Chapter 21 – Nutrition and Digestion
21.1 Animals ingest their food in a variety of ways
All animals eat other organisms. Generally, there are three dietary categories:
 Herbivores (Latin “herba” green crop and “vorous” devouring) eat mainly autotrophs (plants, algae)
 Carnivores (Latin “carne” flesh) eat other animals
 Omnivores (Latin “omnis” all) eat both plants and animals
 Suspension feeders extract food floating in surrounding water.
o Humpback whales feed on krill, clams and oysters feed using the water around them.
o Tube worms use feather-like apparatus to trap food
 Substrate feeders live in or on their food source and eat their way through it.
 Caterpillars eat through the soft green parts of a leaf.
 Earthworms eat as they move through soil.
 Fluid feeders feed by sucking fluids from a host.
o Aphids tap into plants and take out sap.
o Mosquitoes and ticks use a needle-like mouthpiece to drink blood.
 Bulk feeders ingest relatively large pieces of food. They tear off pieces of food and bring it into the mouth
21.2 Overview: Food processing occurs in four stages
 Ingestion is the act of eating.
 Digestion is the breaking down of food into particles small enough for the body to absorb. It occurs in two
phase:
 Mechanical breakdown – food is manually broken down into small parts by chewing
 Hydrolysis – chemical breakdown of food. Digestive enzymes break chemical bonds in macromolecules.
Generally water is added to break down bonds. Food is made of polymers (carbohydrates, proteins,
lipids) that are too large to enter cells. These are broken into their component parts called monomers.
These monomers can then be converted by cells into compounds that are needed by the body
 Absorption – the cells lining the digestive tract absorb the nutrients from food. These nutrients are taken
from the digestive tract by blood cells and transported throughout the body. They are then joined
together to make the macromolecules needed by the body or they are broken down to make energy.
Those not used immediately are converted to fat for storage.
 Elimination is when undigested materials pass out of the digestive tract.
21.3 Digestion occurs in specialized compartments
 Food vacuoles are the simplest digestive compartment. A cell engulfs food by phagocytosis. Sponges
digest their food entirely by food vacuoles, but most animals have internal compartments that allow for
digestion to occur outside of cells, where they can ingest food much larger than that which could be
handled by phagocytosis alone
 Gastrovascular cavity – a digestive compartment with a single opening, the mouth
1. Gland cells lining the cavity secrete digestive enzymes
2. Enzymes break down the tissues of the prey
3. Other cells engulf small food particles
4. Particles are broken down in food vacuoles
5. Anything not digested is expelled from the mouth
 Alimentary canal – a digestive tube with two openings, the mouth and the anus. Food moves in one
direction, and there are specialized regions within the tube to carry out specific functions and absorb
nutrients in sequence
o Pharynx – throat
o Esophagus – attached to pharynx and leads into the next specialized compartment
o Crop – pouch-like organ that softens food and stores it (usually in birds or some insects)
o Stomach or gizzard – store food temporarily, but more muscular so that food can be ground and
churned.
o Intestine – place of chemical digestion and nutrient absorption
o Anus – for expelling unused materials
21.4 The human digestive system consists of an alimentary canal and accessory glands
 Main parts of the canal are the mouth, oral cavity, tongue, pharynx, esophagus, stomach, small intestine,
large intestine, rectum, and anus.
 Digestive glands include: salivary glands, pancreas, and liver.
o They secrete digestive juices that enter the canal from ducts.
o Liver secretions are stored in the gall bladder before release into the intestine.
 Muscles propel food through the canal by peristalsis, which are rhythmic waves of contraction of smooth
muscles. They pyloric sphincter is a ring-like valve that regulates passage of food out of the stomach and
into the small intestine. Nutrients are absorbed by the small intestine over a 5-6 hour period, and the
undigested parts enter the large intestine for a 12-24 hour period. Whatever remains is passed out of the
anus as feces.
21.5 Digestion begins in the oral cavity
Salivary glands deliver saliva through ducts in the oral cavity. Saliva contains substances important in food
processing:
 Glycoprotein protects the mouth and lubricates food for swallowing
 Buffers neutralize food acids and help prevent tooth decay
 Antibacterial agents destroy bacteria that may enter with the food
 Salivary amylase – a digestive enzyme that begins hydrolyzing the starch in food
Mechanical and chemical breakdown begin in the mouth. Chewing is designed to cut, smash, and grind food,
which makes it easier to swallow. Four kinds of teeth exist to help this process:
 Incisors are bladelike and used for biting
 Canine teeth are pointed and used for ripping
 Premolars and molars are used for grinding and crushing
Prominent in the oral cavity is the tongue. It is a muscular organ covered in taste buds. It manipulates food
and forms it into a ball called a bolus, which is pushed into the pharynx and swallowed.
