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6.4. The cell

The cell

Atoms come together to form biomolecules and these group together to form higher units that already have life, which are cellsCells, like all living beings, carry out vital functions in order to keep alive the individual of which they are a part (nutrition and relationship) and to perpetuate it (reproduction).

The cell is the smallest structure that can be considered a living being.

The cell is the structural and functional unit of living beings.

  • The cell is the structural unit of living things because all living things are made up of cells, although not all living things have the same number of cells. According to the number of cells, there are two types of living beings:
    • Unicellular, made up of a single cell. For example, bacteria, some fungi, and some algae.
    • Multicellular, made up of many cells. For example, plants or animals.
  • The cell is the functional unit of living beings because it is the smallest structure that is capable of performing the three vital functions:

Cell theory

The discoveries about cells were made as the technique progressed, allowing the manufacture of new microscopes that facilitated their observation.

The cell theory (Schleiden and Schwann, 1839) postulated that:

  • Every living thing is made up of one or many cells.
  • The cell is the smallest structure that fulfills all vital functions (nutrition, relationship and reproduction).
  • Every cell comes from another cell by division of the same.

Basic structure of cells

All cells are made up of:

  • The plasma membrane is a layer that surrounds the cell. It separates the interior from the cell exterior and is responsible for protecting the cell and regulating the passage of substances between the exterior and the interior of the cell.
  • The cytoplasm is the part of the cell that is surrounded by the membrane. It is made up of an aqueous medium (cytosol) in which cell organelles float. Organelles are structures responsible for carrying out various functions. In the cytosol are the substances necessary for the life of the cell.
  • Genetic material (DNA) is a substance that contains hereditary information and controls the functioning of the cell.

Tipos de células

No se proporciona autor legible por máquina. Mortadelo2005 asumido (basado en reclamaciones de derechos de autor). [Public domain], via Wikimedia Commons

Cell types

Although all cells have the same structure that we have just seen, two types of cells are distinguished:

Size, shape and number of cells

The shape of the cells is very variable, depending on the type of cell, its age and its function. Its shape also depends on whether it is a free cell or is associated with others as part of a tissue.

Cell size  is also highly variable, from 0.001 mm in bacteria (1 μm) to that of an ostrich egg yolk (almost 9 cm), although its size is normally between 0.01 and 0.2 mm.

There is no relationship between the size of the cells and that of the multicellular individual. That is, the cells of a very tall person are no larger than those of a small child, even though their number is much higher.

The cell in other courses

The cell is also studied in other courses: