Structures in Most Eukaryotic Cells

Most eukaryotes have these, but some don't.


All cells need energy to grow, reproduce, and function. Like the organisms they comprise, cells must "eat" to obtain the energy they need. One of the most important types of cellular food is a molecule called glucose, which is a type of sugar and a carbohydrate.

Eukaryotic cells take in glucose through proteins that cross the plasma membrane and then transport it through the cytoskeleton to the mitochondria (mitochondria is plural; the singular is mitochondrion) in the cytoplasm. The mitochondrion is often called the cell's powerhouse.

Take a look at this bad boy:

In the cytoplasm just outside the mitochondria, glucose is broken down into smaller molecules through a process called glycolysis (literally "sugar breaking"), which releases chemical energy. This energy is temporarily captured by specialized molecules and transported through the mitochondrial membranes into the mitochondria. There it is used to make an important molecule called adenosine triphosphate (ATP) through a process known as cellular respiration.

Mitochondria can convert a single molecule of glucose into ~38 molecules of ATP! You can think of each ATP molecule as a unit of stored energy ready to be used by the cell whenever needed. Mitochondria do not mess around with energy storage.

So the main function of all mitochondria is to make ATP, which is the energy source for nearly all cellular functions and processes. You can read more about the details of how mitochondria are involved in cellular respiration in a later unit.

Endoplasmic Reticulum

Endoplasmic reticulum has to be on a "Top Ten Best Terms" list somewhere.

There are two types of endoplasmic reticulum (ER) in eukaryotic cells:

  • Smooth ER (SER)
  • Rough ER (RER)

Picture time!

Both ER types are involved in making important cellular components. The rough endoplasmic reticulum (RER) is mainly responsible for the synthesis and processing of proteins that are either secreted from the cell or that end up stuck in the plasma membrane. Proteins marked for secretion are sent from the RER to the Golgi body (it's next up for explanation, so hang tight) for further processing.

Insulin is an example of a secreted protein processed by the RER. This very large protein is secreted in huge quantities from the pancreas cells in mammals and aids in the uptake and digestion of glucose.

The smooth endoplasmic reticulum (SER) is primarily involved in the synthesis of lipids (aka fatty fat fats) and steroids, both very important components of cell membranes. The lipids made in the SER are combined with phosphorous to make phospholipids, the most abundant component of cell membranes. Steroids, including cholesterol, made in the SER are also important components of cell membranes, because they provide the rigidity and structure needed for the membrane to keep its general shape.

Golgi Bodies

The Golgi body, also called the Golgi apparatus, is simply a flattened stack of microscopic pancakes…wait, scratch that. Those are membrane disks. They may look like pancakes, but no amount of maple syrup could make them appetizing.

Hi there, Golgi B:

In these membranous stacks, called cisternae, proteins that have been marked for secretion in the RER are packaged into vesicles that transport them to the plasma membrane where they are secreted from the cell. The Golgi body also packages the lipids and steroids made in the SER into vesicles.

Packaged lipids and steroids are transported to the edge of the cell, as well as to all organelles within the cell, where they are used to build or repair the cell and organelle membranes. Just remember that the Golgi body likes sticking things into vesicles. In a way, it's like UPS, except that the Golgi body is 75% less likely to chuck, crush, or lose your packages.

Lastly, small portions of the Golgi body cisternae often bud off into small spheres to create lysosomes.

In case you’re wondering, the incessant capitalization of the "G" in "Golgi" is not a word processor error, but a result of the fact that this interesting organelle was named after its discoverer, the preeminent Italian physician Dr. Camillo Golgi. Though remembering to always capitalize Golgi is somewhat annoying, you should be grateful it was discovered by Dr. Golgi and not his long-lost German cousin Dr. Dieter Wolfeschlegelsteinhausenbergerdorff.


Hmm…feels like we just mentioned these guys. Wait, we did!

Lysosomes are small spheres of phospholipids made by the Golgi bodies and are responsible for breaking down cellular debris and material taken into the cell through the process of phagocytosis (the cell's swallowing up of things).

The interior of a lysosome contains many enzymes and is slightly acidic, so material can be digested without harming the rest of the cell. Lysosomes maintain their acidity by pumping protons (hydrogen ions, or H+ ions) across their membranes through integral channel proteins.

Helpful tip: when you see lys– as part of a word, think of cutting, chopping, or breaking down. Lysosomes chop things up, glycolysis is the breaking down of sugar, and so on. There is a method to this wordy madness.


Vesicles are small spheres of phospholipids made by the Golgi bodies and are responsible for transporting proteins, lipids, and steroids to various places throughout the cell, especially to the plasma membrane. The interior conditions of a vesicle are similar to the conditions of the surrounding cytosol, so transported proteins and lipids are not damaged en route to their destinations. Smart, those cells.

Helpful tip: now that you know what lys– means, it will be easy to remember that lysosomes are made by Golgi bodies to break things up, and vesicles are made by Golgi bodies to move things around. Nice.

Brain Snack

The mitochondrion used to be a free living bacterial cell a wicked long time ago. Because of this fact, it has its own genome; however, because it relies so much on its host cell, it has lost many of its genes. Read more here.

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