Chapter 2 — Nutrition In Animals

What is nutrition? Nutrition is the process by which organisms take in food and use it for growth, energy, repair and maintenance. Animals obtain organic material and energy by consuming other organisms (plants, animals or organic remains).

Modes of nutrition relevant to animals

• Holozoic nutrition (most animals): Involves five main steps — ingestion (taking in food), digestion (breaking down food to small molecules), absorption (uptake of digested molecules), assimilation (use of absorbed nutrients for body functions), and egestion (removal of undigested waste). Example: humans, dogs, earthworms.
• Parasitic nutrition: Animals live on or inside a host and obtain nutrition from it, often harming the host. Example: tapeworms, leeches.
• Saprophytic/decomposers (few animals show saprophytic behaviour): Feed on dead organic matter and help recycling. Example: some earthworms and certain insects help decompose matter.

Stages of holozoic nutrition (with short explanation)

• Ingestion: Food is taken into the body (e.g., mouth in humans, beak in birds, phagocytosis in amoeba).
• Digestion: Mechanical and chemical breakdown. Mechanical — chewing or grinding (teeth, gizzard). Chemical — enzymes break complex molecules into simpler ones (starches → sugars, proteins → amino acids, fats → fatty acids + glycerol).
• Absorption: Digested products pass through the gut wall into blood or body fluids (e.g., small intestine in humans; across cell membrane in unicellular organisms).
• Assimilation: Nutrients used to build tissues, for energy, growth and repair.
• Egestion: Removal of undigested and indigestible material as feces.

Special adaptations in animal nutrition

• Ruminants (cows, buffalo): Multi-chambered stomach (rumen, reticulum, omasum, abomasum) to ferment and digest cellulose with help of microbes.
• Birds: Many have a crop (storage) and a gizzard (grinding) instead of teeth.
• Unicellular animals (amoeba): Use pseudopodia to engulf food into a food vacuole where digestion occurs intracellularly.
• Insect gut divisions: Foregut (storage), midgut (digestion and absorption), hindgut (water reabsorption and egestion).

Why nutrition in animals matters

Proper nutrition provides energy for movement, growth, reproduction and maintaining body temperature. Different diets and digestive systems are adaptations to available food and habitat.

Quick summary — Animals are heterotrophs and show mainly holozoic nutrition. The process includes ingestion, digestion, absorption, assimilation and egestion. Various structural and microbial adaptations help animals digest different types of food.

Overview: Nutrition is the process by which organisms obtain and use food to get energy and materials for growth, repair and maintenance. Animals cannot prepare their own food (unlike most plants) and therefore show different modes of nutrition depending on how they obtain and process food.

Major types / modes of nutrition in animals

• Holozoic nutrition: Characteristic of most animals (humans, dogs, earthworms). Food is taken into the body (ingestion), broken down mechanically and chemically (digestion), soluble products are absorbed, useful materials are assimilated and waste is egested. Main steps: ingestion → digestion (mechanical & chemical) → absorption → assimilation → egestion.
• Herbivores, Carnivores and Omnivores (feeding categories within holozoic nutrition):
  • Herbivores eat plants (e.g., cow, deer).
  • Carnivores eat other animals (e.g., lion, eagle).
  • Omnivores eat both plants and animals (e.g., human, bear).
• Ruminant digestion (specialised holozoic): Cows, goats and other ruminants have a four-chambered stomach (rumen, reticulum, omasum, abomasum). They partially digest plant material, regurgitate and chew the cud to break down cellulose with the help of microbes.
• Filter (suspension) feeding: Animals like sponges, many bivalves and baleen whales obtain food particles suspended in water by filtering it—no chewing, but a physical trapping mechanism.
• Parasitic nutrition: Parasites live on or inside a host and obtain nutrients at the host's expense. Examples: tapeworms (intestinal parasites), leeches (external parasites). Parasites may have reduced digestive systems and absorb pre-digested food from the host.
• Symbiotic nutrition / Mutualism: Two organisms live together benefiting each other. Example relevant to animals: termites harbour protozoa in their gut that help digest cellulose; corals host photosynthetic algae (zooxanthellae) that provide nutrients.
• Saprophytic nutrition: Typical of fungi and some bacteria (decomposers) — they secrete enzymes on dead organic matter and absorb nutrients. Saprophytes are generally not animals, but they are important in nutrient cycles that affect animal food supply.

Energy flow and food chains: Animals occupy different trophic levels in a food chain (producers → primary consumers → secondary consumers → tertiary consumers…). Energy flows from lower to higher trophic levels but reduces markedly at each step. This explains why there are fewer large predators than herbivores in an ecosystem.

Key points to remember:

• Most animals are heterotrophs (cannot make their own food).
• Holozoic nutrition involves mechanical and chemical digestion inside the body.
• Parasitic and symbiotic relationships are special ways animals obtain nutrients.
• Energy decreases along the food chain (approx. 10% rule — see formulas).

Definition: Holozoic nutrition is the type of nutrition in which organisms take in solid or liquid food, break it down (digest), absorb the digested products, use them for energy and growth, and remove undigested wastes. Most animals exhibit holozoic nutrition.

