A cell is the smallest unit of life that can divide independently. Every organism is made up out of a cell or multiple cells. Cells can be prokaryote or eukaryote. Where prokaryotes are genetic material with a membrane, eukaryotes also exhibit other components. Prokaryotes are bacteria and other unicellular organisms called archaea and all other cells are eukaryotes. Since human cells are eukaryotic, only eukaryotic cells are discussed in detail here.
There are different kind of cells between and within organisms, but they all have some core components. For human cells, these are the cytoplasm, cytoskeleton, endoplasmic reticulum, Golgi apparatus, lysosomes and peroxisomes, mitochondria, nucleus, plasma membrane and ribosomes.
Cytoplasm is everything within the cell except the nucleus. Cytoplasm is a gel-like fluid where the components of the cell are suspended in. The fluid is called cytosol and consists 70-90% of water. The other 10-30% are soluble proteins, salts (e.g. sodium, potassium) and organic compounds (e.g. sugars, vitamins, fats). The composition of cytosol depends of the state of the cell. When a cell is intoxicated, it needs other types of nutrients than when a cell is dividing. In a way, the function of cytosol to cells can be compared to the function of blood to the human body.
The cytoskeleton gives structure to the cell and gives it the ability to take different shapes. There are three main types of cytoskeletal filaments: microfilaments, microtubules and intermediate filaments. Each filament has its own properties and consists of a different kind of polymer of a protein. Microfilaments are made up out of polymerization of actin and are flexible and relatively strong. Microfilaments provide stability, motility and contractility. Due to microfilaments, cells can divide and move into different kind of shapes. Microtubules are dynamic structures that are formed by the polymerization of two globular proteins named alpha tubulin and beta tubulin. Microtubules are constantly built up and broken down. The main function of microtubules is providing intracellular transport of various products like proteins and enzymes. Microtubules are somehow the highway of the cell. Intermediate filaments are composed out of various of proteins. More than 50 proteins that can take part of intermediate filaments are elucidated. Intermediate filaments basically give mechanical strength to the cell and are therefore comparable with the skeleton of the human body.
The endoplasmic reticulum can be subdivided into the smooth and rough endoplasmic reticulum. The smooth endoplasmic reticulum is primarily responsible for the synthesis of lipids and steroids, metabolism of carbohydrates and steroids, detoxification of alcohol and drugs and the attachment of proteins to the cell membrane. The rough endoplasmic reticulum is caused by ribosomes binding to the endoplasmic reticulum. The binding of the ribosomes is very loose and can be easily disconnected. The main function of ribosomes is manufacturing proteins, so the rough endoplasmic reticulum is the place in the cell where most proteins are made and packed into vesicles for transportation to the Golgi apparatus. Red blood cells and sperm cells do not have an endoplasmic reticulum.
The vesicles with proteins of the rough endoplasmic reticulum fuse with the Golgi apparatus, so the proteins can be modified in the Golgi apparatus. The main function of the Golgi apparatus is post-translational modification of proteins and packing them into new vesicles to deliver them to the place of interest. Examples of post-translational modifications are adding glucose (glycosylation) or phosphor (phosphorylation) to proteins. The purpose of post-translational modification varies widely. For example, glycosylation can be used to stabilize the protein in a particular environment and phosphorylation can be used to activate or suppress the function of the protein at a particular moment.
Lysosomes and peroxisomes break down components that are no longer needed at that moment. They are separated via a membrane with the cytosol. Lysosomes contain a variety of enzymes which break down carbohydrates, lipids, nucleic acids and peptides. The enzymes in a lysosome prefer a slightly acidic environment with a pH of 5. Peroxisomes break down the other molecules which the enzymes in the lysosomes cannot handle. An example are very long chain fatty acids. The molecules are getting into the lysosomes and peroxisomes via endocytosis. Endocytosis is a process where vesicles are gulfed up by the lysosomes and peroxisomes.
Mitochondria are the principal generator of adenosine triphosphate (ATP). ATP is the most important supplier of energy for cells. ATP is like gasoline for the human body. Mitochondria are the only place despite the nucleus who contain DNA. The mitochondrial genome shows similarity with bacterial genomes which gives the hypothesis mitochondria are formed out of bacteria (a prokaryote) taken up by the eukaryotic cell. Around 600 proteins can be translated out of mitochondrial DNA and the regulation can vary depending on the situation of the cell.
The nucleus is the place where the chromosomes are stored. The chromosomes encompass the DNA which contains all the hereditary characteristics. DNA is wrapped around histones and compressed into chromosomes. When a particular piece of DNA needs to be translated into a protein, the place of the piece of DNA needs to be unfolded from the chromosome. Thanks to the compressed chromosomes, lots of information can be stored in a small surrounding. The nucleus contains around 20,000 genes which can be translated into around 90,000 distinct proteins.
The plasma membrane separates the inner environment of the cell with the outer environment. It is composed of a bilayer of phospholipids with embedded proteins. These proteins are called receptors and can pass signals of the outer environments through the inner environment of the cell. Via receptors, different kind of cells can communicate to each other.
Ribosomes are components in the cell where RNA is translated into proteins. RNA is made out of DNA in the cell nucleus.
Cell therapy is the kind of medicine where living cells are brought into a patient to cure the disease. There are different kinds of cell therapy which all have their particular purposes. The main benefits of cell therapy are the high rates of cure and the single treatment. Cell therapy only requires a single injection of cells.