Cell: Structure, Function & Diseases

The cell (Latin cellula) forms the smallest unit of life. Humans are made up of many different types of cells that differ in appearance and function.

What is a cell?

Some organisms, such as bacteria, are made up of only a single cell and are therefore called unicellular organisms. Higher organisms are made up of a large number of cells and are called multicellular organisms. Humans are made up of about ten trillion cells that specialize in different tasks and vary in size and shape depending on the type of cell. For example, there are long, thin nerve cells, spherical red blood cells and round fat cells. The egg cell is the largest cell in humans, measuring 110 to 140 micrometers. Common to all cells is that they contain complete genetic information in the form of DNA (deoxyribonucleic acid), can obtain and use energy, and are able to reproduce by cell division. Cells can assemble to form tissue associations. Of the more than 200 different types of cells in humans, four main tissue groups are composed of epithelial tissue, muscle tissue, connective tissue, and nervous tissue.

Anatomy and structure

Human cells are surrounded externally by the cell membrane. Unlike plant cells, they do not have a cell wall. The size of the cells is not related to the size of an organism. Larger organisms simply consist of a higher number of cells. Inside the cell membrane is the cytoplasm. In the cytoplasm lie various so-called organelles. These include the nucleus, mitochondria, endoplasmic reticulum, Golgi apparatus, lysosomes and peroxisomes. The organelles are specialized for different tasks. The nucleus contains the genetic information in the form of DNA and in humans is demarcated from the cytoplasm by a nuclear envelope. The mitochondria also contain part of the DNA. In the endoplasmic reticulum (ER), a distinction is made between the rough and the smooth ER. Ribosomes are located on the rough ER, which are absent on the smooth ER. Other cell components include the cytoskeleton, RNA (ribonucleic acid), and centrioles. Between the individual cells, outside the cell membrane, is the extracellular matrix.

Function and tasks

The cell membrane serves as an external boundary between the cell and its environment as well as for its protection. It is used to control which substances enter and which leave the cell. It can communicate with neighboring cells via proteins contained in the cell membrane. The cytoskeleton is responsible for the elasticity and stability of the cell. It allows active movement of the cell as well as movement within the cell. Ribosomes are the place in the cell where proteins are synthesized with the help of specific RNA. The Golgi apparatus produces various secretions and is involved in the metabolism of the cell. The lysosomes represent the digestive system of the cell. They contain numerous enzymes with which they can break down foreign and cellular substances. The peroxisomes serve the detoxification. They can utilize oxygen, bind free radicals and can break down various products of metabolism. The centrioles are necessary for cell division and thus cell proliferation. Since each cell can obtain and use energy and multiply, each cell is capable of survival on its own. However, some specialized cells have lost this ability. Depending on their specialization, the cells have different tasks. The specialized cells originally arise from so-called stem cells. Stem cells are general cells of the body that can both proliferate by dividing into new stem cells and develop into specific cell types. When a cell specializes, certain genes are inactivated and others are activated. This results in the formation of the proteins that are specifically required in a particular cell type. As a result, a liver cell, for example, is chemically and structurally different from a nerve cell, even though both carry the same genetic information.

Diseases and disorders

A common disease of cells is cancer. In cancer, the balance between cell division and cell death (called apoptosis), which is regulated by genes, is disturbed. This results in uncontrolled growth of cells and tumors develop.In various neurodegenerative diseases, nerve cells in the brain die. This can depend on age, as in dementia or Parkinson’s disease. However, the age-related loss of cells and their functions is normal to a certain extent and is usually well tolerated by the body. Only when an above-average number of cells die do disease patterns develop. Other neurodegenerative diseases occur independently of age, such as amyotrophic lateral sclerosis (ALS), Huntington’s disease or Creutzfeldt-Jakob disease. In allergic reactions, there is an overreaction of specialized cells of the immune system. In the case of an allergy, these cells fight a substance that is harmless to the body, which leads to the allergic symptoms. A very rare cell disease is hereditary I cell disease, also known as mucolipidosis II. It is a lysosomal storage disease in which one of the enzymes normally found in lysosomes cannot be transported here because of a genetic defect. In mastocytosis or so-called Czernin’s disease, there is a greatly increased proliferation of mast cells. The skin or internal organs can be affected. Complaints are triggered by substances released by the mast cells, primarily histamine.