The cell is the simplest manifestation of life. Its structure can be different from species to species but the majority of its compartments — or organelles — are common within the different phylogenetic kingdoms of living beings. In this topic, we will define and describe the cell and the features and purposes of its organelles.
Cell structure
What is the cell? In the simplest possible terms, it is simply a volume of liquid surrounded by a lipid membrane. For simplicity, you can imagine a drop of fat in water. The cell membrane is made up of a lipid bilayer, and the lipids that form it are organized in a tail-to-tail pattern: hydrophobic tails look at each other inside the lipid membrane, and hydrophilic heads look outward into the water. Because cells exist only in water-based, or aqueous environments, the lipid bilayer is able to protect the cell from the outside world.
The fluid inside the cell is called the cytosol, and it is made up of water with solutes, minerals, and metals. The cytosol contains all the organelles of the cell, and all metabolic processes take place here. The fluid that makes up the cytosol has certain constant properties that are critical to maintaining the health of the cell. The pH of the cytosol and its composition of ions make it a buffer solution that maintains the stability of the cell.
DNA and nucleus
All cells contain DNA, which can either be encapsulated in the nucleus or float freely. Cells that have their DNA in a nucleus are called eukaryotic cells (these are the cells of animals, plants, fungi, and protozoa). Cells that have their DNA free-floating in the cytosol are called prokaryotic cells (these are cells of archaea and bacteria).
The nucleus that is found in a eukaryotic cell is surrounded by the same type of lipid bilayer membrane as the cell itself but contains only DNA with some auxiliary proteins. The nuclear membrane contains nuclear pores, which are necessary for the cell to make proteins according to the instructions recorded in the DNA.
Protein synthesis is carried out by molecular machines called ribosomes. They take instructions from RNA, the messenger intermediate between DNA and protein, and synthesize proteins based on the genetic code. They are formed from two subunits. Ribosomes are very ancient organelles and can be found in all kingdoms of life. Ribosomes can float freely in the cytosol, but they can also be associated with the membranes of other organelles, or they can even be located inside other organelles — for example, inside mitochondria or plastids.
Membranous organelles
Many cell organelles are membranous — their membrane composition is mostly the same as the cell membrane and consists of a lipid bilayer. These membrane can form invaginations, individual vesicles, and other formations that allow them to store and transport substances that the cell needs. They also allow cells to export the proteins they make into the surroundings.
For example, the endoplasmic reticulum is an extensive network of tubules and pockets that synthesizes membranes, secretory proteins, and other substances, as well as working as a storage organelle. In muscles, the endoplasmic reticulum contains a large intracellular supply of calcium because calcium is a mediator of muscle cell contraction and therefore they need a lot of it.
The Golgi complex, named after the Italian scientist Camillo Golgi, is situated next to the reticulum and works as a storage and transport company for the cell. Vesicles are formed in the Golgi complex to carry substances out of the cell. This is how neurotransmitters leave the neuron, for example. The Golgi looks like a stack of disc-shaped sacs of membranes from which vesicles bud. The Golgi complex is associated with lysosomes, which are membrane-enclosed sacs of enzymes that digest cellular macromolecules such as nucleic acids, polysaccharides, lipids, and proteins.
Symbiotic organelles
Cells use mitochondria to make energy, and plant cells also use plastids. Both of these organelles were once free-living, single-celled organisms, so they have their own DNA and ribosomes, can independently divide inside the cell, and also have two membranes. These organelles use their two membranes to create a charge gradient with an external source of energy — sugar or sunlight — which is subsequently converted into ATP.
Plant cells and fungal cells also contain vacuoles — large bladders filled with water and solutes for storing and processing various molecules.
Conclusion
As we can see, there are a lot of different organelles with distinct functions. Most of them can be found in every cell of every organism, but there are some unique ones — such as plastids, which are found only in plants or pilus, which are characteristic features of bacteria.