Algae

Some eukaryotes swallowed sunlight-using-bacteria. Instead of eating them they kept them alive. This was an example of endosymbiosis, and produced a organelle called a chloroplast. These were the simplest plants, called algae ("al-jee"). One of these is called an alga ("al-ga"). They first appeared around 1.5 billion years ago.

Algae could now use sunlight to make food and get energy. At night they still burnt food with their mitochondria, like a protozoan.

Plant cell image courtesy of Mariana Ruiz

Biologists classify algae as belonging to the Protista.

Algae on Soccearth

To get an idea of the size of an alga, imagine a soccer ball blown up to the size of the Earth (Soccearth). The smallest alga would be the size of a factory 1 km across and the largest, for example a seaweed, would be far larger than Soccearth itself.

Plant Cell Walls

Algae and more advanced plant cells are surrounded by a rigid wall made of chains of glucose molecules (making cellulose) and chains of other sugar molecules (making polysaccharides), together with protein containing sugars (glycoproteins).

The wall stops the cell expanding too much and breaking the delicate cell membrane. Note that animal cells, unlike those of bacteria and plants, do not have a cell wall.