Main features / stages:

1. Ingestion: Taking food into the body (e.g., mouth of humans, phagocytosis by amoeba).
2. Digestion: Breakdown of complex food into simple soluble molecules. This can be mechanical (chewing, grinding) and chemical (enzymes). Digestion may be intracellular (inside cells, e.g., amoeba) or extracellular (inside a gut, e.g., humans).
3. Absorption: Simple molecules (glucose, amino acids, fatty acids, glycerol) pass into blood or cells from the gut or food vacuoles.
4. Assimilation: Absorbed materials are used by cells for energy, repair and growth (building new cell components).
5. Egestion: Removal of undigested and unabsorbed waste from the body (faeces).

Examples of digestive processes and enzymes: Carbohydrates are broken by amylase into sugars; proteins by pepsin/trypsin into peptides and amino acids; fats by lipase into glycerol and fatty acids.

Adaptations related to holozoic nutrition: animal teeth differ by diet (incisors, canines, molars), digestive tract length differs (herbivores usually have longer intestines), special stomach compartments in ruminants (e.g., cow) and gizzards in birds for grinding food. These adaptations help ingestion, digestion and absorption.

Where it occurs: Single-celled animals (amoeba, paramecium) perform intracellular holozoic nutrition by forming food vacuoles. Multicellular animals (humans, cows, cats, earthworms) perform mostly extracellular digestion in a digestive tract.

Importance: Supplies energy, raw materials for growth and repair, and maintains life processes.

Overview
Amoeba is a unicellular (single-celled) animal that obtains its food from the surrounding environment. It shows holozoic nutrition — taking in solid or liquid organic food, digesting it, absorbing nutrients and getting rid of waste.

Steps of Nutrition in Amoeba

• Ingestion (Capture of food): Amoeba moves and captures food using pseudopodia (temporary finger-like projections of cytoplasm). The process of engulfing solid food particles is called phagocytosis; liquids can be taken by pinocytosis.
• Formation of Food Vacuole: When a food particle is engulfed, the cell membrane surrounds it and pinches off to form a food vacuole (a membrane-bound sac) inside the cytoplasm.
• Digestion (Intracellular): Digestive enzymes are released into the food vacuole from the cytoplasm. These enzymes break down complex food substances into simpler soluble forms. In Amoeba digestion is intracellular (inside the cell).
• Absorption: The digested soluble nutrients (amino acids, sugars, etc.) diffuse through the vacuole membrane into the cytoplasm and are used for energy, growth and repair.
• Assimilation: The cell uses absorbed nutrients to build cell materials and to get energy.
• Egestion (Removal of waste): Undigested residues remain in the vacuole; the vacuole moves to the cell surface and the waste is removed by fusion with the cell membrane and release outside. Amoeba does not have a permanent anal pore.

Other important features
Amoeba has no mouth, no digestive tract and no specialised organs. It digests food intracellularly. Amoeba also contains a contractile vacuole that helps remove excess water (osmoregulation) but not directly involved in nutrition.

Why single-celled size matters
Efficient exchange of nutrients and wastes by diffusion is possible because the cell is small. For a spherical cell: surface area to volume ratio (SA/V) falls as size increases, so small size helps faster diffusion and supports intracellular digestion.

Overview: Hydra is a simple multicellular animal (a cnidarian) that lives in freshwater. It shows a basic form of heterotrophic nutrition: it captures prey, digests food both outside and inside cells, and expels undigested remains through the same opening.

Body structure (simple form):

• Two tissue layers: outer ectoderm and inner endoderm, with a jelly-like mesoglea between them.
• Radial symmetry and a single opening (mouth) that also serves as the exit for waste (no separate anus).
• Tentacles around the mouth contain specialised stinging cells (cnidocytes) with nematocysts to capture prey.
• A central cavity called the gastrovascular cavity (GVC) acts as the stomach and circulation chamber.

Steps of nutrition in Hydra:

1. Capture: Tentacles sting and paralyse small animals (e.g., tiny crustaceans, insect larvae) using nematocysts.
2. Ingestion: Tentacles push the prey into the mouth and into the gastrovascular cavity.
3. Extracellular digestion: Gland cells (in the endoderm) secrete digestive enzymes into the GVC; large food particles are broken down into smaller soluble molecules.
4. Intracellular digestion: Endodermal cells phagocytose (engulf) food particles or droplets; digestion is completed inside food vacuoles.
5. Absorption and distribution: Nutrients diffuse from the GVC to surrounding cells; because the animal is thin and simple, diffusion suffices to distribute nutrients.
6. Egestion: Undigested remains are expelled back out through the mouth.

Key points:

• Hydra shows both extracellular and intracellular digestion — a characteristic of simple multicellular animals that bridges single-celled and complex animals.
• There is only one opening (mouth); no separate anus.
• Hydra is carnivorous and entirely dependent on other organisms for food (heterotroph).
• Because it lacks circulatory organs, distribution of nutrients relies on diffusion through the thin body.

Why this is important: Studying Hydra helps students understand basic digestive strategies (extracellular + intracellular) and how body structure limits and shapes nutrition. It also shows adaptations for capture (nematocysts) and a simple body plan that supports life without complex organs.

Overview: Nutrition in animals involves ingestion, digestion, absorption, assimilation and egestion. Earthworms and cockroaches are invertebrates but differ strongly in feeding habits, digestive structures and associated systems.