21.6 After swallowing, peristalsis moves food through the esophagus to the stomach
 Openings for the trachea (wind pipe) and esophagus are in the pharynx. Most of the time, the esophagus
is closed by a sphincter so that air can enter the trachea and proceed to the lungs. When a bolus of food is
swallowed, the pharynx triggers the swallow reflex and the esophagus sphincter relaxes, allowing the
food to enter.
 The larynx (voice box) moves up and the epiglottis covers the tracheal opening. After swallowing,
everything moves back into place. The esophagus is a muscular tube that takes food boluses from the
mouth to the stomach. The muscles at the top are under voluntary control, making swallowing voluntary.
Movement after that is involuntary. Muscle contractions, peristalsis, propel food towards the stomach.
21.7 The Heimlich maneuver can save lives
The maneuver removes food that has been inadvertently sent into the trachea
21.8 The stomach stores food and breaks it down with acid and enzymes
 Our stomach is highly elastic and can stretch to take in up to 2L of food and drink. Chemical digestion
occurs in the stomach with the secretion of gastric juice. This juice is made up of mucus, enzymes, and
strong acid. The acid is designed to break apart the cells in food as well as kill any bacteria or microbe
swallowed with the food.
 Stomach interior surface is highly folded and covered with pits that lead down into tubular gastric glands.
Three types of cells are found in the glands, and each secretes one of the digestive juices.
o Mucous cells secrete the mucus that lubricates and protects the lining of the stomach.
o Parietal cells secrete hydrochloric acid
o Chief cells secrete pepsinogen, an inactive form of the enzyme pepsin
 Pepsinogen and HCl are secreted into the lumen (cavity) of the stomach. HCl converts pepsinogen into
pepsin, which activates more pepsinogen to start a chain reaction. Pepsin begins chemical digestion of
proteins by splitting the polypeptide chains into smaller polypeptides. This primes the protein for further
digestion in the small intestine. Gastric juices are not secreted constantly. Their activity is regulated by
nervous and hormonal signals released by entry of food into the stomach.
 Inactive pepsinogen and mucus help protect the stomach lining from being worn away by the action of
the pepsin and HCl . The mixing action of the enzymes and acid result in production of nutrient-rich broth
called chyme. Opening of the sphincter at the base of the stomach allows chyme to enter the small
intestine. Only small amounts of chyme are released at a time, so it can take the stomach 2-6 hours to
empty after a meal.
21.9 Digestive ailments include acid reflux and gastric ulcers
Generally, a coat of mucus protects the stomach from the digestive juices. When this fails, gastric ulcers can
form. In some cases, the spiral shaped bacterium Helicobacter pylori is thought to be a culprit of gastric ulcers.
The bacterium is acid-tolerant and has the ability to burrow below the mucus and release harmful chemicals
21.10 The small intestine is the major organ of chemical digestion and nutrient absorption
 Chemically, digestion of the macromolecules in our diet begins in the small intestine. Nutrients are also
absorbed by the blood in the small intestine. At 6m, it is the longest organ in the alimentary canal.
 Two glandular organs, the pancreas and liver contribute to digestion by the small intestine. The pancreas
produces pancreatic juice (digestive enzymes plus bicarbonate) that neutralizes chyme as it enters the
small intestine. The liver has many functions, one of which is the production of bile that has salts to
emulsify fats. The gallbladder stores the bile until it is needed
 The first 25cm of the small intestine is called the duodenum. It receives the chyme from the stomach and
mixes it with pancreatic juice, bile, and digestive enzymes. All macromolecules entering will be digested.
 Carbohydrate digestion began in the oral cavity. Pancreatic amylase hydrolyzes starch into maltose, a
disaccharide. The enzyme maltase splits that into glucose, a monosaccharide. Other enzymes are used to
digest different sugars. Examples: Sucrase for sucrose, Lactase for lactose
 Protein digestion began in the stomach. Pancreas and duodenum secrete enzymes that break the
polypeptides into amino acids. Trypsin and chymotrypsin break polypeptides into small pieces, while
aminopeptidase and carboxypeptidase break off one amino acid at a time
 Nucleic acids – nucleases hydrolyze the nucleic acids in food. They come from the pancreas and split DNA
and RNA into component nucleotides, which are further broken down into their nitrogenous bases,
sugars, and phosphates
 Lipids – fats are not digested until reaching the duodenum. Fats are insoluble in water, making digestion
more complicated. Bile salts break fat into fat droplets by emulsification. Lipase breaks fat molecules into
fatty acids and glycerol
 Once digestion has occurred, the nutrients must be absorbed. Villi, folds on the intestinal wall, are
covered with projections called microvilli. These folds extend the surface area of the intestine, making
more room to absorb nutrients. Absorption is accomplished by simple diffusion or by pumping them
against a concentration gradient. Capillaries pick up the nutrients and transport them to larger vessels
for dispersal.
