B2.2 Organelles and compartmentalizationIB Biology SL: Revision notes

An organelle is a discrete subunit of a cell that is adapted to perform a specific function. Dividing the work of a cell between organelles is a form of division of labour.

Counted as organelles: nucleus, mitochondria, chloroplasts, vesicles, vacuoles, lysosomes, ribosomes and the plasma membrane. Ribosomes count even though they have no membrane.

Not organelles: the cell wall (outside the plasma membrane, a secreted extracellular structure), the cytoskeleton (a network spread through the whole cell, not discrete) and the cytoplasm (the medium in which organelles sit).

Microscopes showed the structure of organelles, but their functions could only be studied once organelles could be separated. Ultracentrifuges made cell fractionation possible.

1. Tissue is homogenised in a cold, isotonic, buffered solution (cold slows enzymes that would damage organelles; isotonic prevents osmotic bursting; buffer keeps pH stable).
2. Differential centrifugation: spinning at increasing speeds sediments the largest, densest organelles first (nuclei), then mitochondria and lysosomes, then small vesicles and ribosomes.

Each fraction can then be tested for its enzymes and reactions. This illustrates that progress in science often follows the development of new techniques.

In eukaryotes the nuclear membrane separates transcription (in the nucleus) from translation (on ribosomes in the cytoplasm). The advantage is that post-transcriptional modification of mRNA, such as removal of introns, can be completed before the mRNA meets ribosomes, so unmodified pre-mRNA is never translated.

In prokaryotes there is no nucleus, so mRNA may immediately meet ribosomes, and translation can begin while transcription is still in progress.

Membrane-bound compartments in the cytoplasm give two main advantages:

• Concentration of metabolites and enzymes in a small volume, so reactions proceed faster.
• Separation of incompatible biochemical processes, and the ability to keep different conditions (such as pH) in different compartments.

Lysosomes contain hydrolytic enzymes that work best at a low pH (about 5), while the cytoplasm is about pH 7. A cell such as a macrophage engulfs a pathogen into a phagocytic vacuole; lysosomes then fuse with the vacuole, releasing enzymes that digest the contents.

Keeping these enzymes enclosed means they are concentrated with their substrate and work at their optimum pH, and the cell's own molecules are protected from digestion.