HETEROTROPHIC NUTRITION

UNIT 7

Objectives

  • List the different types of heterotrophic nutrition.
  • Describe the structure and state the function of the components of the human alimentary canal.
  • Explain the differences between mechanical and chemical digestion in humans.
  • Discuss digestion, absorption, and assimilation of food in humans.
  • Compare human gastrointestinal system to other holotrophs.

Nutrition

  • Definition: Nutrition is the acquiring of energy and raw materials needed to maintain life.
  • Basic Nutritional Requirements:
      - Energy
      - Building materials.
  • Nutrients: Supplied in the form of chemicals necessary for life.

Nutrients

  • Definition: Nutrients are chemical substances necessary for life.
  • Functions:
      - Provide energy
      - Build and repair tissue
      - Regulate body processes

Elements of Life

  • Earth's crust contains around 100 elements, but 16 are essential for life.
  • Chief Elements of Organic Molecules:
      - H (Hydrogen)
      - C (Carbon)
      - N (Nitrogen)
      - O (Oxygen)
  • Ions and Trace Elements:
      - Na ext{^+} (Sodium)
      - Mg ext{^2+} (Magnesium)
      - Fe (Iron)
      - Cl ext{^-} (Chlorine)
      - Ca ext{^2+} (Calcium)
      - K ext{^+} (Potassium)
      - Zn (Zinc)
      - Cu (Copper)
      - Co (Cobalt)
      - B (Boron)
      - Si (Silicon)
      - I (Iodine)
      - Mn (Manganese)
      - V (Vanadium)
  • Four Most Common Elements in Living Organisms:
      - H, C, O, N, which account for 99% of the mass and number of atoms found in all living organisms.

Essential Nutrients

  • Five nutrients that are essential to life:
      - Carbohydrates
      - Fats (Lipids)
      - Proteins
      - Vitamins
      - Minerals
  • Water and fiber/roughage are essential to life but are NOT considered nutrients.

Modes of Nutrition

  • Nutrition Types:
      - Autotrophic nutrition
      - Heterotrophic nutrition
        - Holozoic nutrition
        - Saprophytic nutrition
        - Parasitic nutrition

Autotrophic Nutrition

  • Definition: Organisms make their own food (complex organic substances) using simple inorganic substances.
  • Examples: Autotrophs (e.g. green plants via photosynthesis).

Heterotrophic Nutrition

  • Definition: Organisms which depend on other organisms or dead organic matter as their food sources.
  • Characteristics: Heterotrophs cannot make their own food and obtain it in organic form.

Holozoic Nutrition

  • Definition: Organisms take in solid organic food from other organisms.
  • Food needs to be broken down into small molecules before being used by the organisms.
  • Holotrophs: Organisms capable of consuming other organisms for food.

Types of Animals Taking Holozoic Nutrition

  • Herbivores (Herbivory): Feed exclusively on plants.
  • Carnivores (Carnivory): Feed exclusively on animals.
  • Omnivores (Omnivory): Feed on both plants and animals.

Saprophytic Nutrition (Saprophytism/Saprotrophy)

  • Definition: Organisms feed on dead organisms or non-living organic matter.
  • Examples: Saprophytes (e.g. fungi, bacteria).
How a Saprophyte Obtains Its Nutrients
  • Example: Mucor (bread mould):
      1. Rhizoids release enzymes into the bread.
      2. Enzymes digest complex organic molecules in bread into simple, soluble molecules.
      3. Digested products are absorbed by the rhizoids.
Importance of Saprophytes
  • Function: As decomposers:
      - Allow essential materials (e.g. Carbon and Nitrogen) to be recycled in the ecosystem.

Parasitic Nutrition (Parasitism)

  • Definition: Organisms (parasites) obtain organic compounds from another living organism of a different species (host).
  • Outcome: Parasite is benefited while the host is harmed.
How a Tapeworm Obtains Its Nutrients
  • Location: Lives inside the small intestine of humans.
  • Mechanism: Food in the small intestine is already digested. The tapeworm can absorb nutrients directly through its body wall by diffusion.

Human Digestion

Introduction

  • Function of the Digestive System: To break down food into nutrients that then pass into the circulatory system and are taken to various parts of the body where needed.

Digestive (GI) Tract Components

  1. Mouth
       - Mechanical processing, moistening, mixing with salivary secretions.
  2. Salivary Glands
       - Secretion of lubricating fluid containing enzymes for carbohydrate breakdown.
  3. Pharynx
       - Muscular propulsion of materials into the esophagus.
  4. Esophagus
       - Transport of materials to the stomach.
  5. Stomach
       - Chemical breakdown of materials via acid and enzymes; mechanical processing through muscular contractions.
  6. Small Intestine
       - Enzymatic digestion and absorption of water, organic substrates, vitamins, and ions.
  7. Large Intestine
       - Dehydration and compaction of indigestible materials in preparation for elimination.
  8. Rectum
       - Compacts and stores feces.
  9. Anus
       - Controls elimination of feces.

