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Photosynthesis A2 Biology Unit 1 Module 3 Life on earth depends on plants for photosynthetic CO2 fixation and O2 evolution What is photosynthesis? All life on Earth depends on photosynthesis. The process is described by the following word and symbol equations: carbon + water dioxide light energy oxygen + glucose light energy 6CO2 + 6H2O 6O2 + C6H12O6 These equations are summaries of a complex two-step process that takes place in the chloroplasts of green plants. The end products are not just glucose, but complex organic molecules such as carbohydrates, amino acids, lipids and nucleic acids. Why do plants photosynthesize? Photosynthesis is an essential biological process. This is because it produces: complex organic molecules needed for growth energy for processes in the organism oxygen, which is then used for respiration. There are not just advantages for the plant itself: If the plant is eaten, the organic molecules are used to provide energy to organisms higher up the food chain. The oxygen produced is released into the atmosphere and is available for other organisms. Revision: leaf structure Cross-section of a leaf 100 mm Mesophyll cells Leaves and photosynthesis Waxy cutin secretion prevents water loss Upper epidermis Pallisade mesophyll – cells contain many chloroplasts Xylem vessel The spongy mesophyll layer has air spaces allowing rapid gas movement Guard cells can become more or less turgid – opening or closing the stoma Phloem sieve tube Phloem companion cell Stoma allows gas exchange Waxy cuticle Revision: the chloroplast Schematic chloroplast membrane-enclosed stroma sealed thylakoid membrane The Chloroplast – originally thought to have come from free-living prokaryotic organisms engulfed by the ancestor cells of plants 2000 million years ago. Chloroplasts form from a sort of ‘stem organelle’ called a leucoplast Ribosomes Starch grain (most efficient way to store glucose made by photosynthesis) Stroma (like cytoplasm) Lipid droplet Like the nucleus and mitochondria a chloroplast has Inner membrane a double membrane Thylakoid An extremely folded inner membrane giving a vast surface area for enzyme catalysed reactions to take place Granum – stack of thylakoid membranes Outer membrane Structure of the chloroplast Thylakoid membrane • chlorophyll • light-harvesting • electron transfer • O2 evolution • energy production Stroma • Rubisco • CO2 fixation • sugar and starch synthesis Pea chloroplast 1 mm Plant photosynthetic tissues Plant anatomy Overview The Light-independent reaction involves fixing carbon by reducing carbon dioxide to glucose. These reactions also require energy, a source of hydrogen, electrons and phosphate. The light-dependent reaction provides all of this! Functions of photosynthetic structures
