Maintaining a balance

Gill Sans Bold Biology HSC Course Stage 6 Maintaining a balance Part 4: Transport in plants IncorporatingOctober2002 AMENDMENTS

Gill Sans Bold Contents Introduction...2 Movement of materials in plants...3 Investigating vascular tissue...5 A different look at vascular tissue...10 Movement of substances through xylem and phloem tissue...12 Suggested answers...17 Additional resources...19 Exercises Part 4...23 Part 4: Transport in plants 1

Introduction So far you have been concentrating on the transportation of materials in animals. In this part you will look at the movement of materials in plants. To study the vessels that carry the materials you will need to purchase some celery, a single sided razor blade and some food dye. In this part you will have the opportunity to learn to: describe current theories about processes responsible for the movement of materials through plants in xylem and phloem tissue. In this part you will have the opportunity to: choose equipment or resources to perform a first-hand investigation to gather first-hand data to draw transverse and longitudinal sections of phloem and xylem tissue Extract from Biology Stage 6 Syllabus Board of Studies NSW, originally issued 1999. The most up-to-date version can be found on the Board's website at http://www.boardofstudies.nsw.edu.au/syllabus_hsc/syllabus2000_lista.html This version October 2002. 2 Maintaining a balance

Gill Sans Bold Movement of materials in plants The native Tasmanian oak tree (Eucalyptus regnans) grows to a height of over 120 metres approximately the height of an 8 10 storey building! Water must reach the leaves of this tree to be used in its photosynthesis and the products of photosynthesis need to be transported from the leaves to other parts of the tree, including the roots. How is this possible? You should be familiar with xylem tissue, which transports water and minerals in the plant, and with phloem, through which the products of photosynthesis are transported within the plant. You also need to know about the stomates or stomata (singular stomate or stoma), which are responsible for the uptake of carbon dioxide from the atmosphere and for transpiration. (See the Additional Resources section for revision material.) Xylem tissue Xylem tissue is made up of long tubular cells which join into pipes. These form the vascular system of ferns, gymnosperms and angiosperms, which are called the vascular plants and they belong to the phylum Trachaeophyta. The cells of the xylem grow to maturity and then die but are kept open by a material called lignin. Lignin is the major component of wood. In woody plants, old xylem cells form growth rings in the stem as the plant grows. bark operative xylem and phloem woody growth rings Tree rings are formed from old xylem cells. Phloem tissue Phloem tissue is also made up of long tubular cells but these cells are living and not thickened with lignin. They contain sieve plates at the ends of the cells so that fluids can move through the cells but cell components remain within each living cell. Part 4: Transport in plants 3

Vascular tissue In young stems of woody plants and the stems of all herbaceous plants (soft herbs), xylem and phloem occur in bundles (vascular bundles). In the roots of plants, xylem and phloem form a cylinder. Here is a three-dimensional diagram of a stem showing the position of the xylem and phloem tissues. cortex epidermis pith FIBRES dead cells with walls thickly coated with lignin are also found around and within the vascular tissue. These provide support to keep the plant erect. xylem cambium phloem vascular bundle Three-dimensional section of a stem from a flowering plant. 4 Maintaining a balance

Gill Sans Bold Investigating vascular tissue A section can be cut across a plant stem or root and this is called a transverse section (TS) or cross-section (XS). If you cut down from top to bottom the section is called a longitudinal section (LS). transverse section (TS) or cross-section (XS) longitudinal section (LS) Diagram showing both transverse and longitudinal sections. The photograph below shows the same cuts through a celery stalk. A transverse section and a longitudinal section of a celery stalk. (Photo: J West) Very thin sections can be cut by a machine called a microtome, which slices the tissue so thinly that light can pass through and it can be viewed using a microscope. The photographs below show vascular bundles in a transverse section from a stem. The other tissues shown in the photographs are of less concern to you and have a variety of functions. They include: cambium produces new xylem and phloem cells as the stem grows Part 4: Transport in plants 5

cortex sometimes called packing cells which have a range of functions but mostly are involved in storing materials (eg. starch) epidermis the outer layer of cells all over the plant pith larger cells similar to the cortex but in the middle of the stem. Cross-section of a stem. (Photograph J West) This photograph is a larger magnification of one vascular bundle. Close-up of a vascular bundle. ( Jane West) 6 Maintaining a balance

Gill Sans Bold Without a microscope, it is difficult to study these tissues in detail. However, you will be able to observe xylem and phloem tissues in a longitudinal section and a transverse section from a soft plant (non-woody or herbaceous) such as celery. Preparing and observing your tissue sections You will need: a fresh stick of celery a single sided razor a glass or container with water food dye (red works well). (Photos: J West) Part 4: Transport in plants 7

