Vacuoles and Storage Organelles

Abstract

The vacuole, a membrane-bound water-filled organelle which contains inorganic ions and organic compounds, is the most prominent organelle in plant cells. The vacuolar membrane, called the tonoplast, contains transport proteins that maintain cytoplasm homeostasis, including proton pumps which stabilize cytoplasmic pH, aquaporins which control water permeability, and ion transporters. The size and number of vacuoles varies greatly depending on cell type and stage of plant development. Mature plant cells generally contain one central or several very large vacuoles. By storing various materials, the central vacuole keeps its water potential as low as that of the cytoplasm, and maintains turgor pressure against the cell wall, which is essential in supporting plants in an upright position. Plant vacuoles also play roles in molecular degradation and storage to maintain a balance between biogenesis and degradation of substances and cell structures. In some specialized cells, vacuoles serve as mediators. In protein-seeds, vacuoles store proteins as protein bodies to be used in germination. In oil-seeds, lipids stored in oil bodies are transported in peroxisomes (glyoxysomes) and metabolized to produce energy for germination.

Biofuels include ethanol and methane generated by fermentation of plant biomass and neutral lipids produced by plants and microalgae. Many photosynthetic algal species have been screened for high lipid content and examined for biofuel production capacity. Such organisms could potentially produce 8–24 times more biofuel per unit area than the best land plants.

The eight figures selected for this chapter illustrate various aspects of vacuoles and oil bodies in algae and plants.