Chapter 50
Nutrition, Digestion, and Absorption
Nutrition, Digestion, and Absorption
Nutrient Requirements
Adaptations for Feeding
Digestion
Structure and Function of the Vertebrate Gut
Control and Regulation of Digestion
The Regulation of Food Intake
Toxic Compounds in Food
Nutrient Requirements
Animals are heterotrophs: They must obtain their nutrition by eating other organisms.
Most plants, some bacteria, and some protists are autotrophs: They trap solar energy through photosynthesis and use that energy to synthesize all of their components.
__________ depend on the organic synthesis carried out by autotrophs.
Nutrient Requirements
Energy from the chemical bonds of food molecules is transferred to the high-energy phosphate bonds of ATP.
When energy from food molecules is converted to ATP much of the energy is lost as heat.
Eventually, all energy transferred to ATP from the chemical bonds of food molecules is released as heat.
Thus, the energy requirements of animals and the energy content of food are described in terms of a measure of heat energy called a calorie.
Nutrient Requirements
A calorie is the amount of __________ needed to raise the temperature of 1 g of water 1°C.
This value is very small in terms of the energy requirements of many animals, so physiologists commonly use the kilocalorie.
Nutritionists
use the kilocalorie as well, but they refer to it as the Calorie (
Scientists are abandoning the calorie in favor of the International System of Units measure of energy called the joule (1 joule = 0.239 calories).
Nutrient Requirements
An animals metabolic rate is the measure of the overall energy needs that must be met by the animals food.
Fats, carbohydrates, and proteins are the components of food that provide that energy.
Basal metabolic rate is the metabolic rate resulting from all of the essential physiological functions that take place in a resting state.
Physical activity adds to the basal energy requirement.
Nutrient Requirements
By comparing calories consumed to calories expended it is possible to construct an energy budget for any set of circumstances an organism may encounter.
Energy budgets can be used to determine a costbenefit analysis for any behavior.
Nutrient Requirements
A study of African sunbirds included a costbenefit analysis of territorial behavior to defend a food source.
The birds could afford to defend a territory only if the food source was rich enough to support the metabolic cost of aggressive behavior.
Nutrient Requirements
Most animals __________ fuel molecules that can be released as needed between meals.
Carbohydrates are stored in liver and muscle cells as __________. The total glycogen store is about one days energy requirements.
The most important form of stored energy in animals is fat. Fat has more energy per gram than glycogen, and it is stored with little associated water, making it more compact.
Protein is not used to store energy, but it can be metabolized as a last resort.
Nutrient Requirements
Animals that do not take in enough food to meet their energy requirements are undernourished.
They must metabolize molecules of their own body to provide the energy they need.
The first storage compounds to be metabolized in this state are glycogen and fat, so that protein loss is minimized for as long as possible.
Eventually, a starving animal must use its own proteins for fuel; the syndrome that results is called kwashiorkor.
This impairs body functions and eventually leads to death.
Nutrient Requirements
Animals that take in more food than is necessary to meet their energy demands are overnourished.
Excess nutrients are __________ as increased body __________. [note, ______ not weight. Do fat people want to loose weight?]
Glycogen reserves are built up, and then additional carbohydrates, proteins, and fats are converted to body fat.
Some species use seasonal overnourishment to survive periods when food is unavailable.
In humans, however, __________ can be a serious health hazard, increasing the risk of high blood pressure, heart attack, diabetes, and other disorders.
Nutrient Requirements
All animals require certain basic organic molecules, called carbon skeletons, that they cannot synthesize themselves.
An example is the acetyl group.
Some carbon skeletons are derived from limited sources, and animals can suffer a deficiency of these materials even if their caloric intake is adequate.
Amino acids are an example of carbon skeletons that can be in short supply.
Nutrient Requirements
Animals can synthesize some amino acids.
Others must be acquired through food and are called essential amino acids; they differ from species to species.
There are eight essential amino acids for humans: isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine.
Most plant foods do not contain all eight amino acids, so vegetarians must eat a complementary mixture of foods (e.g., grains plus legumes) to obtain all eight.
