Nutrition and Digestion in Animals: From Cells to Systems

Posted on Nov 27, 2024 in Biology

Types of Nutrition

Autotrophs have the capacity to use inorganic substances as raw materials for nutrition. Energy acquisition methods include:

  • Chemosynthetic: Using energy from chemical reactions (bacteria).
  • Photosynthetic: Using solar energy (cyanobacteria and plants).

Heterotrophs require organic matter and nutrients (bacteria, fungi, animals). The processing and utilization of nutrients occur inside the cell in a complex chain of metabolic reactions:

  • Catabolism: Degradation of substances and energy generation.
  • Anabolism: Synthesis of molecules and energy consumption.

Capturing Food

Food capture is crucial for heterotrophic organisms, influencing animal structure and complexity. It leads to the development of sensory endings around the capture organ for assessing food’s chemical properties and effective locomotion for predation.

Digestion

Digestion transforms complex foods into simple substances absorbable by cells. It involves:

  • Mechanical Digestion:
    • Crushing food (mouth, gizzard).
    • Movement through the digestive tract.
  • Chemical Digestion:
    • Digestive enzymes transform complex molecules into simpler ones.
  • Bacterial Digestion:
    • Cellulose fermentation in herbivores.
    • Intestinal flora providing vitamin K.

Animals possess a digestive system with:

  • Digestive Tract:
    • Food transit and feces expulsion.
    • Mixing food with digestive enzymes.
    • Regulated passage between cavities through:
      • Peristaltic movements: Circular muscle contractions moving food longitudinally.
      • Sphincters: Thicker muscle rings.
  • Digestive Glands: Develop and secrete juices into the tract to mix with food.

Digestive system processes:

  • Ingestion: Food intake into the digestive tract.
  • Digestion:
    • Extracellular: Occurs in a tract outside the cells.
    • Intracellular: Food particles are engulfed and digested inside cells (protozoa, sponges).
  • Absorption: Passage of nutrients from the digestive tract into the circulatory system.
  • Defecation: Elimination of undigested products (feces).

Gas Exchange

General terms: cellular respiration and external respiration.

The Digestive System of Invertebrates

1. Invertebrates with Intracellular Digestion:

  • Sponges: No specialized digestive organs.
    • Atrial cavity.
    • Choanocytes: Create currents and capture food.
    • Amoebocytes: Digest food.
    • Osculum: Outlet.

2. Invertebrates with Gastrovascular Cavity:

  • Cnidarians (polyps and jellyfish):
    • Single external opening for communication.
    • Gastrovascular cavity for extracellular digestion.

3. Invertebrates with Digestive Tract:

Possess a digestive tube with two openings: mouth and anus.

  • Mollusks (snails):
    • Rasping radula (grinding organ).
    • Salivary glands.
    • Hepatopancreas.
  • Echinoderms (starfish and sea urchins):
    • Mouth on the oral (bottom) side.
    • Anus on the aboral (top) side.
    • Stomach with five pairs of blind sacs for absorption.
    • Aristotle’s lantern (chewing structure).
  • Arthropods (spiders, centipedes, crustaceans, insects):
    • Foregut: Pharynx and esophagus.
    • Midgut: Digestion and absorption; ceca, hepatopancreas.
    • Hindgut: Rectum and anus.
    • Specialization in mouthparts:
      • Chewing: Beetles, grasshoppers.
      • Biting-sucking: Bedbugs, mosquitoes.
      • Licking-chewing: Bees, wasps.
      • Licking: Flies.
      • Sucking: Butterflies.

The Digestive System in Vertebrates

1. Zone of Incorporation: The mouth, with various digestive functions.

2. Zone of Food Transport: Pharynx and esophagus.

3. Zone of Storage: Stomach and sometimes a crop.

4. Zone of Digestion: Stomach, first portion of the small intestine, stomach and intestinal wall cells, pancreas, and liver.

5. Zone of Absorption: Small intestine, large intestine (water only).

Oral Ingestion and Digestion:

  • Teeth: Chewing.
  • Salivation:
    • Dissolves food particles.
    • Moistens and lubricates the food bolus.
    • Hydrolyzes starch to maltose via ptyalin (salivary amylase).
  • Swallowing: Passage of food from mouth to stomach through pharynx and esophagus.
    • Closure of the airway: Soft palate and epiglottis.
    • Bolus progression: Peristalsis.

The Stomach and Gastric Digestion:

  • Peristaltic contractions.
  • Gastric juice actions:
    • Pepsin: Proteins to polypeptides.
    • Hydrochloric acid: Creates acidic environment for pepsin.
    • Mucin: Protects stomach wall.
  • Chyme formation.
  • Progressive stomach emptying through the pyloric sphincter.

Digestion in the Small Intestine and Nutrient Absorption:

  • Complete food digestion.
  • Chyme alkalinization by bile, pancreatic juice, and intestinal juice (enzymes require alkaline environment).
    • Bile: Formed in the liver, stored in the gallbladder.
      • Bile pigments: From red blood cell breakdown.
      • Bile salts: Emulsify fats, aiding lipase action.
    • Pancreatic juice: Produced in the pancreas.
      • Amylase: Starch to maltose.
      • Pancreatic lipase: Fat hydrolysis.
      • Trypsin: Polypeptides to simpler peptides.
    • Intestinal juice: Produced by epithelial cells of the intestinal mucosa.
      • Carbohydrases: Disaccharides to monosaccharides.
      • Intestinal lipase: Fat hydrolysis.
      • Peptidases: Peptides to amino acids.
  • Chyle formation.
  • Nutrient absorption; adaptations increase surface area:
    • Intestinal villi (with absorbent cells and goblet cells).
    • Microvilli.

The Activity of the Large Intestine:

  • Passage of undigested substances.
  • Water absorption and feces formation.
  • Bacterial fermentation (produces odor and vitamin K).
  • Feces accumulation and expulsion through defecation.