Nutrition in Animals

• Why animals need nutrition and how they differ from plants.
• The human digestive system — organs and their functions.
• Digestion in different animals — amoeba, grasshopper, ruminants.
• What absorption and assimilation mean.

Why animals need to eat

• Animals are heterotrophs — cannot make their own food.
• Must eat plants (herbivores), animals (carnivores), or both (omnivores).
• Food provides: energy (carbohydrates, fats), building materials (proteins), regulation (vitamins, minerals).

The human digestive system

Food travels through the alimentary canal (mouth → anus), a ~9 m long tube.

Mouth

• Teeth: incisors cut, canines tear, premolars and molars crush and grind.
• Tongue: mixes food with saliva; taste buds detect sweet, sour, salty, bitter, umami.
• Salivary glands (3 pairs): produce saliva containing:
  • Salivary amylase (ptyalin): enzyme that begins digesting starch → maltose.
  • Water and mucus: lubricates food.
• Bolus: soft ball of chewed, saliva-moistened food.

Oesophagus (Food pipe)

• Muscular tube (~25 cm) connecting mouth to stomach.
• Peristalsis: wave-like muscular contractions push food downward — works even upside down (gravity not needed).
• No digestion here; only transport.

Stomach

• J-shaped muscular bag; can hold ~1.5 litres.
• Gastric glands in stomach wall produce gastric juice containing:
  • Hydrochloric acid (HCl): kills bacteria in food; makes acidic environment (pH ~2).
  • Pepsin: enzyme that digests proteins → smaller peptides (works only in acidic environment).
  • Mucus: protects stomach wall from acid.
• Chyme: semi-liquid mixture of food + gastric juice after churning (~3–4 hours in stomach).

Small intestine

• Longest part (~6–7 m); three sections: duodenum, jejunum, ileum.
• Main site of digestion and absorption.

Duodenum receives:

• Bile from liver (stored in gall bladder): emulsifies fats (breaks large fat droplets into small ones for easier enzyme action); no enzymes in bile.
• Pancreatic juice from pancreas containing:
  • Pancreatic amylase: digests remaining starch → maltose.
  • Lipase: digests fats → fatty acids + glycerol.
  • Trypsin and chymotrypsin: digest proteins/peptides → amino acids.
  • Sodium bicarbonate: neutralises acid from stomach (makes intestine alkaline — pH ~8).

Intestinal juice (from intestine wall itself):

• Maltase, sucrase, lactase: complete carbohydrate digestion → glucose.
• Peptidases: complete protein digestion → amino acids.

Final products of digestion:

Absorption in small intestine:

• Inner wall has villi (finger-like projections) and microvilli (on villi cells) → greatly increase surface area (~200 m²).
• Glucose and amino acids: absorbed into blood capillaries inside villi → transported to liver via portal vein.
• Fatty acids + glycerol: absorbed into lacteals (lymph vessels inside villi) → enter lymph → eventually bloodstream.

Large intestine

• ~1.5 m long; colon + rectum.
• Absorbs water and minerals from undigested matter.
• Bacteria here produce some vitamins (K, B12) — beneficial.
• Undigested matter becomes faeces; stored in rectum; expelled through anus (egestion/defecation).

Liver — largest gland

• Produces bile (stored in gall bladder).
• Detoxifies blood (removes alcohol, drugs, toxins).
• Stores glycogen (glucose reserve).
• Makes blood proteins (albumin, clotting factors).

Pancreas — dual function

• Exocrine: produces digestive enzymes (pancreatic juice → duodenum).
• Endocrine: produces insulin and glucagon (hormones that regulate blood sugar).

Summary: what happens where

Digestion in other animals

Amoeba

• No digestive system.
• Engulfs food particle by pseudopodia (false feet) → forms food vacuole.
• Digestive enzymes secreted into food vacuole → food digested inside → nutrients absorbed into cytoplasm.
• Undigested remains expelled.
• This process = phagocytosis.

Grasshopper

• Mouth → oesophagus → crop (stores food temporarily) → gizzard (grinds food mechanically with hard teeth-like plates) → stomach → intestine → anus.

Ruminants (cow, goat, sheep, buffalo)

• Herbivores that eat grass (high in cellulose — very hard to digest).
• Have 4 stomach chambers: rumen, reticulum, omasum, abomasum.
• Process called rumination ("chewing the cud"):
  1. Grass swallowed quickly → stored in rumen.
  2. Microorganisms in rumen partially digest cellulose.
  3. Food returned to mouth as cud → chewed thoroughly (second chewing).
  4. Re-swallowed → omasum → abomasum (true stomach) → further digestion.
• Humans cannot digest cellulose (no cellulase enzyme) → cellulose passes as dietary fibre.

• Name the enzyme in saliva and what it digests.
• What does HCl do in the stomach? Which enzyme works there?
• Name three enzymes in pancreatic juice and what each digests.
• What are villi? Why do they exist in the small intestine?
• What is rumination? Which animals do it and why?

Open the Practice tab for graded questions on Nutrition in Animals.