1 of 36 © Boardworks Ltd 2009 2 of 36 © Boardworks Ltd 2009 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. 3 of 36 © Boardworks Ltd 2009 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. 4 of 36 © Boardworks Ltd 2009 Leaves and photosynthesis 5 of 36 © Boardworks Ltd 2009 Structure of the chloroplast 6 of 36 © Boardworks Ltd 2009 Plant photosynthetic tissues 7 of 36 © Boardworks Ltd 2009 Plant anatomy 8 of 36 © Boardworks Ltd 2009 Functions of photosynthetic structures 9 of 36 © Boardworks Ltd 2009 10 of 36 © Boardworks Ltd 2009 Photosynthetic pigments A photosynthetic pigment is a coloured biological compound that is present in chloroplasts and photosynthetic bacteria, and which captures light energy for photosynthesis. In plants there are two types of pigments: chlorophylls and carotenoids. They are coloured because they absorb particular wavelengths of light and reflect others. Chlorophyll is the pigment that gives plants their green colour by reflecting green light. Carotenoids reflect red, orange or yellow light. 11 of 36 © Boardworks Ltd 2009 Photosynthetic pigments 12 of 36 © Boardworks Ltd 2009 Photosynthetic pigments 13 of 36 © Boardworks Ltd 2009 Adenosine triphosphate Adenosine triphosphate, or ATP, is an important molecule found in all living cells. It readily diffuses around the cell and provides energy for cellular processes. adenine 3 phosphate groups ribose ATP is made in the light-dependent reaction in photosynthesis from adenosine diphosphate (ADP) and an inorganic phosphate group (Pi). This requires energy. ATP releases energy in the light-independent reaction when a bond between inorganic phosphate groups is broken, producing ADP and an inorganic phosphate group. 14 of 36 © Boardworks Ltd 2009 Photophosphorylation and hydrolysis In the light-dependent reaction: ADP ATP + + energy Adding a phosphate molecule is phosphorylation. In this case, light is used as an energy source so the process is called photophosphorylation. In the light-independent reaction: ATP H2O ADP + + energy The use of water makes this a hydrolysis reaction. 15 of 36 © Boardworks Ltd 2009 NADP and NADPH Nicotinamide adenine dinucleotide phosphate (NADP) is a coenzyme involved in the photosynthesis reactions. The compound is a dinucleotide, containing an adenine base and a nicotinamide base. The nucleotides are joined through their phosphate groups. In addition there is an extra phosphate on the ribose of the adenine-containing nucleotide. nicotinamide adenine ribose NADP can accept electrons to be reduced to reduced NADP, often called NADPH. NADPH can be oxidized back to NADP, releasing electrons. 16 of 36 © Boardworks Ltd 2009 Which molecule? 17 of 36 © Boardworks Ltd 2009 18 of 36 © Boardworks Ltd 2009 Location of the reaction The light-dependent reaction takes place in the thylakoid membranes of the chloroplasts. Situated in these membranes are light-harvesting systems called photosystems. There are two types, photosystem I (PSI) and photosystem II (PSII). Both have chlorophyll at their centres. photosystem light chlorophyll 19 of 36 © Boardworks Ltd 2009 Light-dependent reaction 20 of 36 © Boardworks Ltd 2009 Light-dependent reaction: summary Cyclic photophosphorylation Non-cyclic photophosphorylation photolysis 21 of 36 © Boardworks Ltd 2009 Products of the light-dependent reaction Products of the light-dependent reaction that pass into the light-independent reaction: reduced NADP ATP Products of the light-dependent reaction that leave the plant: oxygen Products of the light-dependent reaction that are re-used in another part of the light-dependent reaction: H+ ions electrons 22 of 36 © Boardworks Ltd 2009 How much do you know? 23 of 36 © Boardworks Ltd 2009 24 of 36 © Boardworks Ltd 2009 Location of the reaction The light-independent reaction takes place in the stroma of the chloroplasts. The light-independent reaction can also be called the “dark reactions”, or the Calvin Cycle. The reaction consists of a cycle of enzyme-controlled processes making complex molecules such as carbohydrates. stroma 25 of 36 © Boardworks Ltd 2009 The light-independent reaction 26 of 36 © Boardworks Ltd 2009 Products of the Calvin cycle Products of the light-independent reaction that pass back into the light-dependent reaction: NADP ADP inorganic phosphate Products of the light-independent reaction that are used in other processes: 27 of 36 triose phosphate – used to build complex carbohydrates, amino acids and lipids. © Boardworks Ltd 2009 Making complex molecules Triose phosphate is a three-carbon sugar that can be used to make a variety of complex biological molecules. Pairs of TP molecules combine to form hexose sugars, such as glucose, some of which may isomerize to form fructose. These monosaccharides can combine to form disaccharides such as sucrose, and polysaccharides such as cellulose and starch. TP can be converted to glycerol and this may be combined with fatty acids to make lipids. 28 of 36 © Boardworks Ltd 2009 How much can you remember? 29 of 36 © Boardworks Ltd 2009 Summary of photosynthesis light lightdependent reactions lightindependent reactions carbohydrates, other complex molecules 30 of 36 © Boardworks Ltd 2009 Photosynthesis: true or false? 31 of 36 © Boardworks Ltd 2009 32 of 36 © Boardworks Ltd 2009 Glossary 33 of 36 © Boardworks Ltd 2009 What’s the keyword? 34 of 36 © Boardworks Ltd 2009 Inputs and outputs to the reactions 35 of 36 © Boardworks Ltd 2009 Multiple-choice quiz 36 of 36 © Boardworks Ltd 2009
