Gill Sans Bold. Biology Preliminary Course Stage 6. Patterns in nature. Part 5: Obtaining and transporting materials in plants

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1 Biology Preliminary Course Stage 6 Patterns in nature Part 5: Obtaining and transporting materials in plants IncorporatingOctober2002 AMENDMENTS

2 Contents Introduction... 2 Autotrophic and heterotrophic cells... 4 Obtaining nutrients in plants... 5 Photosynthesis...5 Function of leaves...11 The stem...13 Roots...16 Transport systems in plants Xylem...19 Phloem...28 Gas exchange in plants Suggested answers Exercises Part Part 5: Obtaining and transporting materials in plants 1

3 Introduction Plants have specialised structures to obtain nutrients from their environment. You may recall that plants and animals obtain nutrients differently. Plants rely on the Sun to manufacture food by a process called photosynthesis. Plants are autotrophic organisms. Animals cannot manufacture their own food; they consume or eat other organisms in order to gain the nutrients they require for life processes. Animals are heterotrophic organisms. Plants and animals have specialised cells, tissues and organs to obtain the nutrition they require and carry out their body processes. Some of these will be investigated in this part. In this part you will be given opportunities to learn to: distinguish between autotrophs and heterotrophs in terms of nutrient requirements identify the materials required for photosynthesis and its role in ecosystems identify the general word equation for photosynthesis and outline this as a summary of a chain of biochemical reactions explain the relationship between the organisation of the structures used to obtain water and minerals in a range of plants and the need to increase the surface area available for absorption explain the relationship between the shape of leaves, the distribution of tissues in them and their role outline the transport system in plants including: root hair cells xylem phloem stomates and lenticels 2 Patterns in nature

4 In this part you will be given opportunities to: plan, choose equipment or resources and perform first hand investigations to gather information and use available evidence to demonstrate the need for chlorophyll and light in photosynthesis perform a first hand investigation and gather first hand data to identify and describe factors that affect the rate of transpiration perform a first hand investigation of the movement of materials in xylem or phloem. Extracts from Biology Stage 6 Syllabus Board of Studies NSW, originally issued The most up-to-date version can be found on the Board s website at This version November To complete the practical activities in this part you will require the following equipment. Alternative exercises have been included. 1 large beaker or saucepan 1 small beaker or glass jar Bunsen burner or hot plate tripod and gauze (if using Bunsen) 250 ml water 50 ml methylated spirit a few soft fleshy leaves such as a geranium aluminium foil a variegated leaf plant iodine solution stick of celery glass of water with food colouring (red/blue works best) knife, small kitchen type hand lens or microscope with lamp glass slides and cover slips if using microscope thin glass tubing or clear plastic tubing Vaseline or petroleum jelly soft, fleshy plant stem eg. Impatiens marker pen or sheet of graph paper scissors retort and clamp or similar. Part 5: Obtaining and transporting materials in plants 3

5 Autotrophic and heterotrophic cells Cells can be classified as either autotrophic or heterotrophic depending on how nutrition is obtained. Autotrophic cells are those which can make their own food (auto = self; trophic = feeding). Plant cells with chloroplasts are autotrophic. The Sun s energy is used to combine simple substances like carbon dioxide and water. These two raw materials are used to make glucose. Glucose can be changed into starch and other more complex substances like cellulose. Heterotrophic cells are those which cannot make their own food (hetero = other). Heterotrophs depend on food made by others. Heterotrophic cells include animal cells, fungal cells and some bacterial cells. Examples of autotrophic cells are: Examples of heterotrophic cells are: Check your answers. Complete Exercise Patterns in nature

6 Obtaining nutrients in plants Photosynthesis Plants carry out the food making process called photosynthesis. In this process plants convert the Sun s energy into chemical energy stored in sugars such as glucose. To do this, plants must have access to materials such as carbon dioxide and water. Oxygen and water are also produced in the process. Inputs carbon dioxide water light Outputs oxygen water sugars What do you remember about photosynthesis? 1 Where does photosynthesis take place in a plant? 2 Name the cell organelle where photosynthesis takes place. Part 5: Obtaining and transporting materials in plants 5

