Digestive Systems I

Learning Objectives

• Energy Principles - Energy cannot be created or destroyed.

• Chemical Structures - Describe the chemical structures of:

  • Lipids

  • Carbohydrates

  • Proteins

• Energy Sources Comparison - Compare and contrast lipids, carbohydrates, and proteins as sources of chemical energy. Lipids provide $9$ kilocalories/gram, while carbohydrates and proteins provide about $4$ kilocalories/gram each.

• Digestion Types - Compare and contrast intracellular and extracellular digestion.

• Gastrovascular Cavity of Hydra - Discuss its functionality.

• Alimentary Canal of Birds - Discuss its organization.

• Mammalian Alimentary Canal General Organization - Compare between different types:

  • Carnivores

  • Herbivores (hindgut and foregut fermenters)

• Human Alimentary Canal - General organization.

  • Oral cavity functions: saliva production, tongue, and taste processing.

  • Process of deglutition (swallowing).

Dietary Needs

• Chemical Energy - Needed as fuel for cellular work.

  • Organic precursors: organic raw materials for biosynthesis (carbon skeletons), derived from ingested carbohydrates, lipids, and proteins.

  • Essential Nutrients - Substances that animals cannot synthesize, including essential amino acids (e.g., histidine) and fatty acids (e.g., omega-3 and omega-6).

  • Metabolic Water - Water produced as a byproduct of oxidative phosphorylation; however, an external source of water is also necessary.

Chemical Energy Overview

• Energy is conserved; it is converted from ingested organic molecules into ATP.

• Different food molecules yield different amounts of ATP, with lipids providing more energy per gram than carbohydrates and proteins.

• Energy Unit: $1$ calorie = energy required to raise $1$ mL of water by $1°C$. Food Calories (capital C) are kilocalories ($1$ kilocalorie = $1000$ calories).

• ATP production examples:

  • Glucose: Approximately $30-36$ ATP molecules per glucose molecule.

  • Amino Acids: Approximately $30$ ATP molecules per amino acid (general statement).

Formation of Lipids

• Glycerol as Backbone - Glycerol: a simple $3$-carbon molecule.

  • Combines with fatty acids:

    • Monoglyceride: Glycerol + $1$ fatty acid.

    • Diglyceride: Glycerol + $2$ fatty acids.

    • Triglyceride (Triacylglycerol): Glycerol + $3$ fatty acids (majority of dietary lipids). "Triacylglycerol" is a more precise term, referring to one glycerol and three acyl groups (fatty acids).

Fatty Acids

• Types of Fatty Acids:

  • Saturated Fatty Acids: No double bonds between carbons; fully saturated with hydrogens, forming linear chains that pack tightly due to van der Waals interactions, making them more solid at room temperature (e.g., stearic acid).

  • Monounsaturated Fatty Acids: Contains one double bond (fewer hydrogen atoms due to double bond). The double bond introduces a kink in the molecule, preventing tight packing and resulting in fewer van der Waals interactions, making them liquid at room temperature (e.g., oleic acid).

  • Chemical structure: long chains of carbon atoms with hydrogens and a carboxyl group (–COOH).

• Steroids: Lipids like cholesterol, which is not broken down for energy due to its stable ring structure, but used as a carbon skeleton for synthesizing steroid hormones (e.g., cortisol).

• Essential Fatty Acids: Omega-3 (alpha linoleic acid) and omega-6 (linoleic acid) fatty acids cannot be synthesized by the body and must be obtained from the diet.

Chemical Energy from Carbohydrates

• Types of Carbohydrates:

  • Monosaccharides: Simple sugars like Glucose (also called dextrose in food labels) and Fructose.

  • Disaccharides: Composed of two monosaccharides (e.g., Sucrose = Glucose + Fructose).

  • Polysaccharides: Glucose polymers, storage forms of energy (e.g., Glycogen in animals, Starch in plants).

