Nutrition, Digestion, and Absorption
Chapter 50 Key Concepts
50.1 Food Provides Energy As Well As Materials for Biosynthesis
50.2 Diverse Adaptations Support Ingestion and Digestion of Food
50.3 The Vertebrate Gastrointestinal System Is a Disassembly Line
50.4 Nutrient Availability Is Controlled and Regulated
Investigating Life: Thrifty Phenotypes
The Pima population of Arizona experienced frequent periods of food deprivation, leading to strong selection pressure for "thrifty genes" involved in digestion and energy storage, as well as efficient conversion of food into fat.
This population now consumes a high-calorie, high-fat Western diet, instead of their traditional diet, and are more sedentary.
If the nutritional values were the same, could different foods have different impacts on health?
These factors contribute to obesity and diabetes, and the Pima people in the United States suffer high rates of obesity, diabetes, and heart disease.
Key Concept 50.1 Learning Objectives
Fats, carbohydrates, and proteins in food provide energy.
Energy is stored in animal bodies as glycogen and fat.
Some small organic molecules required for biosynthesis must come from food, including essential amino acids and fatty acids.
Vitamins are organic compounds that an animal cannot synthesize but are required for healthy cell function.
Animals require a variety of mineral nutrients.
50.1 Food Provides Energy As Well As Materials for Biosynthesis
Animals are heterotrophs: they derive their nutrition by eating other organisms.
Autotrophs synthesize their own components using solar or chemical energy.
Heterotrophs depend on this synthesis and have evolved a variety of adaptations to take advantage of it.
Energy—the capacity to do work—comes in different forms.
Measures of heat energy:
A calorie is the amount of heat needed to raise 1 gram of water 1°C.
A kilocalorie (kcal) = 1,000 calories. A Calorie (Cal) is the same as a kilocalorie.
An animal’s energy needs must be met by ingestion, digestion, and assimilation of food.
Basal Energy Expenditure:
The basal energy expenditure of a human is 1,300 to 1,500 Cal/day for an adult female and 1,600 to 1,800 Cal/day for an adult male.
Physical activity adds to this basal metabolic rate (BMR).
Foods that provide energy are fats, carbohydrates, and proteins.
Caloric value of food and expenditure of energy for any activity can be measured.
Energy budgets compare calories consumed and calories expended and allow a cost–benefit analysis of feeding behavior.
Animals must store food between meals.
Carbohydrates Storage:
Carbohydrates are stored in liver and muscle cells as glycogen—enough for about one day’s energy needs.
Fat Storage:
Fat stores more energy per gram and with little water, which makes it more compact.
Protein Usage:
Proteins are not used for storage but can be metabolized as a last resort.
Metabolism When Taking in too Little Food:
If an animal takes in too little food, metabolism of the body’s own molecules begins.
Glycogen and fat are broken down.
Then proteins are metabolized, starting with blood plasma.
Metabolism of blood proteins leads to edema, a sign of kwashiorkor.
Metabolism When Taking in too Much Food:
If an animal takes in more food than needed, the excess is stored as increased body mass.
Glycogen reserves are built up.
Extra carbohydrates, fats, and proteins are converted to body fat.
Animals require organic molecules to supply carbon skeletons, building blocks for larger organic molecules.
The acetyl group is used to build more complex molecules. Acetyl groups must be obtained from food.
Amino acids are the building blocks of proteins.
Essential amino acids cannot be synthesized by an animal.
Adult humans must obtain 8 essential amino acids from their food. A complementary diet of plant foods can supply all 8 essential amino acids for vegetarians.
Ingested Proteins Breakdown:
Ingested proteins must be broken down to the constituent amino acids before being used by the body.
Proteins are large and not readily absorbed by the gut.
Protein structure and function vary by species.
The immune system would attack protein molecules entering directly from the gut.
Humans also require essential fatty acids from their diet.
Linoleic acid is one that helps synthesize other unsaturated fatty acids, including signaling molecules and membrane phospholipids.
Macronutrients
Macronutrients: Elements required in large amounts, such as calcium.
Micronutrients
Micronutrients: Elements required in tiny amounts, such as iron.
Insufficient iron leads to anemia and is the most common mineral deficiency in the world today.
Vitamins are carbon compounds required for growth and metabolism that cannot be synthesized.
