The Cell

The Cell

The one-celled organism amoeba proteus

A single-celled bacteria of the type: E. coli

A human red blood cell

A plant cell from the leaf of a poplar tree

         The cell is one of the most basic units of life.  There are millions of different types of cells.  There are cells that are organisms onto themselves, such as microscopic amoeba and bacteria cells.  And there are cells that only function when part of a larger organism, such as the cells that make up your body. 

The cell is the smallest unit of life in our bodies.  In the body, there are brain cells, skin cells, liver cells, stomach cells, and the list goes on.  All of these cells have unique functions and features.  And all have some recognizable similarities. 

All cells have a 'skin', called the plasma membrane, protecting it from the outside environment.  The cell membrane regulates the movement of water, nutrients and wastes into and out of the cell.  Inside of the cell membrane are the working parts of the cell.  At the center of the cell is the cell nucleus.  The cell nucleus contains the cell's DNA, the genetic code that coordinates protein synthesis.  In addition to the nucleus, there are many organelles inside of the cell - small structures that help carry out the day-to-day operations of the cell.  One important cellular organelle is the ribosome.  Ribosomes participate in protein synthesis.  The transcriptionphase of protein synthesis takes places in the cell nucleus.  After this step is complete, the mRNA leaves the nucleus and travels to the cell's ribosomes, where translation occurs.  Another important cellular organelle is the mitochondrion.  Mitochondria (many mitochondrion) are often referred to as the power plants of the cell because many of the reactions that produce energy take place in mitochondria.  Also important in the life of a cell are the lysosomes.  Lysosomes are organelles that contain enzymes that aid in the digestion of nutrient molecules and other materials.  Below is a labelled diagram of a cell to help you identify some of these structures.

Prokaryotic Organisms

The simplest of cells, and the first types of cells to evolve, were prokaryotic cells—organisms that lack a nuclear membrane, the membrane that surrounds the nucleus of a cell. Bacteria are the best known and most studied form of prokaryotic organisms, although the recent discovery of a second group of prokaryotes, called archaea, has provided evidence of a third cellular domain of life and new insights into the origin of life itself.
Prokaryotes are unicellular organisms that do not develop or differentiate into multicellular forms. Some bacteria grow in filaments, or masses of cells, but each cell in the colony is identical and capable of independent existence. The cells may be adjacent to one another because they did not separate after cell division or because they remained enclosed in a common sheath or slime secreted by the cells. Typically though, there is no continuity or communication between the cells. Prokaryotes are capable of inhabiting almost every place on the earth, from the deep ocean, to the edges of hot springs, to just about every surface of our bodies.
Prokaryotes are distinguished from eukaryotes on the basis of nuclear organization, specifically their lack of a nuclear membrane. Prokaryotes also lack any of the intracellular organelles and structures that are characteristic of eukaryotic cells. Most of the functions of organelles, such as mitochondria, chloroplasts, and the Golgi apparatus, are taken over by the prokaryotic plasma membrane. Prokaryotic cells have three architectural regions: appendages called flagella and pili—proteins attached to the cell surface; a cell envelope consisting of a capsule, a cell wall, and aplasma membrane; and a cytoplasmic region that contains thecell genome (DNA) and ribosomes and various sorts of inclusions.

Eukaryotic Organisms

Eukaryotes include fungi, animals, and plants as well as some unicellular organisms. Eukaryotic cells are about 10 times the size of a prokaryote and can be as much as 1000 times greater in volume. The major and extremely significant difference between prokaryotes and eukaryotes is that eukaryotic cells contain membrane-bound compartments in which specific metabolic activities take place. Most important among these is the presence of a nucleus, a membrane-delineated compartment that houses the eukaryotic cell’s DNA. It is this nucleus that gives the eukaryote—literally, true nucleus—its name.
Eukaryotic organisms also have other specialized structures, called organelles, which are small structures within cells that perform dedicated functions. As the name implies, you can think of organelles as small organs. There are a dozen different types of organelles commonly found in eukaryotic cells. In this primer, we will focus our attention on only a handful of organelles and will examine these organelles with an eye to their role at a molecular level in the cell.
The origin of the eukaryotic cell was a milestone in the evolution of life. Although eukaryotes use the same genetic code and metabolic processes as prokaryotes, their higher level of organizational complexity has permitted the development of truly multicellular organisms. Without eukaryotes, the world would lack mammals, birds, fish, invertebrates, mushrooms, plants, and complex single-celled organisms.

Understanding what makes up a cell and how that cell works is fundamental to all of the biological sciences. Appreciating the similarities and differences between cell types is particularly important to the fields of cell and molecular biology. These fundamental similarities and differences provide a unifying theme, allowing the principles learned from studying one cell type to be extrapolated and generalized to other cell types.