Earthworm (Detritivore)

• Habitat & feeding: Lives in soil and feeds on dead and decaying plant material, humus and soil particles. It is a detritivore—helps decompose organic matter.
• Mode of ingestion: Takes in soil and organic matter through a small mouth. No jaws; food is pushed in by muscular pharynx.
• Alimentary canal & functions:
  • Buccal cavity & pharynx: takes in and swallows soil/organic matter.
  • Oesophagus: passage to storage organs.
  • Crop: stores food temporarily.
  • Gizzard: muscular grinding organ that crushes soil particles and food.
  • Intestine: chemical digestion (enzymes from intestinal cells) and absorption of soluble nutrients; intestinal wall richly supplied with blood vessels.
  • Anus: egestion of undigested material as casts (rich in nutrients for soil).
• Digestion type: Mechanical (gizzard) + chemical (intestinal enzymes).
• Absorption & transport: Digested nutrients absorbed through intestine into blood (closed circulatory system with dorsal vessel) and distributed to cells.
• Respiration & excretion: Gas exchange by diffusion through moist skin; excretion by nephridia.
• Ecological role: Earthworm casts increase soil aeration, drainage and fertility—important in composting and agriculture (vermiculture).

Cockroach (Omnivore/Scavenger)

• Habitat & feeding: Households, sewers, stored food areas; feeds on a wide variety of organic matter (decaying material, food scraps, paper, glue).
• Mode of ingestion & mouthparts: Chewing type mouthparts with mandibles and maxillae; can tear and chew solid food.
• Alimentary canal & functions:
  • Foregut (mouth, pharynx, oesophagus, crop): mechanical processing and temporary storage.
  • Gastric caeca & midgut: secrete digestive enzymes (carbohydrases, proteases, lipases) and are main sites of digestion and absorption.
  • Hindgut: reabsorption of water and salts; formation of feces.
  • Malpighian tubules: excretory organs that remove nitrogenous wastes and pass them into the gut for elimination.
• Digestion type: Mechanical (mandibles) + chemical (enzymes in saliva, midgut secretions).
• Respiration & circulation: Tracheal system—air enters through spiracles and travels via tracheae to tissues (no blood-borne oxygen transport). Open circulatory system transports nutrients but not oxygen.
• Health & ecological role: Scavenger that helps decompose organic matter but can be a vector for disease and contaminate food in human dwellings.

Key differences (brief): Earthworm ingests soil and relies on muscular gizzard and intestinal enzymes; exchanges gases through skin and uses nephridia for excretion. Cockroach chews food with mandibles, uses gastric caeca and midgut enzymes, breathes via tracheae and excretes wastes through Malpighian tubules.

Steps of nutrition (common terms): Ingestion → Digestion (mechanical + chemical) → Absorption → Assimilation → Egestion. Both animals follow these steps but their organs and methods differ.

Practical implications / classroom links: Observe earthworm casts in gardens or vermicompost; show a preserved/diagrammatic cockroach alimentary canal. Discuss soil fertility improvement by earthworms and public-health aspects of cockroaches.

Overview
Birds are warm-blooded vertebrates that obtain energy and nutrients from food. They have special adaptations for feeding and digestion — no teeth, a beak adapted to diet, and a unique digestive tract (crop and stomach with proventriculus and gizzard). Their high metabolic rate requires energy-rich food and efficient digestion.

Types of food and feeding habits

• Granivores (seed-eaters): strong, conical beaks to crack seeds (e.g., sparrows, parrots).
• Insectivores: slender or pointed beaks to catch insects (e.g., swifts, flycatchers, woodpeckers use chisel-like bills for insects in wood).
• Carnivores / Raptors: hooked beaks and sharp talons to tear flesh (e.g., eagles, owls, hawks).
• Nectarivores: long, narrow beaks or tongues to reach nectar (e.g., hummingbirds, sunbirds).
• Filter feeders: specialized bills to filter small organisms (e.g., flamingos, ducks).
• Piscivores: beaks or bills for catching fish (e.g., kingfishers, pelicans use a pouch).

Digestive system and special structures
Key parts: beak & tongue → oesophagus → crop (storage) → stomach (two parts: proventriculus secretes digestive juices; gizzard grinds food using swallowed stones) → intestines (absorb nutrients) → cloaca. The liver and pancreas produce bile and enzymes to aid digestion.

Functions of crop and gizzard

• Crop: pouch in the oesophagus used to store and soften food; commonly used to carry food for chicks. Some birds (pigeons and doves) produce a nutrient-rich secretion called crop milk to feed young.
• Gizzard: strong muscular stomach that grinds hard food (seeds, shells) with the help of grit or small stones swallowed by the bird.

Adaptations related to nutrition

• Beak shape matches food type — an excellent example of adaptation and niche specialization.
• Fast digestion and efficient nutrient absorption to meet high energy needs (flight demands lots of energy).
• Excretion of nitrogen as uric acid (semi-solid paste) conserves water — useful for flying and nesting in dry places.

Feeding of the young
Many birds feed their chicks by regurgitating pre-digested food. Pigeons and doves produce crop milk (rich in proteins and fats) to feed hatchlings. Parental care strategies vary by species.