Most function as coenzymes or parts of coenzymes.
Required vitamins vary with species (e.g., primates cannot make vitamin C but other mammals can).
Humans require 13 vitamins.
Vitamin Types:
Water-soluble vitamins: eliminated in urine if there is an excess (e.g., vitamin C).
Fat-soluble vitamins: can accumulate to toxic levels in body fat and the liver.
Vitamin D (calciferol) is essential for absorbing and metabolizing calcium. It is a special case because the body can synthesize it (by definition it is a hormone).
Certain lipids are converted to vitamin D by UV light on the skin.
People living at high latitudes may have evolved light skin to facilitate vitamin D production during the short winter days. Dark skin in the low latitudes protects against UV damage.
Dark-skinned Inuit people get ample vitamin D from animal fat (especially whale blubber) and fish oils in their diet.
Nutrient deficiency leads to malnutrition; chronic malnutrition leads to deficiency diseases.
Scurvy—lack of vitamin C
Beriberi—lack of thiamin (vitamin B1) § This disease led to the discovery of vitamins.
Deficiency disease can result from inability to absorb a nutrient. In pernicious anemia, vitamin B12 (cobalamin) is not absorbed in the stomach.
Mineral Deficiencies:
Mineral deficiencies can also lead to disease.
Iodine deficiency leads to hypothyroidism and goiter.
Iron deficiency leads to anemia.
Key Concept 50.1 Learning Outcomes
Describe energy storage adaptations in animal species.
Discuss challenges related to obtaining all essential amino acids from the diet.
Discuss examples of mineral macro- and micronutrients and their functions.
Compare water-soluble and fat-soluble vitamins and their effects on an animal’s body.
Key Concept 50.2 Focus Your Learning
Heterotrophs display a wide diversity of adaptations for acquiring and processing food.
Digestion occurs in a body cavity where secreted enzymes break down large molecules into small molecules that cells can absorb.
Heterotrophs rely on symbiotic bacteria residing in their digestive system to carry out essential tasks in digestion.
50.2 Diverse Adaptations Support Ingestion and Digestion of Food
Heterotrophs acquire nutrition in different ways:
Saprobes absorb nutrients from dead organic matter (e.g., protists and fungi).
Detritivores or decomposers actively feed on dead organic matter.
Predators feed on living organisms.
Herbivores prey on plants.
Carnivores prey on animals.
Omnivores prey on plants and animals.
Filter feeders filter small organisms from an aquatic environment.
Fluid feeders include mosquitoes, leeches, aphids, hummingbirds.
The food of herbivores is low in energy and hard to digest. So they must spend a lot of time feeding and processing their food.
Many have specialized adaptations such as the elephant’s trunk, a long neck, teeth for grinding, and specialized digestive enzymes and processes.
Carnivores have specialized adaptations for finding, killing, and ingesting prey.
They have evolved stealth, speed, power, large jaws, sharp teeth, strong gripping appendages. Detection methods include echolocation and infrared sensing.
Digestion begins with the teeth.
Vertebrate Teeth:
Enamel, composed of calcium phosphate, covers the crown
Dentine, (bony material) in the crown and root
Pulp cavity, contains blood vessels, nerves, and dentine-producing cells
Mammalian teeth are adapted to different diets
In general:
Incisors—used for cutting, chopping, or gnawing
Canines—for stabbing, gripping, or ripping
Molars and premolars—shearing, crushing, or grinding
Digestion usually begins in a body cavity.
Gastrovascular cavities connect to the outside through a single opening—jellyfish and other cnidarians.
Tubular guts have an opening at each end. A mouth takes in food, and wastes are eliminated through the anus.
Different regions in a tubular gut are specialized for particular functions.
Food is broken up in the mouth cavity by teeth, radula (snails), or mandibles (arthropods).
Most birds have gizzards with small stones for grinding food.
Some animals, such as snakes, simply ingest whole prey with little or no fragmentation.
Stomachs and crops are storage chambers that allow for gradual digestion.
Food particles move from the stomach into the intestines.
Food Processing in Intestines:
Most digestion occurs in the intestine; nutrients, water, and ions are absorbed across its walls.
Last segment recovers ions and water and stores undigested waste as feces.
A muscular rectum expels feces.
