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BIOLOGY IN FOCUS MAINTAINING A BALANCE Chapter 2 Transport—dissolved nutrients and gases i`>Ê ,7Ê>À}>ÀiÌÊ,"-" Ê ÜÌ Ê-Ìi« >iÊ"- HSCCOURSE Student Worksheet: Transport in xylem and phloem ■ describe current theories about processes responsible for the movement of materials through plants in xylem and phloem tissue ")/,/'9 ). &/#53 Linked to page 67 On the figure below, highlight the xylem in pink and the phloem in green. Complete the textboxes to outline the processes responsible for the treatment of materials in the xylem and phloem. TRANSPIRATION MASS FLOW sunlight 4 1 2 5 3 6 7 1 Copyright © 2008 McGraw-Hill Australia. Permission is granted to reproduce for classroom use. BIOLOGY IN FOCUS Use the text on pages 64–67 of Biology in Focus HSC Course textbook as well as the material from the next worksheet ‘Assumed Knowledge: Diagrams of xylem and phloem distribution and structure’ to answer the following questions. 1. Name the two transport tissues in plants. _____________________________________________________________________ 2. Identify the substances transported by each tissue and the direction of transport. _____________________________________________________________________ _____________________________________________________________________ 3. List the cells types of which xylem tissue is composed. _____________________________________________________________________ 4. Describe the structure of xylem vessels and tracheids and relate this to their functions. _____________________________________________________________________ _____________________________________________________________________ _____________________________________________________________________ _____________________________________________________________________ 5. List the two main types of cells of which phloem tissue is composed. _____________________________________________________________________ 6. Describe how the structure of each phloem cell type relates to its function. _____________________________________________________________________ _____________________________________________________________________ _____________________________________________________________________ _____________________________________________________________________ 7. Complete the following passage describing how materials are transported in xylem vessels. Use the words below: adhesion, passive, unloading at the sink, active, aplastic, pressure flow, transpiration stream, source-path-sink, cohesion, loading at the source, symplastic, decreases, increases Xylem vessels are not alive so the movement of materials is ___________, not active. ■ ___________: water is drawn up the xylem tubes to replace the loss of water through ___________ from the stomates. 2 Copyright © 2008 McGraw-Hill Australia. Permission is granted to reproduce for classroom use. BIOLOGY IN FOCUS ■ Movement through tubes by capillarity: — ___________: water molecules are polar and stick together. — ___________: water molecules stick to the walls of the vessels. 8. Insert the correct words to describe the theories used to explain how materials are transported in the phloem. Movement of materials is by a mechanism known as ___________ ___________. The flow of materials in phloem is an ___________ process that requires energy. Another name for the mechanism by which it occurs is the ________________________ system that is driven by pressure gradients generated osmotically. (a) ____________________________________________ Amino acids, sucrose and other mineral nutrients are loaded into the phloem in the leaves. There are two theories as to how this may occur: (i) ___________ loading: sugars and other nutrients move in the cytoplasm from the mesophyll cells to the sieve elements. (ii) ___________ loading: sugars and nutrients move along a pathway through the cell walls until they reach the sieve element. Materials would pass into the sieve cell by active transport. (iii) As sugars enter the phloem the concentration of phloem sap ___________ and the osmotic pressure at the source end is high. (b) Materials flow to the sink. At the sink, sugars and materials are actively being removed from the phloem (e.g. roots, stem, flowers or storage areas in the plant). As sugars are actively removed from the phloem, water flows out with them. This __________ pressure in the sieve cells at the sink region. This pressure difference between the source and sink in the phloem drives the phloem sap flow. The direction depends on where the sink areas of the plant are operating. 3 Copyright © 2008 McGraw-Hill Australia. Permission is granted to reproduce for classroom use. BIOLOGY IN FOCUS Answers TRANSPIRATION MASS FLOW sunlight Chloroplast—site of sugar manufacture by photosynthesis Mesophyll cell of leaf 4 1 Loss of water by evaporation Sugar released from illuminated chloroplast. More negative osmotic potential here Cytoplasmic connections between cells 5 Transpiration stream of water up the xylem vessels 2 High hydrostatic pressure in the mesophyll cell leads to bulk export along the sieve tubes Phloem sieve tube Companion cell 3 6 7 Less negative osmotic potential in starch storage cell, e.g. in root Uptake of water from soil Starch grain Water returns to the transpiration stream Cell of root Use the text on pages 64–67 of Biology in Focus HSC Course textbook as well as the material from the next worksheet ‘Assumed Knowledge: Diagrams of xylem and phloem distribution and structure’ to answer the following questions. 