Energy needs and behaviour
Because flight is energy-intensive, birds often show behaviors tied to nutrition: migration to follow food availability, daily foraging routines, and seasonal changes in diet (e.g., fruiting seasons, insect abundance).

Summary
Bird nutrition links form and function: bill and digestive specializations match diet, crop and gizzard perform storage and mechanical digestion, and metabolic adaptations (uric acid excretion, high digestion rates) support an active, flying lifestyle.

Overview: Aquatic animals obtain food from water in many ways. A major group are filter feeders, which feed by straining suspended particles (plankton, detritus, microorganisms) from water. Filter feeding is a form of holozoic nutrition where food is captured, ingested, digested and assimilated.

Mechanisms and adaptations:

• Physical sieving: Large baleen plates (baleen whales) or gill rakers (some fishes) trap tiny organisms while water is expelled.
• Ciliary and mucus trapping: In sponges, choanocytes (collar cells) generate water currents and trap particles in mucus; bivalves (clams, oysters, mussels) use ciliated gills to capture and transport particles to the mouth.
• Feeding appendages: Barnacles use feathery cirri to sweep and capture plankton; some crustaceans (krill) use filtering structures on thoracic limbs.
• Behavioral/positional adaptations: Many filter feeders orient themselves into currents or open their shells to maximize water flow and particle capture.

Feeding process (typical steps): water with suspended food → capture by sieve/cilia/mucus → transport to mouth → ingestion → enzymatic digestion → assimilation for growth, movement and reproduction; wastes are excreted.

Ecological importance: Filter feeders clean water by removing suspended particles, connect primary producers (phytoplankton) to higher trophic levels, and can form reefs/beds (oyster reefs, mussel beds) that provide habitat.

Constraints and trade-offs: Filtration is limited by water particle concentration, filter clogging, and metabolic costs. Many filter feeders show optimal feeding at intermediate particle concentrations and may close up when particles are too scarce or too abundant (to avoid clogging).

What are ruminants? Ruminants are mammals that chew cud and have a specialised stomach with four compartments to digest fibrous plant food (mainly cellulose). Common examples: cow, buffalo, sheep, goat, deer.

Four-compartment stomach & functions

• Rumen – the largest chamber; a fermentation vat where microbes (bacteria, protozoa, fungi) break down cellulose and other complex carbohydrates into simpler compounds.
• Reticulum – works with the rumen; traps denser particles and foreign objects, helps form cud for regurgitation.
• Omasum – absorbs water and some nutrients from the fermented mass; reduces particle size.
• Abomasum – the ‘true stomach’; secretes acids and enzymes (like pepsin) to digest microbial protein and feed proteins before they enter the intestine.

Process of rumination (step-by-step)

1. Eating: Animal grazes or browses and swallows partly chewed plant material.
2. Fermentation: Material enters rumen/reticulum where microbes ferment cellulose into volatile fatty acids (VFAs) and microbial biomass.
3. Regurgitation (chewing the cud): Semi-digested lumps are brought back to the mouth and rechewed to reduce particle size and increase saliva mixing.
4. Reswallowing: Chewed cud passes back to rumen, then to omasum (water absorbed) and abomasum (acid digestion).
5. Small intestine: Final digestion and absorption of nutrients (amino acids, sugars, fatty acids, vitamins).

Role of microbes: Microbes in the rumen produce enzymes (cellulases) that ruminants lack, allowing breakdown of cellulose into VFAs (main energy source), gases (CO2, CH4), and microbial protein. When microbes pass to the abomasum and intestine, they are digested and supply amino acids (protein) to the host.

Important outcomes and advantages:

• Ruminants can use low-quality roughage (grass, straw) as energy by fermenting cellulose.
• They obtain most energy from VFAs (especially acetate, propionate, butyrate) absorbed from the rumen wall.
• Microbial protein is a major protein source, so ruminants can meet protein needs even on poor-quality feeds.

Practical notes: Saliva is alkaline and buffers rumen pH (important for stable fermentation). Sudden diet changes, concentrates overload, or antibiotics may disturb rumen microbes, causing acidosis or reduced digestion.

Note: Some animals (e.g., camels) are pseudo-ruminants with three-chambered stomachs — they practice rumination but differ anatomically.

Introduction
The human digestive system breaks down the food we eat into small absorbable molecules (glucose, amino acids, fatty acids and glycerol), absorbs them into the blood, and removes undigested waste. Digestion involves mechanical processes (chewing, churning) and chemical processes (enzymes and secretions).

Main organs and their roles

• Mouth (Oral cavity): Food is chewed by teeth (mechanical digestion). Saliva from salivary glands moistens food and contains salivary amylase that starts starch digestion.
• Oesophagus: A muscular tube that transports the food bolus from mouth to stomach by wave-like muscle contractions called peristalsis.
• Stomach: Muscular sac that churns food (mechanical) and mixes it with gastric juice (chemical). Gastric juice contains hydrochloric acid (HCl) and the enzyme pepsin that begins protein digestion. Food becomes a semi-liquid called chyme.
• Small intestine: The major site of chemical digestion and absorption. It has three parts: duodenum (receives bile and pancreatic juice), jejunum and ileum (main absorption). The inner surface has villi and microvilli to increase surface area for absorption.
• Accessory organs:
  • Liver: Produces bile (emulsifies fats; not an enzyme).
  • Gallbladder: Stores and concentrates bile, releases it into the duodenum.
  • Pancreas: Secretes pancreatic juice containing enzymes (pancreatic amylase, trypsin, lipase) and bicarbonate to neutralize stomach acid.
• Large intestine (colon): Absorbs water and some minerals, houses beneficial bacteria which synthesize certain vitamins (e.g., vitamin K), and compacts undigested matter into feces.
• Rectum and Anus: Store and then eliminate feces from the body.