Surface area is increased in the parts of the gut that absorb nutrients.
The earthworm gut has an infolding of the gut wall, or typhlosole.
Sharks have a spiral valve—walls of the spiral have a large surface area.
In humans, gut wall has folds with finger- like projections called villi.
Surface cells of villi have smaller projections called microvilli.
The microvilli give the intestine an enormous surface area for absorbing nutrients.
Macromolecules are broken down by hydrolytic enzymes.
They cleave bonds by hydrolysis and are classified according to the substance they hydrolyze:
Proteases
Carbohydrases
Peptidases
Lipases
Nucleases
The gut is colonized by a huge population of microorganisms, mostly bacteria, called the microbiota.
Their cumulative genomes are called the microbiome.
Gut microbiota get nutrition from food passing through the gut and contribute to the host’s digestive processes.
In humans, gut microbiota also produce vitamin K and biotin.
Gut microbiomes are not constant but are influenced by diet and other factors, including medications.
They may be more important than simply facilitating digestion.
Investigating Life: How Does the Gut Microbiome Contribute to Obesity and Metabolic Disease?
Diets rich in saturated fatty acids are implicated in several health concerns, but why are they unhealthy? Does it influence the microbiome?
Hypothesis: The type of fat in the diet can influence the composition of the gut microbiome.
Method:
Raise 2 groups of 15 mice on diets rich in saturated fats (lard) or unsaturated fats (fish oil).
Record body weight each week.
After 11 weeks, compare microbiomes by sequencing the ribosomal RNA genes, and body weight gain.
Conclusions:
The fat composition of the diet influenced the composition of the microbiome.
Mice on the diet containing saturated fats gained more weight and gained weight faster than mice on the isocaloric diet containing unsaturated fats.
Key Concept 50.2 Learning Outcomes
Compare the feeding and digestive adaptations of different species.
Relate structural features of animal guts to their functions and adaptive significance in digestion.
Describe digestive enzymes that are present in animal guts.
Explain the importance of the microbiome in the digestion of food.
Key Concept 50.3 Focus Your Learning
Movement of food through the gut is controlled by CNS reflexes and the enteric nervous system.
A variety of chemical and physical processes take place in different parts of the digestive system to efficiently break down food into forms that the body can use.
Herbivores do not produce the cellulases needed to digest cellulose in their plant diet and thus require microbiota to carry out this task.
50.3 The Vertebrate Gastrointestinal System Is a Disassembly Line
Tissues of the vertebrate gut are arranged in layers:
Lumen: The gut cavity.
Mucosa: Epithelial cells that secrete mucus, digestive enzymes, or hormones; some absorb nutrients through microvilli; in the stomach some secrete hydrochloric acid.
Submucosa: Blood and lymph vessels pick up nutrients.
Nerves in the submucosa have sensory functions and control various secretory functions.
Two layers of smooth muscle are outside the submucosa:
Circular muscle layer: Innermost cells oriented around the gut; constrict gut.
Longitudinal muscle layer: Outermost cells oriented along the gut; shorten gut.
Nerve nets in the submucosa and between smooth muscle layers form the enteric nervous system.
These nerves only form synapses with other nerves in the network.
They are responsible for communication within the gut. The CNS can influence the system, but it is autonomous.
Peritoneum: Membrane surrounding the gut and lining the wall of the cavity.
Includes connective and epithelial tissues that secrete lubricating fluids so organs can easily slide against each other in the body cavity.
Movement of food through gut:
Food is chewed and mixed with saliva.
Tongue pushes a bolus to the soft palate, initiating swallowing—food passes into esophagus.
Food is kept out of the trachea by the closed larynx and epiglottis.
Peristalsis: Waves of muscle contractions that move food toward the stomach.
The upper esophagus is skeletal muscle; the rest is smooth muscle. As food reaches the smooth muscle, the esophagus contracts and pushes the food toward the stomach.
Nerves coordinate esophagus muscles:
Contraction is always preceded by an anticipatory wave of relaxation.
As an area contracts, the region below it relaxes so food does not move upward.
As food moves down, it causes the next region to contract.
The esophageal sphincter, a ring of muscle, controls passage of food into stomach.
Pyloric sphincter—from stomach to intestine
Ileocaecal sphincter—between small and large intestines.
