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• Remember mitochondria talk.

• ATP (Adenosine triphosphate) is body’s energy currency. Created from glucose and other foods in mitochondria

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This is a mitochondrion. Note the folded membrane structure, in which the complexes that perform the energy pricess are embedded.

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• Photosynthesis creates glucose from water, CO2 and SUNLIGHT.

• It also provides the energy needed to generate the ATP plants need for living

• And it provides all the oxygen we breathe as a by-product.

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The phenomenon of photosynthesis was discovered by accident by Joseph Priestley, who is credited with the discovery of oxygen, though neither was really understood for some time afterwards.

• All green plants do it. So do certain bacteria. We will come back to bacteria later. For now, think about plants.

• In plants photosynthesis takes place in chloroplasts. The are organelles within cells, rather like mitochondria. They are bacterium-sized; this is not surprising because like mitochondria that is where they came from. More on that later.

• Chloroplasts use the substance chlorophyll for photosynthesis . It’s what makes plants green (because it absorbs most red and some blue light)

• This is a magnified image of leaf cells. Note the chloroplasts in every cell.

Image of chloroplast. Note the strange multilayered structures.

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• This is a diagram of the inside of a chloroplast.

• You can see now that in 3-dimensions the strange multi-layered structures are in fact piles of flat cylinders which are called thykaloids. Thylakoid comes from Greek word thylakos meaning pouch. The stacks are called Grana

• If you were a small molecule inside the stroma it would be like being in a huge city or a vast factory with towering buildings and masses of machinery.

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• You can compare the structure of a chloroplast and a mitochondrion. They look very different but there are some key similarities:

• Both have a double membrane

• In mitochondria, the inner membrane folds back and forth to increase surface area. The membrane has the biochemical complexes embedded in it

• In chloroplast, the thylakoids themselves are assemblages of membranes with biochemical complexes embedded in them. Like mitochondria, the structure provides a way of containing huge membrane area within confined space.

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• Here is another way to visualise the structure.

• Note the scale – chloroplast is about 5 microns long – human hair is about 100 microns.

• So –what goes on inside a chloroplast?

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First, recap mitochondria.

Note the fact that all the component protein complexes are embedded in the membrane.

Note the way the excited electron is passed along the chain, releasing part of its energy at each step.

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• Here is a process diagram: glucose is broken down, and provides the organic compounds that cell needs like amino acids and lipids.

• The glycolysis process releases protons and electrons as the energy source for the rest of the process.

• The electrons and protons are fed to the respiratory machinery, which outputs ATP with water as by-product.

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• Photosynthesis is the exact reverse of that process.

• It uses light to break up water into Hydrogen and Oxygen; that creates ATP and NADPH as energy currency.

• NADPH (Nicotinamide adenine dinucleotide phosphate) works rather like ATP as a source of energy for further reactions.

• A chain of reactions feeds ATP and NADPH to the carbon fixation process which uses their energy to take CO2 and convert it to organic compounds.

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• This shows where in the chloroplast the various stages happen. The splitting of water and release of oxygen is another process that relies upon a membrane, and that is what the thylakoids contain. Their membranes also contain exactly the same electron transport chains as we saw in mitochondria, to generate ATP and NADPH using the electrons and protons from the first stage. These energy carriers are exported to the fluid that fills the central space, known as the stroma.

• The carbon fixation happens in the stroma, using CO2 comes from atmosphere. The glucose and other compounds produces are exported to the cell.

• Reflect on what is happening here. Water is exceedingly stable and hard to break down. 3 billion years ago some bacteria evolved a mechanism to do exactly that.

• And there is an important additional point. We think of oxygen as a life-giving necessity. In most contexts it is a deadly poison. Because it is so reactive, if let loose it can do untold damage to other chemistry.

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• This diagram shows the steps in the process of stripping electrons from water. The real danger comes from the ionised and activated forms of oxygen that emerge from the chlorophyll’s core chemical process.

• You can see the sequence of intermediates here: hydroxyl ion, hydrogen peroxide, and the superoxide radical - all extremely reactive.

