Video: Eukaryotic Cell Structures

In this video, we will learn how to identify the key organelles in a eukaryotic cell, and describe their functions.

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Video Transcript

In this video, we will learn to identify the organelles of general eukaryotic cells as well as the organelles that are specific to plant cells. We’ll learn about the structures of these organelles and how they’re adapted to their function.

Humans like us and many other multicellular organisms are composed of many different types of cells that carry out many different functions. And each of these individual cells is itself kept alive and functioning by specialized subcellular structures called organelles. Each organelle is specifically adapted to its function. All the different types of eukaryotic cells actually possess the same types of organelles, which is evidence of our shared common ancestry. Let’s take a closer look at these different organelles, their structures, and their functions. Here, we have a diagram of a generalized animal cell, which will show us some of the structures that are common to eukaryotic cells.

All living cells possess a cell membrane. Perhaps, most importantly, the cell membrane controls what can enter and leave the cell. It also defines the boundary of the cell in a similar way to how your skin defines the boundary of your body. The cell membrane is flexible, self-repairing, and selectively permeable. It’s mainly made up of phospholipids, arranged in two sheets called a bilayer. Molecules of cholesterol helped to give the cell membrane of animal cells its structure. And different types of proteins do various jobs, such as moving materials in and out of the cell or sending and receiving signals.

The defining feature of the eukaryotic cell is the nucleus, which has the function of controling all of the various activities of the cell. The nucleus is surrounded by a double membrane, sometimes called the nuclear envelope or nuclear membrane. The nuclear envelope has openings called nuclear pores that allow larger molecules such as RNA to move in and out of the nucleus. The nucleus contains, protects, and regulates the expression of the genetic material, also called DNA, which is how the nucleus is able to control the activities of the cell. The nucleus is filled with a rich, jellylike substance called nucleoplasm.

Often visible within the nucleus is a dense region known as the nucleolus. The primary function of the nucleolus is to make ribosomes. The cytoplasm is the intracellular medium or the fluid that fills the inner space of the cell. The cytoplasm is made of water, proteins, ions, and nutrients, and it’s usually colorless. All of the other organelles and cellular structures are suspended within the cytoplasm. We often think of cytoplasm as a passive and inert substance. But most cellular activity takes place in the cytoplasm, and the volume of the cytoplasm helps to give the cell its structure. So its makeup must be strictly maintained and is an important part of homeostasis for the cell.

All throughout the cytoplasm, there’s a network of proteins called the cytoskeleton. The cytoskeleton gives the cell its shape, holds organelles in place, and directs their movements. It’s also important for the movement and migration of cells as well as cell division. The cytoskeleton consists of microfilaments, microtubules, and intermediate filaments. The mitochondria are organelles that supply the cell with energy by carrying out a process called cellular respiration. Each mitochondrion has a smooth outer membrane and a folded inner membrane. The folds of the inner membrane are called cristae, and the space within the folded inner membrane is commonly known as the matrix. Mitochondria also possess their own DNA, which is evidence that they were once separate prokaryotic organisms.

Ribosomes are tiny organelles that are not surrounded by their own membrane. They’re responsible for the translation of mRNA, also referred to as protein synthesis. Ribosomes are made up of rRNA or ribosomal RNA and proteins that are generated in the nucleolus. One ribosome is made up of two interlocking parts called the small and the large subunit. Eukaryotic ribosomes are referred to as 80S ribosomes because they’re larger than the 70S ribosomes found in prokaryotic cells. The endoplasmic reticulum is a continuous system of folded membranes that forms a network of interconnected spaces within the cell.

We further categorized the endoplasmic reticulum into rough and smooth. The rough endoplasmic reticulum is studded with ribosomes. It usually has a flatter shape and is more closely associated with the nucleus. The rough endoplasmic reticulum is responsible for folding proteins into their final shape. In contrast, the smooth endoplasmic reticulum is not studded with ribosomes. It usually has a more tubular shape and is responsible for the synthesis of certain types of lipids. The Golgi apparatus is a series of flattened membrane sacks. The Golgi apparatus receives finished materials from the endoplasmic reticulum; then it resorts and repackages them into vesicles for delivery to different parts of the cell.

One specialized type of vesicle made by the Golgi apparatus is called a lysosome. Lysosomes are responsible for breaking down and recycling cell materials. One lysosome can contain over 60 different types of enzymes and is usually filled with an acidic fluid. The last organelle present in our generalized eukaryotic animal cell is a vacuole, which is a membrane bound sack used for storage of materials. Different types of eukaryotic cells will have a different selection and quantity of organelles based on their function. So let’s look at a plant cell next.

