Chlamydomonas is a unicellular green alga with two anterior flagella and a single cup-shaped chloroplast. Each cell is about 10 micrometers across. This cell is viewed with Nomarski optics, which gives an image that appears three dimensional. The smooth space at the anterior end, at the bottom of this cell, is the contractile vacuole, which you can see contract in this clip. This organelle is involved in water regulation in Chlamydomonas. By expelling a solution hypotonic to that in the cell, the contractile vacuole counters the osmotic movement of water into the cell. The slightly larger circular structure within the chloroplast at the other end of the cell is the pyrenoid, where starch is deposited. The 3D appearance around the pyrenoid is due to the view of the thylakoid membrane stacks in the chloroplast. Credit: Michael Clayton, University of Wisconsin, Madison.