- All living organisms consist of cells
- Organisms that consist of one cell are called unicellular
- Organisms that contain multiple cells are called multicellular
- Cells are able to produce their own energy, filter the incoming molecules and send out signals and substances, as well as reproduce
- The cells consist of a complex plasma membrane that surrounds the cytoplasm of the cell
- The cytoplasm contains the genetic material of the cell, as well as cellular structures – organelles
- Cells that do not contain a nucleus and organelles are called prokaryotic
- Cells that contain a nucleus and membrane organelles are called eukaryotic
- Plant and animal cells have a number of their own specific organelles
First introduction: the cell
Look around you. Everything around us can be build from smaller parts or building blocks: houses, appliances or clothing. The same goes for all living beings on Earth, both big and small. The building blocks of living beings are called cells. But a cell is much more complex than just a brick, for example. A single cell is able to:
- Consume molecules and use them to produce energy
- Interact with other cells by sending signals
- Produce its own molecules and transport them outside
- Eliminate harmful substances
- Reproduce by making a copy of itself
Cells are very diverse. Here are some examples:
The last picture shows an organism – Paramecium aurelia – that actually consists of only one cell. It moves, hunts and reproduces all by itself. There are millions of such organisms. They are called unicellular organisms.
Humans, animals and plants are different. They consist of multiple cells that perform many different functions. Such organisms are called multicellular.
Despite their diversity, cells have certain things in common. For instance, all cells have three main components:
- a plasma membrane
- cellular structures called organelles
First barrier: plasma membrane
If you suddenly minimize yourself, so you would be as small as a molecule, a cell would not look to you flat and ordinary as it is displayed on the pictures. It would look like a fortress with a complex wall protecting the inner structures against intruders. This wall is called the plasma membrane. As the defence wall of any human-built house, it serves several functions:
- Holds the cell together (i.e. provides structural integrity)
- Protects the cell from the harmful substances
- Regulates what molecules to let through and in what quantity (This property is called semi – permeability)
- Shows the cell’s “flags” that allow other cells to recognize who are they communicating with
- Participates in sending and accepting signal molecules that are used to transmit “messages” in the body
- Provides “anchors” for the inner cellular structures to attach
How can a single structure have so many functions? It is possible because of the way the plasma membranes are formed.
The main components of the plasma membrane are phospholipids. Phospholipids are a class of fat or lipid molecules that are amphiphilic. It means that a single molecule has two parts with different properties. One part of a phospholipid molecule, often referred to as a “tail”, “water-loving” – hydrophilic. It can interact with water. The other part of this molecule, often called a “tail” is hydrophobic – it does not interact with water at all. So, when a structure made of many amphiphilic lipid molecules interacts with water, the hydrophilic “heads” all turn towards the water, and the hydrophobic parts hide from it. Just like a hedgehog that hides from the enemy, putting all his needles outside and hiding the vulnerable stomach. And what would happen if this lipid-made structure is surrounded with water? It forms a ball!
In the case of the plasma membrane, there are two layers of lipid molecules. One is located on the outside of the cell and interacts with the environment around it. The other layer is under the first one, interacting with the contents of the cell itself. This structure is called a lipid bilayer. Both areas contain water, so the bilayer displays the water-soluble “heads” on each side, hiding the hydrophobic “tails” inside. And this way, the bilayer always holds a certain shape. Bilayer also serves as a filter: small polar molecules can get through the phospholipids wall relatively easily, as well as water or certain types of fats. The molecules of this type are also very mobile, so the lipid bilayer is flexible and fluid. In order to make this structure sturdier, there are other lipids added, for instance, cholesterol. Cholesterol helps make the membrane more robust and less permeable to molecules.
But what if the cell still needs those molecules that can’t pass through the layers by themselves? There are special assistants that help get these molecules through – proteins!
Proteins are embedded in the bilayer. Sometimes, they are located only on one side. Those proteins are mostly receptors. Their main role is serving as sentries – they interact only with molecules that have a certain shape, in a key-lock fashion. Receptors can serve both as “sensors”, to accept signals and as “flags”, to demonstrate the cell’s identity.
Other proteins go through the entire membrane bilayer. They also are quite complex and consist of many parts. They are called transmembrane proteins. They are the ones who act as channels that allow molecules that can’t go through the bilayer by themselves to pass through.
There is a third component to the plasma membranes: carbohydrates. They are usually attached to certain proteins (glycoproteins) or lipids (glycolipids) that are located on the outer layer of the membrane. Usually, glycoproteins and glycolipids are important in the cell-to-cell recognition and for additional stability of the whole membrane.
As you can see, a plasma membrane is a complex structure. It consists of a flexible bilayer made from phospholipids, and there are multiple proteins, as well as cholesterol embedded in it. It very much resembles a complex mosaic. That is why bilayer is often described by a fluid mosaic model:
But even more important are the inner contents of the cell that the membrane protects: the cytoplasm.
The inner sanctuary: the cytoplasm
All living cells have a cytoplasm. Cytoplasm is a complex, gel-like substance contained within the plasma membrane. There are many molecules in the cytoplasm: water, proteins, lipids, carbohydrates and smaller molecules. The cytoplasm also contains cellular structures. These structures are called organelles. In more complex cells organelles are formed by membranes similar to the plasma membrane. In simpler cells, there are no membrane organelles and even the “brain” of the cell, the DNA that contains all the genetic information necessary for the cell, is present directly in the cytoplasm. Based on these features, the scientists divide all cells into two main types:
These cells are very small. They do not have many organelles either. Their main components are:
- plasma membrane
- Genetic material – DNA, that is located in the cytoplasm directly
- Ribosomes – small organelles that contain nucleic acids and proteins and participate in the protein production in the cell
Eukaryotic cells are much bigger than prokaryotic ones. Their main features are the presence of a nucleus and other membrane organelles.
- nucleus – a central major organelle that protects the genetic material of the cell – the DNA;
- Mitochondria or chloroplasts – special organelles that produce energy for the cell;
- Endoplasmic reticulum – a system of membranes that is responsible for the production of proteins and lipids;
- Golgi complex – another special system of membrane structures responsible for packaging of proteins and lipids produced by the endoplasmic reticulum.
Both plant and animal cells are eukaryotic. As plants and animals function differently, they also have a different number of organelles, though you may find a nucleus, endoplasmic reticulum and ribosomes in both cell types. In most multicellular organisms cells often get specialized functions, so they may have different shapes and slightly different organization from the overall “cell map”.
References and further reading:
[1.] https://ghr.nlm.nih.gov/primer/basics/cell What is a cell?
[2.] https://courses.lumenlearning.com/boundless-biology/chapter/components-and-structure/ Fluid mosaic model
[3.] https://commons.wikimedia.org/wiki/File:Cell_membrane_detailed_diagram_en.svg Detailed diagram of the cell membrane
[4.] https://commons.wikimedia.org/wiki/File:Neuron_-_Nerve_Cell_01.png Nerve cell picture
[5.] https://commons.wikimedia.org/wiki/Category:Red_blood_cells#/media/File:201304_red_blood_cell.png Red Blood cell picture
[6.] https://upload.wikimedia.org/wikipedia/commons/thumb/c/cb/Paramecium.jpg/148px-Paramecium.jpg Paramecium aurelia photo
[7.] G. M. Cooper (2nd edition). The Cell: A molecular approach. Sunderland (MA), Sundauer Associates; 2000.