Digestive System Functions

  • Acquire nutrients from the environment.
  • Anabolism: Uses raw materials to synthesize essential compounds.
  • Catabolism: Decomposes substances to provide energy cells need to function.

Actions of Digestive (GI) Tract

  • Ingestion: Occurs when material enters via the mouth. May require specialized mouthparts (e.g. proboscis, stylet).
  • Mechanical Digestion/Processing: Crushing/shearing to facilitate movement through the tract.
  • Chemical Digestion: Breakdown of food using enzymes (chemicals) into small organic compounds for absorption.
  • Secretion: Release of water, acids, buffers, enzymes, and salts by the epithelium of GI tract and glandular organs.
  • Absorption: Movement of organic substrates, electrolytes, vitamins, and water across digestive epithelium.
  • Egestion/Elimination: Removal of undigested or waste material from an organism.
1. The Mouth
  • Teeth Types:
      - Incisor: Cutting and grinding; 8 total.
      - Canine: Piercing and tearing; 4 total.
      - Pre-molar: Grinding; 8 total.
      - Molar: Grinding; 12 total.
      - Total: 32 in an adult human.
Mouth Functionality
  • Receipt of food to be broken down physically (teeth) and chemically (enzymes).
  • Saliva:
      - Provides optimal pH 7 for salivary amylase (initiates starch digestion).
      - Contains water, mucus (protects the lining and lubricates food), salivary amylase (breaks down starch into maltose), buffers (neutralize acids), and lysozyme (lyses oral bacteria).
Tongue
  • Forms chewed food into a bolus (ball of food) before swallowing.
  • Taste Buds: Detect taste (bitter, sour, sweet, etc.).
Pharynx
  • A region between the mouth, esophagus, and trachea.
  • Passageway for food, water, and air; involved in the reflex action of swallowing.
Epiglottis
  • A flap of tissue that covers the glottis (opening of the trachea).
  • Function: Prevents food from entering trachea while swallowing.
Esophagus
  • Transfers the bolus from the mouth to the stomach via peristalsis (rhythmic smooth muscle contractions).
  • Secretes mucus for lubrication.
Peristalsis
  • Along the digestive tract, food moves through rhythmic cycles of activities (peristalsis).
  • Consists of waves of muscular contractions that propel a bolus along the length of the tract.
Cardiac/Esophageal Sphincter
  • Circular muscle at the junction of the esophagus and stomach.
  • Function: Relaxation allows bolus to enter the stomach, contraction prevents stomach chyme from re-entering the esophagus.
Stomach
  • J-shaped muscular and glandular organ on the left side of the body below the diaphragm.
  • Characteristics:
      - Contains deep folds (rugae) that increase surface area for food and fluids.
      - Can stretch to accommodate 2L of food (muscular pouch).
      - Mechanically churns bolus (Physical Digestion) and mixes it with gastric juice (stomach acid, mucus, and enzymes).
Chemical and Mechanical Digestion in Stomach
  • Chemical Digestion: The acid kills invading bacteria or viruses. The enzymes break down proteins and lipids.
  • Mucus: Protects the stomach lining from acid damage.
  • Cellular Composition:
      - Lined with simple columnar epithelium that secretes large amounts of mucus.
      - Gastric glands (parietal cells and chief cells) in the wall secrete HCl, intrinsic factor (for Vitamin B12 absorption), and pepsinogen (inactive enzyme activated to pepsin by HCl).
Pyloric Sphincter
  • Circular muscle at the junction of the stomach and duodenum.
  • Function: Controls the amount of chyme entering the duodenum.

Small Intestine

  • Chyme is approximately 6 meters (size of a tennis court) long.
  • Majority of digestion and absorption occurs in the small intestine.
  • Divided into three parts:
      - Duodenum: First 30 cm receives acid chyme from stomach, pancreatic juice, and bile; initiates lipid emulsification.
      - Jejunum: Major site for nutrient absorption, lined with villi for increased surface area.
      - Ileum: Last portion; compacts leftovers before passing into the large intestine.
Small Intestine Continued
  • Absorbs nutrients/monomers through villi.
  • Receives secretions from gall bladder and pancreas.
  • Long Length: Allows time for enzymatic reactions and increases absorptive area.
Digestive Juices in the Small Intestine
  • Produces intestinal juice containing water, mucus, maltase, and peptidase.
  • Optimal pH: 8-9 for enzymes like pancreatic amylase, lipase, trypsin, and nucleases.
  • Chemical Digestion: Completes digestion of carbohydrates, proteins, nucleic acids, and lipids.
Absorption Mechanism
  • Nutrients absorbed into the bloodstream via villi.
  • Villi Structure: Have capillary beds and lacteals (for fatty acids).
  • Epithelial cells contain microvilli for increased absorption surface area; thinness aids in diffusion.
Coordination of Secretion and Absorption
  • Absorption takes about 5 hours from duodenum to ileum end.
  • Mucosa movements increase absorptive effectiveness by stirring and changing environment around epithelial cells.