Nutrient Requirements
Proteins are digested in the gut into amino acids before being used by the body:
Macromolecules are not readily absorbed by the cells of the gut, but the smaller monomers are.
A protein that functions well in one species might not function well in another.
Foreign proteins entering the body directly from the gut would be attacked by the __________ system.
Nutrient Requirements
Humans are able to synthesize almost all of their required lipids using acetyl groups obtained from food.
Essential fatty acids such as linoleic acid must be obtained through a dietary source.
Essential fatty acids are needed to make other fatty acids, components of signaling molecules, and membrane phospholipids.
Nutrient Requirements
Elements required in large amounts are called macronutrients; elements required in only tiny amounts are called micronutrients.
Calcium is an example of a macronutrient. A 70-kg person contains about 1.2 kg of calcium.
Humans require 8001,000 mg of calcium per day in their diet.
Iron is an example of a __________.
Although humans require only about 15 mg of iron per day in their food, insufficient iron is the most common nutrient deficiency in the world today.
Nutrient Requirements
The vitamins are another group of essential nutrients.
Vitamins are carbon compounds that animals require for normal growth and metabolism, but cannot synthesize for themselves.
Most vitamins function as coenzymes.
Scurvy is a condition caused by a vitamin C deficiency in primates. Other mammals can make their own.
Nutrient Requirements
The list of necessary vitamins varies from species to species. Humans require 13 vitamins.
Water-soluble vitamins (e.g., vitamin C), when ingested in excess, are __________ the the urine.
Fat-soluble vitamins (A, D, E, and K) can accumulate in body fat and may build up to toxic __________ if taken in excess.
Vitamin D (calciferol) is a special case. It can be __________ in the presence of ultraviolet light on the skin, but it must be acquired through diet in individuals with inadequate exposure to the sun.
The need for vitamin D may have been an important factor in the evolution of __________ __________.
Nutrient Requirements
A chronic shortage of any nutrient produces a state of deficiency called malnutrition.
Chronic malnutrition leads to a characteristic deficiency disease.
Deficiency diseases can also result from an inability to absorb or process an essential nutrient.
For example, pernicious anemia (the failure of red blood cells to mature), is caused by B12 deficiency, usually brought on by the persons inability to absorb the vitamin.
Adaptations for Feeding
Heterotrophic organisms can be classified by how they acquire their nutrition.
Saprobes, mostly protists and fungi, absorb nutrients from dead organic matter.
Detritivores, such as earthworms and crabs, actively feed on dead organic material.
Predators are animals that feed on living organisms:
Herbivores prey on plants.
Carnivores prey an animals.
Omnivores prey on both.
Adaptations for Feeding
Filter feeders, such as clams and blue whales, prey on small organisms by filtering them from the aquatic environment.
Fluid feeders obtain food from the fluids produced by various organisms, including blood and nectar.
Adaptations for Feeding
Because the energy content of vegetation is low, herbivores must process large amounts of it; thus many herbivores spend a great deal of their time feeding.
The trunk of the elephant and the long neck of the giraffe are examples of the striking adaptations for feeding found in many herbivores.
Many types of mouthparts have evolved in invertebrates for ingesting plant material.
The teeth of herbivorous vertebrates have evolved to process coarse plant matter.
Adaptations for Feeding
Carnivores evolved stealth, speed, power, large jaws, sharp teeth, and strong gripping appendages as means of capturing and killing prey.
Carnivores have also evolved remarkable means of detecting prey. Pit vipers, for example, sense infrared radiation from the warm bodies of their prey.
The adaptations for killing and ingesting prey are diverse and highly specialized.
Adaptations for Feeding
Teeth are adapted for the acquisition and initial processing of specific types of foods.
All mammalian teeth have the same basic structure, but many adaptations for specific diets have evolved.
Mammalian teeth are composed of a hard enamel of calcium phosphate, a layer of bony material called dentine, and a pulp cavity containing blood vessels, nerves, and cells that produce the dentine.
Digestion
Most animals digest their food __________.
Food is taken into a body cavity that is continuous with the outside environment, where it is acted on by enzymes secreted by the animals.
