Understanding Cells: Structure, Function, and Chemical Composition
The cell is the fundamental unit of life, capable of autonomous function. All living organisms are composed of cells, and it is generally accepted that no entity is considered alive without at least one cell. Some microscopic organisms, such as bacteria and protozoa, are single-celled, while animals and plants are composed of millions of cells organized into tissues and organs. Although viruses and cell-free extracts perform many functions of living cells, their lack of independence, growth, and reproductive capacity excludes them from being considered living organisms. Cell biology studies the molecular constitution of cells and how they cooperate to form complex organisms like humans. Understanding the cells that constitute the healthy human body, how it develops, and what goes wrong with aging and disease is essential.
General Characteristics of Cells
Cells vary in shapes and sizes. Some smaller bacterial cells have a cylindrical shape, less than one micron (µm) in length (1 micron equals one millionth of a meter). At the opposite extreme are nerve cells, complex in shape with thin extensions that can reach several meters in length (the neck of a giraffe is a spectacular example). Almost all plant cells are between 20 and 30 µm long, have a polygonal shape, and possess a rigid cell wall. Animal tissue cells are usually compact, between 10 and 20 µm in diameter, with a deformable surface membrane that is often highly folded.
Despite the many differences in appearance and function, all cells are enclosed by a membrane called the plasma membrane, which contains a water-rich substance called cytoplasm. Inside the cells, numerous chemical reactions occur, enabling them to grow, produce energy, and eliminate waste. All these reactions are collectively called metabolism (derived from a Greek word for change). All cells contain hereditary information encoded in deoxyribonucleic acid (DNA). This information directs the cell’s activity and ensures reproduction and the passing of traits to offspring. These and numerous other similarities (including many identical or nearly identical molecules) demonstrate an evolutionary relationship between current cells and the first cells that appeared on Earth.
Chemical Composition of Cells
In living organisms, all processes adhere to the laws of chemistry and physics. The chemistry of living things, studied in biochemistry, is dominated by carbon compounds and characterized by reactions occurring in aqueous solutions within a narrow temperature range. The chemistry of living organisms is highly complex, more so than any other known chemical system. It is dominated by polymers – large, coordinated molecules formed by chains of chemical subunits. The unique properties of these compounds allow cells and organisms to grow and reproduce. The main types of macromolecules are:
- Proteins: Formed by linear chains of amino acids.
- Nucleic acids (DNA and RNA): Consisting of nucleotide bases.
- Polysaccharides: Composed of sugar subunits.
Prokaryotic and Eukaryotic Cells
Prokaryotic cells are cells without a distinct cell nucleus, meaning their DNA is dispersed in the cytoplasm. Cells that do have a nucleus within the cytoplasm are called eukaryotic cells. The more familiar and complex life forms, which constitute the domain Eukarya, are eukaryotes.
Almost without exception, prokaryotes are unicellular, consisting of a single cell. The term “prokaryotic” refers to organisms of the empire Prokaryota, which coincides with the concept of the kingdom Monera in older, though still popular, Whittaker or Copeland classifications. Prokaryotes are divided between the domains Bacteria and Archaea.
Eukaryotic cells are defined by having their fundamental genetic material (genetic information) enclosed within a double membrane, the nuclear envelope, which defines a cell nucleus.
The alternative to the eukaryotic cell organization is the prokaryotic cell. In prokaryotic cells, the hereditary material is not found within distinct compartments called organelles within the cytoplasm. Eukaryotic cells do not have a compartment around the plasma membrane (periplasm), as prokaryotes do.