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1.3. The cell

The cell, unit of life

The cell (from the Latin cellula, diminutive of cella ,"hollow") is the morphological and functional unit of all living beings. It is the smallest part that can be considered as having life, since it can perform the three vital functions of living beings: nutrition, relationship and reproduction.

All the living creatures are formed from cells. Some single-celled organisms are made up of a single cell. Others, multicellulous organisms are made up of many cells.

Cell shape and size

The shape and size of cells is highly variable, depending on cell type, age, and function.

The shape of the cell also depends on whether it is free or part of a tissue. It has the shape that allows it to perform its function with the lowest possible energy expenditure. Cells that have free life, floating in a liquid medium, tend to be more or less spherical, due to surface tension. On the other hand, those that are part of the tissues have a polyhedral shape. Others, like neurons, have a star shape to facilitate the establishment of connections between them. Or fibrillar, like muscular, or flat, like epithelial.

The size of cells is also highly variable and depends on their function. It varies from 0.001 mm of bacteria (1 mm) to that of the yolk of an ostrich egg (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 not larger than those of a small child.

Basic structure of cells

All cells have a basic structure made up of three parts:

Todas las células tienen ADN, membrana plasmática y citoplasma.

  • The plasma membrane

The plasma membrane is a thin covering that surrounds the cell, delimits it and isolates it from the external environment. It is responsible for regulating the entry and exit of substances, to protect the cell, to detect external stimuli and to communicate the cells with each other.

  • Cytoplasm

The cytoplasm is the space surrounded by the plasma membrane in which cellular metabolic processes take place. It is made up:

- The cytosol: aqueous liquid that contains many organic molecules.

- Cellular organelles. All cells have ribosomes, although the rest of organelles it may have depends on the type of cell.

  • The genetic material (DNA)

The DNA (deoxyribonucleic acid) contains genetic or hereditary information transmitted to daughter cells.

Types of cell organization

Although as we have seen in cell theory, all living things are made up of cells, not all cells are the same. According to the degree of complexity and organization, two types of cells are distinguished:
  • Prokaryotic cell. They do not have the genetic material (DNA) separated from the cytoplasm by a membrane. They are the smallest and simplest cells, although they can perform their vital functions. Bacteria are prokaryotic organisms.
  • Eukaryotic cell. They have the genetic material protected by a membrane, the nucleus, which separates it from the cytoplasm. These cells are larger and more complex than prokaryotic cells. Except for bacteria (prokaryotes), all other living beings (algae, protozoa, fungi, plants and animals) are eukaryotic organisms.
The ribosomes are organelles unique present in all cells (prokaryotes and eukaryotes). Eukaryotes, on the other hand, have more organelles.


Comparación entre célula procariota y célula eucariota

By No machine-readable author provided. Mortadelo2005 assumed (based on copyright claims). [Public domain], via Wikimedia Commons

All the living creatures are formed from cells. Depending on the number of cells they are made of, they can be of two types:
  • Unicellular organisms. They are made up of a single cell that performs all vital functions. They can be prokaryotes (bacteria) or eukaryotes (cyanophyte algae, protozoa, and some algae). Sometimes they live in colonies, groups of organisms in which functions are distributed among them. But each organism can live independently, performing all its vital functions.
  • Multicellular organisms. They are all eukaryotes, made up of many cells that have arisen from a single cell. For this reason, all the cells of the organism have the same genetic information (DNA) although they are different.
Unlike unicellular organisms, multicellular cells cannot survive in isolation, since they are specialized cells for a specific function but have lost some capacities for other functions. Cells collaborate with other cells to form tissues to perform a certain function. Animals, plants, fungi and many algae are multicellular.

The cell in other courses

The cell is also discussed in the following courses:


 

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Biology and Geology teaching materials for Compulsory Secondary Education (ESO) and Baccalaureate students.