Cell Organelles - Online Tutor, Practice Problems & Exam Prep

Cells contain organelles, which are specialized structures with distinct functions. Eukaryotic cells have membrane-bound organelles, including the nucleus, endoplasmic reticulum (ER), Golgi apparatus, and lysosomes. The nucleus stores DNA and regulates material passage via nuclear pores. The rough ER aids in protein folding, while the smooth ER detoxifies and synthesizes lipids. The Golgi apparatus modifies and packages proteins for secretion. Ribosomes, essential for translation, consist of large and small subunits, differing in size between eukaryotes and prokaryotes. The cytoskeleton, composed of actin filaments, intermediate filaments, and microtubules, supports cell structure and transport.

Eukaryotic Endomembrane System

We're going to talk about some relevant cellular organelles. So all cells have organelles, and that includes eukaryotic and prokaryotic cells, and organelles can be thought of as the organs of a cell, and all they really are are subcellular structures or components with specialized functions. As you guys already know, eukaryotic cells have several membrane-bound organelles, whereas prokaryotic cells do not. These membrane-bound organelles can be grouped together as part of a system known as the endomembrane system. The endomembrane system functions include modifying, transporting, and secreting cellular materials. Recall that secreting just means to release into the environment.

Let's take a look at our example below. At the bottom of the diagram, there's a plasma membrane separating the outside of the cell from the inside of the cell. Inside the cell, we have this big purple structure that everybody knows is the nucleus. Inside the nucleus, we have the DNA. The functions of the nucleus include storing and protecting the DNA. The nucleus has a membrane known as the nuclear envelope, and the nuclear envelope has these structures in it called nuclear pores that control the passage of materials into and out of the nucleus.

Extending off of the nuclear envelope is the endoplasmic reticulum, which can be abbreviated as ER. Recall there are actually two types of ERs. The first is the rough ER, which is externally studded with ribosomes, and the ribosomes are shown as these brown dots. There are a bunch of brown dots on the exterior surface of the rough ER. The rough ER functions include assisting a protein take its 3D shape, as well as modifying the protein. The other type of ER is the smooth ER, which does not have ribosomes studded on its surface. The smooth ER functions include detoxifying a cell from alcohols and chemicals, as well as making carbohydrates and lipids.

Over here in this teal colored structure, we have the Golgi apparatus, which functions include modifying and tagging proteins and lipids that it receives, as well as packaging and shipping these proteins and lipids to their final destinations either within the cell or out to the plasma membrane so that those materials are secreted. That leads us to our next organelle, which is the plasma membrane. We've talked about this organelle in our previous videos, and we know that it acts as a barrier and controls the passage of materials into and out of the cell.

We also have these structures called vesicles, and vesicles are essentially membrane bubbles that store and transport materials. You can see there are a bunch of vesicles throughout the cell. Vesicles have the ability to bud off, so we can see over here, there's a vesicle budding off from the ER, and the vesicles also have the ability to fuse. Here, we have a vesicle fusing with the Golgi apparatus and a vesicle over here fusing with the plasma membrane. Some vesicles are specialized vesicles known as lysosomes. Over here, we have a lysosome shown in purple, and these are vesicles that contain digestive enzymes that break down foreign particles or recycle materials within the cell.

These organelles could show up at different points throughout our biochemistry course, so it's good to touch up on them a little bit. Now, in our next video, we're going to talk about ribosomes, so I'll see you guys in that video.

Ribosomes

So you guys already know that all cells contain ribosomes. And ribosomes are structures that are directly involved in the process of translation. And recall that translation is simply the process of protein synthesis or the process of building a protein using RNA. And a single ribosome actually has 2 different subunits. The first is a large ribosomal subunit and the second is a small ribosomal subunit. And ribosomes are actually a big mixture of several components. They are a mixture of several different ribosomal RNA molecules or rRNA molecules as well as a mixture of several different protein molecules. And we already know that eukaryotic and prokaryotic ribosomes differ. Eukaryotic ribosomes are larger 80S ribosomes with a 60S large subunit and a 40S small subunit, and prokaryotic ribosomes are smaller 70S ribosomes with a 50S large subunit and a 30S small subunit. So you might be thinking, how am I supposed to memorize all these subunits? And I could tell you what helps me is to list the multiples of 10 from 30 up until 80. And so when we do that, notice that the first two that are listed here are the small subunits, the next 2 that are listed are the completely intact ribosomes. And so for the eukaryotic, components, they're always the larger number in each set, and the prokaryotic is always the smaller number in each set.

And so, if we look at our example below, what we'll see is that ribosomes are indeed involved in the process of translation. There's a protein being synthesized using RNA as a template. And so when we look at our block over here on the right, we see that we have the eukaryotic ribosome, which again, consists of 2 different subunits, a large and a small subunit. And when they're complexed together, when it's completely intact, it creates an eighty S subunit. When the large is separate and independent, it's a 60 S subunit. And when the small is independent and separate, it's a 40 S subunit. Now, for the prokaryotic ribosome, we have the same deal. We have a large and a small subunit. Now overall, this ribosome is much smaller. So when it's completely intact, it's a 70S ribosomal subunit. When the large is separated out, it's a 50S subunit. And when the small is separated out, it's a 30S subunit. And so it's important to memorize these subunits because they're going to come into play later on in our course. And so in the next video, we're going to talk about the cell's cytoskeleton. So I'll see you guys in that video.