1. Name the two transport tissues in plants. Xylem and phloem 2. Identify the substances transported by each tissue and the direction of transport. Xylem: transports water and mineral ions upwards from the roots to the leaves. Phloem: transports organic materials (particularly sugar, amino acids and hormones) up and down the stem to other parts of the plant. 3. List the cells types of which xylem tissue is composed. Parenchyma and fibre cells 4 Copyright © 2008 McGraw-Hill Australia. Permission is granted to reproduce for classroom use. BIOLOGY IN FOCUS 4. Describe the structure of xylem vessels and tracheids and relate this to their functions. Xylem vessels are hollow—their end walls break to form a tube for easy flow of water and dissolved mineral ions. Vessels can be up to several metres long and are strengthened with lignin (in spiral, ring or net patterns), making them impermeable to water and giving them support. Pits in the walls enable water and solutes to pass through from one xylem element to another. Tracheids transport water and mineral ions. They are long, thin tubes with overlapping cells. They have pits in the walls so water can pass through from one tracheid to another. 5. List the two main types of cells of which phloem tissue is composed. Sieve cells and companion cells 6. Describe how the structure of each phloem cell type relates to its function. Sieve cells are joined end-to-end to form a series of connecting tubes. Sieve plates (membranes with pores) are found at the junction of sieve cells. This allows for the transport of phloem sap from one sieve cell to another. Mature sieve cells have a large internal space to allow for movement of sap. Companion cells are linked to sieve cells by plasmodesmata. They are thought to take on metabolic functions for the sieve plate. 7. Complete the following passage describing how materials are transported in xylem vessels. Use the words below: adhesion, passive, unloading at the sink, active, aplastic, pressure flow, transpiration stream, source-path-sink, cohesion, loading at the source, symplastic, decreases, increases Xylem vessels are not alive so the movement of materials is passive, not active. Transpiration: water is drawn up the xylem tubes to replace the loss of water through evaporation from the stomates. ■ Movement through tubes by capillarity: — cohesion: water molecules are polar and stick together. — adhesion: water molecules stick to the walls of the vessels. ■ 8. Insert the correct words to describe the theories used to explain how materials are transported in the phloem. Movement of materials is by a mechanism known as translocation. The flow of materials in phloem is an active process that requires energy. Another name for the mechanism by which it occurs is the source–path–sink system that is driven by pressure gradients generated osmotically. 5 Copyright © 2008 McGraw-Hill Australia. Permission is granted to reproduce for classroom use. BIOLOGY IN FOCUS (a) Loading at the source Amino acids, sucrose and other mineral nutrients are loaded into the phloem in the leaves. There are two theories as to how this may occur: (i) Symplastic loading: sugars and other nutrients move in the cytoplasm from the mesophyll cells to the sieve elements. (ii) Apoplastic loading: sugars and nutrients move along a pathway through the cell walls until they reach the sieve element. Materials would pass into the sieve cell by active transport. (iii) As sugars enter the phloem the concentration of phloem sap increases and the osmotic pressure at the source end is high. (b)Path–sink Materials flow to the sink. At the sink, sugars and materials are actively being removed from the phloem (e.g. roots, stem, flowers or storage areas in the plant). As sugars are actively removed from the phloem, water flows out with them. This reduces pressure in the sieve cells at the sink region. This pressure difference between the source and sink in the phloem drives the phloem sap flow. The direction depends on where the sink areas of the plant are operating. Note to teacher: An additional worksheet is provided overleaf to assist weaker students create a flow chart showing pressure flow in the xylem and phloem. 6 Copyright © 2008 McGraw-Hill Australia. Permission is granted to reproduce for classroom use. BIOLOGY IN FOCUS MAINTAINING A BALANCE Chapter 2 Transport—dissolved nutrients and gases i`>Ê ,7Ê>À}>ÀiÌÊ,"-" Ê ÜÌ Ê-Ìi« >iÊ"- HSCCOURSE Additional worksheet: Pressure flow in the xylem and phloem ")/,/'9 ). &/#53 Linked to page 67 Student task Select words from the list below to complete the flow chart provided. pressure difference, loading, sink, decreases, increases, removed Flow chart outlining pressure flow in phloem ______________ occurs at the source (symplastic/apoplastic) Ð As sugar enters the phloem, the concentration of the fluid in the phloem ______________, and so the osmotic pressure increases Ð Materials flow to the ______________ Ð Sugars and water are ______________ at the sink Ð Pressure ______________ in sieve cells in sink region Ð ______________ ______________ drives the phloem sap flow 1 Copyright © 2008 McGraw-Hill Australia. Permission is granted to reproduce for classroom use. BIOLOGY IN FOCUS Answer Loading occurs at the source (symplastic/apoplastic) Ð As sugar enters the phloem, the concentration of the fluid in the phloem increases, and so the osmotic pressure increases Ð Materials flow to the sink Ð Sugars and water are removed at the sink Ð Pressure decreases in sieve cells in sink region Ð Pressure difference drives the phloem sap flow 2 Copyright © 2008 McGraw-Hill Australia. Permission is granted to reproduce for classroom use.