Prokaryotic Ribosomes and Inclusion Bodies
Imagine a factory floor where proteins are the products. The ribosome is the assembly machine that reads the blueprint (mRNA) and links amino acids together. In a prokaryotic cell, there is no separate "packaging room" (no membrane-bound organelles), so the machines and the raw material stores sit directly in the cytoplasm.
The Ribosome: The Protein Factory
A prokaryotic ribosome is a massive complex of RNA and protein, built from two subunits that clamp onto mRNA. The 70S designation comes from its sedimentation rate in a centrifuge — the "S" stands for Svedberg units, a measure of size and shape, not a simple sum of masses. The large subunit is 50S, the small is 30S; together they make 70S.
Prokaryotic ribosomes are 70S (50S + 30S subunits). Eukaryotic ribosomes are 80S (60S + 40S). This difference is why some antibiotics (like streptomycin) can target bacterial ribosomes without harming human cells.
Now, a single ribosome can translate one mRNA, but that is slow. In a prokaryotic cell, as soon as the 5' end of an mRNA emerges from transcription, ribosomes hop on. Multiple ribosomes translate the same mRNA simultaneously, each at a different point along the strand. This structure is called a polysome (polyribosome). It looks like a string of beads under an electron microscope — each bead is a ribosome, and the string is the mRNA.
Polysomes massively boost protein output. One mRNA can be read by 10–20 ribosomes at once, producing many copies of the same protein in the time it takes one ribosome to finish.
Inclusion Bodies: The Storage Rooms
Prokaryotes have no membrane-bound organelles like lysosomes or vacuoles. Instead, they store reserve materials directly in the cytoplasm as inclusion bodies. These are simply aggregates of molecules, often visible under a light microscope as granules. They are not surrounded by a membrane — they are just dense clumps of stuff.
The key point: inclusion bodies are non-membrane bound and serve as stockpiles for when nutrients run low.
Three common types you need to know:
| Inclusion Body | What It Stores | Why It Matters |
|---|
| Glycogen granules | Polymer of glucose | Energy reserve — broken down when carbon is scarce |
| Phosphate granules (volutin) | Inorganic polyphosphate | Phosphate and energy reserve — used for ATP synthesis later |
| Gas vacuoles | Air-filled protein shells | Buoyancy — lets cyanobacteria float to optimal light depth |