Digestion and Absorption: CBSE Class 11 Biology

The Human Digestive System: A Journey Through the Alimentary Canal

The human digestive system consists of the alimentary canal and the associated glands. The alimentary canal is a long, muscular tube that begins at the mouth and ends at the anus.

1. Mouth and Oral Cavity: Digestion starts here. The teeth perform mechanical digestion (mastication). Our heterodont dentition (incisors, canines, premolars, molars) is specialized for cutting, tearing, and grinding. The tongue manipulates food and mixes it with saliva from the salivary glands. Saliva contains the enzyme salivary amylase, which begins the chemical digestion of starch into maltose.

1. Pharynx and Oesophagus: The chewed food, now a 'bolus', passes through the pharynx (a common passage for food and air) into the oesophagus. A cartilaginous flap called the epiglottis prevents food from entering the windpipe. The oesophagus pushes the bolus down to the stomach through wave-like muscular contractions called peristalsis.

1. Stomach: This J-shaped organ stores food and continues digestion. The stomach wall has gastric glands that secrete gastric juice containing hydrochloric acid (HCl), pepsinogen, and mucus. HCl creates a highly acidic environment (pH 1.8) that kills bacteria and activates pepsinogen into its active form, pepsin. Pepsin begins the digestion of proteins into smaller peptides (proteoses and peptones). Mucus protects the stomach lining from the acid. The food, now a semi-liquid paste called chyme, is released in small amounts into the small intestine.

1. Small Intestine: This is the main site for digestion and absorption. It's divided into three parts: the duodenum, jejunum, and ileum. The duodenum receives secretions from the liver (bile) and the pancreas (pancreatic juice). Bile emulsifies fats (breaks large fat globules into smaller droplets), increasing the surface area for enzymes to act. Pancreatic juice contains enzymes like trypsinogen, chymotrypsinogen, amylase, and lipase. The intestinal wall also secretes enzymes. By the end of this stage, all macromolecules are broken down into their simplest forms: glucose, amino acids, fatty acids, and glycerol.

1. Large Intestine: The undigested and unabsorbed substances pass into the large intestine. Its main function is to absorb water, minerals, and certain drugs. It also secretes mucus which helps in holding the waste particles together and lubricating it for easy passage. The undigested, unabsorbed waste, called faeces, enters the rectum and is expelled through the anus.

Key Enzymes and Their Roles in Digestion

Salivary Amylase (Ptyalin)

Source: Salivary glands (in the mouth). Action: Breaks down starch (a complex carbohydrate) into maltose (a disaccharide).

Pepsin

Source: Gastric glands (stomach), secreted as inactive pepsinogen. Action: Begins the digestion of proteins into smaller polypeptides called proteoses and peptones in an acidic medium.

Trypsin

Source: Pancreas, secreted as inactive trypsinogen into the small intestine. Action: Digests proteins, proteoses, and peptones into smaller peptides.

Pancreatic Lipase

Source: Pancreas (acts in the small intestine). Action: The principal enzyme for fat digestion; it breaks down triglycerides (fats) into fatty acids and monoglycerides.

Dipeptidases & Disaccharidases (e.g., Maltase, Lactase, Sucrase)

Source: Intestinal wall (brush border enzymes). Action: These are final-stage enzymes. Dipeptidases break dipeptides into amino acids. Disaccharidases break down disaccharides (like maltose, lactose, sucrose) into monosaccharides (like glucose, galactose, fructose).

Step-by-Step: How Nutrients are Absorbed

1. Step 1: Simple Diffusion — Small, nonpolar molecules move from an area of high concentration to low concentration across the cell membrane without any help. Small amounts of monosaccharides like glucose, amino acids, and some electrolytes like chloride ions are absorbed this way.
2. Step 2: Facilitated Diffusion — Some nutrients, like glucose and most amino acids, are absorbed with the help of carrier proteins in the cell membrane. This process does not require energy but is dependent on the concentration gradient. Fructose is a key example of a nutrient absorbed via facilitated diffusion.
3. Step 3: Active Transport — This process moves nutrients against their concentration gradient, from a lower to a higher concentration. It requires energy in the form of ATP. Many nutrients, including large amounts of glucose, amino acids, and electrolytes like Na+, are absorbed into the blood by this mechanism.
4. Step 4: Absorption of Fats — This is a unique process. Fatty acids and glycerol, being insoluble, cannot be absorbed directly into the blood. First, bile salts help form them into small, water-soluble droplets called micelles. These micelles move into the intestinal cells (mucosa). Inside the cells, they are re-formed into triglycerides and coated with protein to form small globules called chylomicrons. These chylomicrons are then transported into the lymph vessels (lacteals) in the villi, which eventually release them into the bloodstream.

Exam Traps and Key Points to Remember

Emulsification vs. Digestion: A very common point of confusion. Bile emulsifies fats; it does not digest them. Emulsification is a physical process of breaking large fat globules into smaller ones. The chemical digestion of fat is carried out by lipases.

Zymogens (Inactive Enzymes): Remember that powerful protein-digesting enzymes like pepsin and trypsin are secreted in an inactive form (pepsinogen and trypsinogen). This prevents them from digesting the very cells that produce them. Know the activators: HCl activates pepsinogen to pepsin, and enterokinase (from the intestinal mucosa) activates trypsinogen to trypsin.

Site-Specific pH: The pH of the digestive tract varies dramatically and is crucial for enzyme function. Pepsin works only in the highly acidic environment of the stomach (pH 1.8-2.0). Salivary amylase works at a neutral pH (6.8) in the mouth. Most enzymes in the small intestine, like trypsin and lipase, function in an alkaline medium.

Role of the Large Intestine: The large intestine does not carry out significant digestion. Its primary roles are the absorption of water and electrolytes, and the formation and storage of faeces.

Practice Questions with Solutions

• [object Object]
• [object Object]
• [object Object]
• [object Object]

Frequently Asked Questions