Some cells have a greater proportion of specific organelles dependent on their function for example muscle cells have a large amount of mitochondria as muscles require large amounts of energy, as do sperm cells in order to swim well.
Structure of the Cell Membrane.
Another important organelle is the cell membrane as it controls how substances move in and out of the cell wall. The fluid mosaic model was first proposed by S. J. Singer and G. L. Nicolson in 1972 to describe the structure of cell membranes (Singer and Nicolson 1972). It states that the structure of the cell membrane consists of two layers of phospholipids with their polar, hydrophilic (water loving) head forming the inner and outer layer of the membrane and the non polar, hydrophobic (water repelling) layer sandwiched in between the hydrophilic heads. This layer is known as the lipid bilayer. This water repellent layer acts as a barrier between the inside and outside of the cell, making it highly impermeable and preventing most molecules from passing freely into or out of the cell. Within the phospholipid bilayer are various types of proteins and cholesterol molecules. The cholesterol helps the cell membrane to maintain its fluidity, and also assists integral proteins to carry out their functions, such as pumping substances in or out of the cell, communicating with other cells in close proximity or responding to hormones from cells that may be further away in the body by way of receptor molecules. An example of this is glycoprotein which is protein with at least one carbohydrate molecule attached to it. Using insulin as an example only cells within the liver and muscles have receptors for insulin, (making the cells specific) which allows the hormone to be released into the entire body without causing damage to other non specific cells. .
There are two main ways in which substances can both enter and leave the cell these are known as passive and active transport.
