A vesicle leaving the Golgi can travel several micrometres to the cell surface. Meanwhile the nucleus stays in place and the cell changes shape. These observations make sense once you distinguish three jobs: bearing tension, providing a track and generating force. Different proteins perform each job.

Which fibres do you see?

Actin filamentIntermediate filamentMicrotubule~7 nm · two strands~10 nm · rope-like~25 nm · hollow tube
Figure 1. Three filament architectures. The sketches compare construction; the diameters are approximate.

Actin monomers assemble into polarized filaments. Under the plasma membrane a changing mesh of actin supports the cortex. Local assembly can push the cell edge forward; myosin acting on existing filaments can pull it inward. A finger-like microvillus contains an actin bundle, whereas the core of a cilium contains microtubules.

Intermediate filaments are cable-like assemblies. Keratin in epithelial cells helps resist a pull transmitted from a neighbouring cell through a junction. Lamins support the animal nuclear envelope. These fibres lack the plus/minus polarity used by transport motors. Their protein composition varies among tissues.

Microtubules form when tubulin dimers assemble into longitudinal rows that close around a lumen. Their two ends differ structurally and kinetically. A plus sign identifies polarity, not an electric charge. Growing ends explore the cytoplasm and can switch abruptly to shortening. This allows a cell to rearrange its tracks during division.

Centrosome, centriole or centromere?

One of nine tripletsPericentriolar materialMicrotubules nucleate here
Figure 2. A centriole in cross-section. Each coloured group represents one microtubule triplet. The surrounding matrix belongs to the centrosome.

In many animal cells the centrosome lies near the nucleus. It contains two centrioles, commonly at roughly right angles, embedded in pericentriolar material. The matrix recruits gamma-tubulin complexes that start new microtubules. Draw the pair inside a shaded matrix; drawing an arrow from every fibre directly into a centriole gives the wrong impression.

A centromere is a region of a chromosome. The kinetochore built there attaches to a spindle microtubule. A centrosome helps organize a spindle pole. To label a diagram, first ask whether the pointer ends on chromatin, on the attachment complex, or at the pole.

A journey from the Golgi to the cell edge

− end+ endMembrane vesicleMotor← DyneinMany kinesins →
Figure 3. A polar microtubule acts as a track. Many kinesins move cargo towards the plus end; cytoplasmic dynein generally moves towards the minus end.

In a typical radial array the minus ends cluster near the centrosome and many plus ends extend toward the edge. A vesicle can recruit a motor through adaptor proteins. ATP drives the motor through cycles of attachment and movement. Motors can change, detach or reverse the net direction of a cargo. A trip towards the nucleus is evidence about direction only after the track orientation is known.

Follow the cargo back to the Golgi and vesicle lesson to distinguish cargo sorting from transport.

From one dividing cell to two daughters

Before animal mitosis the centrosome duplicates. The two organizing regions separate as spindle microtubules interact. Kinetochore attachments must connect sister chromatids to opposite poles before faithful separation. Other spindle microtubules overlap or contact the cell cortex. A spindle is therefore more than two fibres attached to one chromosome.

After chromosome segregation, an actomyosin ring constricts the animal cell. In flowering plants a phragmoplast guides vesicles toward the developing cell plate. Most flowering-plant somatic cells organize microtubules without conventional centriole-bearing centrosomes. Compare these steps in mitosis and the cell cycle.

Read an experiment

Suppose a fluorescent cargo travels 12 µm in 6 s before treatment and 3 µm in 6 s afterwards. Its mean speed falls from 2 to 0.5 µm/s, a 75% decrease. Speed alone cannot distinguish a damaged track from a stalled motor. Image labelled tubulin and cargo in the same living cells, retain a solvent control, and compare several cells. Use the same frame interval and temperature. Report these example values as teaching data, not observed measurements.

Frequently asked questions

Does a centriole pull a chromosome? Spindle microtubules, motors and attachment complexes generate and transmit forces. The centriole is part of an organizing structure.

Can I see the nine triplets with a school microscope? A conventional school microscope cannot resolve individual triplets. A labelled focus identifies position, while an electron micrograph can show architecture.

Where does this fit? Review the cell organelle overview before comparing tracks and cargo.

20 explained practice questions

1. Which polymer builds a contractile ring?

  1. Keratin
  2. Tubulin
  3. Cellulose
  4. Actin
Reveal answer and reasoning

Actin. Myosin pulls on actin filaments around the equator during animal cytokinesis.

2. Which protein family forms hollow intracellular tracks?

  1. Tubulin
  2. Collagen
  3. Lamin
  4. Myosin
Reveal answer and reasoning

Tubulin. Alpha- and beta-tubulin form dimers that assemble into microtubule walls.

3. Which filament class commonly resists stretching in epithelia?

  1. Cellulose microfibrils
  2. Intermediate filaments
  3. Actin filaments
  4. Spindle microtubules
Reveal answer and reasoning

Intermediate filaments. Keratin intermediate filaments distribute mechanical stress through connected cells.

4. What is the approximate outside diameter of a microtubule?

  1. 10 µm
  2. 100 µm
  3. 25 nm
  4. 7 nm
Reveal answer and reasoning

25 nm. The outside diameter is about 25 nm. Diameter and filament length are different measurements.

5. What surrounds the centrioles and nucleates many microtubules?

  1. Nuclear lamina
  2. Cell cortex
  3. Golgi lumen
  4. Pericentriolar material
Reveal answer and reasoning

Pericentriolar material. Nucleation complexes are concentrated in the pericentriolar matrix.

6. Which unit has nine microtubule triplets?

  1. Centriole
  2. Actin cable
  3. Typical motile ciliary shaft
  4. Contractile ring
Reveal answer and reasoning

Centriole. A centriole has nine triplets. A typical motile axoneme has outer doublets.

