<P> 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 more details ) . Calcium is used to make calcium carbonate ( found in chalk ) and calcium phosphate , two compounds that the body uses to make teeth and bones . This means that too much calcium within the cells can lead to hardening ( calcification ) of certain intracellular structures , including the mitochondria , leading to cell death . Therefore , it is vital that calcium ion levels are controlled tightly , and can be released into the cell when necessary and then remove from the cell . </P>
<P> 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 more details ) . Calcium is used to make calcium carbonate ( found in chalk ) and calcium phosphate , two compounds that the body uses to make teeth and bones . This means that too much calcium within the cells can lead to hardening ( calcification ) of certain intracellular structures , including the mitochondria , leading to cell death . Therefore , it is vital that calcium ion levels are controlled tightly , and can be released into the cell when necessary and then remove from the cell . </P>
where are calcium ions stored in a muscle fiber
where are calcium ions stored in a muscle fiber
The sarcoplasmic reticulum ( SR )
The sarcoplasmic reticulum ( SR )