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YUMMY!!! Sigh, I wish it’s time for dinner already. I am so hungry! Hmmm, I wonder what we are having tonight!? WOW!!! What a pretty flower!!!!! Hey! I wonder if plants need to eat too!? If they do, then how do they get their food? Of course we eat!!! And we are able to make our own food. That is why we are called AUTOTROPHS! Hmmm, I thought you learned all about this already!!! Do you remember how we can make our own food??? Things needed: • Light • Carbon dioxide • Water • Chlorophyll Things produced: • Carbohydrates (which can be used to form fats and proteins) • Oxygen Photosynthesis • As one can see, plants need to obtain carbon dioxide in order to carry out photosynthesis • They also release oxygen as a by-product • The process by which plants exchange oxygen and carbon dioxide is called ___________ Gas Exchange • Plants exchange gases by diffusion • Where does gas exchange occur in plants? Internal Structure of Leaf Gas Exchange • Gas exchange mainly occurs in the leaves • How do gases diffuse into and out of the leaves? Stomata Stomata Gas Exchange • Gas exchange can also take place in the stems and roots • Herbaceous plants – diffusion through stomata on stem surface • Woody plants - stomata when young - lenticels when matured Lenticels • Gases cannot penetrate the protective cork layer • Lenticels are loosely-packed masses of cells in the bark of a woody plant, visible on the surface of a stem as raised spots, through which gas exchange occurs Lenticels Lenticels Gas Exchange in Roots • The epidermis is usually just one cell thick. Root epidermal cells lack a thick cuticle which would interfere with water uptake. Moreover, there is no stomata present as the cell membrane is very thin and therefore gases can directly diffuse into and out of the cells Adaptation of Leaves to Photosynthesis Adaptation of Leaves The leaf is thin Decreases diffusion distance for gases Adaptation of Leaves Numerous stomata on lower epidermis Allows rapid gaseous exchange with the atmosphere Adaptation of Leaves Guard cells control the size of stomata In presence of light, stomata open widely to allow the diffusion of carbon dioxide and oxygen Guard Cells • When turgor develops within the two guard cells, the outer walls bulge out and force the inner walls into a crescent shape. This opens the stomata. When the guard cells lose turgor, the elastic inner walls regain their original shape and the stomata closes Adaptation of Leaves Spongy mesophyll cells are loosely packed with numerous large air spaces Allows rapid diffusion and free circulation of gases throughout the leaf Adaptation of Leaves Most cells in the leaves are surrounded by a layer of water Allows gases to dissolve and diffuse into and out of the cells Gas Exchange Carbon Dioxide Oxygen Oxygen Carbon Dioxide Photosynthesis Respiration What will be the net gas exchange between the leaf and its surrounding air? Rate of Gas Exchange The rate of gas exchange is different throughout the day due to a change in light intensity What is going on here? Light Intensity • Night – plants carry out RESPIRATION and release CARBON DIOXIDE Light Intensity Light Intensity • Night – plants carry out RESPIRATION and release CARBON DIOXIDE • Early morning – PHOTOSYNTHESIS begins to take place as light intensity increases Rate of photosynthesis < Rate of respiration Net release of CARBON DIOXIDE Light Intensity Light Intensity • Around 6:00 a.m. – light intensity increases even more Rate of photosynthesis = Rate of respiration Release of CO2 = Uptake of CO2 That is, there is NO net gas exchange This is referred to as the COMPENSATION POINT Light Intensity Light Intensity • Afternoon – light intensity further increases Rate of photosynthesis > Rate of respiration Net uptake of CARBON DIOXIDE Net uptake of carbon dioxide reaches a maximum in early afternoon Light Intensity Light Intensity • Evening – light intensity begins to decrease At a certain time period, there will again be a net release of CARBON DIOXIDE when plants only carry out RESPIRATION at night Light Intensity Similarly, we can study the relationship between light intensity and the exchange of OXYGEN Critical Thinking 8.1 (p. 11) Question 1. Does a plant release or absorb oxygen at night? Ans: A plant absorbs oxygen at night Critical Thinking 8.1 (p. 11) Question 2. When the light intensity gradually increases in the morning, will there be any changes in the exchange of oxygen? Why? Ans: The rate of oxygen uptake would gradually decrease and the rate of oxygen release would gradually increase. It is because photosynthesis begins to occur when light intensity gradually increases in the morning Critical Thinking 8.1 (p. 11) Questions 3. Why is there a compensation point? Ans: Compensation point refers to the light intensity at which there is no net gas exchange 