The cytoskeleton

The cytoskeleton is a network of protein fibres running through the cytoplasm of eukaryotic cells. It has three kinds of fibre: microfilaments made of actin, intermediate filaments, and microtubules made of tubulin. Together they give the cell its shape and mechanical strength, act as tracks that move organelles and vesicles around the cell, and let cells move and divide.

What is the cytoskeleton?

A cell is not a bag of loose parts floating in liquid. The cytoskeleton is a framework of fibres that holds organelles in place and keeps the cell in shape, much like the poles of a tent, but it can also be taken apart and rebuilt in seconds as the cell changes shape or moves.

OCR A Level Biology A lists the importance of the cytoskeleton among the parts of cell structure, including providing mechanical strength to cells, aiding transport within cells and enabling cell movement (2.1.1 j). For the other parts of a cell, see animal and plant cells.

The three types of cytoskeleton fibre

The sizes are from OpenStax Biology. Every fibre is far too thin to see with a light microscope; they show up in electron micrographs.

Microfilaments

Microfilaments are the thinnest fibres: two intertwined strands of the protein actin. They can be taken apart and rebuilt quickly, which lets a cell change shape and crawl along. In muscle cells, actin filaments slide past myosin filaments to make the muscle contract, and in animal cells a ring of microfilaments pinches the cell in two at the end of cell division (cytokinesis).

Intermediate filaments

Intermediate filaments are rope-like fibres that resist pulling forces. They are purely structural, take no part in movement, and help hold the nucleus and other organelles in position. Keratin, the protein in skin, hair and nails, forms intermediate filaments.

Microtubules

Microtubules are the widest fibres: hollow tubes built from the proteins α-tubulin and β-tubulin. They resist squashing, and they act as tracks. Motor proteins "walk" along them carrying vesicles and organelles, using energy from ATP. Microtubules also make the spindle fibres in cell division, the centrioles, and the core of cilia and flagella.

Functions of the cytoskeleton

1. Mechanical strength and shape

The fibres support the cell membrane and resist forces that would distort the cell. That is why cells keep a definite shape, such as the biconcave disc of a red blood cell, and why skin cells, packed with intermediate filaments, can be stretched without tearing.

2. Transport within the cell

Vesicles move along microtubules. For example, vesicles carrying newly made proteins travel from the Golgi apparatus to the cell surface membrane for secretion. Motor proteins use ATP for each step, so this transport needs energy from respiration.

3. Cell movement

• Cilia and flagella: both have a core of nine pairs of microtubules arranged in a ring around two central microtubules (the "9 + 2" arrangement). Flagella are long and few, like the tail of a sperm cell. Cilia are short and many, like those that sweep mucus out of the airways.
• Crawling cells: white blood cells change shape by building and breaking down microfilaments as they move towards pathogens.
• Muscle contraction: actin and myosin filaments slide past each other.

4. Cell division

During mitosis, microtubules form the spindle. Spindle fibres attach to the chromosomes and pull the chromatids to opposite poles. In animal cells the spindle is organised by the centrioles. See mitosis for the stages.

Check your understanding.

Exam questions with answers

A drug stops microtubules from forming. Suggest why it stops cells dividing.

Without microtubules, no spindle can form, so the chromatids cannot be separated and pulled to opposite poles. Mitosis stops at metaphase. Drugs like this, such as colchicine, are used to study cell division.

Explain why cells that secrete a lot of protein need many mitochondria.

Making proteins on ribosomes uses ATP, and the vesicles that carry the finished proteins to the cell surface are moved along microtubules by motor proteins, which also use ATP. Mitochondria release the energy for making that ATP in aerobic respiration.

Common mistakes

• Mixing up the fibres. Microfilaments are actin and thinnest; microtubules are tubulin, hollow and widest.
• Thinking the cytoskeleton is fixed. Microfilaments and microtubules are constantly built and taken apart.
• Saying cilia and flagella are made of actin. Their core is microtubules in a 9 + 2 arrangement.
• Forgetting ATP. Movement along the cytoskeleton by motor proteins needs energy.
• Calling the cytoskeleton an organelle with a membrane. It is a network of protein fibres with no membrane.

Practise with a short timed paper

Cell structure is in the first year of A-level Biology. The daily A-level Biology: Year 1 paper is a short timed paper of 8 questions, with a new one every day. Explore the organelles of a cell in the Cell Simulator or the Cell Biology course.

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