Science Chp- 10.pdf

Science Chapter 10 Photosynthesis The following topics are discussed in this chapter: 5 Conversion of solar energy into chemical energy 5 Steps of photosynthesis in plants 5 Carbon bonding 10.1 Photosynthesis You...

Science Chapter 10 Photosynthesis The following topics are discussed in this chapter: 5 Conversion of solar energy into chemical energy 5 Steps of photosynthesis in plants 5 Carbon bonding 10.1 Photosynthesis You know that seedlings are produced from seeds. These seedlings gradually grow larger and physically develop into full-grown plants. For many years, scientists have wondered what exactly contributes to this physical growth of plants over time. Is it water, nutrients obtained from the soil, light, or something else? Now we know that plants make their own food using sunlight, water and carbon dioxide from the air. This process is called photosynthesis, and not only plants depend on the food produced in this process, but all living beings on Earth depend on it directly or indirectly. The most important thing is that in this process, plants release oxygen gas as a surplus by- product, and we all survive by taking that oxygen. On the eve of the creation of the Earth, our atmosphere was composed of nitrogen, carbon dioxide, and methane. But there was no oxygen then. As you have known from a previous chapter, about 2.5 billion years ago, a prokaryotic unicellular organism called cyanobacteria began to produce oxygen and carbohydrates, or sugars, through photosynthesis using sunlight and carbon dioxide. This ability later developed in algae. Algae, plankton, and fully developed plants now work together to ensure the supply of oxygen in our atmosphere, which has slowly transformed our atmosphere into its Academic year 2024 current state. In this chapter, we will try to understand this very important process in nature called photosynthesis. 148  Photosynthesis 10.2 Conversion of Solar Energy into Chemical Energy You already know that the leaves Adenine of plants appear green because Phosphate group of chlorophyll. The meaning of something being green is that it can absorb other colours of light but cannot absorb the green colour. So you must understand that during photosynthesis, the chlorophyll of Ribose the plant leaves absorbs the other colours it needs from the light Energy emission and returns the green colour. This absorbed light energy, processed by Figure 10.1: Energy is generated by breaking chlorophyll, is used to complete the the phosphate bonds of ATP. subsequent steps of photosynthesis. This chemical energy is stored in two molecules called ATP (Adenosine Triphosphate) and NADPH (Nicotinamide Adenine Dinucleotide Phosphate). In a living organism, ATP stores energy for all reactions and supplies it as required. For this reason, ATP is also known as the biological coin of the cell (Figure 10.1). When you use your muscles, that energy is also supplied by this energy-storing biomolecule called ATP. The three phosphate groups in ATP are linked by strong chemical bonds, which store energy. When this chemical bond breaks and releases a phosphate group, energy is released for chemical reactions. (Image/Figure) When there are two phosphates instead of three, it is called ADP (ADP: Adenosine Diphosphate). Therefore, we can say that an important step in photosynthesis is to convert ADP to ATP, thereby converting solar energy into chemical energy. In the case of NADPH, hydrogen (H) is added to NADP to convert solar energy into chemical energy in the chloroplast, and here too, the hydrogen (H) of NADPH can be Academic year 2024 released to obtain the necessary energy to complete the photosynthesis process. 10.3 The Site of Photosynthesis Process The mesophyll tissue of the leaf is the primary site for the photosynthesis process. 149  Science Terrestrial green plants absorb water from the soil through the roots and reach the chloroplast of the mesophyll tissue of the leaf, and through the stomata, they take in carbon dioxide from the air, which reaches the chloroplast of the mesophyll tissue. The process of photosynthesis takes place from the beginning to the end in cytoplasmic organelles called chloroplasts. Structure of Chloroplasts: The structure of the chloroplast is illustrated in Figure 10.3. It is roughly 1–2 μm thick and 5–7 μm in diameter. The chloroplast is oval in shape and contains two membranes: an outer membrane and an inner membrane. Between the outer and inner membranes, there is a space of approximately 10-20 nm width called the intermembrane space. The space within the inner membrane is known as the stroma. In the granum (plural: grana) region of chloroplasts, the light of photosynthesis is absorbed and chemical energy is Mesophyll cell Mesophyll Tissue Chloroplast Stroma Thylakoids Stroma Academic year 2024 Figure 10.2: Structure of Chloroplast 150  Photosynthesis produced, and in the stroma region, the reaction of forming carbohydrates from carbon dioxide is carried out using that chemical energy. Some of the organelles inside the chloroplast required for photosynthesis are: Chlorophyll: It is a green photosynthetic pigment, and being a pigment, Chlorophyll absorbs light at a specific wavelength. Thylakoids: They are composed of flat sac-like structures in the chloroplast. They hang within the stroma, where light energy is converted into chemical energy. Chlorophyll is present on the surface of thylakoids. It is to be noted that cyanobacteria do not have chloroplasts but have thylakoids, and on their surface, there are chlorophyll and other photosensitive pigments. Grana (plural granum): Granum is formed by the aggregation of many (10 to 20) thylakoids, which are the site of the conversion of light energy into chemical energy. 