Cells and Their Compositions

(cytology is the study of cells and the cell theory states that all living things are composed of cells and that all cells arise only from other cells)

The cell is the basic structural unit of life

A cell is the smallest unit capable of independent existence which can reproduce itself.

  • All living organisms, with the exception of viruses, are composed of one or more cells.
  • Single-cell organisms; such as, bacteria, protozoa, and other micro-organisms are termed unicellular, while plants and animals that contain many cells are termed multicellular.
  • Highly complex organisms; such as, human beings consist of billions of cells, all of which are adapted to carry out specific functions; for example, groups of these specialized cells are organized into tissue and bodily organs.
  • Although cells may differ widely in size, appearance, and function, their essential features are similar.
  • Almost all cells perform biochemical processes, generate and process energy, and store genetic information to be passed down to future generations of cells.
  • Advanced single-celled and multicelled organisms have nuclei that carry the DNA, primitive cell; such as, bacteria, have no nucleus and carry their DNA loosely in coils throughout the cells
  • The number of cells in an organism varies; for example, bacteria are single-celled organisms; the average adult human body has about one hundred trillion cells.
  • Cells are the structural unit of all organisms and come in a range of sizes and shapes.
  • Animal cells contain only an outside cell membrane.
  • In most fungi, bacteria, and plants; there is a rigid cell wall outside the membrane.
  • Almost all cells perform biochemical processes, generate and process energy and store genetic information to be passed down to future generations of cells.

  • Each cell is composed of a mass of jelly like substance called cytoplasm, surrounded by a membrane.
  • The cytoplasm contains ribosomes (particles), which carry out protein synthesis, and DNA, the coded instructions for the behavior and reproduction of every cell.
  • In eukaryote cells, those of protozoa, fungi, plants, and animals, the DNA is organized into chromosomes and is contained within a clearly defined nucleus, which is surrounded by a double membrane.
  • Some cells; such as mammalian an red blood cells, lose their nuclei as they mature.
  • The cytoplasm also contains other membrane-bound structures called organelles; such as, mitochondria and chloroplasts, which carry out specific functions.
  • In prokaryote cells, those of bacteria and cyanobacteria, or blue-green algae, the DNA forms a simple loop and there is no nucleus.
  • The prokaryotic cell also lacks organelles (specialized structures found inside cells that carry out a specific life process or function), although it possesses many ribosomes.
  • Each eukaryote has a surrounding membrane, that is a thin layer of protein and fat which restricts the flow of substances in and out of the cell and encloses the cytoplasm, a jellylike material containing the nucleus and other structures (organelles); such as, mitochondria (an organelle containing enzymes responsible for producing energy).
  • In general, plant cells differ from animal cells in that the membrane is surrounded by a cell wall made of cellulose.
  • They also have larger vacuoles (fluid-filled pouches) and contain chloroplasts that convert light energy into chemical energy for the synthesis of glucose.
  • The cell membrane is a thin, continuous layer, made up of fat (phospholipid) and protein molecules, that encloses the cell or organelles within a cell.
  • The nucleus is usually the largest and most prominent structure in the eukaryote cell.
  • Its function is to house and to pass on genetic information to future generations of cells, and to direct and to control the activities of the cell according to its own genetic instructions.

—Compiled from information found in
The New York Public Library Science Desk Reference;
Patricia Barnes-Svarney, Editorial Director;
A Stonesong Press Book; New York; 1995; page 102.
Scientific American Science Desk Reference;
Helicon Publishing Ltd.; New York; 1999; pages 349-350.

For more information about cell words, check out the following:

A cross reference of word units that are related, directly or indirectly, to "cell, cells, cell nucleus": celli-; cellulo-; cyto-; endothelio-; gameto-; glio-; kary-, karyo-; neuro-.