These enzymes break the food down into nutrient molecules that are absorbed by the cells lining the cavity.
Gastrovascular cavities, such as those found in the cnidarians, are the simplest digestive systems.
Digestion
Most animals have a tubular gut with a mouth that takes in food and an anus for waste excretion.
The tubular gut has different regions that are specialized for different functions.
All regions within the tubular gut are outside of the body of the animal; only by crossing the plasma membranes of cells lining the gut do nutrients enter the body.
Digestion
The anterior end of the gut consists of the mouth and the buccal cavity (mouth cavity).
Food is usually broken up here into smaller fragments by structures such as teeth or a radula.
Stomachs and crops are storage chambers that enable animals to ingest large amounts of food and digest it at leisure.
The next section of the gut is called the midgut, or intestine. Most materials are digested and absorbed here.
Specialized glands secrete some digestive enzymes into the intestine, and cells in the gut wall itself secrete other digestive enzymes.
Digestion
The hindgut recovers water and ions and stores undigested wastes (feces).
A muscular rectum near the anus assists in the expulsion of feces (defecation).
Many species have colonies of endosymbiotic bacteria within their hindguts.
These bacteria obtain nutrients from the food passing through the hosts gut and contribute to the digestive processes of the host.
Digestion
The parts of the gut that absorb nutrients have evolved large __________ areas to maximize nutrient absorption.
Vertebrates have a gut wall that is richly folded, with individual folds bearing fingerlike projections called villi, which in turn have projections called microvilli.
Together, villi and microvilli present an enormous surface area for the absorption of nutrients.
Digestion
Hydrolytic enzymes break down protein, carbohydrate, and fat macromolecules into their simplest monomeric units.
Digestive enzymes are classified according to the substances they hydrolyze:
Carbohydrases hydrolyze carbohydrates.
Proteases hydrolyze proteins.
Lipases hydrolyze fats.
Nucleases hydrolyze nucleic acids.
Digestion
The prefixes exo- and endo- are used to indicate where the enzyme cleaves the moleculeat the ends of the molecule or at internal sites, respectively.
Most enzymes are produced in an inactive form known as a zymogen, so that they cannot digest the cells that excrete them.
Zymogens are usually activated by another enzyme or conditions within the environment in which they are secreted.
Structure and Function of the Vertebrate Gut
The digestive tract of vertebrates is a tubular gut that runs from mouth to anus.
Compartments of the gut are specialized to perform different functions.
Structure and Function of the Vertebrate Gut
The vertebrate gut has four layers of tissue throughout its length.
The cavity of the gut is called the lumen.
Starting from the lumen, the first layer of tissue is the mucosa.
Cells of the mucosa have secretory and absorptive functions.
Just outside the mucosa is the second layer of cells, the submucosa, which contains blood and lymph vessels that carry absorbed nutrients to the rest of the body.
Structure and Function of the Vertebrate Gut
There are two layers of smooth muscle cells external to the submucosa.
The innermost layer is the circular muscle layer and has cells that are oriented around the gut.
The longitudinal muscle layer is the outermost layer and has cells that are oriented along the length of the gut.
A fibrous coat called the serosa surrounds the gut.
All abdominal organs are covered and supported by a tissue called the peritoneum.
Structure and Function of the Vertebrate Gut
When food enters the mouth it is chewed and mixed with the secretions of salivary glands.
When the bolus of food makes contact with the back of the mouth, the reflexive action of swallowing is initiated.
Swallowing involves muscles propelling food through the pharynx (where the mouth cavity and the nasal passages join) and into the esophagus (the food tube).
Structure and Function of the Vertebrate Gut
Food in the esophagus is pushed toward the stomach by a wave of smooth muscle contraction, called peristalsis.
Peristalsis causes the lower esophageal sphincter, which normally is constricted to prevent movement of food from the stomach to the esophagus, to relax.
The pyloric sphincter governs the passage of stomach contents into the intestine.
Structure and Function of the Vertebrate Gut
Carbohydrate digestion begins in the mouth, where amylase is secreted with saliva and mixed with the food as it is chewed.
