23.4 Leaves. Lesson Objectives. Lesson Summary

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1 23.4 Leaves Lesson Objectives Describe how the structure of a leaf enables it to carry out photosynthesis. Explain how gas exchange in leaves relates to homeostasis. Lesson Summary Leaf Structure and Function The structure of a leaf is optimized to absorb light and carry out photosynthesis. Most leaves have a thin, flattened part called a blade, which is attached to the stem by a thin stalk called a petiole. Leaves are made up of the three tissue systems. Leaves are covered on their top and bottom surfaces by epidermis. The epidermis of nearly all leaves is covered by a waxy cuticle, which protects tissues and limits water loss. The vascular tissues of leaves are connected directly to the vascular tissues of stems. Xylem and phloem tissues are gathered together into bundles called leaf veins that run from the stem throughout the leaf. The area between leaf veins is filled with a specialized ground tissue known as mesophyll, where photosynthesis occurs. Photosynthesis happens in the mesophyll, which has two specialized layers: The palisade mesophyll is beneath the upper epidermis. The cells are closely packed and absorb light. Beneath this layer is a loose tissue called the spongy mesophyll, which has many air spaces between its cells. These air spaces connect with the exterior through small openings called stomata. Stomata allow carbon dioxide, water, and oxygen to diffuse in and out of the leaf. The mesophyll cells lose water by evaporation. This loss of water through leaves is called transpiration. Transpiration helps to cool the leaves, but also threatens their survival during droughts. Gas Exchange and Homeostasis A plant s control of gas exchange is one of the most important elements of homeostasis. Plant leaves allow gas exchange between air spaces in the spongy mesophyll and the exterior by opening their stomata. Plants maintain homeostasis by keeping their stomata open just enough to allow photosynthesis to take place but not so much that they lose an excessive amount of water. Guard cells are highly specialized cells that surround the stomata and control their opening and closing depending on environmental conditions. Wilting results from the loss of water and pressure in a plant s cells. The loss of pressure causes a plant s cell walls to bend inward. When a plant wilts, its stomata close so the plant can conserve water. Lesson 23.4 Workbook A Copyright by Pearson Education, Inc., or its affiliates. All Rights Reserved. 368

2 Leaf Structure and Function For Questions 1 4, complete each statement by writing the correct word or words. 1. The structure of a leaf is optimized for the purposes of absorbing light and carrying out photosynthesis. 2. The epidermis of nearly all leaves is covered by a waxy cuticle. 3. The vascular tissues of leaves are connected directly to the vascular tissues of stems. 4. The area between leaf veins is filled with a specialized ground tissue known as mesophyll. For Questions 5 10, match the description with the leaf structure. Description F 5. A layer of mesophyll cells that absorb light that enters the leaf D 6. Small openings in the epidermis B 7. The thin, flattened part of a leaf A 8. A bundle of xylem and phloem tissues in a leaf C 9. A stalk that attaches a leaf to a stem E 10. A loose tissue with many air spaces between its cells Gas Exchange and Homeostasis 11. Why can t stomata be kept open all the time? Structure A. leaf vein B. blade C. petiole D. stomata E. spongy mesophyll F. palisade mesophyll Water loss would be so great that few plants would be able to take in enough water to survive. 12. Complete the flowchart that summarizes how guard cells help maintain homeostasis. Guard cells are forced into a curved shape when water pressure increases. The thick inner walls of the guard cells pull away from one another, opening the stoma. Water is lost by transpiration. Guard cells straighten out when water pressure decreases. The inner walls of the guard cells pull together, closing the stoma. Lesson 23.4 Workbook A Copyright by Pearson Education, Inc., or its affiliates. All Rights Reserved. 369

3 For Questions 13 17, write the letter of the correct answer on the line at the left. D 13. Which is likely to happen to a plant if it starts losing more water than it can take in? A. It will reproduce. B. It will flower. C. It will grow. D. It will wilt. B 14. Which is a plant that has narrow leaves with a waxy epidermis? A. cactus B. spruce C. rock plant D. rose bush C 15. A pitcher plant s leaves are adapted for A. conducting photosynthesis. B. limiting transpiration. C. catching and digesting insects. D. pollination and fertilization. C 16. A rock plant adapts to hot, dry conditions by having very few A. thorns. B. leaves. C. stomata. D. nutrients. A 17. A cactus s thorns are actually its A. leaves. B. stems. C. roots. D. bark. 18. The inside of the glass or plastic walls of a greenhouse full of plants is very wet on cool days. Where does this water come from? The inside surfaces of the walls of a greenhouse are wet because of water vapor from the air condensing on the cool glass or plastic. The water vapor in the air comes from the plants through the process of transpiration. Lesson 23.4 Workbook A Copyright by Pearson Education, Inc., or its affiliates. All Rights Reserved. 370

