Briefly describe the structure and function of the membrane-bound organelles present in a mammalian cell.

Nucleus - This is the largest organelle within the cell and contains all the cell's genetic information (with the exception of the mitochondrial genome). Structurally, it has a double membrane, known as the nuclear envelope, which tightly regulates the passage of substances in and out of the nucleus. Within the nuclus, there is a smaller, dense spherical structure called the nucleolus which is the site of ribosome synthesis. 

Endoplasmic Reticulum (ER) - The ER is a continuation of the nucleus and is formed of flattened membrane bound sacs. It can be further subdivided both morphologically and functionally into the smooth ER and the rough ER. The smooth ER is responsible for lipid biosynthesis. The rough ER is studded with ribosomes (the organelles responsible for translation) and is the site of protein synthesis for proteins which would be too dangerous to release directly into the cytoplasm. 

Golgi Apparatus - This is formed from stacks of flattened membrane. It is responsible for the post-translational folding and modification of proteins synthesized by the cell. 

Lysosome - This acidified organelle contains digestive enzymes and is important in breaking down contents which the cell may have endocytosed (e.g. pathogens). 

Mitochondria - This double membrane bound organelle is sometimes known as the 'powerhouse of the cell' and is responsible for all ATP produced by aerobic respiration. Its inner membrane is highly folded to form cristae which contains the complexes of the electron transport chain necessary for ATP production.