PLANT TRANSPORT EXAM+SOLUTIONS.doc
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9 The chart shows the results obtained from an investigation to determine the effect of light intensity on the tension in xylem vessels in the leaves of a plant. (a) Describe and explain the effects of increasing light intensity on the tension in the xylem vessels in the leaves. ...................................................................................................................................................... ...................................................................................................................................................... ...................................................................................................................................................... ...................................................................................................................................................... ...................................................................................................................................................... ...................................................................................................................................................... ...................................................................................................................................................... ...................................................................................................................................................... ...................................................................................................................................................... ...................................................................................................................................................... (b) Explain why it was important to keep the humidity constant during the investigation. ...................................................................................................................................................... ...................................................................................................................................................... ...................................................................................................................................................... ...................................................................................................................................................... (2 marks) (c) Explain how the negative pressure in the xylem vessels of the leaves causes water to move up the plant from the xylem in the roots. ...................................................................................................................................................... ...................................................................................................................................................... ...................................................................................................................................................... ...................................................................................................................................................... ...................................................................................................................................................... ...................................................................................................................................................... ...................................................................................................................................................... ...................................................................................................................................................... (4 marks) (d) Many tropical plants have a specialised method of photosynthesis. Explain how this method adapts the plant to tropical conditions. ...................................................................................................................................................... ...................................................................................................................................................... ...................................................................................................................................................... ...................................................................................................................................................... ...................................................................................................................................................... ...................................................................................................................................................... ...................................................................................................................................................... ...................................................................................................................................................... (4 marks) SOLUTIONS Question 9 (a) (i) 1. (when light intensity is increased) tension in the xylem becomes greater / more negative / stronger; 2. (this increase) takes place over ≈100 second; 3. then levels out; 4. stomata open (more); 5. increased evaporation / transpiration; 6. therefore the water potential of leaf cells becomes more negative / lower; 7. therefore more water moves from xylem to surrounding cells; 8. down a water potential gradient; 9. correct ref. to hydrogen bonds / cohesion; 5 max (b) humidity will affect (the rate of) evaporation / transpiration; increased humidity / humid conditions decreases rate of water loss; 2 (c) 1. continuous / leaf to root column of water; 2. H-bonds; 3. cohesion; 4. column under tension / pull transmitted; 4 (d) 1. method is C4 photosynthesis / correct ref. to a 4-carbon molecule; 2. carbon dioxide stored in a molecule; (reject 3-carbon molecule/RuBP ) 3. stomata close during day / hot conditions; 4. reducing water loss; 5. low carbon dioxide concentration inside leaves; 6. carbon dioxide released from molecules for photosynthesis; 7. allows efficient use of high light intensity; allow 8. effect of inhibition of RUBISCO is overcome; 4 max Total 15 9 (a) Describe and explain how water moves via the apoplastic and symplastic pathways from the soil to the xylem in a root. ...................................................................................................................................................... ...................................................................................................................................................... ...................................................................................................................................................... ...................................................................................................................................................... ...................................................................................................................................................... ...................................................................................................................................................... ...................................................................................................................................................... ...................................................................................................................................................... ...................................................................................................................................................... ...................................................................................................................................................... ...................................................................................................................................................... ...................................................................................................................................................... ...................................................................................................................................................... ...................................................................................................................................................... ...................................................................................................................................................... (6 marks) (b) The graphs show the daily changes in environmental temperature and light intensity, and changes in the diameter of the trunk of a pine tree. Use information from the graphs, and your knowledge of the cohesion-tension theory of water movement through a plant, to explain why the diameter of the trunk is smallest at midday. ...................................................................................................................................................... ...................................................................................................................................................... ...................................................................................................................................................... ...................................................................................................................................................... ...................................................................................................................................................... ...................................................................................................................................................... ...................................................................................................................................................... ...................................................................................................................................................... ...................................................................................................................................................... ...................................................................................................................................................... ...................................................................................................................................................... ...................................................................................................................................................... ...................................................................................................................................................... ...................................................................................................................................................... ...................................................................................................................................................... (6 