Anal sphincter relaxes to allow defecation.
Chemical Digestion:
Salivary glands secrete amylase that breaks down carbohydrates.
Gastric pits in the stomach have 3 types of secretory cells:
Mucus secreting cells: mucus protects tissues from the acids and enzymes.
Chief cells secrete pepsinogen, the inactive form of the protease pepsin.
Low pH of the stomach denatures proteins, and converts pepsinogen to the active form, pepsin.
Pepsin activates other pepsinogen molecules—autocatalysis, a positive feedback process.
Parietal cells secrete HCl; keeps stomach pH below 1.
Carbonic anhydrase catalyzes formation of from , which dissociates to and .
is exchanged for in gastric pit lumens.
leaks back into the cells and is continually returned to the stomach.
Chyme: Mixture of gastric juices and partly digested food.
Some things are absorbed from the stomach, including alcohol and caffeine, but most absorption occurs in the small intestine.
Stomach empties slowly, allowing small intestine to work on a little material at a time.
Small Intestine:
Most chemical digestion occurs in the small intestine.
Duodenum—site of most digestion
Jejunum and
Ileum—carry out most absorption
Liver:
Liver: Synthesizes bile salts from cholesterol and secretes them as bile.
Bile also contains phospholipids and bilirubin (breakdown product of hemoglobin).
Bile flows through the hepatic duct to the duodenum and through the cystic duct to the gallbladder where bile is stored.
Fat entering the duodenum signals the epithelial cells to release the hormone cholecystokinin (CCK). This hormone stimulates the walls of the gallbladder to contract rhythmically and squeeze bile into the duodenum.
Bile emulsifies fats in the chyme.
One end of the molecule is lipophilic and the other end is hydrophilic.
Lipophilic ends merge with fat droplets and keep them from sticking together, forming micelles.
This enlarges the surface area exposed to lipases that digest fat.
Pancreas:
The pancreas is both an endocrine and exocrine gland.
Exocrine: Secretes digestive juices to the duodenum via the pancreatic duct.
Pancreas produces many digestive enzymes, including lipases, amylases, proteases, and nucleases.
Protease enzymes are released in inactive forms called zymogens.
Zymogens cannot digest the pancreas and its ducts before reaching the gut.
The zymogen trypsinogen is activated by enterokinase to produce the active protease trypsin, which can activate other zymogens.
The pancreas also secretes to neutralize chyme in the intestine.
Intestinal enzymes function best at neutral or slightly alkaline pH.
In the small intestine, epithelial cells secrete various enzymes to cleave peptides, disaccharides, and lipids.
Many humans stop producing lactase after childhood and cannot digest lactose.
The lactose is metabolized by bacteria in the large intestine, causing gas, diarrhea, and cramps.
Epithelial cells absorb nutrients and inorganic ions by many mechanisms.
and other ions are actively transported; the osmotic concentration gradient that is created is important for water absorption.
Water moves through spaces between cells and carries nutrients in solution—solvent drag or bulk transport.
Fructose transporters facilitate diffusion; a concentration gradient must be maintained—fructose is converted to glucose in the cells, so fructose concentration is always low.
Symporters combine transport of nutrient molecules with as it diffuses down its concentration gradient.
Products of fat digestion are lipid-soluble and pass through the microvilli membranes.
In the cells, fats are re-formed into chylomicrons that pass into lacteals, vessels of the lymph system, before entering the blood stream.
Blood leaving the digestive tract goes to the liver via the hepatic portal vein.
Liver cells absorb nutrients and store them or convert them for use.
Glycogen is synthesized from sugars, proteins are made from amino acids, and lipids can be used to make lipoproteins.
Large Intestine (Colon):
The large intestine, or colon, absorbs water and ions and produces feces.
Too much water absorption in the colon leads to constipation; too little leads to diarrhea.
In the colon, segmentation movements are due to contractions of circular muscles that repeatedly divide the colon into separate segments.
This causes the chyme to slosh around in the colon and promotes absorption of ions and water by increasing contact with the epithelium.
Herbivores:
Herbivores cannot produce cellulases to break down cellulose.
They rely on the microbiota in their digestive tracts.
Ruminants have four-chambered stomachs:
The rumen and reticulum contain microorganisms that break down cellulose by fermentation.
