Organelle

An organelle (pronounced /ɔrɡəˈnɛl/) is the general concept of a specialized subunit within a cell with a specific function. As an example, a chloroplast is a photosynthetic organelle, that is specialized in carrying out photosynthesis in plants and certain microorganisms and has its own specialized chloroplast DNA. Mitochondria are the analogue of the latter in animal cells and also have their own mitochondrial DNA

Most organelles are protected by their own membrane(s), as shown in the following schematic diagram of an 'abstract' organelle.



See also:

 * Schematic Drawing of Cell Organelles
 * Cross-section of an Animal Cell

Animal cell organelles
Different cell organelles present in an animal cell are as follows (presumed to be in the order of their importance)

1. The Cell Nucleus contains the DNA genetic material of the organism, usually coupled with histones in higher animals. Both DNA replication and the RNA synthesis based on the DNA template occurs in the nucleus in a controlled manner. The nucleus of a cell regulates the activities of the other cell organelles in the cell and it is protected by anuclear membrane with nuclear pores, or channels, via which the nucleus communicates with the cytoplasm. The biosynthesis of ribosomal RNA (r-RNA) occurs within the nucleolar organizer within the nucleus, with the latter being considered as a sub-organelle.

2. The Endoplasmic Reticulum (ER)is a network of interconnecting pathways that enable the transport of molecules in a cell. The rough ER has ribosome molecules attached to its membrane surface, whereas the smooth ER does not have ribosome molecules attached to its surface.

3. Ribosomes or Palade bodies are involved in protein biosynthesis and consists of two sub units, a large and a small one, both containing r-RNA. Protein biosynthesis occurs primarily in the ribosomes and is controlled by specific enzymes attached to specific amino acids. The ribosomes may be found either freely floating in the cytoplasm or sometimes attached to the ER. In bacterial cells, ribosomes are attached to the cytoplasmic membranes, inside the cell.

4. The Golgi Apparatus - The Golgi apparatus is involved with processing and packaging of the molecules synthesized by the cell mainly the proteins ready for secretion. The ER transports the proteins in their crude form to the golgi appratus. The golgi apparatus packages the proteins developing them into primary, secondary and tertiary proteins respectively.

5. The mitochondria are the `power house' of animal cells as they organize and generate energy within the cells. A mitochondrion has a double membrane--somewhat similar to a Gram-negative bacterium-- that is involved in the controlled energy production within the cell. The energy is generated biochemically in the nmitochondrion through ATP breakdown. Mitochondria also have their own genetic material called mitochondrial DNA, which is interestingly circular, being therefore similar in its biotopology to the bacterial, single chromosome. Biochemical energy is produced from certain biochemical reactions involving or 'cycles' simpler carbohydrates such as glucose and sucrose through the process called cellular respiration.

6. Lysososmes - Lysososmes are sometimes called the `suicide bags' of the cell because they are involved in clearing the anabolic, or `waste' debris from the cell. The contain, therefore, numerous hydrolytic enzymes that are breaking down the cell `debris' to simpler component molecules,and also process the toxins that would otherwise accumulate in the cell. Lysososmes are also capable of engulfing damaged organelles, viruses, bacteria, and also certain food particles.

7. Vacuole are large storage organelles floating in the cytoplasm that accumulate the excess water or food, and are present in large numbers withing the cell.