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Instruction 1-9

Cell Membrane | Enzymes | Prokaryotic and Eukaryotic Cells | RNA's Role | The Role of the Endoplasmic Reticulum and the Golgi Apparatus | Energy Capture and Storage | Mitochondria | Macromolecules | What Determines the Eukaryotic Cell's Shape? | Summary

What Determines the Eukaryotic Cell's Shape?
CA GR 9-12  Biology 1.j.

A few Instructions back, you learned about prokaryotic and eukaryotic cells. You learned that eukaryotic cells are the cells that complex organisms (like us) are made of. And you learned that one of the main differences between the two kinds of cells is their structure. A eukaryotic cell always has a nucleus and organelles, while a prokaryotic cell doesn't.

We will now talk about what gives eukaryotic cells their shape, the cytoskeleton. The cytoskeleton doesn’t just give these cells their shape. It is also involved in cell movement.
The Cytoskeleton

The word cytoskeleton is easy to remember since we human beings are made up of eukaryotic cells -- and we have skeletons too. Cyto means cell, so we are talking about the skeleton of the cell. It gives the cell its shape. The cytoskeleton is a moving three-dimensional (3-D) structure that fills the cytoplasm.

We should probably remind you here that the three parts of a eukaryotic cell are the membrane, the nucleus and the cytoplasm (the “cell fluid” in between the membrane and the nucleus).
Incidentally, plant cells also have something extra outside the plasma membrane called the cell wall. This cell wall controls the cell’s shape and protects it from the outside world. Many mature plants have both primary and secondary cell walls, with the secondary cell wall being the innermost portion.
Anyhow, back to the cytoskeleton. The cytoskeleton acts as both skeleton and muscle -- and it does three very important things:

  • It gives the cell its shape
  • it helps moves vital substances around the cytoplasm, and
  • It helps give the cell its motility (movement).

The Fibers that make it up

Three main fibers make up the cytoskeleton, those are:

We’ll tell you a little bit about each of them.

Microfilaments are fine, thread-like fibers, 3-6mn in diameter. They are mostly made up of a spherical, globular protein molecule called actin. Actin is responsible for cell contraction and movement Microfilaments help a cell change its shape by adding units at one end while they take units away at another end. If you’ve ever seen an amoeba move, you have witnessed this activity.

Remember actin and another protein called myosin, these filaments will come into play when we talk about muscle contraction in a later lesson.

Microtubules are tiny cylindrical tubes, 20-25mn in diameter. They are composed of two kinds of a globular protein called tubulin (alpha tubulin and beta tubulin).

These microtubules act as a scaffold to determine the cell’s shape and provide a set of “tracks” for cell organelles and vesicles to use inside the cytoplasm. They also help the flagella help the cell move (remember vesicles and flagella from previous Instructions?). The cilia and flagella are actually made up of these microtubules.

They also help with a kind of cell division called mitosis, which you’ll be learning more about in an upcoming instruction. They also help with a kind of cell division called mitosis, which we will discuss more in an upcoming instruction.

Intermediate Filaments The intermediate filaments are about 20nm in diameter and provide tensile strength for the cell. Tensile means stretching, so what these intermediate filaments do is help the cell stretch and contract. That's kind of like flexing the muscle in your arm to show how strong you are. These filaments are made up of fibrous proteins instead of globular like the other two fibers.

Video Instruction
*Availability of You Tube video links may vary. eTAP has no control of these materials.

Experiments for Home and Classroom

This is fascinating, and ambitious, web site that invites students to take virtual tours of cells and all their structures -- including the cytoskeleton and its component parts (with helpful worksheets for guidance along the way). Tours are available in English, Spanish, French and Russian. Start with the Instruction page. Note: this cell tour was also suggested in a previous Instruction. Click:  

To begin the tour, return to the home page at: 


Reading List
from the California Department of Education
  Rensberger, Boyce: Life Itself: Exploring the Realm of the Living Cell


for Students, Parents and Teachers

Now let's do Practice Exercise 1-9 (top).


You have now completed Lesson 1 on Cell Biology and are ready to do the Problem and Test sections.

You may wish to review any or all of the topics before answering the questions that follow.

Good luck!

Next Page:  Problems (top)