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B CLASS-X EPISODE-2 STATE SYLLABUS LIFE PROCESSES PHOTOSYNTHESIS – FACTORS In the last episode we have studied the importance of Photosynthesis. In this episode we shall examine the factors of Photosynthesis. The factors of Photosynthesis are a) Carbon dioxide. b) Water c) Sun light d) Chloroplasts for synthesis of Carbohydrates. The factors “a” and “c” i.e., CO2 and Sun light are external factors while the factors “b” and “d” are internal factors. Let us now examine the importance of external factors. 1. Light:- Light is a form of energy. What do the light rays contain? The light rays have tiny particles known as “Photons”. The energy is carried by a photon is called ”Quantum”. Light is an electromagnetic radiation. It has many components with different wave lengths. Comic rays, gamma rays, X rays, ultra violet rays, (UV) infrared rays (IR) and radio waves are some of the components of light. The wave length of visible light lies between 400-700mm ie., lies between UV and IR. As the wave length increases there is decrease in the energy quantum. See the image 1 From this image we can understand that the wave length of visible light is between 100mm to 1000mm. Violet has a minimum wave length of 100mm while red has 700mm. Properties of Light:Light has seven different colours. The colours as we see in the rainbow are Violet, Indigo, Blue, Green, Yellow, Orange, Red, or VIBGYOR, in short. Show rainbow When light strikes any object, the light rays undergo reflection, refraction, transmission or absorption. This depends on the nature of object. When the light is absorbed the energy present in photons is transferred to the object. For photosynthesis maximum energy is required. Light rays with small wave lengths would not be useful. Only the light rays with maximum wave lengths are useful. Therefore the length of light becomes a critical factor. In a spectrum photosynthesis will be more in the regions of red and blue rays. Chlorophy reflects green light. Hence no photosynthesis is possible in green light. The chlorophyll molecules, located on the thylakoid membranes of the chloroplast, absorb light energy and initiate photosynthesis. The importance of light can be understood by the experiment that follows. 1.Keep a potted plant in dark for two three days. After two or three days the leaves become free of starch. 2.Cut a design in the center of light screen. Place the light screen on one of the leaves. The center of the screen. 3.Place the pot in sun light for about 5 hours. 4. Pluck the leaf with light screen and remove the light screen. 5. Dip the plucked leaf in iodine solution to test for the presence of starch. 2 6. The areas where starch is present turns blue. You will observe that the area other than the area where leaf was covered by the light screen turn blue. Animate Story Board 1. (image) Potted plant kept in dark. 2. Light screen(image) place light screen on one leaf with a design in center. 3. (image) keep the pot in sun light. 4. (image). 5. (image). 6. (image). The experiment proves that photosynthesis can take place only in the presence of light. As certain part of the leave was covered and did not receive light, photo synthesis did not take place and no starch has formed. The other important factor for photosynthesis is carbon dioxide. Let us conduct an experiment to test the importance of CO2 for photosynthesis. 3 Animate Story Board 1.Pot image 1.Take a potted plant with long leaves 2.Take a bottle with a cork. 3.Split the cork vertically 4.Keep the pot in dark for two to three days. The leaves will be free from starch. 2.Bottle Cork.(image) 3.Cork up down 4.Potted Plant (image) 5.Take 5 – 10 ml of potassium hydroxide in the bottle. This will dissolve the CO2 present in the bottle. 5. Potassium Hydroxide taken in bottle (image). 6. After three days insert a leaf into the bottle through the slit in the cork without detaching it from the plant. Keep the bottle in sun light for 4 to 5 hours. The mouth of the bottle should be made air tight by applying Vaseline or grease. This prevents any air going into the bottle. 7. After 4-5 hours detach the leaf from the plant and dip it in iodine solution to test for the presence of starch. 8. You will observe that the part of the leaf outside the bottle exposed to environment turns blue. The part of leaf does not turn blue. 6. Potted Plant and bottle kept in sun light (image) 7. leaf becomes blue in colour. (image) 8. leaf testing (image) 4 this proves that the plant of leaf in the bottle did not receive CO2 while the one outside the bottle received CO2 . Hence starch was synthesized by Photosynthesis. Thus we know that CO2 and light are both essential external factors for Photosynthesis. How does the leaf absorb CO2? Are there any special structures? Leaves have natural openings in their epidermic. These are called STOMATA. It is through this stomata the leaves exchange gases i.e., absorb CO2 and release O2. The efficiency of photosynthesis depends on the number of stomata on leaves. More the number of stomata more photosynthesis and lesser number of stomata reduced photosynthesis occurs in plants. Epidermal peels of a leaf showing stomata and guard cells Let us now examine the internal factors of photosynthesis. All green plants have in their leaves special membrane bound organells called chloroplasts. The chloroplasts are disc or spindle shaped. They are filled with colourless area called STROMA. In the stroma are a number of stacks of structures known as THALAKOIDS. 5 The thalakoids are connected to one another by structures called LAMELLAE. Chloroplast In the lipid part of thalakoid a green pigment CHLOROPHYL is present. Due to this pigment the chloroplasts are green in colour. The molecules in the pigment absorb light rays of a specific wave length i.e., Red and blue rays. Chlorophyll and other accessory pigments in the thalakoids membranes organize to form REACTION CENTERS called photo system-I and Photo system-II (PSI and PSII). The pigments harvest solar energy and pass it into the reaction centers. They absorb photons and convert CO2 in the air to sugar and release O2. some enzymes present in stroma are utilized during this process. If you dip a leaf in iodine solution in the evening you will observe that some areas of the leaf turn blue. If you observe a leaf you will find that some areas are green while some are not. Dip this leaf in iodine solution in the evening. You will observe that the green areas turn blue indicating presence of starch and hence photosynthesis. The green colour in these areas is due to presence of chlorophyll. 6 The area of leaf which does not turn blue indicates absence of Photosynthesis due to absence of chlorophyll. This proves that chlorophyll is essential for Photosynthesis. Story 1. 2. 3. Animate Board Take a Green leaf. Show patches of non green or pale areas. Dip the leaf in iodine solution. Take out the leaf. Show the leaf in blue colour except the pale areas which remain pale green. Fig 1.2.5 state syllabus X page no.8 In the next episode we will examine the mechanism of Photosynthesis. 7