If you’re getting ready for the AP Biology exam, then you’re probably spending quite a bit of time studying cell structure. But keeping all the structures straight and understanding each one’s function can be confusing!
That’s why we’re breaking down cell structures for you, starting with the endoplasmic reticulum, or ER. In this article, we’ll teach you everything you need to know about the endoplasmic reticulum, including how it functions in a cell. We’ll even break down the differences between the rough endoplasmic reticulum and the smooth endoplasmic reticulum!
Ready? Then let’s get started!
A Quick Introduction to Cell Structure
The endoplasmic reticulum is an essential part of a cell. In your biology class, you probably learned that cells are the building blocks of all life...including humans! Obviously, that makes cells super important, which is why it’s also important to understand how they function.
Because plants and animals are complex creatures, the structure of a cell is complex, too. Each cell is made up of many individual parts, each of which has a job within the cell itself! Some help keep everything in one place (like the cell membrane), some produce energy to power the cell (the mitochondria), and there are even parts that help keep the cell clean (lysosomes)!
These different structures found within cells are called organelles. The endoplasmic reticulum is an organelle that can be found in both eukaryotic and prokaryotic cells. Just keep in mind that not all cells have endoplasmic reticulum! For example, red blood cells don’t have endoplasmic reticuli, even though they’re an important part of animal biology.
Endoplasmic Reticulum Definition
The endoplasmic reticulum is defined as an organelle that is made up of a series of phospholipid membranes. In fact, the membranes that make up the endoplasmic reticulum can account for half of a cell’s total membrane structure in animal cells! These membranes are called cisternae, are shaped like tubes or sacs, and are continuous with the outer membrane of the cell’s nucleus. That’s a fancy way of saying that the endoplasmic reticulum is attached to the nucleus itself.
Endoplasmic Reticulum Function
Now let’s talk a little bit more about how an endoplasmic reticulum functions.
In general, the endoplasmic reticulum helps with the synthesis, folding, modification, and transport of proteins and lipids. The endoplasmic reticulum does this through ribosomes that are attached to its membrane walls. (We’ll talk more about how this works a little later). The endoplasmic reticulum also stores calcium and releases it when the cell needs it. In fact, many of the proteins and lipids made by the endoplasmic reticulum are used by other organelles in the cell.
One of the best ways to understand—and remember!—what the endoplasmic reticulum does is to think of it like a factory. In a manufacturing plant, people take raw materials and make it into something new and usable, which they then ship to other stores, manufacturers, and suppliers around the world. Just like a real-world factory, the endoplasmic reticulum builds the “products” a cell needs to function, then “ships” them to where they need to go, when they need to go there.
Endoplasmic Reticulum Appearance
So what does the endoplasmic reticulum look like, exactly? Well, do you remember the mazes that you could find in coloring books when you were a kid? The endoplasmic reticulum looks a lot like that! The cisternae stretch out and away from the cell nucleus in a series of folds and tubes, and they extend throughout the cell almost like a highway system.
So when you’re looking at a cell diagram, look for the maze-like structure that’s attached to the cell nucleus. That’s the endoplasmic reticulum!
You might notice that cell diagrams often picture some areas of the endoplasmic reticulum with bumps, while other sections look smooth. That’s because endoplasmic reticulum is actually comprised of two pieces: the rough endoplasmic reticulum and the smooth endoplasmic reticulum. Knowing how these different areas work is important to understanding the function of the endoplasmic reticulum as a whole.
Rough Endoplasmic Reticulum
The rough endoplasmic reticulum, or RER, gets its name from the ribosomes embedded in its surface...which make it look rough! The rough endoplasmic reticulum is situated closest to the nucleus—in fact, it’s attached to the nuclear envelope—so that molecules can move directly between the membranes.
The ribosomes that are attached to the walls of the rough endoplasmic reticulum function just like free ribosomes would. That means that they synthesize proteins, which provide the energy needed for a cell to operate. The process of creating proteins is called translation.