What to do: Place an end of a freshly cut stick of celery, with its leaves still attached, into a container containing a strong solution of food dye. Leave it overnight. For a transverse section Take a piece of the celery stalk and use a hard-backed razor blade to cut across it. This makes a transverse section. (Be careful not to cut yourself! If you do use a double-sided blade, put some tape over one side to make it safer to use.) Cutting a transverse section through the celery stick. (Photo: J West) Look at the cut end, either with the unaided eye or using a hand lens. You should see the vascular tissue (xylem and phloem) as bundles. The red dye, which will have moved up through the xylem cells, will highlight the bundles. 8 Maintaining a balance

Gill Sans Bold The darker regions are the vascular bundles. (Photo: J West) If you have a powerful hand lens you may be able to distinguish separate xylem cells. You will see what looks like the ends of very small tubes. If you do have access to a microscope (and know how to use it or can get someone to show you), you can cut fine transverse sections of the celery using the razor blade. Place a thin slice on a slide in a drop of water, cover it with a coverslip then view it under the microscope with x100 total magnification. (Can you remember which lenses you need to use? Yes, the x10 eyepiece lens and x10 objective lens.) The image you see will not be as clear as shown in the photographs here, but you should see that the xylem cells are very large with thick walls. It is hard to distinguish the phloem unless you stain it, as has been done in the slides used for the photographs. In Exercise 4.1, draw a simple diagram of your transverse section of the celery, showing the positions of the xylem and phloem in the vascular bundles. For a longitudinal section Cut off a piece of celery about 2 cm long. Cut a longitudinal section down through the piece, making sure that you are cutting through at least one vascular bundle. Use your hand lens, if you have one, to look closely at the long tubes. In Exercise 4.2, draw a simple diagram of your longitudinal section showing the position of the xylem and phloem in the vascular bundle. A different look at vascular tissue In your investigation, you have studied vascular tissue from a stem. Your observations (and diagrams) would be slightly different if you had prepared a transverse section and longitudinal section of a root instead. Part 4: Transport in plants 9

The following photograph shows a transverse section of a root. Compare it with the photograph of the transverse section of a stem earlier in this part. By comparing the photographs, you will be able to identify the different types of cells present in the root crosssection. Then draw a simplified diagram of the transverse section of the root in the space below its photograph. Label the cells that you have identified. (Do not attempt to draw all the cells! Draw an outline and label the areas of different cell types.) Cross-section of a root (Photograph Jane West). 0 Maintaining a balance

Gill Sans Bold Check your answer. Did you notice that the xylem vessels in the root are arranged in a cross or star shape at the centre of the root? The phloem vessels are in bundles around the xylem cells. Here is a close-up of the vascular bundle at the centre of a transverse section of a root. Close-up of the vascular bundle of a root (Photograph Jane West). Part 4: Transport in plants 11

Movement of substances through xylem and phloem tissue The xylem and phloem tissues in the plant are responsible for the transport of materials around the plant. These materials include water, minerals and the products of photosynthesis. Movement of water in plants Water moves into the plant through the large surface area of the root hairs. It moves from the soil, where it is usually in a higher concentration than in the plant. This process is called osmosis. Water moves from cell to cell within the plant by osmosis until it reaches the xylem cells. 2 Maintaining a balance

Gill Sans Bold Several theories arose for how water moved in the xylem once it reached this tissue. (Refer also to the Additional resources.) Originally biologists thought that the pressure created by osmosis in the roots pushed water up through the xylem. However, once it was possible to calculate pressures inside plants it was concluded that this pressure was not strong enough to push water up to reach the leaves of tall trees like the Tasmanian oak tree. It was also suggested that water crept up the very fine tubes of the xylem, a bit like ink moving through the fibres of blotting paper. This movement is called capillarity but again it was worked out that this movement would still not move water to the top of a tall tree. The suggestion that the water was pumped by the cells using energy from cellular respiration (active transport) was quickly abandoned. Since xylem cells are dead, they cannot carry out cellular respiration to produce the required energy. So how is it done? The theory which has most support is called the transpiration-tensioncohesion theory. This is how it appears to work. Water enters the roots by osmosis and reaches the xylem by the same process (by moving from an area of higher concentration to an area of lower concentration). A continuous column of water molecules occurs in the xylem. Water molecules stick together by a process called cohesion, where the positive end of one water molecule is attracted to the negative end of the next one in the xylem cells. As one water molecule evaporates (in transpiration) from the leaf surface through the stomate, another one is pulled up the column of water in the xylem by the negative pressure (tension) created, to replace it. In turn, another molecule of water moves by osmosis into the bottom of the xylem to replace the one that has moved up. In this way, microscopic columns of water continue to move up the xylem vessels, being essentially pulled up from the leaves (transpiration-cohesion-tension theory) rather than being pushed up from the roots (root pressure, capillarity and active transport theories). The movement of water from the roots through the xylem to the leaves is referred to as the transpiration stream. So is there evidence for this theory? Yes, there is; indeed some good evidence. Very accurate measurements of the stems of plants show that the stems actually shrink very slightly while the plant is transpiring, indicating pulling from the top. If water were being pushed from the bottom, you would expect a slight expansion of the stem as a result of that positive pressure. Water continues to move through a plant whose roots have been cut off (eg. cut flowers). If root pressure were the mechanism for water movement, you might expect the removal of the roots to significantly reduce water movement. Plants which have been chilled or poisoned to kill all living cells continue to conduct water. This rules out active transport as a mechanism of water transport. The figure below summarises the theory of the functioning of the transpiration stream. Part 4: Transport in plants 13

