Organelles and compartmentalization

Biology · B Form and function. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.

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The idea

An organelle is a discrete part of a cell that is adapted to carry out a particular function. Some have a double membrane (nucleus, mitochondrion, chloroplast), some a single membrane (lysosome, Golgi apparatus, vesicles) and ribosomes have none.

Membranes divide a eukaryotic cell into compartments. This is compartmentalization. Each compartment can have its own conditions, enzymes and substrates can be concentrated together, and harmful substances can be kept away from the rest of the cell.

The nuclear membrane separates transcription from translation, so mRNA can be modified before it meets a ribosome. In prokaryotes there is no nucleus, so translation begins while the mRNA is still being made.

Rules to remember

Organelles include the nucleus, mitochondria, chloroplasts, ribosomes, lysosomes, Golgi apparatus, endoplasmic reticulum, vesicles and the plasma membrane.
Not organelles: the cell wall, the cytoskeleton and the cytoplasm.
Lysosomes keep digestive (hydrolytic) enzymes inside a membrane so they do not digest the cell itself.
A phagocytic vacuole holds an engulfed particle; lysosomes fuse with it and their enzymes digest the contents safely.
Enzymes and substrates held together in a compartment are more concentrated, so pathways run faster.
Cell fractionation: break open the cells, then spin in an ultracentrifuge to separate the organelles by size and density.
Larger, denser organelles settle first: nuclei, then mitochondria, then ribosomes.

Common mistake

Students list the cell wall, cytoskeleton or cytoplasm as organelles. They are not discrete subunits with a specific function, so the syllabus does not count them.

Worked example

Liver cells are broken open and spun at increasing speeds. Pellet 1 forms at low speed, pellet 2 at medium speed and pellet 3 only at very high speed. Which pellet would use the most oxygen?

Large, dense organelles settle at low speed, so pellet 1 is mostly nuclei.
Mitochondria are smaller and settle next, in pellet 2.
Ribosomes are the smallest and need the highest speed, so they are in pellet 3.
Aerobic respiration, which uses oxygen, takes place in mitochondria.

Answer: Pellet 2, because it contains the mitochondria.

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