marks) (c) Describe and explain three ways in which the leaves of xerophytic plants may be adapted to reduce water loss. ...................................................................................................................................................... ...................................................................................................................................................... ...................................................................................................................................................... ...................................................................................................................................................... ...................................................................................................................................................... ...................................................................................................................................................... (3 marks) SOLUTIONS (a) Apoplastic . Via cell walls / spaces external to cell membrane / external to cytoplasm / between cells; As far as endodermis / Casparian strip / layer of wax; Caused by transpiration pull; Cohesion / hydrogen-bonding between water molecules; Symplastic . Through cell surface membrane (of epidermis / root hair cell) / ref. vacuoles membrane; High to low Ψ / Ψs; Diffusion / osmosis; Cell-to-cell via plasmodesmata / via strands of cytoplasm; Secretion / active transport of ions into xylem by endodermis; OR Active uptake of ions from soil at epidemis; Lowers Ψ / Ψs in xylem / increases osmosis into xylem; [If symplast & apoplast are confused . max 5 marks] max 6 (b) 1. Diameter of trunk minimal at warmest / brightest time of day / midday = warmest / brightest; 2. Stomata open in light more water loss; 3. Water evaporates more when warm / more heat energy for water evaporation; 4. Hydrogen-bonding between water molecules; 5. Cohesion (/ described) between water molecules; 6. Adhesion (described) between water molecules and walls of xylem vessels; 7. (Xylem) pulled inwards by faster flow of water / pulled in by tension; 8. Reduced pressure at leaves / top of plant / pull from top / from leaves / tension from leaves / from top of plant due to transpiration / evaporation; 9. Water pulled up plant; max 6 (c) Feature Explanation Think cuticle / wax layer waterproof / impermeable; Sunken stomata saturated layer of still air outside; Hairy saturated layer of still air outside; Leaves small / reduced to spines / needles reduced S.A. for water loss; Leaves roll up in dry weather less S.A. for water loss / stomata covered / saturated region of still air; Reduced number of stomata reduced S.A. for water loss; CAM (/ Crassulacean Acid Metabolism) stomata closed in light / in warm / only open in dark / when cool; 3 features but no explanations . max 1 mark max 3 Total 15 marks (a) Explain how water enters a plant root from the soil and travels through to the endodermis. ...................................................................................................................................................... ...................................................................................................................................................... ...................................................................................................................................................... ...................................................................................................................................................... ...................................................................................................................................................... ...................................................................................................................................................... ...................................................................................................................................................... ...................................................................................................................................................... ...................................................................................................................................................... ...................................................................................................................................................... (5 marks) (b) From the root, water is transported upwards through the stem. Explain how evaporation from the leaves can cause the water to move upwards. ...................................................................................................................................................... ...................................................................................................................................................... ...................................................................................................................................................... ...................................................................................................................................................... ...................................................................................................................................................... ...................................................................................................................................................... ...................................................................................................................................................... ...................................................................................................................................................... (4 marks) (c) In daylight, most of the water evaporates from the leaves but some is used by the plant. Describe the ways in which this water could be used by the plant. ...................................................................................................................................................... ...................................................................................................................................................... ...................................................................................................................................................... ...................................................................................................................................................... ...................................................................................................................................................... ...................................................................................................................................................... ...................................................................................................................................................... ...................................................................................................................................................... ...................................................................................................................................................... ...................................................................................................................................................... ...................................................................................................................................................... ...................................................................................................................................................... (6 marks) SOLUTIONS (a) water enters root hair cells; by osmosis; because active uptake of mineral ions has created a WP gradient; water moves through the cortex; (by osmosis) down a WP gradient; through cell vacuoles and cytoplasms / symplastic pathway; through cell walls / apoplastic pathway; max 5 (b) WP in leaf cells decreases / becomes more negative; therefore water moves out of xylem (into surrounding tissues) by osmosis; this creates a pull/tension on the water in xylem; which is in a continuous column / water molecules cohere; cohesion due to H bonding; column doesn’t break because of adhesion with xylem walls; max 4 (c) (water is used in) the light-dependent reactions of photosynthesis; electrons from water enable ATP production / H+ are used to reduce NADP / produces O2 ; (water can be used in) hydrolysis reactions within the plant; to create turgor; as a solvent for transport; as a medium for chemical reactions; component of cells / cytoplasm; 6 Total 15 marks (a) Root pressure is a force that is partly responsible for the movement of water through xylem in stems. Explain how the active transport of mineral ions into xylem vessels in the roots results in water entering these vessels and then being moved up the xylem tissue. ...................................................................................................................................................... ...................................................................................................................................................... ...................................................................................................................................................... ...................................................................................................................................................... ...................................................................................................................................................... ...................................................................................................................................................... ...................................................................................................................................................... ...................................................................................................................................................... ...................................................................................................................................................... ...................................................................................................................................................... (5 marks) (b) The presence of an air bubble in a xylem vessel in the stem blocks the movement of water through that vessel. Use the cohesion-tension theory to explain