Animal Physiology — Nutrition and Digestion

"Every cell in your body needs NUTRIENTS. The digestive system is how you GET them."

1. Nutrition — How Organisms Obtain Food

2. The Human Digestive System — Step by Step

Step 1 — Mouth (Buccal Cavity)

Teeth: 4 types — Incisors (8, cutting). Canines (4, tearing). Premolars (8, crushing). Molars (12, grinding). Adult dental formula: 2-1-2-3 / 2-1-2-3 = 32.

Saliva: Secreted by 3 pairs of salivary glands (parotid, submandibular, sublingual). ~1-1.5 L/day. Contains SALIVARY AMYLASE (ptyalin) — begins STARCH → MALTOSE digestion. 'Chew bread for 2 minutes. It starts tasting SWEET — the amylase is breaking starch into sugar!'

Step 2 — Oesophagus (Food Pipe)

~25 cm muscular tube. Food moves by PERISTALSIS — rhythmic contraction and relaxation of muscles pushing food DOWNWARD. 'Peristalsis works AGAINST gravity. You can swallow while standing on your head!'

Step 3 — Stomach

J-shaped muscular bag. Inner lining has GASTRIC GLANDS that secrete GASTRIC JUICE: HCl (kills bacteria, provides acidic pH ~2 for pepsin). Pepsinogen (inactive) → activated by HCl → PEPSIN (active protease: Proteins → Peptides). Mucus (protects stomach lining from HCl and pepsin — prevents self-digestion). Food stays 3-4 hours → semi-liquid CHYME.

Step 4 — Small Intestine

LONGEST part (~6-7 m). Three sections: Duodenum, Jejunum, Ileum. Site of COMPLETE DIGESTION and ABSORPTION.

Absorption — The Villi

The inner wall of the small intestine has MILLIONS of finger-like projections called VILLI (singular: villus). Each villus contains BLOOD CAPILLARIES and a central LACTEAL (lymph vessel). Villi increase surface area ~600 times. Glucose and amino acids → absorbed into BLOOD. Fatty acids and glycerol → absorbed into LYMPH.

Step 5 — Large Intestine

Wider but SHORTER (~1.5 m). No villi. No digestive enzymes. Functions: Absorption of WATER and MINERALS (Na⁺, K⁺). Hosts BENEFICIAL BACTERIA (E. coli) that produce VITAMINS (B complex, K). Faeces formation → Rectum → Anus (EGESTION).

3. Key Enzymes Summary

4. Common Mistakes to Avoid

1. 'Bile contains enzymes' — Bile has NO enzymes. It EMULSIFIES fats (physical, not chemical).
2. 'Digestion happens only in the stomach' — It BEGINS in the mouth CONTINUES in the stomach, and is COMPLETED in the small intestine.
3. 'Absorption happens in the stomach' — VERY LITTLE absorption in the stomach (only alcohol, some water, aspirin). VAST MAJORITY in the SMALL INTESTINE.
4. 'Enzymes work at any pH' — Each enzyme has an OPTIMAL pH. Pepsin = pH 2 (stomach). Trypsin = pH 8 (small intestine).

5. AP SSC Exam Focus

Nutrition in AP Context

• Rice-based diet: Andhra Pradesh is the Rice Bowl. Rice is primarily CARBOHYDRATE — broken down by amylase starting in the mouth, then fully digested in the small intestine to glucose.
• Gongura and leafy greens: Rich in ROUGHAGE (fibre) — essential for digestion. Fibre is NOT digested but helps move food through the intestines and prevents constipation.
• Spicy Andhra cuisine: Spices like chilli stimulate gastric juice secretion. Moderate spice intake aids digestion — excessive intake can irritate the stomach lining.

Deficiency Diseases — A Crucial Connection

• Protein deficiency → KWASHIORKOR (swollen belly, stunted growth — common in children with inadequate protein intake).
• Protein + calorie deficiency → MARASMUS (extreme wasting, 'skin and bones').
• Iron deficiency → ANAEMIA (reduced haemoglobin — weakness, pale skin). Affects ~50% of women in India. Solved by iron-rich foods (leafy greens, dates, jaggery) and iron supplements.
• Vitamin A deficiency → NIGHT BLINDNESS. Vitamin C → SCURVY. Vitamin D → RICKETS. 'Understanding nutrition is understanding HOW to stay healthy. The digestive system chapter connects directly to public health in India.'

