Plant Cell Vacuoles
Mature plant cells contain a single large central vacuole and it is the most conspicuous compartment of the cell which occupies nearly 50-70% of the total cell volume. On the contrary, meristematic cells are lacking in such vacuoles. Plant cell vacuoles are distinct and characteristic in having a single unit membrane called tonoplast, which separates the vacuolar content from the rest of the cytoplasmic fluid. The liquid present within the vacuole in called cell sap this contains a host of inorganic and organic compounds.
Cell derivatives of meristems; first undergo expansion, and then differentiation. During this stage many smaller vacuoles arise from cytoplasmic membranes of both RER and SER and fuse with one another to form a large central vacuole.
Tonoplast membranes at their cytoplasmic surfaces are associated with polysomal complexes engaged in protein synthesis. In the course of time the central vacuole gets loaded with wide variety chemical components. The most perplexing situation is that during transformation of mature parenchyma cells into meristematic cells the central vacuole disappears. What happens to the vacuolar components is not known.
Plant vacuoles can be identified from other membranous vesicles by vital staining techniques. The neutral red stain being basic in its properties binds components of cell vacuole. These are highly inducible enzymes. ER is involved in the intracellular transportation of various components like proteins, lipids, carbohydrates and others via golgi. Some of ER membranes, containing specific proteins within their lumen, are pinched off into vesicles which later fuse with each other or get processed via Golgi complex and get integrated with specific cell organelles. Even cellular vacuoles of various kinds and dimensions show metabolic activities like amino acid metabolism, fatty acid oxidation etc.
The loading of variety of components in to cell vacuoles bring about changes in the osmotic potential of the cell sap. As a consequence of this, water may freely enter into protoplast; this has been taken as evidence to argue that tugour changes within the cell sap is mainly responsible for the growth of the negative turgour pressure. But now it is known that the growth of the cell takes place without development of turgour pressure.
Vacuolar expansion and concentration is another dynamic feature, where vacuoles play a predominated role in the development of young buds. The swelling of vacuole forces the cell wall to bulge into a bud. Similarly in the case of stomata, guard cells contain a number of small contracted vacuoles, but at the time of opening, the same vacuoles fuse and enlarge into a large central vacuole in the guard cells, thus they bring about the movement of guard cells. Nonetheless, it is speculated that hormones like Abscisic acid and cytokinin play important roles in closing and opening of stomata
Inorganic substances found in the vacuole show variation from cell type to cell type. For example, more than 90% of the total cellular Mg2+ ions are found within the vacuole. On the contrary, the total concentration of calcium ions and copper ions is just 6% and 2% respectively. But the most common ions like K+ ions are equally distributed between cytoplasm and vacuoles. In certain cases more than 40% of the total phosphates are found in vacuoles and most of it is in the form of polyphosphates called
It is really interesting to observe the presence of a wide variety of hydrolysing enzymes within the vacuoles. The common enzymes found are carboxypeptidase, RNase, DNase, phosphotases, b-glycosidase, alfa and beta–amylase etc. The concentration and composition of such enzymes vary from cell to cell and species to species, strangely leaf cells of Solanaceae members like tomato, potato, contain significant amount of protease inhibitors within their vacuoles. How such variety of enzymes and other components are retained and released is a mystery.
Other organic compounds:
Plant cell vacuoles also contain a good number of organic carboxylic acids, amino acids, amides, mucilage, anthocyanins, flavones, gums, alkaloids, anthocyanin and other pigments and even tannins. In certain plants like citrus, the vacuoles are filled with highly acidic citrate compounds whose pH is about 2.5 and curiously enough such acidic pH is prevented from inactivating cytosolic components by tonoplast membranes and provides a distinct compartmentalization. In some cases more than 25 per of the total amino acid pool is found in vacuoles.
Aleurone vacuoles: Certain vacuoles in seeds or grains of various pulses and cereals store a variety of proteins; such granular components are referred as aleurone grains. Inulin, legume, vanillin, glycerin, etc. are some of the common proteins found as storage products in Aleurone vesicles. Such vesicles are derived from RER-SER membrane transitions. Most of the stored or aggregated nonliving structures are called ergastic substances.
Spherosomes and starch vacuoles: Similar to Aleurone vesicles, lipids and starch are also stored in special vacuoles called spherosomes and amylosomes. Spherosomes are derived from RER SER transitional endomembranes. Such membranous vesicles are endowed with enzymatic system from synthesis interconversion. On the other hand, starch is synthesized in chloroplasts but transported to be stored in amyloplasts. Such starch containing membranous organs are found in large numbers in storage tissues like root tubers, stem tubers, cotyledons and endosperms of seeds and grains. The role of pyrenoids found in lower algal cells and their role in storing starch is very interesting.
Vacuoles are highly dynamic. They store many inorganic and organic compounds including a host of enzymes. At the same time they are engaged in protein synthesis at the cytosolic surfaces of tonoplast where they of found as polysomal complex. Some of the ions are transported across tonoplast membranes against concentration gradient. In crassulacian plants carboxylic acids are released into cytoplasm at nights. The diurnal behavior of vacuoles is a unique feature for succulents. The synthesis and aggregation of various substances like calcium oxalate crystals, raphides and other ergastic is very common for plant central vacuoles.
Central vacuole with peripheral cell organelles; http://biology.unm.edu/
Isolated central vacuole
Tonoplast with peripheral protoplasm