Food then travels to the omasum where it is concentrated by water absorption.
The abomasum is the true stomach. Microorganisms are also digested by the host, providing protein.
Some herbivores have a cecum—a fermentation chamber off the large intestine.
Two types of feces are produced: one is pure waste and one contains cecal material with nutrients.
Coprophagy: Reingestion of cecal feces.
In humans, the cecum is the appendix, which has no function.
Key Concept 50.3 Learning Outcomes
Compare anatomical differences based on feeding behavior and diet.
Explain the mechanisms that move food through the vertebrate digestive system.
Describe the role of acid and how it is produced in the stomach.
Discuss autocatalysis and its role in digestion.
Explain how bile assists in digestion.
Explain observations about the presence of microorganisms found in the digestive tracts of herbivores.
Compare the roles of microbiota in the digestive systems of different vertebrates.
Key Concept 50.4 Focus Your Learning
Hormones control actions of the digestive tract and its accessory organs.
The liver interconverts and stores fuel molecules.
The pancreas secretes hormones that help control glucose levels in the blood.
Body mass and food intake behavior is influenced by stimulatory and inhibitory hormones that activate neurons in the acruate nucleus region of the hypothalamus in the brain.
50.4 Nutrient Availability Is Controlled and Regulated
Digestion is governed by neuronal and hormonal controls.
Many autonomic reflexes coordinate activity in different regions of the digestive tract.
The enteric nervous system coordinates movement of food; it also exchanges information with the CNS.
Digestive Hormones:
Secretin from the duodenum causes pancreas to secrete bicarbonate ions.
Cholecystokinin causes gallbladder to release bile, stimulates pancreas, slows stomach action.
Gastrin stimulates stomach movements and secretion of digestive juices.
Animals do not eat continuously; they exist in one of two states:
Absorptive state—after a meal when food is in the gut and nutrients are absorbed.
Postabsorptive state—stomach and small intestine are empty and metabolism runs on internal reserves.
The liver can interconvert fuel molecules.
Monosaccharides can be converted into glycogen or fats during the absorptive state; this can be reversed in the postabsorptive state.
Gluconeogenesis is the conversion of amino acids and other molecules into glucose.
Exercising muscles produce pyruvate and lactate that can enter the blood and get taken up by the liver and converted to glucose.
The liver controls fat metabolism by lipoprotein production.
Lipoproteins are the most abundant fuel reserve in the body; they also move fats in the blood.
Chylomicrons are lipoproteins produced in the small intestine.
Lipoproteins produced in the liver:
High-density lipoproteins (HDLs) remove cholesterol from tissue and carry it to the liver.
Low-density lipoproteins (LDLs) transport cholesterol around the body.
Very-low-density lipoproteins (VLDLs) transport triglycerides to fat cells.
Insulin is released by the pancreas (from beta cells) during the absorptive period, when blood glucose rises.
Insulin promotes uptake and utilization of glucose for metabolic activities or synthesis of glycogen and fat from excess glucose.
Glucagon is produced by alpha cells in the pancreas and has the opposite effect: it stimulates liver cells to break down glycogen and carry out gluconeogenesis.
Glucose enters cells by facilitated diffusion, thus a concentration gradient is required and blood glucose levels must be regulated.
Glucose transporters are sequestered in cytoplasmic vesicles until insulin binds to receptors on the cell surface and triggers insertion of transporters into the cell membrane.
Insulin also controls how cells use the glucose they take up.
In adipose cells, insulin inhibits lipase and promotes fat synthesis from glucose.
In liver cells, insulin activates glucokinase, which phosphorylates glucose as it enters, so it cannot diffuse back out again.
Insulin inhibits glucose phosphatase (which if active would enable glucose to leave the cell), activates glycogen synthase, and activates enzymes that increase the flow of glucose into glycolysis.
In the postabsorptive state, insulin levels fall.
To maintain blood glucose levels, liver cells break down stored glycogen, releasing glucose into the blood.
Lipase activity increases in liver and adipose tissue, releasing fatty acids.
Most cells use fatty acids for fuel in the postabsorptive state.
Cells of the nervous system do not switch fuels and require a constant supply of glucose.
Most neurons do not require insulin, but a concentration gradient is needed to drive facilitated diffusion of glucose.