21.11 The liver processes and detoxifies blood from the intestines
 The liver is located between the intestines and the heart. Capillaries from the small and large intestines
converge into veins that lead into the hepatic portal vein. This vein transports nutrients absorbed by the
intestines directly to the liver. One function is to remove excess glucose and convert it to glycogen, which
will be stored in the liver. By converting glucose, the liver plays a key role in regulating metabolism.
 Liver cells use the nutrients they receive to make new substances, such as proteins, that are needed by
the body. Some of these proteins are the ones needed for blood clotting and maintenance of osmotic
balance. The liver also modifies and detoxifies substances absorbed by the digestive tract before they are
carried by the heart to the rest of the body. The liver produces bile, which helps in the digestion of fat.
21.12 The large intestine reclaims water and compacts the feces
 The large intestine, colon, is 1.5m long and 5 cm in diameter. It joins the small intestine at a T-shaped
junction and a sphincter controls the passage of material from the small to the large intestine. One part of
the T-shape is the cecum. The appendix is an extension of the cecum and plays a role in making a mass of
white blood cells that contribute minorly to immunity. Despite this, the appendix is prone to infection,
appendicitis, and if that occurs, it must be removed surgically.
 The colon’s main function is to absorb water. About 7L of fluid enters the lumen of the digestive tract
daily and 90% of that is reclaimed into blood and tissues, with the small intestine absorbing most of it.
The large intestine finishes the job. As water is absorbed, the remaining waste solidifies and moves
through the colon by peristalsis. This waste, feces, consists of indigestible materials such as cellulose
from plant fibers.

Many prokaryotes inhabit the large intestine and help put vitamins (vitamin K, folic acid, several B
vitamins, and biotin) into the bloodstream from the colon. The end of the colon is the rectum, where feces
are stored until elimination. If the colon becomes irritated, constipation or diarrhea can result.
21.13 Evolutionary adaptations of vertebrate digestive systems relate to diet
 The structure and function of an animal’s digestive system are adapted to the kind of food the animal
eats. Generally, herbivores and omnivores have longer alimentary canals than carnivores.
 A longer canal allows more nutrients to be absorbed from vegetation, which is more difficult to digest. It
also provides more surface area for this absorption of nutrients, which are more concentrated in
vegetation
 Herbivores also have chambers to house more microbes (bacteria and protists) that are able to break
down components in the diet that the animal cannot. Some nutrients made by the microbes are absorbed
by the cecum and colon, but many are lost in feces because they do not go through the small intestine
 Ruminant mammals (cattle, sheep, deer) have an elaborate system for cellulose digestion. This process
can involve rumination (“chewing the cud”) to soften plant fibers and make them more accessible to
microbes in the gut.
21.14 An animal’s diet must provide sufficient energy
 The energy content of food is measured in kilocalories (1kcal = 1,000calories). Calories listed on food
labels are actually in kilocalories, but written as C for Calories.
 Metabolism must go through several processes to keep the animal alive. This includes cellular
maintenance, heartbeat, breathing, and endothermy. The number of kcal (Calories) needed for an animal
at rest is called the basal metabolic rate (BMR). Adult females have a BMR of 1300-1500kcal daily. Adult
males have a BMR of 1600-1800kcal daily
 The more strenuous the activity, the more kcal the body needs. When people take in more kcal than we
need, the body stores the extra. Our liver and muscles store excess sugars as glycogen. Most of us store
about one day’s worth of extra glycogen. Other excess energy is stored as fat. Even in a low fat diet, the
liver converts extra carbs and proteins to fat.
21.15 An animal’s diet must supply essential nutrients
 Essential nutrients must be obtained preassembled because the animal cannot make them from raw
materials. An individual who is missing one or more essential nutrients is considered malnourished. If
they are chronically deficient in calories, they are considered undernourished
 Our cells make fats and lipids by combining fatty acids with other molecules like glycerol. However, there
are essential fatty acids we cannot make and they must be supplied in the diet. Linoleic acid must be
supplied from the diet, and it is critical in the making of phospholipids of cell membranes. Twenty amino
acids are needed to make proteins. Eight of these we are not able to synthesize (nine in infants). They
must be obtained from the diet. The body cannot store excess amino acids, so this can be problematic.
The easiest way to obtain these amino acids is through eating meat and animal products such as eggs,
milk and cheese
 The key to being a healthy vegetarian is to eat a variety of plant foods that can supply all eight of the
essential amino acids. Combinations of legumes and grains provide the right balance. The staple of beans
and rice is an example.
