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Biology – Communication, Homeostasis and EnergyModule 3 – Photosynthesis 1) THE IMPORTANCE OF PHOTOSYNTHESISPhotosynthesis – the process whereby light energy from the Sun is transformed intochemical energy and used to synthesise large organic molecules from inorganicsubstances.Autotrophs and HeterotrophsAutotrophs – organisms that can synthesise complex organic molecules, i.e.carbohydrates, lipids, proteins, nucleic acids and vitamins, from inorganic moleculesand an energy source.Photoautotrophs – organisms that can photosynthesise. Their source of energy issunlight and their raw materials are CO2 and water.Heterotrophs – cannot make their own food but digest complex organic moleculesinto simpler soluble ones, from which they synthesise complex molecules such aslipids, proteins and nucleic acids.Both heterotrophs and autotrophs can respire the molecules made duringphotosynthesis. And they can also use the oxygen released for respiration.6CO2 + 6H2O (+light energy) C6H12O6 + 6O2 2) THE STRUCTURE AND FUNCTION OF CHLOROPLASTSStructure - disc-shaped, between 2 – 10 micrometres long - double membrane, or envelope - intermembrane space, about 10 – 20 nanometres wide - outer membrane permeable to many small ions - inner membrane is selectively permeable, and contains transport proteins - the membrane is folded into lamellae, stacked to form grana - between the grana are intergranal lamellaeThe stroma is a fluid-filled matrix, containing the necessary enzymes for the light-independent stage of photosynthesis, along with starch grains, oil droplets, DNA andprokaryote-type ribosomes.
The grana are stacks of thylakoids (flattened membrane compartments), which arethe sites of light absorption and ATP synthesis during the light-dependent stage ofphotosynthesis.Function - the inner membrane can control the exchange of substances between the cytoplasm and the stroma. - the many grana provide a large surface area for photosynthetic pigments, electron carriers and ATP synthase enzymes. - the photosynthetic pigments are arranged to allow maximum absorption of light energy, special structures known as photosystems. - the fluid-filled stroma contains enzymes required for the light- independent (or dark) stage of photosynthesis. - the grana are surrounded by the stroma, allowing ready diffusion of products required in the light-independent stage of photosynthesis. - chloroplasts, using chloroplast ribosomes and DNA, can synthesis some of the required proteins for photosynthesis.Photosynthetic PigmentThese pigments absorb certain wavelengths of light. Many pigments work together,in funnel-shaped photosystems, to capture as much light energy as possible.Chlorophyll contains a long hydrocarbon chain and a pophyrin group: - light hits chlorophyll, causing electrons associated with magnesium to excite. - there is chlorophyll a – P700 and P680 – and each has different absorption peaks for red light at a different wavelength. - Chlorophyll a also absorbs blue light, at 450nm wavelength. - Chlorophyll b absorbs light of wavelengths 500nm and 640nm.Accessory Pigments – absorb light wavelengths not well absorbed by chlorophylland pass the associated energy to the base of the photosystem, chlorophyll a.
1) THE LIGHT- DEPENDENT STAGE OF PHOTOSYNTHESISThis stage takes place on the thylakoid membranes of the chloroplasts, inphotosystems I and II (PSI and PSII), which occur mainly on the intergranal lamellaeand almost exclusively on the granal lamellae, respectively.Role of WaterIn PSII, there is an enzyme that can PHOTOLYSE water into H+ ions, electrons andoxygen.2H2O 4H+ + 4e- + O2The hydrogen ions are used in chemiosmosis to produce ATP, and are alsoaccepted by a co-enzyme NADP, which becomes reduced NADP, important in thelight-independent stage.The electrons replace these lost by oxidised chlorophyll.Photophosphorylation (cyclic)Light hits chlorophyll in photosystem I (P700), causing the excitation of twoelectrons, which pass to an electron acceptor, and back to the chlorophyll moleculefrom which they were lost.As the electrons pass through an electron carrier system, it creates a proton gradient across the thylakoid membrane, causing the protons to flow down the gradient, through channels associated with ATP synthase enzymes. This is known as CHEMIOSMOSIS. It produces a force which binds ADP to a phosphate group to form ATP. P
P ADENINE P P ADENINE P RIBOSE RIBOSE ADP (adenosine diphosphate) ATP (adenosine triphosphate) Photophosphorylation (non-cyclic) 1) Light strikes PSII, exciting a pair of electrons that leave the chlorophyll molecule from the primary pigment reaction centre. 2) The electrons pass along a chain of electron carriers and the energy released is used to synthesise ATP. 3) Light has also struck PSI and a pair of electrons has been lost. 4) These electrons, along with protons (from PSII, lost in photolysis) join NADP, forming reduced NADP. 5) The electrons from the oxidised PSII replace electrons lost from PSI 6) Electrons from photolysed water replace those lost by the oxidised chlorophyll in PSII. 7) Protons from photolysed water take part in chemiosmosis to make ATP and are then captured by NADP, in the stroma.
1) THE LIGHT-INDEPENDENT STAGE OF PHOTOSYNTHESIS
The light-independent stage of photosynthesis takes place in the stroma of thechloroplasts. This stage is also known as The Calvin Cycle, as the sequence ofevents was worked out by Melvin Calvin between 1946 and 1953. It involves theproducts of the light-dependent stage, so although light is not directly required in theCalvin cycle, the process would soon cease without the presence of light.Carbon dioxide is a source of carbon and oxygen for the production of all largeorganic molecules. The molecules are used as either structures or for energystores/sources by every carbon-based life form on the planet. 1) CO2 diffuses into the leaf through the open stomata, where it then diffuses throughout the airspaces in the mesophyll. It diffuses through the thin cellulose cell walls, the plasma membrane, the cytoplasm and then the chloroplastic envelope, eventually reaching the stroma. 2) In the stroma, an enzyme catalysed reaction takes place, combining the carbon-dioxide with a pentose sugar, ribulose bisphosphate (RuBP), which is a carbon dioxide acceptor. The enzyme which catalyses this reaction is ribulose bisphosphate carboxylase- oxygenase, otherwise known as rubisco. It is thought to be the most abundant protein on the entire planet, consisting of 16 polypeptide chains: 8 identical long chains and 8 identical short chains. The long chains form the active site while the short chains maintain the 3D shape, or tertiary structure, of the protein.
3) The rubisco-catalysed reaction produces two molecules of a 3-carbon compound, glycerate 3-phosphate, referred to at this point as GP; the CO2 has now been fixed.4) ATP and reduced NADP from the light-dependent stage (see above) are used in the next process. GP is reduced and phosphorylated into another 3-carbon compound, known as triose phosphate (TP).5) Five out of every 6 molecules of TP are recycled by phosphorylation, using ATP from the light-dependent stage of photosynthesis, to three molecules of the pentose sugar, RuBP.6) Some GP is used to make amino acids and fatty acids and oils.7) Pairs of TP molecules can combine to form hexose sugars, such as glucose.8) TP can be converted to glycerol.