Answer:  

Dr. Fred Shih at USDA-ARS Rice Research Unit kindly answers the question above.
  
  
I may be oversimplifying the matter, but let me try:
  

As rice contains about 90% starch, rice-cooking is essentially the reaction of starch in water at elevated temperatures. Starch granules adsorb water and swell as the heating continues. When a certain temperature (call gelatinization temperature) is reached, the cell wall of the granule breaks and the starch turns viscous (gelatinized). That is essentially what happens during the cooking of rice. There are two types of starch in rice, amylose and amylopectin. Long grain rice is rich in amylose, whereas short grain rice is rich in amylopectin. Cooked-rice properties vary, depending on the type of rice or the ratio of amylose and amylospectin in the starch. Thus, as we all know, when well cooked, long grain rice is much less sticky than the short grain rice.

Structure of rice grain   Courtesy of International Starch Institute

  
Other than starch, which is carbohydrate, rice also contain some protein and fiber. As for nutrients in rice, they are mostly in the outer layer of rice called bran. Unfortunately, the bran is removed almost totally in the milling process. That's why we should eat brown rice (the rice with bran) not polished rice. 
  
  
Acknowledgement
We would like to thank Dr. Fred Shih for his kind answer.