The largest organelle in plant cells containing the cell sap is the vacuole. Plant cells usually have a single vacuole that can take up 80 percent or more of the volume of a cell.
A membrane called the tonoplast surrounds the vacuole. As the plant cell grows, smaller vacuoles fuse together to form one large central vacuole.
One of the main functions of the vacuole is storage. The plant cell vacuole stores cell sap, which is a general term used to describe the various possible contents of a vacuole. Some plant cell sap consists of plant pigments, while others contain proteins to nourish a seed, salts and minerals, or chemicals that taste bad to insects. Another function of the vacuole is to lend support and structure to the plant cell.