Unit 4, Lesson 13
Lesson 13 - Taking a Deeper Look at Cellular Structure
(An optional lesson about the cellular cytoskeleton)
Student Performance Objectives
1. List and describe the locations of the 3 major types of cytoskeletal elements found
2. Describe the functions of the microfilaments, intermediate filaments and the microtubules
found in the cell's cytoplasm.
A. A cell is not a watery soup with organelles floating around at random.
B. Cells have a cytoskeleton composed of:
1. Microfilaments, of which actin is the specific protein type. About 6/1000 of a micron
a. Microfilaments form a network at the junction of the outermost border of the
cytoplasm and the inner border of the cell membrane phospholipids. The
association of microfilaments and phospholipids holds the inner membrane
in place thus acting as a membrane stabilizing force. Appropriately, this band
of microfilaments underlying the cell membrane is called the membrane
b. Microfilaments extend upward from the cytoplasm into microvilli, those
cellular extension seen best in intestinal epithelial cells and kidney tubular
epithelial cells where they increase absorptive surface area. The filaments can
then pull down on the microvillus, physically propelling absorbed materials
deeper into the cell for processing.
2. Intermediate filaments, of which keratin is a major type, are seen in epidermal cells.
Slightly thicker than microfilaments - about 8-10/1000 of a micron thick
a. Extend throughout the cytoplasm like a spider's web.
b. Help to maintain cell shape by resisting mechanical stresses placed on the cell.
3. Microtubules, which are more complex structures composed of the protein, tubulin. Significantly
larger than the other cytoskeletal elements: 25/1000 of a micron.
a. Each microtubule is composed of the globular protein, tubulin, arranged into 13 tiny filaments that
form a cylinder.
b. The cylinders radiate from the cell's centrosomal region throughout the cell like the intermediate
filaments, only they are larger.
(1) Anchor organelles in place in specific cytoplasmic regions.
(2) Allow organelles, like chromosomes during mitosis, to glide to
different locations as required. The mitotic spindle and centrioles are composed of
(3) Work with the intermediate filaments to help maintain cell shape.
(4) Make up the inner structure of cilia and flagella and are responsible
for their movements.