7 3 Describe the conditions necessary for photosynthesis to take place. Check your answers. Plants are producers as they make their own food. Plants are the first step in a food chain. They use light energy to produce carbohydrates like glucose. These carbohydrates are eaten by animals (herbivores and omnivores). In turn, herbivores are eaten by carnivores and so on through the food chain. So, in an ecosystem photosynthesis is an important step in the flow of energy. The phases of photosynthesis There are two main stages or phases in the photosynthetic process. These are called the light and dark phases. The light phase During this phase, light is absorbed by chlorophyll and the splitting of water molecules occurs. Water molecules are split to form oxygen and hydrogen ions. Light acts on the chlorophyll. The energy is converted from light to chemical energy. These reactions take place in the grana of the chloroplast. Many enzymes are used to carry out the process. chlorophyll a light ENERGY oxygen released splits H 2 O H+ to the second phase Simplified light phase of photosynthesis. 6 Patterns in nature

8 The dark phase The dark phase is often called the fixation of carbon phase. Carbon dioxide is fixed into glucose molecules using hydrogen ions and energy obtained during the light phase. hydrogen from light phase energy from light phase H + + CO 2 5C sugar many steps Simplified dark phase of photosynthesis. This phase also involves a number of steps in the reaction where specific enzymes are required. This series of reactions occur in the colourless fluid of the chloroplast called the stroma that surround the grana. Light is not required for these reactions. Glucose is synthesised during the dark phase. 1 Outline what happens in the light phase of photosynthesis. 2 Outline what happens in the dark phase of photosynthesis. Check your answers. Word equations are used to describe reactions. They can also be used as a summary for complex pathways. The general word equation for photosynthesis is shown below. light energy carbon dioxide + water sugar + oxygen chlorophyll You can see from the previous diagrams that photosynthesis is a complex process. So, this equation is a summary of the biochemical reactions that make up the process of photosynthesis. You do not need to learn these biochemical reactions. Part 5: Obtaining and transporting materials in plants 7

9 A general equation for photosynthesis is: 6CO 2 +6 H 2 O C 6 H 12 O 6 + 6O 2 Radioactive tracing has shown that a more correct equation for photosynthesis is: 6CO H 2 O C 6 H 12 O 6 +6O 2 + 6H 2 O Complete Exercise 5.2. Why do most plants appear green? Photosynthetic pigment is a mixture of a number of different pigments, including chlorophyll. Chlorophyll absorbs mostly blue violet and red light and reflects green. This characteristic of reflecting green light is why most leaves appear green in colour. Engelmann s experiment Thomas Englemann was a German biologist who investigated the wavelength of light used by plants during photosynthesis. He used bacteria to detect the presence of oxygen by observing the change in bacteria numbers in different light environments. The environments were created by projecting the whole spectrum onto a filament of algae. The results indicated that red and violet wavelengths are absorbed by chlorophyll pigments, resulting in the increased production of oxygen and increased numbers of bacteria in the corresponding regions. The need for chlorophyll and light in photosynthesis In this activity you will be investigating the need for chlorophyll and light in photosynthesis. The first experiment examines the production of starch as an indicator that photosynthesis has occurred in parts of a leaf. The second experiment focuses on the production of starch in areas of leaves that do not contain chlorophyll. 8 Patterns in nature

10 Variegated leaves have areas that do not contain chlorophyll (Photo J West). Aim The aim of this experiment is to demonstrate the need for chlorophyll and light in photosynthesis. Materials required: 1 large beaker or saucepan 1 small beaker or glass jar Bunsen burner or hot plate tripod and gauze (if using Bunsen) 250 ml water 50 ml methylated spirit a few soft fleshy leaves such as a geranium aluminium foil a variegated leaf plant (leaves with a mixture of colours choose one that is a mixture of yellow and green) iodine solution. Method: 1 Place aluminium foil over one half of your leaf on the plant, secure with paperclips and leave overnight. Part 5: Obtaining and transporting materials in plants 9

