Kylie+R.

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 * cell information**

A cell is the smallest living thing (unless you count [|viruses]), and all bigger living things are made of cells. Both [|plants] and [|animals] are made of cells. We don't know yet whether there are cells on any other [|planet] besides Earth. On Earth, the first cells got started about 3.5 billion years ago, about 2.5 billion years after the [|Earth] first formed. These early cells were pretty simple - a [|molecule] with a wrapper around it to keep it safe. Many scientists think that life on Earth first began about [|four billion years ago] when [|amino acids] developed into [|RNA], a larger molecule made out of amino acids that has the ability to reproduce itself, and also to make [|proteins]. RNA was very unstable though - it often broke apart or changed the order of its atoms. After a while, some RNA evolved into [|DNA], which was a more stable, though more complicated, version of the same idea.

One cell of a plant (magnified 22,500 times) Thanks to Ohio State University A cell is the smallest living thing

photosynthesis

Chloroplasts (seen through a microscope) Photosynthesis is the way some [|cells] on [|Earth] pull in [|light] from the [|Sun] and use it to make energy to repair and reproduce themselves. The first cells could not photosynthesize (foe-toe-SIN-the-size). They got food by breaking apart [|hydrocarbon] molecules. But a little more than [|three billion years ago], some of the prokaryote cells evolved to be able to pull [|electrons] out of the [|sunlight] that fell on them from [|space]. This turned out to be a very efficient method of getting food for the cell, and so more of the cells that lived this way survived. Pretty soon most of the cells on Earth got their food through photosynthesis. When sunlight reaches the [|Earth], these cells mix the sunlight with molecules of [|carbon dioxide] and [|water]. The cells store the energy from the sunlight in bigger [|molecules] made of [|carbon] and [|oxygen] atoms (from the carbon dioxide) and [|hydrogen] atoms (from the water). The photosynthesis process ends up with some [|oxygen] left over, which the cell doesn't need and shoots back out of the cell. The early photosynthesizing cells took in so much [|carbon dioxide] and shot out so much oxygen that by [|2.2 billion years ago], Earth's [|atmosphere] had a lot of oxygen in it - instead of 1% oxygen, Earth's atmosphere went up to about 20% oxygen, as it is now (20 out of every 100 [|atoms] in the air are oxygen). People call this the [|oxygen revolution]. At first, only the blue-green [|prokaryotes] could photosynthesize. But when other cells ate the blue-green prokaryotes, they found out that it was more useful to let the blue-green prokaryotes live inside them and keep making energy from sunlight, than it was to destroy them. Gradually more and more cells began to have lots of blue-green prokaryotes living inside them, and by about [|two billion years ago] some of these blue-green prokaryotes lost the ability to live on their own and evolved into [|chloroplasts] (KLOR-oh-plasts) that could only live inside other cells (This is very much like what happened to [|mitochondria] about the same time.).