• So the complex of proteins that surround the chlorophyll as it splits the water molecule the grabs the ionised oxygen atoms and wraps them up tight until 4 electrons have been removed and the oxygen can be safely released as O2 molecules.

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• This is the basic structure of the chlorophyll molecule, though it occurs in a variety of forms.

• The magnesium atom at the top is the heart of the process of stripping electrons from water molecules. The blue shows where a cloud of electrons are effectively shared, and are the source of excitons when light strikes.

• The long hydrophobic ensures that it is firmly embedded in the thylakoid membrane.

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• Remember that the purpose of the exercise from the plant’s point of view is the production of organic molecules from carbon dioxide.

• To achieve that it has to produce molecules of ATP and NADPH carrying highly energised electrons to drive the fixation cycle, and that energy has to come from light.

• It turns out that a single photon of visible light does not give enough energy for that goal. So the process involves TWO steps, each requiring the absorption of a photon.

• The first step, in Photosystem II, strips electrons from water, creating oxygen as a by-product.

• The energised electrons are then passed along a chain of carrier molecules to the second step in Photosystem I where they get a second boost from a second photon. That gives them enough energy to create NADPH as a source of energy for further reactions - in this case using carbon dioxide to create sugars, lipids etc.

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• It’s actually even more subtle than that. • This diagram shows one of the two photosystem complexes. It has lots of

chlorophyll molecules most of which are known as antennae. Once excited by light, they pass energy along the chain to feed the pair of special chlorophyll molecules in the reaction centre which is where the critical process takes place.

• And there is s further mystery. The efficiency of the energy capture process is close to 100%. How is that possible?

• If the excitation followed a random path through the antennae, you would expect it to get lost quite often. But it doesn’t. Why not?

• There is increasing evidence that a process similar to a quantum computer is at work.

• The excitation (exciton) seems to use quantum coherence to follow all paths simultaneously and “select” the one that gets it most quickly to the reaction centre.

• Read more about this in Jim Al-Khalili’s excellent book “Life on the Edge” about quantum biology.

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• And this shows the whole process with two photosystems. • Electrons are excited by the first with oxygen released, the electrons are passed

along to the second where they get another kick from another photon, and ATP and NADPH are fed into the stroma (the liquid in the body of the chloroplast) to provide energy for the carbon fixation cycle.

• The movement of the electrons along the chain embedded in the membrane is very much like mitochondria.

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This is the Calvin cycle.. Like all other enzymes, Rubisco is a catalyst: it does its work bringing the component chemicals together, and is finally regenerated to start another loop.

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There is yet a third version of photosynthesis, named after the Crassulaceae - the Jade Plants shown in second picture

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• So how did all this start? It is now believed that the process originated in what are now called cyanobacteria (once known as blue-green algae). Probably about 3 billion years ago.

•• Why did it happen? The ability to harvest energy from sunlight rather than relying on

whatever heat and chemicals happened to drift pass gives you a big advantage. So once it started by chance, evolution made sure it prospered.

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• Once photosynthesis started, the earth started to change. • At first, the oxygen would have been quickly mopped up by the iron and manganese

in the seawater. The geological record shows this happening in bands of rocks laid down on what was the seabed.

• Eventually all the iron had been used up, and then the oxygen started to get into the atmosphere.

• At some point between 1 and 2 billion years ago eukaryotes formed and the first mitochondria were created when an archaeon swallowed a methane-producing bacterium. There is still debate about the exact order of events. But by about 1 Bn years ago fully-formed eukaryotes powered by mitochondria were growing fast.

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• At some point in this period, a eukaryote- presumably still a single-celled creature -repeated the same trick that had led to mitochondria but this time with a cyanobacterium, and the result was the first chloroplasts.

• After this came the development of multicellular organisms, both with and without chlorplasts.

• Meanwhile the concentration of oxygen in the atmosphere, and free oxygen in the sea water, is steadily growing as a result of all the photosynthesis..

• And around 541M years ago there came the Cambrian Explosion - the point at which there is a huge growth in the number and variety of living organisms.

• The driving force for the Cambrian explosion is still hotly debated, but it seems likely that the increasing oxygen concentration was at least a factor.

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LUCA = last universal common ancestor

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