Final answer:
Phytoplankton float due to the presence of lipids in their cells, which are part of the phospholipid bilayer in the plasma membrane, helping to maintain buoyancy.
Step-by-step explanation:
Phytoplankton can float because their cells contain lipids, which are less dense than water. Lipids, often in the form of phospholipids, are important components of the cell's plasma membrane. These lipids can help some species of phytoplankton to maintain buoyancy in the water column. While chlorophyll is essential for photosynthesis and responsible for giving them their green color, it is not directly involved in their flotation. Phospholipids form a bilayer that is part of the plasma membrane, providing the necessary structure to contain the cellular components, yet still allowing for buoyancy within the aquatic environment.