leaf water moves along xylem into leaf water evaporates into air spaces in spongy mesophyll water vapour escapes through stomata water drawn up xylem in the stem stem water around soil particles water enters root hairs by osmosis Movement of water in plants. water enters xylem in root Do Exercise 4.3. Movement of minerals in plants Most minerals (ions) are dissolved in water in the soil. They move into the plant through root hairs and then travel through xylem tissue. Some mineral ions enter the plant by diffusion, when they are in high concentration in the soil but are in lower concentration inside the plant roots. However, most are moved from the soil into the root by active transport. Some minerals are recirculated within the plant through the phloem. For example, it has been shown that radioactively marked phosphorus is transported from old leaves that are dying to new leaves which require this mineral for their growth. However, other minerals do not seem to be recycled (eg. calcium). These minerals need to be constantly taken up from the soil. Movement of products of photosynthesis Biologists studied the ways that radioactively marked sugars move throughout plants. Some observations include that: 4 Maintaining a balance

Gill Sans Bold their movement in the phloem is very rapid (as quickly as 1 metre per hour) the direction of movement can be reversed movement of materials can be in different directions in different parts of the same vascular bundle. These observations need to be explained by any theory of movement of products of photosynthesis in the phloem (usually called translocation). Obviously, the movement of materials in phloem is different from that in xylem. Unlike xylem cells, phloem cells are living so material stops moving through them when phloem cells die. This suggests that the translocation of products of photosynthesis through phloem tissue is an active process that demands the use of energy. The following diagram is a summary of the current theory for how translocation of sugars works. This theory is called the pressure flow theory. Part 4: Transport in plants 15

site of sugar production in leaves Sugars are pumped into the phloem cell due to active transport. This results in a higher sugar concentration inside the phloem cells. HIGH osmotic pressure Water moves into the phloem cells due to osmosis as a result of the higher sugar concentration. movement of sugar and water through the phloem along an osmotic pressure gradient Sugars are pumped out of phloem cells by active transport. Water moves out of phloem cells by osmosis due to more sugars in the surrounding cells. LOW osmotic pressure site of use of sugars in fruit, flowers, root and stem Diagram representing the pressure flow theory for the translocation of sugars in vascular plants. 6 Maintaining a balance

Gill Sans Bold Suggested answers A different look at vascular tissue epidermis small cells in cortex large cells in cortex xylem phloem Part 4: Transport in plants 17

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Gill Sans Bold Additional resources This information comes from the Preliminary module, Patterns in Nature. Vascular bundles This is the term used to describe the groups of conducting tissue in a stem. Each bundle contains three types of tissue: xylem, phloem and cambium. Xylem Xylem forms long tubes up to 1 m in length. They are made up of dead cells, thickened with woody material, with cross walls that have broken down. They are known as xylem vessels. Xylem gives support, strength and rigidity to the stem, and transports water and mineral ions upwards from the roots to the leaves. Note: Water and mineral ions travel only in one direction in the xylem upwards. Phloem Phloem consists of living sieve-tube cells forming long columns. There are perforations in the cell walls so that the cytoplasm of the cells connects along the tubes. Associated with the sieve-tube cells are companion cells and other supporting tissue. Organic materials including sugars, amino acids and hormones are transported by the living sieve-tube cells of phloem tissue. This movement is called translocation. Materials move both up and down the plant through phloem tissue. The movement is too fast to be caused by diffusion only. There are several theories suggesting possible forces involved but the exact mechanism remains unknown. Cambium Cambium cells are capable of cell division. They divide to form cells which become new xylem and phloem tissue. In older stems, division of cambium cells results in a continuous ring of vascular tissue. Part 4: Transport in plants 19

