Number 342 Transport in Plants This Factsheet: Explains why plants need a transport system Describes what plants transport Describes the tissues which carry out transport Outlines the position of the xylem and phloem in a plant Explains how the xylem s structure is related to its function Explains the movement of water through xylem Describes the detailed structure of phloem Explains how the structure of phloem is related to its function. Why Plants Need a Transport System As plants get bigger: They have a greater number of cells or volume The surface area of the plant, relative to the volume, gets smaller Plants, like animals, need to take in materials from their surroundings. They also need to remove waste products. A multicellular plant cannot get all the materials it needs by simple diffusion through its surface alone. Plants need a transport system to move materials around. What Do Plants Transport? Plants make their own nutrients by the process of photosynthesis. The green pigment (chlorophyll) in their cells, absorbs light energy. Plants use light energy to create essential compounds like glucose and amino acids. The raw materials for photosynthesis are: Water Carbon dioxide Water is an essential component of plants: It is needed for photosynthesis It provides structural support for the plant It contains dissolved mineral ions The water is transported in tissue called xylem. Figure 1 The main plant organs Flower Terminal bud Glucose is the sugar that is created by photosynthesis. Glucose is needed for: Respiration Growth and repair of new tissues Cell division Reproduction Glucose is transported around the plant. The glucose is converted to sucrose. Sucrose is transported in the phloem. Plant Transport Tissue There are three main plant organs: Roots These are found underground and have several roles: To anchor plants in the ground Absorb water through the root hairs Store food for the plant Stems The stem is above ground and has a couple of roles: Support the branches, leaves and flowers Carry out photosynthesis Leaves The leaves contain chlorophyll and have several other roles: Inside the plant, there is a vascular system. The vascular system Absorb sunlight energy for photosynthesis Provide a large surface area for the exchange of gases consists of the xylem and phloem tissue. The vascular tissue is Allow water vapour to diffuse out into the air during transpiration responsible for transporting materials around the plant. Some plants produce flowers. These are the organs of sexual reproduction. 1 Leaves Stem holding side branches and leaves Main root Side roots covered in root hairs Axillary bud
Figure 2 The location of the xylem and phloem in a plant Xylem and phloem forming the veins in the leaf Xylem and phloem in the leaf stem Xylem and phloem shift position in the stem. They are now under the epidermis Phloem tissue (coloured blue) Xylem tissue (coloured red) Xylem and phloem form a central core of vascular tissue in the root To observe the vascular tissue from another angle, the root and stem can be cut across. This cut is described as being transverse. Transport in the Xylem Xylem tissue is specially adapted to carry water and dissolved minerals. Water from the soil is absorbed into the plant through the root hairs. Exam Hint: If a question asks which part of a plant absorbs water, it is important to be precise. Roots can be impermeable and covered in a layer of corky material. It is the root hairs which absorb the water. Remember also that root hairs are complete single cells The xylem has many important characteristics: Xylem tissue is dead it forms the woody part of plants Xylem tissue is made of microscopic, hollow tubes called xylem vessels. Lignin or wood is laid down in the walls of xylem vessels, keeping the tubes strong, stopping them from collapsing, and waterproofing the vessels Xylem vessels have no contents so there is nothing to slow down the movement of the water The movement of water through the xylem is passive, meaning that energy is not required The water moves in one direction only in the xylem Water can move from one xylem vessel to the next through tiny holes called pits Figure 3 The position of xylem and phloem in cut sections of the root and stem Cut Section Y - Y in stem Phloem Y Y Xylem Epidermis X X Cut section X - X in root Parenchyma cells Phloem in arms of xylem Central core of xylem Root epidermis The root has been cut across from X to X. The stem has been cut from Y to Y. Endodermis Root hairs 2
