You have learned about cells before. Now you need to refresh your memory, and focus on those aspects of cells that will help you with Anatomy & Physiology.
As you review cells, keep in mind that you only need to focus on animal cells, since humans are animals. You may visit a web site that describes prokaryotic cells or plant cells, and those are not cells you need to be concerned with for this course.
When you look at a cell, like the one in this cartoon, keep in mind that this is a "model" cell. As we progress through the semester, we will study a variety of cell types. Each cell type is a variation on the theme presented here.
Looking at this fake cell, you probably notice certain prominent aspects of it right away. Generally, it is surrounded by a membrane, which defines the boundaries of the cell. Also, the fluid environment inside the cell, the cytoplasm, seems different from that outside the cell. We will discuss each of these items in more detail-- but only to refresh your memory on important points since you should have already learned most of this before. The above , so that you can go to any one particular cell region later and review it. On your first visit to this page, you may want to just continue reading below.
When looking at cells in lab, you won't see nearly as much in them as in this model cell; that is because some of the organelles depicted here are only visible with an electron microscope.
?Do you know which ones are visible with light microscopes like in lab?
This page proceeds in the following order:
If you want to check out another resource for cell terminology, check out the Biology Hypertextbook online.
A really cool web page dedicated to the cell is The Virtual Cell. You should check it out after you review the material that follows.
Cells are surrounded by a membrane. This membrane has many functions, some of which are listed here:
The first two items, maintaining and protecting are rather simple. You'll see in next week's web page that it can take on these roles because its primary component is lipid, and so everything that is soluble in water sees the membrane as a general barrier.
Selective permeability means that the membrane will let some things cross it, while not permitting other things to cross. This is important, because we need things like oxygen and glucose to be able to cross the membrane, but we don't want toxins crossing the membrane. In order to understand selective permeability, you need to understand passive and active transport, as well as endocytosis and exocytosis. These concepts can seem quite difficult, but they are rather straightforward. We will spend more time on them throughout the semester. But for a little more information on them now, link to the selective permeability page.
Signal transduction is recognizing and then responding to cues. Cells need to be able to do this. For example, cells of your digestive tract need to be able to recognize food on their surfaces and then respond to this food by taking it in and making digestive enzymes. Another example is that bone cells need to be able to recognize growth hormone as it circulates by, so that the bone cell can then respond to it by growing and dividing. What does a cell need for signal transduction? It needs receptors on the surface of its membrane to bind to the signaling molecules. Then it also needs enzymes within the membrane (or special channels will also work, as you'll see in our study of the nervous system) in order to start generating a chemical response to the signal. Remember, cells are the smallest living things, so when their parts have to respond, their parts are chemicals, and chemicals respond with chemical reactions-- thus, the need for enzymes in signal transduction.
Note that I did not mention cell shape or movement. These things have nothing to do with the membrane... they are functions of the cytoskeleton. Please keep that in mind.
Throughout the year in this course, we will spend more time on items and issues that relate to the cell membrane than any other part of the cell. Therefore, you will need to try to really understand the concepts that relate to the cell membrane; then you will be able to expand on these concepts later in the semester.
Finally, it will be important for you to understand the structure of a membrane. I'll go into this a bit more next week when we discuss the organic molecules.
The nucleus is a membrane-bounded region that contains the cell's genetic information (genome) within it. The membrane that surrounds it is called the nuclear envelope because it is a double membrane with a thin space between the two membranes (like the thin space between the two papers of an envelope). There are numerous pores in the membrane that allow passage of material into and out of the nucleus, called nuclear pores. The nucleus also contains the nucleolus, which is just a particularly busy region of the cell's DNA, where ribosomes are being made. Since there is so much activity in the region of the DNA to make ribosomes, there are lots of proteins and RNA there; because of all the molecules in that tiny region of the genome, it looks like a dark spot when viewed in the microscope.
There is a lot of DNA in the genome, and it all has to fit into the tiny nucleus. Therefore, the DNA is packed up into chromatin. The DNA can also be packed into chromosomes, which is a much tighter kind of packing, but that is only done when the cell is dividing. All of the rest of the time the DNA is in the form of chromatin. The chromatin never leaves the nucleus.
The cytoplasm is the solution within the membrane, called the cytosol, plus all the organelles (except the nucleus) that are in it. That means that it is the cytosol, rER, sER, Golgi Apparatus, ribosomes, mitochondria, and lysosomes. There are other organelles, but we won't worry about them.
The cytosol is important, because it contains many dissolved materials. Simple sugars, ions, amino acids, nucleotides (including ATP), and other molecules (like vitamins) are dissolved in the fluid cytosol. These items are all necessary for the cell. Students tend to forget that stuff is in the cytosol, since the stuff is dissolved and we can't see it. But if, for example, any of us got a cup of tea that had salt dissolved in it, we would quickly realize that even things that can't be seen are crucial! In the cell cartoon above, all of the green-colored interior is the cytosol.
Animal cells are eukaryotic cells, and they contain many functional areas within the cytoplasm in independent compartments, called organelles. Most organelles are membrane-bounded; the only exception is the ribosome. You should be able to see how this compartmentalization allows one organelle to do one thing while another organelle does something different. Therefore, having organelles allows for segregation of function within a cell.
Each organelle has a particular function within the cell. However, a few of the organelles are all involved in different aspects of making proteins. It takes quite a few organelles to accomplish the task of making proteins, since proteins come in many varieties and have many functions. The rest of the organelles have other functions, like destroying waste or making energy. For other pages that describe organelles, check out the MIT cell bio page.
The four main organelles that are important in making proteins, in the order they are used, are: 1) nucleus; 2) ribosomes; 3) rough endoplasmic reticulum (also known as the rER); and 4) Golgi apparatus. Since we have already discussed the nucleus, we can pick up this description there.
The DNA in the nucleus holds the plans for all the proteins that a cell will ever need to make. But this DNA doesn't leave the nucleus, and the rest of the protein-synthesizing machinery is in the cytoplasm. So, within the nucleus, the DNA is used to make RNA. RNA can slip out the nuclear pores and enter the cytoplasm.
1. Organelles found in all eukaryotic cells (in both animal and plant cells!)
Vesicles: How do proteins and other materials shuttle around in the cell? They move around within vesicles. Vesicles are just little spheres of membrane with stuff inside, or you can think of them as membrane packets. Vesicles are formed when membrane pinches off organelles. For example, vesicles form off the rER as a bit of membrane pinches off with protein inside it; that vesicle can then deliver its protein to the Golgi apparatus by fusing with the Golgi's membrane. This pinching and fusing of membrane is the same thing that happens with endocytosis and exocytosis. Vesicles are not really considered organelles, but they are important for how membranous organelles function.
What does the cytoskeleton do?
1) amoeboid movement, which is kind of a creeping
along of a cell (if you want to see a cell move by amoeboid movement, check out the very
last listing on this web
page for the movie "Time lapse of a locomoting human anaplastic astrocytoma
cell"... but only do this when you have time!)
The cytoskeleton is made from three main components, each of which is build from long filaments of protein. The three cytoskeletal components are:
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