Nutrition and Digestion: Chapter 21 Study Notes

Nutrition and Digestion

Overview

Nutrition and digestion are essential biological processes that allow animals to obtain energy and nutrients from their environment. This chapter explores the diversity of animal diets, the mechanisms of food processing, and the structure and function of the human digestive system.

Types of Animal Diets and Feeding Mechanisms

Animal Diets

• Herbivores: Eat plants. Examples include cattle, gorillas, sea urchins, and snails.

• Carnivores: Eat meat. Examples include lions, owls, whales, and spiders.

• Omnivores: Eat both plants and animals. Examples include humans, roaches, raccoons, and crows.

Feeding Mechanisms

• Suspension feeders: Filter small food particles from water (e.g., manta rays).

• Substrate feeders: Live in or on their food source and eat their way through it (e.g., caterpillars).

• Fluid feeders: Suck nutrient-rich fluids from a living host (e.g., mosquitoes).

• Bulk feeders: Ingest large pieces of food (e.g., birds eating fish).

Stages of Food Processing

Four Stages of Food Processing

• Ingestion: The act of eating.

• Digestion: Breaking down food into molecules small enough for absorption. This includes mechanical and chemical digestion.

• Absorption: Uptake of digested nutrients by cells lining the digestive tract.

• Elimination: Removal of undigested materials from the digestive tract.

Example: A cat eats food (ingestion), chews and digests it (digestion), absorbs nutrients (absorption), and expels waste (elimination).

Enzymatic Breakdown of Food Molecules

• Proteins are broken down by protein-digesting enzymes into amino acids.

• Polysaccharides are broken down by carbohydrate-digesting enzymes into monosaccharides.

• Nucleic acids are broken down by nucleic acid-digesting enzymes into nucleotides.

• Fats are broken down by fat-digesting enzymes into fatty acids and glycerol.

Digestion: Intracellular vs. Extracellular

Intracellular Digestion

• Food particles are engulfed by phagocytosis.

• Food vacuoles fuse with lysosomes containing hydrolytic enzymes.

• Few animals, such as sponges, use this mechanism exclusively.

Extracellular Digestion

• Most animals digest food in specialized compartments.

• Cnidarians and flatworms have a gastrovascular cavity with a single opening.

• Most animals have an alimentary canal running from mouth to anus, with specialized regions for digestion and absorption.

Advantage of an alimentary canal: Allows for sequential processing of food and specialization of digestive regions.

The Human Digestive System

Structure and Function

• Consists of the oral cavity, pharynx, esophagus, stomach, small intestine, large intestine, rectum, and anus.

• Accessory organs include salivary glands, liver, gallbladder, and pancreas.

Digestion in the Oral Cavity

• Mechanical digestion: Chewing cuts, smashes, and grinds food.

• The tongue: Tastes food, shapes it into a bolus, and moves it toward the pharynx.

• Lingual lipase: Secreted by tongue cells, begins lipid breakdown.

• Salivary glands release:

  • Amylase: Begins hydrolysis of starch.

  • Glycoprotein: Moistens and lubricates food.

  • Buffers: Neutralize acids.

  • Antibacterial agents: Kill some bacteria ingested with food.

Swallowing and Movement to the Stomach

• The pharynx opens to the esophagus (digestive) and trachea (respiratory).

• The swallowing reflex moves food into the esophagus and prevents entry into the trachea.

• Peristalsis: Rhythmic contraction of muscles moves food through the esophagus.

Example: The epiglottis closes over the trachea during swallowing to prevent choking.

The Heimlich Maneuver

• Forceful elevation of the diaphragm pushes air into the trachea, dislodging food during choking.

• Immediate action is required to prevent brain damage or death due to lack of oxygen.

Stomach Function and Protection

• The stomach secretes gastric juice (contains pepsinogen, HCl, and mucus).

• Pepsinogen is converted to pepsin by HCl, which digests proteins.

• Mucus protects the stomach lining from acid and enzymes.

• New cells lining the stomach are produced every few days to replace damaged ones.

• Gastric juice secretion is regulated by nerve signals and hormones (e.g., gastrin).

• Negative feedback inhibits gastrin release when stomach contents become too acidic.