    • Storage as polymers (glycogen) avoids osmotic problems caused by storing individual glucose molecules.

• Types of Glycosidic Linkages:

  • Alpha 1-4 Linkage: Found in glycogen and starch; digestible by animals. Hydroxyl groups line up on one side of the chain.

  • Beta 1-4 Linkage: Found in cellulose and chitin; cannot be directly digested by vertebrates due to the lack of the enzyme cellulase. Features alternating hydroxyl groups, creating extensive hydrogen bonds that make the structure very stable and difficult to break down enzymatically.

Structural Polysaccharides

• Chitin: Found in invertebrates and insect exoskeletons.

• Cellulose: Major structural component in plants; humans and other vertebrates cannot directly digest cellulose for energy, relying on symbiotic microbes (e.g., in herbivores) to break it down using cellulase enzymes.

Chemical Energy from Proteins

• Structure: Composed of amino acids joined by peptide bonds. Amino acids differ by their R-side groups.

• Proteins fold into primary, secondary (alpha-helix, beta-pleated sheet stabilized by hydrogen bonds), tertiary (involving covalent disulfide bonds and electrostatic interactions), and quaternary structures.

• Digestion Challenge: Folded proteins expose only surface peptide bonds to digestive enzymes. Stomach acid (hydrochloric acid) denatures proteins, making them more accessible for enzymatic digestion (e.g., by pepsin, trypsin).

Essential Vitamins and Minerals

• Vitamins:

  • Fat-soluble vitamins (A, D, E, K): Accumulate in adipose tissue and are retained by the body. Overconsumption can lead to toxicity. Vitamin D is crucial for calcium absorption.

  • Water-soluble vitamins (B complex, C): Eliminated in urine, making toxicity less common. B vitamins act as coenzymes. Vitamin C is essential for collagen biosynthesis; deficiency causes scurvy.

• Minerals:

  • Sodium: Major determinant of extracellular fluid volume; essential for maintaining fluid balance.

  • Calcium: Key for muscle contraction, neurotransmitter release, and bone structure (e.g., hydroxyapatite).

  • Iron: Central atom in the heme group, essential for oxygen transport in respiration.

Digestion Process

• Types of Digestion:

  • Intracellular Digestion: Occurs within specialized organelles, typically food vacuoles, which fuse with lysosomes containing hydrolytic enzymes (e.g., in Paramecium, where bacteria are ingested via phagocytosis).

  • Extracellular Digestion: Involves regions like gastrovascular cavities (e.g., Hydra) or complete alimentary canals, where enzymes are secreted into a lumen to break down food outside cells.

Gastrovascular Cavity Mechanism (e.g., Hydra)

• Single opening (mouth) for ingestion and elimination.

• Tentacles with nematocysts immobilize prey (e.g., daphnea water flea).

• Gland cells secrete enzymes into the gastrovascular cavity for initial extracellular digestion of larger prey.

• Nutritive cells then take up smaller food particles by phagocytosis, followed by intracellular digestion inside food vacuoles that fuse with lysosomes.

• Nutrients are distributed via diffusion and body movements, as Hydra lacks a circulatory system.

Alimentary Canal Structure in Birds

• Two openings: mouth and anus, with specialized regions for digestion and absorption.

• Anatomical features include:

  • Crop: For temporary storage of food.

  • Stomach: Produces acid and enzymes to partially digest and soften food (e.g., insect exoskeletons).

  • Gizzard (Gastric Mill): Muscular organ for grinding food particles; birds may ingest small stones to aid this mechanical breakdown.

  • Further digestion and absorption occur in the intestine.

Mammalian Alimentary Canal General Organization

• Differs significantly between carnivores and herbivores due to their diets.

• Carnivores: Tend to have a small cecum and dentition adapted for ripping and tearing meat. Digestion is relatively straightforward, involving stomach acid (denaturation) and enzymes (pepsin, trypsin, chymotrypsin).

