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2.4 |
Main Organ for Photosynthesis |
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Photosynthesis is needed by plants because:
- The photosynthetic product is glucose,the primary source for cellular transpiration used to synthesise energy
- The glucose produced can be used to from cellulose
- The glucose produced can be converted into sucrose and transported to other parts of plant for metabolism or storage in the form of starch
- Glucose can be converted into amino acids,proteins or fats for metabolism or storage
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Chemical reaction of overall photosynthesis process |
\(12\text{H}_2\text{O}+6\text{CO}_2\xrightarrow[\text{chlorophyll}]{\text{light energy}}\text{C}_6\text{H}_{12}\text{O}_6+6\text{O}_2+6\text{H}_2\text{O}\)\(12\text{H}_2\text{O}+6\text{CO}_2\xrightarrow[\text{chlorophyll}]{\text{light energy}}\text{C}_6\text{H}_{12}\text{O}_6+6\text{O}_2+6\text{H}_2\text{O}\)
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Main organ in photosynthesis by having some adaptations:
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Leaf shape |
- Broad-increases surface area to absorb more sunlight
- Thin-allow penetration of sunlight to reach lower layer of cells in leaf
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Epidermis |
- Transparent-allow sunlight to penetrate into leaf
- Cuticle-to reduce water loss from leaf
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Palisade mesophyll |
- Cells which have abundant of chloroplasts-can carry out photosynthesis
- Cylindrical shape of cells-more cells can be arranged compactly
- Cells arranged compactly-absorb sunlight maximally for photosynthesis
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Spongy mesophyll |
- Cells which have chloroplasts-can carry out photosynthesis
- Irregular shaped of cells and loosely arranged-to form air spaces for transpiration and gaseous exchange
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Xylem |
- Transport water and mineral salts from roots to mesophyll for photosynthesis
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Phloem |
- Transport organic products from leaves to other parts of plants
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Stoma |
- Pore that formed from two guard cells
- For gaseous exchange and transpiration
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Structure of chloroplast:- |
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- Photosynthesis takes place in chloroplasts
- Photosynthesis consists of light-dependent reactions and light independent reactions
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The light-dependent reactions take place in thylakoid:
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- In light-dependent reactions,chlorophyll absorbs light energy to becomes active and then releases electrons
- At the same time,photolysis of water occurs in which a water molecule is splitted into hydrogen ion,oxygen and molecules
- The oxygen gas produced is released into atmosphere
- The electron released from water molecule will replace the electrons released from the chlorophyll
- The electrons flow from chlorophyll eventually received by hydrogen ions and NADP to form hydrogen atoms and NADPH
- During the flow of electrons from chlorophyll to hydrogen ions,some energy released from the electrons is used to synthesis ATP
- The hydrogen atoms,NADPH and ATP produced from the light-dependent reactions will be used by the light independent reaction in stroma
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The light independent reactions occur in stroma and do not required sunlight:
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- Carbon dioxide gas diffuses into chloroplast
- The hydrogen atom , NADPH and ATP produced from the light-dependent reactions are used to reduce the carbon dioxide to produce glucose and water.
- The reduction process requires enzymes found in the stoma.
- The glucose produced form starch by condensation.
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The simillarities between the light-dependent and light independent reactions:
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- Occurs in daytime
- Occurs in chloroplasts
- Involves chemical reactions with enzymes
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Perbezaan antara tindak balas bersandarkan cahaya dan tindak balas tidak bersandarkan cahaya:
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Light-dependent reaction |
Light-independent reaction |
Require light energy |
Do not require light energy |
Occurs in granum and thylakoid |
Occurs in stroma |
Photolysis of water takes place |
Photolysis of water does not take place |
Produce ATP and NADPH |
Do not produce ATP and NADPH |
Produce oxygen and water molecule |
Produce glucose |
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Factors affecting the rate of photosynthesis |
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Carbon dioxide concentration:
- The increase in carbon dioxide concentration increases the photosynthesis rate as long as there are no other limiting factors such as surrounding temperature and light intensity.
- At P, photosynthesis rate is constant.
- As the concentration of carbon dioxide increases after P, the rate of photosynthesis remains unchanged.
- This is due to light intensity becoming the limiting factor.
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Light intensity:
- Light is needed in the light-dependent reaction.
- If the concentration of carbon dioxide and temperature are constant, the rate of photosynthesis increases until it reaches its maximum point at noon.
- Graph I shows that the rate of photosynthesis increases with the increase of light intensity until it reaches a light saturation point at P.
- After point P, the increase in light intensity (from P to Q) is no longer increases the rate of photosynthesis because it is limited by other factors such as temperature and carbon dioxide concentration.
- Graph II shows when the concentration of carbon dioxide in the environment is increased to 0.13%, the rate of photosynthesis also increases.
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Temperature:
- The reactions in photosynthesis are catalysed by enzymes.
- Therefore, changes of surrounding temperature will affect enzyme activity and also the rate of photosynthesis.
- The optimum temperature is different for different plant species but in general, the optimum temperature is between 25°C to 30°C.
- A very high temperature denatures the enzymes and the process of photosynthesis is stopped.
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The effect of different light intensities and light colours on the rate of photosynthesis |
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- The rate of photosynthesis in plants is different throughout the day.
- Other than the light intensity factor, the rate of photosynthesis is also affected by the colour of light.
- Light spectrum consists of seven colours in a certain sequence (violet, indigo, blue, green, yellow, orange and red).
- Each colour has a different wavelength.
- The rate of photosynthesis is the highest in red and blue light.
- All of the red light is absorbed by chlorophyll.
- The blue light is absorbed by carotenoid pigments before being transferred to the chlorophyll.
- These two lights have enough amount of energy to excite electrons in the light-dependent reaction.
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