Ribosomes, the cytoskeleton, cilia and centrioles

Ribosomes

Ribosomes are tiny granular structures. George Palade first saw them, as dense particles, under the electron microscope in 1953. They are made of ribonucleic acid (RNA) and proteins. They are not surrounded by any membrane.

Ribosomes are where proteins are made. They lie free in the cytoplasm and on the rough endoplasmic reticulum. Mitochondria and chloroplasts have their own as well.

• Eukaryotic cells have 80S ribosomes in the cytoplasm. Prokaryotic cells have 70S ribosomes.
• S is the Svedberg unit, a unit of the sedimentation coefficient. It shows how fast a particle settles when spun very fast in a centrifuge. So it tells us about the size and the density, though only indirectly.
• Each ribosome has two subunits, a larger and a smaller one.

The S values do not add up like masses: 50 + 30 is 80, not 70. How fast a particle settles depends on its shape as well as its mass. When two subunits join, the shape changes too.

The cytoskeleton

A network of fine protein fibres runs all through the cytoplasm. It is the cytoskeleton. It is made of three kinds of fibres:

• microtubules, hollow tubes of protein;
• microfilaments, the thinnest;
• intermediate filaments, in between.

The cytoskeleton gives the cell mechanical support, helps it move, and keeps its shape.

Cilia and flagella

Cilia (singular, cilium) and flagella (singular, flagellum) are hair-like outgrowths of the cell membrane.

• Cilia are small. They work like oars, moving either the cell or the fluid around it.
• Flagella are longer. They move the cell.
• The flagellum of a prokaryote is built quite differently from these.

Under the electron microscope, a cilium or a flagellum is covered by the plasma membrane. Its core is the axoneme. It has many microtubules running along its length.

• Nine doublets of microtubules sit round the edge.
• Two single microtubules sit in the centre.
• This is the 9 + 2 array.

The two central tubules are joined by bridges and enclosed by a central sheath. A radial spoke joins the sheath to one tubule of each doublet, so there are nine radial spokes. Linkers join the doublets to each other. Both cilia and flagella grow from a centriole-like structure, the basal body.

Centrosome and centrioles

The centrosome usually contains two cylindrical centrioles. They are surrounded by a shapeless pericentriolar material, and they lie at right angles to each other.

Each centriole is built like a cartwheel.

• It has nine evenly spaced fibrils of the protein tubulin round the edge.
• Each fibril is a triplet of tubules, and neighbouring triplets are linked.
• The central part, the hub, is made of protein too. Radial spokes of protein join it to the triplets.

Centrioles form the basal bodies of cilia and flagella. In animal cells, they also give rise to the spindle fibres, which form the spindle apparatus during cell division.

A cilium and a centriole in cross-section

Switch between the two and count the tubules.

Try it in Python

The program lists four places where ribosomes are found, with their kind and their subunits.

Practise with new numbers

Common mistakes

• "Ribosomes are wrapped in a membrane." They have no membrane at all.
• "60S + 40S makes 100S." S values do not add. The whole ribosome is 80S.
• "Mitochondria have 80S ribosomes because they are in a eukaryotic cell." Mitochondria and chloroplasts have 70S ribosomes.
• "A centriole has a 9 + 2 pattern." That is the axoneme of a cilium or a flagellum. A centriole has nine triplets and no central pair.
• "Cilia and flagella are the same length." Cilia are short and many. Flagella are longer and fewer.

Remember these

• Ribosomes: RNA and protein, no membrane, first seen by George Palade.
• 80S = 60S + 40S in the eukaryotic cytoplasm; 70S = 50S + 30S in prokaryotes, mitochondria and chloroplasts.
• S is the Svedberg unit of the sedimentation coefficient.
• The cytoskeleton: microtubules, microfilaments, intermediate filaments. It supports, moves and shapes the cell.
• Axoneme: nine doublets and a central pair, the 9 + 2 array, with nine radial spokes. Basal bodies at the base.
• Centriole: a cartwheel of nine tubulin triplets round a hub. Two centrioles at right angles form a centrosome.
• Centrioles form basal bodies and, in animal cells, the spindle fibres.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

1. What are ribosomes made of, and what do they do?

2. What does the S in 80S stand for, and what does it measure?

3. Name the three kinds of fibre in the cytoskeleton and give two of its jobs.

4. Describe the 9 + 2 array of an axoneme.

5. Give two jobs of centrioles.