11 2 Place in direct sunlight for several hours. 3 Place your variegated plant in sunlight for several hours. 4 Pick the leaves from both plants and place the leaves in a beaker with the water. Remove the foil from the soft leaf. Heat the water and leaves gently, until they go very limp. 5 Turn off the heat source. 6 Remove the leaves from the water and place into the small beaker or jar with the methylated spirit. Care should be taken using methylated spirit, as it is a flammable substance. Avoid contact with a naked flame. Even methylated spirits vapours are flammable. 7 Place the small beaker or jar into the hot water and allow to stand for approximately five minutes. The green pigment should be extracted from the leaves after this time. If the methylated spirit has not become very dark green, stir the leaves and leave for a few extra minutes. 8 After there has been sufficient chlorophyll extracted, remove the leaves and wash them in water. 9 Now place your two leaves onto a white surface and flood them with iodine. Remember that iodine turns blue black in the presence of starch and starch is produced in areas that are actively photosynthesising. 10 Patterns in nature

12 Results In the areas that produce starch and are carrying out photosynthesis the iodine will turn blue black or purple. In the yellow areas of variegated plants there is no chlorophyll so these areas should not be coloured. In the leaf that was covered with aluminium foil the light would not have got under the foil and photosynthesis would not have occurred. Conclusion The experiment demonstrated that photosynthesis does not occur unless there is both light and chlorophyll present. Do Exercise 5.3 now. Function of leaves Go outside into the garden or take a walk to a park. Look at the leaves on the plants and sketch three different ones on your own paper. Note, on your drawings, three similarities and three differences in the leaves you selected. You will have noticed that most leaves are thin and flat. There are a large range of leaf shapes. Being thin and flat means that leaves have a large surface area to volume ratio which is important for the absorption of light, oxygen and carbon dioxide. Part 5: Obtaining and transporting materials in plants 11

13 epidermis cuticle palisade mesophyll layer spongy mesophyll cells containing chloroplasts xylem phloem vascular bundle stomate cell wall air space Cross section of a leaf. Source: Messel, H (chair). (1963.) Science for high school students. The Foundation for Nuclear Energy. University of Sydney. The chloroplasts are located in the mesophyll (middle leaf) region of the leaf. Gases enter and leave the leaf through the stomates. Therefore, the structure of the leaf ensures that the photosynthetic cells that contain chlorophyll are close enough to the top of the leaf to receive light and close enough to the stomates to gain the gases they require. 1 Outline the features of leaves. 2 Identify the major role of leaves. 12 Patterns in nature

14 3 a) Identify the tissue where photosynthesis occurs in leaves. How does the location of this tissue assist in photosynthesis? b) Xylem carries water up through the plant. How does the location of this tissue help in photosynthesis? c) Xylem is associated with phloem in plants. Predict a role related to photosynthesis for phloem in plants.? 4 Can you suggest a reason why leaves are thin and flat? (Hint: Think about the effects of the SA:V.) Check your answers. The stem You have looked at the structure of leaves. This is where photosynthesis occurs. But how does the products of photosynthesis get to the other parts of the plants. To answer that question you need to look at the structure of the stem. Sketch a plant from your garden and label the stem, leaves, flowers and buds. Use your own paper. Stems can be recognised because they have leaves and buds. Most stems are above ground, forming part of the shoot system of plants. Some plants have underground stems and their leaves may be reduced to scales. Although the arrangement varies with different types of plants, stems usually form a complex branching pattern. The leaves are spaced along them to gain maximum exposure to sunlight. Part 5: Obtaining and transporting materials in plants 13

15 Tissues in stems Stems are usually green when young and can carry out photosynthesis. They may become woody when older. At the tip of each stem is a terminal bud, a growing point for the plant. Stems also support the flowers and fruit of the plant. One of the main functions of stems is transport. Internally, stems contain tubes of conducting tissue, the xylem and phloem. This vascular tissue carries materials between the shoot and root systems. The conducting tissue is arranged in a ring or scattered throughout the stem tissue. The outer covering of stems, the epidermis, forms an impermeable layer protecting the inner cells and preventing water loss. There are stomates for the exchange of gases on young green stems and lenticels which serve the same purpose on woody stems. Cells in the cortex and pith usually store food but may also contain chloroplasts and photosynthesise. There are air spaces between cells for the circulation of gases. Some stems are hollow with little or no pith. Cross section of a stem showing vascular bundles. (Photo Jane West) 14 Patterns in nature

