Lesson Explainer: Eukaryotic Cell Structure

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

Every plant, animal, fungus, and protist on the planet is made of at least one eukaryotic cell. While the cells of these organisms are all very different, the majority of them have certain basic features in common. The small structures found within cells (or subcellular structures) that carry out specific functions are called organelles. The types and abundance of organelles that a cell possesses are the first clue as to what type of cell it is and what it can do.

Definition: Organelle

An organelle is a subcellular structure that carries out a specific function.

Cells can be sorted into two large groups: eukaryotic and prokaryotic. Figure 1 below shows a simple diagram of each.

The most notable feature of a eukaryotic cell is the nucleus. The possession of a nucleus is the main distinction between prokaryotic cells, such as bacteria, and eukaryotic cells, such as the trillions of cells that make up your body. In addition to this nucleus, your cells possess many other structures that have other important jobs. We call cells like the ones your body is made of animal cells. Let’s take a look at the different organelles in a typical animal cell, as shown in Figure 2.

The cell (plasma) membrane is the outer layer of the cell. It forms the boundary that distinguishes the cell from its surroundings. The cell membrane is made of two layers of molecules called phospholipids, which is why we call this type of membrane a phospholipid bilayer since there are two layers of molecules (bi- means “two”). We can also see some different types of other molecules like proteins embedded in the plasma membrane that also play an essential role in the membrane’s main function. The structure of the cell membrane is shown in Figure 3. The phospholipid bilayer is selectively permeable, meaning that some things are allowed to easily pass through while others are not.

Key Term: Cell Membrane

The cell membrane is composed of a phospholipid bilayer and embedded molecules and separates the internal and external environment of the cell.

The cytoplasm is the jelly-like fluid that fills the cell. It is made primarily of water, along with proteins, ions, and nutrients. Rather than being an organelle itself, the cytoplasm is the fluid that all the organelles are suspended in. Although we often think of the cytoplasm as inert and passive, many cellular activities and important chemical reactions take place within the cytoplasm. The volume of the cytoplasm also helps give the cell its structure. The cytoplasm is like the air inside a balloon: it fills the cell to give it a three-dimensional shape. The cytoplasm and the nucleus of the cell are sometimes collectively referred to as the protoplasm.

Key Term: Cytoplasm

The cytoplasm is a fluid that fills the internal space of the cell and is the site of many chemical reactions.

The cytoskeleton is a network of proteins throughout the cytoplasm. The term cyto- means “cell,” so the cytoskeleton literally translates to the cell’s skeleton. The cytoskeleton is made of microfilaments, intermediate filaments, and microtubules. These proteins hold other organelles in place so that they do not just randomly float around in the cytoplasm. You can see a simple outline of this in Figure 4. The cytoskeleton also works like tracks that the organelles can use to move from one place to another. The cytoskeleton is what allows certain cells, for example, species that belong to a genus of single-celled eukaryotes called Amoeba, to move around on their own. It also plays an important role in cell division.

Key Term: Cytoskeleton

The cytoskeleton is a network of protein filaments within the cell that positions the organelles, provides structural support, and allows some cells to move.

The nucleus contains and protects the genetic material, or DNA, which is found in long strands wrapped around proteins. These molecules of DNA and their associated proteins are called chromatin. The nucleus also controls the expression of that DNA, which is how it controls the activities of the cell. A basic diagram of the nucleus is provided in Figure 5.

Key Term: Nucleus

The nucleus is a key component of eukaryotic cells and is the organelle that stores and protects the DNA.

The nucleus has a double membrane, meaning that there are actually two phospholipid bilayers instead of just one, as you can see illustrated in Figure 5.

The nuclear membrane, sometimes called the nuclear envelope, has special openings called nuclear pores. These nuclear pores allow some large molecules, like RNA and proteins, to pass through, but not the chromatin. The nucleus contains its own special filling called nucleoplasm, which is very similar to the cytoplasm. It also has a scaffolding of structural proteins called the nuclear matrix, which is very similar to the cytoskeleton. At the center of the nucleus, there is a dense region called the nucleolus. This nucleolus makes the rRNA, or ribosomal RNA, which is the major component of ribosomes.

Example 1: Identifying the Parts of the Nucleus in an Electron Micrograph

The micrograph shows a cell viewed under a transmission electron microscope. A circular nucleus is visible.

Which of the following is not part of the structure of a nucleus?

1. Nuclear envelope
2. Nucleolus
3. Nucleoplasm
4. Nuclear pores
5. Nuclear cristae

Answer

The nucleus is the defining feature of the eukaryotic cell. It distinguishes eukaryotic cells from prokaryotic cells, which do not possess a nucleus.

