CELL THEORY

Cell Theory: A Core Principle 

Cell Theory is one of the basic principles of biology. Credit for the formulation of this theory is given to German scientists Theodor Schwann (1810–1822), Matthias Schleiden (1804–1881), and Rudolph Virchow (1821–1902).

The Cell Theory states:

• All living organisms are composed of cells. They may be unicellular or multicellular.
• The cell is the basic unit of life.
• Cells arise from pre-existing cells. (They are not derived from spontaneous generation.)

The modern version of the Cell Theory includes the ideas that:

• Energy flow occurs within cells.
• Heredity information (DNA) is passed on from cell to cell.
• All cells have the same basic chemical composition.

Cell Basics

All living organisms in the kingdoms of life are composed of and depend on cells to function normally. Not all cells, however, are alike. There are two primary types of cells: eukaryotic and prokaryotic cells. Examples of eukaryotic cells include animal cells, plant cells, and fungal cells. Prokaryotic cells include bacteria and archaeans.

Cells contain organelles, or tiny cellular structures, that carry out specific functions necessary for normal cellular operation. Cells also contain DNA (deoxyribonucleic acid) and RNA (ribonucleic acid), the genetic information necessary for directing cellular activities.

Cell Reproduction

Eukaryotic cells grow and reproduce through a complex sequence of events called the cell cycle. At the end of the cycle, cells will divide either through the processes of mitosis or meiosis. Somatic cells replicate through mitosis and sex cells reproduce via meiosis. Prokaryotic cells reproduce commonly through a type of asexual reproduction called binary fission. Higher organisms are also capable of asexual reproduction. Plants, algae, and fungi reproduce through the formation of reproductive cells called spores. Animal organisms can reproduce asexually through processes such as budding, fragmentation, regeneration, and parthenogenesis.

Cell Processes: Cellular Respiration and Photosynthesis

Cells perform a number of important processes that are necessary for the survival of an organism. Cells undergo the complex process of cellular respiration in order to obtain energy stored in the nutrients consumed. Photosynthetic organisms including plants, algae, and cyanobacteria are capable of photosynthesis. In photosynthesis, light energy from the sun is converted to glucose. Glucose is the energy source used by photosynthetic organisms and other organisms that consume photosynthetic organisms.

Cell Processes: Endocytosis and Exocytosis

Cells also perform the active transport processes of endocytosis and exocytosis. Endocytosis is the process of internalizing and digesting substances, such as seen with macrophages and bacteria. The digested substances are expelled through exocytosis. These processes also allow for molecule transportation between cells.

Cell Processes: Cell Migration

Cell migration is a process that is vital for the development of tissues and organs. Cell movement is also required for mitosis and cytokinesis to occur. Cell migration is made possible by interactions between motor enzymes and cytoskeleton microtubules.

Cell Processes: DNA Replication and Protein Synthesis

The cell process of DNA replication is an important function that is needed for several processes including chromosome synthesis and cell division to occur. DNA transcription and RNA translation make the process of protein synthesis possible.