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# 4.13 Organelle biogenesis and additional roles

There is not sufficient space in this course to describe all the properties of organelles in detail, so before leaving the topic, it is important to make two general points: firstly, organelles are dynamic structures; and secondly, they have additional roles to those described above.

New organelles form by the growth and division of existing organelles, processes that are independent of nuclear division. The formation of new organelles requires the production of new phospholipids, most of which are assembled in the SER. The new phospholipids are then incorporated into the different organelles in two ways. The first is by fusion of vesicles derived from the ER; the Golgi and lysosomes form in this way. The second is by direct incorporation of phospholipids into the organelle membrane via special membrane proteins; the mitochondria, chloroplasts and peroxisomes incorporate new phospholipids in this way, as they do not fuse with vesicles.

In addition to their main roles described in this course, many organelles are also involved in other functions. One such function is regulated cell death, often referred to as programmed cell death, one form of which is known as apoptosis. Programmed cell death is of importance in ensuring appropriate cell numbers, particularly during the development of animals and in disease. The cell membrane, mitochondria and the ER are all involved in programmed cell death.

Another function of the ER and the mitochondria is in the regulation of the levels of calcium in the cell. Calcium plays an important role in cell signalling.

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