Nutrients, Vitamins, and Digestion

• Nutrients are either essential or non-essential
• Essential nutrients are required for survival, like oxygen, carbon, hydrogen, and nitrogen
• Non-essential nutrients are not required for survival

Vitamins

• Vitamins are either fat-soluble or water-soluble
• Fat-soluble vitamins include A, D, E, and K
• Water-soluble vitamins include the B vitamins and vitamin C; excess amounts are flushed through urine and sweat

Macronutrients vs. Micronutrients

• Macronutrients are needed in large amounts for energy and building structures: carbohydrates, proteins, and fats
• Micronutrients are needed in small amounts for various physiological functions: vitamins and minerals

Dietary Classifications

• Herbivores eat plants
• Carnivores eat meat
• Omnivores eat both plants and meat

Digestion

• Fortified foods have additional nutrients added
• Digestion involves ingestion, digestion, absorption, and elimination
• Chemical digestion uses enzymes and chemical processes (e.g., amylase, bile, HCl)
• Mechanical digestion involves the physical breakdown of food (e.g., chewing)

Digestive Tract Pathway

• Food travels through the mouth, oral cavity, pharynx, esophagus, and stomach
• In the stomach, food mixes with gastric juice and becomes chyme
• Chyme enters the small intestine (duodenum, jejunum, ileum) for chemical digestion and nutrient absorption
• Finally, it enters the large intestine (cecum, ascending, transverse, descending, sigmoid colon, rectum, anus) for water reabsorption and waste elimination

Parts of the Digestive System

• The alimentary canal includes the mouth/oral cavity, pharynx, esophagus, stomach, small intestine, and large intestine
• Accessory organs/glands include the liver, gallbladder, and pancreas

Alimentary Canal Functions

• Mouth/Oral Cavity: Ingestion and initial chemical (salivary amylase) and mechanical digestion (chewing) occur
• Pharynx & Esophagus: Transport food via peristalsis
• Stomach: Secretes gastric juice (mucus, HCl, pepsinogen) and churns food into chyme; contains rugae and sphincters
• Small Intestine: The duodenum receives pancreatic enzymes and bile; the jejunum and ileum are sites for most digestion and absorption through blood vessels and lacteals in villi
• Large Intestine: Reabsorbs water, houses bacteria (producing vitamin K), and stores feces

Accessory Organs/Glands Functions

• Liver: Produces bile for emulsifying fats, regulates nutrient distribution, and stores glycogen
• Gallbladder: Stores and releases bile into the duodenum
• Pancreas: Secretes digestive enzymes (amylase, lipase, proteases) and hormones (insulin and glucagon)

Animal Feeding Mechanisms

• Dietary adaptations include herbivory, omnivory, carnivory, and specialized ruminant digestion (like in cows)

Production of Gastric Juice

• Gastric glands produce mucus, HCl, pepsinogen, and pepsin
• Mucus lubricates and protects the stomach lining
• HCl creates a low pH (around 2) to denature proteins and activate pepsinogen
• Pepsinogen is an inactive enzyme precursor converted into pepsin
• Pepsin is a protease that digests proteins into polypeptides

Stomach Secretions and Cell Types

• Mucus Cells: Secrete protective mucus
• Chief Cells: Secrete pepsinogen and gastric lipase
• Parietal Cells: Secrete HCl and intrinsic factor
• Other cells include mucous neck cells (mucus and bicarbonate), enterochromaffin-like cells (histamine), D cells (somatostatin), and G cells (gastric hormone)

Enzyme Release and Function

• Salivary Amylase: Converts starch into maltose
• Pepsin: Breaks down proteins into smaller polypeptides in the stomach
• Pancreatic Enzymes: Amylase (carbohydrates), lipase (lipids), and proteases (proteins) act in the small intestine
• Disaccharidases (maltase, lactase, sucrase) further digest sugars

Digestive System Structures

• Villi: Increase the small intestine’s surface area for nutrient absorption and contain blood vessels and lacteals
• Rugae: Folds in the stomach mucosa that allow expansion and increase surface area
• Sphincters: Muscular valves (e.g., esophageal/cardiac and pyloric sphincters) that control the passage of food between sections

Digestion and Absorption of Fats

• Bile emulsifies fats into micelles
• Fatty acids are absorbed by epithelial cells, reassembled into triglycerides, then packaged with proteins in the Golgi to form chylomicrons, which enter lacteals and the lymphatic system