4 23.5 Transport in Plants Lesson Objectives Explain the process of water movement in a plant. Describe how the products of photosynthesis are transported throughout a plant. Lesson Summary Water Transport The pressure created by water entering the tissues of a root push water upward in a plant stem, but this pressure is not enough. Other forces are much more important. The major force is provided by the evaporation of water from leaves during transpiration. Its pull extends into vascular tissue so that water is pulled up through xylem. Both the force of attraction between water molecules, cohesion, and the attraction of water molecules to other substances, adhesion, help with water transport. The effects of cohesion and adhesion of water molecules are seen in capillary action, which is the tendency of water to rise in a thin tube. Capillary action is important because xylem tissue is composed of tracheids and vessel elements that form hollow, connected tubes. Nutrient Transport The leading explanation of phloem transport is known as the pressure-flow hypothesis. Active transport moves sugars into the sieve tube from surrounding tissues. Water then follows by osmosis, creating pressure in the tube at the source of the sugars. If another region of the plant needs sugars, they are actively pumped out of the tube and into the surrounding tissues. Pressure differences move the sugars to tissues where they are needed. Changes in nutrient concentration drive the movement of fluid through phloem tissue in directions that meet the nutritional needs of the plant. Water Transport For Questions 1 2, refer to the Visual Analogy of clowns being pulled up a ladder compared to water being pulled up a tree. 1. In the visual analogy of the climbing circus clowns, what makes it possible for the falling clowns to pull others up the ladder? The rope that ties the clowns to each other transfers the force of gravity pulling on the falling clowns back through the chain to the ones still climbing. Lesson 23.5 Workbook A Copyright by Pearson Education, Inc., or its affiliates. All Rights Reserved. 371

5 2. How are water molecules similar to the clowns? SAMPLE ANSWER: A force between water molecules is keeping them together and transferring the pull of transpiration back through the rising column of water. 3. Complete the table about the types of attraction between molecules. Attraction Between Molecules Type of Attraction Cohesion Adhesion Definition The attraction between molecules of the same substance The attraction between unlike molecules For Questions 4 8, complete each statement by writing the correct word or words. 4. Water cohesion is especially strong because water molecules tend to form hydrogen bonds with each other. 5. The tendency of water to rise in a thin tube is called capillary action. 6. The height to which water can rise in a tube is determined by its diameter. 7. Vessel elements in xylem form many hollow, connected tubes through which water moves. 8. The pull of transpiration extends from the leaves to the roots of a plant. Nutrient Transport 9. According to the pressure-flow hypothesis, why must sieve-tube elements in phloem be living cells? Sugars are moved across cell membranes in phloem tissue by active transport. This process requires ATP, which is made by the cell during cellular respiration. Only living cells can conduct cellular respiration. 10. Where sugar concentration is high, what is the source of water taken in by phloem? The water drawn in by osmosis in areas of high sugar concentration comes from xylem tissue. 11. How does the structure of the vascular bundles in stems and roots and of the veins in leaves make the process of pressure-flow possible? The vascular bundles and veins in leaves contain both xylem and phloem. The two are close together, which means that water can move from xylem to phloem without having to pass through many cells. Lesson 23.5 Workbook A Copyright by Pearson Education, Inc., or its affiliates. All Rights Reserved. 372

6 12. Complete the flowchart that summarizes the movement of sugars in plants. Photosynthesis produces a high concentration of sugars in cells called source cells. Sugars move from a source cell to phloem, and water moves into the phloem by the process of osmosis. Water moving into the phloem causes an increase in sieve tubes. pressure inside the The pressure causes fluid to move through phloem toward sugars are less concentrated. sink cells, where 13. What is one importance of the cell walls of xylem to the capillary action that occurs during transpiration? The cell walls of the tracheids and vessel elements in the xylem are made of cellulose, and water adheres very strongly to cellulose. 14. According to the pressure-flow hypothesis, what process prompts rapid spring growth in a plant? Chemical signals stimulate phloem cells in the roots to pump sugars back into phloem sap. These sugars are raised into stems and leaves to support growth. 15. Leaves range in size from very large to very tiny. In what type of environment would you expect to find the most plants with very large leaves? Very small leaves? Explain. Plants that live in warm, wet places tend to have larger leaves. They take in a lot of water and need to give off a lot of water by transpiration to maintain homeostasis. Plants that live in hot, dry places tend to have very tiny leaves. They conserve water by limiting the amount of surface area through which transpiration can occur. Lesson 23.5 Workbook A Copyright by Pearson Education, Inc., or its affiliates. All Rights Reserved. 373

