SO1BI Chapter 6: Nutrition in Plants PDF

# Chapter Six: Nutrition in Plants ## Introduction Plants are organisms that can manufacture their own food, and therefore are called autotrophs. Other organisms that can manufacture their own food include some bacteria and protoctists. Although plants can make their own food, they require some ch...

# Chapter Six: Nutrition in Plants ## Introduction Plants are organisms that can manufacture their own food, and therefore are called autotrophs. Other organisms that can manufacture their own food include some bacteria and protoctists. Although plants can make their own food, they require some chemical elements from the environment. Such chemical elements are important for normal growth and development. In this chapter, you will learn about photosynthesis process and its importance, as well as the structure of a leaf in relation to photosynthesis. You will also learn roles of essential mineral elements to plants. The competencies developed will enable you to explore the importance of plant nutrition to life. ## Think The world without nutrition in plants ## Concept of nutrition Nutrition is the process of feeding and utilising of food for energy provision, growth, development, repair and maintenance of the overall body health. Nutrition also refers to the study of the relationship between diet, health, and diseases. There are two major types of nutrition based on how organisms obtain their food. These are autotrophic nutrition and heterotrophic nutrition. In this chapter, you will learn about autotrophic nutrition. ## Autotrophic nutrition Autotrophic nutrition is a type of nutrition in which organisms manufacture their own food using available energy sources. An organism that manufactures its own food is called an autotroph. The term ‘autotroph’ originates from two Greek words ‘autos’ which means ‘self’ and ‘trophy’ which means ‘feed’; hence ‘autotrophy’ means ‘self-feeding’. This means that autotrophs are organisms that are capable of making their own food for their own use and for other organisms. Examples of autotrophic organisms include green plants, algae, and some bacteria such as cyanobacteria. The autotrophs can be divided into two groups based on how they obtain their energy. These are chemoautotrophs and photoautotrophs. ## Chemoautotrophs Chemoautotrophs are organisms that obtain their energy from chemical substances like hydrogen sulphide, iron, methane, and ammonia. They utilise these chemicals to make their own food in the form of carbohydrates through a process called chemosynthesis. Examples of chemoautotrophs are some bacteria (archaebacteria) that live in harsh environments. Such environments include the deep sea and around volcanic sites where there is no sunlight and where many other organisms cannot survive. ## Photoautotrophs Photoautotrophs are organisms that obtain their energy from sunlight in order to make their own food. They make their food using water and carbon dioxide in the presence of chlorophyll and sunlight through a process called photosynthesis. ## Photosynthesis Photosynthesis is the process by which green plants, some bacteria and protoctists make their own food using water and carbon dioxide in the presence of sunlight energy. Organisms that make their own food are called primary producers or autotrophs. They are the primary source of food for all other organisms. During the process of photosynthesis, six molecules of carbon dioxide and twelve molecules of water combine to form one molecule of glucose (a simple sugar), six molecules of water, and six molecules of oxygen. The leaf is the main site for photosynthesis in plants. However, photosynthesis can take place in other green parts of the plant, such as stem. The following chemical equation represents the photosynthesis process in plants. $6CO_2 + 12H_2O \longrightarrow C_6H_{12}O_6 + 6O_2 + 6H_2O$ Photosynthesis produces a six-carbon sugar molecule (a hexose) called glucose. Plants convert these hexose sugars into other carbohydrates, such as complex sugars, starch and cellulose. Plants are also capable of converting hexose sugars into other organic compounds, such as proteins and fats. The food formed by plants during photosynthesis is stored in the form of starch. ## Structure of the leaf in relation to photosynthesis The internal and external structures of a leaf make it well adapted for photosynthesis. In most plants, the leaf is the main site for photosynthesis, although some plants, such as cactus (plural is cacti) use their stems for photosynthesis. The leaf structure provides a very efficient means for absorbing carbon dioxide and sunlight. ### External structure of a leaf The external features of a leaf can be viewed using either a hand lens or with the naked eye. Figure 6.1 shows the external structure of a leaf. * Lamina or leaf blade: The expanded portion of a leaf. It has a large surface area. This maximizes the absorption of sunlight energy and carbon dioxide. The