Dentition

• Dentition: Refers to the structure and arrangement of teeth (incisors, canines, premolars, molars; with components like the crown, enamel, root, cementum, and dentin)
• The type of dentition correlates with dietary needs (cutting, grinding) based on what is consumed

Mutualistic Adaptations

• Humans have adaptations in dentition and enzyme secretion for an omnivorous diet
• Herbivores often feature longer digestive tracts and mutualistic bacteria to break down plant material

Ruminant Digestion

• Cows possess a four-chambered stomach and use rumination (regurgitating and re-chewing food) to ferment and digest tough plant fibers

Appetite-Regulating Hormones

• Ghrelin: Secreted by the stomach and triggers hunger
• Leptin: Produced by adipose tissue and suppresses appetite; levels fall with decreased body fat
• PYY: Secreted by the small intestine after meals and suppresses appetite and counters ghrelin
• Insulin also helps reduce hunger by promoting glucose uptake

Glucose Homeostasis

• Maintained through the balance of insulin (lowers blood glucose by moving it into cells) and glucagon (raises blood glucose by releasing it from glycogen)

Diabetes Mellitus

• Type I: Insulin-dependent; the pancreas produces little or no insulin
• Type II: Often lifestyle-related with insulin resistance
• Gestational: Occurs during pregnancy

Macromolecules

• Carbohydrates: Monosaccharides (glucose, fructose, galactose), disaccharides (sucrose, maltose, lactose), and polysaccharides (starch, glycogen, cellulose) provide energy and structure
• Lipids: Fats, oils, triglycerides, phospholipids, and steroids store energy and build cell membranes
• Proteins: Made of amino acids; function as enzymes, structural elements, antibodies, and transporters
• Nucleic Acids: DNA and RNA store and transmit genetic information

Carbohydrates

• Provide quick energy and are made of carbon and water
• Simple carbohydrates: Single sugars (monosaccharides)
• Complex carbohydrates: Composed of multiple sugar units (polysaccharides)
• Monosaccharide: A single sugar unit (e.g., glucose, fructose, galactose)
• Disaccharide: Two monosaccharides joined together (e.g., sucrose, maltose, lactose)
• Dehydration Synthesis: Joins two monosaccharides by removing a water molecule to form a disaccharide
• Hydrolysis: Breaks disaccharides into monosaccharides by adding water

Glycosidic Linkages

• 1-2 Glycosidic Linkage: Involves the 1st carbon of one sugar and the 2nd carbon of another (as seen in sucrose formation from glucose and fructose)
• 1-4 Glycosidic Linkage: Involves the 1st carbon of one sugar and the 4th carbon of another (as seen in lactose formation from galactose and glucose or maltose formation from two glucose units)

Sugar Molecules

• Fructose: C₆H₁₂O₆; pentagon-shaped; carbons counted counter-clockwise starting at the top left
• Glucose: C₆H₁₂O₆; hexagon-shaped; the 4th carbon has an OH on the bottom
• Galactose: C₆H₁₂O₆; hexagon-shaped; the 4th carbon has an OH on the top
• Maltose: Formed from two glucose molecules and uses the enzyme maltase
• Sucrose: Formed from glucose and fructose and uses the enzyme sucrase
• Lactose: Formed from glucose and galactose and uses the enzyme lactase

Fats (Lipids)

• Formed by a glycerol molecule + 3 fatty acids to create triglycerides
• Formation Reaction: Dehydration synthesis (removes 3 water molecules, one per fatty acid) vs. hydrolysis (adds water to break bonds)
• Saturated Fat: All carbons in the chain have only single bonds
• Unsaturated Fat: Contains one or more double bonds
• Glycerol: Contains hydroxyl (OH) groups
• Fatty Acids: Contain carboxyl groups
• Trans Fatty Acids: Produced via hydrogenation, which changes the structure of the hydrocarbon chains

Proteins

• Proteins are built from amino acids and joined by peptide bonds formed via dehydration synthesis between the amino and carboxyl groups
• Dipeptide: Two amino acids linked together
• Polypeptide: Three or more amino acids linked together
• The variable R group distinguishes each amino acid and determines its properties
• Essential Amino Acids: Cannot be synthesized by the body and must be obtained from food
• Non-Essential Amino Acids: Can be synthesized by the body
• Each amino acid has a common backbone (amino group, carboxyl group, and hydrogen attached to a central carbon) plus its unique R group
• Complete Proteins: Contain all essential amino acids, often found in animal products
• Incomplete Proteins: Lack one or more essential amino acids, commonly found in individual vegan sources such as nuts, seeds, legumes, or grains