Cells are the smallest units in any living thing and are considered the building blocks of life. Some organisms are made of just a single cell, whilst others, like us humans, are made of trillions of cells; however, all cells come from other (pre-existing) cells. All cells are similar in their overall build and structure but can vary greatly depending on how they store their genetic information. They also vary in their functions. Plant cells, for example, have a lot of similarities with animal cells but also have their own specific features, mainly for storing water. We will talk more about organelles, the subunits of cells, in the article cell structure.

Cell examples

Some examples of cells include animal cells such as a neurone (a nerve cell), a stem cell (special human cells with the ability to differentiate into any other type of cells), and a muscle cell, vital building blocks of our muscles.

Fig. 1 - Different cell types illustration

Functions of cells

Cells have many different functions. Some of the most important ones include:

• Production of energy sources: Cells need energy to carry out functions within the cell and the whole organism.

• Protein production: Proteins are needed for almost everything in the cell and the organism. The process is called protein synthesis.

• Division: Almost all eukaryotic cells can replicate themselves through mitosis, meaning that the whole cell, along with its genetic material, is copied.

  • Prokaryotic cells reproduce in different ways, with the most common one being a process called binary fission, which is similar to mitosis.

• Transport: A cell can move materials in and out of a cell through the cell membrane in order to carry out functions both inside and outside the cell. There are different types of transport mechanisms: active and passive. The active transport uses ATP to bring molecules in or out of the cell that otherwise could not get past the cell membrane. Passive transport is for molecules that follow a concentration gradient. These molecules then move to the side of the cell membrane on which fewer of these molecules are present. This process is known as osmosis and is related to the concept of diffusion.

• Recognition: Cells can communicate with each other due to recognising certain molecules on both cell surfaces. Through these molecules, cells can recognise other cells but also foreign material. This is an important feature of the immune system when fighting infections and producing antibodies.

Types of cells

There are two types of cells:

• Prokaryotes, which are often single-celled organisms.

• Eukaryotes, which are more complex and composed of many cells (multicellular).

As the type of organism is usually defined by the cell, cells are divided into animal cells, plant cells, bacteria or archaea. Lastly, the actual cell can be categorised histologically - for example, skin cells or liver cells.

Cells and living organisms

Let's discuss why cells are the smallest building blocks of life. Many organisms are unicellular, meaning that they only have a single cell. These are usually prokaryotes. However, there are also more complex single-celled organisms with eukaryotic cells (yeast and algae are single-celled eukaryotes).

We know that animals and plants are alive, but single-celled organisms like bacteria are considered living as well, even though we can't see them with the naked eye. How do we know those small organisms are alive? They are looked at under a microscope and studied to see if they meet the criteria that have been defined to determine that something is a living organism.

This is especially interesting when it comes to studying the treatment of diseases. Some disease-causing particles are viruses and prions (usually harmless proteins with abnormal structures that cause diseases), which are not categorised as living. However, bacteria are technically living, so treating diseases caused by bacteria is a very different process.

Criteria for life in cells and organisms

• Order: Cells and parts of cells have a specific order every time. The organelles in a cell are always organised in a similar way. For instance, skin cells are organised in rows.

• Reproduction: Cells can produce offspring or new cells, sexually or asexually. In eukaryotes, new cells are formed through mitosis. Prokaryotes usually reproduce through binary fission. On a larger scale, animals reproduce and produce young.

• Growth: For example, the growth of a plant from a seed to a tree.

• Use of energy: An organism produces energy sources such as ATP, which is needed for various reactions, for example, muscle movement.

• Homeostasis: Maintaining stable conditions within a body or cell. The inside of a cell always has a specific pH so that reactions can take place at optimal conditions.

• Response to stimuli: Movement and enzyme production. For example, when plants grow in the direction of sunlight, the plant responds to sunlight’s stimuli. The same happens with unicellular organisms. When a prokaryote encounters a different environment, it responds to those environmental changes (stimuli) by, for example, producing enzymes.

• Ability to adapt: Bacteria have not always been resistant to antibiotics; this has come through their ability to adapt through genetic mutation.

Cells are considered the building blocks of life because they are the smallest unit to meet these criteria. Even a single-celled prokaryote is considered a living organism.