lamina is also thin so that carbon dioxide gas can diffuse and sunlight energy can penetrate over a short distance to reach cells. * Mid-rib and veins: These contain vascular tissues, namely xylem and phloem. Xylem transports water and dissolved minerals while phloem transports manufactured food in the plants. * Petiole or leaf stalk: Attachs the leaf to the branch or stem. It keeps the lamina in a position that will enable it to get maximum amount of sunlight. ### Internal structure of a leaf The internal features of a leaf can be viewed under a microscope. Figure 6.2 shows the internal structure of a leaf. * Cuticle: The outermost transparent and waxy layer of the leaf. It allows light to penetrate into the photosynthetic cells. It also protects the leaf from injury, pests, and excessive loss of moisture. In order to allow gaseous exchange, there is no cuticle on the stomata. * Epidermis: The outermost layer of cells found on both lower and upper surfaces of a leaf. It is transparent and only one cell is thick, hence it allows sunlight to penetrate the leaf easily. The epidermis has pores called stomata. * Stomata: Small pores in the epidermis. They allow oxygen and carbon dioxide to diffuse in and out of the leaf. Stomata are surrounded by guard cells that close and open the pores. * Mesophyll: It is made up of palisade layer and spongy layer. The palisade mesophyll is made up of cells that are elongated and arranged at right angles to the surface of the leaf. It is found just below the upper epidermis. Being near the upper epidermis, the palisade cells are exposed to maximum sunlight. This enables the cells to absorb maximum amount of sunlight energy. The spongy mesophyll is just above the lower epidermis. It is a loosely constructed layer of irregularly shaped cells separated by large intercellular air spaces that connect with each other and linked to the atmosphere through the stomata pores. The air spaces in the spongy mesophyll provide an immediate source of carbon dioxide. ## The process of photosynthesis Photosynthesis takes place in cell organelles known as chloroplasts. These are mostly found in the green leaves. Chloroplasts contain chlorophyll that is responsible for trapping sunlight energy that is used during photosynthesis. Photosynthesis takes place in two stages, namely light reaction and dark reaction. ### The light reaction or light dependent reaction stage This stage takes place in specialised structures of the chloroplast called grana. The grana (singular is granum) contain chlorophyll that absorbs light energy from the sun. The light dependent reaction changes light energy into chemical energy. The formed energy is stored in a chemical compound called ATP (Adenosine Triphosphate). This energy is used in the dark reaction stage of photosynthesis. Light energy causes photolysis, a chemical process whereby water molecules ($H_2O$) are split into hydrogen ions ($H^+$) and hydroxyl ions ($OH^-$). This can be represented using the following equation. $H_2O \longrightarrow H^+ + OH^-$ The hydroxyl ions undergo further reactions to produce water and oxygen. Some oxygen is released into the atmosphere. The rest of the oxygen is used for respiration in the plant. Hydrogen ions are used in the dark reaction stage to synthesise food together with carbon dioxide from the air. ### The dark reaction or light independent reaction stage This stage takes place in the stroma in the absence of light. The stroma is a colourless matrix of fine material found in the chloroplast. During the dark reaction stage, hydrogen molecules ($H_2O$) are split into hydrogen ions ($H^+$) and hydroxyl ions ($OH^-$). This can be represented using the following equation. $H_2O \longrightarrow H^+ + OH^-$ The hydroxyl ions undergo further reactions to produce water and oxygen. Some oxygen is released into the atmosphere. The rest of the oxygen is used for respiration in the plant. Hydrogen ions are used in the dark reaction stage to synthesise food together with carbon dioxide from the air. ## Essential and non-essential elements in plants There are many chemical elements which are known to be important for plant growth and development. These chemical elements are divided into two major groups, namely essential and non-essential elements. The essential elements are vital for plant development and survival. The plant cannot complete its life cycle without them. These include nickel (Ni), carbon (C), hydrogen (H), oxygen (O), nitrogen (N), phosphorus (P), calcium (Ca), iron (Fe), manganese (Mn), potassium (K), magnesium (Mg), sulphur (S), boron (B), copper (Cu), zinc (Zn), molybdenum (Mo), and chlorine (Cl). The non-essential elements are used to stimulate growth in plants. Examples of non-essential elements include lithium (Li), iodine (I), sodium (Na), mercury (Hg), silver (Ag), tin (Sn), radium (Ra), silicon (Si), bromine (Br), and cobalt (Co). The essential elements can also be divided into two groups, which are macroelements