All major classes of structural molecules in living organisms, such as proteins, carbohydrates, and fats, contain oxygen, as do the major inorganic compounds that comprise animal shells, teeth, and bone. Oxygen in the form of O2 is produced from water by cyanobacteria, algae and plants during photosynthesis and is used in cellular respiration for all complex life.
Everything that the cell needs or has to get rid of has to go through the cell membrane, the amount of which relates to the surface area. Therefore, the cell's ability to either get substances from the outside or eliminate waste is related to the surface area. Secondly, how much food and other material from the outside and how much waste the cell has to get rid of, is related to the volume.
Therefore, as a cell gets bigger there will come a time when its surface area is insufficient to meet the demands of the cell's volume and the cell stops growing, or it reproduces ( mitosis ).
A series of growth disorders can occur at the cellular level and these underpins much of the subsequent course in cancer, in which a group of cells display uncontrolled growth and division beyond the normal limits, invasion (intrusion on and destruction of adjacent tissues), and never metastasis (spread to other locations in the body via lymph or blood).
Mitochondria are sometimes described as "cellular power plants" because they generate most of the cell's supply of adenosine triphosphate (ATP), used as a source of the chemical energy. In addition to supplying cellular energy, mitochondria are involved in a range of other processes, such as signaling, cellular differentiation, cell death, as well as the control of the cell cycle and cell growth
