# Lecture 16: Nutrition and Digestion
## General Biology II — Organismal, Evolution & Ecology
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## Learning Objectives
By the end of this lecture, students will be able to:
1. Describe the essential nutrients required by animals and their dietary sources
2. Compare digestive strategies across animal phyla (intracellular vs. extracellular digestion)
3. Trace the path of food through the human digestive tract and describe the function of each organ
4. Explain the chemical digestion of carbohydrates, proteins, and lipids, including the enzymes involved
5. Describe the mechanisms of nutrient absorption in the small intestine
6. Explain hormonal and neural regulation of digestion
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## Lecture Content
### I. Nutritional Requirements
As heterotrophs, animals must obtain organic molecules from their food to fuel metabolism, build structures, and carry out essential biochemical reactions. Certain nutrients are classified as essential because animals cannot synthesize them and must acquire them from the diet. **Essential amino acids** -- eight to ten that humans cannot manufacture, including lysine, tryptophan, and leucine -- must come from protein-containing foods. **Essential fatty acids**, specifically the omega-3 (linolenic acid) and omega-6 (linoleic acid) families, are required for membrane function and signaling. **Vitamins** are organic molecules needed in small amounts: the water-soluble B vitamins serve as coenzymes in metabolic pathways, and vitamin C is essential for collagen synthesis and functions as an antioxidant, while the fat-soluble vitamins A (vision and growth), D (calcium absorption), E (antioxidant), and K (blood clotting) serve diverse roles. **Minerals** are inorganic elements with critical functions: calcium for bones and muscle contraction, iron for hemoglobin, sodium and potassium for nerve function, iodine for thyroid hormone, and zinc as an enzyme cofactor.
The three macronutrients provide the bulk of dietary energy. Carbohydrates are the primary energy source at approximately 4 kcal/g. Proteins supply amino acids for building proteins, also at about 4 kcal/g. Lipids (fats) serve as concentrated energy stores, membrane components, and insulation at approximately 9 kcal/g. Malnutrition can result from either undernutrition or overnutrition. Caloric deficiency leads to conditions such as kwashiorkor (protein deficiency) and marasmus (caloric starvation), while excess caloric intake contributes to obesity, type 2 diabetes, and cardiovascular disease. Specific nutrient deficiencies produce characteristic diseases: scurvy from vitamin C deficiency, rickets from vitamin D deficiency, anemia from iron deficiency, and goiter from iodine deficiency.
### II. Overview of Digestive Strategies
Digestive strategies across the animal kingdom reflect the fundamental challenge of breaking large food molecules into absorbable units. **Intracellular digestion** occurs when food particles are engulfed by phagocytosis and digested within food vacuoles using lysosomal enzymes; this strategy is found in sponges and some protists. **Extracellular digestion** breaks food down outside cells within a digestive compartment. The simplest version is the gastrovascular cavity, a single-opening chamber (where the mouth also serves as the anus) found in cnidarians and flatworms. The more advanced complete digestive tract (alimentary canal) is a tube with a separate mouth and anus, allowing one-way food flow and regional specialization of different segments for different digestive functions. This tube-within-a-tube design characterizes most animal phyla.
Animals have also evolved diverse feeding strategies. Suspension (filter) feeders strain food particles from water (sponges, bivalves, baleen whales). Substrate feeders live in or on their food source (earthworms, leaf miners). Fluid feeders suck nutrient-rich liquids (mosquitoes, leeches, hummingbirds). Bulk feeders ingest large pieces of food (most animals).
### III. The Human Digestive System
#### A. Oral Cavity
Digestion begins in the mouth, where mechanical digestion through chewing (mastication) breaks food into smaller pieces, increasing the surface area available for enzymatic attack. Chemical digestion begins simultaneously: salivary amylase starts breaking starch into maltose, and lingual lipase initiates a minor amount of fat digestion. Saliva itself is a complex fluid containing water, mucus for lubrication, bicarbonate to buffer acids, lysozyme for antibacterial defense, and amylase. The tongue shapes the chewed food into a bolus for swallowing.
#### B. Pharynx and Esophagus
Swallowing (deglutition) is a coordinated reflex during which the epiglottis folds over the trachea to prevent food from entering the airway. The esophagus, a muscular tube, moves the bolus to the stomach through peristalsis -- rhythmic waves of smooth muscle contraction. The lower esophageal sphincter (cardiac sphincter) at the junction with the stomach normally prevents the backflow of acidic stomach contents; when it malfunctions, acid reflux results.
#### C. Stomach
The stomach is a muscular, J-shaped organ that stores food, mixes it vigorously with gastric juice, and begins protein digestion. Gastric juice is a potent cocktail of hydrochloric acid (HCl), secreted by parietal cells to create a pH of approximately 2, and pepsinogen, secreted by chief cells and activated by the acid into pepsin, a powerful endopeptidase that cleaves proteins. The acid denatures dietary proteins and kills most ingested microbes, while intrinsic factor, also produced by parietal cells, is essential for the later absorption of vitamin B12 in the ileum. A thick layer of mucus secreted by mucous cells protects the stomach lining from self-digestion. The churning action of the stomach's muscular walls mixes food with gastric juice to produce chyme, an acidic, partially digested slurry. The pyloric sphincter regulates the release of chyme into the duodenum.
#### D. Small Intestine
The small intestine is the principal site of both chemical digestion and nutrient absorption. Its three regions -- the duodenum (approximately 25 cm), jejunum (approximately 2.5 m), and ileum (approximately 3.5 m) -- are specialized for different aspects of this work. Most chemical digestion occurs in the duodenum, which receives secretions from the pancreas and liver.