The primary function of the nucleus is to store and protect the DNA, which is coiled around proteins to form long strands called chromatin. The nucleus is surrounded by a nuclear envelope, which is a double membrane, meaning that there are two phospholipid bilayers instead of just one. This double membrane possesses special openings called nuclear pores, which allow certain molecules to pass through easily. At the center of the nucleus, there is a dense region called the nucleolus, which is responsible for making the rRNA, or ribosomal RNA, which ribosomes are mainly made of. The nucleus is filled with a rich liquid substance called the nucleoplasm and scaffolded by a network of proteins known as the nuclear matrix.

The term cristae actually refers to a fold in a phospholipid membrane that creates smaller compartments, like those we see in the mitochondria. The nucleus does not possess cristae.

Using this information, we can conclude that the structure that is not part of the nucleus is nuclear cristae.

Ribosomes are tiny structures that are responsible for making proteins, which can be found freely in the cytoplasm or attached to the rough endoplasmic reticulum. The ribosomes translate messenger RNA (mRNA), which carries the genetic code from the DNA in the nucleus, into a string of amino acids called a polypeptide chain. This polypeptide usually needs to be modified by other organelles, such as the endoplasmic reticulum and Golgi apparatus, to become a functional protein, for instance, an enzyme. Ribosomes consist of two ribosomal subunits, one large and one small, as seen in Figure 6. These subunits are made of rRNA, or ribosomal RNA, which is made in the nucleolus, as well as proteins.

Ribosomes are not surrounded by a phospholipid bilayer, so they are referred to as non-membrane-bound (nonmembranous) organelles. In fact, this made some scientists argue that they cannot be considered organelles at all.

Unlike other membrane-bound (membranous) organelles, ribosomes can be found in both eukaryotic and prokaryotic cells. The ribosomes of eukaryotic cells are larger than those of prokaryotic cells. S is the unit used to measure ribosomes. Eukaryotic ribosomes are referred to as 80 S ribosomes, whereas prokaryotic ribosomes are called 70 S ribosomes.

Key Term: Ribosomes

Ribosomes are nonmembranous structures that act as the site of protein synthesis in the cell.

Example 2: Recalling an Organelle from a Description

State the eukaryotic organelle being described: This organelle contains RNA and is the site of protein synthesis.

Answer

This question provides a description of an organelle and asks us to recall the name of the organelle that fits this description. The question specifies that the answer will be a eukaryotic organelle, which means it is one of the many organelles that we can find in a eukaryotic cell. Eukaryotic cells generally possess many of the organelles that prokaryotic cells do, along with several more complex, membrane-bound organelles.

The key clue here is that this organelle is the site of protein synthesis. The organelle responsible for the synthesis of proteins in all cells, prokaryotic or eukaryotic, is the ribosome. Ribosomes are tiny organelles that are composed of two subunits. These subunits are made of rRNA and proteins. The ribosomes translate the genetic code in a molecule of mRNA into a polypeptide, or a strand of amino acids. This polypeptide is then eventually folded into a functional protein.

This means that the organelle that contains RNA and is the site of protein synthesis is the ribosome.

The endoplasmic reticulum, often shortened to just ER, is an interconnected network of folded membranes found throughout the eukaryotic cell. The ER plays a role in the formation and transport of proteins and lipids and is further divided into two types, as shown by Figure 7: rough and smooth.

The outer surface of the rough endoplasmic reticulum has many ribosomes attached to it, so you can tell it plays a role in protein synthesis. This is also what gives the rough ER its lumpy shape and name. The passages of the rough ER generally have a flatter shape and are joined to the outer membrane of the nucleus. The main function of the rough ER is to fold proteins into their final shape.

The smooth endoplasmic reticulum does not have ribosomes on its surface, which is why it is called smooth. Its passages generally have a more tubular shape and are generally found farther from the nucleus. The smooth ER primarily plays a role in the synthesis of lipids and is also involved in converting toxins into less toxic compounds that can then be excreted.

Key Term: Rough Endoplasmic Reticulum

The rough endoplasmic reticulum is a series of folded membranes, or flattened sacs, that is covered with ribosomes and is associated with the production of proteins.

Key Term: Smooth Endoplasmic Reticulum

The smooth endoplasmic reticulum is a series of tube-like structures with folded membranes that is not covered with ribosomes and is associated with the production of lipids.

