HKDSE Ch.10 Transpiration, Transport and Support in Plants Worksheet Answer Key PDF

HKDSE Ch.10 Transpiration, transport and support in plants Answer 10.1 Transpiration A. Where does transpiration occur? Question: How do human lose heat to Question: Will plant lose heat to the environment? the environment in hot condition? transpiration Evaporatio...

HKDSE Ch.10 Transpiration, transport and support in plants Answer 10.1 Transpiration A. Where does transpiration occur? Question: How do human lose heat to Question: Will plant lose heat to the environment? the environment in hot condition? transpiration Evaporation of sweat → bring a cooling stomata effect. cuticle lenticels Question: Identify the sites of transpiration in the diagram below: stomata cuticle lenticel B. Process of transpiration Analogy Question: Are there any similarities between water transport in a plant and drinking through a straw? Drinking → Transpiration Sucking force → Transpiration creating a force (transpiration pull) Straw → xylem vessels In both cases, a continuous water column is maintained (cohesive force between water molecules; adhesive force between the wall and water molecule) 1  HKDSE Ch.10 Transpiration, transport and support in plants  Water diffuses out of the mesophyll cells to form a water film on the cell surface.  Water of the water film evaporates to form water vapour. The water vapour moves into the air space among the mesophyll cells.  Water vapour in the air space diffuse through the stomata to the atmosphere. Creation of transpiration pull a During transpiration, mesophyll cells near the air space lose water to the air space. Their water potential (lowers). b Water moves from neighbouring cells into these cells by osmosis. This is repeated across the layer of mesophyll cells. c Eventually, water moves out of the xylem vessels by osmosis to replace the water loss in mesophyll cells. Transpiration pull is created to pull water up the xylem vessels from roots. Significance of transpiration Question: Estimate the height of the tallest tree on Earth. Circle your choice. ~50m ~100m ~400m Question: What are the characteristics of the vessel (i.e. xylem) for water transport to a high position? Hollow (little resistance for water transport) Thick wall (high strength preventing collapse) Question: What is the significance of transpiration? Put a ‘✓’ in the correct box(es).  Transpiration creates transpiration pull, which enables carbon dioxide to be transported from the roots to the leaves for photosynthesis. ✓ Transpiration creates transpiration pull, which enables water to be transported from the roots to other parts of the plant. ✓ Transpiration creates transpiration pull, which enables minerals to be transported from the roots to other parts of the plant. ✓ Transpiration pull facilitates the absorption of water by roots. ✓ Produce a cooling effect on plants. 2  HKDSE Ch.10 Transpiration, transport and support in plants Question: With reference to the adaptation, describe the features of leaves of terrestrial plants to prevent excessive water loss. Feature of leaves of terrestrial plants Adaptation Epidermis covered with a waxy cuticle Cuticle is almost impermeable to water. This reduces water loss through leaf surfaces. Few or no stomata on the As the leaves are usually oriented horizontally, their upper upper epidermis of most dicotyledonous epidermis faces the sun and is hotter than the lower epidermis. leaves The small number of stomata on the upper epidermis helps reduce water loss by evaporation. Guard cells are present to control the At night, the need for gas exchange in plants decreases. Most opening and closing of stomata stomata close to reduce water loss from the leaves. Question: Name cell A and cell B. Question: Is it at daytime or at night when stoma closes? Discuss the functional cell A: epidermal cell significance for the closure of stoma. At night. The closure of stoma reduce water cell B: guard cell loss by transpiration when the need for gas exchange decreases in the absence of photosynthesis. Question: Draw a labelled diagram of a pair of cell B when stoma opens. (For reference only – the diagram below is different from that of photomicrograph given) Title: A diagram showing a pair of cell B (guard cells) Question: With reference to t hickness of the cell wall of cell B, explain the mechanism for the opening and closing of stoma. The opening and closing of stoma is controlled by changing the turgidity of guard cells. Guard cells have a thicker inner wall and a thinner outer wall. When guard cells take up water from the surrounding cells and become turgid, the outer side expands more than the inner side. As a result, the guard cells bend and stoma opens. Question: Apart from low light intensity and with reference to the mechanism of opening and closing of stoma, state another condition of plant resulting the closure of stoma. Plant is deficit in water 3  HKDSE Ch.10 Transpiration, transport and support in plants E. Distribution of stomata on leaves of different types of plants Type of plant Distribution of stomata and other features on leaves Terrestrial Epidermis covered with cuticle. dicotyledonous Question: What is the significance of this? plants Cuticle is almost impermeable to water. This reduces water loss through leaf surfaces.  