why. ...................................................................................................................................................... ...................................................................................................................................................... ...................................................................................................................................................... ...................................................................................................................................................... ...................................................................................................................................................... ...................................................................................................................................................... ...................................................................................................................................................... ...................................................................................................................................................... (4 marks) (c) Water vapour diffuses through open stomata into the atmosphere. Describe two structural adaptations of the leaves of xerophytes that reduce this loss. Using Fick’s law, explain how these two adaptations reduce the rate of diffusion of water vapour into the atmosphere. ...................................................................................................................................................... ...................................................................................................................................................... ...................................................................................................................................................... ...................................................................................................................................................... ...................................................................................................................................................... ...................................................................................................................................................... ...................................................................................................................................................... ...................................................................................................................................................... ...................................................................................................................................................... ...................................................................................................................................................... ...................................................................................................................................................... ...................................................................................................................................................... (6 marks) SOLUTIONS Quality of Communication The answers to all sections of this question require the use of continuous prose. Quality of language should be considered in crediting points in the scheme. In order to gain credit, answers should be expressed logically and unambiguously, using scientific terminology where appropriate. (a) Any five from: Water potential in xylem reduced (by entry of ions); Water potential gradient established between xylem and surrounding cells; Plasma membranes of surrounding cells are partially permeable; Water enters xylem by osmosis; Volume of water in xylem increases; Cannot move back due to gradient; Pressure in xylem increases (and forces water upwards); max 5 (b) Any four from: Evaporation from leaves / transpiration; Water in xylem under tension*/negative pressure/pulled up; Water molecules cohere*/stick together/form hydrogen bonds; [Ignore: references to adhesion] So water a single column; Air bubble breaks column / prevents cohesion; max 4 [*Note: just mentioning the cohesion-tension theory is not enough] (c) 2 marks per feature for relating Fick’s law to reducing water loss – max. 2 features reduced number of stomata; reduced surface area; thick waxy cuticle; increases diffusion distance; leaves reduced to spines; reduced surface area ; (epidermal) hairs; reduce diffusion gradient; sunken stomata; reduce concentration difference; curled leaves; reduced concentration gradient; Statement of Fick’s law: Rate of diffusion EQUALTOSurface area of exchange surface x concentration difference across surface DIVIDE BY thickness of exchange surface ; Low surface area, low concentration difference and high thickness/equivalent reduce loss / candidate clearly relates features to equation to show how rate is reduced; max 6 Total 15 marks Some organisms are adapted for living in hot, dry environments. (a) The drawing shows a cactus, which is a xerophytic plant. (i) Explain how each of the features, A, B and C, adapt this cactus to life in a hot desert. ............................................................................................................................................ ............................................................................................................................................ ............................................................................................................................................ ............................................................................................................................................ ............................................................................................................................................ ............................................................................................................................................ ............................................................................................................................................ ............................................................................................................................................ ............................................................................................................................................ ............................................................................................................................................ ............................................................................................................................................ ............................................................................................................................................ (6 marks) S (ii) Explain how feature B could have evolved to be present in all plants of this species. ............................................................................................................................................ ............................................................................................................................................ ............................................................................................................................................ ............................................................................................................................................ ............................................................................................................................................ ............................................................................................................................................ ............................................................................................................................................ ............................................................................................................................................ ............................................................................................................................................ ............................................................................................................................................ (4 marks) (b) Explain what causes the activity of reptiles living in a desert to vary greatly over a twenty-four hour period. ...................................................................................................................................................... ...................................................................................................................................................... ...................................................................................................................................................... ...................................................................................................................................................... ...................................................................................................................................................... ...................................................................................................................................................... ...................................................................................................................................................... ...................................................................................................................................................... ...................................................................................................................................................... ...................................................................................................................................................... (5 marks) SOLUTIONS Quality of Communication The answers to all sections of this question require the use of continuous prose. Quality of language should be considered in crediting points in the scheme. In order to gain credit, answers should be expressed logically and unambiguously, using scientific terminology where appropriate. (a) (i) A Root system: Can absorb water from a wide area; Quickly/ before water drains away; B Leaves reduced to spines: Reduces leaf area/ reduces number of stomata; Reduces water loss by transpiration; C Stem cells contain chlorophyll: Can photosynthesise; Compensates for reduced photosynthesis by leaf; 6 (ii) Mutation in allele(s) determining leaf shape in original population; Those with leaves reduced to spines have an advantage due to reduced water loss; Survive to reproduce better than other forms; Increase in frequency with each generation; Pass on advantageous alleles; Advantageous alleles increase in frequency with each generation; max 4 (b) Reptiles are ectotherms; [Reject: cold blooded] Body temperature varies with that of environment; Temperature of desert fluctuates greatly over 24 hours; Metabolic reactions controlled by enzymes; Enzyme activity/ metabolic rate changes with body temperature; Speed of bodily actions dependent on metabolic rate/ enzyme activity; Reptiles seek shade/ water when hot/ reduce contact with hot surface; Seek sun when cool; max 5 Total 15 marks