The Golgi apparatus, also called Golgi body, is a series of flattened membrane sacs called cisternae, as shown in Figure 8. It functions to package the correct combinations of proteins, lipids, and other chemicals and deliver them to the areas of the cell where they are needed. For this reason, the Golgi apparatus is sometimes even referred to as the post office of the cell. In plant cells, there are smaller dispersed arrays of Golgi-type vesicles called dictyosomes. Figure 8 below shows how the endoplasmic reticulum and Golgi apparatus interact in a typical animal cell.

Proteins and lipids are passed from the endoplasmic reticulum to the Golgi apparatus in transport vesicles. A vesicle is a small, membrane-wrapped package of materials within the cell. There, they fuse with and move through various layers of the Golgi apparatus from one end to the other. Finally, they are packaged into new secretory vesicles for delivery of materials out of the cell by exocytosis, or to form lysosomes.

Key Term: Golgi Apparatus (Golgi Body)

The Golgi apparatus packages lipids or proteins received from the endoplasmic reticulum and delivers them throughout the cell.

Lysosomes are specialized, membrane-bound vesicles that are made by the Golgi apparatus. They function to break down and recycle cell materials. One lysosome can contain more than 60 different types of digestive enzymes, and the fluid inside is usually quite acidic. The enzymes and acid work together to break different compounds down into components that can then be once again used by the cell.

Key Term: Lysosomes

Lysosomes are specialized vesicles filled with enzymes that break down and recycle old cellular structures or components.

Mitochondria (singular: mitochondrion) are the main site of cellular respiration in the eukaryotic cell. The mitochondria are responsible for converting glucose into usable cellular energy in the form of ATP. For this reason, the mitochondria are often referred to as the powerhouse of the cell. Each mitochondrion possesses two layers of membranes, as you can see in Figure 9. It has a smooth outer membrane and a folded inner membrane. The folds of the inner membrane are called cristae, and these folds increase the surface area available for the respiration reactions to take place. The space within the folded inner membrane is known as the matrix.

Key Term: Mitochondria (Singular: Mitochondrion)

Mitochondria are the main site of cellular respiration within eukaryotic cells.

The cell membrane, cytoplasm, cytoskeleton, nucleus, rough and smooth endoplasmic reticulum, Golgi apparatus, and mitochondria can be found in most eukaryotic cells. This includes animal and plant cells.

However, plants are different from animals. They have different characteristics. For example, they make their own nutrients and are stationary. This means that the cells of plants have some features that are different from those of animals cells, which support these different characteristics.

Let’s have a look at some organelles found in plant cells but not in animal cells.

The cell wall is a rigid structure surrounding the cell membrane, as shown in Figure 10. It is a tough outer layer that gives the plant cell its shape. The cell wall of plant cells is mostly made of a carbohydrate called cellulose. The cell wall also provides support for the plant. Since plants are stationary organisms and do not possess skeletons like some animals, their rigid cell walls help keep them upright and allow them to point their leaves toward the Sun.

Key Term: Cell Wall

The cell wall is a rigid outer layer that provides structural support to the plant cell.

Example 3: Identifying the Functions of Eukaryotic Organelles

The following is a list of eukaryotic organelle functions.

1. Synthesizing and transporting lipids
2. Providing mechanical strength for a plant cell
3. Providing the site for the aerobic stages of respiration
4. Synthesizing proteins
5. Maintaining the shape and structure of a plant cell

1. Which functions in the list provided are carried out by the smooth endoplasmic reticulum?
2. Which functions in the list provided are carried out by the mitochondria?
3. Which functions in the list provided are carried out by the cell wall?

Answer

This question lists several different functions, or jobs, of cell organelles and asks us to match these functions to the organelles in each question. Our answer may be one or more than one of the listed functions.

In order to answer this question, it may be most effective to determine which organelle is described by each of the listed functions and then use that information to answer our three questions.

Synthesizing and transporting lipids is the job of the smooth endoplasmic reticulum. The endoplasmic reticulum is a series of folded membranes that form a network of interconnected compartments within the cell. The rough endoplasmic reticulum is studded all over with ribosomes and is associated with the production and transport of proteins. This is easy to remember because the ribosomes on the rough endoplasmic reticulum are the site of protein synthesis. The smooth endoplasmic reticulum, on the other hand, is responsible for the production and transport of lipids.

Providing mechanical strength for a plant cell is the job of the cell wall. The cell wall is a thick layer that surrounds the cell membrane of the plant cell. It is made of cellulose, which makes it very strong and rigid. The cell walls of plant cells are joined closely together, giving the plant tissue its overall structure.