7. Which structure includes centrioles and their surrounding matrix?

  1. Kinetochore
  2. Centrosome
  3. Centromere
  4. Nucleolus
Reveal answer and reasoning

Centrosome. The centrosome is a whole organizing region. Each centriole is one component.

8. What binds spindle microtubules at a chromosome?

  1. Basal body
  2. Myosin head
  3. Kinetochore
  4. Centrosome
Reveal answer and reasoning

Kinetochore. The kinetochore assembles on centromeric chromatin and binds microtubules.

9. Which motor commonly carries cargo towards microtubule minus ends?

  1. Myosin II
  2. Actin
  3. Keratin
  4. Cytoplasmic dynein
Reveal answer and reasoning

Cytoplasmic dynein. Motor direction is defined relative to track polarity. Dynein usually moves towards the minus end.

10. Which motors typically carry cargo towards microtubule plus ends?

  1. Kinesins
  2. Lamins
  3. Myosins
  4. Integrins
Reveal answer and reasoning

Kinesins. Many kinesins are plus-end directed, although some family members differ.

11. Which process alternates growth and rapid shortening of a microtubule?

  1. DNA replication
  2. Dynamic instability
  3. Osmosis
  4. Endocytosis
Reveal answer and reasoning

Dynamic instability. Individual microtubules can switch between growth and shrinkage without the whole array disappearing.

12. Which structure stiffens the inner face of the nuclear envelope in animal cells?

  1. Basal lamina
  2. Golgi matrix
  3. Nuclear lamina
  4. Cell wall
Reveal answer and reasoning

Nuclear lamina. Nuclear lamins form an intermediate-filament network beneath the inner nuclear membrane.

13. Which motor works on actin tracks?

  1. Dynein
  2. Kinesin
  3. Tubulin
  4. Myosin
Reveal answer and reasoning

Myosin. Myosins convert ATP-driven conformational changes into force along actin.

14. What provides the spindle in flowering-plant cells?

  1. Microtubules organized at other sites
  2. Keratin replacing tubulin
  3. A cellulose spindle
  4. Absence of mitosis
Reveal answer and reasoning

Microtubules organized at other sites. Plant cells can nucleate and organize spindle microtubules without an animal-type centrosome.

15. What is the motor energy source for vesicle transport?

  1. DNA
  2. ATP
  3. Cellulose
  4. Cholesterol
Reveal answer and reasoning

ATP. Motor proteins couple ATP hydrolysis to repeated steps on a suitable track.

16. Which structure anchors the base of a cilium?

  1. Lysosome
  2. Cell plate
  3. Basal body
  4. Centromere
Reveal answer and reasoning

Basal body. The basal body organizes the microtubule-based core of a cilium.

17. Which arrangement describes a typical motile ciliary axoneme?

  1. Nine inner triplets
  2. Actin ring around DNA
  3. Two nuclei
  4. Central pair of singlets
Reveal answer and reasoning

Central pair of singlets. The classic 9+2 pattern consists of nine outer doublets and two central singlets; exceptions occur.

18. Which technique can resolve individual 25 nm tubes directly in cross-section?

  1. Transmission electron microscopy
  2. Unaided observation
  3. Low-power hand lens
  4. Ordinary brightfield at 40×
Reveal answer and reasoning

Transmission electron microscopy. Electron microscopy provides the resolution needed for this fine structural comparison.

19. Which control best tests the effect of a drug dissolved in a solvent?

  1. Untreated dead cells only
  2. Same solvent without drug
  3. Different solvent plus drug
  4. Twice the drug
Reveal answer and reasoning

Same solvent without drug. The matched vehicle control separates drug effects from effects of the solvent.

20. Which observation distinguishes lost motion from lost tracks?

  1. Only total cell mass
  2. Only medium colour
  3. The filament network itself
  4. Only the nucleus count
Reveal answer and reasoning

The filament network itself. Cargo and track images show whether tracks persist when movement stops.

Test yourself with 20 new prompts

Choose one answer per question, then check your score.

1. An animal cell divides its nucleus but cannot constrict its equator. Which filament would you examine first?
2. A purified fibre contains repeating alpha/beta dimers. Which subunit family is present?
3. A skin-cell sheet tears under tension after a keratin mutation. Which network has failed?
4. An electron micrograph shows a hollow fibre roughly how wide?
5. Centrioles remain visible but gamma-tubulin recruitment fails. Which region normally recruits it?
6. A transverse section shows nine peripheral groups of three tubes. What was cut?
7. What would an investigator isolate to collect an animal cell’s main microtubule-organizing centre?
8. The centromeric DNA is intact, but its protein attachment complex is absent. What is missing?
9. A vesicle travels inwards on a radial array with minus ends at the centre. Which motor fits?
10. A motor moves outwards on a conventional radial array. Which family includes many such motors?
11. A single filament lengthens, abruptly shrinks and then grows again. What is being recorded?
12. Knockdown of lamins distorts nuclear shape. Which layer is affected?
13. A plant organelle moves along a labelled actin bundle. Which motor family is a candidate?
14. A root-tip cell has no conventional centriole pair but separates chromosomes. What explains this?
15. A purified motor stops stepping when which fuel is removed?
16. A centriole docks at the surface before cilium assembly. What does it act as?
17. A shaft contains nine outer doublets. Which additional feature is typical of the classic motile pattern?
18. Two nearby tubules merge in a conventional light image. Which method can resolve their ultrastructure?
19. One group receives drug plus solvent. Which comparison isolates the drug effect?
20. Cargo stalls after motor inhibition. What should be imaged alongside cargo?

Further reading

OpenStax Biology 2e, The Cytoskeleton · PreachBio Cell Biology hub