Respiration

• Plants

Respiration in plants is essential as it provides metabolic energy and carbon skeletons for growth and repairs. As such, respiration is an essential component of a plant's carbon budget. Depending on species and environmental conditions, it consumes 25-75% of all the carbohydrates produced in photosynthesis,even more at extremely slow growth rates. Respiration in plants can also proceed in a manner that produces neither metabolic energy nor carbon skeletons, but heat.

Taken from: http://www.cplbookshop.com/contents/C2066.htm

• Animals

In complex animals, where the cells of internal organs are far from the external environment, respiratory systems facilitate the passage of gases to and from internal tissues. In such systems, when there is a difference in pressure of a particular gas on opposite sides of a membrane, the gas diffuses from the side of greater pressure to the side of lesser pressure, and each gas is transported independently of other gases.

• Humans
  

Gas exchange in humans takes place in the lungs. Breathing is the procedure in which oxygen reaches the lungs. During inhalation muscular action lowers the diaphragm and raises the ribs; atmospheric pressure forces air into the enlarged chest cavity. In exhalation the muscles relax and the air is expelled. This combined rhythmic action takes place about 12–16 times per minute when the body is at rest. The rate of breathing is controlled mainly by a respiratory center in the brain stem that responds to changes in the level of hydrogen ion and carbon dioxide in the blood, as well as to other factors such as stress, temperature changes, and motor activities. Some residual air always remains in the lungs, but with each breath an additional quantity of fresh air, called tidal air, is inhaled.

Taken from: http://www.factmonster.com/ce6/sci/A0860708.html