16 Close up of a vascular bundle showing xylem, cambium and phloem (Photo Jane West) Vascular bundles Vascular bundles are groups of conducting tissue in a stem. Each bundle contains three types of tissue: xylem, phloem and cambium. Xylem forms long tubes up to 1 m in length. They are dead cells (they have no nucleus). These long tubes are known as xylem vessels. Xylem vessels are thickened with woody material, with cross walls that have broken down. 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 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 Part 5: Obtaining and transporting materials in plants 15

17 sieve tube cells of phloem tissue. This movement is called translocation. Materials move both up and down through the plant in the phloem. The movement is too fast to be caused only by diffusion. There are several theories suggesting possible forces involved but the exact mechanism remains unknown. Cambium cells are capable of cell division. They divide to form cells which become new xylem and phloem tissue. In older stems division of the cambium cells results in a continuous ring of vascular tissue. Roots You have now read about how and why plants transport water. Have you asked yourself where they get the water? Roots do not photosynthesise but grow through the soil anchoring the plant and supplying the plant with water and mineral ions. To do this roots have to have an extensive surface area to be able to absorb water. The drawing below shows a young root covered in root hairs. The root hairs greatly increase the surface area of the root so that water can pass from the soil into the plant. As well as root hairs plants have different types of roots. The two main types of roots are fibrous roots and tap roots. 16 Patterns in nature

18 fibrous taproot 1 Outline the differences between taproots and fibrous root systems. 2 What structures of root systems increase surface area to improve water uptake? 3 Explain how the large surface area of roots assists in the survival of plants in dry weather. Check your answers. Complete Exercise 5.4. Part 5: Obtaining and transporting materials in plants 17

19 Transport systems in plants You have already looked at some of the structures involved in the transport system of plants. Answer these questions below for revision. 1 What is the role of the root, stem and leaf in flowering plants? 2 Plants have a system of vascular bundles to transport sugars, gases and water within the plant. The term vascular bundle is used to describe the conducting tissue in a stem. What type of tissue is found in a vascular bundle? Check your answers. xylem tubes move water up the plant from the roots phloem tubes move sugars dissolved in water throughout the plant 18 Patterns in nature

20 Like all multicellular organisms, plants need to transport materials from one place to another. Sugars are produced by the process of photosynthesis in the leaves. Every cell in a plant requires sugars for respiration. So, sugars are transferred from where they are produced to where they are needed. The same is true for water and minerals. These are taken into the plant through the root hairs and are needed by every cell in the plant. This transport function is carried out by xylem tissues (for water and minerals) and phloem (for sugars). Cambium cells are capable of cell division. They divide to form cells, which become new xylem and phloem tissue. In older stems, division of the cambium cells results in a continuous ring of vascular tissue. Xylem Xylem forms long tubes up to one metre in length. They are made up of dead cells, thickened with woody material (lignin), the cross walls 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. Movement of water and dissolved chemicals takes place in xylem vessels which form part of the vascular bundles within roots, stems and leaves. Detailed information on the processes involved in the movement of substances through the xylem can be found in the Additional resources section of this part. Stems can be recognised because they have leaves and buds. Most stems are above ground, forming part of the erect shoot system of plants. Some plants have underground stems and their leaves may be reduced to scales. Stems are usually green when young and can carry out photosynthesis. They may become woody when older. At the tip of each stem is a terminal bud, a growing point for the plant. Stems also support the flowers and fruit of the plant. Part 5: Obtaining and transporting materials in plants 19

21 In this activity you will be investigating the movement of water through the plant. Materials required: stick of celery glass of water with food colouring (red/blue works best) knife, small kitchen type hand lens or microscope with lamp glass slides and cover slips if using microscope. What you will do: 1 Place a stick of celery into the glass of coloured water for a few hours. Place the celery into the glass. (Photo: J West) 2 Remove celery from water and cut in half, carefully, cutting away from fingers. Using the hand lens, examine the stem and the location of the coloured water. 20 Patterns in nature

22 Cut across the stem. (Photo: J West) The coloured liquid is seen in the xylem. (Photo: J West) 4 Where in the stem is the coloured water located? 5 Name the tissue in which the water is located. Part 5: Obtaining and transporting materials in plants 21

