Photosynthesis, crash course

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Photosynthesis - a crash course 

So, your exam's a week away and you've not revisied photosynthesis? Sounds familiar... I appologise now if these notes seem a bit disjointed, I've written them around the OCR exam sylabus so Ive had to tick all the boxes and some of them dont flow together beautifully- hence the clunkyness of it. On the up side, this is everything you need to know! Here is a complete set of notes covering the entire OCR A2 sylabus on photosynethesis: 

"photosynethesis" is the process by which light energy from the sun is transformed nto chemical energy and used to synthesise large organic molecules from inorganic substrates" 

Auto Vs. Hetro 

The first thing the sylabus demands is that you should know the difference between Autotrophs and Hetrotrophs. Living organisms, when talking about feeding, are grouped into two groups. Broadly speaking, those who can make their own food are termed Autotrophs and those which cant are called Hetrotrophs. However, that wont get you any marks in an exam as its waaaay too vague. Autotrophs can synthesise complex organic molecules (Carbohydrates, lipids, proteins, etc...) from inorganic substrates and an external energy source. Organisms which use the sun's light as their external energy source are known as photoautotrophs (light-self-feeding). Thats all plants, but thats only if you're really looking to impress. Animals, as you know, are not green and can't make their own food (especially true if you're a bloke). So, animals are Hetrotrophs- they digest complex organic molecules to form simple inorganic substrates which they can then make into the molecules they want. 

Photosynthesis is the process by which photoautotrophs produce complex organic molecules from inorganic substrates. These processes requires light. 


Both photoautotrophs and hetrotrophs relase chemical energy by respiring the products of photosynthesis, albeit indirectly in animals. Photosynthesis is therefore vital to both types of organism, providing the materials required for respiration and ultimately life. In plants, the process of photosynthesis is a two stage process called the light dependant reaction and the light independant reaction. Both take place in an organelle we know and love, the Chloroplast. 

The humble chloroplast: 

Photosynthesis takes place within organelles known as the chloroplast, a disc shaped organelle between 2-10 um long. Ill cover the detail you need to A2 briefely, but you probably know all this for AS and if not, there are a billion zillion pcitures/diagrams/pretty scribbles of them all over google so you could look there if you really want to. 

They are surrounded by a double membrane, or envelope with a space of about 10-20nm between. The outer membrane is permiable to all sorts of excietment and will let all manner of small ions into it, however, the inner membrane is far less permiable and has transport proteins embedded in it. The innter membrane is folded into stacks of thylakoid membranes which are known as grana (granum- single). Importantly, there are two main reions within a chloroplast: The stroma (site of light independant reaction) and the grana (ite


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