Cytology - Part V

Plasma membrane (cell membrane)

The plasma membrane is clearly not a passive envelope surrounding the cell, but is highly selective in what it allows to enter or exit. One of its essential functions is the maintenance of homeostasis. All cells need to regulate their internal environment by constantly removing waste products of metabolism and actively bringing in needed resources.

The primary component of the plasma membrane is a double layer of phospholipids.

The bilayer mentioned above is common to all cellular membranes. (Taken from the WWW Cell Biology Course)

What distinguishes one membrane from another is the proteins associated with or embedded in the phospholipids, which in turn is related to the membrane’s function.

membrane structure

Go to the University of Virginia for a professionally executed illustration of the cell membrane

Internal Support: The Cytoskeleton.

What at one time was called the "cell matrix" or "ground substance" the seemingly clear regions of the cytoplasm have yielded their secrets with the use of fluorescent labeling, detergents and the high voltage transmission electron microscope.

Today we know that much of the cytoplasm's matrix is laced with an intricate, mazelike network of proteinaceous microtubules intermediate filaments and microfilaments which constitutes the cytoskeleton.

The cytoskeleton regulates cell shape, transport, motility, and integrity. It consists of 3 protein filament systems noted above and a large number of associated proteins that regulate each system.

In summary the major functions of the cytoskeletal filaments and tubules are:

  • to act as a supporting framework for organelles
  • to provide for cell movement
  • to change or maintain cell shape
  • to anchor the plasma membrane

The actin microfilaments and myosin motor microfilaments can be found wherever the cell membrane needs support and movement. This includes:

  • microvilli of intestine cells,
  • cytoplasmic streaming, and
  • transport or movement of organelles


Intermediate filaments of the epithelium are composed of the protein keratin, which lends strength to the microvilli as shown in the diagram below.

Microtubules are the largest of the cytoskeletal elements. A globular protein, called tubulin, is the building block of the microtubules, the tiny hollow tubes found throughout the cytosol.

Each tubulin molecule is actually a dimer, composed of two spherical polypeptides.

Tubulin dimers are assembled into spiraling rows quickly forming each microtubule. This modular structure allow the cell to rapidly convert the associated cytoplasm between a sol (solid) or a gel (fluid) state. _________________________________________________________________

Microtubules play a role in:

  • cytoskeletal structure
  • intracellular material transport
  • maintenance of cell shape
  • cellular organization
  • cell division and movement of chromosomes.

Specific cellular structures which are composed of microtubules are:

In cell division microtubules cross the cell from pole to pole, taking on a spindle shape. Each chromosome has numerous microtubules attached to its centromere (actually the kinetochore). Thus microtubules help to separate and move the daughter chromatids to the opposite poles of the cell.

The shape of specialized cells such as nerve cells will revert to a simple sphere when their microtubules are chemically disrupted.

Learn more and about the cytoskeleton and practice some multiple choice questions at the Biology Project at Arizona State.

Modified July 10, 2005