14.2. Homeostasis in plants
A subsection of Biology, 9700, through 14. Homeostasis
Listing 10 of 36 questions
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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Woolly foxglove, Digitalis lanata, shown in A, and common oleander, Nerium oleander, shown in B, are plants grown for the attractive flowers that they produce. Both plants are poisonous, as their leaves produce toxic organic compounds known as cardiac glycosides. Cardiac glycosides have a powerful effect on the action of cardiac muscle. A B N. oleander is able to grow in very dry conditions. The leaves have adaptations to reduce water loss by transpiration. State the term used to describe a plant, such as N. oleander, that has adaptations to allow it to grow in conditions where water is in short supply. Aphids are small insects that feed on plant fluids using piercing and sucking mouthparts. When aphids feed on the sap present in vascular tissue of leaves and stems, a sugary liquid called honeydew is passed out of the gut. The honeydew can be analysed to find out what is present in the sap. State the name of the vascular tissue from which the aphids feed. An investigation found that aphids feeding on D. lanata produced honeydew containing cardiac glycosides. Suggest why cardiac glycosides were present in the sap from the vascular tissue. Cardiac glycosides have an effect on the movement of ions into and out of cardiac muscle cells. The outcome is an increased ability for the cells to contract. Investigations into the action of the cardiac glycoside oleandrin, extracted from N. oleander, have shown that it acts to prevent the correct functioning of Na/K‑ATPase, a membrane transport protein. Na/K‑ATPase has a role as an enzyme and as a transport molecule. • ATPase is an enzyme that catalyses the hydrolysis of ATP to ADP and inorganic phosphate. • Energy released from this hydrolysis is used to transport sodium ions (Na+) out of cardiac muscle cells and potassium ions (K+) into the cells. Explain what is meant by the hydrolysis of ATP. Name the type of transport mechanism involved in the transport of Na+ and K+ across the cell surface membrane of cardiac muscle cells. Oleandrin is a non‑competitive reversible inhibitor of ATPase. Describe the mode of action of oleandrin and explain how this will affect ion movement through Na/K‑ATPase transport proteins of the cell surface membranes of cardiac muscle cells. Digoxin, a cardiac glycoside extracted from D. lanata leaves, can be purified and used as a drug to treat some heart disorders. Examples of these heart disorders are: • atrial fibrillation, where the normal rhythmic cardiac cycle is disrupted • heart failure, where cardiac muscle is contracting weakly. Describe the sequence of events occurring in the left side of the heart during one normal cardiac cycle. Include reference to blood pressure changes. The first event in the sequence is described for you. The left atrium fills with blood during relaxation of the left atrium and left ventricle. Suggest how the health of a person with heart failure can be improved by treatment with the drug digoxin.
9700_w18_qp_22
THEORY
2018
Paper 2, Variant 2
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Questions Discovered
36