23 6 Draw a sketch of your observations. If you have access to a microscope, prepare a slide of a cross section of the celery stem. Observe this specimen under the microscope and look at it. (A cross section is produced by cutting across, not lengthways.). 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. The concentration of solutes in the soil water is lower than inside the root hair cells. Water will move from an area of high water concentration (in the soil) to an area of low water concentration (within the root hair cells). root hair soil particles water Water moves into the plant from the soil through the root hairs. 22 Patterns in nature

24 One of the main functions of stems is transport of substances around the plant. Stems contain tubes of conducting tissue or vascular bundles, which consist of the xylem and phloem, that carry materials between the shoot and root systems. The conducting tissue is arranged in a ring or scattered throughout the stem tissue. Transpiration When stomates are open gases including carbon dioxide can diffuse into a plant. At the same time, however, water molecules can diffuse into the air because of the higher water concentration inside the plant. Water evaporates from the cell surfaces, diffuses through the intercellular spaces and out through the stomates. This diffusion of water from a plant is called transpiration. Water loss by transpiration is unavoidable by a plant with the stomates open. The water lost needs to be replaced by uptake through the roots. There is a constant upward flow of water through a plant. This is known as the transpiration stream. If water loss exceeds water intake, the stomates close and cells lose their turgidity. The stems and leaves wilt and the plant may die. Transpiration is an important part of the mechanism by which water and mineral ions are transported from the roots to the stems and leaves. The evaporation of water has a cooling effect on the plant, particularly the leaves. Factors affecting transpiration The structure of the plant has an effect on the transpiration rate. Stomates may be open or closed. When they are closed the transpiration rate drops and diffusion occurs at a much slower rate through the cuticle. Normally, stomates are open during the day for the exchange of gases in photosynthesis and closed at night. Some plants have special features (adaptations) to reduce the transpiration rate. Structural features may include a very thick cuticle, sunken stomates, hairs on the leaf or a reduction in leaf surface area. Physiological features may include the closure of the stomates or rolling up of the leaf to reduce surface area, during the day when the temperature is high. There are a number of external (environmental) factors that affect transpiration in a plant. These are temperature, humidity, wind, light and soil. Part 5: Obtaining and transporting materials in plants 23

25 In high temperatures, diffusion is more rapid (warm air holds more water than cold air). If the atmosphere is saturated with water vapour (conditions of high humidity) transpiration is decreased. Moving air increases the transpiration rate. Water vapour is carried away from the leaf and a high diffusion gradient maintained. Light intensity affects stomate opening and this in turn affects the transpiration rate. The water content of the soil and the solute concentration affect the rate at which water can be taken up by a plant. Measuring transpiration A potometer is an instrument which can be used to measure the rate of transpiration. There are several varieties of potometers. In a potometer, water is run into the apparatus through a glass funnel. A soft, fleshy twig or branch is pushed into the tubing which must be completely filled with water (it may assist to colour the water so that the movement can be easily seen). This must be a tight fit, otherwise water will run out of the capillary tube and the instrument will not function correctly. The area around the neck of the tubing where the plant has been inserted needs to be sealed, using Vaseline or petroleum jelly. soft fleshy plant funnel or well water base very thin gradated glass tubing Experimental set up to measure the rate of transpiration. 24 Patterns in nature

26 As time passes, the thin thread of water moving along the capillary tube towards the plant will be visible. A scale fitted behind the capillary tube helps in the measurement of the rate of transpiration. It is possible to accelerate the process by changing the environmental conditions of the plant. For example, by using a fan it is possible to simulate windy conditions. Other variables (factors) such as temperature can be investigated for their effect on the rate of transpiration. The effect of the environment on transpiration Optional activity If you have access to the equipment below then carry out the experiment. If not, answer the questions at the end of the experiment in the results section. The aim of this experiment is to compare the rate at which a leaf loses water, that is, transpires, under different conditions. Materials required: thin glass tubing or clear plastic tubing (look at the diagram to see what it could look like) Vaseline or petroleum jelly soft fleshy plant stem eg. Impatiens marker pen or sheet of graph paper scissors retort and clamp or similar. What you will do: 1 Fill the funnel with water. 2 Insert the soft, fleshy branch into the tubing 3 Smear Vaseline, paraffin or fat around the join between the stalk and the tubing. This join must be airtight when removed from the water. 4 Check that there is no water leaking from your potometer at any point. Water is most likely to escape in the area where the stalk is placed into the tubing. 5 Set up a second potometer in the same way, but this time omit the leafy branch. The second potometer is used as the control. What is the function of a control? Part 5: Obtaining and transporting materials in plants 25

