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What Is a Cell Made Of?

Last Updated: 29.10.20

 

We have prepared an informative article that can teach you all that you need to learn about cells so that you can put your digital dissecting microscopes to good use. You can also take a quick look at some of the other articles we have available and learn more about compound microscopes with camera and many other instruments.

What is a cell and what is it made of?

According to the British Society for Cell Biology, a cell is the basic unit of life as we know it, namely the smallest unit that is capable of reproducing independently. The name “cell” is older than you might imagine, as it was first suggested by Robert Hooke back in 1665 and it comes from the Latin ‘cella’ which means chamber or storeroom.

He came up with this name after he used a very early version of a microscope to look at a piece of cork. All living things, be them animal or plants are made of these building blocks. When they work in concert with one another they can create complex living things such as animals and humans, which are multicellular organisms.

No matter the type of organism, be it multicellular or unicellular, cells are very small and impossible to see without an optical microscope. While cells share many common features, they look very different depending on the roles they play.

Take for example nerve cells – they have long and thin extensions that can reach for meters in order to transmit signals as quickly as possible. On the other hand, brick-shaped plant cells have a rigid outer layer that provides the structural support that plants such as trees require.

If you are wondering about the composition of these building blocks, in the case of a human cell, two-thirds of it is water. The rest of the cell is a mixture of lipids, proteins, carbohydrates, enzymes, and other molecules.

If you are looking to find even more about the structure of a cell, we will explore the main components of a cell, such as the membrane, cell wall and nucleus, together with some of the other lesser-known components below.

 

Cell membrane and cytoplasm

The cell membrane, also known as the plasma membrane, is a structure that separates the space inside the cell from the outside of it. Both plant and animal cells have this component which surrounds and protect the cytoplasm.

The cytoplasm is a gel-like fluid inside the cell and it is the medium for chemical reactions. The functions for cell expansion, replication and growth are done in this structure of the cell. Inside the cytoplasm, materials move by a process known as diffusion.

Now back to the cell membrane, this structure is composed of a double layer of special lipids that are called phospholipids. They prevent water-loving (hydrophilic) substances from entering or escaping the cells. Furthermore, the cell membrane is also studded with proteins which serve various functions.

Some of the proteins can determine what substances are allowed and not allowed to cross the membrane, while others function as simple markers to help identify the cell as part of the same organism or to identify it as foreign. Some also serve the role of communicators by sending and then receiving signals from any neighboring cells and the environment.

 

Cell wall

The cell wall is a non-living and rigid layer that is found on the outside of the cell membrane and that surrounds the cell entirely. Plants, fungi, and bacteria all have cell walls and in plants, the wall is composed of cellulose. In the case of humans and animals, their cells do not have a cell wall, just cell membranes.

The main function of a cell wall is to form a framework that can prevent over expansions. Other roles include providing mechanical strength and support to control the direction of cell growth, help regulate growth and diffusion, and help the cells communicate with one another via the channels found between plant cell walls.

 

Nucleus and nucleolus

The nucleus is the control center and there you can find the deoxyribonucleic acid (the famous DNA), which is the genetic material of the cell. There is also the nucleolus, a region found in the nucleus that is dense in ribonucleic acid (RNA). The nucleus is the brain of the cell, so to speak, and it will determine how the cell will eat, move, function and reproduce.

Despite its name, the nucleus is not always in the center of the cell. It can be easily observed as a big dark spot found somewhere in the middle of the cytoplasm. You can’t find it near the edge of a cell since it would be too dangerous of a place for it to reside.

Even if the nucleus is an important part of a cell, not all will have one. If a cell does not have a defined nucleus, the DNA will most likely be found floating around the cell in a region known as the nucleoid. In this sense, cells are categorized into eukaryotic (that have a defined nucleus) and prokaryotic (that don’t have a defined nucleus).

What other components are there?