and microelements. The macroelements are those elements that are required by plants in relatively large amounts. They are also known as macronutrients. These include phosphorus (P), potassium (K), nitrogen (N), sulphur (S), magnesium (Mg), calcium (Ca), carbon (C), oxygen (O), and hydrogen (H). Among the macroelements, some are non-minerals, such as carbon, oxygen and hydrogen, that are obtained from the air. Other are minerals, such as magnesium, calcium, phosphorus and potassium. Both non-mineral and mineral elements are essential for normal plant growth and metabolic activities. For example, carbon is a major constituent of plants, hydrogen is required during photosynthesis and oxygen is required during respiration. The essential mineral elements are mainly absorbed by plants from soil. They usually dissolve in water and are absorbed by plants in form of ions. The ions enter the plant through roots and are transported upward through the vascular system. Table 6.1 shows various sources of macroelements in plants. | Macroelement | Sources | |---|---| | Phosphorus | Obtained from commercial fertilisers, crop residues, and soil minerals | | Potassium | Obtained from soil minerals, organic materials, and commercial fertilisers | | Nitrogen | Obtained from commercial fertilisers and from the air. Rhizobium bacteria in the roots of leguminous plants fix nitrogen from the air for use by the plants | | Sulphur | Obtained from rainwater, gypsum, and commercial fertilisers | | Magnesium | Obtained from soil minerals, organic materials (such as compost), commercial fertilisers, and lime | | Calcium | Obtained from lime, gypsum, and commercial fertilisers | ## Functions of macroelements in plants Each mineral element has a specific function in the plant body. Some are used in the formation of building materials for proteins and carbohydrates in plants while others play important roles in other metabolic activities of the plant. The deficiency of such elements will eventually affect plant growth and metabolic activities. Table 6.2 shows the functions, signs of deficiency, and effects of excess macroelements in plants. | Macroelement | Functions | Signs of deficiency | Effects of excess | |---|---|---|---| | Nitrogen | * Synthesis of proteins and transfer of materials. * Increases the rate of photosynthesis * Increases seed and fruit production * Speeds up growth rate * Germination and growth of seeds * Production of flowers and fruits * Growth of roots | * Slow growth * Yellowish or light green leaves * Reduced yield of fruits and seeds * Poorly developed leaves | * Very dark green succulent leaves * Break down of vascular tissue and restricted transport of water * Easily damaged stems because of excess sap| | Phosphorus | * Germination and growth of seeds * Production of flowers and fruits * Growth of roots | * Reduced plant growth * Delayed development * Small leaves that drop early | * Reduces the plant's ability to take up required micronutrients, particularly, iron and zinc | | Potassium | * For steady growth * Increases resistance to diseases * Useful in protein formation * Ripening of seeds and fruits | * Bluish-green or purplish leaves * Scorched brown leaf edges and tips * Rolling of leaves * Susceptibility to diseases| * Deficiency of calcium and magnesium | | Magnesium | * Formation of chlorophyll * Activation of plant enzymes necessary for metabolism | * Yellowish leaves * Leaves fall without withering | * Stunted growth * Lack of food needed for growth and development due to impaired photosynthesis | | Calcium | * Formation of cell walls * Increases mechanical strength of the plant * Growth of roots * Normal transport and retention of other elements in the plant | * Poorly developed roots with weak tips * Curling of leaf margins * Hooked leaf tips * Internal decay | * Reduced uptake of essential elements, particularly, phosphorus and magnesium | | Sulphur | * Production of protein * Formation of chlorophyll * Improved root growth and seed production | * Slow growth * Small leaves that roll up and are stiff and brittle * Premature shedding of leaves * Tips of buds die | * Premature ageing of plants | ## Microelements in plants Microelements are essential mineral elements that are needed for plant growth and survival but only in very small quantities. However, deficiency or excess of microelements in plants can have effects on physiological activities of plants. These elements are also referred to as trace elements. They include boron, copper, iron, chlorine, manganese, zinc, nickel, and molybdenum. Table 6.3 shows functions of microelements in plants, deficiency signs, and effects of their excess levels. | Macroelement | Functions | Signs of deficiency | Effects of excess | |---|---|---|---| | Boron | * Aids in the production of sugar and starch * Aids in water intake by cells * Important for seed and fruit development * Keeps Calcium in a soluble form | * Distorted or growing tips dead * Hollow stems * Deformed fruits * Discoloured leaves| * Scorched leaves * Falling off of leaves | | Copper | * Important for normal growth and development * Aids in the formation of proteins | * Bluish-green leaves * Withering of leaves * Leaves failing to unfold * Distorted growth tips | * Iron deficiency * Suppressed growth | | Chlorine | Important for plant metabolism | * Wilting * Stumpy roots | * Scorched leaf edges | | Iron | * Important for the formation of chlorophyll * Important in the transportation of oxygen | * Yellowing of leaves between the veins | * Brown spots on leaves | | Manganese | * Catalyst for enzyme action * Required for the formation of chlorophyll | * Yellowing of leaves between veins * Grey spots on leaves | * Iron deficiency * Brown spots on leaves surrounded by a pale circle | | Molybdenum | * Helps in the formation of nitrogen nodules in legumes | * Yellow spots on leaves * Dead spots on leaves * Distorted or dead growing tips | * Pale-green leaves * Rolled leaf margins * Stunted growth | | Zinc | * Important for plant growth and maturity * Helps in the formation of proteins | * Yellowing of leaves between veins * Appearance of purple or dark spots on leaves * Small, deformed leaves * Reduced fruiting | * Iron deficiency * Pale green leaves | ## Importance of photosynthesis Photosynthesis is a vital process for the continuous survival of plants and other organisms on earth. The importance of photosynthesis includes production of oxygen, reduction of atmospheric carbon dioxide, conversion of solar energy into chemical energy, and production of food. * Production of oxygen Photosynthesis produces oxygen. This replenishes the atmospheric oxygen that has been used during burning, respiration, rusting, and other processes. All aerobic organisms require oxygen for respiration. * Reduction of atmospheric carbon dioxide The process of photosynthesis uses carbon dioxide from the atmosphere. This process reduces atmospheric carbon dioxide, since it is converted into carbohydrate in the presence of water, sunlight, and chlorophyll. Being a by-product of respiration, carbon dioxide is harmful if excessive amounts accumulate in the atmosphere. * Conversion of solar energy into chemical energy Photosynthesis converts sunlight energy into chemical energy. The sun is the primary source of all forms of energy used in life processes. This energy is used by plants as well as animals. It is made available to living things through photosynthesis. Animals obtain energy by feeding on plants or animals that eat plants. Thus, sunlight energy is converted into chemical energy that is stored in organic food molecules. The energy stored in organic food molecules is then released into the cells during respiration. This energy is used in life processes, such as movement, reproduction, sensitivity, growth, and excretion. * Production of food Photosynthesis produces food for the plants. Some animals, such as cattle, goats, and zebra depend directly on plants as their source of food. Such animals are called herbivores. Other animals called carnivores, such as lions, hyenas, and leopards feed directly on herbivores by killing and eating them. Animals, such as human beings and monkeys depend on both plants and animals as their source of food. These are called omnivores. The extra food produced by plants is stored in various plant organs. The stored food is used by the plants during adverse conditions, such as drought when the plant cannot synthesise adequate amounts of food. The underground storage organs of the plants are formed from modified stems, leaves or roots. There are several types of underground storage organs, such as bulbs, tubers, roots, corms, and rhizomes. * Bulbs: This is an underground storage organ formed from the modification of the plant stem and leaves. An onion is an example of a bulb, as shown in Figure 6.9. * Tubers: This is a fleshy storage organ formed from either a stem or a root. An examples of a stem tuber is Irish potato while an example of a the root tuber is sweet potato (See Figure 6.10) * Taproot: Some taproots such as those of carrot, sugar beet, and beetroot are specialised for food storage (See Figures 6.11). * Corm: This is an underground storage organ formed from the plant stem. It is a mass of solid tissues with a dry papery cover made of modified leaves. Examples of corms are yams, cocoyams and crocuses (See Figure 6.12). * Rhizome: A rhizome is a swollen underground stem, which grows horizontally on the surface or just below the surface. An example is ginger, as shown in Figure 6.13. ## Revision exercise 6 **Section A** **Choose the correct answer.