HKDSE Biology: Photosynthesis Notes PDF

HKDSE BIOLOGY: Connecting Concepts Revision notes 21 Photosynthesis 21.1 An overview of photosynthesis 1. Photosynthesis is the process by which complex organic molecules are made from simple inorganic molecules using light energy. 2. The word equation for ph...

HKDSE BIOLOGY: Connecting Concepts Revision notes 21 Photosynthesis 21.1 An overview of photosynthesis 1. Photosynthesis is the process by which complex organic molecules are made from simple inorganic molecules using light energy. 2. The word equation for photosynthesis is: light energy captured carbon dioxide + water glucose + oxygen by chlorophyll 3. Photosynthesis is important in: sustaining plant life providing the basic food source for most organisms and maintaining the energy flow in ecosystems helping to maintain the balance of oxygen and carbon dioxide in the atmosphere 21.2 The requirements for photosynthesis 4. The presence of starch in leaves is regarded as evidence of photosynthesis. Starch can be tested by iodine test. 5. Destarching is the removal of starch from the leaves of a plant by leaving it in the dark for at least 24 hours. This is done before an investigation to make sure that any starch detected at the end of the investigation must have been produced during the investigation. 6. Chlorophyll, light, carbon dioxide, and water are necessary for photosynthesis. © Aristo Educational Press Ltd. 21-1 HKDSE BIOLOGY: Connecting Concepts Revision notes 21.3 The site of photosynthesis 7. Leaves are the main photosynthetic organs in plants. The structure of a leaf is adapted for photosynthesis: Structure Description Adaptation for photosynthesis of leaf Leaf blade Broad and flat Provides a large surface area to absorb light Thin Allows light and carbon dioxide to reach the photosynthetic cells easily Midrib Contains vascular The xylem transports water bundles (xylem and and minerals to the leaf, phloem) while the phloem transports the carbohydrates produced by photosynthesis away from the leaf Branches into an The network supports the extensive network of leaf blade to spread out to veins maximize the area exposed to sunlight Ensures efficient transport of materials Palisade Located at the upper Maximizes the amount of mesophyll part of a leaf light absorbed for photosynthesis Cells are closely packed and contain numerous chloroplasts © Aristo Educational Press Ltd. 21-2 HKDSE BIOLOGY: Connecting Concepts Revision notes Structure Description Adaptation for photosynthesis of leaf Spongy Cells are loosely Allows gases to diffuse mesophyll packed with lots of air freely inside the leaf spaces between them Upper Contains numerous Guard cells (with epidermis stomata, each stoma chloroplasts) control the and lower is surrounded by a pair opening and closing of the epidermis of guard cells stoma to regulate the passage of gases and water vapour into and out of the leaf The upper epidermis Reduces water loss from the of terrestrial leaf so more water is retained dicotyledonous plants for photosynthesis contains fewer The cuticle and epidermis stomata than the (except guard cells) are lower epidermis transparent to allow light to penetrate and reach the photosynthetic mesophyll beneath Covered by a thin Reduce water loss from the layer of waxy cuticle leaf so more water is retained for photosynthesis © Aristo Educational Press Ltd. 21-3  HKDSE BIOLOGY: Connecting Concepts Revision notes 8. Chloroplasts are organelles where photosynthesis takes place in plant cells. The structure of a chloroplast is adapted for photosynthesis: Structure of Description Adaptation for photosynthesis chloroplast Thylakoid Stacking of thylakoids Provides a large surface into grana area in a limited space to pack more chlorophyll Grana are Allows efficient transport interconnected of photosynthetic products within the chloroplast Stroma Jelly-like fluid Contains enzymes that catalyse reactions in the Calvin cycle Contains starch grains as a temporary store of carbohydrates produced by photosynthesis starch outer Three-dimensional view grain membrane inner membrane thylakoid stroma granum © Aristo Educational Press Ltd. 21-4 HKDSE BIOLOGY: Connecting Concepts Revision notes 21.4 The process of photosynthesis 9. Photosynthesis occurs in two stages: Stage Site of occurrence Processes involved Photochemical Thylakoids of A chlorophyll molecule reactions (light- chloroplasts absorbs light energy and dependent) some of its electrons are excited to a higher energy level. As electrons pass along the electron transport chain, some of the energy released is used to synthesize ATP from ADP and phosphate (photophosphorylation). Some of the light energy is used to split water into hydrogen and oxygen (photolysis of water). Hydrogen is accepted by NADP to form NADPH. Oxygen is used in respiration or released as a gas. © Aristo Educational Press Ltd. 21-5 HKDSE BIOLOGY: Connecting Concepts Revision notes Stage Site of occurrence Processes involved Calvin cycle Stroma of Carbon dioxide combines (light- chloroplasts with a 5-C carbon dioxide independent) acceptor (carbon dioxide fixation) to form two molecules of a 3-C compound. The 3-C compound is reduced to triose phosphate, using energy from ATP and hydrogen from NADPH. Most triose phosphate molecules are used to regenerate the 5-C carbon dioxide acceptor. This requires energy from ATP. 10. Photochemical reactions and the Calvin cycle are interdependent: Photochemical reactions produce ATP and NADPH to be used in the Calvin cycle. The Calvin cycle regenerates ADP and NADP to be reused in photochemical reactions. 21.5 Conversion and use of photosynthetic products 11. Some of the triose phosphate formed in the Calvin cycle is synthesized into glucose. Glucose may be used in respiration to release energy, or may be converted to other carbohydrates, such as starch (for storage), sucrose (for transport) and cellulose (the major component of cell walls). © Aristo Educational Press Ltd. 21-6 HKDSE BIOLOGY: Connecting Concepts Revision notes 12. Intermediate compounds of photosynthesis can be synthesized into glycerol and fatty acids, which combine to form lipids. 13. Intermediate compounds of photosynthesis can combine with minerals to form amino acids, which can then become proteins. 21.6 Factors affecting the rate of photosynthesis 14. The rate of photosynthesis is affected by environmental factors. The factor that is in least supply limits the rate of photosynthesis, and is called the limiting factor. 15. When light intensity is the limiting factor, increasing the light intensity increases the rate of photosynthesis because more energy is supplied to the photochemical reactions. 16. When carbon dioxide is the limiting factor, increasing the carbon dioxide concentration increases the rate of photosynthesis because more carbon dioxide is available for carbon fixation in the Calvin cycle. © Aristo Educational Press Ltd. 21-7