Performance Objectives:
After completing this lesson, students will be able to:
- Describe the differences between intracellular and extracellular digestion, and between mechanical and chemical digestion
- Describe the advantages of "assembly line" digestion in higher animals
- Describe the parts of a typical vertebrate digestive system, including the function of each region.
ENERGY AND METABOLISM:
- Calories and Work: Energy is the ability to do work. It is measured in calories.
A calorie is the amount of energy required to raise the temperature of one gram of water by one degree Celsius (or one degree Kelvin).
Food energy is usually measured in kilocalories. One kilocalorie, equal to 1000 calories, is the energy required to raise the temperature of
a kilogram of water one degree Celsius. (IMPORTANT: the "calories" that you see on food labels are really kilocalories.)
(In the SI or Metric System, energy is measured in Joules; a calorie is equivelent to 4.184 Joules, and a kilocalorie is 4184. Joules.)
- Basal metabolic rate (BMR) is measured in calories of energy used per minute, per gram or kilogram of body weight. "Basal" means that it is measured at rest (lying awake).
Any activity or exercise increases the metabolic rate above this basal value, by twofold or more for vigorous activity like running.
- Metabolic rate and size: For animals of the same general shape, volume (and also mass)
are approximately proportional to the cube of body length.
If Length (L) doubles, and width doubles, and height doubles, then volume and mass increase 8-fold (2x2x2 = 8). However, surface area increases in proportion to the
square of body length. (A sphere of radius
r has a surface area of
4πr2/3.)
The heat generated by muscle activity varies approximately in proportion to body mass, which is proportional to volume or to L3, but
the rate of head LOSS across the body's surface varies with surface area and is thus proportional to L2.
For this reason, an animal twice as large generates 8 times the internal heat, but only loses it 4 times as fast, and so is much more able
to maintain its body temperature simply as a consequence of size. (The ratio of surface to volume is proportional to L2/L3, or 1/L.) This is thought to be one reason why many dinosaurs were so big— it was
an adaptation to conserve heat. On the other hand, very small animals need to maintain a much higher metabolic rate (and therefore eat more in proportion
to their body size) in order to compensate for rapid heat loss at small sizes.
This relationship helps explain the graph in the illustrations below.
Illustrations: Energy
ENERGY, TEMPERATURE, and ENVIRONMENTAL ADAPTATIONS:
BODY TEMPERATURE:
- Poikilotherms: Most animals are poikilotherms, meaning that their body temperature varies and is usually close to that
of their surroundings. Most of the time, they are "temperature conformers" whose body temperature matches their surroundings, but they may have
behavioral mechanisms to regulate their body temperatures by sunning themselves to warm up of by burrowing underground or seeking ponds to
avoid the dangers of overheating. Metabolism in poikilotherms varies with temperature: they tend to be slow and sluggish in the cold,
and more active in warm conditions. Very few reptiles or amphibians live north of the U.S./Canada border. Fishes living in very cold waters often
have special "anti-freeze" compounds in their blood and body fluids.
- Homeotherms: Mammals and birds are homeotherms, meaning that their body temperature is fairly constant and usually
warmer than their surroundings. To maintain higher temperatures, homeotherms must maintain much higher metabolic rates than poikilotherms
of comparable size. Homeotherms must also have good body insulation (hair, fur, blubber, or feathers) to minimize heat loss. Some dinosaurs,
pterosaurs (extinct flying reptiles), and synapsids (mammal-like reptiles) may have been homeotherms-- several dinosaurs had feathers, and pelycosaurs
had sail-like membranes that may have been used to collect or to dissipate heat. Heat-regulating adaptations include:
- Sweating, which cools the body by evaporation to prevent overheating
- Counter-current blood flow to the limbs, which minimizes heat loss (see the accompanying illustration using the link above)
- Temporary hyperthermia in gazelles and camels, to minimize heat loss to hot surroundings
- Hibernation in small mammals
NUTRITION IN ANIMALS:
- Calories and Work: Energy is the ability to do work. It is measured in calories.
A calorie is the amount of energy required to raise the temperature of one gram of water by one degree Celsius (or one degree Kelvin).
