Membrane-Bound Organelles

Since thousands of chemical reactions occur at any given time within a cell, it is imperative for the cell to segregate the chemical reactions so as to avoid "chemical chaos" from taking place inside the cell.

The cell, therefore, has evolved 2 strategies to overcome this problem:

1. Assembling of enzymes that serially catalyze particular reactions into large protein complexes
2. Enclosing different enzymes and the metabolic processes they are responsible for in distinct compartments or membrane-bound organelles

The protein composition within an organelle is meticuously controlled and maintained by the cell through means of a highly selective process called protein sorting (next post).


Membrane-bound Organelles

Almost all eukaryotic cells contain a set of membrane-bound organelles namely:
Cytosol - protein synthesis and metabolic pathways

• Nucleus - DNA, RNA synthesis and main genome
• Endoplasmic Reticulum - lipid synthesis, also synthesis of proteins
• Golgi apparatus - sorting and packaging of proteins for delivery
• Lysosomes - degradation
• Peroxisomes - oxidation of molecules which are toxic to the cell
• Endosomes - sorting and delivery of proteins and other material
• Mitochondria - main site for oxidative phosphorylation
• Chloroplasts - photosynthesis

It is worthwhile to note that mitochondria and chloroplasts are each enclosed by double membranes (called innner and outer membranes each with a different protein composition). In fact, these membranes are the sites where oxidative phosphorylation and photosynthesis take place! This explains a lot about their coming into existence.


Origin of Mitochondria and Chloroplasts

Evolutionary origin of mitochondria, Albert et al.

Apart from the presence a double membrane, another distinguishing feature of mitochondria and chloroplasts is that they posses their own DNA and can also synthesize some of their own proteins.

This led researchers to believe that their origin is result of them being "engulfed" by ancient eukaryotic cells and the two shared a symbiotic relationship.

This also explains the resemblance of their genomes and some proteins to those of bacteria.

On the other hand, the membranes of endoplasmic reticulum, Golgi apparatus as well as nuclear membranes seem to have been a result of the invagination of the plasma membrane making all of them a part of the endomembrane system.