Equation for HCl secretion: $\mathrm{CO_2 + H_2O \xrightarrow{carbonic\ anhydrase} H_2CO_3 \rightarrow HCO_3^- + H^+}$

Digestive Disorders

• Acid reflux: Chyme moves back into the esophagus, causing heartburn.

• Gastroesophageal reflux disease (GERD): Chronic acid reflux damages the esophagus.

• Ulcers: Open sores in the stomach lining, often caused by Helicobacter pylori infection.

Small Intestine: Digestion and Absorption

Structure and Function

• Duodenum: First part of the small intestine; mixes chyme with digestive juices.

• Enzymes from the pancreas and intestinal wall digest food molecules.

• Bile (from liver, stored in gallbladder) emulsifies fats for enzyme action.

• Villi and microvilli increase surface area for nutrient absorption into capillaries and lymph vessels.

Enzymatic Digestion in the Small Intestine

Food Type	Enzymes	Products
Carbohydrates	Pancreatic amylase, maltase, sucrase, lactase	Monosaccharides
Proteins	Trypsin, chymotrypsin, peptidases	Amino acids
Nucleic Acids	Nucleases, other enzymes	Nitrogenous bases, sugars, phosphates
Fats	Bile salts, lipase	Fatty acids, glycerol

Absorption Mechanisms

• Secondary active transport (cotransporter): Uses ATP to move nutrients against concentration gradients.

• Facilitated diffusion: Moves nutrients along concentration gradients.

Liver Function

Processing and Detoxification

• Blood from the intestines enters the liver via the hepatic portal vein.

• The liver regulates nutrient levels, detoxifies substances, and synthesizes blood proteins.

• Converts glucose to glycogen, stores and releases sugars, synthesizes proteins and lipoproteins, modifies toxins, and produces bile.

Example: The liver acts as a go-between for the digestive and circulatory systems.

Digestion and Absorption of Lipids

Bile and Lipid Transport

• Bile salts emulsify large fat globules into smaller droplets.

• Lipase digests fats into monoglycerides and fatty acids.

• Absorbed lipids are packaged into chylomicrons for transport via the lymphatic system.

Large Intestine: Water Reclamation and Feces Formation

Function

• Reclaims water and compacts feces.

• Contains the cecum and appendix.

• Bacteria in the colon produce vitamins and aid in digestion.

• Absorbs vitamins and water into the bloodstream.

• Feces are stored in the rectum until elimination.

Disorders: Diarrhea (too little water reclaimed), constipation (too much water reclaimed).

Mutualistic Adaptations

• Humans and intestinal bacteria coexist in mutualistic symbiosis.

• Bacteria produce vitamins and regulate immune and epithelial function.

• Over 400 bacterial species are found in the human digestive tract.

Nutrition: Dietary Requirements and Health

Energy and Nutrient Needs

• Diet must provide chemical energy (measured in kilocalories), raw materials for biosynthesis, and essential nutrients.

• Metabolic rate: Rate of energy consumption, including basal metabolic rate (BMR) and energy for activities.

Equation: $1\ \text{kcal} = 1,000\ \text{calories}$

Essential Nutrients

• Essential fatty acids: Used to make phospholipids; found in seeds, grains, vegetables.

• Essential amino acids: Used to make proteins; found in meats, eggs, milk, cheese.

• Vitamins: Organic molecules required in small amounts, often as coenzymes.

• Minerals: Inorganic nutrients required in small amounts for various functions.

Example: Beans and wheat together provide all essential amino acids.

Diet, Obesity, and Disease

Obesity and Health Risks

• Obesity is defined as a high body mass index (BMI).

• Leads to type 2 diabetes, cancer, and cardiovascular disease.

• Overnourishment is a major concern in industrialized countries.

Regulation of Appetite

• Hormones regulate appetite via the brain's satiety center.

• Ghrelin: Triggers hunger before meals.

• Insulin and PYY: Suppress appetite after meals.

• Leptin: Produced by fat tissue, suppresses appetite and regulates body fat levels.

Diet and Cardiovascular Disease

• Low-density lipoproteins (LDL): Associated with blocked blood vessels and heart attacks.

• High-density lipoproteins (HDL): May decrease risk by transporting excess cholesterol to the liver.

• Decrease 'bad cholesterol' by avoiding saturated fats and hydrogenated oils.

• Increase 'good cholesterol' by eating unsaturated fats and avoiding hydrogenated oils.