27 6 Expose the potometer and the control to the following conditions and measure the time taken for the water to move 2 cm along the glass tube. Conditions to which the experiment and control and to be exposed are: cool and shady (in a room away from a window or draught) cool and windy (use a fan for creating wind ) hot and shady (use a radiator) hot and windy (use a radiator plus a fan). 7 Draw up a table of results on your own paper and enter your measurements. 26 Patterns in nature

28 Results: 1 Outline the conditions where you would expect the transpiration rate to be greatest. 2 Which type of conditions causes plants to wilt? 3 Why do people need to top up the water in vases with cut flowers? 4 Not all the water lost from vases of flowers is taken up by the plants. Explain. Conclusion Transpiration rate is affected by different external conditions. For each of the conditions below describe the rate of transpiration (fast, medium, slow). cool and shady cool and windy hot and shady hot and windy Explain briefly how nutrients are obtained and transported around plants. Your answer should include the names of the main structures and processes involved. Check your answer. Part 5: Obtaining and transporting materials in plants 27

29 Summary of processes of water transport Several processes appear to be involved in the upward movement of water in plants. 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 some way as a blotter soaks up water. Capillarity: capillarity is the rise of water in thin tubes by forces of adhesion and cohesion. The 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: this refers to the upward movement of water caused by the pressure from water moving into the root as a result of osmosis. Transpiration cohesion: the transpiration cohesion theory proposes that 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: is the loss of water in the form of a liquid from openings on the leaves. Phloem Phloem tissue like xylem tissue, consists of tube like cells. In phloem these cells are called sieve cells and they form long columns. When these cells mature they lose their nuclei. There are perforations in the cell walls at the end so that the cytoplasm of the cells can connect along the tubes. These are called sieve plates. Associated with the sieve tube cells are companion cells (which retain their nuclei and cytoplasm) and other supporting tissue. 28 Patterns in nature

30 Phloem tube. Translocation Organic materials including sugars, amino acids and hormones are transported by the living sieve tube cells of the phloem tissue. This movement is called translocation. The material that flows through phloem is called sap. The approximate composition is 30% plant sugars and 70% water. Materials move both upwards and downwards through the plant. The movement is too fast to be caused only by diffusion. There are several theories suggesting possible forces involved however the exact mechanism remains unknown. Probably the most widely accepted explanation for the mechanism of phloem translocation is the pressure flow hypothesis of Ernst Much, which was proposed in Leaves and roots can be sources of nutrients; nutrients are unloaded into stem apexes, flowers, fruits and roots. Movement of sap through the phloem results in pressure within the cells. When aphids stick a feeding tube into the phloem, the sap is forced through the aphid s body. Complete Exercise 5.4. Part 5: Obtaining and transporting materials in plants 29

31 Gas exchange in plants Like multicellular animals, multicellular plants usually have specialised tissues for gas exchange. You will look mainly at angiosperms (flowering plants) and algae (seaweeds and their relatives) in this section. Respiration is the process by which energy is released for use by the cell. All plant cells respire. Plant cells respire aerobically (most of the time). This means that they use oxygen gas in the process and release carbon dioxide gas as a waste product. Some plant cells produce glucose by the process of photosynthesis. During photosynthesis carbon dioxide is used and oxygen is released. During daylight a plant respires as well as carrying out photosynthesis. In sunlight, plants: release more oxygen from photosynthesis than their cells use in respiration use all the carbon dioxide released by their cells in respiration in photosynthesis take in additional carbon dioxide from the atmosphere to satisfy the needs of photosynthesis. At night, plants: do not photosynthesise take in oxygen gas from the atmosphere for respiration. release carbon dioxide gas as a product of respiration. Overall plant metabolism results in the release of more oxygen than carbon dioxide. If a plant was placed in a sealed container for several days and nights the composition of air in the container would change. Even though plants do not photosynthesise at night, they still release more oxygen during photosynthesis than they absorb by respiration. 30 Patterns in nature