Cytoskeleton

In this video, we're going to do a quick recap on the cytoskeleton. Recall that it consists of actin and intermediate filaments as well as microtubules. You can think of the cytoskeleton as the skeleton predominant component of specific cell structures, such as flagella and cilia, that allow a cell to move through its environment. It's also important for the transportation of materials inside the cell. Because it can respond to specific factors on the outside of the cell, it can be involved in signaling. Notice below in the diagram, we have the actin filaments down below, which are also known as microfilaments and these are the smallest of the three. We have intermediate filaments up above here, which are intermediate in size. We have microtubules or tubulin, which is shown in the middle here, and they are the largest of the three. Notice that the intermediate filaments are projected throughout the entire cell and the microfilaments are usually found on the perimeter of the cell. We can see that over here in this diagram as well, where the intermediate filaments are projected throughout the entire cell, the microfilaments or the actin are found on the perimeter of the cell, and the microtubules or the tubulin are found projected through specific points of the cell. Recall that the microtubules are very important for nuclear division of eukaryotes or mitosis. Over here, what we have is an image that shows you what these components look like under a microscope. Scientists can stain these different components, and the blue represents the nucleus, the green represents microtubules, and the red represents intermediate filaments. The last thing I want to leave you guys with is that these orange structures over here are the mitochondria. Mitochondria and chloroplasts are both very relevant organelles, but we're going to talk about them in some of our later videos when we talk about the topic of the endosymbiotic theory. This concludes our lesson on the cytoskeleton, and I'll see you guys in the practice videos.

Which of the following contains an incorrect match of the organelle function/description?

• A. Ribosomes: conduct the process of translation
• B. Rough Endoplasmic Reticulum: lipid synthesis
• C. Smooth Endoplasmic Reticulum: alcohol/chemical detoxification
• D. Lysosome: specialized vesicles with digestive enzymes

Which of the following contains a correct match?

• A. 80S: Eukaryotic large ribosomal subunit
• B. 60S: Prokaryotic large ribosomal subunit
• C. 40S: Eukaryotic small ribosomal subunit
• D. 70S: Eukaryotic large ribosomal subunit

Here’s what students ask on this topic:

What are the main functions of the endoplasmic reticulum (ER) in a cell?

The endoplasmic reticulum (ER) is a crucial organelle in eukaryotic cells, consisting of two types: rough ER and smooth ER. The rough ER is studded with ribosomes, which gives it a 'rough' appearance. Its main functions include assisting in the folding and modification of proteins. The smooth ER, on the other hand, lacks ribosomes and is involved in the synthesis of lipids and carbohydrates, as well as detoxifying the cell from harmful substances like alcohol and chemicals. Both types of ER play essential roles in maintaining cellular function and homeostasis.

How do ribosomes differ between eukaryotic and prokaryotic cells?

Ribosomes in eukaryotic and prokaryotic cells differ primarily in size and composition. Eukaryotic ribosomes are larger, with an 80S size, consisting of a 60S large subunit and a 40S small subunit. Prokaryotic ribosomes are smaller, with a 70S size, comprising a 50S large subunit and a 30S small subunit. These differences are significant because they affect the process of protein synthesis and are also targeted by certain antibiotics that can inhibit bacterial ribosomes without affecting eukaryotic ribosomes.

What is the role of the Golgi apparatus in a cell?

The Golgi apparatus is a vital organelle in eukaryotic cells responsible for modifying, tagging, packaging, and shipping proteins and lipids. It receives these molecules from the endoplasmic reticulum (ER) and processes them to ensure they are correctly folded and functional. The Golgi apparatus then sorts and packages these molecules into vesicles, which are transported to their final destinations either within the cell or to the plasma membrane for secretion. This organelle is essential for maintaining cellular organization and function.

What are the components of the cytoskeleton, and what are their functions?

The cytoskeleton is composed of three main types of filaments: actin filaments (microfilaments), intermediate filaments, and microtubules. Actin filaments are the smallest and are primarily found on the cell's perimeter, providing structural support and enabling cell movement. Intermediate filaments are of medium size and are distributed throughout the cell, offering mechanical strength. Microtubules are the largest and are crucial for cell division (mitosis) and intracellular transport. Together, these components maintain cell shape, enable movement, and facilitate the transport of materials within the cell.

What is the function of lysosomes in a cell?

Lysosomes are specialized vesicles within eukaryotic cells that contain digestive enzymes. Their primary function is to break down and recycle cellular waste, foreign particles, and damaged organelles. By digesting these materials, lysosomes help maintain cellular health and homeostasis. They can also play a role in programmed cell death (apoptosis) by releasing their enzymes into the cytoplasm, leading to the controlled destruction of the cell.