Pathway of food (step-by-step)

1. Mouth: ingestion, chewing, partial starch digestion by salivary amylase → bolus.
2. Oesophagus: bolus moved to stomach by peristalsis.
3. Stomach: protein digestion begins (pepsin) → chyme.
4. Duodenum (small intestine start): bile emulsifies fats; pancreatic enzymes continue digestion; intestinal enzymes (maltase, sucrase, lactase, peptidases) finish breakdown to absorbable units.
5. Jejunum & ileum: absorption of nutrients through villi into blood (sugars, amino acids) and lymph (fatty acids/glycerol as chylomicrons).
6. Large intestine: water absorption, vitamin absorption from gut bacteria, formation of feces.
7. Rectum & anus: storage and elimination (egestion).

Enzymes, actions and conditions

• Salivary amylase: starch → maltose (works around neutral pH in mouth).
• Pepsin (gastric): proteins → peptides (works in acidic pH in stomach ~pH 1.5–3.5).
• Pancreatic amylase: starch → disaccharides (duodenum).
• Trypsin and other proteases: peptides → smaller peptides/amino acids (duodenum/jejunum).
• Lipase + bile (emulsification): triglycerides → fatty acids + glycerol (duodenum/jejunum).
• Intestinal brush border enzymes (maltase, sucrase, lactase, peptidases): disaccharides → monosaccharides; peptides → amino acids (at intestinal lining).

Absorption and transport

Monosaccharides and amino acids enter blood capillaries in villi and travel to the liver via the hepatic portal vein. Fatty acids and glycerol form chylomicrons, enter lacteals (lymph vessels), and reach the bloodstream via the lymphatic system.

Common disorders (brief)

• Indigestion/heartburn: acid reflux from stomach into oesophagus.
• Constipation: slow movement in large intestine, excess water absorption.
• Diarrhea: reduced water absorption or infection; rapid passage of feces.
• Lactose intolerance: lack of lactase causing milk sugars to be undigested.

Key idea: Digestion is a coordinated process involving mechanical actions, enzymes and secretions; each organ has a specific role and the small intestine is the main site of nutrient absorption.

Overview
Digestive glands secrete juices (enzymes, acids, bile, mucus) that chemically break down food into small absorbable molecules. Major digestive glands: salivary glands, gastric (stomach) glands, pancreas, liver (bile), and intestinal glands. Each gland produces specific secretions working at particular pH and places in the alimentary canal.

1. Salivary glands (in the mouth)

• Location: Under the tongue, near the jaw and cheeks (parotid, submandibular, sublingual).
• Secretion: Saliva — mostly water, mucus, and the enzyme salivary amylase (ptyalin).
• Function: Moistens food, helps form a bolus, and starts chemical digestion of starch (polysaccharides → maltose).
• pH: Slightly acidic to neutral (~6.5–7.5). Amylase works best near neutral pH.

2. Gastric glands (in the stomach)

• Location: Lining of the stomach (gastric pits/glands).
• Secretion: Gastric juice — contains hydrochloric acid (HCl), pepsinogen (inactive), mucus, and in young mammals rennin (chymosin).
• Function: HCl creates strongly acidic environment (kills microbes, denatures proteins) and converts pepsinogen to active pepsin, which starts protein digestion (proteins → peptides). Mucus protects stomach lining from acid. Rennin helps coagulate milk in infants to slow its passage.
• pH: Very acidic (~1.5–3.5); pepsin works best at low pH.

3. Pancreas

• Location: Behind the stomach, opens into small intestine (duodenum) via pancreatic duct.
• Secretion: Pancreatic juice — alkaline fluid containing enzymes: pancreatic amylase (starch → maltose), trypsin and chymotrypsin (proteins → peptides; secreted as inactive precursors trypsinogen etc.), and pancreatic lipase (fats → fatty acids + glycerol).
• Function: Completes carbohydrate digestion, continues protein digestion, and digests fats (works best in slightly alkaline conditions).
• pH: Alkaline (~8.0–8.5) to neutralize stomach acid.

4. Liver and bile

• Location: Upper right abdomen; bile stored in gallbladder and released into duodenum.
• Secretion: Bile — bile salts, bile pigments (bilirubin), cholesterol; not an enzyme.
• Function: Emulsifies fats (breaks large fat droplets into smaller droplets), increasing surface area for lipase action; helps absorption of fat-soluble vitamins (A, D, E, K).

