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The sarcoplasmic reticulum is a unique intracellular membrane structure that is found in the skeletal muscle cells. Its function is vital to the contraction and relaxation of muscles. The sarcoplasmic reticulum is made up of stacks of convoluting membrane that forms an enclosed environment in the muscle cells. The main function of the sarcoplasmic reticulum is to act as storage for calcium ions in the enclosed environment that it forms. Calcium pumps in the membrane of the sarcoplasmic reticulum facilitate the influx and outflow of the calcium ions. This regulation of calcium ions is vital because calcium ions take part in the process of moving the actin and myosin filaments in the muscle cells. This essay plans to explain and elaborate about the structure and function of the sacroplasmic reticulum, in particular looking at the semi permeable properties of its membrane, as well as the calcium ion protein pumps in the membrane.

The sarcoplasmic reticulum in the muscle cells forms a large network of lateral cisternae that surround the myofibrils. Traverse tubules made up by invaginations of the plasma membrane are often found beside the sarcoplasmic reticulum. The semi permeable nature of the sarcoplasmic reticulum means that it is a membrane that allows only certain molecules and ions to pass through. Like the plasma membrane, the sarcoplasmic membrane is also made up of a fluid phospholipid bilayer with proteins embedded all over the layer. The common features of the lipid molecules are that they are amphiphatic, meaning that they have a hydrophobic non polar end and a hydrophilic polar end. Due to these properties, the phospholipids molecules aggregate together in aqueous solution to from a continuous, fluid lipid bilayer (1). The hydrophilic phosphate heads are able to form favourable hydrogen bonds or electrostatic interactions when exposed to water whilst the hydrophobic fatty acid tails of each molecules form hydrophobic interactions with each other, away from the water. The molecules cluster together so as to minimize the free energy and thus such arrangements form spontaneously in aqueous environments such as the human body because they are energetically favourable (1).

The lipid bilayer membrane is semi permeable, and forms the enclosed environment of the sarcoplasmic reticulum. unscrambledthoughts.wordpress.com Being a continuous, enclosed layer, large molecules such as proteins, glucose, urea, are unable to pass through the membrane freely by diffusion. The region of high hydrophobicity in the membrane also prevents charged molecules or ions to pass through the membrane freely.

However, the sarcoplasmic reticulum allows the movement of calcium ions to pass through. How does this happen? Although the membrane is mainly impermeable, it consists of transport proteins that are found throughout the whole membrane. They allow the movement of essential molecules such as ions to pass through the membrane. A transport protein found on the sacroplasmic reticulum is the ATP dependent calcium pump (6). With the structure and properties of the sacroplasmic reticulum membrane already written, now the attention turns to the transport protein, in particularly to the structure, how it interacts with the surrounding molecules in the membrane, and how it regulates the transport of calcium ions.