Eukaryote

A eukaryote is an organism whose cells contain a nucleus and other organelles enclosed within the cell membrane. For example, protozoa_, fungi_, plants_, and animals.

Contents

1   Etymology

The presence of a nucleus gives eukaryotes their name, which comes from the Greek ευ (eu, "well") and κάρυον (karyon, "nut" or "kernel").

2   Substance

static/images/cell_diagram.gif

Diagram of a cell.

Eukaryotic cells are typically much larger than those of prokaryotes. They have a variety of internal membranes and structures, called organelles, and a cytoskeleton composed of microtubules, microfilaments, and intermediate filaments, which play an important role in defining the cell's organization and shape.

2.1   Nucleus

The nucleus is a minute spherical body, imbedded in the protoplasm, its size having little relation to that of the cell. It is surrounded by a well-defined wall, the nuclear membrane; this encloses the nuclear substance (nuclear matrix), which is composed of a homogeneous material in which is usually embedded one or two nucleoli. [1]

The contents of the cell nucleus are separated from the cytoplasm, and are then called the nucleoplasm.

The nucleus of eukaryotic cells is composed primarily of protein anddeoxyribonucleic acid, or DNA. The DNA is organized into linear units calledchromosomes, also known as chromatin when the linear units are not obvious. Functional segments of the chromosomes are referred to as genes. Approximately 100,000 genes are located in the nucleus of all human cells. Nuclear proteins belong to a class of proteins called histones. The chromosome is coiled around the histones.

2.2   Organelles

An organelle is a specialized subunit within a cell that has a specific function. The name organelle comes from the idea that these structures are to cells what an organ is to the body (hence the name organelle, the suffix -elle being a diminutive).

2.2.1   Cytoskeleton

The cytoskeleton is an interconnected system of fibers, threads, and interwoven molecules that give structure to the cell. The main components of the cytoskeleton are microtubules, microfilaments, and intermediate filaments. All are assembled from subunits of protein.

2.2.2   Endoplasmic reticulum

The endoplasmic reticulum is a series of membranes extending throughout the cytoplasm of eukaryotic cells. In some places, the ER is studded with submicroscopic bodies called ribosomes. This type of ER is referred to as rough ER. In other places, there are no ribosomes. This type of ER is called smooth ER. The ER is the site of protein synthesis in a cell. Within the ribosomes, amino acids are actually bound together to form proteins. Cisternae are spaces within the folds of the ER membranes.

2.2.3   Golgi body

The Golgi body is a series of flattened sacs, usually curled at the edges. In the Golgi body, the cell's proteins and lipids are processed and packaged before being sent to their final destination. To accomplish this function, the outermost sac of the Golgi body often bulges and breaks away to form droplike vesicles known as secretory vesicles.

2.2.4   Mitochondrion

static/images/mitochondria.jpg

The mitochondrion (plural mitochondria) is a membrane bound organelle found in most eukaryotic cells. The word mitochondrion comes from the Greek μίτος, mitos, i.e. "thread", and χονδρίον, chondrion, i.e. "granule".

Mitochondria convert glucose (blood sugar) into adenosine triphosphate, which the cells uses a source of `chemical energy`_.

The number of mitochondria in a cell can vary widely by organism, tissue, and cell type. For instance, red blood cells have no mitochondria, whereas liver cells can have more than 2000

Although most of a cell's DNA is contained in the cell nucleus, the mitochondrion has its own independent genome.

3   Production

Cell division in eukaryotes differs from that in prokaryotes. There are two types of division processes. In mitosis, one cell divides to produce two genetically identical cells. In meiosis, which is required in sexual reproduction, one diploid cell (having two instances of each chromosome, one from each parent) undergoes recombination of each pair of parental chromosomes, and then two stages of cell division, resulting in four haploid cells (gametes). Each gamete has just one complement of chromosomes, each a unique mix of the corresponding pair of parental chromosomes.

4   References

[1]Henry Gray. Anatomy of the Human Body: Embryology. http://www.bartleby.com/107/2.html