10.4 The Process of Photosynthesis: Photosynthesis is a physiological process in which chlorophyll in the cells of living plants captures light energy and stores it as chemical energy in organic molecules called ATP and NADPH. The chemical energy is then used to carry out the photosynthesis reaction. The reaction is as follows: Light 6CO2 + 12H2O chlorophyll C6H12O6+602 + 6H2O You can see from the above chemical reaction that in the photosynthesis process, 1 molecule of hexose sugar (glucose) is produced from 6 molecules of CO2 and 12 molecules of H2O. Here, H2O is oxidized to release O2, while carbon dioxide is reduced by combining with hydrogen. Therefore, photosynthesis is called a complex oxidation- reduction process. Although there are many steps in the process of photosynthesis, it can be divided into two main phases: light dependent and light independent (Figure Academic year 2024 10.3). 151  Science 10.4.1 Light Dependent Phase The phase of the photosynthesis process in which light energy is converted into chemical energy and stored in ATP and NADPH is called the light-dependent phase. Light is Light Dark phase Thylakoid (center of light phase) Light Calvin dependent phase cycle Stroma (center of dark phase) Starch Figure 10.3: Light-dependent and dark phases of photosynthesis inside a chloroplast essential for this part. The chemical reaction is shown below: Light 2ADP + 2Pi + 2NADP + 4H2O Chlorophyll 2ATP + 2NADPH + H+ + 2H2O + O2 The above reaction shows inorganic phosphate as Pi. Reactions in the photodependent phase of photosynthesis take place in the grana of chloroplasts in the presence of light. The main events at this level include: Activation of Chlorophyll: Chlorophyll plays an important role in this reaction. Chlorophyll Academic year 2024 molecules become active and energetic by absorbing photons of sunlight. Photolysis: Active chlorophyll molecules split water to produce oxygen, hydrogen, and electrons. The generated oxygen is released into the environment through leaf stomata. 152  Photosynthesis Photophosphorylation: In this process, the compound ADP (Adenosine Diphosphate) present in the leaf cell combines with inorganic phosphate (Pi) in the presence of sunlight to form the high-energy compound ATP (Adenosine Triphosphate). Formation of reduced NADPH: The compound NADP present in the leaf cells combines with hydrogen ions, and forms reduced NADPH, which serves as a source of energy. 10.4.2 Light-independent Phase In the light-dependent phase of photosynthesis, ATP and NADPH are produced using light energy. These are used as an energy source to produce carbohydrates or sugars by taking carbon dioxide from the air in the light-independent phase. After receiving energy from ATP and NADPH, they are converted to ADP and NADP respectively and again store energy in the light-dependent phase. The reactions take place cyclically in the stroma of the chloroplast with the help of enzymes and do not require light. The cycle is named the Calvin cycle after its discoverer, Dr. Melvin Calvin. Photosynthesis converts carbon from its gaseous state into carbohydrates that other organisms on Earth can use. This is called carbon fixation. You may remember that life on Earth is carbon-based and carbon fixation in the Calvin cycle is one of the essential events for sustaining life. Photosynthesis does not always operate at maximum speed; the intensity of light, the amount of carbon dioxide, temperature, and the amount of water can also affect the rate of photosynthesis. 10.5 Importance of Photosynthesis The main significance or importance of photosynthesis can be summarized into three points: 1. Capturing the solar energy and converting it into stable energy in food: The sun is the source of all energy on Earth. During photosynthesis, green plants absorb solar energy and convert Academic year 2024 it into chemical energy, storing within ATP molecules. Later, this energy is transformed into potential energy within the food produced. This energy is used by plants for various metabolic processes. Heterotrophic organisms acquire the necessary energy by consuming plant-based food. The energy contained in coal, petrol, etc., is solar energy trapped in plants 153  Science from many years ago. 2. Conversion of glucose into carbohydrates and transportation to storage organs: The simple sugar glucose is produced during the process of photosynthesis. It is then converted into carbohydrates and stored in various storage organs of the plant, such as fruits, roots, and seeds. Proteins, fats etc. are synthesized from glucose. Heterotrophs consume these plant foods directly or indirectly. Therefore, the food produced by photosynthesis is the main source of food. 3. Maintaining the Balance of Oxygen and Carbon Dioxide in the Environment: The natural balance of gases in the atmosphere includes approximately 0.04% CO2 and 21% O2. During photosynthesis, green plants help maintain the balance of O2 and CO2 in the environment by absorbing carbon dioxide and releasing oxygen. Academic year 2024 154