4. What will happen to the exchange of oxygen when the light intensity further increases? Ans: The rate of oxygen release would increase as light intensity increases Critical Thinking 8.1 (p. 11) Question 5. Draw a graph to show the relationship between light intensity and the exchange of oxygen of a plant. Critical Thinking 8.1 (p. 11) INVESTIGATION #1 Studying the effect of light intensity on gas exchange in leaves using hydrogencarbonate indicator Introduction to Investigation • In this investigation, you will study the effect of light intensity on gas exchange in leaves • Green leaves will be put into different light intensities, and the level of carbon dioxide will be estimated by using hydrogencarbonate indicator solution • Note: Increase in CO2 – Orange to Yellow Decrease in CO2 – Orange to Purple Procedure Please refer to pages 7 and 8 in your textbook A B C D Results Table Colour of hydrogencarbonate indicator solution after one hour Tube A Tube B Tube C Tube D INVESTIGATION #2 Studying the effect of light intensity on the gas exchange of a plant using a data logger Introduction to Investigation • In this investigation, you will study the effect of light intensity on the gas exchange of a water plant using a data logger • Gas exchange in plants is affected by both the rates of respiration and photosynthesis • You can measure the rate of oxygen released by a water plant by measuring the change in pressure in an enclosed set-up • A data logger and a low-pressure sensor can be used Procedure Please refer to pages 8 and 9 in your textbook Results Table Distance between the lamp and the conical flask (cm) 20 50 80 110 Initial pressure Final pressure Change in pressure per minute Discussion 1. What is the purpose of putting a water trough between the conical flask and the lamp? Ans: It is used to reduce the heating effect of the lamp. The result obtained is mainly due to the influence of the light intensity Discussion 2. What is the purpose of using dilute sodium hydrogencarbonate solution in the conical flask? Ans: It provides carbon dioxide for the plant to carry out photosynthesis Discussion 3. What is the relationship between the light intensity and the distance between the conical flask and the table lamp? Ans: The shorter the distance between the lamp and the conical flask, the stronger is the light intensity Discussion 4. What is the relationship between the pressure in the conical flask and the light intensity in this experiment? Ans: The stronger the light intensity, the faster is the increase in pressure detected in the conical flask. The reason is that the rate of photosynthesis increases with light intensity, and the rate of oxygen release also increases Photosynthesis Oxygen Synthesis of Fats carbon dioxide and water photosynthesis carbohydrates (e.g. glucose) fatty acids glycerol Combine to form fats and oils for construction of cell membranes and as a food storage Synthesis of Proteins carbon dioxide and water photosynthesis carbohydrates (e.g. glucose) mineral salts from soil (e.g. NO3-, SO42-) amino acids join together to become protein molecules Mineral Requirements in Plants • In order to synthesize amino acids (i.e. proteins), plants must absorb minerals through the roots • Minerals that are required in large quantities: nitrogen, phosphorus, potassium, magnesium, sulphur and calcium • Other minerals are also required but in a lesser amount: copper, zinc and iron • A constant supply of minerals is necessary for the healthy development of a plant INVESTIGATION #3 Investigating the effects of minerals on plant growth using potted plants Introduction to Investigation • In this experiment, you will investigate the effects of different minerals on plant growth • Some of the plants will be watered with a solution lacking certain essential minerals, such as nitrogen and magnesium • How will a lack of minerals affect the growth of a plant? Procedure Please refer to pages 12 and 13 in your textbook A B C Discussion 1. Why do we use seedlings of similar size? Ans: It is because seedlings of different size may differ in nutrient requirements, making it difficult to compare the results 2. What differences in appearance of seedlings between pots A and B can you find at the end of the experiment? Ans: Seedlings in pot A grow healthy, but those in pot B show poor growth and small, yellowing of leaves Discussion 3. What differences in appearance of seedlings between pots A and C can you find? Ans: The seedlings in pot A grow healthy, but those in pot C also show poor growth and yellowing of leaves 4. Why do we use sand but not garden soil in the pots? Ans: As garden soil may contain different minerals that plants need, accurate result of the effects of different minerals on plant growth may not be obtained Discussion 5. What conclusion can you make from this experiment? Ans: Both nitrogen