NOTES FOR BIOLOGY 1002

SECTIONS 004, 005, 006

Spring 2006

DR. STEVEN POMARICO

CHAPTER 41

DIGESTION AND HUMAN NUTRITION

Remember the basics about nutrition.

Nutrition in organisms involves four main steps:

         -Acquisition of nutrients

         -Digestion, if required

         -Distribution of nutrients throughout the body.

         -Synthesis of molecules for the organism's body.

Animal cells require a continuous supply of energy. One of the main functions of nutrition in animals is to act as an energy source.

We didn’t see this aspect when we talked about nutrition in plants because they rely on photosynthesis for their energy needs.

This difference is seen in the starting material.

The large organic molecules that animals start their nutrition with are filled with energy.

Nutrient energy is measured in calories (amount of energy needed to raise the temperature of I gram of water by 1 ^o C).

Food energy content is measured in Calories. 1 Calorie = 1 kilocalorie (1000 calories).

The average adult needs 1500 Calories per day at complete rest. Exercise (and mental activity) greatly boosts caloric requirements.

The categories of nutrients that animals consume reflect this need for energy supplying foods.

The two basic methods of digestive are:

         -intracellular digestion

         -extracellular digestion

Intracellular digestion is digestion within a single cell. This type of digestion is found in sponges and protistians.

Intracellular digestion involves endocytosis (food intake), formation of a food vacuole, digestion of the food by lysosomes, and exocytosis (waste dumping)

Extracellular digestion varies from incomplete digestive systems in a simple sac (See fig 41.3) to complete digestive systems in a tube open at both ends and running through the body.

All of these variations use a gastrovascular cavity. The cells lining the gastrovascular cavity engulf small particles or absorb molecules.

Because the digestive system of higher animals (nematode worms through vertebrates) is a one-way tube there is more specialization of the system.

Complete Digestive Systems

There are five functions that a complete digestive system must accomplish:

         Mechanical processing and motility

         Secretion

         Digestion

         Absorption

         Elimination

Mechanical processing and motility

         food is broken into smaller pieces and moved into the cavity

                   Gizzards, Teeth, Churning action of digestive cavity

Secretion

         digestive fluids and enzymes

Digestion

         breakdown of the large molecules into small subunits

Absorption

         transport of small nutrient molecules into the body fluid

Elimination

         expelling indigestible materials from the body.

HUMAN DIGESTION (See fig 41.6)

Begins in the mouth where mechanical and chemical breakdown of food begins.

         32 adult teeth (See fig 41.7)

         3 pairs of salivary glands

Saliva has several functions:

         Contains the enzyme amylase which begins breaking down starch to sugars

         Kills some bacteria with antibiotics.

         Lubricates food

         Dissolves some food molecules so the taste buds in the tongue can recognize                              them.

The tongue manipulates the food into a mass and pushes it back to the pharynx

The pharynx connects mouth and the esophagus.

The epiglottis blocks the opening to the trachea while directing the food to the esophagus.

The esophagus is a muscular tube connecting mouth and stomach

In the esophagus, more mucus further lubricates the food that is pushed along by peristalsis (sequential contractions of circular muscles).

The stomach is an expandable muscular sac

The stomach has three major functions:

         Storage chamber

         Mechanical breakdown of food

         Chemical digestion.

The storage chamber function is accomplished by two rings of muscles. One between stomach and small intestine (pyloric sphincter) the other between the stomach and the esophagus (cardiac sphincter). These sphincter muscle keep the food in the stomach and act to regulate the passage of chyme (partially digested food and digestive secretions) into the small intestine.

The churning of the stomach continues the mechanical breakdown that was begun by the mouth.

The chemical breakdown comes from chemicals secreted by the stomach:

         Gastrin

         Hydrochloric Acid

         Pepsinogen

Gastrin is a hormone that stimulates the secretion of hydrochloric acid by specialized stomach cells.

Pepsinogen is the inactive form of a protease (an enzyme that breaks proteins into smaller peptides). The pepsinogen is converted to the active form (called pepsin) by the hydrochloric acid in the stomach.

The reason the stomach doesn't self-digest is because of a protective layer of mucus which is continuously secreted by glands. If this layer breaks down (do to abuse or infection) an ulcer can occur.

Peristaltic waves, about 3 per minute, move chyme into small intestine about a teaspoon per contraction

Under normal conditions a stomach will empty in 2-6 hours

Little absorption occurs through the stomach: water, alcohol, and some drugs. A full stomach slows alcohol absorption

Most digestion and nutrient absorption occurs in the small Intestine.

