Chapter 8 – Nutrition
After studying this chapter, you should be able to:
1. compare nutrition in simple organisms with nutrition in more complex organisms.
2. describe how food is digested and absorbed by the human digestive system; describe the functions of the
mouth, esophagus, stomach, small intestine, liver, gallbladder, pancreas, large intestine, rectum, and anus.
3. list the principle digestive enzymes, where they are produced, the type of food they act upon, and the end
products of enzymatic breakdown.
4. compare autotrophs and heterotrophs.
5. define the term calorie and explain how the energy content of food is measured.
6. describe the functions of the six basic types of nutrients found in the human diet.
7. distinguish between mechanical breakdown and chemical digestion of food.
8. contrast intracellular digestion and extracellular digestion.
9. compare digestive processes in protozoa, the hydra, the earthworm, and the grasshopper.
8.1 – The Process of Nutrition
The bottom line is all cells require energy to survive. They obtain energy from the chemical bonds in ATP,
which was originally formed from food containing nutrients. Nutrients are not found in all food, in fact, some
substances humans ingest have very little or no nutrients other than water. Nutrients are things like proteins,
carbohydrates, fats, vitamins, minerals, and water. Minerals are things like iron, calcium, phosphorus, and
iodine. It is important that your food contain the proper amounts of nutrients and minerals so that your cells
have all they need to maintain, regulate, grow, and repair themselves.
Autotrophs (photosynthetic organisms) obtain organic nutrients (proteins, fats, carbohydrates, and nucleic acids)
by synthesizing them from simple inorganic substances. The exceptions to this rule are the chemotrophs, which
don’t need light to make their own food, just chemicals.
Heterotrophs (can’t make their own organic nutrients) obtain organic nutrients by ingesting food that contains
these substances already synthesized. They eat plants and/or animals.
As you learned in chapter six, the cell’s energy is stored in the ATP molecule, which was made from food.
Interestingly, we have found that the amount of energy released by cellular respiration is the same as would be
released by simply burning the food in a calorimeter (an instrument used to measure the calorie content of
foods). Refer to Figure 8-2, on page 150 for an illustration of the calorimeter. One calorie is equivalent to 4.18
joules (the unit of energy). It is also the amount of heat that will raise the temperature of one gram of water one
degree Celsius. We use the kilocalorie, which is 1000 calories, also written as one Calorie, because foods
contain so many calories. Calories are units to measure energy, which is given off by breaking down food
during cellular respiration. One gram of carbohydrate, protein, and fat, give off 4, 4, and 9 Calories of heat
respectively. Fat has more than twice as many calories per gram.
Caloric requirements vary with the individual. The young and active require more calories than the old and
inactive. Males require more calories than females.
On page 153, you will see the “Food Pyramid Guide”. You should know the six different food groups and the
recommended number of servings from each group. Planning your diet according to the pyramid will ensure
that you obtain all the nutrients, vitamins, and minerals your cells need to survive. This in turn, will lead to
healthy body systems and a healthy organism.
Note: Fiber, obtained from cellulose found in fruits and vegetables, is an important stimulant of the muscles in
your digestive tract. Eating fiber ensures these muscles remain strong and healthy and can reduce the chances
of getting colon and rectal cancers.
8.2 – Adaptation for Nutrition and Digestion
Food must be broken down into small particles that can pass through the cell membrane. Mechanical digestion
is the physical breaking down of food, which starts in the mouth. Your teeth crunch and grind food down into
smaller particles. Then your stomach and intestines pulverize it further using enzymes, which is called
chemical digestion, until it is small enough to diffuse through membranes. We call the process of passing
through the membrane absorption.
Protists are unicellular, eukaryotic cells, which carry out intracellular digestion (meaning, it happens inside the
cell). The ameba, for example, is attracted to food by sensing certain chemicals in its environment and follows
the scent. The ameba uses its pseudopods to surround the food particle and takes the food into the cell. Part of
the cellular membrane is used to contain the food so that it is separate from the rest of the cell, once inside.
Then, a lysosome arrives on the scene, fuses with the membrane, and digests the food. Indigestible material is
brought to the cellular membrane. There the membranes fuse and the contents are expelled. The paramecium
has cilia that sweep food along the oral groove, refer to page 156, and into the gullet. From there, the process is
similar to the ameba in that the gullet pinches off forming a food vacuole and is fused with a lysosome so that
digestion can occur. Indigestible material exits through the anal pore.
