Digestive System: Small Intestine

Digestion in the Small Intestine

• The small intestine is the longest part of the alimentary canal, fitted into a compact space due to extensive coiling.
• The length of the small intestine varies among animals, depending on their diet, with herbivores having a longer small intestine to allow for cellulose digestion.
• The small intestine is the site of complete digestion of carbohydrates, proteins, and fats.

Role of Liver and Pancreas

• The liver secretes bile juice, which makes the acidic food from the stomach alkaline, allowing pancreatic enzymes to act.
• Bile juice breaks down fats into smaller globules, increasing the efficiency of enzyme action.
• The pancreas secretes pancreatic juice, containing enzymes like trypsin for protein digestion and lipase for fat breakdown.

Digestion and Absorption

• The walls of the small intestine contain glands that secrete intestinal juice, which converts proteins to amino acids, complex carbohydrates to glucose, and fats to fatty acids and glycerol.
• The inner lining of the small intestine has numerous finger-like projections called villi, which increase the surface area for absorption.
• The villi are richly supplied with blood vessels, taking the absorbed food to every cell of the body for energy, tissue building, and repair.

Unabsorbed Food and Waste Removal

• Unabsorbed food is sent to the large intestine, where its wall absorbs more water from the material.
• The remaining material is removed from the body via the anus, with the exit of waste material regulated by the anal sphincter.

Nutrition in Organisms

• Food material is used in cells to provide energy for various life processes.

Cellular Respiration

• Glucose, a six-carbon molecule, is broken down into a three-carbon molecule called pyruvate in the cytoplasm.
• Pyruvate may be converted into ethanol and carbon dioxide through anaerobic respiration (in the absence of oxygen), which occurs in yeast during fermentation.
• Pyruvate may also be broken down using oxygen in the mitochondria, resulting in three molecules of carbon dioxide and water, through aerobic respiration.
• Aerobic respiration releases a lot more energy than anaerobic respiration.

Breakdown of Pyruvate

• In the absence of oxygen, pyruvate is converted into lactic acid, a three-carbon molecule, in muscle cells.
• The build-up of lactic acid in muscles during sudden activity causes cramps.

Energy Generation

• The energy released during cellular respiration is used to synthesise ATP (adenosine triphosphate).
• ATP is broken down to release a fixed amount of energy that drives endothermic reactions in the cell.
• ATP is used to fuel all other activities in the cell.

Learn about the small intestine, its structure, function, and adaptations in different animals based on their diet.