across a semi-permeable membrane into the highly concentrated area in the roots of the plant.
Upon absorption of water in the roots, water is then carried up the plant from the roots through the xylem through the cohesion-tension mechanism. During transpiration, water is lost from the plant leaves through evaporation. This loss creates tension/negative pressure, usually of about 2 MPa depending on the relative humidity of the surrounding environment, on the surface of the leaves. This tension causes uptake of water from the roots through the xylem. Specifically, the presence of water on the mesophyll cells ‘surfaces inside the leaf increases the concentration on the primary cell wall. The wet cellular walls around the stomata are then left exposed, which leads to evaporation of water into the atmosphere. In the process, the film on the surface of these mesophyll cells decreases. This creates the tension that pulls the water up the xylem.
At night, when the stomata are closed and there is no transpiration through evaporation. Therefore, according to the cohesion tension mechanism, water is held in the xylem vessels and the plant leaves by cohesive and adhesive forces of water molecules.
Qn. 3 Xylem is often described as a “vulnerable pipeline”, but vulnerable to what? Describe the common problems that plants face when it comes to water transport through the xylem and using three examples, discuss the coping mechanism put in place.
The xylem is described as the vulnerable pipeline because of numerous changes and disturbances in the flow of xylem water. These disturbances often result in a break of the xylem’s hydraulic continuity that makes it possible for the uptake of water through the tension-cohesion mechanism.
First, the xylem embolism caused by gases from the adjacent tissues that enter the xylem vessels through pores reduces the hydraulic conductance of in the xylem. The xylem embolism is brought about by the drop in xylem pressure, which is rampant in times of high transpiration rates, such as during drought. The evaporation of water on the leaf surfaces increases the possibility of transition of xylem water into water vapor that could compromise the xylem hydraulic conductance.
In times of drought, the xylem is most vulnerable because it results in the fluctuations of the levels of water uptake in the xylem. This seasonal reduction and increase in water levels during rain can result in the plant’s natural death as a whole. In some instances, when the percentage of water uptake falls drastically and the hydraulic conductance fails completely for weeks, or even months, the plant may die. Additionally, this hydraulic conductance may also fail on normal days and in normal environments due to dynamic daily water stress.
However, plants cope with xylem vulnerability by avoiding xylem embolism through the control of the stomata. However, some plants also exercise reversal of the xylem embolism in the case of gas bubbles some develop new xylem vessels to cope with xylem vulnerability and enable its continuity.
Qn4. Nutrients are essential for the development of plants and both the uptake and assimilation are regulated according to plant’s needs. Discuss how plants can regulate nutrients uptake and assimilation at the root level. What are the mechanisms in place for the assimilation of nutrients and transport to the xylem?
The assimilation of nutrients is done through the process of diffusion wherein nutrients move from a surrounding environment of high concentration into the roots of plants where the concentration of nutrients is low. These minerals are usually dissolved in water for uptake in the xylem. The uptake of minerals takes place in three processes, which include simple diffusion, facilitated diffusion, and active cellular transportation of minerals. Examples of these nutrients include potassium, calcium, Nitrogen, Ammonium, and magnesium, among many other nutrients depending on plant deficiency.
In the case of simple diffusion, solutes are transported in a solution and moved across a semi-permeable membrane. In simple diffusion, solutes and the molecules of water are held by temporary hydrogen bonds. Water containing the soluble minerals moves from a region of low concentration to a region high concentration following the concentration gradient. Simple diffusion does not involve the use of transportation proteins since minerals are moved passively.
Additionally, facilitated diffusion is also responsible for the movement of minerals in plants along the concentration gradient. It often involves transportation proteins in the movement of mineral substances across cellular membranes. However, it does not require high energies such as Adenosine Triphosphate since the movement of minerals is along the concentration gradient. This type of diffusion includes both passive transportation as in simple diffusion and active transportation of minerals.
Lastly, the active transportation system is responsible for the movement of minerals against the concentration gradient. Adenosine Triphosphate provides the necessary energy to transportation proteins that are responsible for the active transportation of minerals from a region of high concentration to a region of low concentration.
Qn. 5 Hormones play an important role in plant functions. Gibberillins (GA) and Abscisic acid (ABA) are both found in seeds. Describe the development of a seed embryo from dormancy to germination, clearly stating the role of those two hormones during the process.
Gibberillins (GA) and Abscisic acid (ABA) hormones are phytohormones that regulate the process of germination in an antagonistic manner. The ABA is responsible for the maintenance of dormancy in seeds both before and after maturity. On the other hand, the GA facilitates the germination of these seeds after maturity during favorable environmental conditions. Therefore, when ABA concentration in a mature seed is high, the concentration of the GA hormone is proportionately low and, when ABA is low, GA is high. For instance, when a mature seed of a specific plant require elongated chilling to reduce its quantities of ABA or rather break dormancy, the application of GA hormones exogenously could break its dormancy and make it ready for germination when planted.
The figure above illustrates the process from dormancy to germination of a seed.
Qn. 6 Plants can be classified into three major groups depending on their photosynthetic pathway and for some plant species photorespiration can occur. Which category of plants is more likely to experience photorespiration? Explain what photorespiration is, when it occurs and what the metabolic cost for the plant is.
Plants can be classified on the basis of their carbon fixation. According to their photosynthetic pathway, the three categories of plants include C3, C4, and Crassulacean acid Metabolism. Photorespiration is a wasteful process in which RuBisCO enzymes oxygenate RuBP. It is wasteful because oxygen is used instead of carbon dioxide. C3 category of plants is likely to experience photorespiration because they have no special means of combating it. Photorespiration reduces the yield of crops by significant percentages that it may create significant losses to the farmer. Photorespiration often occurs in times of high temperatures when the environment is mostly hot and dry.