Photosynthesis is, simply speaking, the process of converting sunlight into usable energy. In one way or another, virtually all food on this planet comes originally from plants, which in turn get this energy from the Sun.¹ Sunlight is food for plants. Many people confuse plant food with fertaliser, and it is also a common misconception that plants breathe in carbon dioxide and breathe out oxygen. In fact plants "eat" sunlight and carbon dioxide and, well, um... excrete oxygen. They do also take in oxygen and give out carbon dioxide just like humans do, but in much lower quantities than is given out by photosynthesis²

Where it happens

Photosynthesis takes place exclusively inside organelles³ known as chloroplasts. Chloroplasts are a type of plastid, as are mitochondria, and are similar in many respects to them. Chloroplasts are green because they contain chlorophyll, and this is why most plants are green. They contain two main parts:

1. The stroma
2. The thylakoid membrane system

The thylakoid membrane system is a series of membranes which are folded up into stacks called grana.⁴ These basically look like a pile of records stacked up on one another, joined to other stacks by more membranes. No one really knows why they are like this.

How it happens

The actual chemical process of photosynthesis is very complicated, and in fact is not completely understood even today. The basic formula for the process is this:

6CO₂ + 12H₂O + Light Energy --> C₆H₁₂O₆ + 6O₂ + 6H₂O
This basically says that carbon dioxide is glued to water using sunlight, which produces sugar as an end product.

Photosynthesis takes place in two main steps - the light reactions, and the dark reactions.⁵

The light reaction

This is so named because it requires light energy to proceed. It is here that the energy of the sun is captured. This occurs on and inside the thylakoid membrane.

1. Sunlight hits an integral-membrane protein (or IMP) called photosystem II. This causes a molecule of water to be split up into H⁺ (hydrogen) ions and oxygen. A photosystem is simply a complicated name for a protein (lump of stuff) which is stuck through the membrane and does something when sunlight hits it. In this case it breaks water into its component atoms.
2. The hydrogen ions are released into the thylakoid space,⁶ and the electrons captured by this process are passed along an electron transport chain, very similar to that of mitochondria. As in mitochondria, these elctrons power IMP "pumps" which push even more H⁺ ions into the thylakoid space.
3. The electrons ends up at photosystem I.⁷ Photosystem I receives light, and one of its electrons jumps off, converting NADP⁺ to NADPH. This creates an electron "hole" which the incoming electron from photosystem II fills. NADP⁺ is simply an electron carrier. It grabs an electron (and a hydrogen ion) to become NADPH and then wanders over to where that electron is needed, then the H falls off and it becomes NADP⁺ again.
4. H⁺ ions are quite anti-social. There are a lot of them inside the thylakoid space compared to the outside and, like all chemicals, will try to move to where there is the lowest concentration of them. As well as this, they are all positively charged, so there is even more incentive to escape. However, in order for them to get out, they have to pass through another IMP known as ATP synthase. This is basically a turnstile. In order for the H⁺ ions to get out, they must pass through this protein, and in doing so their energy is used to convert ADP to ATP.⁸

The dark reaction

The dark reaction consists of the Calvin cycle, which occurs in the stroma (the parts which aren't in the thylakoid membrane) of the chloroplast.

This is a very complex series of chemical reactions in which 3 molecules of CO₂ is put in, along with 9 molecules of ATP to power it, and 6 molecules of NADPH to provide hydrogen, and is eventually converted to something called G3P (glyceraldehyde 3-phosphate). This is later converted to other sugars such as glucose (C₆H₁₂O₆).

How it started

Like the mitochondria, chloroplasts belong to a group of organelles called plastids. These are quite unusual in that they contain their own DNA, are capable of synthesising their own proteins (to some extent) and are capable of self-replicating. This has led some scientists to believe that they were initially free living bacteria, which got "captured" by another cell, and started a mutualistic relationship with them. This does seem to have some substance, as plastid DNA is a loop rather than a string (much like bacterial DNA) and the ribosomes⁹ plastids use have a lot more in common with bacterial ribosomes than with the ribosomes used in the rest of the cell.