If blood glucose level is very low, glucagon is released and stimulates the liver to break down glycogen and begin gluconeogenesis.
Under conditions that stimulate glucagon release, the effects of low insulin are already in play.
The hypothalamus provides signals of hunger or satiety and governs how much food is eaten.
The arcuate nucleus integrates a variety of feedback signals that influence food intake and metabolism.
Three proteins reflect the body’s energy balance: insulin, leptin, and ghrelin.
Leptin is released by fat cells in proportion to how much lipid they contain. It acts as a satiety signal.
Insulin and leptin activate neurons in the arcuate nucleus that inhibit feeding.
Ghrelin is released by the stomach when it is empty and has the opposite effect on the arcuate nucleus.
Another signal in feeding regulation is the enzyme AMP-activated protein kinase (AMPK).
AMPK is produced by starved cells and stimulates oxidation of substrates to replenish ATP.
Insulin and leptin decrease AMPK activity in the hypothalamus, and ghrelin increases it.
Key Concept 50.4 Learning Outcomes
Explain how hormones control the rate of delivery of substrate to the small intestine and the appropriate secretions of digestive juices.
Describe metabolic interconversions that take place in liver and muscle to meet the body’s demands for glucose.
Differentiate between the classes of lipoproteins.
Compare the effects of insulin on activities in skeletal muscle cells, adipose cells, and liver cells.
Cite evidence that supports the role of leptin as a satiety signal and ghrelin as a hunger signal.
Interpret results from experiments designed to elucidate the mechanisms underlying regulation of body mass and food intake behaviors.
Investigating Life: Thrifty Phenotypes
Studies have shown that there are genetic contributions to the propensity for obesity, but generally, obesity is a multigenic trait that is highly influenced by the environment, including physical activity and diet.
If the nutritional values were the same, could different foods have different impacts on health?
Diet can influence the composition of the gut microbiome, which can influence nutritional and other health effects of the diet.
When the Pima switched from traditional to a high-fat Western diet, it could have resulted in changes in their microbiomes that have contributed to obesity and diabetes. Transcribed and Formatted by: [Your Name]
50.1 Food Provides Energy As Well As Materials for Biosynthesis
50.2 Diverse Adaptations Support Ingestion and Digestion of Food
50.3 The Vertebrate Gastrointestinal System Is a Disassembly Line
50.4 Nutrient Availability Is Controlled and Regulated
Investigating Life: Thrifty Phenotypes
Pima population: food deprivation favored "thrifty genes". Western diet/lifestyle $\rightarrow$ obesity, diabetes.
Key Concept 50.1 Learning Objectives
Food: fats, carbs, proteins = energy. Stored as glycogen & fat. Need essential amino acids/fatty acids. Vitamins & minerals required.
50.1 Food Provides Energy As Well As Materials for Biosynthesis
Heterotrophs eat others. Autotrophs make own food. Energy = work capacity.
Measures of heat energy:
Calorie: raises 1g water 1°C. Kilocalorie (Cal) = 1,000 calories.
Basal Energy Expenditure:
BMR: 1300-1800 Cal/day. Activity increases needs. Energy from fats, carbs, proteins. Track energy budgets. Store food between meals.
Carbohydrates Storage:
Glycogen in liver/muscle (1 day's energy).
Fat Storage:
More energy/gram, compact.
Protein Usage:
Not stored, last resort.
Metabolism When Taking in too Little Food:
Break down glycogen, fat, then protein $\rightarrow$ edema (kwashiorkor).
Metabolism When Taking in too Much Food:
Store excess as fat. Need C skeletons from food (acetyl groups). Essential amino acids (8 in adults) from diet. Complementary plant proteins work.
Ingested Proteins Breakdown:
To amino acids. Immune system attacks intact proteins. Need essential fatty acids (linoleic acid).
Macronutrients
Large amounts (calcium).
Micronutrients
Tiny amounts (iron, $\rightarrow$ anemia). Vitamins (coenzymes) can't be synthesized. 13 needed.
Vitamin Types:
Water-soluble: excess in urine (Vit C). Fat-soluble: toxic build-up (Vit D from UV).
Mineral Deficiencies:
Iodine $\rightarrow$ goiter. Iron $\rightarrow$ anemia.