Plant cells are also eukaryotic cells, so they contain many of the same organelles that we find in animal cells. Unlike animals, plants are stationary organisms. So plant cells have a rigid outer layer called a cell wall that gives them their shape and their structure. The cell wall of plants is made of cellulose, which makes it different from the cell walls of bacteria and fungi. Many plant cells also contain chloroplasts, which are the site of photosynthesis. Photosynthesis is how plants make glucose from sunlight, which is another adaptation that allows them to be stationary, since they don’t have to move around to find food.

The chloroplast possesses an outer membrane and an inner membrane. The liquid filling the inner membrane is called stroma. The chlorophyll that facilitates photosynthesis is stored in coin-shaped sacks called thylakoids. And these thylakoids are arranged in stacks, which are referred to as granum. The special characteristics of chloroplasts allow them to efficiently carry out photosynthesis. Chloroplasts also contain their own DNA, which is evidence that they were once separate prokaryotic organisms. Often, the largest feature in the plant cell is the large central vacuole. The central vacuole has two main functions; it’s responsible for storing water and nutrients.

Another function of the central vacuole is to fill the cell and maintain pressure on the cell wall so that the cell doesn’t lose its shape. Not only is each organelle adapted to its function, but the quantity and selection of organelles within a cell is adapted to the function of the cell. A highly active cell that uses a lot of energy, such as a muscle cell or a nerve cell, will have many mitochondria within it. In contrast, a less active cell that uses less energy will have fewer mitochondria, such as a skin cell. In a similar fashion, the top layer of cells within a leaf will have many chloroplasts in order to capture sunlight and carry out photosynthesis. In contrast, the cells in the root of the plant, which are not exposed to sunlight, will have no chloroplasts at all.

Now that we’ve learned about the structures within eukaryotic cells and how they’re adapted to their functions, let’s try a practice question.

The following is a list of eukaryotic organelle functions. (I) Processing and transporting proteins. (II) Coordinating activities of the cell, including cell division and protein synthesis. (III) Providing the site for the aerobic stages of respiration. (IV) Providing the site for the reactions of photosynthesis. (V) Storing genetic material as DNA. Which functions on the list provided are carried out by the nucleus? Which functions on the list provided are carried out by the chloroplast? Which functions on the list provided are carried out by the Golgi apparatus?

This question is asking us to match one or more of the five listed functions to the organelles in each of the questions. In order to answer this question, we’ll review the structure and function of the organelles listed, in order to make it easier to match them to the functions provided in the question. And since one of the questions mentions chloroplast, and I know that a chloroplast is an organelle that’s only found in a plant cell, I’ll use a plant cell as my example.

So let’s start with the nucleus. The nucleus has several functions. It stores and protects our DNA. It controls and directs the activities of the cells. And the nucleolus, which is this denser section in the middle, makes the rRNA that ribosomes are made out of. So now, let’s match up what we know with the functions in the list. The nucleus coordinates the activities of the cell, including cell division and protein synthesis. And it’s also storing genetic material as DNA.

Next, we’ll take a look at the chloroplast. The chloroplast is a green organelle within the plant cell, and it gets its color from a chemical called chlorophyll. Chlorophyll is necessary for the process of photosynthesis. And photosynthesis is when plant cells transfer the energy found in sunlight into the glucose molecule, which they then use to carry out cellular respiration and generate cellular energy. Now, we can take a look at our list. The function of the chloroplast is providing the site for the reactions of photosynthesis.

Finally, let’s take a look at the Golgi apparatus. The Golgi apparatus looks like a stack of flat membrane bound sacks. It performs the function of packaging and delivering proteins in structures called vesicles. It also makes a specialized vesicle called a lysosome, which is responsible for breaking down and recycling old cell materials. So now we’re ready to take a look at our list. And the function of the Golgi apparatus is processing and transporting proteins.

Next, let’s wrap up our lesson by reviewing what we’ve learned. In this video, we learned about the structures that you find in eukaryotic cells. We learned about how these specialized structures, called organelles, are adapted to their function. And we learned about how different cells may have a different variety and number of certain structures. For example, plant cells possess chloroplasts, a cell wall, and a large central vacuole, while animal cells do not.

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