32 So, what is the evolutionary significance of plants releasing more oxygen than carbon dioxide? Prior to the evolution of photosynthesising organisms, the Earth s atmosphere was significantly different as it contained no free oxygen gas. It probably contained gases like methane as well as higher levels of carbon dioxide than today. As free oxygen, produced by photosynthesis, became available it could react with the methane to produce carbon dioxide. That carbon dioxide was then available for photosynthesis. Since photosynthesis produces a net amount of oxygen, the atmospheric oxygen levels were able to gradually increase. Photosynthesis has changed the composition of the atmosphere of the planet. This must surely be one of the most significant change made by living things on the planet. 1 Imagine that you have placed a plant into an airtight container. There is air in the container, but it cannot escape from the container. The container is placed outside and left in sunlight for eight hours. What would you expect to happen to the composition of the air in the container? 2 Return to the plant in the sealed container. What would happen to the composition of the air in the container at night? Check your answers. Gas exchange in leaves Green plants require gas exchange for two purposes: provision of carbon dioxide gas for photosynthesis provision of oxygen gas for respiration. When these gases are not available within the cells of a plant then the gases need to be brought in from the surrounding atmosphere. The leaf is one of the most important gas exchange sites. Cells in leaves respire and are also some of the most important cells involved in the process of photosynthesis. Part 5: Obtaining and transporting materials in plants 31

33 You will already know that the outer surface of a leaf has a waxy covering (cuticle). The cuticle is a most unsatisfactory surface for gas exchange. However, it does prevent excessive water loss. Gases enter the leaf through tiny holes called stomates. Cells in the interior of a leaf do not have a waxy covering. In most plants, there are more stomates on the underside of the leaf than on the upper surface. This reduces the amount of water that can be lost through the stomatal openings. Look at the diagram of a cross section of a leaf following. cuticle epidermal cell palisade mesophyll many chloroplasts in cytoplasm of cell xylem and phloem cells in leaf vein spongy mesophyllwith fewer chloroplasts in cytoplasm air space epidermis cuticle The stomate is shown on the lower surface of the leaf. The two guard cells of a stomate surround a pore. When the guard cells are turgid (full of fluid) they are curved like a banana. The curve of the two turgid guard cells creates the opening that allows gases into and out of the leaf. When the guard cells are flaccid they collapse, sealing the stomate. When stomates are closed, water vapour and gases cannot pass into or out of the leaf. 32 Patterns in nature

34 Leaf epidermis. Note the stomates are all open. Many consider stomate closure to be an adaptation to prevent desiccation or drying out. Explain. Check your answer. Where does gas exchange occur? Gas exchange in plants occurs on the surface of each cell in the leaf. Moisture on the outside of the cells allows gases to dissolve into fluid. Once dissolved, gases can diffuse into the cell. Wastes are removed by a similar method. The waste gases diffuse to the moisture on the outside of each cell and from there to the gases in the cavities within the leaf. For cells that are not immediately adjacent to a leaf cavity, gases are passed by diffusion from cell to cell to deliver gases to cells deeper in the leaf. Stomates open into cavities and there are considerable air spaces within the leaf. Because the cells within the leaf are so tiny, the surface area to volume ratio is high for each cell adjacent to a leaf cavity. Just as in lungs, gills and insect tracheoles a high surface area to volume ratio is important for gas exchange in plants. Part 5: Obtaining and transporting materials in plants 33

35 Gas exchange is required for all plant cells, even the ones that are not photosynthesising. Gas exchange in stems As stems are often thick, a different structure is required to allow gases to pass through outer coverings such as bark. This structure is the lenticel. Lenticels are a loose association of cells with many intercellular spaces between them. These spaces allow oxygen to pass from the atmosphere to the respiring cells within the stem. Lenticels also allow waste carbon dioxide to leave the plant. lenticels Lenticels can be found on a woody stem. lenticel Lenticels allow gas exchange to occur. Root hairs are sufficiently moist, small and thin to allow adequate gas exchange between the gases in the soil air and the roots. 34 Patterns in nature

