Chlorella is a genus of single-celled green algae, belonging to the phylum Chlorophyta. It is spherical in shape, about 2 to 10 Âµm in diameter, and is without flagella. Chlorella contains the green photosynthetic pigments chlorophyll-a and -b in its chloroplast. It depends on photosynthesis for growth and multiplies rapidly, requiring only carbon dioxide, water, sunlight, and a small amount of minerals. Chlorella has been researched as a potential food because it is high in protein and other essential nutrients. When dried, it is about 45 percent protein, 20 percent fat, 20 percent carbohydrate, and 10 percent various minerals and vitamins. However, because it is a single-celled alga, harvest had posed practical difficulties for its large-scale use as a food source. Methods of mass production are now being used to cultivate it in large artificial circular ponds. It has been eaten in times of famine in areas such as China during the failed Great Leap Forward, often being grown in human urine. The cell walls of Chlorella are made of cellulose and are very strong and so they are normally pulverized to improve digestibility. The name Chlorella is taken from the Greek word chloros meaning green and the Latin diminutive suffix ella meaning small and was named by a Dutch biologist. The German biochemist Otto Heinrich Warburg received the Nobel Prize in Physiology or Medicine in 1931 for his study on photosynthesis in Chlorella. In 1961 Melvin Calvin of the University of California received the Nobel Prize in Chemistry for his research on the pathways of carbon dioxide assimilation in plants using Chlorella. In recent years researchers have made less use of Chlorella as an experimental organism because it lacks a sexual cycle and, therefore, the research advantages of genetics are unavailable.