5. Intestinal glands (small intestine)

• Location: Lining of the small intestine (crypts of Lieberkühn; brush border enzymes on epithelial cells).
• Secretion: Intestinal juice (succus entericus) — contains enzymes such as maltase, sucrase, lactase (disaccharidases), and peptidases (dipeptidases, aminopeptidases) and enterokinase.
• Function: Final stages of digestion — disaccharides → monosaccharides (glucose, fructose, galactose) and peptides → amino acids, ready for absorption.
• Activation: Enterokinase activates trypsinogen → trypsin in the intestine, which then activates other pancreatic proteases.

How secretions work together (summary)
Digestion is cooperative: salivary amylase begins carbohydrate digestion in mouth; stomach acid and pepsin start protein breakdown; pancreatic enzymes and intestinal enzymes complete carbohydrate, protein and fat digestion in the small intestine while bile emulsifies fats for efficient lipase action. The end products absorbed are mainly glucose (or other monosaccharides), amino acids, glycerol and fatty acids.

Protection and regulation
Mucus protects organs (especially stomach) from self-digestion. Secretion is regulated by nervous signals (sight, smell, taste of food) and hormones (e.g., gastrin stimulates gastric secretion; secretin and cholecystokinin regulate pancreatic and bile secretions).

Common points to remember

• Bile = emulsifier (not enzyme).
• Many proteases are secreted as inactive zymogens (e.g., pepsinogen, trypsinogen) and activated where safe.
• Different enzymes have different optimum pH and locations adapted to those pH values.

Introduction
Teeth are hard, white structures in the mouth that help animals take in and break down food. In humans and many animals teeth are adapted for different functions such as biting, tearing and grinding. Humans are diphyodont (two sets of teeth: milk and permanent) and heterodont (different types of teeth).

Types of Teeth in Humans

• Incisors: Front teeth (flat, chisel-shaped) used for biting and cutting food (8 in permanent dentition).
• Canines (Cuspids): Pointed teeth for tearing food (4 in permanent dentition).
• Premolars (Bicuspids): Transitional teeth for tearing and grinding (8 in permanent dentition).
• Molars: Broad teeth at the back for grinding food (12 in permanent dentition, including 4 wisdom teeth).

Structure of a Tooth

• Crown: The visible part of the tooth above the gum.
• Neck: The narrow part where crown meets the root at the gum line.
• Root: Anchors the tooth into the jaw bone (may be single or multiple).
• Enamel: Hard, outermost layer of the crown (hardest substance in the body).
• Dentin: Beneath enamel, a hard tissue that supports enamel and surrounds the pulp.
• Pulp Cavity: Central soft area containing blood vessels and nerves (supplies nutrients and sensation).
• Cementum: Covers the root and helps attach the tooth to the periodontal ligament.
• Periodontal Ligament: Fibres that hold the tooth in the socket.

Milk (Deciduous) vs Permanent Teeth
Milk teeth start erupting at about 6 months and are usually complete by 2–2.5 years (20 teeth). Permanent teeth begin replacing milk teeth from about 6 years and continue into adolescence (32 teeth).

Teeth in Other Animals (Adaptations)

• Carnivores (e.g., lion): Large canines for killing/holding prey; sharp carnassial teeth for slicing meat.
• Herbivores (e.g., cow): Broad, flat molars for grinding; some lack upper incisors (cow has dental pad).
• Rodents (e.g., rat, rabbit): Prominent incisors that continuously grow and are worn down by gnawing.
• Fish and reptiles (e.g., shark): Often homodont (all teeth similar) adapted to catch and cut prey; many replace teeth continuously.

Functions of Teeth

• Biting and cutting (incisors).
• Tearing and holding food (canines).
• Crushing and grinding food (premolars and molars).
• Helps in speech (pronunciation of certain sounds) and in maintaining facial shape and appearance.

Care of Teeth (brief)
Brush twice daily, limit sugary foods, visit dentist regularly, use fluoride toothpaste to protect enamel and prevent cavities.

Key points: Types of teeth are matched to diet and mode of feeding; a tooth’s layered structure (enamel → dentin → pulp) supports its mechanical and sensory roles.

Overview: Digestion is the process by which food is broken down into small, soluble molecules that the body can absorb and use. In animals, digestion involves mechanical breakdown, chemical breakdown by enzymes, absorption of nutrients, assimilation into body cells, and egestion of undigested material.

Main steps of digestion:

• Ingestion: Taking food into the mouth.
• Mechanical digestion: Chewing in the mouth (teeth and tongue), churning in the stomach, and grinding in organs like the gizzard in birds. Mechanical actions increase the surface area of food.
• Chemical digestion: Enzymes and secretions break large food molecules into smaller molecules. This occurs in the mouth, stomach, and small intestine.
• Movement (peristalsis): Wave-like muscular contractions in the alimentary canal that move food along (from oesophagus to intestines).
• Absorption: Small soluble molecules (glucose, amino acids, fatty acids, glycerol, vitamins, minerals) are absorbed mainly in the small intestine through villi into blood and lymph.
• Assimilation: Absorbed molecules are taken up by cells to build tissues, produce energy, and support growth.
• Egestion: Removal of undigested wastes through the rectum and anus.

