14. Homeostasis
A section of Biology, 9700
Listing 10 of 131 questions
The tomato plant, Solanum lycopersicum, does not tolerate periods of drought (water shortage). Researchers have produced a tomato plant that has an improved tolerance of drought. The researchers measured the width and the length of open stomata in plants that are tolerant of drought and tomato plants that are not tolerant. is the formula used to calculate the size of an open stoma (stomatal aperture). stomatal aperture = width of open stoma length of open stoma shows the mean stomatal aperture of the two groups of tomato plants. shows the rates of transpiration of the two groups of tomato plants when kept in identical conditions of drought. 0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40 0.00 0.50 1.00 1.50 2.00 2.50 3.00 3.50 drought tolerant mean stomatal aperture transpiration rate / mmol water m–2s–1 non-drought tolerant drought tolerant non-drought tolerant The nuclei of plants produce small lengths of RNA known as microRNAs. MicroRNAs in guard cells have been shown to prevent the synthesis of some proteins. The guard cells of the drought-tolerant tomato plants produced more microRNA molecules than the guard cells of the non-tolerant plants. MicroRNA molecules do not prevent transcription but interact with messenger RNA (mRNA). Suggest how this microRNA can interact with mRNA to prevent the production of proteins in guard cells of S. lycopersicum.
9700_w19_qp_23
THEORY
2019
Paper 2, Variant 3
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The leaves of many plants have stomata that show a regular daily rhythm of opening and closing over a period of 24 hours. Explain why it is important for plants to open and close their stomata in a daily rhythm. shows the results of an experiment to monitor this rhythm over three days and nights for the plant Arabidopsis thaliana. The percentage of open stomata is shown. Each day consisted of 14 hours of light (white bar) and each night consisted of 10 hours of darkness (black bar). time / hours open stomata / % day night Key shows that the percentage of open stomata increases in the first seven hours of the experiment. Describe the sequence of changes that occurs in the guard cells that leads to the stomata opening. The experiment was repeated with A. thaliana plants that were left in darkness from 14 to 96 hours. The results are shown in . time / hours open stomata / % day night Key With reference to , explain what shows about the role of genes and the role of the environment in controlling the rhythm of stomatal opening and closing in A. thaliana.
9700_w24_qp_41
THEORY
2024
Paper 4, Variant 1
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The leaves of many plants have stomata that show a regular daily rhythm of opening and closing over a period of 24 hours. Explain why it is important for plants to open and close their stomata in a daily rhythm. shows the results of an experiment to monitor this rhythm over three days and nights for the plant Arabidopsis thaliana. The percentage of open stomata is shown. Each day consisted of 14 hours of light (white bar) and each night consisted of 10 hours of darkness (black bar). time / hours open stomata / % day night Key shows that the percentage of open stomata increases in the first seven hours of the experiment. Describe the sequence of changes that occurs in the guard cells that leads to the stomata opening. The experiment was repeated with A. thaliana plants that were left in darkness from 14 to 96 hours. The results are shown in . time / hours open stomata / % day night Key With reference to , explain what shows about the role of genes and the role of the environment in controlling the rhythm of stomatal opening and closing in A. thaliana.
9700_w24_qp_43
THEORY
2024
Paper 4, Variant 3
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The enzyme glutamyl-tRNA reductase (GluTR) is present in many bacteria to make a product which is essential to their survival. GluTR acts on the substrate glutamyl-tRNA, which is composed of the amino acid glutamic acid attached to a tRNA. shows the structure of glutamyl-tRNA and another compound called glutamycin. CH2 H2N COOH glutamyl-tRNA NH2 O O OH rest of tRNA N N O N N tRNA portion CH2 CH3 CH3 H2N COOH glutamycin OH O N OH N N O N N N Explain how glutamycin can act as an inhibitor for the enzyme GluTR. The bacteria take in glutamic acid by active transport. Describe the process of active transport. In leguminous plants, glutamic acid is also essential for the formation of root nodules containing the bacterium Rhizobium. Name the stage in the nitrogen cycle carried out by Rhizobium. Suggest advantages of Rhizobium living in root nodules of leguminous plants. Explain the role of Rhizobium in the growth of leguminous plants.
9700_s14_qp_23
THEORY
2014
Paper 2, Variant 3
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Hakea spp. are xerophytic plants native to Australia. The leaves of Hakea have adaptations for a xerophytic mode of life. is a photomicrograph of a transverse section of part of a leaf of H. laurina. waxy cuticle stoma sub-stomatal air space Describe the xerophytic features of the waxy cuticle and the stoma shown in and explain how these features adapt the plant to a xerophytic mode of life. When the availability of phosphate ions and other soil nutrients is limited, a number of changes occur in the roots of Hakea spp.: • Regions of meristematic tissue are active for a few days. • Root clusters are formed. A root cluster is a dense arrangement of tiny side roots known as rootlets. • Most of the epidermal cells of the rootlets are root hair cells. • Rootlets release compounds into the soil that make phosphates and other mineral ions more soluble for uptake. • Uptake of phosphate ions and the absorption of water from the soil increases. Meristem cells have a similar role to stem cells in animals. Suggest how meristematic tissue activity leads to the formation of root clusters. Organic anions (negatively charged organic compounds) are released into the soil by rootlets. The concentration of these organic anions can become higher in the soil solution than in the rootlet cells. Suggest and explain how the concentration of organic anions in the soil solution can become higher than in the rootlet cells. Explain how the formation of root clusters can lead to an increase in the uptake of phosphate ions and absorption of water from the soil solution.
9700_s22_qp_23
THEORY
2022
Paper 2, Variant 3
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is a photomicrograph of a longitudinal section (LS) through a root tip. Two different regions are visible: • the root apical meristem • the root cap. Cells in the root cap synthesise a gel-like, sticky secretion known as mucilage, which is important in reducing friction between soil and the growing root. It is composed mainly of polysaccharides and also contains some amino acids and enzymes. root apical meristem cells 1mm root cap cells Describe three differences, visible in , between root apical meristem cells and root cap cells. Mucilage acts as a glue to bind tiny soil particles together, forming small clumps close to the root. These small clumps help to maintain the soil water around the root tip and prevent the loss of water. With reference to the cohesive and adhesive properties of water, suggest and explain how the formation of small clumps of soil helps to maintain the soil water around the root tip. Enzymes present in mucilage catalyse the breakdown of organic compounds in the soil. This increases the presence of mineral ions in the soil. State the term used to describe enzymes that act outside the cells that synthesise them. The polysaccharides and amino acids present in the mucilage are a source of nutrients for soil microorganisms that live in the area surrounding the root. Some of these microorganisms can break down soil compounds to release mineral ions. Explain what is meant by a polysaccharide. The soil microorganisms use amino acids to synthesise proteins. All of the twenty different amino acids that are present in proteins have the same general structure. Draw the general structure of an amino acid in the space provided and use this drawing to explain how it is possible to have many different amino acids. Mineral ions are usually present in the soil in very low concentrations. The action of mucilage enzymes and soil microorganisms can help to increase the presence of mineral ions. Root hair cells are specialised for the uptake of these mineral ions and for the absorption of water from the soil. Suggest and explain how the presence of mineral ions in the root hair cell can increase the absorption of water by the root hair cells.
9700_s23_qp_22
THEORY
2023
Paper 2, Variant 2
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Questions Discovered
131