Key Concept 50.1 Learning Outcomes
Describe storage, amino acid sources, minerals, vitamins.
Key Concept 50.2 Focus Your Learning
Adaptations for food. Digestion in cavity w/enzymes. Symbiotic bacteria assist.
50.2 Diverse Adaptations Support Ingestion and Digestion of Food
Nutrition:
Saprobes: dead matter
Detritivores: feed on dead
Predators: live organisms
Herbivores: plants
Carnivores: animals
Omnivores: plants & animals
Filter feeders: filter water
Fluid feeders: mosquitoes
Herbivores: low energy, hard to digest. Carnivores: adaptations to kill. Digestion starts with teeth.
Vertebrate Teeth:
Enamel, dentine, pulp. Adapted to diets.
In general:
Incisors: cut. Canines: stab. Molars: grind. Gut types: gastrovascular (1 opening) or tubular (2 openings). Birds: gizzards. Stomachs/crops store food. Intestines do most digestion.
Food Processing in Intestines:
Absorb nutrients in intestine. Increase surface area: typhlosole, spiral valve, villi, microvilli. Enzymes: proteases, carbohydrases, peptidases, lipases, nucleases.
Microbiota help digestion. Produce vitamin K, biotin. Influenced by diet.
Investigating Life: How Does the Gut Microbiome Contribute to Obesity and Metabolic Disease?
Saturated fats unhealthy? Hypothesis: affects microbiome.
Method:
Mice w/sat. vs unsat. fats. Track weight weekly. Compare microbiome rRNA and weight.
Conclusions:
Diet fat affects microbiome & weight gain.
Key Concept 50.2 Learning Outcomes
Compare adaptations, gut structures, enzymes, microbiome importance.
Key Concept 50.3 Focus Your Learning
Momentum: CNS reflexes & enteric NS. Herbivores: microbiota digest cellulose.
50.3 The Vertebrate Gastrointestinal System Is a Disassembly Line
Gut layers: lumen, mucosa, submucosa, muscle layers, enteric NS, peritoneum. Esophagus moves food via peristalsis. Sphincters control passage.
Chemical Digestion:
Salivary amylase digests carbs. Stomach: mucus cells, chief cells (pepsinogen $\rightarrow$ pepsin), parietal cells (HCl). Chyme produced. Small intestine digests most nutrients.
Small Intestine:
Duodenum, jejunum, ileum.
Liver:
Bile (from cholesterol) emulsifies fats.
Pancreas:
Exocrine: enzymes (zymogens activated to trypsin). neutralizes chyme. Lactase digests lactose.
Large Intestine (Colon):
Absorbs water/ions. Segmentation mixes chyme.
Herbivores:
Microbiota digest cellulose. Ruminants: 4-chamber stomach. Cecum.
Key Concept 50.3 Learning Outcomes
Compare anatomy, movement, acid/bile roles, autocatalysis, microbiota.
Key Concept 50.4 Focus Your Learning
Hormones control digestion, liver interconverts fuels, pancreas regulates glucose, hypothalamus affects food intake.
50.4 Nutrient Availability Is Controlled and Regulated
Neuronal/hormonal control. Enteric NS & CNS coordinate.
Digestive Hormones:
Secretin, CCK, gastrin. States: absorptive (after meal), postabsorptive (empty).
Liver interconverts fuels. Gluconeogenesis: make glucose from other sources. Muscles: pyruvate $\rightarrow$ glucose. Lipoproteins transport fats. Chylomicrons from intestine. HDLs remove cholesterol. LDLs transport cholesterol. VLDLs transport triglycerides.
Insulin ($\beta$ cells): glucose uptake. Glucagon ($\alpha$ cells): glycogen breakdown & gluconeogenesis. Glucose entry needs transporters. Insulin activates/inhibits enzymes to control glucose use. Postabsorptive: insulin $\downarrow$, use fats for fuel. Nerves need constant glucose. Hypothalamus: hunger/satiety. Arcuate nucleus integrates signals. Leptin (fat cells): satiety. Ghrelin (stomach): hunger. AMPK: energy balance.
During starvation, the body uses stored biological molecules in the following order: glycogen, fat, and then protein. The metabolism of blood proteins leads to edema, a sign of kwashiorkor.
