Plant Nutrition Unit 6 PDF

Unit 6 - Plant nutrition 6.1 Photosynthesis Photosynthesis Green plants make the carbohydrate glucose from the raw materials carbon dioxide and water At the same time oxygen is made and released as a waste product The reaction requires energy which is obtained by the pigment chloro...

Unit 6 - Plant nutrition 6.1 Photosynthesis Photosynthesis Green plants make the carbohydrate glucose from the raw materials carbon dioxide and water At the same time oxygen is made and released as a waste product The reaction requires energy which is obtained by the pigment chlorophyll trapping light from the Sun So photosynthesis can be defined as the process by which plants manufacture carbohydrates from raw materials using energy from light It can be summed up in the following equation: 6.2 Chlorophyll Chlorophyll Chlorophyll is a green pigment that is found in chloroplasts within plant cells It reflects green light, giving plants their characteristic green colour Chlorophyll absorbs light energy; its role is to transfer energy from light into energy in chemicals, for the synthesis of carbohydrates, such as glucose Photosynthesis will not occur in the absence of chlorophyll 6.3 Use & Storage of Carbohydrates Use & Storage of Carbohydrates How are the products of photosynthesis used? The carbohydrates produced by plants during photosynthesis can be used in the following ways: Converted into starch molecules which act as an effective energy store Converted into cellulose to build cell walls Glucose can be used in respiration to provide energy Converted to sucrose for transport in the phloem As nectar to attract insects for pollination Plants can also convert the carbohydrates made into lipids for an energy source in seeds and into amino acids (used to make proteins) when combined with nitrogen and other mineral ions absorbed by roots 6.4 Minerals in plants Minerals in Plants Photosynthesis produces carbohydrates, but plants contain many other types of biological molecule; such as proteins, lipids and nucleic acid (DNA) As plants do not eat, they need to make these substances themselves Carbohydrates contain the elements carbon, hydrogen and oxygen but proteins, for example, contain nitrogen as well (and certain amino acids contain other elements too) Other chemicals in plants contain different elements as well, for example chlorophyll contains magnesium and nitrogen This means that without a source of these elements, plants cannot photosynthesise or grow properly Plants obtain these elements in the form of mineral ions actively absorbed from the soil by root hair cells ‘Mineral’ is a term used to describe any naturally occurring inorganic substance  Mineral ion Function Deficiency Magnesium Magnesium is needed to Causes yellowing between make chlorophyll the veins of leaves (chlorosis) Nitrate Nitrates are a source of Causes stunted growth nitrogen needed to make and yellowing of leaves amino acids (to build proteins) 6.5 Investigating the Need for Chlorophyll, Light & Carbon Dioxide Investigating the Need for Chlorophyll Although plants make glucose in photosynthesis, leaves cannot be tested for its presence as the glucose is quickly used, converted into other substances and transported or stored as starch. Starch is stored in chloroplasts where photosynthesis occurs so testing a leaf for starch is a reliable indicator of which parts of the leaf are photosynthesising. Leaves can be tested for starch using the following procedure: A leaf is dropped in boiling water to kill the cells and break down the cell membranes The leaf is left for 5-10 minutes in hot ethanol in a boiling tube. This removes the chlorophyll so colour changes from iodine can be seen more clearly The leaf is dipped in boiling water to soften it The leaf is spread out on a white tile and covered with iodine solution In a green leaf, the entire leaf will turn blue-black as photosynthesis is occurring in all areas of the leaf This method can also be used to test whether chlorophyll is needed for photosynthesis by using a variegated leaf (one that is partially green and partially white) The white areas of the leaf contain no chlorophyll and when the leaf is tested only the areas that contain chlorophyll stain blue-black The areas that had no chlorophyll remain orange-brown as no photosynthesis is occurring here and so no starch is stored Care must be taken when carrying out this practical as ethanol is extremely flammable, so at that stage of the experiment the Bunsen burner should be turned off. The safest way to heat the ethanol is in an electric water bath rather than using a beaker over a Bunsen burner with an open flame Investigating the Need for Light The same procedure as above can be used to investigate if light is needed for photosynthesis Before starting the experiment the plant needs to be destarched by placing in a dark cupboard for 24 hours This ensures that any starch already present in the leaves will be used up and will not affect the results of the experiment Following destarching, a leaf of the plant can be partially covered with aluminium foil and the plant placed in sunlight for a day The leaf can then be removed and tested for starch using iodine The area of the leaf that was covered with aluminium foil will remain orange-brown as it did not receive any sunlight and could not photosynthesise, while the area exposed to sunlight will turn blue-black This proves that light is necessary for photosynthesis and the production of starch Investigating the Need for Carbon Dioxide Destarch two plants by placing in the dark for a prolonged period of time Place one plant in a bell jar which contains a beaker of sodium hydroxide (which will absorb carbon dioxide from the surrounding air) Place the other plant in a bell jar which contains a beaker of water (control experiment), which will not absorb carbon dioxide from the surrounding air Place both plants in bright light for several hours Test both plants for starch using iodine The leaf from the plant placed near sodium hydroxide will remain orange-brown as it could not photosynthesise due to lack of carbon dioxide The leaf from the plant placed near water should turn blue-black as it had all necessary requirements for photosynthesis 6.6 Investigating the Rate of Photosynthesis Investigating the Rate of Photosynthesis The plants