Prokaryotic cells

The first life on Earth came in the form of a prokaryotic cell. For two billion years prokaryotic cells were the only living things on Earth and spread to almost every corner of the planet. Today they are still the most abundant and diverse organisms on Earth and more prokaryotes are found in one handful of soil than all the humans that have ever existed.

A prokaryotic cell is one of the two types of cells that make up all the trillions of organisms that live on Earth, the other type being eukaryotic cells. Although prokaryotic cells appear far less advanced than eukaryotic cells, prokaryotic organisms outperform eukaryotes in many ways.

Prokaryotic cells make up the organisms found in the two Kingdoms of life known as Bacteria and Archaea. Bacteria and archaea are literally found almost everywhere on Earth and in far greater abundance than any eukaryotic organism such as plants and animals.

These microscopic organisms maybe tiny but their impact to life on Earth is enormous. In fact, all life would perish from our planet if prokaryotic organisms were to disappear.

Although they receive a bad rap from the media and pharmaceuticals, the majority of prokaryotes are either harmless or actually help eukaryotes, such as animals and plants, to survive and only a small number of species are responsible for serious illnesses.

Why are prokaryotic cells important?

Prokaryotes are important to all life on Earth for a number of reasons. They play a critical role in the recycling of nutrients by decomposing dead organisms and allowing their nutrients to be re-used.

They are also important for many metabolic processes. Bacteria in our guts and mouth help with the digestion of food by breaking down difficult to digest carbohydrates and other compounds. Recent research has suggested that the microbial community in our guts can be a significant factor contributing to obesity.

Structure of prokaryotic cells

Prokaryotic cells are simple in structure. They are small cells enclosed in a plasma membrane and a cell wall. The inside of the cell contains little more than DNA, ribosomes and the cytoplasm.

Prokaryotic cells do not have a nucleus

The defining difference between prokaryotic and eukaryotic cells is the lack of a ‘true’ nucleus in prokaryotic cells. The word ‘prokaryotic’ means ‘before nucleus’ and eukaryotic cells are thought to have evolved from prokaryotic cells.

Instead of DNA being enclosed in a nuclear membrane, as it is in eukaryotic cells, the DNA of prokaryotic cells is tightly coiled in a region of the cell called the ‘nucleoid’. The nucleoid is not a ‘true’ nucleus because the DNA is not surrounded by a membrane. Prokaryotic cells have a single ring of DNA and contain roughly 0.1% of the DNA of a typical eukaryotic cell.

Prokaryotic cells don’t have organelles

Prokaryotic cells are far less organised than eukaryotic cells. In eukaryotic cells, special cellular structures called organelles are used to perform specific functions such as respiration and digestion of waste. Prokaryotic cells lack these organelles which reduces the efficiency of the cells to perform certain functions. Some specialised prokaryotic cells do have networks of membranes that allow the cell to perform functions such as photosynthesis and cellular respiration.

Prokaryotic cells are surrounded by a plasma membrane and a cell wall

Plasma membranes are a feature common to all cells, both eukaryotic and prokaryotic. They surround the internal material of cells such as the DNA and cytoplasm.

Plasma membranes are made from fat-like molecules called phospholipids. Carbohydrates and proteins are also scattered through the phospholipid membrane to help move substances in and out of the cell and to allow cells to communicate with each other.

The cell wall is very important to a prokaryotic cell. It helps in a number of ways such as maintaining the shape of the cell, providing protection and preventing the cell from bursting open. The cell wall is found on the outside of the plasma membrane and defines the boundary of the cell, similar to the way skin defines the boundary of a person.

Prokaryotic cell walls are far more complex than the cell walls of plants. They are an essential part of bacterial cells and are often the focus of modern antibiotics. If an antibiotic is able to breakdown the complex structure of the cell wall, they are likely to be effective at killing off populations of bacteria.

Cytoplasm and ribosomes

The general space inside the plasma membrane of a cell is known as the cytoplasm. It is filled with a jelly-like substance called ‘cytosol’.

Ribosomes are one of the few features that can be found inside the cytoplasms of prokaryotic cells, aside from DNA. They are small structures made from protein and RNA that are not surrounded in a membrane. Because they lack a membrane they are not considered to be organelles. Ribosomes are not found in all prokaryotic cells.

Flagella, pilli and fimbriae

There are a few different structures that can extend out from the cell wall of prokaryotic cells. These include flagella, pilli and fimbriae.

Flagella make it possible for prokaryotic cells to move and are found in around 50% of known bacteria species. They are completely different to the flagella found in the eukaryotic cells of animals and fungi. Pilli and fimbriae are external growths that bacterial cells use to stick to each other or to the substrate they are living on.

Size of prokaryotic cells

Prokaryotic cells are tiny. They are about 10% as wide as the eukaryotic cells of plants, animals or fungi. The diameter of a prokaryotic cell is usually between 1-10 μm, whereas a typical eukaryotic cell is between 10-100 μm.

The smallest bacteria, called mycoplasmas, can be as little as 0.1 μm in diameter. When you consider that 1000 μm = 1 mm or 0.4 in, then it would take 25,000 mycoplasmas lined up next to each other to make a line 1 inch long (2.5 cm long).

Prokaryotic cells have never evolved to produce multicellular organisms as eukaryotic cells have. All prokaryotes are single-celled microorganisms. Often cells aggregate together to form colonies. A colony of prokaryotic cells often make changes to their surrounding environments that benefits the individual cells.

The size of a prokaryotic cell is restricted by its metabolism. Because prokaryotic cells lack organelles, their cellular processes aren’t as efficient as eukaryotic cells, which is why they are unable to grow as large.

Increases in the size of a cell lead to increases in nutrient and energy demands. Nutrients enter a cell through the plasma membrane but as a cell gets bigger, the volume of the cell increases faster than the surface area of the plasma membrane and energy demands increase more than the energy supply.

To counteract the difference between the supply and demand of nutrients and energy, cells must become more efficient at using their supply of nutrients and energy. Eukaryotic cells with organelles have evolved far more efficient methods for using their supply of nutrients and energy and so they have been able to grow much larger than prokaryotic cells.

Metabolism of prokaryotic cells

Prokaryotic organisms have evolved a wide range of ways of to take energy from the environment. Compared to eukaryotic cells that have only evolved to transfer energy through photosynthesis and respiration, prokaryotic cells can obtain energy through photosynthesis, respiration, nitrogen fixation, denitrification, sulfate reduction and methanogenesis. These words may not mean anything to you but they illustrate the diversity of ways prokaryotic cells can take energy from their surrounding environment.

Last edited: 9 March 2016