Organelles

MITOCHONDRIA

Mitochondria provide the energy a cell needs to move, divide, produce secretory products, contract - in short, they are the power centers of the cell. They are about the size of bacteria but may have different shapes depending on the cell type.

Mitochondria are membrane-bound organelles, and like the nucleus have a double membrane. The outer membrane is fairly smooth. But the inner membrane is highly convoluted, forming folds called cristae. The cristae greatly increase the inner membrane's surface area. It is on these cristae that food (sugar) is combined with oxygen to produce ATP - the primary energy source for the cell.

RIBOSOMES

Ribosomes are found in the cytoplasm and on the endoplasmic reticulum. Their job is to make proteins needed in the cell for everyday life. The nucleus tells the ribosomes which proteins to make, so that the cell can work efficiently. The nucleus also makes the ribosomes and sends them out for work.


ENDOPLASMIC RETICULUM

Throughout the eucaryotic cell, especially those responsible for the production of hormones and other secretory products, is a vast amount of membrane called the endoplasmic reticulum, or ER for short. The ER membrane is a continuation of the outer nuclear membrane and its function suggests just how complex and organized the eucaryotic cell really is.

When viewed by electron microscopy, some areas of the endoplasmic reticulum look "smooth" (smooth ER) and some appear "rough" (rough ER). The rough ER appears rough due to the presence of ribosomes on the membrane surface. Smooth and Rough ER also have different functions. Smooth ER is important in the synthesis of lipids and membrane proteins. Rough ER is important in the synthesis of other proteins.


Information coded in DNA sequences in the nucleus is transcribed as messenger RNA. Messenger RNA exits the nucleus through small pores to enter the cytoplasm. At the ribosomes on the rough ER, the messenger RNA is translated into proteins. These proteins are then transferred to the Golgi in "transport vesicles" where they are further processed and packaged into lysosomes, peroxisomes, or secretory vesicles.

GOLGI APPARATUS

The Golgi apparatus is a membrane-bound structure with a single membrane. It is actually a stack of membrane-bound vesicles that are important in packaging macromolecules for transport elsewhere in the cell. The stack of larger vesicles is surrounded by numerous smaller vesicles containing those packaged macromolecules. The enzymatic or hormonal contents of lysosomes, peroxisomes and secretory vesicles are packaged in membrane-bound vesicles at the periphery of the Golgi apparatus.