Absorption

  • Process: Food taken from intestine into blood, delivered to cells for assimilation.
  • Water and minerals absorbed from upper part of the small intestine.
  • Nutrients are chemically altered by liver or cellular secretions.

Assimilation

  • Definition: The process of nutrients becoming part of the body, converting to cellular substance.
  • Involves chemical alteration of substances in blood.

Appendix

  • Vestigial organ located at the junction of small and large intestines
  • May have roles in the lymphatic system; thought to aid digestion via bacterial presence.

Large Intestine/Colon

  • Length: Up to 1.5 meters, width: 7.5 centimeters.
  • Contains smooth muscle fibers for peristalsis and dehydration/compaction of waste.
Functions of Large Intestine
  • Absorbs water from waste material; prevents diarrhea.
  • Houses E. coli bacteria which synthesize vitamins (B, K), growth factors, and further breakdown undigested materials.
Rectum
  • Last 20 cm of the colon, controlled by sphincter muscle, compacts/stores feces.
Anus
  • Opening of rectum, surrounded by sphincter muscles for control during defecation.

Accessory Organs of the Digestive System

Pancreas

  • Oblong gland (12.5 cm long) behind the stomach made of clusters of glandular cells.
  • Functions:
      - Secretion of pancreatic juice containing H2O, NaHCO3, enzymes (e.g. amylase, lipase, trypsin).
      - Neutralizes acid chyme with NaHCO3.
      - Splits DNA and RNA (ribonuclease and deoxyribonuclease).

Liver

  • Large internal organ (1.5 kg), highly vascularized, located under diaphragm.
  • Functions:
      - Produces bile necessary for emulsifying lipids, stored in gallbladder.
      - Maintains glucose levels (stores excess glucose as glycogen).
      - Produces blood/plasma proteins from amino acids (e.g. albumin).
      - Detoxifies blood by removing harmful substances.
      - Converts hemoglobin from worn-out RBCs for bile production.

Gall Bladder

  • Pear-shaped sac beneath the liver that stores and concentrates bile.

Types of Digestive Systems

Basic Types

  1. Monogastric: Simple stomach.
  2. Ruminant: Multi-compartmented stomach (e.g. cattle, sheep).
  3. Hind Gut Fermentor: Simple stomach with large and complex large intestine (e.g. horses).

Ruminant Digestive System

  • Definition: Digestive system allows extraction of nutrition from cellulose, supported by symbiotic microorganisms.
  • Structure:
      - Rumen: Major fermentation chamber; incomplete digestion leads to multiple ingestion cycles.
      - Reticulum: Transfers food to rumen; causes regurgitation during rumination.
      - Omasum: Not fully understood; absorbs residual nutrients like volatile fatty acids (VFA).
      - Abomasum: True stomach, secretes HCl and digestive enzymes similar to simple stomach animals.

Digestive Tract - Ruminants and Hind Gut Fermentors

  • Ruminant Example (Cattle):
      - Rumen, reticulum, omasum, abomasum, small intestine, cecum, colon, rectum, anus.
  • Hind Gut Fermentor Example (Horses):
      - Similar layout with a large, complex large intestine supplementary to fermentation.

Comparison of Digestive Systems

  • Monogastric:
      - Limited fermentation with minimal nutrient extraction from cellulose.
  • Ruminants:
      - Efficiently digest cellulose through fermentation in the rumen.
  • Hind Gut Fermentors:
      - Large cecum ferments cellulose, allowing for better nutrient extraction.

Human vs Rabbit vs Cow vs Bird Digestive Systems

  • Human: Simple digestive system with single stomach.
  • Cow: Four-compartment stomach (rumen, reticulum, omasum, abomasum) for complex digestion.
  • Rabbit: Similar setup with large cecum which stores food for fermentation.
  • Bird: Gizzard processes foods mechanically.

Digestive System Comparison Summary

  • Functionality Insights:
      - Herbivores exhibit specialized adaptations for cellulose digestion (e.g. larger intestines, specialized stomachs).
      - Carnivores have simpler systems, optimized for protein and fat digestion with concentrated stomach acid.
      - Omnivores demonstrate mixed adaptations catering to varied diets, thus possessing more flexible digestive systems.

References

  • Various detailed educational resources and textbooks covering the digestive systems across various species and nutritional modes.
  • A significant emphasis on molecular biology and physiology linking digestion with nutrient absorption and assimilation processes respective to heterotrophic nutrition.