Multicellular organisms carry out extracellular digestion (meaning, it takes place outside the cells). Refer to
Figure 8-9 for a diagram of the hydra. The hydra waits for its food (a water flea or something) to come into
contact with its tentacles. When it does, the nematocysts release their long threads and wrap around the food
and injects it with poison to paralyze it. The tentacles bring the food to the mouth and into the gastrovascular
cavity where specialized cells in the endoderm secrete enzymes to break down the food. The cells then absorb
nutrients. Some of the cells in the endoderm engulf the partially digested food and finish the job intracellularly
using vacuoles and lysosomes. Waste products from the ectoderm are diffused out through all the cells that
make up the hydra. Waste products from the endoderm are passed back up to the mouth and expelled.
Complex multicellular animals, such as the earthworm, carry out extracellular digestion. Refer to page 158.
The earthworm’s body plan is called the “tube within a tube”. The inner tube is called the alimentary canal and
has two openings, the mouth and the anus. Food travels in one direction into the mouth and out of the anus.
Food is brought to the mouth by the sucking action of the pharynx. From there, it passes to the esophagus and
then to the crop (for storage). Food then enters the gizzard for grinding (mechanical digestion). From there it
enters the intestine, which contains cells that secrete digestive enzymes (chemical digestion). The products are
then absorbed by the cells that line the intestine and carried around the body by the circulatory system.
The grasshopper has a few more organs than the earthworm, but the digestive process is similar. First, chewing
occurs in the mouth and food is mixed with saliva secreted by salivary glands (chemical digestion). Then the
food passes through the esophagus and to the crop. From there, to the gizzard for grinding. From there, to the
stomach for more chemical digestion and absorption into the bloodstream. Undigested material continues to
pass through the rectum, where water is absorbed and the rest is sent to the anus and eliminated.
8.3 – The Human Digestive System
Food enters the mouth, where mechanical and chemical digestion begins. The teeth and tongue physically
break food down into smaller particles while the salivary glands secrete salivary amylase, an enzyme the breaks
down starch (a polysaccharide) into maltose (a disaccharide). The food material is shaped into a bolus and then
pushed to the pharynx by the tongue and the swallowing reflex takes over to send the bolus down the
esophagus. To prevent the bolus from entering your windpipe or trachea, a flap of tissue called the epiglottis
blocks the opening.
Peristalsis (wave-like muscle contractions) sends the food down the esophagus and to the stomach where it is
pushed through the cardiac sphincter (a valve controlling food into the stomach). Once in the stomach, food is
broken down further mechanically (by the muscular contractions of the stomach walls) and chemically (by the
secretion of gastric juice).
The lining of the stomach contains pyloric glands, which secrete mucus to cover and protect the stomach lining
from the acidic environment, and gastric glands, which secretes HCl and pepsinogen (together they make up
gastric juice). The HCl destroys most of the bacteria and the pepsinogen, activated by the HCl, becomes the
active enzyme pepsin which breaks down protein into short chains of amino acids or polypeptides. Protein is
only partially digested in the stomach. The gastric juice changes the food material into chyme, a thin soupy
The chyme passes to the small intestine through the pyloric sphincter two to six hours after a meal. Most of the
chemical digestion and absorption of simple sugars, amino acids, fatty acids, glycerol, vitamins, and minerals
takes place in the small intestine. Refer to page 164 for an illustration of the lining. The lining contains villi,
which contain microvilli, which absorb the nutrients and then diffuse to the blood vessels, which pick up the
nutrients and transport them to all the cells of the body.
The fatty acids and glycerol (digested from the fat molecule) are absorbed into the lymphatic system by the
lacteals in the villi.
The secretions of the small intestine are alkaline(base has pH >7). The chyme is mixed with pancreatic juice
from the pancreas, which digests polypeptides from the proteins (digested by proteases such as trypsin and
chymotrypsin into smaller polypeptides or dipeptides), starches (turned into maltose by amylase), and lipids
(broken down by lipase into fatty acids and glycerol). Lipid digestion is helped by bile. Bile, secreted by the
liver, passes through the bile duct directly from the liver, or from the gallbladder where it is stored, to reach
the small intestine where it emulsifies fats and oils.
Peptidase, secreted by intestinal glands in the small intestine, finish the digestion of the small polypeptides into
amino acids. Intestinal glands also secrete enzymes that digest disaccharides: maltose, sucrose and lactose are
broken down into simple sugars by maltase, sucrase and lactase, respectively.
The large intestine is next and the material passes from the small intestine to the large intestine through a
sphincter. NO chemical digestion occurs in the large intestine, only the absorption of water, and also the
absorption of vitamins( B and K), which are produced by the bacteria that live in the large intestine. Lastly, the
waste material is eliminated as feces or stool as it passes from the rectum through the anus.