Those in All Eukaryotic Cells
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Remaining organelles found in all eukaryotic cells

Smooth Endoplasmic Reticulum (sER)

    Not only do cells have rER, but they also have smooth endoplasmic reticulum (sER). The sER has a completely different function from the rER.  The sER has absolutely nothing to do with protein synthesis (because it has no ribosomes attached). Instead, this organelle is involved with making lipids. Any cells that have to make a lot of lipids (like cells that produce steroids such as testosterone or estrogen) will have a lot of sER.

    Why does it seem to have the same name?  Because it looks very much like the rER but without the ribosomes.  The term endoplasmic reticulum sounds complicated, but it means what it is--

bullet"endo-" means inside
bullet"-plasmic" represents the fact that it is made of a lot of plasma membrane
bullet"reticulum" means a network

So, the endoplasmic reticulum means basically "a network of membranes inside a cell."  That's all that the sER looks like.


    These organelles are also found in all eukaryotic cells.   Mitochondria are very important organelles, because they are responsible for generating almost all of the cellular energy (called ATP) that the cell needs. The chemical reactions that lead to the production of energy by the mitochondria are collectively called cellular respiration. Why? When you think of respiration, you think of breathing, right? Well, why do you breathe? Of course! To take in oxygen gas from the environment. Did you ever stop and ask yourself why you needed to get that oxygen gas inside of you? It turns out that the oxygen is used within each cell of your body, within the mitochondria of the cells, in order to make ATP. So, cellular respiration is just the term that is used for those chemical reactions whereby your cells use oxygen gas to make ATP. Let's now talk a bit more about cellular respiration and the structure of mitochondria.

    You know that you eat sugars (carbohydrates, see below) in order to get energy.  And now you also know that you breathe in order to get energy. The chemical reaction of cellular respiration is depicted here. So, if you have sugar monomers and oxygen gas, you should be able to generate ATP! Right? Well, that's actually how it works:


    You are not responsible for the details of the above diagram (or those that are included below as explanations of the above diagram). I only included it so that you would see exactly what is involved in making energy. Glucose is the carbohydrate monomer (the monosaccharide) that is used to make energy. You already know that we exhale carbon dioxide as waste, and we have to remove water waste from our body as well. ATP stands for adenosine triphosphate. ADP is just adenosine diphosphate, and so it needs one more phosphate group to be turned into a triphosphate. That's all that the Pi is.

What does a mitochondrion look like? (note that mitochondrion is for one of these organelles, and that mitochondria is for more than one mitochondrion). It is unusual because it is surrounded by two independent membranes, like a double barrier. The outer membrane is pretty smooth. The inner membrane is all folded up inside to make membranous ridges called cristae. In the lumen of the inner membrane we find something really unusual: the mitochondrion has its own DNA, its own ribosomes, and makes its own proteins!


These organelles are the waste centers of the cell, and it is also absolutely necessary for cells to use lysosomes in order to access the material that they take in through endocytosis.  I am going to explain both of these roles here.

First of all, lysosomes look just like vesicles. They are simply membrane-bounded, spherical organelles. But the thing that makes them special is what they have inside their lumen. They contain powerful, hydrolytic enzymes!  Enzymes help with chemical reactions (as you will see in a later web page) and are made of protein. The enzymes in lysosomes are hydrolytic enzymes, which mean that they facilitate hydrolysis reactions. Remember, hydrolysis reactions break large molecules down into smaller molecules, like monomers.

Since lysosomes contain powerful, hydrolytic enzymes, anything that the lysosome takes inside of it will get broken down. This can be handy if the cell needs to destroy old organelles or digest large food particles.  We will come back to this after we see how large food particles get inside of cells.

Finally, lysosomes are in almost all eukaryotic cells.  Some plant cells use another organelle (the central vacuole) to do the job of a lysosome, so they don't have a separate lysosome.  But no matter what, getting rid of wastes and breaking materials down is an essential function of a cell and all cells have methods to do this.


    Don't worry about this one.


2006 STCC Foundation Press, content by Dawn A. Tamarkin, Ph.D.

Last changed: January 21, 2007