Providing a site for the aerobic stages of respiration is the job of the mitochondria. The term aerobic means that a process needs oxygen in order to occur. Mitochondria use oxygen and glucose to make ATP, in a process called cellular respiration. ATP is a molecule that stores cellular energy to be used by the cell to fuel other vital chemical reactions. Cellular respiration generates carbon dioxide and water as by-products.

Synthesizing proteins is the job of the ribosomes. Ribosomes are tiny structures made of two subunits, which translate mRNA, or messenger RNA, into a chain of amino acids called a polypeptide. This polypeptide is then usually shaped and folded into a functional protein.

Maintaining the shape and structure of the plant cell is the job of the cell wall and the cytoskeleton. It can also be considered a function of the large central vacuole found within a plant cell. The cell wall gives the cell its shape, and the fluid-filled vacuole applies pressure on the inside of the flexible cell membrane, which presses it outward, filling the space within the rigid cell wall.

Using this information regarding the functions of the organelles described, we can match the functions to the organelles in the questions.

Part 1

The smooth endoplasmic reticulum synthesizes and transports lipids, or option I only.

Part 2

The mitochondria provide a site for the aerobic stages of respiration, or option III only.

Part 3

The cell wall provides mechanical strength and gives the plant cell its shape and structure, or both options II and V.

Plastids are found in plant cells but not in animal cells. They possess a double membrane, so they are surrounded by two phospholipid bilayers instead of one. There are three main types of plastids, classified according to the type of pigment they possess: chromoplasts, leucoplasts, and chloroplasts. Chromoplasts are plastids that make and store pigments, such as the red, yellow, and orange pigments that give fruits and flowers their bright colors. Leucoplasts are white or colorless plastids that possess no pigments and store materials like starch and fat.

Chloroplasts are plastids that contain the green pigment chlorophyll. Chloroplasts are the site of photosynthesis, in which plant cells use the energy in sunlight to convert carbon dioxide and water into glucose and oxygen. This glucose is later used in cellular respiration by the mitochondria.

Chloroplasts, shown in Figure 11, have an outer and an inner membrane. The fluid inside the inner membrane is called the stroma. Within the stroma, there are stacks of coin-shaped sacs. These sacs are filled with chlorophyll and are called thylakoids. These stacks of thylakoids are called grana (singular: granum).

Key Term: Chloroplasts

Chloroplasts are the site of photosynthesis within the plant cell.

Plant cells possess a large central vacuole. Under a microscope, this vacuole is considered a defining feature of plant cells. It stores water and some other materials in a mixture that is sometimes called cell sap. It helps give the cell its shape by maintaining pressure against the inside of the cell wall. It also fills up the cell, pushing the contents of the cytoplasm toward the outside, helping the chloroplasts access the sunlight they need to carry out photosynthesis. Animal cells also have vacuoles, but they are smaller and larger in number and are usually involved in metabolic or cellular transport processes.

Key Term: Large (Central) Vacuole

The large vacuole stores cell sap and provides the plant cell with its structure.

There are also some cases of organelles being found in animal cells but not plant cells. Centrioles are non-membrane-bound organelles that are found in a region of the cell near the nucleus called the centrosome, a region that is not present in plant cells. We can see a longitudinal and cross-sectional view of a centriole in a typical animal cell in Figure 12.

Each centriole is made up of nine groups of microtubules arranged into triplets (or threes), as you can see in Figure 12. There are connecting fibers between each of the nine triplets of microtubules.

During cell division, filaments called spindle fibers extend from the centrioles and help pull the replicated genetic material to opposite ends of the cell. Interestingly, mature nerve cells are an example of a type of animal cell that does not contain centrosomes.

The organelles within a cell both fulfill the needs of the cell and also determine what the cell can do. While all eukaryotic cells possess mitochondria, animal cells that consume more energy, such as muscle cells, have more mitochondria than those that need less. In plants, cells that are not exposed to light, such as root cells, do not possess any chloroplasts. You can tell a lot about the life of a cell by looking closely at its organelles. A brief summary of the types of cells that we may expect to have large numbers of certain organelles is outlined in Table 1.

Table 1: A table summarizing where certain organelles are found in large amounts in the body.

The diagram below summarizes the types of cellular structures we have discussed in this explainer.

Let’s now review what we have learned about the structure of eukaryotic cells in this explainer.

Key Points

• The organelles of typical eukaryotic cells are the cell membrane, nucleus, cytoplasm, ribosomes, endoplasmic reticulum (rough and smooth), Golgi apparatus, lysosomes, mitochondria, and cytoskeleton.
• The organelles specific to plant cells are the large vacuole, cell wall, and chloroplasts.
• Each organelle possesses a structure that is directly related to its function.