36 1 With the aid of examples, explain why gas exchange surfaces have a high surface area to volume ratio. 2 Outline the role of stomates in: a) gas exchange b) prevention of desiccation. 3 A number of plants growing in arid parts of Australia close their stomates during the heat of the day. Their stomates are only opened during the evening, early morning and late afternoon. Make a hypothesis to explain these observations. Check your answers. Complete Exercise 5.5. Part 5: Obtaining and transporting materials in plants 35

37 36 Patterns in nature

38 Suggested answers Autotrophic cells and heterotrophic cells Autotrophic cells include plant cells and some bacterial cells. Heterotrophic cells include animal cells, fungal cells and some bacterial cells. Photosynthesis 1 Photosynthesis takes place in the green parts of plants, especially the leaves. 2 Photosynthesis occurs in the chloroplasts. 3 Light provides the energy that drives the process, photosynthesis. Chlorophyll (in the chloroplasts), carbon dioxide and water are necessary materials. The phases of photosynthesis 1 Light is absorbed by chlorophyll and water molecules are split into hydrogen and oxygen. 2 Carbon fixation and glucose is synthesised in the dark phase of photosynthesis. Function of leaves 1 Leaves are mostly green, thin and flat. They have a network of veins. 2 The major role of leaves is to provide the plant with food. Photosynthesis occurs mainly in the leaves. 3 a) Photosynthesis occurs in mesophyll (spongy and palisade). The mesophyll is located between the layers of epidermis. Mesophyll has access to carbon dioxide that enters through the stomata. Part 5: Obtaining and transporting materials in plants 37

39 b) Xylem provides water for photosynthesis. c) Phloem transports the products of photosynthesis to the rest of the plant. 4 The flat shape increases the SA:V ratio. This means there is a large surface area available for light absorption for photosynthesis. Because leaves are thin, the mesophyll is close to the epidermis. So, carbon dioxide can easily absorb to the mesophyll for the process. Roots 1 Tap roots have a main central root from which the root hairs grow. Fibrous roots do not have a central root, but simply a collection of fine roots spreading out. 2 Roots have root hairs on their surface that increase the surface area. 3 The increased surface area enables the plants to spread out within the soil to gather available moisture and dissolved nutrients. This aids in the survival of the plants, particularly in times/areas when there may be a water shortage. Transport systems in plants 1 Roots absorb water and dissolved nutrients from the soil. The stem supports the plant and the leaf is the site of photosynthesis. 2 Each vascular bundle contains three types of tissue: xylem, phloem and cambium. (Note: cambium is not a conducting tissue.) The effect of the environment on transpiration Nutrients are absorbed through the root hairs on the root system of a plant. They are transported via the xylem by adhesive and cohesive forces, to the leaves. The movement of water upward through the plant is called transpiration. From the leaves, the products of photosynthesis are transported via the phloem to the rest of the plant. The movement of sugars around the plant is called translocation. 38 Patterns in nature

40 Exercises Part 5 Exercises 5.1 to 5.5 Name: Exercise 5.1 Outline the main difference between autotrophs and heterotrophs. Exercise 5.2: The phases of photosynthesis a) Write down the general word equation for photosynthesis. b) Explain why this equation can be thought of as a summary of a chain of biochemical reactions. Exercise 5.3: Photosynthesis a) List the materials required for photosynthesis. b) What is the role of photosynthesis in the ecosystem? Part 5: Obtaining and transporting materials in plants 39

41 Exercise 5.4: Absorption of water and minerals in plants Plants obtain water and minerals through their root systems. Roots are generally long and thin. Root hairs are found along the tips of growing roots. How does the structure of the root system affect a plant s ability to obtain water and minerals? Exercise 5.5: Transport systems in plants a) Multicellular plants and animals have transport systems. Why is this necessary? b) Describe the movement of water through a plant starting with root hairs and finishing with the water leaving the plant. _ c) You have learned that materials move upwards in xylem. Substances move upward and downward in the phloem. How do we know this? Briefly describe the evidence for movement of substances in the xylem or the phloem. You may need to consult a biology textbook and another source such as the Internet. _ 40 Patterns in nature