Pancreatic secretions, delivered via the pancreatic duct, include a battery of digestive enzymes: pancreatic amylase (continuing starch digestion), trypsin, chymotrypsin, and carboxypeptidase (digesting proteins, secreted as inactive zymogens and activated by enterokinase), pancreatic lipase (digesting triglycerides into fatty acids and monoglycerides), and nucleases (digesting DNA and RNA). Bicarbonate ions (HCO3-) neutralize the acidic chyme, raising the pH to 7-8 to create optimal conditions for enzyme activity.
Bile, produced by the liver and stored in the gallbladder, is not an enzyme but an emulsifier. Bile salts break large fat droplets into smaller ones, increasing the surface area available for lipase activity. Bile also carries bilirubin (a waste product of hemoglobin breakdown) and cholesterol for excretion.
Brush border enzymes on the microvilli of the intestinal epithelium complete the final stages of digestion: maltase, sucrase, and lactase hydrolyze disaccharides into monosaccharides, while aminopeptidase and dipeptidase cleave small peptides into individual amino acids.
Absorption in the small intestine is facilitated by an enormous surface area -- approximately 200 square meters -- created by three levels of structural amplification: circular folds of the intestinal wall, finger-like villi projecting from the mucosal surface, and microvilli on the apical surface of each epithelial cell. Monosaccharides and amino acids are absorbed into the capillary network within each villus via active transport and facilitated diffusion, then travel through the hepatic portal vein to the liver. Lipid absorption follows a different route: fatty acids and monoglycerides enter epithelial cells, are reassembled into triglycerides, packaged into chylomicrons (lipoprotein particles), and released into lacteals (lymphatic capillaries within each villus), eventually reaching the bloodstream through the lymphatic system. Water, ions, and vitamins are absorbed throughout the small intestine.
A detailed cross-sectional diagram of the small intestine wall showing the hierarchy of surface area amplification. Panel A: The overall intestinal tube with circular folds (plicae circulares) visible on the inner surface. Panel B: Magnified view of the mucosal surface showing finger-like villi projecting into the lumen. Each villus is labeled with its internal structures: a capillary network for absorbing amino acids and sugars, a central lacteal (lymphatic vessel) for absorbing fats, and a thin layer of epithelial cells. Panel C: Further magnification of a single epithelial cell showing microvilli (brush border) on the apical surface, with brush border enzymes labeled. Arrows show the absorption pathways: sugars and amino acids into capillaries (then to hepatic portal vein), and fatty acids into the lacteal (then to lymphatic system). Goblet cells secreting mucus are also shown interspersed among the absorptive cells.
#### E. Large Intestine (Colon)
The large intestine absorbs water and electrolytes from the remaining indigestible material and forms, stores, and eventually eliminates feces. Its major regions include the cecum (with the appendix), ascending colon, transverse colon, descending colon, sigmoid colon, rectum, and anal canal. The gut microbiome -- trillions of bacteria residing in the colon -- ferments undigested dietary fiber into short-chain fatty acids, produces vitamin K and some B vitamins, and plays important roles in immune function and overall health. The appendix may serve as a reservoir for beneficial gut bacteria, helping to repopulate the colon after disturbances such as diarrheal illness. The defecation reflex is triggered by distension of the rectum and controlled by both an internal involuntary sphincter and an external voluntary sphincter.
### IV. Regulation of Digestion
Digestion is regulated by a sophisticated interplay of hormonal and neural signals. **Gastrin**, secreted by G cells in the stomach in response to food, stimulates HCl and pepsinogen secretion. **Secretin**, released by duodenal cells in response to acidic chyme, stimulates pancreatic bicarbonate secretion and inhibits gastric acid production. **Cholecystokinin (CCK)**, released by duodenal cells in response to fats and proteins, stimulates pancreatic enzyme secretion and gallbladder contraction for bile release, and promotes the sensation of satiety. **Gastric inhibitory peptide (GIP)** stimulates insulin release and inhibits gastric activity. **Ghrelin**, the "hunger hormone" secreted by the empty stomach, stimulates appetite, while **leptin**, secreted by adipose tissue, signals satiety and helps regulate long-term energy balance.
Neural regulation involves the enteric nervous system -- sometimes called the "second brain" -- which consists of intrinsic nerve plexuses (the myenteric and submucosal plexuses) embedded in the gut wall that can coordinate digestive function independently. The parasympathetic nervous system, acting through the vagus nerve, stimulates digestion ("rest and digest"), while the sympathetic nervous system inhibits it ("fight or flight").
A schematic of the hormonal regulation of digestion. The diagram shows the stomach, duodenum, pancreas, gallbladder, and liver. Arrows indicate hormone release and target actions: (1) Food in the stomach stimulates G cells to release gastrin, which stimulates HCl secretion (positive feedback loop within the stomach). (2) Acidic chyme entering the duodenum stimulates S cells to release secretin, which targets the pancreas (bicarbonate release) and stomach (inhibits acid secretion). (3) Fats and proteins in the duodenum stimulate I cells to release CCK, which targets the pancreas (enzyme release) and gallbladder (contraction/bile release). (4) An inset shows ghrelin released from the empty stomach signaling the brain (hypothalamus) to stimulate hunger, and leptin released from adipose tissue signaling the brain to suppress appetite. Each hormone is color-coded with matching arrows to its target organ.
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