Key organs and their functions (human example):

• Mouth: Teeth chew food (mechanical); saliva containing salivary amylase begins starch digestion to maltose (chemical).
• Oesophagus: Transports food to stomach by peristalsis.
• Stomach: Muscular churning and acidic gastric juice (HCl) activate pepsin to begin protein digestion to peptides; acidity also kills many microbes.
• Small intestine: Major site of chemical digestion and absorption. Pancreatic enzymes (amylase, lipase, proteases) and intestinal enzymes (maltase, sucrase, peptidases) finish digestion. Bile from liver (stored in gallbladder) emulsifies fats to aid lipase action.
• Large intestine: Absorbs water and some salts; houses beneficial bacteria that ferment undigested materials and synthesize some vitamins; forms feces.

Important enzymes and their roles:

• Salivary amylase: starch → maltose
• Pepsin (stomach): proteins → peptides
• Pancreatic amylase: starch → maltose
• Proteases (trypsin, chymotrypsin, peptidases): peptides → amino acids
• Lipase: emulsified fats → glycerol + fatty acids

Special adaptations in other animals:

• Cows and other ruminants: Four-chambered stomach (rumen, reticulum, omasum, abomasum) with microbes in the rumen that digest cellulose; cud is regurgitated and rechewed (chewing cud).
• Birds: Crop stores food, proventriculus secretes enzymes, gizzard mechanically grinds food using swallowed stones.
• Earthworms: Crop stores food and gizzard grinds it; digestion occurs in intestine and absorption through intestine lining.

Why digestion matters: Proper digestion supplies energy (glucose, fatty acids) and raw materials (amino acids, minerals, vitamins) for growth, repair, and maintaining body functions. Problems in any step (enzyme deficiency, poor absorption, infection) reduce nutrition.

Overview
After food is digested into small soluble molecules, three linked processes take place: absorption, assimilation and egestion. These are essential stages of nutrition in animals.

Absorption
Absorption is the movement of digested food (nutrients) from the alimentary canal into the body fluids (blood or lymph). In humans and many animals most absorption occurs in the small intestine. The inner wall of the small intestine has folds covered with villi and microvilli. These structures increase the surface area hugely and help rapid absorption.

• What is absorbed: monosaccharides (glucose), amino acids, small peptides, fatty acids and glycerol (after emulsification and breakdown), vitamins, minerals and water.
• How it happens: small molecules enter blood capillaries in a villus by diffusion, facilitated diffusion or active transport. Fatty acids and glycerol are re‑assembled into fats and enter the lacteal (lymph vessel) as chylomicrons.
• Large intestine: absorbs water, salts and some vitamins (e.g., vitamin K produced by gut bacteria).

Assimilation
Assimilation is the process by which absorbed nutrients are taken up by body cells and used to build cell components, produce energy, or stored for later use. It includes synthesis of proteins from amino acids, conversion of excess glucose into glycogen or fat, and use of nutrients in cellular respiration to release energy (ATP).

• Examples of assimilation: amino acids → body proteins (for growth and repair), glucose → glycogen (storage in liver and muscle), fatty acids → body fats (adipose tissue).
• Control and regulation: hormones (e.g., insulin) and enzymes regulate how nutrients are used or stored.

Egestion
Egestion (also called defaecation in higher animals) is the removal of undigested and unabsorbed food materials from the alimentary canal. In humans, indigestible remains (fibre, some pigments) become faeces and are expelled through the rectum and anus.

• Note the difference: egestion removes undigested food; excretion removes metabolic wastes (e.g., CO2, urea).
• Examples: birds and reptiles may eliminate wastes differently (cloaca), unicellular organisms expel undigested material through an anal pore or by exocytosis.

Why these processes matter
Together these steps make nutrients available as building blocks and fuel for cells. Efficient absorption (large surface area, healthy villi), correct assimilation (hormonal balance, enzymes), and timely egestion (removal of bulk) are required for good nutrition and health.

Introduction
Microorganisms (bacteria, fungi, protozoa, yeast) play vital roles in nutrition — both for other organisms and in ecosystems. They help break down complex substances, make nutrients available, aid digestion, and are used to prepare many foods.

1. Decomposers and Nutrient Recycling
Many microbes act as decomposers: they break down dead plants and animals and waste into simpler inorganic substances (minerals, CO2, water) that plants can absorb. This recycling of nutrients keeps soil fertile and maintains the flow of matter in ecosystems.

2. Microbes in Food Production
Some microorganisms are used by humans to produce food by fermentation. Examples include yeast in bread (produces CO2 to make dough rise) and bacteria/yeasts in curd, cheese, yogurt, idli/dosa batter, vinegar and some alcoholic drinks. Fermentation converts sugars into acids, gases or alcohol which preserve food and produce characteristic flavours.

3. Microbes in Animal Digestion
In many animals, microbes live in symbiosis in the digestive tract and help digest food that the host cannot break down by itself. Examples: ruminant animals (cow, buffalo, goat) have microbes in the rumen that digest cellulose from grass into simpler substances the animal can absorb. In humans, gut bacteria help digest some foods and synthesize certain vitamins (e.g., vitamin K and some B vitamins).

4. Microorganisms in the Nitrogen Cycle
Certain bacteria convert atmospheric nitrogen (N2) into forms plants can use (like ammonia) — a process called biological nitrogen fixation. Other microbes convert ammonia into nitrates (nitrification) or return nitrogen to the atmosphere (denitrification). These microbial actions are essential for making nitrogen available to plants and thus supporting nutrition up the food chain.

