A number of junior and senior high school textbooks still give the impression that the cell is just a bag of small objects that float around inside, like the image used for this series of articles. The truth is nothing floats freely in the cell. Everything is connected and transported from one location to another in a very complex system.
Contained in eukaryotic cells (cells that contain a nucleus) is a dynamic structural system known as the cytoskeleton (cyto = cell). The cytoskeleton is very multi functional. It is responsible for such things as maintaining cell shape, changing cell shape, muscle contraction, movement of crawling cells, and the segregation of chromosomes during mitosis. Another function of the cytoskeleton is the transport of food and water from the cell membrane to the many locations within the cell and the transportation of waste products back to the cell membrane. It is also involved in the intracellular movements of organelles in the cell.
The cytoskeleton is also responsible for the organization of the thousands of proteins contained in the cell. Some proteins work together in a functional complex of 5 to 10 proteins. The cytoskeleton helps keep those proteins grouped together and then keeps those groups near other groups that associate with them. Like large cities, cells contain many specialized services and functions that are concentrated in various areas throughout the cell. All of these are interconnected by a vast communication and transportation system much like the city streets, telephones, water and sewer systems in a city. The cytoskeleton is the mechanism that accomplishes all of these tasks.
Similar in one way to our skeletons which are made up of three primary components, bone, cartilage and marrow, the cytoskeletons of cells is also made up of three basic parts. These parts consist of different types of protein units that form three basic types of protein filaments: actin filaments, microtubules and intermediate filaments.
Actin filaments are formed by actin protein subunits. The actin filament is smallest of the three types of filaments as it is made up of only 2 helical strands of actin protein. Unlike microtubules that act individually, actin filaments form bundles and work together in networks. While they are generally found throughout the cell, their concentrations are higher near the outer edges of the cell just near the cell membrane. Actin filaments near the cell membrane will cross-link with each other to form the cell cortex. This cell cortex latticework is controlled by certain nucleation sites in the plasma membrane. Different regions of the membrane direct the function of specific actin filament structures. The communication and control is always changing with the conditions in and out of the cell creating a highly dynamic network. Certain signals from outside the cell can cause the nucleation sites in the membrane to signal the actin cell cortex that lies just beneath the membrane to react in such a way as to cause the cell membrane to send out long narrow microspikes. Other signals will cause the same actin cortex to form long thin ridges in the cell membrane. And yet another signal can cause the cell cortex to start pulling the cell membrane inward to form vacuoles that surround water and food. In mitosis, this inward pull on the cell membrane by the actin cortex will cause the cell to divide in two.
Actin filaments also work with other protein units like myosin. Actin and myosin filaments are stacked as seen in the photomicrograph. The two fibers slide back and forth against each other resulting in the contraction and relaxation of the muscle fibers. The actin and myosin filaments can work together with incredible speed. Just think of how fast you can move the different parts of your body, each of which is the result of the interaction of the actin and myosin filaments. The process is far more complicated than the simple explanation provided here.
Microtubules are the largest of the cytoskeletal filaments. It is a hollow tubular structure formed by tubulin protein subunits. They originate from the centrosome, which generally lies near the nucleus of the cell. Each centrosome can have several hundred microtubules growing out in all directions. They are polar structures having positive and negative charges. The positive end grows outward by adding more tubulin subunits to the end of the tube. At any given time, the end of the microtubules can undergo a change to negative which will cause that end to start losing subunits and shorten back towards the centrosome. In this way of growing and shrinking, the microtubules transport various items to different locations around the cell. These tubes can reach all the way to the cell membrane.
Intermediate filaments, named because they are intermediate in size between the actin filaments the microtubules, are formed by a number of related proteins such as vimentin and lamin. They are thin ropelike filaments and can be found throughout the cell and nucleus. Some intermediate filaments form a meshwork like lattice within the nucleus. This lattice is called the nuclear lamina. Cells such as epithelia are subject to greater stress, will have a higher concentration of intermediate filaments to give them the strength they need to counter the stress they are under. Added strength is provided when the intermediate filaments link from cell to cell at specialized junctions. The linking of cells together also occurs in nerve cells and many different types of muscle cells.
The descriptions above of the actin filaments, microtubules and intermediate filaments barely scratch the surface of their functions and roles in the life and division of cells. My intention is to give you a tiny peek into just one more area of the so-called simple cell to show you how really complex they are. By demonstrating how complex the cell really is, I pray you grasp the total impossibility of them ever having formed by random chance with no intelligence whatsoever behind it.
I find it so hard to believe that so many people study the complexity of the cell down to the molecular level and still fail to see the awesomeness of our Creator God. Isn’t it interesting that the smallest things under the microscopic give us such a huge look at an infinite God?