How water travels up plant stems Why does water move upwards in plants? Some of Australia s tallest trees, such as the Mountain Ash in Victoria and Tasmania, are more than 100 metres tall. How does water move to this height, defying gravity? Several processes seem to be involved in the upward movement of water. The processes include: adhesion capillarity root pressure transpiration-cohesion guttation. Adhesion Adhesion refers to the forces of attraction which exist between different types of particles. Using tissues or a cloth to mop up water works because of the force of attraction between dissimilar particles, the cloth particles and the water particles, or the tissue particles and the water particles. A piece of plastic would not be used to soak up water because the forces of attraction between the plastic particles and water particles are very weak while those between cloth and water are much stronger. In plants, cellulose acts like blotting paper or a cloth. Cell walls are made of cellulose. They help the plant to absorb water from the soil. Capillarity Capillarity is the name given to the action by which the surface of a liquid (usually water) is elevated when in contact with a solid surface by attraction of molecules between the liquid and solid surfaces. The water particles at the top of a column of water help to pull up the water particles beneath them. When the liquid is in a narrow vessel, the level of water will rise quickly, but capillarity can also occur in structures such as soils, causing a rise in the water table. The xylem vessels in plants extend from the roots to the leaves. They are extremely minute tubes or capillaries and water rises up them partly by a process of capillarity. Root pressure If the stem of a well-watered plant is cut off, water can be seen to come out of the plant from the severed xylem vessels. If a glass tube were attached to the cut stem with a piece of rubber tubing, water would be seen to rise up the glass tube. It can rise from several centimetres to more than one metre. This rise is said to be caused by root pressure. Root pressure is caused by the intake of water due to osmosis. Scientists have found that root pressure is too small to account for the rise of water upward in plants which are taller than several metres. 20 Maintaining a balance

Gill Sans Bold Transpiration-cohesion At the moment, the best theory which attempts to explain the upward movement of water in plants is the transpiration-cohesion theory. Water is continually leaving the plant via the leaves. This loss of water, in the form of water vapour from the leaves, is called transpiration. Now, as each water molecule passes out of the leaf, more water is drawn up, because of forces of cohesion (attraction) between water particles. The columns of water in plants can be likened to chains of beads. As one bead (water molecule) is pulled out of the leaf, the whole chain of beads (water molecules) is pulled up a little. As more beads (water molecules) are pulled out of the plant, more enter the plant in the root region. So, there is this continual stream of water molecules upwards in plants. Guttation You may have noticed, occasionally, drops of water (not dew) on certain outdoor plants and indoor plants, such as a Monstera deliciosa commonly called Monsteria (mon-stear-e-ah) or fruit salad plant. Generally, plants lose water in the form of water vapour; that is, water in the form of a gas. However, when humidity is high and plants are well watered, they may lose water as drops of liquid water. This process is called guttation. Many plants have special openings through which drops of water are forced out. These special openings may be found along the edges of leaves or near the ends of their veins. It is thought that high root pressure may cause guttation. Summary Several processes appear to be involved in the upward movement of water in plants. They include: adhesion forces of attraction between different particles are called forces of adhesion. The cellulose cell walls in plants soak up water by this process, in much the same way as a blotter soaks up water capillarity the rise of water in thin tubes by forces of adhesion and cohesion. Water rises up thin tubes because of attraction between the particles of the plant and water particles (adhesion) and because of the attraction between the water particles themselves (cohesion) root pressure the upward movement of water caused by the pressure from water moving into the root as a result of osmosis transpiration-cohesion the loss of water molecules from the leaves (that is, transpiration) results in the upward movement of more water molecules since these molecules are attracted to each other by forces of cohesion guttation the loss of water in the form of a liquid from openings on the leaves. Water enters the plant through the roots. The roots are covered by fine root hairs which increase the surface area for absorption of water. The root hairs are single celled extensions of the root epidermis (surface or outer layer of the root). Water enters the root hair by diffusion since the concentration of solutes in the soil water is lower than their concentration inside root hair cells. Water will move from an area of high water concentration (in the soil) to an area/region of low water concentration (within the root hair cells). Part 4: Transport in plants 21

root hair soil particles water Water moves into the plant from the soil through the root hairs. One of the main functions of stems is transport of substances around the plant. Internally, stems contain tubes of conducting tissue or vascular bundles, which consist of xylem and phloem, that carry materials between the shoot and root systems. The conducting tissue can be arranged in a ring or scattered throughout the stem tissue cortex. 22 Maintaining a balance

Gill Sans Bold Exercises Part 4 Exercises 4.1 to 4.3 Name: Exercise 4.1: For a transverse section Draw a transverse section of a celery stem in the space below. Remember to carefully label the diagram, including the xylem and phloem. Exercise 4.2: For a longitudinal section Draw a longitudinal section of a celery stem in the space below. Carefully label the diagram, including the xylem and phloem. Part 4: Transport in plants 23

Exercise 4.3: Movement of water in plants Outline three theories that have been used to explain the movement of water through xylem tissue. What is the accepted theory today and what evidence is there for this theory? 24 Maintaining a balance