** 1. Which of the following is NOT a type of an underground storage organ? * Bulb * Seed * Corm * Rhizome 2. The process by which plants and some bacteria use the energy from sunlight to produce glucose is called * Photolysis * Hydrolysis * Photosynthesis * Plasmolysis 3. Boron, copper, iron, chlorine, manganese, molybdenum and zinc are examples of: * microelements * macroelements * non-mineral elements * organic elements 4. The part of a leaf that provides a large surface area for maximum absorption of sunlight and carbon dioxide is: * petiole * stomata * lamina * midrib 5. Write **TRUE** for correct statements and **FALSE** for incorrect statements in the spaces provided. * The leaf is the main site for photosynthesis in plants. **TRUE** * Essential elements are necessary for plant growth, development, and reproduction. **TRUE** * Carbon, oxygen, hydrogen, and nitrogen are non-minerals. **FALSE** * Rhizobium bacteria helps to fix nitrogen from the air into the soil. **TRUE** * Microelements are needed by plants in large quantities. **FALSE** * Copper is an example of trace elements. **TRUE** * Lamina is the external part of a leaf while midrib is the internal part. **TRUE** * A bulb is an underground storage organ formed from the plant roots. **TRUE** * Sunlight energy is not needed during photosynthesis. **FALSE** * Unlike carnivores, autotrophs make their own food in the form of carbohydrates. **TRUE** **Section B** 1. Briefly explain the following terms: * Photosynthesis: The process by which green plants, some bacteria, and some protoctists use light energy to synthesize foods from carbon dioxide and water. * Palisade mesophyll: A layer of elongated, tightly packed cells found beneath the upper epidermis of a leaf. * Photolysis: The splitting of water molecules into hydrogen ions and hydroxyl ions in the presence of light during the light-dependent stage of photosynthesis. 2. Differentiate the light reaction stage from the dark reaction stage of photosynthesis. * The light-dependent stage: Takes place in the grana of chloroplasts and utilizes light energy from the sun. It produces ATP and splits water molecules into hydrogen ions and hydroxyl ions. * The dark-independent stage: Takes place in the stroma of chloroplasts and utilizes the ATP produced in the light-dependent stage. Carbon dioxide is fixed into organic compounds using hydrogen ions, resulting in the production of glucose. 3. Describe how a plant leaf is adapted to photosynthesis. * A plant leaf is well-adapted for photosynthesis. It has a broad, flat lamina (to maximize sunlight absorption), a thin structure (to facilitate gas diffusion), vascular tissues (xylem and phloem for transporting water and nutrients), stomata (pores on the epidermis that allow gas exchange), and chloroplasts (contain chlorophyll for trapping light energy). 4. Why is photosynthesis a vital process for all living organisms? * Photosynthesis is essential for the production of organic food, and release of oxygen into the atmosphere. It is the primary source of energy for all living things, supporting food chains on Earth and making life as we know it possible. 5. Why is it necessary to destarch a leaf in an experiment investigating the importance of photosynthesis to plants? * Destarching ensures the plant's leaves do not contain stored starch, which could interfere with the accurate analysis of starch production during the experiment. 6. Answer the following questions: * Mention any two sources of sulphur in plants. * Rainwater * Commercial fertilizers * Give any two signs of sulphur deficiency in plants. * Slow growth * Small and curled leaves * List down at least three functions of sulphur in plants. * Production of proteins * Formation of chlorophyll * Improved root growth and seed production * Why should a leaf be boiled in alcohol when testing for starch? * Boiling in alcohol kills the leaf cells and stops further chemical reactions, preventing any further production of starch. * List down the necessary conditions for photosynthesis. * Sunlight * Carbon Dioxide * Water * Chlorophyll 7. Study the two plant leaves in Figure 6.15 and then answer questions (a) and (b). *(a) For each of the signs shown in figures 6.15 (a) and (b), name the macroelement that is in limited supply.* * (a) Magnesium * (b) Nitrogen *(b) Outline the effects of excessive amounts of the macroelement stated in (a) above.* * Excess magnesium: May cause stunted growth, lack of food needed for growth and development, and impaired photosynthesis. * Excess nitrogen: May cause succulent leaves, breakdown of vascular tissue, restricted transport of water, easily damages stems due to excess sap. 8. An agricultural officer advised farmers to apply fertiliser that contains nitrogen, phosphorous and potassium in their farms with mineral deficiency. Which signs were shown by plants that made the agricultural officer give such advice to farmers? * The agricultural officer would suggest applying a fertiliser containing nitrogen, phosphorous, and potassium to address several symptoms: * Yellowing of leaves is often a sign of a nitrogen deficiency. * Poor plant growth and stunted root development can indicate a lack of phosphorus. * Scorched brown leaf edges and tips, as well as rolling of leaves, are often associated with insufficient potassium.