Green alga. Interference contrast light micrograph of the alga Cosmarium sp., one of a number of unicellular organisms belonging to the group of desmids. The cell of a desmid is in two halves, separated by a narrow waist. At cell division the cell splits in two at the waist, with each half generating a replica of itself to restore the original shape. The colour of the cell is due to the presence of green chlorophyll pigments contained within the chloroplasts. These are employed in a chemical process called photosynthesis to convert sunlight, water and carbon dioxide into starch.