In the previous articles on nutrition, we discussed the basics of nutrition including autotrophic and heterotrophic nutrition. In this article, we will discuss human nutrition.
Human beings are heterotrophs. We depend on other living organisms of food. The food consumed by us varies in complexity from simple sugars like glucose to complex carbohydrates, fats and proteins. The individual cells of the human body can only utilise elemental molecules like glucose, amino acids and fatty acids. Therefore, the complex molecules in our diet need to be broken down before absorption. Humans have developed a specialised organ system to accomplish this task- the digestive system.
The digestive system begins in the mouth or oral cavity and terminates in the anus. From the mouth to anus, the digestive system is one long tube. However each part of this tube is distinct, in both anatomy and functional characteristics.
The oral cavity is the first part of the digestive tract. There are some major functions performed by the mouth and most involve preparing the food for the digestive process.
Have you ever wondered why our mouth fills up with fluid when we see or smell food? This fluid is called saliva, and its function is twofold. The primary function is to moisten the food to enable easy swallowing. Secondly, the saliva contains many enzymes, which are biological catalysts, which begin the process of digestion. ?Many glands line the oral cavity; these are called salivary glands. There are the parotid glands on both sides of the jaw that produce the bulk of saliva. Then there is the submandibular gland located beneath the floor of the mouth and then there are many small salivary glands that line the oral cavity.
The oral cavity also contains teeth whose primary function is to masticate the food into a soft, semisolid consistency. Semisolid food is easy to swallow, and it also ensures mixing of the food with enzymes.
Once the food is moistened and chewed, it is taken and transported to the stomach through the oesophagus. The oesophagus is a long muscular tube whose sole purpose transportation of food from the mouth to the stomach. Transportation of food to the stomach is not dependent upon gravity. One can eat even in a lying position, so how does the oesophagus transport food to stomach. It does so by a series of co-ordinated movements called peristalsis. In fact, all segments of the digestive system express this form of action. Now consider a food bolus that is being swallowed. The muscles behind the bolus push the food forward while the muscles ahead of the bolus relax, thus ensuring seamless transport of food from one end to another. The digestive system has developed an intricate web of nerves which co-ordinate this process. This web is called the enteric nervous system.
The next stop for food is the stomach. The stomach is a hollow muscular tube that expands as food enters the stomach. The stomach secretes acid and some enzymes like pepsin. The acidic environment in the stomach serves two purposes. One, it helps to kill ingested bacteria, secondly, it facilitates the action of enzymes like pepsin (an enzyme that aids digestion of proteins).
Acidity is a common problem in humans. You might have seen many of your family or even yourself suffering from burning sensation in the stomach, especially when one is hungry. This is due to the secretion of acid by the stomach. So, how does the stomach protect itself from the action of acid? The cells lining the stomach produce a slimy fluid called mucus. The mucous coats the surface of the stomach and prevents damage to the lining of the stomach by acid. When this mucous becomes depleted, then symptoms of acidity occur (of course there are many other reasons for acidity).
From the stomach, the food then travels to the intestines. The intestine is a long hollow tube that lies within the abdomen. There are three parts of the gut- duodenum, jejunum and ileum.
Two principal digestive organs secrete their enzymes into the first part of the intestine- liver and pancreas. Liver secretes bile that helps with the digestion of fat. Fat is a very complex molecule and is not soluble in water while the enzymes can only act in a water-based medium. Bile acts on the fat like soap acts on grease. It emulsifies the fat to make it water soluble. It also increases the surface area for enzymes to act.
Also, the pancreas also secretes its enzymes into the duodenum. Pancreas also secretes copious amounts of dilute bicarbonate into the duodenum. This bicarbonate neutralises the acid (remember stomach secretes acid) and enables enzymes like trypsin and chymotrypsin to act on the proteins.
The food then travels along the length of the intestine where carbohydrates, proteins and fats are broken down by the above enzymes into their elemental forms, namely glucose, amino acids and fatty acids. They then travel to the ileum, where they enter the bloodstream by absorption through the cells lining the ileum. ?The cells lining the intestine have fine hair like projections called villi. These increase the surface area of the intestine manifold and help with the absorption of nutrients.
From the ileum, the food bolus, which now only contains indigestible fibre, moves to the colon. Here the food residue is compacted by absorption of water and then expelled out of the body through the anus.
To summarise, the humans are dependent on other living beings for their nutrition. The human digestive system does the task of breaking the food into its basic unit?and its absorption. The alimentary is a highly evolved and well-organised system with different components. Each organ has a particular role to play in human nutrition as discussed above. Students of class 10 science are advised to familiarise themselves with the basics of human nutrition before their board exams.
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