Stomatal density on the upper epidermis is lower than that on the lower epidermis. Question: What is the significance of this? The upper epidermis is directly illuminated by the sun and its temperature is higher. Few or no stomata on it helps reduce water loss due to transpiration. Terrestrial  The stomata are almost equally distributed on both leaves of the leaves. monocot Question: With reference to the orientation of leaves, explain the significance of such plants stomatal distribution. As the leaves are oriented vertically, both sides of the leaves receive similar sunlight. They lose water at a similar rate. Submerged  Few or no stomata on both the upper and lower epidermis plants  Not covered with cuticle  Dissolved gases, water and minerals diffuse directly into the leaves through all surfaces e.g. hydrilla Floating plants Question: How is the floating leaf adapted to its environment? Annotate the features in the photomicrograph below. e.g. water lily 4  HKDSE Ch.10 Transpiration, transport and support in plants F. Measurement of the rate of transpiration A potometer can be used to estimate the rate of transpiration of a plant. Question: In setting up this experiment, the lower end of the shoot should be cut under water. Why? Avoid blockage of xylem by the air bubble formed during cutting. Question: The initial position of the air bubble is recorded 5 minutes after immersing the leafy shoot in water. Why? Allow time of equilibrium for the plant to adapt to the new set of conditions. (important step for investigation involving living things.) Question: Give one assumption for using this set-up to measure the transpiration rate. The rate of transpiration is the same as the rate of water absorption / the water absorbed us used for transpiration only. A weight potometer is comprised of two parts: (1) the burette which is used to measure the amount of water absorption (A) by a leafy shoot, and (2) the balance which is used to measure the amount of water loss (B) of the leafy shoot due to transpiration. Question: An oil layer is added in burette. Why? To prevent the evaporation of water Question: What do you expect the difference of (A) and (B)? What’s the significance? A( amount of water absorption) is slightly higher than B (amount of water loss). The difference is the amount of water retained by the shoot which is used for photosynthesis, growth and other metabolic activities. G. Environmental factors affecting the rate of transpiration [Concept check] The rate of transpiration is determined by three factors: (1) The rate of evaporation of water (2) The rate of diffusion of water vapour out of stomata; and (3) The degree of opening of stomata 5  HKDSE Ch.10 Transpiration, transport and support in plants Light intensity a The rate of transpiration is low in darkness. Reason: The stomata are closed. Only a small amount of water transpiration rate of b vapour diffuses out of the leaves. b The rate of transpiration increases when the light intensity a increases. light intensity Reason: As light intensity increases, the stomata open wider. The cross-sectional area for the diffusion of water vapour increases. Water vapour diffuses out of the leaves more rapidly. Put a ‘✓’ for the related factors Factor (1)  Factor (2) ✓ Factor (3) ✓ Temperature ◼ The rate of transpiration increases when the temperature transpiration increases. rate of Reason: As temperature increases, the rate of evaporation of water from the surfaces of mesophyll cells increases. This increases the concentration gradient between the air space and temperature the surrounding air. As a result, water vapour diffuses out of the leaves more rapidly. Put a ‘✓’ for the related factors Factor (1) ✓ Factor (2) ✓ Factor (3)  6  HKDSE Ch.10 Transpiration, transport and support in plants Air movement a The rate of transpiration is low in still air. transpiration b rate of c Reason: Water vapour accumulates around the stomata. This decreases the concentration gradient of water vapour between the a air space and the surrounding air. The rate of diffusion of water wind speed vapour out of the leaves decreases. b The rate of transpiration increases when the wind speed increases. Reason: Wind blows away water vapour around the stomata. This helps maintain a steep concentration gradient of water vapour between the air space and the surrounding air. Water vapour diffuses out of the leaves more rapidly. c The rate of transpiration decreases when the wind is too strong. Reason: Under strong wind, water may be lost from the plant too rapidly, causing most stomata to close. Less water vapour diffuses out of the leaves. Put a ‘✓’ for the related factors Factor (1)  Factor (2) ✓ Factor (3) ✓ Relative humidity ◼ The rate of transpiration decreases when the relative humidity of transpiration the surrounding air increases. rate of Reason: A higher relative humidity of the surrounding air decreases the concentration gradient of water vapour between the air space and the surrounding air. Less water vapour diffuses relative humidity out of the leaves. Put a ‘✓’ for the related factors Factor (1)  Factor (2) ✓ Factor (3)  Question: Besides environmental factors, structure of plants also affect the rate of transpiration. List three examples that lead to a higher rate of transpiration. (1) the surface area of leaves is larger. (2) the cuticle is thinner. (3) the stomatal density is higher. 