Vacuoles and Microbodies

Vacuoles

Vacuoles are membrane-bound, fluid-filled sacs found prominently in plant cells; in animal cells, when present, they tend to be fewer in number and smaller in size. A typical plant cell has two or three permanent vacuoles, though in some large plant cells a single large vacuole, called the central vacuole, occupies the central part of the cell and can take up as much as 90% of its total volume.

Vacuoles are bound by a semipermeable membrane called the tonoplast, which keeps the composition of the vacuolar fluid, or cell sap, different from that of the surrounding cytosol -- the exact composition of cell sap varies between cell types. The cell sap of a central vacuole acts as a storehouse for various ions and, as a result, is hypertonic to the cytosol. Small vacuoles in the seeds of certain plants store organic material such as proteins, while other vacuoles store excretory products together with compounds that are harmful or unpalatable to herbivores, giving the plant a degree of protection; the attractive colours of many petals also come from pigments stored in vacuoles. Vacuoles can also form through phagocytosis, as food vacuoles that take in food or foreign particles, and in freshwater unicellular organisms such as Paramoecium, contractile vacuoles carry out excretion and osmoregulation. More generally, vacuoles help a plant cell maintain its turgidity.

Microbodies …