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1.  Hello 2.  Digestion 3.  Respiration

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Digestion

The digestive system, also called the alimentary canal or gastrointestinal tract, begins with the mouth, from where food enters, and ends with the anus, from where solid waste material leaves the body. The primary functions of the organs of the digestive system are threefolds. First, complex foods material that is taken into the mouth is digested, or broken down mechanically and chemically in to an absorbable form, as it travels through the gastrointestinal tract. Digestive enzymes are substances that speed up chemical reactions and help in the breakdown(digestion) of complex nutrients. Complex carbohydrates are digested to simple sugar such as glucose; complex proteins reduced to simpler amino acids; and large fat molecules are broken down to fatty acids and glycerol. Second, all the digested foods are absorbed into blood stream by passing through the walls of the small intestine. In this way, simpler foods are carried to the tissue where they are oxidized to release energy. Amino acids are also used in the growth and development of cells. Fatty acids and triglycerides are also absorbed through walls of small intestine but enter lymphatic vessels rather than blood vessels. The third function of the digestive system is to eliminate the solid undigested waste that cannot be absorbed into the bloodstream. These solid wastes, called as faeces, are concentrated in the large intestine and finally pass out the body through the anus. Activity of digestive system can be grouped under 5 main headings:

1. Ingestion: It is the process of taking food into gastrointestinal tract(GI tract).
   
2. Propulsion: It moves the food content along alimentary tract.
   
3. Digestion: It involves the breaking down the complex foods into simple and absorbable forms. It is of two types: mechanical and chemical digestion.
   
4. Absorption: It involves taking up the digested foods materials by the blood and transport to all the cells.
   
5. Elimination: It involves egestion of undigested food content through anus.
   

Respiration

After digestion, the digested foods are absorbed and transported along with blood to the various tissues, where they produce energy. But these foods do not produce energy unless they oxidised with oxygen. The oxygen is brought to the tissues from lungs through the process of respiration. Respiration can be defined as a biochemical process by which digested foods are oxidised to liberate energy. Therefore, respiration is an energy releasing process. It takes place in the tissues. The energy is essential to carry out all metabolic activities of the body. It is a catabolic process. Overall respiration involves two processes:

• Exchange of respiratory gases (O₂ and CO₂) (between lungs and blood).
  
• Oxidation of food stuff to release energy (between blood and tissues).

Cellular respiration occurs in mitochondria via respiratory chain. Energy is released in a stepwise process. It is temporarily stored in the cells in the form of adenosine tri-phosphate (ATP) molecules which are the energy currency of the cells. Whenever, a cell requires energy, ATP is broken down into ADP, a phosphate group and energy.

                           ATP → ADP + iP + Energy    [iP = Inorganic phosphate]