Two important concepts will be discussed in this chapter. One is that of the membrane-enclosed organelles which are thought to have evolved in at least two ways:

  1. The nuclear membrane and the membrane of the endoplasmic reticulum, Golgi apparatus, endosomes, and lysosomes are believed to have originated by invagination (inner folding) of the plasma membrane. These membranes and the organelles they enclose (except for the nucleus) are all part of the endomembrane system.

    Simplified illustration of invagination.

  2. Mitochondria and chloroplasts are unique because they have their own DNA that loops in a circle like that of bacteria. And they manufacture many of their own proteins and reproduce by binary fission so they may have evolved in the sense that they were prokaryotes (primordial single-cells) that became so dependent on their host that they are now a part of the cell.

 

The other important concept is the fact that a typical mammalian cell contains about 10,000 different kinds of proteins. For a cell to work properly, each of these many proteins must be sent to the correct place. The process of directing each newly made polypeptide to particular destination is known as protein targeting or protein sorting.

The proper organelle destination for a protein is determined by signals in the amino acid sequence that will ultimately bind to a receptor protein involved in importing that protein into the organelle.

In some cases, there is a single sequence at the N-terminal which are called signal sequences or signal peptide. And in other cases there are one or more internal sequences that are not in a continuous series called signal patch. Because this sequence is composed of two separate elements, it is referred to as bipartite (being or having two-bi parts-partite).

Different types of signal sequences.

Signal patch are formed because when the protein is made, it is folded and since a fold can bring different regions together (think of smudged paper that can bring the corners of the paper close to the center), the juxtaposition of amino acids from far regions that are physically separated before the protein folds will contain the signal in series (now imagine colouring the smudged paper. After unfolding it, the paper will appear to be coloured in patches). Btw, protein can be synthesized inside the cell or come from outside, and in those two cases it will still have a sequence.

All for now.