• Herbivores: Require elaborate structures and symbiotic microbes to digest cellulose (beta 1-4 linkages).

  • Hindgut Fermenters (e.g., Koalas): Have a very long, elaborate cecum where microbes (mostly bacteria) ferment cellulose using cellulase. The animal relies on these microbes for glucose extraction.

  • Foregut Fermenters/Ruminants (e.g., Cows): Possess a multi-chambered stomach system.

    1. Rumen: Large fermentation chamber packed with anaerobic microbes that break down cellulose.

    2. Reticulum: Acts as a size selection chamber. Large particles are regurgitated to the oral cavity for further chewing (chewing the cud).

    3. Omasum: Primarily reabsorbs water.

    4. Abomasum (True Stomach): Similar to a monogastric stomach, producing acid and enzymes for digestion.

    5. Food then passes to the small intestine for further digestion and absorption.

  • Ruminants rely entirely on microbes for cellulase activity, as vertebrates do not produce this enzyme.

Human Alimentary Canal Overview

• Major components: Oral Cavity, Esophagus, Stomach, Small and Large Intestines, Rectum, Anus.

• Accessory organs: Salivary Glands, Liver, Gallbladder, Pancreas.

• Sphincters: Control food movement:

  • Upper Esophageal Sphincter: Skeletal muscle, controls entry to the esophagus, part of the swallowing reflex.

  • Lower Esophageal Sphincter: Smooth muscle, prevents reflux of stomach contents into the esophagus.

  • Pyloric Sphincter: Controls movement from the stomach to the small intestine.

  • Anal Sphincter: Controls elimination of waste.

Oral Cavity Functions

• Digestion: Mechanical through teeth (omnivorous dentition for grinding and biting), tongue, and jaw movement; initial chemical digestion with salivary amylase for carbohydrates.

• Saliva Production:

  • Glands: Parotid, Sublingual, Submandibular (all exocrine glands).

    • Parotid Gland: Rich in enzymes (salivary amylase) for carbohydrate digestion.

    • Sublingual Gland: Rich in mucins, contributing to viscous, lubricating saliva.

    • Submandibular Gland: Mixed secretion of enzymes and mucins.

  • Functions:

    • Lubrication: Mucins in saliva help lubricate food (bolus) and protect the GI tract lining from abrasion.

    • Digestion Initiation: Salivary amylase begins carbohydrate breakdown.

    • Maintaining Microbiome: Prevents overgrowth of harmful bacteria that cause tooth decay.

    • Solubilizing Taste Molecules: Essential for taste perception.

• Saliva Composition: Mostly water, becoming hypoosmotic as ions (sodium, potassium, chloride) are reabsorbed in the duct, while water is not.

• Taste Processing: Taste buds on the tongue detect five primary tastes:

  • Sweet, Umami (savory), Bitter: Detected by specific G protein-coupled receptors (GPCRs) on Type 2 taste cells. Sweet and umami indicate energy-rich foods; bitter is an evolutionary avoidance mechanism for toxins.

  • Sour, Salt: Detected by ion channels on Type 3 taste cells. Sour involves hydrogen ion channels; salt involves sodium channels (epithelial sodium channels).

Deglutition (Swallowing Process)

1. Initiation: Tongue pushes the chewed food (bolus), coated with mucins, against the soft palate/back of the oral cavity, triggering stretch receptors and initiating the involuntary swallowing reflex in the brainstem.

2. Airway Protection: Breathing briefly halts, and the epiglottis (cartilage) folds down to cover the trachea, preventing food/liquid from entering the airway.

3. Esophageal Transport: The bolus enters the esophagus past the relaxed upper esophageal sphincter. Peristaltic waves (rhythmic contractions of smooth muscle, aided by gravity but not dependent on it) propel the bolus down the esophagus towards the stomach. This process involves the enteric nervous system, stimulated by distention of the esophageal wall.