1.0 Digestion: The Journey Begins
Digestion is the process of converting complex, insoluble food into simple, soluble molecules that can be absorbed into the bloodstream. It involves the Alimentary Canal and Associated Glands.
1.1 The Teeth (Mastication)
Humans have Diphyodont (two sets: milk and permanent) and Heterodont (different types) dentition.
[attachment_0](attachment)| Type | Number (Adult) | Primary Function |
|---|---|---|
| Incisors | 8 | Cutting and Biting |
| Canines | 4 | Tearing food |
| Premolars | 8 | Crushing and Grinding |
| Molars | 12 | Grinding (includes wisdom teeth) |
1.2 Chemical Action of Saliva
Saliva contains Salivary Amylase (Ptyalin). Its primary role is the chemical breakdown of starch.
Starch + Salivary Amylase → Maltose (Sugar)
- pH: Saliva works best at a slightly alkaline or neutral pH.
- Mucin: A slippery substance that lubricates food into a Bolus.
- Lysozyme: An enzyme that kills bacteria.
The outer covering of the tooth crown is Enamel. It is the hardest substance in the human body. Below the enamel lies the Dentine, which forms the bulk of the tooth. If asked what contains blood vessels and nerves, the answer is the Pulp Cavity.
2.0 The Oesophagus and Stomach
2.1 The Oesophagus (Food Pipe)
The Oesophagus is a 25 cm long muscular tube. It does not produce any digestive enzymes. Its sole purpose is to transport the food bolus to the stomach.
Peristalsis is the involuntary wave of muscular contraction and relaxation that pushes food downward. This ensures food reaches the stomach even if you are upside down!
2.2 The Stomach: Chemical Churning
The stomach is a J-shaped muscular bag. It secretes Gastric Juice, which is a mixture of water, Hydrochloric acid, and enzymes.
| Component | Function |
|---|---|
| HCl (Acid) | Kills bacteria; Activates Pepsinogen into active Pepsin. |
| Pepsin | Breaks down Proteins into Peptides. |
| Rennin | Curdles milk protein (in infants) for better digestion. |
| Mucus | Protects the stomach lining from being digested by its own acid. |
The food is converted into a semi-liquid, pulpy mass called Chyme. This is then released in small amounts into the small intestine through the Pyloric Sphincter.
If asked why the stomach doesn't digest itself, always mention the protective Mucus layer. If asked for the functions of HCl, remember it serves two purposes: 1. Sterilizing food and 2. Providing an acidic medium (pH ~2) for protein enzymes to function.
3.0 The Small Intestine & Associated Glands
The small intestine is about 7 meters long and consists of three parts: the Duodenum (C-shaped), Jejunum, and Ileum. It receives secretions from two major glands.
3.1 The Liver: The Metabolic Factory
The liver is the largest gland in the body. It produces Bile, which is stored in the Gall Bladder.
- Emulsification: Breaking large fat globules into tiny droplets to increase surface area for enzymes.
- Alkalinity: It neutralizes the acidic chyme from the stomach, making it alkaline so pancreatic enzymes can act.
3.2 The Pancreas: The Enzyme Powerhouse
The Pancreas secretes Pancreatic Juice into the duodenum via the pancreatic duct. This juice contains three vital enzymes:
| Enzyme | Acts On | Product |
|---|---|---|
| Pancreatic Amylase | Leftover Starch | Maltose |
| Trypsin | Proteins/Peptides | Amino Acids |
| Steapsin (Lipase) | Emulsified Fats | Fatty Acids & Glycerol |
In exams, you might be asked why pancreatic juice is alkaline. It contains Sodium Bicarbonate, which neutralizes stomach acid. This is essential because Trypsin and Amylase cannot function in an acidic environment.
Bile contains no digestive enzymes at all. Its role is purely mechanical (emulsification) and chemical (pH adjustment). This is a top-tier "True or False" question in ICSE Biology!
4.0 Final Digestion & Absorption
4.1 Succus Entericus (Intestinal Juice)
The walls of the small intestine secrete intestinal juice containing a group of enzymes that complete the digestion of all food components.
| Enzyme | Acts On | End Product |
|---|---|---|
| Erepsin | Peptides | Amino Acids |
| Maltase | Maltose | Glucose |
| Lactase | Lactose (Milk sugar) | Glucose + Galactose |
| Sucrase | Sucrose (Cane sugar) | Glucose + Fructose |
4.2 Adaptation for Absorption: The Villi
The small intestine is specially adapted for absorption. Its inner surface is folded into millions of tiny finger-like projections called Villi.
- Great Surface Area: Millions of villi increase the surface area significantly.
- Single Cell Layer: The epithelium is very thin, allowing rapid diffusion.
- Blood Capillaries: To absorb Glucose and Amino acids.
- Lacteal (Lymph Vessel): To absorb Fatty acids and Glycerol.
In "Identify the function" questions, remember that while most nutrients go straight to the blood, fats (Fatty acids and Glycerol) are absorbed into the Lacteals. These are central lymph vessels found in the middle of each villus.
Absorption is purely the physical process of nutrients entering the blood/lymph. The subsequent use of these nutrients by cells to produce energy or new tissues is called Assimilation.
5.0 The Final Stages: Large Intestine & Beyond
After the small intestine has absorbed most of the nutrients, the remaining watery mass enters the large intestine. No digestion occurs here; the focus shifts to recovery and waste management.
5.1 The Large Intestine
The large intestine is wider but much shorter (about 1.5 meters) than the small intestine. It consists of the Caecum, Colon, and Rectum.
- Water Absorption: Reabsorbs most of the water from the undigested food, turning it into semi-solid faeces.
- Storage: The Rectum temporarily stores faeces until they are expelled.
- Egestion: The process of eliminating undigested food through the anus.
5.2 Assimilation: Putting Nutrients to Work
Assimilation is the conversion of absorbed food into the living matter (protoplasm) of the cells. This is how the body grows and repairs itself.
| Nutrient | Assimilated Use |
|---|---|
| Glucose | Used for energy (respiration). Excess stored as Glycogen in the liver. |
| Amino Acids | Used for building new cells, enzymes, and repairing tissues. |
| Fatty Acids | Used as a reserve energy source and stored as fat under the skin. |
5.3 The Liver: Post-Digestive Functions
- Deamination: The liver breaks down excess amino acids into urea (to be excreted by kidneys).
- Detoxification: It neutralizes toxins, alcohol, and drugs from the blood.
- Glycogenesis: Converts excess glucose into glycogen for future use.
ICSE examiners often trap students here. Excretion is the removal of metabolic waste (like Urea or CO2 produced inside cells). Egestion is the removal of undigested food residue that never entered the body's cells. They are not the same!
- Can you explain the function of the Appendix (a vestigial organ in humans)?
- Do you know why the large intestine is called "large" despite being shorter?
- Can you define Deamination and where it occurs?
- Do you understand the difference between Bolus, Chyme, and Faeces?