5. Harmful Effects
Not all roles are beneficial: some microbes spoil food (reducing its nutritional value) and some cause diseases that affect nutrition (e.g., intestinal infections causing poor absorption or loss of nutrients). Proper hygiene and food storage control harmful microbes.

Summary
Microorganisms are indispensable to nutrition through decomposition and nutrient recycling, food fermentation, aiding digestion and vitamin production, and driving parts of the nitrogen cycle. They can also be responsible for spoilage and disease if uncontrolled.

What it means: Nutrition is the process by which organisms obtain and use food to grow, repair tissues, and obtain energy. Nutrition and health are closely linked: good nutrition maintains healthy body functions and prevents diseases, while poor nutrition causes illness, stunted growth, weakness and reduced resistance to infection.

Components of a healthy (balanced) diet and their functions

• Carbohydrates – main source of energy (eg, rice, wheat, potatoes).
• Proteins – build and repair tissues, make enzymes and hormones (eg, pulses, eggs, meat, milk).
• Fats – concentrated energy, insulation and fat-soluble vitamin absorption (eg, oils, ghee, nuts).
• Vitamins – regulate body processes and prevent deficiency diseases (eg, vitamin C in citrus, vitamin D from sunlight and dairy).
• Minerals – required for body structure and function (eg, calcium for bones, iron for blood).
• Fibre – helps digestion and prevents constipation (eg, whole grains, fruits, vegetables).
• Water – essential for all metabolic reactions, temperature regulation and excretion.

Effects of poor nutrition

• Undernutrition – insufficient calories/proteins leading to thinness, stunted growth, weakened immunity.
• Deficiency diseases – specific lack of nutrients: scurvy (vitamin C), rickets (vitamin D), anemia (iron), night blindness (vitamin A).
• Overnutrition – excess intake, especially of fats and sugars, causing obesity and related problems like diabetes and heart disease.

Hygiene and food safety: Clean, safe food and safe drinking water are essential for nutrition to result in good health. Contaminated food/water causes infections (diarrhea, cholera) that reduce nutrient absorption and lead to malnutrition. Good habits include washing hands before eating, storing food properly, cooking food thoroughly and drinking boiled or treated water.

Prevention and practical tips: Eat a balanced plate (cereals, pulses/protein, vegetables, fruits, dairy, small amounts of fats), include a variety of foods, ensure regular meals, encourage outdoor play (for vitamin D), and practise food hygiene. Regular health check-ups and vaccinations also support nutrition and health.

Overview: Practical activities in the chapter 'Nutrition in Animals' help students observe how animals obtain and break down food. Core skills practiced include making observations, using simple chemical tests, controlling variables and drawing conclusions about digestion and feeding adaptations.

Key experiments commonly done in Class 7:

1. Action of saliva (salivary amylase) on starch
   Aim: To show that saliva starts digestion of starch to sugar.
   Materials: Starch solution (or mashed bread/potato), clean spit or extracted saliva, iodine solution, Benedict's reagent, test tubes, water bath/boiling source, dropper.
   Procedure (short): Prepare two test tubes each with equal starch solution. Add fresh saliva to tube A and nothing (or water) to tube B as control. Incubate at body/room temperature for 10–20 minutes. Test small drops from each tube with iodine (tests for starch) and Benedict's reagent (test for reducing sugar; heat gently for Benedict's). Also include a third tube where saliva is boiled before adding (enzyme inactivated) as another control.
   Observations & conclusion: Iodine in tube A turns less intensely blue/black (starch reduced) and Benedict's test gives positive colour (green → brick red) showing appearance of reducing sugar. Boiled-saliva tube behaves like control. Conclusion: Saliva contains an enzyme (α‑amylase) that hydrolyses starch to sugars; enzyme activity is destroyed by heat.
2. Iodine test for starch in foods
   Aim: To detect starch in vegetables/foods.
   Procedure: Place a small piece or extract of the food on a white tile. Add a drop of iodine solution. A blue-black colour indicates starch.
3. Simple peristalsis model
   Aim: To demonstrate movement of food by peristalsis.
   Materials: Flexible tube or long balloon, small marble or bead, clamps or fingers.
   Procedure: Place bead in tube and squeeze sequentially to push it forward. Observation: Sequential contractions move the bead forward, similar to peristalsis in the alimentary canal.
4. Studying teeth and feeding adaptations
   Activity: From charts, models or safe skull samples, identify incisors, canines, premolars and molars. Measure shape and size, relate to diet (herbivore, carnivore, omnivore) and describe chewing behaviour (e.g., grinding vs tearing).

Safety and controls: Always use proper hygiene when handling saliva; use spatulas and avoid ingestion. Include negative and positive controls (boiled saliva or plain water) to confirm enzyme action. Heat and acid/alkali affect enzyme activity — handle hot equipment with care.

Interpretation tips: A decrease in starch (less blue-black with iodine) together with a positive Benedict's test (colour change after heating) indicates enzymatic breakdown of starch to reducing sugars. Compare fresh saliva with boiled saliva to show the role of enzymes.