 Dietary Deficiencies: A diet chronically lacking in calories or essential nutrients can lead to
malnutrition. Living in industrialized countries, there is generally plentiful food; while in places of
extreme poverty, food may be unavailable. The most common type of malnutrition is protein deficiency
and may come from a diet that is dependent on plant staples such as rice, corn, or potatoes. In children,
this can lead to impaired physical and mental development.
21.16 A proper human diet must include sufficient vitamins and minerals
 Vitamins are organic nutrients that we must obtain through our diet, but they are needed in smaller
quantities than the essential amino acids. Most vitamins have catalytic functions and are used repeatedly
in metabolic reactions. They are needed in small quantities, but deficiencies have serious repercussions.
Excesses can be toxic. Water-soluble vitamin (B, C) excesses can be eliminated in urine. Fat soluble
vitamins (A, D, E, K) build up in body fat and can become toxic
 Minerals are simple inorganic compounds needed in small amounts. They must be acquired from the diet.
Some of these include calcium, iodine, potassium, sodium, iron, and chlorine. A healthy diet usually
includes enough vitamins and minerals for most people. These diets meet the Recommended Daily
Allowances (RDA).
21.17 Food labels provide nutritional information
The Food and Drug Administration (FDA) requires that certain information be listed on food label.
Ingredients are listed in order from greatest amount (by weight) to least. Nutrition facts are defined
according to FDA standards. Calories are listed per serving and as a percentage of the daily value. This is
based on a 2000 C (kcal) per day diet.
21.18 Dietary deficiencies have a number of causes
 Malnutrition - the health problem caused by improper or insufficient diet. Living in industrialized
countries, it is hard to relate to starvation, but more than 800 million people are dealing with it around
the world. The most common malnutrition is protein deficiency, insufficient intake of one or more amino
acids. Animal products provide these amino acids, but they are expensive. Children lacking in protein will
develop poorly physically and mentally, with a condition termed kwashiorkor (meaning “rejected one”)
 In America, undernutrition may be self-inflicted. Many, mostly women, are affected by anorexia nervosa
or bulimia. Anorexia is self-starvation from fear of gaining weight. Bulimia is binge eating followed by
purging. Both are centered around an obsession with body weight and shape and could result in death
21.19 The human health problem of obesity may reflect our evolutionary past
 The World Health Organization (WHO) recognizes obesity as a global problem. The availability of
fattening food and a more sedentary life style has increased obesity in many populations. In the US,
obesity has doubled to 30% and 35% of people are overweight.
 Obesity contributes to other health problems such as diabetes, breast and colon cancer, and
cardiovascular disease. Because of this epidemic, research has increased into why some people are obese
while others do not struggle at all with their weight. Several genes have been identified, but more
research needs to be done.
21.20 Scientists use a variety of methods to test weight loss claims
 As weight concerns rise, so do the number of fad diets. However, just because a diet is popular does not
make it healthy. Many of the newer and more popular diets advocate a reduction of carbohydrates. The
RDA for carbs is much higher than the 20g of carbs allowed on low carb diets. While many people have
lost weight using this method, this can lead to water loss.
 Replacing carbs with higher fat items also can contribute to heart disease and kidney problems. A
reduction in fruit and vegetable intake cuts back on the number of vitamins and minerals taken in as well.
The low carb diet came in and replaced the low fat diets seen earlier. A healthy diet includes a certain
amount of fat, and low fat diets make it hard to get the essential fatty acids and proteins it needs. This
leads to problems absorbing fat-soluble vitamins. A small amount of fat is essential to a healthy immune
system.
 Overall, the body requires a healthy balance of nutrients. The best weight control is exercise and a diet
that includes all of the essential nutrients.
21.21 Diet can influence cardiovascular disease and cancer
 Certain factors in disease risk are unavoidable. Diet is a behavior we can control to help decrease our risk
for certain diseases, such as cardiovascular disease. Diets high in saturated fat and cholesterol are linked
to cardiovascular disease, so minimizing these intakes can help.
 Trans fat – adding hydrogen to make vegetable oil into a solid fat. These are dangerous fats linked to
heart disease, and doctors recommend the lowest levels as possible. Any food containing hydrogenated
or partially hydrogenated fats will contain trans fat.
 Cholesterol – another contributor to heart disease. Low-density lipoproteins – correlate with blocking
vessels, high blood pressure and heart attacks. High-density lipoproteins – can decrease the risk of
blockage by carrying cholesterol to the liver for breakdown. Exercise seems to lead to higher HDL and
smoking seems to lower the HDL level.
 Diets high in trans fat and saturated fat cause LDL levels to raise and have a negative effect on cardio
health. Eating mainly unsaturated fats tends to lower LDL levels and raise LDL levels. They also have
vitamin E, which may help reduce vessel blockage.