In preparation for the biology GRE subject test I need to re-learn (or learn) general biology. My upper division work has been focused on evolution and computational techniques, so I am very rusty on most other biological topics.
Today I went over organelle structure and function in eukaryotes (organisms that have membrane enclosed structures within their cells such as animals, plants, protists, and fungi). As well as containing the chromosomes, the nucleus contains the nucleolus which is the production site for ribosomal subunits. These subunits are then shipped out of the nucleolus, then the nucleus and into the cytoplasm. The ribosomal subunits are destined to either float freely in the cytoplasm where they eventually link up over messenger RNA (mRNA) to make proteins destined for the cytoplasm, or they attach to the walls of the endoplasmic reticulum (ER) where they produce proteins that are destined for a membrane or export out of the cell. It is also interesting to note that the mitochondria and chloroplast produce their own set of robosomes that more closely resembles procharyotic(bacteria, archea, etc) ribosomes.
The destination of proteins is determined by their amino acid sequence, structure, and sometimes post-translational modifications. Proteins that are to be secreted out of the cell have a hydrophobic signal sequence called the signal peptide.
Lysosomes are interesting. They have enzymes inside of them which digest proteins, carbohydrates and nucleic acids. These enzymes function well in an acidic environment where the pH is about 5 (a cell's pH is closer to 7 meaning it is neutral). This fact keeps the overall cell safe because the enzymes aren't able to digest the cell in its higher pH environment. However inside the lysosome, the pH is kept at 5 so that the enzymes are able to do their work. That is a very interesting adaptation.
Peroxisomes are similar and break down fats and harmful chemicals like alcohal. These organelles do so through the production and degredation of peroxide, which if allowed to come in contact with DNA would be very dangerous. Thus the reactions are kept within the peroxisome.
The mitochondria and chloroplast are cool. They have their own DNA, and produce many of their own proteins. They are basically like a small unicellular organism that lives within the cell and produces the energy needed for the cellular function. Biologists theorize that these organelles were once seperate organisms that entered into a symbiotic relationship where they exchanged energy production for protection and a stable environment.