15:1/ X-ray Analyses of Proteins and Nucleic Acids 269 



physical form to the structures they surround, but also can use energy 

 to transport molecules actively against electrochemical gradients. 

 (Active transport is discussed in Chapter 23.) 



Proteins are found in abundance within the formed elements, the 

 contractile elements, the nucleus, and the endothelial reticulum shown 

 in Figure 1. Some of these proteins serve structural purposes and 

 others act as catalysts for the numerous biological reactions which must 

 occur at controlled rates if the cell is to live. Other proteins are found 

 in the liquid parts of the cytoplasm and nucleoplasm ; these are probably 

 mostly enzymes, although some may be concerned with the osmotic 

 balance of the cell. The number and variety of different proteins seem 

 almost limitless. 



The nucleic acids are also found both in the cytoplasm and in the 

 nucleoplasm. Those in the cytoplasm are all of a type called Ribo- 

 Nucleic Acids (RNA) ; they probably exist primarily as small collections 

 of RNA along the endothelial reticulum. Within the nucleus, there are 

 two types of nucleic acids. One type is RNA, similar to that found in 

 the cytoplasm; the other type, the Deoxijribose Nucleic Acids (DNA), 

 occurs only in the nucleoplasm. Evidence was presented in the pre- 

 ceding chapter to show that DNA is associated with the transmission of 

 genetic information. During cell division (see Chapter 10), the DNA 

 in the nucleus of most plant and animal cells is organized into long 

 threads, called chromosomes. The chromosomes contain both protein and 

 DNA; they are referred to as nucleoproteins. 



The RNA, in contrast with the DNA, is not restricted to the chromo- 

 somes or the cell nucleus. It is believed that DNA controls the synthesis 

 of RNA which in turn controls the synthesis of proteins. Thus, both 

 DNA and RNA act as biological catalysts, controlling ultimately the 

 synthesis of protein-enzymes. These, in turn, control the rates of most 

 other chemical reactions within the cell. The role of DNA is discussed 

 further in Chapter 25. (As stated in Chapter 14, in the case of many 

 plant viruses and some animal viruses, the genetic information necessary 

 to build new virus particles is carried by RNA rather than DNA.) 



There are many other types of molecules within the cell besides 

 proteins and nucleic acids. In the typical living cell, there are more 

 molecules of water than of any other compound. Water makes up as 

 much as 80 per cent of the cell weight in some cases. The fatlike mole- 

 cules, that is, lipids, have already been mentioned in connection with 

 membranes but are by no means restricted to the membranes. Rather, 

 lipids are found in varying roles throughout the cell. Those in fat 

 globules are used to store energy, and a few lipids are hormones. How- 

 ever, the role of most lipids is unknown. Structures of a few lipids are 

 shown in Figure 2. The typical lipid has a molecular weight between 



