Rita Mary King - Biology Made Simple

LEVELS OF ORGANIZATION

Biosphere: Regions of Earths crust, water, and atmosphere inhabited by living things

Ecosystem: A community and its environment

Community: Populations of different species living in the same area

Population: Group of organisms of the same species living in the same area

Multicelled Organism: An individual organism made up of cells organized as tissues, organs, and organ systems

Organ System: Two or more organs interacting to perform a particular function

Organ: A unit composed of tissues, combined to perform a specialized task or tasks

Tissue: Cells and substances combined to perform a specialized function

Cell: Smallest unit with the ability to live and reproduce

Organelle: Intracellular compartment surrounded by a protective membrane

Molecule: A unit made up of two or more atoms of the same or different elements

Atom: Smallest unit of an element that retains the properties of the element

Subatomic Particle: Fundamental unit of matter such as proton, neutron, and electron

In the Hall of Fame for Biological Theories, the Cell Theory is among the super stars. Its seeds were planted during the seventeenth century following the invention of the microscope. Robert Hooke (16351703) was well known in his day as the Curator of Experiments for the Royal Society of London. He had developed an excellent compound microscope and he used it to observe small organisms. Hooke noted box-like structures in a thin slice of cork. He called them cells since they reminded him of the cells in a monastery. Within a decade, the Royal Society asked Hooke to review the work of Antonie van Leeuwenhoek. The latter had observed living cells when he viewed microbes in pond water.

In 1665, Robert Hooke published a book of his observations called, Micrographia. He illustrated it with detailed drawings of microscopic views of insects, plants, feathers, and, yes, those famous cork cells. Samuel Pepys praised it as, the most ingenious book that I ever read in my life.

In 1838, Matthias Schleiden proposed that, like cork, all plants were composed of cells. A year later, Theodor Schwann took it even further. He believed the cell was the basic functioning unit of all living things. The cell theory was officially born in 1868, when pathologist Rudolf Virchow combined the earlier ideas and went on to show that cells are the result of division of preexisting cells.

Life will always remain something apart, even if we should find out that it is mechanically aroused and propagated down to the minutest detail.

Rudolf Virchow, 1855

The cell is the lowest level of biological structure capable of performing all of the activities of life. The invention of the electron microscope in the 1950s has allowed us to determine the ultrastructure of cells. All have a plasma membrane and, at some stage in their development, contain deoxyribonucleic acid, better known as DNA. Structurally, cells can be divided into two groups, prokaryotes and eukaryotes.

Prokaryotes were the first cells on Earth. Archaebacteria and bacteria are prokaryotic cells. They are generally between 0.1 and 10 m in size. Their DNA is not separated from the rest of the cell by a membrane-bound nucleus and they lack membrane-bound organelles. Most prokaryotic cells have tough external cell walls. There is no cytoskeleton (a lattice-like network of proteins that holds a cell together and gives it its shape) or cytoplasmic streaming (the flow of cytoplasm from one cell region to another). Metabolism can take place in the presence of oxygen (aerobic) or not (anaerobic), depending upon the cell type and species. Cell division is accomplished by binary fission.