Plant Science Flashcards
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. It is covered by a thick waxy cuticle. 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 O2release) 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.
Identify modifications of roots, stems and leaves for different functions: bulbs
Bulbs: These are modified leaf bases which serve as food storage and thereby enable the plant to survive adverse conditions.These leaf bases may look like scales or they may extend over and encircle the centre of the bulb (onion). At the base of the bulb, a modified stem can be seen. Roots grow from the underside of the base while the new stems and leaves arise from the upper side of the base. An example of a bulb is an onion bulb.
Identify modifications of roots, stems and leaves for different functions: stem tubers
Stem Tubers: These are modified stems which serve as food storage. The stem extends into the ground and forms enlarged, swollen structures which we call stem tubers. Stem tubers are used to store nutrients and therefore allow the plant to survive winter as well as other adverse conditions. They also serve as a mean of asexual reproduction as new plants develop from these stem tubers. An example of a stem tuber is a potato.
Identify modifications of roots, stems and leaves for different functions: storage roots
Storage Roots: These are modified roots which serve as food storage. They also allow the plant to survive adverse conditions. An example of a storage root is a carrot.
Identify modifications of roots, stems and leaves for different functions: bulbs, stem tubers, storage roots and tendrils.
Tendrils: These are modified leaves. They are slim and provide attachment as well as support. In doing so they allow plants to climb upwards. They will rotate in the air until they reach a solid structure to which they can attach to. An example of plants with tendrils are grape vines.
Dicotyledonous plants have what kinda meristem?
State that dicotyledonous plants have apical and lateral meristems.
Compare growth due to apical and lateral meristems in dicotyledonous plants.
The plant meristem is a type of tissue found at several locations on plants. This tissue is composed of cells which are totipotent. This means that these cells are able to divide and make all the types of cells of that particular plant at any given time. Meristem tissue allows continuous growth and the formation of new organs. Apical meristems are found at the tips of roots and shoots. The apical meristem is responsible for the elongation of roots and stems. It allows the stem to grow taller and the roots to increase in length. Also, the shoot apical meristem allows the formation of new leaves and flowers. The growth in height of the stem is important for photosynthesis while the lengthening of the roots is important for the plant to anchor deep into the soil and it is also vital for the uptake of water and nutrients found in deeper soil layers. The growth taking place at apical meristems is called primary growth. In addition, plants also grow by increasing the diameter of their stems and roots. This is called secondary growth and is a result of cell devision in the lateral meristems. It allows extra xylem and phloem tissue production and it also provides stability for the plant to grow taller.
Explain the role of auxin in phototropism as an example of the control of plant growth.
Tropisms are directional movement responses which occur due to external environmental stimuli. The direction of the stimulus affects the direction of movement. Tropisms can either be negative or positive. Positive tropisms are the directional movement towards the stimulus while negative tropisms are the directional movement away from the stimulus. Examples of stimuli causing tropisms in plants are gravity and light. Roots will grow towards gravity while the plant shoot will grow upwards in the opposite direction. The directional movement of plants in response to light is called phototropism. As seen with gravity, the plant’s roots will grow away from the light, into the soil (negative phototropism) while the plant shoot will grow towards the light (positive phototropism). Positive phototropism seen at the tips of plant shoots is made possible due to plant hormones called auxins. Auxins are produced at the tips of plant shoots and then translocate to the darker side of the shoot tip and stem which is receiving less light. This translocation is made possible via auxin efflux carriers which are unevenly distributed in the plant tissue. Once auxins reach the shaded side of the plant, they cause the elongation of cells so that the shaded side grows faster than the brighter side, thereby promoting the bending of the plant shoot towards the light. Auxins do so by binding to auxin receptors on cells. The binding of auxin causes the transcription of certain genes within those cells and therefore the production of specific proteins which affect growth. Auxins allow the expelling of protons (hydrogen ions) into the cell walls of the cells on the shaded side, decreasing the pH inside the cells and in doing so activate specific enzymes which break down cellulose microfibrils within the cell wall. This loosens the cell wall and allows cell elongation. So to conclude, auxins are very important in the control of plant growth towards the light and thereby allow the plant to increase its rate of photosynthesis.
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.
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.
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.
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.
Define transpiration.
Transpiration is the loss of water vapour from the leaves and stems of plants.
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.
How do guard cells regulate transpiration?
Guard cells can regulate transpiration by opening and closing stomata.
