Eukaryotic Cells

What is a eukaryotic cell?

There are four main types of eukaryotic cells: plant, animal, fungi and protozoan cells. In this article, we will mainly cover animal and plant cells. Unlike prokaryotes which do not have a nucleus, all eukaryotes have a nucleus.

Eukaryotic cell diagram

Eukaryotic cells are quite varied: for starters, there are four main types of eukaryotic cells, each with particular characteristics that make them different from the rest. If we focus only on animal cells, the variety just increases: neurones, muscle cells, and skin cells, are all part of the same main group but they are all extremely different in shape and the location and proportion of organelles.

However, we've included the general diagram for an animal and a plant eukaryotic cell to help you understand the main components of eukaryotic cells.

Fig. 1. Two types of eukaryotic cells: a plant and an animal cell, respectively. As you can see, although they have a lot of things in common (importantly, the nucleus), they also have some differentiating factors: plants have chloroplasts and a cell wall, while animal cells have centrosomes.

Eukaryotic cell structure

Eukaryotic cells are extremely different from one another. Depending on the type (animal, plant, fungal or protozoan cell) and the specific function, they can have different organelles, or a different distribution or proportion of them. However, there are some key components that are shared by all or most eukaryotic cells:

• Nucleus: The nucleus is a membrane-bound organelle that houses the cell's genetic material, the DNA. It serves as the "brain" of the cell, directing its activities and ensuring the proper functioning of the cell.

• Mitochondria: These organelles are known as the "powerhouses" of the cell because they generate the energy needed for cellular activities.

• The endomembrane system: from the nucleus to the plasma membrane, the membranes of the cell organelles are all connected. The nuclear membrane is directly connected to the endoplasmic reticulum (ER), involved in the synthesis, folding, and modification of proteins. The ER in turn connects with the Golgi apparatus by the exchange of vesicles, and the Golgi apparatus sends some vesicles to the plasma membrane too, to secrete substances or to regenerate parts of the plasma membrane.

• Ribosomes: ribosomes are the protein producers of the cells, and prokaryotes also have them. They are not membrane-bound.

• Peroxisomes: Peroxisomes are vesicles that contain enzymes that detoxify harmful substances and reactive oxygen species.

• Cytoskeleton: the cytoskeleton is a complex and interconnected protein structure that gives the cell structural support, helps with transporting molecules and vesicles around the cell and is needed for cell motility. Prokaryotes also have a cytoskeleton, but it's much less complex that the eukaryotic version.

• Cell wall: animal cells do not have a cell wall, but plant, fungal and protozoan cells do. In each case, they are made of a different substance. Plants' cell wall is made of cellulose, while fungal ones are made of chitin. The protozoan cell wall can be made of either molecule, and some protozoans have no cell wall at all.

Each type of eukaryotic cell can have a different combination of organelles or cellular structures, as represented in the following diagrams:

Fig.2. Animal cell example.

Fig. 3. Plant cell example.

Fig. 4. Protozoan cell example.

Fig. 5. Fungal cell example.

What are the differences between prokaryotic and eukaryotic cells?

As mentioned, the main differences between eukaryotic cells and prokaryotic cells are that eukaryotes have a nucleus. Instead of a nucleus, prokaryotes have loose chromosomes that contain DNA information that are floating in the cytoplasm.

Bacteria and other cells can also contain plasmids - small, circular DNA. Interestingly, these are separate from the main prokaryotic chromosome and will replicate independently. Almost like a mind of its own! Plasmids often provide a genetic advantage and rarely have essential genes - this is where antibiotic resistance can occur. In addition, cells can exchange these plasmids via bacterial conjugation. Prokaryotes are "smart" with their adaptations.

Below you will find a table showing the main differences between eukaryotic and prokaryotic cells, also known as the ultrastructure or the composition of eukaryotic cells.

Fig. 6. Prokaryotic cell. Can you spot the differences between a eukaryotic cell and a prokaryotic one? Apart from the most obvious structural differences, there are more. For example, the cell wall of bacteria is made of a different substance than that of plant cells.

Cell nucleus

Because the presence of the nucleus is the most important difference between eukaryotic and prokaryotic cells, we'll have a closer look at this crucial organelle.

Fig. 7. The structure of the nucleus. Note that the membrane has pores, which are important because they allow the exchange of nucleic acids and protein complexes from one side of the membrane to the other.

The parts of the nucleus are:

• The nuclear envelope or membrane is a double layer of plasma membrane surrounding the nucleus. It connects directly to the endoplasmic reticulum. It's a semipermeable membrane, so it only lets in certain substances.
• Nuclear pores act as a passageway for larger molecules, such as messenger RNA (mRNA). There are 3000 nuclear pores in a nucleus, each with an approximate diameter of 40 to 100 nm. Contrary to what the name might suggest, they are not holes in the membrane, but rather breaks in the plasma membrane filled with a protein complex that regulates what can come in or out of the nucleus.
• Nucleoplasm is similar to a cell's cytoplasm. It is a jelly-like liquid surrounding the nucleolus.
• The nucleolus is a special region of the nucleus where ribosomal RNA (rRNA) is produced. The nucleolus is also where ribosomes are assembled
• Chromatin is the less condensed form of DNA compared to chromosomes.