We briefly mentioned DNA and RNA – these are nucleic acids that help express the genetic code of the cell. DNA contains all the information needed to build and maintain the cell, while RNA acts as a messenger and carries instructions from the DNA.

While nucleic acids are very important, they are not the only ones responsible for the expression and preservation of genetic material and that’s why special proteins called enzymes are used. With the help of proteins, the cell can replicate the genome and accomplish cell division.

Carbohydrates are another type of organic molecules. They are classified into simple and complex carbohydrates, and the former are used for the cell’s immediate energy demands while the latter serve as intracellular energy stores.

Finally, the lipids or the fat molecules are the components of the cell membranes and other structures that are involved in energy storage, and they help relay signals to a cell’s interior.

When someone asks the question ‘what are cells made of?’, it is easy to answer using the simple explanation. However, understanding cells and how they work is not just about theory and remembering some terms that are more or less academic. There is a more interesting way to look at cells. 

If you think of the universe and its complexity, you could say that we are a small part of it, just like a cell is a small part of the human body. Think of the body as the universe. From this perspective, the cell is like a person that has a body, and all bodies have organs. 

All of these organs have a purpose and there are some parts of the cell that act just like organs. They are called organelles. We have already talked about the plasma membrane, which is similar to the skin and the nucleus, which is similar to the human brain. But there are other components that have specific roles and we are going to analyze them as well.

It is an interesting exercise to analyze different types of cells under the microscope and see the differences between them and also the similarities. Analyzing how a cell works and what happens inside of it can be very interesting, but watching how they interact with one another is even better.

 

Mitochondria

‘Mitochondria’ is actually the plural of ‘mitochondrion’ and you have probably heard about it on the internet as the powerhouse of the cell. This is because the mitochondria have the capability of producing energy.

In our cells, we have a lot of mitochondria, but their quantity differs according to the type of tissue that the cell is a part of. For example, the liver cells are 20 percent made of mitochondria while the heart cells are up to 40 percent made of mitochondria. This is pretty interesting and, using a microscope, you can differentiate one from the other.

But how did the mitochondria become the powerhouse of the cell? Well, about 2 billion years ago, the organisms on Earth were made of single cells and some of them were aerobe while others were anaerobe. The anaerobe ones were harmed by the oxygen in the atmosphere and, at some point, to survive, they consumed an aerobic cell. 

The aerobic cell, which we now know as the mitochondrion, survived and because it could use oxygen to provide energy for the anaerobic cell, they both lived happily ever after. This is why, if you look at mitochondria through the microscope, you will see that they have an inner membrane and an outer one, similar to an actual cell. 

However, it is considered an organelle because the cell has evolved over time. The inner membrane is responsible for the production of ATP which is known as a molecule that is the fuel of the processes happening in our bodies.

If you decide to study a sperm cell and an egg cell, you will see that the sperm cell has arguably fewer mitochondria than the egg cell. Our DNA is composed of nucleus DNA and mitochondrial DNA.

Due to the fact that the mitochondrial DNA in egg cells is nonrecombinant, which means it does not combine with other mitochondrial DNA (in this case, from sperm cells of the fathers), all our mitochondrial DNA comes exclusively from our mothers.

 

Endoplasmic reticulum and ribosomes

The endoplasmic reticulum, that we are going to refer to as ER, is a system of membranes inside the cell, called intracellular membranes, that have the role of transporting but also of producing substances. These membranes are not the same as the plasma membrane. 

These membranes are attached to the nucleus membrane and can be covered with ribosomes, we call this the rough ER or RER, or it can not have ribosomes on its outer layer, and we call this the smooth ER or SER. 

A ribosome is an organelle that is present in plants and animals, including in humans and has the role of protein synthesis. Even if species are different from one another, the ribosomes have the same structure in all eukaryotes (cells with a nucleus). So if you decide to compare an animal cell with a plant cell, you will be surprised to see the similarity between plants and animals.

