CELLS AND TISSUES 



47 





Fiyure 3.7. Stages in iiiU()m,> ul a cell li'oin a salaiiiaiRlci hcait grown in tissue cul- 

 ture aiul photographed by phase microscopy. The numbers are clock readings. (Courtesy 

 of L. Wang.) (Maxiraow and Bloom: Textbook of Histolog) .) 



must first consider some of the basic physical concepts of energy and 

 molecular motion. 



13. Energy 



Energy may be defined as the ability to do work, to produce a 

 change in matter. It may take the form of heat, light, electricity or mo- 

 tion. Physicists recognize two kinds of energy: potential energy, the 

 ability to do work owing to the position or state of a body, and kinetic 

 energy, the capacity to do work possessed by a body because of its 

 motion. A rock at the top of a hill has potential energy; as it rolls down- 

 hill the potential energy is converted to kinetic energy. 



Energy derived ultimately from solar energy is stored in the mole- 