Essential amino acids: Amino acids that cannot be synthesized by the animal and must be obtained from their diet. Adult humans need 8 essential amino acids.
Macronutrients: Elements (such as calcium) required in large amounts.
Micronutrients: Elements (such as iron) required in tiny amounts; iron deficiency leads to anemia.
Vitamins: Carbon compounds required for growth and metabolism that cannot be synthesized by the animal; function as coenzymes
Nutritional classifications of heterotrophic organisms:
Saprobes: Absorb nutrients from dead organic matter.
Detritivores (Decomposers): Feed on dead organic matter
Predators: Feed on living organisms.
Herbivores: Prey on plants.
Carnivores: Prey on animals.
Omnivores: Prey on plants and animals.
Filter feeders: Filter small organisms from an aquatic environment.
Fluid feeders: Include mosquitoes, leeches, aphids, and hummingbirds.
Mammalian teeth are adapted to different diets:
Incisors: Used for cutting, chopping, or gnawing.
Canines: Used for stabbing, gripping, or ripping.
Molars and premolars: Used for shearing, crushing, or grinding.
Examples of how different animal phyla improve the surface area of their digestive tracts:
Earthworm: gut has an infolding of the gut wall, or typhlosole.
Sharks: Have a spiral valve; walls of the spiral have a large surface area.
Humans: Gut wall has folds with finger-like projections called villi. Surface cells of villi have smaller projections called microvilli.
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Principle activities of the stomach:
Mucus-secreting cells: Mucus protects tissues from the acids and enzymes.
Chief cells: Secrete pepsinogen, the inactive form of the protease pepsin. Low pH of the stomach denatures proteins and converts pepsinogen to the active form, pepsin.
Parietal cells: secrete HCl; keeps stomach pH below 1. Carbonic anhydrase catalyzes formation of from , which dissociates to and . is exchanged for in gastric pit lumens, and leaks back into the cells.
The activities of these cells complement each other. Mucus protects the stomach lining from acid (secreted by parietal cells) and pepsin. Pepsin (activated by HCl) breaks down proteins.Pepsin: A protease that breaks down proteins in the stomach. It is produced from pepsinogen in the presence of low pH (HCl).
Pepsinogen: The inactive precursor of pepsin secreted by chief cells. It is converted to pepsin in the presence of low pH (HCl) or by autocatalysis.
The three sections of the small intestine are:
Duodenum
Jejunum
Ileum
The principal function of the small intestine is that most chemical digestion occurs there, and nutrients water, and ions are absorbed across its walls.
Bile: Synthesized by the liver from cholesterol and secreted. Emulsifies fats in the chyme, increasing the surface area exposed to lipases that digest fat.
The functions of trypsin:
Produced by pancreas as Trypsinogen then activated by enterokinase to produce the active protease trypsin, which can activate other zymogens.
The pancreas also secretes to neutralize chyme in the intestine.
The functions of chymotrypsin produced by the pancreas are not explicitly provided in the text. Chymotrypin is an inactive zymogen. Which is then cleaved to active chymotrypsin by trypsin.
Ruminants have four-chambered stomachs:
Rumen and reticulum: Contain microorganisms that break down cellulose by fermentation.
Omasum: Food then travels here, where it is concentrated by water absorption.
Abomasum: The true stomach where microorganisms are also digested by the host.
Insulin-Glucagon System: Regulates blood glucose levels.
Insulin: Released by the pancreas (from beta cells) during the absorptive period, when blood glucose rises. Insulin promotes the uptake and utilization of glucose for metabolic activities or synthesis of glycogen and fat from excess glucose.
Glucagon: Produced by alpha cells in the pancreas and has the opposite effect; it stimulates liver cells to break down glycogen and carry out gluconeogenesis.
Absorptive state: After a meal, food is in the gut and nutrients are absorbed.
Postabsorptive state: The stomach and small intestine are empty, and metabolism runs on internal reserves.
Hormone Functions:
- Leptin: Released by fat cells in proportion to how much lipid they contain; acts as a satiety signal.
- Ghrelin: Released by the stomach when it is empty and has the opposite effect on the arcuate nucleus (hunger signal).
- Insulin: Activates neurons in the arcuate nucleus that inhibit feeding during the absorptive state.