Secretions of the stomach kill microorganisms that are taken in with food and begin the digestion of proteins.
An endopeptidase called pepsin is the major enzyme produced by the stomach.
Pepsin is secreted by cells in the gastric glands as a zymogen called pepsinogen.
Structure and Function of the Vertebrate Gut
Hydrochloric acid (HCl) maintains a pH of 1 3 in the stomach fluid, which activates the conversion of pepsinogen to pepsin.
Newly formed pepsin activates other pepsinogen molecules in a process called autocatalysis.
Mucus secreted by the stomach mucosa coats the walls of the stomach and protects them from being eroded and digested by HCl and pepsin.
When walls of the stomach are exposed directly to HCl and pepsin, an ulcer can result.
Structure and Function of the Vertebrate Gut
The muscles in the walls of the stomach contract to churn its contents and mix them with the stomach secretions.
This mixture of gastric juice and partly digested food is called chyme.
Peristaltic contractions of the stomach push the chyme toward the bottom end of the stomach and into the beginning of the intestine through the pyloric sphincter.
Structure and Function of the Vertebrate Gut
In the small intestine, the digestion of carbohydrates and proteins continues, and the digestion of fats and the absorption of nutrients begins.
The small intestine of an adult human is more than 6 m long and has an inner surface area of about 550 __________.
The small intestine has three sections: The duodenum is the initial section and is the site of most digestion.
The jejunum and the ileum carry out 90 percent of the absorption of nutrients.
Structure and Function of the Vertebrate Gut
The liver and the pancreas provide many of the specialized enzymes required for digestion.
The liver produces bile, which aids in fat digestion.
Bile is secreted from the liver and flows through a branch of the hepatic duct to the gallbladder, where it stored until it is needed.
When fat enters the duodenum, bile is squeezed into the common bile duct, where it flows into the duodenum.
Structure and Function of the Vertebrate Gut
Bile is an emulsifier a substance that prevents oil droplets from aggregating.
Bile emulsifies fats in the chyme and greatly increases the surface area of the fats that are exposed to lipases.
Bile molecules have lipophilic end and a lipophobic end.
The small fat particles surrounded by bile molecules are called micelles.
Structure and Function of the Vertebrate Gut
The pancreas is a large gland that lies just beneath the stomach and functions as both an endocrine and exocrine gland.
The exocrine tissues of the pancreas produce a number of digestive enzymes, released as zymogens.
Trypsinogen is activated in the duodenum by enterokinase, which is produced in the cells lining the duodenum.
Active trypsin can activate other trypsinogen molecules by autocatalysis.
Structure and Function of the Vertebrate Gut
The pancreas also produces a secretion rich in bicarbonate ions, which neutralize the pH of the chyme from the stomach.
This process is essential because intestinal enzymes function best at a neutral or slightly alkaline pH.
Structure and Function of the Vertebrate Gut
Only the smallest products of digestion can be absorbed through the mucosa of the small intestine.
The final digestion that produces these absorbable products takes place among the microvilli.
Mucosal cells within the microvilli produce peptidases that cleave peptides into tri- and dipeptides and individual amino acids.
They also produce maltase, lactase, and sucrase, which cleave common disaccharides into monosaccharides.
Structure and Function of the Vertebrate Gut
The mechanisms by which cells lining the intestine absorb nutrient molecules are diverse and not completely understood.
Carrier proteins actively transport many inorganic ions into cells.
Carrier proteins also transport amino acids, glucose, and galactose, in conjunction with diffusion of sodium ions. A symport in the mucosal cell plasma membrane binds the Na+ and the nutrient molecule. The process is called sodium cotransport.
Structure and Function of the Vertebrate Gut
The process of fat absorption does not involve carrier proteins.
Lipases break fats down into diglycerides, monoglycerides, and fatty acids, which are able pass through the plasma membrane of microvilli.
Once in the cells they are resynthesized into water-soluble packets called chylomicrons. These pass to the lymph vessels in the submucosa.
They enter the blood through the thoracic ducts.
Structure and Function of the Vertebrate Gut
Bile does not get absorbed into the cells, but shuttles back and forth between gut contents and microvilli.