Respiratory System — A Preview (Linked to Digestion)

Digestion provides GLUCOSE. Respiration USES glucose to make ATP. C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + ATP. Organs: Nasal cavity → Pharynx → Larynx (voice box) → Trachea (windpipe, C-shaped cartilage rings) → Bronchi → Bronchioles → Alveoli (tiny air sacs, site of gas exchange). 'Alveoli provide a HUGE surface area (~70 m² — roughly a tennis court) for gas exchange with capillaries.'

Deep Dive — Enzyme Action and Specificity

Enzymes are BIOLOGICAL CATALYSTS — they speed up reactions without being consumed. Each enzyme is SPECIFIC to ONE substrate (lock-and-key model). Enzyme + Substrate → Enzyme-Substrate Complex → Enzyme + Product. 'An enzyme is like a key that fits only ONE lock. Amylase fits starch but not protein. Trypsin fits protein but not starch. This specificity is crucial for coordinated digestion.'

Effect of temperature on enzymes: Activity INCREASES with temperature up to an OPTIMAL point (~37°C body temperature). Above optimum: activity DROPS sharply — enzyme is DENATURED (loses its 3D shape). 'High fever (>42°C) can denature human enzymes. That's why prolonged high fever is dangerous.'

Effect of pH on enzymes: Each enzyme has an optimal pH. Pepsin (stomach): pH ~2. Trypsin (small intestine): pH ~8. Salivary amylase (mouth): pH ~7. 'HCl in the stomach denatures salivary amylase — so starch digestion stops once food enters the stomach. This is why you should chew thoroughly!'

Digestion in Other Organisms

Amoeba (Holozoic, single-celled) : Phagocytosis — pseudopodia engulf food → food vacuole → lysosomes fuse with food vacuole (deliver digestive enzymes) → digestion → absorption into cytoplasm → egestion by exocytosis.

Ruminants (Cow, Buffalo, Goat) : 4-chambered stomach — Rumen (largest — cellulose digestion by bacteria/protozoa → volatile fatty acids), Reticulum, Omasum, Abomasum (TRUE stomach — secretes gastric juice). 'Cows regurgitate and re-chew cud. The rumen is essentially a FERMENTATION VAT — symbiotic microbes digest cellulose that no animal can digest on its own.'

The Liver — The Body's Chemical Factory

Located in the upper right abdomen (~1.5 kg in adults). Functions beyond bile: Detoxifies ammonia → urea (urea cycle). Converts excess glucose → glycogen (glycogenesis — insulin-dependent). Converts glycogen → glucose when blood sugar is low (glycogenolysis — glucagon-dependent). Synthesises plasma proteins (albumin, clotting factors like fibrinogen and prothrombin). Stores vitamins A, D, B₁₂ and iron. Breaks down old RBCs (Kupffer cells). Produces HEAT (major thermogenic organ). 'The liver is the only internal organ that CAN REGENERATE. Up to 70% can be removed — it will grow back. This makes partial liver transplants possible.'

The Pancreas — Dual Organ

Exocrine function (98% of pancreas): Secretes pancreatic juice with digestive enzymes (trypsinogen, chymotrypsinogen, lipase, amylase). Endocrine function (Islets of Langerhans, ~2%): α-cells → glucagon (raises blood glucose). β-cells → insulin (lowers blood glucose). 'Damage to β-cells → insulin deficiency → Type 1 DIABETES MELLITUS. Understanding the pancreas explains one of India's most common diseases.'

Balanced Diet — Components and Importance

Peristalsis — The Mechanism

Controlled by the ENTERIC NERVOUS SYSTEM ('second brain' — ~500 million neurons in the gut). Circular muscles contract BEHIND food → push forward. Longitudinal muscles contract to SHORTEN the segment ahead. WAVE of contraction → food moves only FORWARD. 'Peristalsis works in one direction. The ONLY exception: VOMITING — reverse peristalsis (antiperistalsis) — coordinated by the vomiting centre in the medulla oblongata.'

Comparison — Mechanical vs Chemical Digestion

Quick Self-Test

1. Dental formula of adult human? (Answer: 2-1-2-3 / 2-1-2-3 = 32 teeth.)
2. Name the enzyme in saliva and its function. (Answer: Salivary amylase/ptyalin — starch → maltose.)
3. Bile is produced by ___ and stored in ___. (Answer: Liver; gall bladder.)
4. Where does maximum absorption occur? (Answer: Small intestine — via villi.)
5. Optimal pH for pepsin? (Answer: ~2 — acidic, provided by HCl in stomach.)
6. Name two protein-deficiency diseases. (Answer: Kwashiorkor and Marasmus.)