The Cell: Cytosol and Cytoskeleton

A cell’s cytoskeleton is a network of intracellular protein fibers that provides structural support, anchors organelles, and aids intra- and extracellular movement. The cytosol is the liquid inside the cell membrane that surrounds the organelles and cytoskeleton. The cytosol is a complex solution where many biochemical processes take place.

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The cytoskeleton is composed of different protein fibers that extend through the cytosol. Three different types of fiber proteins provide the framework along which molecular motor proteins move.

Protein fibers

  • Microfilaments (actin filaments): 
    • Structure:
      • Smallest
      • Most common intracellular protein
      • 2 filaments made of approximately 7 nm actin polymers (F-actin) coiled around each other (double helix)
      • Growth is directional (positive and negative end).
    • Function: 
      • Disassembles and reassembles rapidly (treadmilling), allowing cell to move
      • Provides track for motor protein myosin 
      • Provides some structural support to cell
    • Examples:
      • Muscle contractions
      • Assemble along the circumference of a cell during cytokinesis and tighten to pinch it into 2 daughter cells
      • Macrophage movement within body
      • Cytoplasmic streaming (movement of cytoplasm within the cell)
  • Intermediate filaments: 
    • Structure:
      • Medium
      • 10 nm in diameter
    • Composed of several different fibrous proteins
    • Function:
      • Maintenance of cell shape
      • Anchor organelles
      • Structural components of nuclear lamina
    • Examples:
      • Keratin protects against mechanical stress.
      • Cell–cell connection and intercellular communications
      • More stable than microfilaments
      • Stronger tensile strength
  • Microtubules
    • Structure:
      • Largest
      • 25–100 nm spiral made up of alpha and beta tubulin
    • Function:
      • Anchor on centriole near nucleus
      • Directional growth: addition and subtraction of tubulin subunits
      • Highways for transportation within cell by molecular motors
    • Examples:
      • Assemble and provide framework for chromosome movement during cell division
      • Form cilia and flagella, allowing movement through extracellular matrix

Molecular motors

Specialized proteins “walk” along the microtubule highways, transporting organelles, substrates, or chromosomes within the cell. 

  • Structure:
    • Protein complexes:
      • Transport vesicle
      • Connector protein
      • Motor protein
    • Different complex based on direction of transportation:
      • Dynein (connector dynactin) moves from the periphery toward the nucleus.
      • Kinesin moves from the nucleus toward the periphery.
  • Function:
    • Convert chemical energy into kinetic energy
    • Transport substances along microtubule “highways”
    • Assembly walks down microtubule by ATP hydrolysis.
  • Examples:
    • Walk along mitotic spindle to separate sister chromatids during cell division
    • Transport neurotransmitter-filled vesicles down neuron axons 
Molecular motor moving along microtubule

Molecular motor moving along microtubule

Image by Lecturio.


  • Cytosol is a liquid found inside cells (fluid matrix):
    • Water (70%)
    • Dissolved ions (pH 7.0–7.4)
    • Small proteins
    • Large, water-soluble macromolecules
  • Surrounded by cell membrane in prokaryotic cells
  • Part of cytoplasm (all material within cell membrane except nucleus)
  • Site of eukaryotic cellular processes:
    • Signal transduction
    • Cytokinesis
    • Glycolysis
    • Pentose phosphate pathway
    • Protein biosynthesis
    • Gluconeogenesis
    • Transport of metabolites and vesicles
  • Organizational areas within cytosol (components do not mix randomly):
    • Concentration gradients: (e.g., calcium) can be created near open channel
    • Protein complexes: form to pass substrate along metabolic pathway
    • Protein compartments: form to separate contents from cytosol (e.g., proteasome enzymes within proteasome)
    • Microdomains created by cytoskeleton: can prevent larger proteins from accessing areas of cytosol (sieving)


  1. Alberts B, Johnson A, Lewis J, et al. (2002). Molecular Biology of the Cell. 4th edition. New York: Garland Science. How Cells Regulate Their Cytoskeletal Filaments.
  2. Alberts B, Johnson A, Lewis J, et al. (2002). Molecular Biology of the Cell. 4th edition. New York: Garland Science. The Cytoskeleton and Cell Behavior.
  3. Lodish H, Berk A, Zipursky SL, et al. (2000). Molecular Cell Biology. 4th edition. New York: W. H. Freeman. Section 18.1, The Actin Cytoskeleton.

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