Bile is synthesized from cholesterol.
One major way that cholesterol leaves the body is through unresorbed bile in the feces.
Some kinds of fiber will bind with bile, decreasing its absortion by the ileum and helping to lower cholesterol levels.
Structure and Function of the Vertebrate Gut
Peristalsis pushes the contents of the small intestine into the large intestine, or colon.
The colon absorbs water and ions, producing semisolid feces from indigestible material.
Too much water absorption results in __________ and too little water absorption results in __________.
Large populations of bacteria live in the colon, including Escherichia coli, which synthesizes vitamin K and biotin that are absorbed across the wall of the colon.
Prolonged intake of antibiotics can lead to vitamin deficiency because the antibiotics kill the normal intestinal bacteria.
Structure and Function of the Vertebrate Gut
Intestinal bacteria produce gases such as methane and hydrogen sulfide as by-products of anaerobic metabolism.
A large percentage of the material in feces consists of cell walls of dead bacteria.
Structure and Function of the Vertebrate Gut
Cellulose is the principle organic compound in the diets of herbivores.
Most herbivores cannot produce cellulases, the enzymes that hydrolyze cellulose.
They rely on microorganisms living within their digestive tracts to digest cellulose for them.
The cud chewers, or ruminants, have digestive tracts that are specialized to maximize the benefits of their endosymbiotic microorganisms.
Ruminants derive a significant amount of protein from digestion of their endosymbiotic microorganisms.
Structure and Function of the Vertebrate Gut
Ruminants have four specialized stomach compartments:
The contents of the rumen (the cud) are periodically regurgitated into the mouth for rechewing.
The rumen and reticulum have anaerobic microorganisms that break down cellulose by fermentation.
The omasum concentrates the food by water absorption.
The true stomach is the abomasum, which secretes hydrochloric acid and proteases.
Structure and Function of the Vertebrate Gut
Some other mammalian herbivores have microbial chambers in a branch off of the large intestine called the cecum.
The cecum empties directly into the large intestine, so the absorption of the nutrients is incomplete.
To deal with this problem, animals such as rabbits reingest some of their own feces in a process known as coprophagy.
These animals typically produce two types of feces, one that consists mostly of cecal material that they reingest, and one consisting of pure waste.
In humans, the vestigial cecum is the appendix.
Control and Regulation of Digestion
The digestive tract has an intrinsic nervous system that allows neural messages to travel from one region of the digestive tract to another without being processed by the central nervous system.
Control and Regulation of Digestion
Hormones control many digestive functions.
The first hormone ever discovered came from the duodenum and was called secretin. It stimulates the pancreas to secrete a solution rich in bicarbonate ions.
The hormone cholecystokinin is secreted by the small intestine mucosa in response to the presence of fats and proteins in the chyme.
This hormone stimulates the gallbladder to release bile and the pancreas to secrete digestive enzymes.
Control and Regulation of Digestion
When food is present, cells in the lower region of the stomach secrete the hormone gastrin into the blood.
Gastrin stimulates the secretions and movements of the stomach.
Gastrin release is inhibited when the stomach contents become too acidic (negative feedback).
Control and Regulation of Fuel Metabolism
After an animal eats, food is present in the gut and nutrients are being absorbed for a period of time after the meal, called the absorptive period.
In the postabsorptive period, the animal must gain the energy and materials it requires for metabolism and biosynthesis from internal reserves.
Control and Regulation of Fuel Metabolism
The liver __________ fuel molecules.
If glucose is abundant, the liver converts it to glycogen and fats.
When glucose in blood decreases, the liver delivers glucose back to the blood.
The liver can convert certain amino acids and some other molecules into glucose, a process called gluconeogenesis.
The liver also controls fat metabolism through production of lipoproteins.
Control and Regulation of Fuel Metabolism
Lipoproteins transport fats in the aqueous circulatory system.
They consist of a core of fat and cholesterol covered by a protein that makes them water-soluble. The largest are the chylomicrons.
Lipoproteins are classified according to their densitythe more fat, the lower its density.