342. Transport in Plants Exam Hint: Always refer to the hollow tubes of the xylem as vessels. They are not cells. Exam Hint: When answering a question on the structure of xylem, never state that xylem vessels are made of wood. This sounds as if the entire vessel is a woody block. Make it clear that it is the walls of the vessels that are strengthened with lignin or wood. Structure of Xylem Tissue Figure 4 How lignin is laid down in xylem vessels Three adjacent xylem vessels Direction of water flow Pits in the side walls of the vessels allow water to pass in and out Lignin in the walls of the vessels is laid down in different patterns Figure 5 A light microscope section through xylem and an electron microscope section through xylem Light microscope section of xylem in the centre of a root Phloem tissue Electron microscope section of xylem vessels Xylem vessels stained red in Parenchyma cells two dimensions. They can be packed with starch seen as hollow tubes Xylem vessels seen in three dimensions. The rings of lignin are clearly visible Movement of Water in Xylem 1. Soil water moves down a water potential gradient into the root hairs by osmosis (see Figure 6). 2. Water moves across the root cortex. There are two pathways that the water can take. These are: The apoplast pathway. The symplast pathway (See Figure 7). The water then moves into the xylem through the border pits in the vessels. 3. Once it gets inside the xylem, water always moves upwards. The water moves passively by two methods: Capillarity Transpiration pull 3
Figure 6 How water moves into a root hair cell Water surrounding the soil particles is at a higher water potential than the vacuole of the root hair cell Soil particles Vacuole of root hair cell has a lower water potential than soil water Soil water moves down a water potential gradient into the root hair cell by osmosis Root hair cell can actively pump mineral ions into the vacuole. This helps lower the water potential Water moves from cell to cell by osmosis Figure 7 How water moves across the cortex to the xylem Plasmodesma is a cytoplasmic bridge between cells Water moves from cell to cell through plasmodesmata Mineral ions carried in the water are concentrated here. This helps set up a water potential gradient from the soil to the xylem Water enters the cell via cell membrane and then enters cytoplasm Water moves through the cellulose cell walls Cells of root cortex Symplastic pathway Apoplastic pathway Casparian strip is a layer of waterproof material in the cells of the endodermis. This stops the apoplast pathway Xylem vessel Capillarity Water molecules stick to each other. Each molecule forms weak hydrogen bonds with the surrounding water molecules. This is called cohesion. Water molecules are also attracted to other molecules. In the xylem, water will stick to the walls of the vessels. This is called adhesion. 4
These sticky properties mean that water molecules will form a chain inside a narrow tube. The whole chain of water molecules can be pulled up the tube, under tension. The pull is caused by water molecules evaporating out of the xylem vessel. This movement up the tube is called capillarity. Figure 8 How water molecules move up through narrow xylem vessels Water molecules evaporating off the chain Water molecules stick together to form a chain. This is cohesion Water molecules stick to the walls of xylem vessel.this is adhesion Water molecule Transpiration Pull Transpiration is the loss of water vapour from the leaves. The water vapour diffuses out through pores called stomata. As water is lost from the leaves, there is less water in the mesophyll cells. The water potential in the mesophyll cells becomes lower than the water potential in the xylem vessels. This creates a pulling force called transpiration pull. Transpiration pull draws columns of water up through the xylem vessels. Figure 9 How water moves from the xylem vessel and diffuses out of a leaf section during transpiration Upper epidermal cells covered in waxy cuticle Palisade mesophyll cells Spongy mesophyll cells Xylem vessel carrying water Water vapour diffusing out of xylem and into air spaces Guard cell opens and closes stoma Lower epidermal cells Water vapour diffusing out of stoma (pore) in lower epidermis Exam Hint: Mesophyll tissue is photosynthetic tissue. It contains chloroplasts. Exam Hint: When describing transpiration, it is important to state that it is the movement of water vapour or evaporation, not the movement of water. 5
Transport in the Phloem Phloem carries sugars and other metabolites in solution. Phloem tissue has important characteristics: Unlike xylem, phloem is living tissue Phloem is more difficult to identify on a microscope slide because it is thin walled and easily distorted there is less phloem than xylem in plants Phloem tissue consists of two different types of cell known as sieve tube elements and companion cells The transport of sugars and metabolites in phloem is an active process, meaning that the energy of ATP is required The sugar carried by the phloem is sucrose Phloem can transport materials in two directions in a plant Sugars are loaded into the phloem at a source Sugars are unloaded from the phloem at a sink The movement of materials through the phloem is called translocation Structure of Phloem Tissue There are two main cell types in phloem: Companion cells Sieve tube elements Sieve tube elements are linked together to form vertical sieve tubes. The walls between neighbouring sieve tube elements are punctured by holes, forming a sieve plate. The holes allow the sugary sap to flow through. Sieve tube elements are free of organelles. This allows the contents to flow easily. Exam Hint: Always refer to sieve tube elements, not sieve cells or phloem cells. A sieve tube element is part of a sieve tube. Companion cells contain all the organelles that would be found in a plant cell. The companion cell provides the ATP for the active process of translocation. Figure 10 The phloem of a stem (as seen under a light microscope) Companion cell containing nucleus mitochondria, ribosomes and other organelles Waxy cuticle Sieve plate. This is a cross wall between adjacent sieve tube elements. It is pierced with holes to allow materials to flow between one sieve element and the next. Epidermis Sieve tube element 6
