Need for Food

Through photosynthesis, plants convert light energy to chemical energy in food. Animals must consume plants or other animals that consume plants to get this energy.

Nutrients are chemical substances in the food we eat and they provide energy and raw materials for cell growth, cell division, metabolism and repair. In addition, it helps organisms stay healthy, as it provides substances the cells cannot produce.

Main Types

There are three main types of nutrients : carbohydrates, fats and proteins.

Purpose

The main function of the digestive system is to break down food substances into small and simple food substances. Why is this necessary?

Physical vs Chemical

This breakdown of food occurs via two processes : physical digestion and chemical digestion. This table summarises the essential characteristics of each type.

What are they?

Enzymes are a type of protein that speeds up rate of reaction. Enzymes are very important and are heavily involved in chemical digestion. Here are some examples of enzymes in the body.

How they work

The targets of enzyme action are called substrates. These substrates bind to a highly specific active site on the enzyme. This is where it is broken down. Enzymes remain chemically unchanged after the entire process and can be reused repeatedly.

There are two models of enzyme action : lock and key and induced fit. Both models state that the substrate must fit the active site. Since the active site has unique configuration, each enzyme can only act on one type of substrate.

Parts of the System

The human digestive system consists of the gut or alimentary canal and the organs (liver and pancreas) associated with it. The alimentary canal involves a tube running from the mouth to anus.

Peristalsis is a series of wave-like constructions that moves food along the gut. Mucus along the gut acts as lubricant. Digestion occurs in the gut. Organs such as salivary glands and the liver secrete substances involved in digestion.

The Mouth

Food enters the body through the mouth (ingestion). In the mouth there are:
* teeth - chewing action (physical digestion)breaks down food into smaller pieces, increasing surface area for enzymes to act more efficiently
* salivary glands - secrete saliva containing salivary amylase (chemical digestion) via salivary ducts
* tongue - helps mix food with saliva; taste buds help identify food.

The Oesophagus

The oesophagus is a muscular tube connecting the mouth to the stomach. The oesophagus wall has two antagonistic muscles that move the bolus (food) along the gut via peristalsis.

Peristalsis is the alternate waves of muscular contractions and relaxations in the wall of the alimentary canal, as seen below.

The Stomach

The stomach is a J-shaped distensible bag, with thick and well-developed muscular walls. The churning action of the stomach wall causes food to break up further into smaller pieces (physical digestion). It also helps mix food well with gastric juice.

Gastric juice is a dilute solution of hydrochloric acid (pH 2), water, and proteases. The proteases digest proteins into amino acids (chemical digestion). The hydrochloric acid provides an acidic environment optimal for the proteases to function. It also kill germs and parasites.

Food remains in the stomach for 2 to 6 hours and is liquefied to form chyme.

The Small Intestine

The small intestine is a long muscular tube with average length 6 metres. The liver, gall bladder and pancreas are associated with it. The presence of chyme triggers production of pancreatic juice, intestinal juice and bile.

Bile is produced by the liver and stored in the gall bladder. Bile breaks up fat molecules into smaller fat globules by emulsification, increasing surface area (physical digestion) and speeding up fat digestion.

Pancreatic juice is secreted by the pancreas and intestinal juice by the small intestine. They contain enzymes like amylase, maltase, carbohydrases, proteases and lipase (chemical digestion).  

The table below summarizes the enzymes found in small intestines and its origin.

The Large Intestine

The large intestine is about 1.5 metres long, shorter but broader than the small intestine. The main function of the colon is to absorb water and mineral salts from undigested food.

Villus

Absorption is when digested food is absorbed into the body.

Villus are finger like structures in the small intestine. They increase surface area for more efficient absorption and are supported by a network of blood capillaries for efficient extraction of nutrients.

The process

Carbohydrates, proteins and fats are absorbed via diffusion and active transport into the blood stream.

They pass through the wall of the small intestine, which consists of one-cell thick epithelial cells. This allows quick absorption of the nutrients into the bloodstream.

The following table shows how the small intestine is adapted for efficient absorption.

Putting it all together

The entire process of digestion and absorption has been broken down. Now, this video will allow you to visualise how the entire process works!

Food-borne Disease

Consuming unhygienic or contaminated food will lead to vomiting, diarrhoea, fever, and other symptoms of food-borne disease.

Bacteria in the food cause infections in the intestine and produce toxins which spread to cause infections elsewhere in the body.

Excessive Alcohol Consumption

Alcoholic drinks contain ethanol, which is toxic to the body, and does not give us energy. Excessive alcohol consumption will have biological and social impacts on the individual and society. The effects are summarized in the table below.