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.