The small intestine is about 1-2 inch diameter and about 10 feet long

Secretions from the liver, pancreas, gall bladder and small intestine cells aid the digestion process

The liver is the largest organ in the digestive tract.

The livers role in digestion:

         Produces bile stores it in the gallbladder, and releases it into small intestine                    through the bile duct

Bile is a complex mixture of bile salts, other salts, water, and cholesterol

Bile salts act as detergents for emulsification or dispersal of fats so they can be digested further.

The Pancreas has 2 types of cells:

         -produces the hormones insulin and glucagon for blood sugar regulation

         -produces digestive pancreatic juices

The pancreatic juices contain:

         Sodium bicarbonate that neutralizes the acidic chyme.

         Three types of digestive enzymes:

                   Amylases: break down carbohydrates.

                   Lipases: break down fats.

                   Proteases: break down proteins and peptides

The cells of the walls of the small intestine also produce digestive enzymes:

         Proteases: convert peptides into amino acids

         Sucrase, lactase, and maltase: convert disaccharides into monosaccharides

Absorption in the small intestine is aided by the presence of villi and microvilli along the inside on the small intestine. (See fig 41.10)

These structures increase the surface area and create an area for the close placement of capillary beds. This configuration (we saw this in the lung) allows for the greatest diffusion of material (nutrients) into the bloodstream.

After the small intestine any remnants move into the large intestine.

The large Intestine is about 3 inches in diameter and about 5 feet long. Most of this length is the colon and the final 6 inches is the rectum.

Bacteria living in the large intestine use the leftovers from digestion to produce vitamins needed by the body (vitamin B₁₂, thiamine, riboflavin, vitamin K)

Final absorption of water and salts

The result is a semisolid feces consisting of indigestible wastes and dead bacteria are transported by peristalsis to the rectum where defecation occurs,

The five major categories of animal nutrients are:

         1) Lipids.

         2) Carbohydrates.

         3) Proteins.

         4) Minerals.

         5) Vitamins.

Lipids:

         fats and oils

         phospholipids

         cholesterol

As an energy source, lipids provide more energy/gram than any other nutrient source.

         fat (9 Calories/gram)

         carbohydrates or proteins (4 Calories/gram)

Because of this, we also store our “extra” Calories as fat. This offers some advantages

         -more calories with less weight (3600 Calories/pound)

         -fats are hydrophobic and do not cause accumulation of water

         -functions as insulation because fat conducts less heat

         -protective padding around important organs

Fats also supply:

         -certain hormones

         -cell components (membranes)

         -nerve cell coverings

Most fats can be synthesized by animals but some cannot and therefore have to be obtained in our diet these are essential fatty acids. In humans linoleic acid is an essential fatty acid.

Carbohydrates

         sugars

         starches

         cellulose

         glycogen

Carbohydrates are the principal energy storage molecules in plants.

Animals also store carbohydrates as a quick energy source. In the liver and muscles of humans carbohydrates are stored as glycogen (animal starch)

Proteins

Proteins provide the amino acids building blocks for the construction of proteins.

Of the 20 amino acids that are commonly found in proteins in humans 10 either cannot be synthesized by our bodies or are made in too low of an amount and must be supplied by our diet.

These amino acids are essential amino acids

Specialized amino acids also act as hormones and neurotransmitters.

Minerals

Like plants, animals require these small inorganic molecules. We obtain most from food or drinking water.

Essential Minerals (See table 41.3)

         -bones

         -teeth (calcium, magnesium, phosphorus)

         -nerve impulse conduction

         -muscle contraction (sodium and potassium)

         -oxygen transport (iron for hemoglobin)

Vitamins

Vitamins are diverse types of organic compounds needed in small amounts.

Most cannot be made by humans (except vitamin D) and therefore must be supplied by our diet (See table 41.2).

There are two categories of vitamins:

         water-soluble vitamins

         fat-soluble vitamins

The water-soluble vitamins cannot be stored and must be continually supplied.

         -vitamin C, the B vitamins, and other

The fat-soluble vitamins are stored in body fat

         -vitamin A (forms pigments for vision)

         -vitamins D, E, and K (needed for normal blood clotting)

If a little is good, then a lot must be better. NOT TRUE!!!

Vitamins A and D are toxic if intake is excessive.