One the ribosomes have synthesized a protein, they are “labeled” with a specific final destination. Some proteins are sent to the Golgi apparatus, while others are secreted into the cell exterior or kept within the membrane of the rough endoplasmic reticulum itself.
There are certain proteins that are sent into the space within the rough endoplasmic reticulum. This space, which is also called the lumen, is where certain proteins are folded, modified, and assembled. Some of these proteins will have sugar groups added to them to form glycoproteins. Likewise, some of these new proteins will be transported out of the endoplasmic reticulum, while others will stay inside the endoplasmic reticulum to perform functions there.
The lumen is also where the endoplasmic reticulum does its “quality control.” When misfolded or otherwise incorrect proteins accumulate in the lumen, the unfolded protein response (or UPR) is triggered. This tells the cell to reduce the amount of protein its producing while enhancing the endoplasmic reticulum’s protein-folding ability. If the problem doesn’t correct itself, it triggers apoptosis, or programmed cell death.
One super cool note: the ribosomes of the rough endoplasmic reticulum aren’t permanently attached to the membrane itself. That means that new ribosomes can detach and attach depending on the proteins the cell needs!
Smooth Endoplasmic Reticulum
Unlike the rough endoplasmic reticulum, the smooth endoplasmic reticulum doesn’t have any ribosomes attached to it. That makes it look smooth—which is how it gets its name!
The smooth endoplasmic reticulum’s function is almost exclusively to make lipids, like phospholipids and cholesterol. How these lipids are used depends on the cell type. Lipids can be used to create new cell membranes, create hormones, and store energy.
The smooth endoplasmic reticulum also helps detoxify the cell by converting toxic organic chemicals into safer, water soluble products. Fun fact: when there are lots of toxins present, the smooth endoplasmic reticulum can double its surface area to help clear them out. It will then return to normal size after the toxins have been removed. Liver cells have large amounts of smooth endoplasmic reticulum for this very purpose!
Finally, there’s a type of specialized smooth endoplasmic reticulum called the sarcoplasmic reticulum. The sarcoplasmic reticulum is found in muscle cells and is used to store calcium ions that muscles need to function. When muscles experience sustained activity, the sarcoplasmic reticulum can release the stored calcium ions to help the muscles function.
If you’ve read through this guide and still would like to know more about how the endoplasmic reticulum works, here are a few other resources that you can check out.
Khan Academy has tons of free resources on all sorts of topics including cell structure. Their video on the endoplasmic reticulum is really helpful, and they have articles about it on their website, too.
The British Society for Cell Biology
The British Society for Cell Biology is a British non-profit organization dedicated to advancing cell biology research, which includes sharing knowledge and information. One of the ways they do this is through educational material, which they share on their website. Their softCell e-Learning portal has tons of good information about all the organelles of a cell, including the endoplasmic reticulum.
Who said studying has to be boring? The CrashCourse channel on YouTube—hosted by none other than John and Hank Green of VlogBrothers fame—is all about creating fun and informational educational content. Hank’s series on animal cells is a great resource, and the fourth video in the series (Eukaryopolis!) gives you an overview of the endoplasmic reticulum.
Need to brush up on more than just the endoplasmic reticulum before tackling the AP Biology exam? Grabbing a workbook or textbook might be your best bet. Here’s a curated list of the best AP Biology books to help you study harder and smarter.
If you’re feeling overwhelmed by the AP Biology exam, you’re not alone. There are lots of students who find this test tricky! That’s why we’ve put together a complete AP Biology review guide. It breaks down all of the topics that might appear on the exam, so you can figure out exactly what you need to study. (It also includes some great study tips, too!)
The best way to figure out if you’re prepared for the AP Biology test is to take a practice exam. Here’s a list of every AP Biology practice test available. And the best news? They’re free!
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Ashley Sufflé Robinson has a Ph.D. in 19th Century English Literature. As a content writer for PrepScholar, Ashley is passionate about giving college-bound students the in-depth information they need to get into the school of their dreams.