7  HKDSE Ch.10 Transpiration, transport and support in plants Question: What do Area X and Area Y represent Question: Compare the size of Area X and Area Y. respectively? Explain the significance of the difference. Area Y is larger than Area X. The plant has a net Area X: net amount of water lost by the plant from uptake of water during the 24-hour period. Some 0600 to 1600 hours water is retained in the plant for photosynthesis, Area Y: net amount of water absorbed by the plant formation of new cells and other metabolic activities. from 1600 to 0600 hours 10.2 Absorption of water and minerals in plants A. Structure of the root Question: Label the structures of root in the diagram below. root hair epidermis cortex xylem phloem vascular bundle Root cap ▲ Longitudinal section of a dicotyledonous root ▲ Transverse section of a dicotyledonous root B. Process of absorption of water by roots  In the cortex, water travels from cell to cell through three routes: a Water moves along a water potential gradient through the cytoplasm of cells by osmosis. b Water moves along the same water potential gradient through the vacuole of cells by osmosis. c Water travels from one cell to another freely through the cell wall. Question: Which route (4a, 4b, 4c) does most water travel? Route 4c (through the cell wall) a this movement is easier and faster with the least resistance.  As water is drawn away from the root hair cells to the neighbouring cortical cells, the water potential of the root hair cells becomes lower. As the water potential of the soil water is (higher) than that of the root hair cells, water in the soil enters the root hair cells by osmosis. 8  HKDSE Ch.10 Transpiration, transport and support in plants Question: Suggest the processes by which these inorganic ions are absorbed into the root hair cells. Nitrate ions Sulphate ions Active transport. Diffusion (and active transport) As the concentration of nitrate ions is lower in As the concentration of sulphate ions is higher the soil than that in the root hair cells, nitrate in the soil than that in the root hair cells, ions are absorbed into the root hair cells against sulphate ions are mainly absorbed into the root the concentration gradient by active transport. hair cells along the concentration gradient by diffusion. Active transport can also take place at the same time irrespective of the concentration gradient. Question: Chemical fertilizers contain minerals that promote plant growth. What would be the adverse effect to the plant if too much fertilizers are applied to the soil? T his would increase the mineral concentration in the soil which lower the water potential in the soil. As a result, water cannot be absorbed into the roots by osmosis. The plant cannot get adequate water intake. D. Adaptive features of roots for absorption of water and minerals Question: What are the adaptive features of roots for the absorption of water and minerals? Feature Adaptation Highly branched and have numerous Provide a large surface area for the absorption root hairs. of water and minerals Long and fine root hairs Allows root hairs to grow between soil particles to absorb water and minerals Epidermis not covered by cuticle and made up of one layer Allows water and minerals to pass through the of thin-walled cells only epidermis into the root easily Many mitochondria in root hair cells Ensures enough energy is supplied to absorb minerals from the soil by active transport 10.3 Transport in plants A. Types of vascular tissues Xylem and phloem are vascular tissues specialized for the transport of substances in flowering plants. Question: Put a ‘✓’ for the function(s) of xylem and phloem. Xylem Phloem Transports water and minerals ✓  Provides support for plants ✓  Transports organic nutrients  ✓ 9  HKDSE Ch.10 Transpiration, transport and support in plants Differentiation Question: What are the changes to be made for a typical plant cell (top) to become a xylem vessel? (1) Thickened and lignified cell walls → provide high strength to prevent vessels from collapsing under the great tension of the transpiration pull / withstand the negative pressure of the transpiration pull. (2) Degeneration of all cell contents (including nucleus and cytoplasm) → becoming dead cells (3) Degeneration of end walls → xylem vessels are joined end to end (2) + (3) → hollow tube which allows the passage of water with little resistance (4) Become elongated Question: What are the changes to be made for a typical plant cell (top) to become a sieve tube? (1) Degeneration of nucleus and reduced cytoplasm to the edge → living cells but metabolism supported by companion cell, allow organic nutrient to move along with little resistance (2) Modified end walls to form sieve plates → with pores to allow organic nutrient to pass through. (3) Become elongated Question: Describe the relationship