7 Chapter Vocabulary Review For Questions 1 2, refer to the diagram. 1. What are the names of the two parts of a leaf indicated in the diagram? A. Guard cells B. Stoma 2. What process do the structures control? transpiration B. A. For Questions 3 9, match the description with the tissue or cell type. Description D 3. Ground tissue specialized for photosynthesis G 4. Layer of ground tissue that encloses the vascular cylinder B F E A C 5. Thick-walled cells in ground tissue 6. Dermal tissue in leaves and young plants 7. Region of actively dividing unspecialized cells 8. Very thick-walled cells that make ground tissue such as seed coats tough and strong 9. Thin-walled cells in ground tissue Tissue and Cell Types A. sclerenchyma B. collenchyma C. parenchyma D. mesophyll E. meristem F. epidermis G. endodermis For Questions 10 16, complete each statement by writing the correct word or words. 10. Most leaves have a flattened part called a blade, which is attached at a node on the stem by a petiole. 11. The root hairs increase a root s surface area for absorption, while the root cap protects the growing tip of the root. 12. The cells of the palisade mesophyll are tightly packed, but many air spaces separate the cells of the spongy mesophyll. 13. The meristem between xylem and phloem cells is called vascular cambium and forms wood by secondary growth. 14. In a mature stem, the tissues outside the vascular cambium make up the bark ; the tissues include phloem, cork, and the cork cambium. 15. Water is drawn to the material in cell walls by the process called adhesion. 16. Monocot stems have scattered vascular bundles while dicots form a ringlike pattern around the pith. Chapter 23 Workbook A Copyright by Pearson Education, Inc., or its affiliates. All Rights Reserved. 374

8 THE HOLLOW TREE In the Chapter Mystery, you learned about a tree that begins growing in the branches of other trees. Its roots get nutrients from materials that collect in the folds in the host tree s bark. Learning Scale Drawings Engineers often look to nature for inspiration. Early designers of airplane wings examined birds. Modern camera lens engineers are using the human eye for inspiration. Similarly, an engineer interested in extracting water or oil from the ground might look to a plant s roots for ideas. The engineer might start by making a model of a root. Often, the first step in building a model is making a scale drawing of the object. A scale drawing is a drawing that is the same shape but not the same size as the actual object. Maps and blueprints are examples of scale drawings. Scale drawings are often used to show objects that are too large or too small to be shown in detail in their actual sizes. For example, the root used for the blueprint below is actually 1/1,000 the size of the drawing. This makes the scale of the drawing 1 cm = cm. Epidermis Root hairs Cortex Endodermis Phloem Xylem Continued on next page Chapter 23 Workbook A Copyright by Pearson Education, Inc., or its affiliates. All Rights Reserved. 375

9 Themes Science Literacy 1. Considering the scale of the root blueprint, approximately how long would the root hairs on the actual root be? Students answers may vary, but should be near cm and cm. 2. If you were building a working model of a root, what properties would you want the root hairs to have? They should be long and thin, and capable of absorbing water. 3. Suppose the root blueprint is reduced to half its current size. What would the scale then be? 1 cm = cm 4. The root used to draw the blueprint is very small, thus the drawing is bigger than the actual object. Give an example of a plant part that you would most likely need to draw on a smaller scale if you were making a blueprint of that part. SAMPLE ANSWER: a tree trunk 5. Suppose you were going to use the root blueprint to build a model. Give some examples of materials you might use when making your model. Explain your answer. SAMPLE ANSWER: I might use small tubes to represent the vascular structures of the roots so that liquids could flow through them. I might use a ground cloth that allows water to pass through it in one direction only to construct the root s epidermis, thus mimicking the structure and function of an actual epidermis. Scale Drawing of a Leaf The skills used in this activity include problem identification, formulation, and solution; creativity and intellectual curiosity; and self-direction. Collect a leaf from a plant. Use a hand lens or microscope to examine the leaf closely. Then make a scale drawing of the leaf. The scale you use in your drawing should be appropriate to the size of the leaf you chose. If you collected a small leaf, your scale drawing should be larger than the actual leaf. The opposite should be true if the leaf you collected is very big. Make the drawing as detailed as possible. Be sure to label your drawing with the scale you used. You should also label any structures you recognize. Share your drawing with the class. Have the class calculate the actual size of the leaf using the scale you provided. Evaluate students drawings on accuracy and neatness. Make sure students included a scale on their drawings. Chapter 23 Workbook A Copyright by Pearson Education, Inc., or its affiliates. All Rights Reserved. 376