and magnesium are important to plant growth. Insufficient supply of these minerals would affect plant development Note to Experiment • A solution containing ALL the minerals that are required by a plant is called a complete culture solution • A solution which lacks certain essential minerals for plant growth is called a deficient culture solution • Water cultures can be set up for the investigation of the effects of minerals on plant growth Hi! It’s me again. Hmmm, there are a few things that I still don’t understand. You mean, in addition to carbon dioxide, water and sunlight, plants also need to take in…arrr…what are those things called again? Oh…MINERALS…in order to grow healthily? Can someone PLEASE tell me how are these minerals important to plants? And what will happen if the plants do not take in these minerals? Nitrogen • Nitrogen is needed for the synthesis of amino acid (which are the building blocks for proteins) Structure of Amino Acid Proteins in Plants Proteins are important for the synthesis of various plant structures: • Cell membrane Cell Membrane Proteins in Plants Proteins are important for the synthesis of various plant structures: • Cell membrane • Cytoplasm Cytoplasm • Reaction catalyst • In various structures of the cell Proteins in Plants • • • • Proteins are important for the synthesis of various plant structures: Cell membrane Cytoplasm Enzyme Hormone Plant Hormones • Chemicals made in one part of the plant that move to another part of the plant where, at very low concentrations, they regulate growth and/or development • Many different types of hormones • e.g. promotion of growth, promotion of cell division, etc. Other Functions of Nitrogen • DNA (in making the nitrogenous base) • Chlorophyll Nitrogen in Soil • Usable forms of nitrogen include nitrate (NO3-) and ammonium (NH4+) • Nitrate is the more common form of nitrogen that is absorbed be plants from soil • However, most of the nitrogen in soil is NOT present as nitrate nor as ammonium • Nitrogen in soil must therefore be converted to the usable forms by soil microorganisms Nitrogen Deficiency A deficiency in nitrogen will result in: • Small and weak plants • Stunted growth • Yellowish leaves (Chlorosis) Nitrogen Deficiency Magnesium • Most of the magnesium in the soil exists in forms which are not directly available to plants • Magnesium is taken up by plants as magnesium ions (Mg2+) • Magnesium is an essential component of chlorophyll Magnesium in Chlorophyll Magnesium • Most of the magnesium in the soil exists in forms which are not directly available to plants • Magnesium is taken up by plants as magnesium ions (Mg2+) • Magnesium is an essential component of chlorophyll • Magnesium also plays a role in enzymes activation, protein synthesis, etc. Magnesium Deficiency A deficiency in magnesium will result in: • Chlorosis • Growth can be reduced also Magnesium Deficiency Minerals Soil Plant Minerals in soil are taken up by plants, and can be released back into the soil by decomposition Minerals • Crops take up minerals from soil • When crops are harvested, minerals are removed from soil • Soil can also be washed away by rain water • If there is a lack of minerals in soil, the production of crops might be affected • How can farmers prevent the depletion of minerals in soil? Fertilizers • Fertilizers are added to soil to replace the loss of minerals • Two kinds of fertilizers can be used: - Natural fertilizers - Chemical fertilizers Natural Fertilizers • Organic fertilizers • Made from organic substances, such as manure (animal wastes) and dead bodies of plants and animals • Organic compounds in it are decomposed by the bacteria in soil to form mineral salts Chemical Fertilizers • “Man-made” fertilizers • Made with inorganic compounds • Can result in pollution of the environment, such as algal bloom Comparison between natural and chemical fertilizers Natural fertilizers Chemical fertilizers Contain humus which can improve soil texture No humus so cannot improve soil texture Humus • Humus is the organic portion of soil, brown or black in color, consisting of partially or wholly decayed plant and animal matter that provides nutrients to plants and increases the ability of soil to retain water Comparison between natural and chemical fertilizers Natural fertilizers Chemical fertilizers Contain humus which can improve soil texture No humus so cannot improve soil texture Less soluble in water so less likely to be washed away Very soluble in water so more likely to be washed away Comparison between natural and chemical fertilizers Natural fertilizers Chemical fertilizers Much cheaper Very expensive Less soluble in water so more difficult to be absorbed Very soluble in water so easier to be absorbed Time is needed for the decomposition to complete before nutrients are available to plants More readily to be used by the plants