The nucleus is usually one of the most prominent features in eukaryotic cells. The vacuole in plants is usually bigger, but there are multiple stainings that are designed to detect the nucleus.

How big are eukaryotic cells?

The size of eukaryotic cells varies quite a bit. Eukaryotic cells are usually bigger than prokaryotic cells, ranging from 10–100 µm, making them up to 1000 times bigger than prokaryotic cells. When referencing cell size, we are referring to the diameter. Animal cells are usually up to 30 µm, while plant cells can reach 100 µm.

The shape of eukaryotic cells varies immensely. Generic animal cells are typically depicted as round. However, we know that the membrane around animal cells is fluid and mostly made of phospholipids, meaning that the shape of the animal cell is irregular, and usually adapted to its function: neurones and muscle cells have particular shapes to aid their role in the body.

On the other hand, a plant cell has a more restricted shape similar to a cube/rectangle due to the presence of a cell wall.

Examples of eukaryotic cells

The definition for eukaryotic cells (cells that have a defined nucleus) is so general, that as you can imagine there are plenty of examples of eukaryotic cells. We can use these examples to better understand the variability of eukaryotic cells, and how the function of a cell influences the location and presence of organelles. Here are some broad cell type categories to illustrate how cell shape can vary:

Fig. 8. Even though the generic animal cell is shown as a round cell, neurones and muscle cells, which are animal cells, have a completely different shape.

Specialised eukaryotic cells - muscle cell structure and function

Let's compare the types of muscle cells to explain how function conditions the structure and organelles present in a cell.

Muscle cells are, as the name indicates, cells that form the muscle fibres of our body. There are three types of muscle cells:

1. Skeletal muscle cells: these are the type of muscle cells that are responsible for voluntary movement and are attached to the bones of the skeleton. Skeletal muscle cells are long and cylindrical in shape and contain multiple nuclei. Skeletal cells are striated.

2. Smooth muscle cells: these muscle cells are found in the walls of internal organs, such as the stomach and intestines and are responsible for involuntary movement. Involuntary movement means that you don't realise or consciously order a part of your body to move, but it's still moving. For example, the intestines make wave-like movements to move the food down the digestive tract, known as peristalsis. Smooth muscle cells are spindle-shaped and contain a single nucleus.

3. Cardiac muscle cells: cardiac muscle (cardiomyocytes) cells are responsible for heart contraction and blood pumping. They are shorter and thicker than skeletal muscle cells and contain a single, central nucleus. Cardiomyocytes are capable of contracting independently, without the need for neuronal stimulation, although the contraction is still due to changes in membrane polarity. Cardiac muscle is also striated.

Fig. 9. Types of muscle cells and their key characteristics.

Even though they have a lot of differences, muscle cells also share some traits compared to other cell types. They are:

• Contractile: they can contract or become shorter.
• Excitable: they react to changes in membrane polarity.
• Extendable: they can be stretched.
• Elastic: they can return to their original shape and size.

However, their specific function (bone, involuntary or heart movement) conditions the cell's shape and structure.

Skeletal muscle cells are very long compared to other muscle cells because they need that larger length to have enough attachment to the bones they move and to generate the force to pull or push them to allow you to move. Because they are so large, they need several nuclei to swiftly coordinate throughout the cell and contract or relax the striated muscle.

Fig. 10. Skeletal muscle cell. Note the presence of multiple cell nuclei in the same fibre, and the lines following the length of the muscle cell. Source: Flickr.

Skeletal and cardiac muscle cells are called "striated" because under the microscope they appear to have stripes. This is because they have sarcomeres which are the basic contractile unit of these cells. Sarcomeres are highly organised protein complexes made of myosin and actin that lengthen and shorten to contract or elongate the muscle cell. When this happens coordinatedly with the cells of a whole muscle, the muscle contracts or relaxes. Sarcomeres are crucial when strong and fast contractions are necessary. Myoglobin is also essential in these two types of cells due to the rate of contraction that is sometimes needed. Myoglobin is an oxygen-bound protein that helps deliver oxygen to the mitochondria within the cells and thus avoids oxygen deprivation when muscles are generating a lot of energy.

Because cardiomyocytes are not as big as skeletal muscle cells, they can have a single nucleus. It is essential that they coordinate perfectly to avoid any issues with the heart's pumping rate, and this is achieved more easily with one nucleus in this case.

Fig. 11. Cardiac muscle cells. Notice the difference between the skeletal fibres and the cardiomyocytes. Cardiac muscle cells only have one nucleus, although they are still striated. Source: Flickr.

Smooth muscle cells, however, don't have sarcomeres, and thus, do not have the striated look under the microscope. They still have an arrangement of filaments that allow them to contract, but their distribution is different. They also do not have myoglobin. Therefore, the contraction speed of smooth muscle is much slower.

Fig. 12. Smooth muscle cells. You can clearly see in the image the spindle shape of the cells, as well as that they only have one nucleus and no stripes. Source: Flickr.

We hope that you now understand clearly what a eukaryotic cell is, and how function always determines structure, even at the very basic of biological levels!