where does calcium ion storage occur in a cell
endoplasmic reticulum mitochondria
title: Calcium in biology text: in different ways. However, in certain circumstances, its action may be more general. Ca ions are one of the most widespread second messengers used in signal transduction. They make their entrance into the cytoplasm either from outside the cell through the cell membrane via calcium channels (such as calcium-binding proteins or voltage-gated calcium channels), or from some internal calcium storages such as the endoplasmic reticulum and mitochondria. Levels of intracellular calcium are regulated by transport proteins that remove it from the cell. For example, the sodium-calcium exchanger uses energy from the electrochemical gradient of sodium by coupling the influx of title: Calcium in biology text: physiological significance in humans is dubious. Calcium storages are intracellular organelles, that constantly accumulate Ca ions and release them during certain cellular events. Intracellular Ca storages include mitochondria and the endoplasmic reticulum. Characteristic concentrations of calcium in model organisms are: in "E. coli" 3mM (bound), 100nM (free), in budding yeast 2mM (bound), in mammalian cell 10-100nM (free) and in blood plasma 2mM. In vertebrates, calcium ions, like many other ions, are of such vital importance to many physiological processes that its concentration is maintained within specific limits to ensure adequate homeostasis. This is evidenced by human plasma calcium, which is title: Calcium in biology text: Calcium in biology Calcium ions (Ca) play a vital role in the physiology and biochemistry of organisms and the cell. They play an important role in signal transduction pathways, where they act as a second messenger, in neurotransmitter release from neurons, in contraction of all muscle cell types, and in fertilization. Many enzymes require calcium ions as a cofactor, those of the blood-clotting cascade being notable examples. Extracellular calcium is also important for maintaining the potential difference across excitable cell membranes, as well as proper bone formation. Calcium levels in mammals are tightly regulated, with bone acting as the major title: Sarcoplasmic reticulum text: Sarcoplasmic reticulum The sarcoplasmic reticulum (SR) is a membrane-bound structure found within muscle cells that is similar to the endoplasmic reticulum in other cells. The main function of the SR is to store calcium ions (Ca). Calcium ion levels are kept relatively constant, with the concentration of calcium ions within a cell being 100,000 times smaller than the concentration of calcium ions outside the cell. This means that small increases in calcium ions within the cell are easily detected and can bring about important cellular changes (the calcium is said to be a second messenger; see calcium in biology for title: Calcium in biology text: contents. This is particularly important in developing fruits. Without calcium, the cell walls are weak and unable to hold the contents of the fruit. Some plants accumulate Ca in their tissues, thus making them more firm. Calcium is stored as Ca-oxalate crystals in plastids. Ca ions are usually kept at nanomolar levels in the cytosol of plant cells, and act in a number of signal transduction pathways as second messengers. The U.S. Institute of Medicine (IOM) established Recommended Dietary Allowances (RDAs) for calcium in 1997 and updated those values in 2011. See table. The European Food Safety Authority (EFSA) uses