usually used are Elodea or Cabomba - types of pondweed As photosynthesis occurs, oxygen gas produced is released As the plant is in water, the oxygen released can be seen as bubbles leaving the cut end of the pondweed The number of bubbles produced over a minute can be counted to record the rate The more bubbles produced per minute, the faster the rate of photosynthesis A more accurate version of this experiment is to collect the oxygen released in a test tube inverted over the top of the pondweed over a longer period of time and then measure the volume of oxygen collected This practical can be used in the following ways: Investigating the effect of changing light intensity This can be done by moving a lamp different distances away from the beaker containing the pondweed Investigating the effect of changing temperature This can be done by changing the temperature of the water in the beaker Investigating the effect of changing carbon dioxide concentration This can be done by dissolving different amounts of sodium hydrogen carbonate in the water in the beaker Care must be taken when investigating a condition to keep all other variables constant in order to ensure a fair test For example, when investigating changing light intensity, a glass tank should be placed in between the lamp and the beaker to absorb heat from the lamp and so avoid changing the temperature of the water as well as the light intensity 6.7 Investigating Gas Exchange Investigating Gas Exchange Plants are respiring all the time and so plant cells are taking in oxygen and releasing carbon dioxide as a result of aerobic respiration Plants also photosynthesise during daylight hours, for which they need to take in carbon dioxide and release the oxygen made in photosynthesis At night, plants do not photosynthesise but they continue to respire, meaning they take in oxygen and give out carbon dioxide During the day, especially when the sun is bright, plants are photosynthesising at a faster rate than they are respiring, so there is a net intake of carbon dioxide and a net output of oxygen We can investigate the effect of light on the net gas exchange in an aquatic plant using a pH indicator such as hydrogencarbonate indicator  This is possible because carbon dioxide is an acidic gas when dissolved in water Hydrogencarbonate indicator shows the carbon dioxide concentration in solution The table below shows the colour that the indicator turns at different levels of carbon dioxide concentration Several leaves from the same plant are placed in stoppered boiling tubes containing some hydrogencarbonate indicator The effect of light can then be investigated over a period of a few hours Results from a typical experiment are shown in the table below: Tube Content Conditions Indicator Conclusion s turns A Leaf Light Purple There is a net intake of carbon dioxide by a leaf in light B Leaf Dark Yellow There is a net intake of oxygen by a leaf in the dark C No leaf Light Red This is the control - the two other tubes can be compared with it 6.8 Photosynthesis Chemical Equation Balanced Photosynthesis Chemical Equation (Extended) The balanced chemical equation for photosynthesis is: The light energy is converted into chemical energy in the bonds holding the atoms in the glucose molecules together 6.9 Limiting Factors Limiting Factors (Extended) If a plant is given unlimited sunlight, carbon dioxide and water and is at a warm temperature, the limit on the rate (speed) at which it can photosynthesise is its own ability to absorb these materials and make them react  However, most often plants do not have unlimited supplies of their raw materials so their rate of photosynthesis is limited by whatever factor is the lowest at that time So a limiting factor can be defined as something present in the environment in such short supply that it restricts life processes There are three main factors which limit the rate of photosynthesis: Temperature Light intensity Carbon dioxide concentration Although water is necessary for photosynthesis, it is not considered a limiting factor as the amount needed is relatively small compared to the amount of water transpired from a plant so there is hardly ever a situation where there is not enough water for photosynthesis Temperature As temperature increases the rate of photosynthesis increases as the reaction is controlled by enzymes However, as the reaction is controlled by enzymes, this trend only continues up to a certain temperature beyond which the enzymes begin to denature and the rate of reaction decreases Light intensity The more light a plant receives, the faster the rate of photosynthesis This trend will continue until some other factor required for photosynthesis prevents the rate from increasing further because it is now in short supply At low light intensities, increasing the intensity will initially increase the rate of photosynthesis. At a certain point, increasing the light intensity stops increasing the rate. The rate becomes constant regardless of how much light intensity increases as something else is limiting the rate The factors which could be limiting the rate when the line on the graph is horizontal include temperature not being high enough or not enough carbon dioxide. Carbon dioxide concentration Carbon dioxide is one of the raw materials required for photosynthesis This means the more carbon dioxide that is present, the faster the reaction can occur This trend will continue until some other factor required for photosynthesis prevents the rate from increasing further because it is now in short supply The factors which could be limiting the rate when the line on the graph is horizontal include temperature not being high enough or not enough light 6.10 Leaf structure Leaf Structure & Adaptations for Photosynthesis Leaf structure Pathway of carbon dioxide from the atmosphere to chloroplasts by diffusion: atmosphere → air spaces around spongy mesophyll tissue → leaf mesophyll cells → chloroplast 6.11 Identifying Leaf Structures in a Dicotyledonous Plant Identifying Leaf Structures in a Dicotyledonous Plant You will be expected to identify the following structures in the leaf of a dicotyledonous plant: Chloroplasts Cuticle  Guard cells Stomata Upper and lower epidermis Palisade mesophyll Spongy mesophyll Air spaces Vascular bundles (xylem and phloem) Unit 7 - Human Diet & Digestion