Introduction
Digestion and absorption are critical processes in human physiology that convert food into nutrients and energy necessary for cellular functions. This document outlines the key concepts of digestion and absorption as per the International Baccalaureate (IB) Biology Higher Level (HL) syllabus.
6.1.1 Digestion
Overview of Digestion
Digestion is the process by which large, insoluble food molecules are broken down into smaller, soluble molecules that can be absorbed into the bloodstream. The human digestive system consists of the alimentary canal and accessory organs.
Alimentary Canal
- Mouth: Initiates mechanical digestion through chewing and chemical digestion via salivary amylase.
- Esophagus: Transports food to the stomach.
- Stomach: Uses gastric juices containing hydrochloric acid and pepsin to break down proteins.
- Small Intestine: Completes digestion with enzymes from the pancreas and bile from the liver.
- Large Intestine: Absorbs water and forms feces.
Accessory Organs
- Salivary Glands: Produce saliva containing amylase.
- Pancreas: Produces digestive enzymes and bicarbonate.
- Liver: Produces bile to emulsify fats.
- Gall Bladder: Stores and concentrates bile.
Key Processes
- Mechanical Digestion: Physical breakdown of food (e.g., chewing).
- Chemical Digestion: Enzymatic breakdown of macromolecules into monomers.
Accessory organs do not directly process food but produce substances essential for digestion.
6.1.2 Villi & Absorption
Structure of the Small Intestine
The small intestine is adapted for absorption through structures called villi and microvilli, which increase the surface area.
Villi
- Microvilli: Tiny projections on epithelial cells that further increase surface area.
- Lacteal: Lymphatic vessel within each villus that absorbs fats.
- Capillaries: Absorb amino acids and monosaccharides.
Absorption
Absorption is the movement of digested food molecules from the digestive system into the blood and lymph.
Key Absorbed Substances
- Monosaccharides: Glucose, fructose, galactose.
- Amino Acids: Building blocks of proteins.
- Lipids: Fatty acids, monoglycerides, glycerol.
