In unit three you were introduced to some of the basic structural and functional organization of eukaryotic cells. This unit expands that theme and concentrates on the complex network of fibers and other proteins that form the internal cytoskeleton of cells and the interconnections between cells. Most of the structures you studied in Unit 3 were constructed with the basic phospholipid bilayer and associated membrane proteins. The elements of the cytoskeleton are quite different and much more variable. They have a wide range of functions including support of the cell and its organelles, movement of material within the cell, movement of the cells themselves, and the regulation of cell processes.
Learning Objectives: the successful student will be able to ...
- name and describe the physical properties of the three primary components of the cytoskeleton.
- name the major functions of the cytoskeletal elements and explain the dynamic changes they undergo when active.
- describe the complex organization of the centrosome and centrioles and explain their function.
- describe in detail the structure of the cilia and flagella and how this structure functions to produce cell movement.
- name and describe the structure and functions of the various intercellular junctions between animal and plant cells.
- describe the role of cytoskeletal elements in the production and maintenance of plant cell walls and animal extracellular matrix.
Lesson One: Basic elements of the cytoskeleton.
Your cells have much in common with your entire body. Just as you need a boney skeleton to support and protect your body and its organs and help you move your body parts, cells have an internal cytoskeleton that performs many of the same functions. There are three basic components that make up the cytoskeleton: the microtubules, microfilaments, and the intermediate filaments. Each is constructed with different proteins, have different properties, and functions. Go to the Kimball pages on the cytoskeleton and review the material there. For each of the three elements of the cytoskeleton understand the types of protein monomers involved in constructing these tubules and filaments, their location and function in the cell. In particular, understand that these structures are not static; they grow and shrink in response to the demands of the cell.
- Microfilaments (Actin or Thin filaments).
- Intermediate filaments.
- Microtubules (Thick filaments).
Lesson Two: Microtubule Organizing Centers, Centromeres, and Centrioles.
Microtubules are anchored to and grow from microtubule organizing centers (MTOG's). In animal cells these are called centromeres and these contain centrioles. Go to the Kimball page on the centromere. Don't worry at this point about the details of cell division; just understand that the centromere plays an important role in the organization of the microtubules that move the chromosomes (the spindle fibers). Appreciate the 9X3 arrangement of the microtubules of the centrioles in the centrosome. Go to this link at St. Olaf College for a good summary of centromere structure and function.
Lesson Three: Cilia and Flagella.
One of the most important functions of the cytoskeleton is movement. In this section we study the structures that cell use to move from one place to another in their environment (e.g. protistans such as Paramecium and sperm cells) or to move material across their surfaces (e.g. lungs). In the next lesson we study the movement of material within the cell itself. The Kimball page provides a brief summary of cilia and flagella structure. Note the similarity to centrioles. Cilia and flagella have a 9+2 arrangement of microtubules. A more extensive treatment can be found at the University of Arkansas for Medical Sciences. Note the arrangement of the microtubules in the cilia and their relationship to the dynein protein arms. These are the proteins involved in the movement mechanisms of the cilia and flagella. Here is a short animation reviewing this material.
Homework, Due 19 October . Read this short entry from the the Encyclopedia of Science. Compare the similarities and differences among the three types of flagella described in the article. Email me the answer using your Fontbonne account.
Lesson Four: Intercellular Connections.
Most cells to not live and function independently of the other cells around them. In fact, many cells have structures that bind them together for support and communication. Be able to identify and describe these various intercellular connections and understand their basic functions. Go to the Kimball pages and read the descriptions of Tight Junctions, Adherens, Gap Junctions, Desmesomes, and Plasmodesmata. Be able to give examples of where these types of intercellular connections are found (plant vs animal and types of tissue) and describe their functions. This short animation from Pearson is a nice overview of these structures.
Homework, Due 25 October . Read the posting "Simple sponges provide clues to origin of nervous system" from the blog Not Exactly Rocket Science. Using the comment section below, post your thoughts on the following questions. Feel free to also comment on your classmates' answers.
- What does the presence of PSD proteins in sponges tell us about the origins of the complex human nervous system?
- What is an exaptation and how does it provide the process of evolution greater flexibility in responding to the demands of a changing environment?
- How might some of the intercellular junctions you just studied serve a similar function to the PSD proteins and other molecular elements of the synapses in the nervous system? Be specific and give an example.