Figure 11 How a sieve tube element and its companion cell are linked Sieve plate Perforations in sieve plate Sap flowing through sieve tube element Companion cell containing organelles Plasmodesmata are cytoplasmic bridges between the companion cell and the sieve tube element The companion cells and sieve tube elements work together to transport nutrients around the plant. Nutrients are loaded into the phloem at a source. The nutrients are taken to a sink where they are unloaded. A source may be: A photosynthetic cell which creates sugars A storage organ like a root A sink may be: Part of the plant which can store sugars. For example, a root, a tuber or a bulb can store sugar as insoluble starch Part of the plant which requires sugars for growth and reproduction. For example, a meristem (a root tip or a shoot tip) Movement of Nutrients in Phloem Loading at a Source Figure 12 How sucrose is loaded into the sieve tube element at the source 4. 5. Companion cell supplies ATP 3. 1.. 2. Chloroplasts in photosynthetic cell create glucose. This cell is the source Glucose molecules are converted to sucrose for transportation 1. Hydrogen ions are pumped into the leaf cell. This requires ATP created by the mitochondria in the companion cells. 2. The hydrogen ions combine with the sucrose molecules. These two molecules diffuse back into the companion cell together. They move down a concentration gradient. This is called co-transport. 3. There is a higher concentration of sucrose in the companion cell than the sieve tube element. The sucrose moves down a concentration gradient. The sucrose moves through plasmodesmata into the sieve tube element. 4. Water from the xylem vessel moves into the phloem. The water moves down a water potential gradient. 5. A high hydrostatic pressure is created by the water and sugars entering the phloem. This pressure helps move the phloem sap along the sieve tube. 7
Unloading at A Sink Figure 13 How sucrose is unloaded at a sink 3. Companion cell 1. 2. Root cell. This is the sink Starch grains 1. Sucrose molecules diffuse down a concentration gradient into the companion cell. 2. Sucrose diffuses into the root cell. The sucrose is converted into insoluble starch for storage. 3. The water moves out of the sieve tube element and returns to the xylem. If the stored starch is needed by the plant, the sink becomes a source. The starch is converted back into sucrose and is loaded into the phloem. Practice Questions 1. The diagram shows how water passes across the root of a plant. Pathway 1. R. Pathway 2. S. T. Q. Xylem (a) Name the process by which water enters cell Q from the soil. (b) Identify pathway 1 and pathway 2 on the diagram. (c.) State which letter in the diagram represents the following: (i) a root hair cell (ii) the endodermis (d) What is the Casparian strip? (e) Explain how this strip helps control the flow of water in the root. 8
2. The table is a comparison of xylem vessels and phloem sieve tube elements. Answer the questions in the feature column. Insert the correct answer in the xylem vessel or sieve tube element column. Feature Xylem vessel Sieve tube element Cells living or dead Bordered pits present or absent Lignin present or absent Names of substances transported Direction of transport Transport active or passive Located towards the centre or the outside of the root Answers 1. (a) Osmosis (b) Pathway 1 = apoplast; Pathway 2 = symplast. (c) Q = root hair cell; S = endodermis (d) Casparian strip is a layer of waterproof material (suberin). This layer forms a complete band in the walls of each endodermal cell. (e) Casparian strip blocks the apoplast pathway. This stops water moving through the cellulose cell wall of the endodermis. All water entering the xylem has to go through the symplast pathway. 2. Feature Xylem vessel Sieve tube element Cells living or dead Dead Living Bordered pits present or absent Lignin present or absent Present Present Absent Absent Names of substances transported Direction of transport Water and minerals Up through plant in one way system (from root to stem to leaf) Water, sucrose, amino acids, herbicides Two directions. Both up and down Transport active or passive Located towards the centre or towards the outside of the root Passive Centre Active Outside Acknowledgements: This Biology Factsheet was researched and written by Margaret Royal and published in September 2017 by Curriculum Press. s may be copied free of charge by teaching staff or students, provided that their school is a registered subscriber. No part of these Factsheets may be reproduced, stored in a retrieval system, or transmitted, in any other form or by any other means, without the prior permission of the publisher. ISSN 1351-5136 9