Digestion Definition

With regards to digestion definition, it can be defined as the breaking down of large particles of indissoluble food molecules into tiny water dissolving molecules. A body quickly absorbs these small molecules of food into blood plasma.

Also, in some organisms, their small intestine helps in absorbing the small food molecules into blood stream. Moreover, digestion is a kind of catabolism which is often classified into two procedures depending on the breaking down of food. The processes are:

• Mechanical Digestion

This process involves physical breakdown of big chunks of food into smaller bits, which the enzymes can digest easily.

• Chemical Digestion

In case of chemical digestion, the digestive enzymes break food particles into smaller molecules which the body can utilise.

Do It Yourself

1. Name the organs related to digestive system in humans?

(a) Pancreas and Salivary Glands 

(b) Liver and Salivary Glands 

(c) Pancreas and Liver 

(d) Liver, Pancreas and Salivary Glands

2. Single-cell organisms exhibit a form of digestion referred to as:

(a) Ingestion 

(b) Phagocytosis 

(c) Absorption 

Process of Digestion in Humans

Considering the system of digestion for human beings, mechanical digestion starts as soon as someone eats food and starts mastication (chewing) in the presence of saliva. Furthermore, saliva is a fluid secretion from salivary glands containing salivary amylase. This enzyme aids in digestion of starch present in food.

Moreover, saliva also consists of hydrogen carbonate and mucus. The former one gives suitable pH (alkaline) conditions so that amylase can work, and the latter one helps in lubricating food.

After the actions of starch digestion and mastication, food takes up the shape of tiny circular slurry mass known as a bolus. Next, it moves to the stomach by travelling through the oesophagus with the support of an action called peristalsis.

When food reaches stomach, gastric juices present in it begins the digestion of proteins. It primarily consists of HCL (hydrochloric acid) and pepsin. However, renin is also present in gastric juices in small children like toddlers and infants.

The two chemicals – hydrochloric acid and pepsin can affect and cause damage to the stomach wall. So, stomach secretes mucus which provides a greasy layer against the harming effects of HCL and pepsin. Simultaneously, digestion of protein also occurs where mechanical mixing takes places by the action of peristalsis. Additionally, this process assists in further mixing up of food with digestive enzymes.

Furthermore, the thick fluid which forms after a considerable amount of time (human beings take 1 to 2 hours, cats take 3 to 4 hours, and dogs require 4 to 6 hours) is known as chyme. It moves to duodenum after pyloric sphincter valve opens. Moreover, in duodenum, chyme mixes up with enzymes secreted by pancreas and bile from liver. Then, it travels through small intestine where digestion still continues.

Additionally, when digestion of chyme is fully done, it gets absorbed into blood. Moreover, 95 per cent of absorption of nutrients takes place in small intestine. Also, minerals and water again undergo reabsorption in colon (large intestine), which has pH a bit acidic (5.6 ~ 6.9). 

However, bacteria present in colon produces few vitamins like vitamin K and biotin, which also gets absorbed in colon by blood. Finally, waste material eliminates during defecation from rectum.

Digestive Hormones

In general, five hormones help in the digestive system for mammals; they are –

• Gastrin.

• Secretin.

• Cholecystokinin (CCK).

• Gastric inhibitory peptide (GIP).

• Motilin.

Variations are there over vertebrate organisms like for example in birds.

Did You Know?

Unicellular organisms like the amoeba, exhibit a kind of digestion known as phagocytosis. In this process, phagocytes (here amoeba) ingest food particles with the help of plasma membrane and enables digestion.

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