The name of the ER defines a network in the inside part of the cytoplasm. ‘Endo’ comes from endoplasm, which is the inside part of the cytoplasm, and reticulum actually means ‘fine network’. The ER is also known as the transportation system of the cells, but it also produces substances inside the cells.

The rough ER is responsible for producing proteins and the smooth ER produces steroids, fats, and phospholipids. After producing these substances, the ER transports them through the cell in order for them to be efficiently exported.

 

Lysosomes

A lysosome is a membrane-bound organelle that is present in animal cells and the interesting thing about it is that it was discovered only in 1949. The lysosome is also known as the destructive guy inside a cell. Well, it is not actually that bad if you think about its function, which is to digest the waste product of the cells using enzymes.

This organelle contains a membrane that has the role of keeping all of the enzymes inside of it. If these enzymes were to get out, they would digest the entire cell. So, you see, the cell is not such a boring place to be in, and all sorts of things happen inside.

The lysosomes are in close connection to the endoplasmic reticulum because the rough ER is the place where they get these enzymes. They are like a baby that gets the milk it needs to function properly from its mother. However, they are more like business partners than forming a parent-child relationship.

The Golgi apparatus

The Golgi apparatus, or Golgi body, sits near the endoplasmic reticulum and nucleus because it has the role of modifying and transporting lipids and proteins. It also transforms them. If you put a plant cell under the microscope, you will see that it has up to hundreds of Golgi bodies, but don’t be fooled, many cells have only one or two Golgi bodies.

This organelle is a membrane-bound organelle and is made of stacked flattened membranes, called cisternae. 

 

Eukaryotic cells vs prokaryotic cells

There are a lot of organisms on the planet and some are more similar to humans than others. Take a look at bacteria and fungi, which one do you think has cells that are more similar to human cells? If your answer is fungi, you are right.

Bacteria and archaea make up a special category of organisms that are made of one cell and are called prokaryotes. You would think that at the opposite end of the line are the eukaryotes, organisms like plants, animals, humans, and fungi, for example. But eukaryotes can be unicellular and also multicellular.

If you thought that your cells have more in common with bacteria than with fungi, you were wrong. The thing that makes us similar is the structure of the cells which is different from prokaryotic cells.

There are a lot of similarities between these types of cells. They both carry DNA and both have a cell membrane, cytoplasm, and ribosomes, but the interesting part is when we talk about the differences.

Prokaryotic cells are different from the eukaryotic cells because most of them have cell walls, but not all of the eukaryotic cells have these walls. For example, animal cells do not have cell walls, so if you compare an animal cell with a bacteria you will be able to see this difference. 

Also, even if they carry DNA, they do not have a nucleus. They have, however, a nucleoid, which is made of a piece of DNA that has two strains, and another very important difference that you will see between prokaryotic cells and eukaryotic cells is that prokaryotic cells do not have any of the membrane-bound organelles. 

So, this means that, besides the nucleus, they do not have mitochondria, endoplasmic reticulum, or Golgi apparatus. You might think that examining a prokaryotic cell under the microscope would be more boring than examining a eukaryotic one and this would be true if you consider looking at them separately, but many scientists like to study their interactions.

 

What is the structure of a virus?

While you can classify different unicellular organisms into eukaryotes and prokaryotes, you will be surprised to find out that viruses are not cells. They are a lot smaller, that is why they can be observed only by using electron microscopes. 

Viruses do not have a membrane but, instead, they have a protein coat. This coat is called a capsid. They also have genetic material that can be either DNA or RNA which is protected by the protein coat. However, the structure of viruses can differ and the important thing about them is that they can not reproduce if they do not have a host. 

If you were to compare viruses and bacteria in terms of the structure under the microscope, you would first see that bacteria are larger than viruses.

 

Ioana Moldovan

Ioana’s professional experience in the optics field has helped her understand the value of passing her knowledge forward. Her curious personality helps her gather useful information for her readers and her goal is to make technical information fun and accessible to everyone.

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