Control and Regulation of Fuel Metabolism
High-density lipoproteins (HDL) are about 25% cholesterol. They carry cholesterol to the liver where it is used to make bile.
Low-density lipoproteins (LDL) are 5060% cholesterol. They transport cholesterol to tissues around the body for biosysthesis and storage.
Very-low-density lipoproteins (VLDL) contain mostly triglyceride fats, which are transported to fat cells.
A high ratio of LDL to HDL is a risk factor for atherosclerotic heart disease.
Control and Regulation of Fuel Metabolism
During the absorptive period, blood glucose levels are high and the pancreas releases insulin.
Insulin stimulates uptake of glucose and its conversion to glycogen by liver and muscle cells.
It also stimulates fat cells to take up glucose and convert it to fat and stimulates all cells to preferentially use glucose for fuel.
Another pancreatic hormone, glucagon, has the opposite effect; it stimulates the liver to produce glucose by gluconeogenesis.
Control and Regulation of Fuel Metabolism
During the postabsorptive period, the processes are reversed.
The liver breaks down glycogen to supply glucose to the blood. Liver and adipose cells also supply fatty acids to the blood and most cells switch to using fatty acids for fuel.
The central nervous system does not switch fuel sources during the postabsorptive period; it always relies on blood glucose.
Because other cells shift to fatty acids, glucose is reserved for use by neuronal tissue.
The Regulation of Food Intake
The amount of food that an animal eats is governed by sensations of __________ and __________, which are influenced by the hypothalamus.
When the medial hypothalamus of rats is damaged, they will increase their food intake and become obese.
If the lateral hypothalamus is damaged, they will decrease their food intake and become skinny.
Regulation of body weight involves feedback information, and there is evidence that levels of blood glucose and insulin, as well as signals from fat metabolism, influence hunger and satiety.
The Regulation of Food Intake
A mutation in the gene that codes for the protein leptin results in their eating enormous amounts of food and becoming obese.
When leptin was injected into these mice, they ate less and lost body fat.
There are leptin receptors in the regions of the hypothalamus that control hunger and satiety.
Interestingly, most obese humans have higher than normal levels of leptin. In this case, it is likely that they have receptors with reduced sensitivity.
The recently discovered hormone ghrelin is produced by the stomach and binds to receptors in the hypothalamus, stimulating appetite.
Toxic Compounds in Food
Many plant and animal tissues contain toxins, and humans add millions of tons of synthetic toxic compounds to the environment each year.
The physical and chemical properties of a toxic compound can affect its retention within a biological system.
Compounds that can dissolve in water may be quickly metabolized and __________.
Lipid-soluble compounds tend to be metabolized more slowly and are often stored in the body for a long time.
Toxic Compounds in Food
Many pesticides and other lipid-soluble toxins can become more and more concentrated in predators that eat contaminated prey.
This process is called bioaccumulation.
The toxin load is passed up the food chain from the prey to the predator, putting long-lived top predators such as eagles or bears at particular risk.
Toxic Compounds in Food
The detoxification systems responsible for metabolizing natural chemicals can often also metabolize synthetic chemicals.
Liver enzymes called cytochrome P450s are responsible for much of this detoxification.
P450s are less specific in their abilities to bind substrates than are most enzymes and can thus catalyze reactions with a wide range of compounds.
Some synthetic chemicals cannot be metabolized, and if these chemicals are lipid soluble they can __________.
Toxic Compounds in Food
Polychlorinated biphenyls, or PCBs, are a group of synthetic chemicals that appear to mimic hormones.
Response to PCBs in the diet varies among species.
In rhesus monkeys, PCBs have been shown to alter reproductive cycles, decrease weight gain in infants, depress the immune system, and increase the incidence of death among developing embryos.
Although the risks of PCBs and DDT are now clear, it is often difficult to make a causal connection between an environmental toxin and its specific health effects in a population.
Animation 50.1 The Digestion and Absorption of Fats
Animation 50.2 Insulin and Glucose Regulation
Video 50.1 Prey capture by a kingfisher
Video 50.2 Release of saliva
Video 50.3 Endoscopic view of the digestive system
Video 50.4 X-ray view of the digestive system