between the rate of transpiration and stem diameter. The diameter of the stem decreases as the rate of transpiration increases, or vice versa. Question: It is known that the change in stem diameter is related to the diameter of the xylem vessels. With reference to the way in which water is transported along the stem, explain the relationship between the rate of transpiration and stem diameter described in the previous question. Water is transported up the stem by transpiration pull / pulling force / negative pressure. This force increases with transpiration rate and pull the xylem vessel’s walls inwards, thus reducing the diameter of the xylem as well as the stem. B. Distribution of vascular tissues Xylem and phloem are arranged in large, separate strands called vascular bundles in flowering plants. In leaves  Vascular bundles are found in the large central midrib and the network of small veins.  Xylem lies at the top of phloem. In stems  Vascular bundles are separate and usually arranged in a ring at the periphery.  Xylem is located in the inner region and phloem in the outer region. Question: What is the significance of such arrangement in stem? Such arrangement gives greater mechanical strength to the periphery of the stems to resist bending strain imposed by wind and passing animals. In roots  Vascular bundles are located at the centre. Question: What is the significance of such arrangement in root? Such arrangement allows easier bending and gives greater mechanical strength to the centre of the roots to resist pulling strain. 10  HKDSE Ch.10 Transpiration, transport and support in plants C. Process of transport of water and minerals  Water is lost from leaves through transpiration. Transpiration pull is created.  Water (with dissolved minerals) is drawn up the xylem vessels by transpiration pull as a continuous stream.  Water in the soil is absorbed into the root by osmosis. Dissolved minerals are absorbed by active transport. D. Translocation of organic nutrients in plants ◼ Organic nutrients (carbohydrates) are synthesized in the leaves by photosynthesis and then transported by phloem to other parts of the plant where they are used or stored. ◼ The transport of organic nutrients in plants is called translocation. It is an active process (i.e. requires energy). The ringing experiment Question: With reference to the distribution of xylem and phloem in the dicotyledonous stems, explain why it is possible to remove phloem by cutting away a ring of bark without affecting xylem. In dicot stem, vascular bundles are arranged in a ring at the periphery. Phloem is located in the outer ring in each vascular bundle while xylem is located in the inner region. Question: Explain why sugar accumulated in tissue just above the ring after a few weeks. Carbodydrates are continuously produced in the leaves of the tree by photosynthesis. Carbohydrates will be converted to sugar (sucrose) but the downward transport of the sugar was stopped by the removel of phloem. Sugar accumulated in the tissue just above the ring. Question: Explain why swelling of the stem was less prominent in winter. As the leaves of the tree had fallen / less duration of daytime / lower light intensity in the winter, less sugar is produced by a lower rate of photosynthesis. The amount of sugar being transported down the stem decreased. Question: The photo on the left shows the famous hollow tree in Lai Chi Wo in Hong Kong. The centre of its tree trunk has rotted away, leaving a large hole. Can water and organic food transported between the roots and the leaves? Explain briefly. Yes. The vascular tissues are located at the periphery of the stem. The transport of water by xylem and the translocation of organic food by phloem remains unaffected. Note: This famous hollow tree in Lai Chi Wo is still alive and growing actively. 11  HKDSE Ch.10 Transpiration, transport and support in plants 10.4 Support in plants Question: What is the importance of support in terrestrial plants? Put a ‘✓’ for the correct choice(s). ✓ To hold the leaves in the best position to receive the maximum amount of light for photosynthesis ✓ To lift up the flowers to facilitate pollination ✓ To lift up the fruits and seeds to facilitate their dispersal Support in plants is mainly provided by the turgidity of thin-walled cells and the rigidity of thick-walled cells. A. Turgidity of thin-walled cells When water supply is adequate (a) The thin-walled cells become turgid and press against each other. The plant stands upright. When water supply is inadequate (b) The thin-walled cells become flaccid. The plant wilts. B. Rigidity of thick-walled cells  Thick-walled cells are mostly xylem vessels. Their cell walls contain lignin, which makes them hard and rigid.  Support provided by thick-walled cells is independent of the water content of the plant.  As woody plants grow, more lignin is deposited in the cell walls of the older xylem. The older xylem finally becomes wood, which provides great support to the plants. 12  HKDSE Ch.10 Transpiration, transport and support in plants Question: How are the cell types of the young dicot stem adapted to support? Label the cell type involved in support. 13