Digestion of Food – Class 11 | Chapter – 16 | Biology Short Notes Series PDF

Digestion of Food: Digestion is the process of breakdown of large food molecules that are insoluble into small food molecules that are water-soluble. After the breakdown and conversion process of the food molecules, these are absorbed into the blood. Digestion is carried out by our digestive body organs, including the large intestine, small intestine, and the stomach. These body organs make up the digestive system of our body. 

Catabolism is a metabolic process in which large molecules are broken down into smaller molecules. Digestion is a type of catabolic process. The catabolism aspect is the breakdown of large food molecules into smaller food molecules that are soluble, and the nutrients generated by the food are absorbed by the bloodstream. Digestion is divided into two processes: mechanical digestion and chemical digestion. Refer to the official website of Vedantu or download the app for an elaborate and comprehensive explanation.

Types of Digestion of Food

Mechanical Digestion

Mechanical digestion refers to the breakdown of large food pieces into smaller food particles that are accessed by digestive enzymes. In chemical digestion, digestive enzymes help in the breakdown of large molecules that are insoluble into smaller molecules that are soluble.

In the human digestive system, the digestion process starts with mastication. Mastication is the chewing of food in the mouth. The food is crushed and grounded by teeth. Mastication is the mechanical digestion which occurs when the food is mixed with saliva. Saliva that is secreted by the salivary glands produces a kind of an enzyme called salivary amylase, which helps in digesting the starch present in food. Saliva also contains mucus which acts as a lubricant for the food that we eat. During mastication, when the food is mixed with saliva, it forms bolus. The bolus that is formed is then swallowed down the oesophagus from where through the action of peristalsis, it reaches the stomach.

Chemical Digestion

In the stomach, the gastric juice that contains hydrochloric acid and pepsin act upon the food particles to make simpler forms. The stomach further releases enzymes which act upon the food particles to absorb nutrients. The stomach breaks down the molecules and churns the bolus that was swallowed in the presence of gastric juice, pepsin, mucus and other enzymes. The food is not fully digested yet. It is partially digested in the stomach. The partially digested food is then entered in the duodenum, which is the first section of the small intestine. In the duodenum, the semi-digested food is called chyme. Chyme is a semi-liquid food that is passed from the stomach to the small intestine. The major part of the digestion takes place from here onwards. 

In the intestine, bile, pancreatic juice and intestinal juice are secreted, which helps in further digestion and metabolism. In the inner walls of the intestine, villi are lined, and the epithelial cells contain microvilli that support the absorption of nutrients and micronutrients. The storage capacity of the nutrients and micronutrients is increased by the villi as it increases the surface area of the intestines. From the small intestine, the food moves to the large intestine. In the large intestine, the digestion process is slowed down. The slow passage of the food helps in its fermentation. The fermentation of the food is carried out by the human gastrointestinal microbiota, which is also known as gut flora or gut microbiota. Water is absorbed from the food, and the rest of the waste which is stored as faeces is removed by the defecation process. In the defecation process, the waste material is expelled out of the body through the anal canal and anus.

Phases of Digestion of Food

The digestion process can be divided into three phases- the cephalic phase, the gastric phase, and the intestinal phase.

  • The cephalic phase involves not just the digestive system but also the nervous system and the endocrine system, which collaborates to help the secretion of gastric acid. The sight of the cephalic phase is basically the sight of smell and food. 
  • The gastric phase takes place in the stomach. It generally takes about three to four hours. With the presence of food, the pH level decreases, and this causes the enlargement of the stomach, which is called distension. There can be many other reasons for distention. Distension accelerates the secretion of various enzymes in the stomach – gastric juices and acetylcholine. 
  • In the intestinal phase, the food moves to the duodenum, and from there the partially digested food is moved further to complete its full digestion.

The digestive system is complemented by the excretory system of our body. When we eat something, the food is swallowed through the digestive organs, but most of the nutrients are processed in the kidney, which is the fundamental unit of the excretory system. The unwanted materials like urea, ammonia and others are not retained but moved forward to the intestines, and from there the waste materials are expelled out of our body through the anal canal and anus. 

Functions of the Digestive Organs

Organ Major functions Other functions
  • Ingests food
  • Chews and mixes food
  • Begins chemical breakdown of carbohydrates
  • Moves food into the pharynx
  • Begins breakdown of lipids via lingual lipase
  • Moistens and dissolves food, allowing you to taste it
  • Cleans and lubricates the teeth and oral cavity
  • Has some antimicrobial activity
  • Propels food from the oral cavity to the esophagus
  • Lubricates food and passageways
  • Propels food to the stomach
  • Lubricates food and passageways
  • Mixes and churns food with gastric juices to form chyme
  • Begins chemical breakdown of proteins
  • Releases food into the duodenum as chyme
  • Absorbs some fat-soluble substances (for example, alcohol, aspirin)
  • Possesses antimicrobial functions
  • Stimulates protein-digesting enzymes
  • Secretes intrinsic factor required for vitamin B12 absorption in small intestine
Small intestine
  • Mixes chyme with digestive juices
  • Propels food at a rate slow enough for digestion and absorption
  • Absorbs breakdown products of carbohydrates, proteins, lipids, and nucleic acids, along with vitamins, minerals, and water
  • Performs physical digestion via segmentation
  • Provides optimal medium for enzymatic activity
Accessory organs
  • Liver: produces bile salts, which emulsify lipids, aiding their digestion and absorption
  • Gallbladder: stores, concentrates, and releases bile
  • Pancreas: produces digestive enzymes and bicarbonate
  • Bicarbonate-rich pancreatic juices help neutralize acidic chyme and provide optimal environment for enzymatic activity
Large intestine
  • Further breaks down food residues
  • Absorbs most residual water, electrolytes, and vitamins produced by enteric bacteria
  • Propels feces toward rectum
  • Eliminates feces
  • Food residue is concentrated and temporarily stored prior to defecation
  • Mucus eases passage of feces through colon

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By Team Learning Mantras