Organelles of Cells and Their Functions

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The human body is composed of more than 100 trillion cells, of which there are a number of varieties. Cells are composed of small, specialized structures or organelles, each with their own unique role. The functions of the cells organelles in humans are associated with diseases when certain organelles of cells and their functions are not working properly.

The Nucleus

The nucleus is a large membrane bound organelle. Most cells have a single nucleus though skeletal muscle can have more than one (multinucleate) or some cells such as red blood cells have no nucleus (anucleate). The nucleus contains the genetic material and is where DNA replication and RNA transcription occur. The nucleus is bound by a membrane which contains nuclear pores. These pores allow RNA molecules and proteins to move in and out of the nucleus. However, this process is selective and is energy dependent.


This is a specialized region within the nucleus where RNA and protein components of ribosomes are assembled.


Ribosomes can either float freely within the cell or are bound to the endoplasmic reticulum. Ribosomes are the sites where protein molecules are synthesized from amino acids and RNA. Ribosomes read the messenger RNA (sequence copied from the DNA sequence) and this is used to generate the protein. Ribosomes are made from complexes of proteins and RNAs two subunits.

Endoplasmic Reticulum

Endoplasmic reticulum is a network of membranes, involved in protein synthesis, the modification of a number of molecules and the regulation of calcium concentration. The rough endoplasmic reticulum is where ribosomes attach to the surface and is the location where proteins are synthesized. The smooth endoplasmic reticulum is the area where lipids and steroids are synthesized and where calcium is regulated.

Golgi Apparatus

Golgi apparatus is a sac surrounded by a membrane which modifies and sorts proteins into vesicles for secretion or transport. These vesicles then deliver the proteins to other cell organelles and to the plasma membrane for cell secretion.

Lysosomes and Peroxisomes

These organelles are found in cells, and their function is essential in maintaining the cell’s chemical balance and removal of unwanted debris. Lysosomes and peroxisomes are membrane bound organelles which contain enzymes that remove unwanted particles. The lysosome contains digestive enzymes which break down bacteria and cell debris and play a role in the cell’s immune system. Peroxisome maintains the chemical balance within the cell.


Mitochondria, like the nucleus, contain genetic material. However, the genetic material in the mitochondria is only inherited from the mother. These organelles are the power house of the cell and involve the synthesis of ATP. Mitochondria are surrounded by two membranes - a smooth outer membrane and a folded inner membrane.


Centrosomes produces the microtubules, a component of the cytoskeleton which directs transport through the endoplasmic reticulum and Golgi apparatus. It is composed of two centrioles, which in cell division separate and assist in the mitotic spindle formation.


The endosome is a membrane bound vesicular structure located between the cell’s plasma membrane and the Golgi apparatus. Its role is to organize and direct vesicular traffic by directing vesicles for degradation to the lysozyme or secretion to the plasma membrane.


The cytoskeleton is a ‘skeleton’ made of different proteins. It plays a role in transport within the cell, cell division, cell shape and cell motility.


Biology OnLine: Cell Structure (

Ivy Rose Holistic: Histology and Cell Structure (