Food energy is usually measured in kilocalories. One kilocalorie, equal to 1000 calories, is the energy required to raise the temperature of
a kilogram of water one degree Celsius. (IMPORTANT: the "calories" that you see on food labels are really kilocalories.)
(In the SI or Metric System, energy is measured in Joules; a calorie is equivelent to 4.184 Joules, and a kilocalorie is 4184. Joules.)
As explained above, small mammals need to eat more in proportion to their body weight, just to compensate for metabolic heat loss.
- Carbohydrates: Carbohydrates include sugars and starches. The majority of caloric energy in most diets usually comes from carbohydrates.
- Monosaccharides are single-unit sugars, usually occurring in ring-like molecular shapes.
Ribose and Deoxyribose are 5-carbon sugars that occur in nucleic acids like DNA and RNA.
The nutritionally important monosaccharides are all 6-carbon sugars with the formula C6H12O6.
They include glucose, fructose and galactose.
All carbohydrates are digested into individual monosaccharides that the body then absorbs and uses for energy.
- Disaccharides are sugars containing two rings and usually 12 carbons. Sucrose (table sugar) is made of glucose and fructose units;
it can be split into a moleculae of fructose and a molecule of glucose by adding water, a process called hydrolysis (literally,
"splitting by water") that usually takes place with the help of an enzyme. The most familiar enzymes are named after
the molecule that they split: sucrase splits sucrose into glucose plus fructose. Lactose is another common
disaccharide. The enzyme lactase splits lactose into glucose plus galactose.
- Polysaccharides are made of many sugar units linked together as a polymer (meaning a chain of similar molecular units).
Starch and cellulose are two polysaccharides, each made of multiple glucose units. Humans and many other mammals
have enzymes to help digest starch, but no mammal has enzymes to digest cellulose without the help of symbiotic gut bacteria.
- Amino acids and proteins: Amino acids all contain a central carbon, bonded to a hydrogen atom, a —COOH carboxyl group,
an —NH2 amino group, and a "side chain" that differs from one amino acid to another. Twenty common amino acids occur
in proteins; other (less common) amino acids can be made from these twenty. Eight of the twenty common amino acids are
considered "essential" in human nutrition (nine in infants) because they are all needed to build proteins, but the body can convert these eight into all the others.
A protein is built of a chain (or polymer) of many amino acids. The simplest proteins are coiled into a helix (called an "alpha" helix) in which one
twist of the coil includes three amino acid units. Most proteins bend or fold into compact and complex shapes by various physical and chemical forces.
For example, some amino acids are positively charged, while others are negatively charged, and the positive and negative charges attract one another
(while two positive charges or two negative charges will repel one another), bending the protein chain out of shape.
Proteins are digested by splitting them into individual amino acids. Once absorbed by the body, the individual amino acids are used to
build proteins that the body needs, especially for growth and for tissue repair.
- Lipids, meaning fat-soluble (or water-insoluble, nonpolar) compounds. The most common dietary lipids are triglycerides, made of
a molecule of glycerol linked to three fatty acid chains. Gram for gram, lipids provide more caloric energy than carbohydrates.
They are also used by the body to synthesize important membrane components, hormones, and certain brain chemicals. (Overconsumption, however,
can lead to obesity, heart disease, diabetes, clogged blood vessels (atherosclerosis), and other problems.)
- The above food components are called macronutrients-- things that the body needs in significant quantities. The vitamins and minerals, on the other hand,
are called micronutrients because they are only needed in very small amounts, usually measured in milligrams (sometimes even micrograms).
- Vitamins are complex organic compounds needed only in tiny amounts. Most vitamins function as parts of enzymes that get re-used at the
molecular level, over and over again, so that a tiny amount can make a big difference.
- Minerals are individual chemical elements (usually metallic elements) required in tiny amounts. In most cases, the minerals are important
parts of molecules (like enzymes) that get re-used again and again, so a little bit goes a long way. Calcium and iron are two of the most
important elements required in human diets. Eating a good VARIETY of foods is usually the best way to make sure you are getting
all the important mineral nutruents. Milk products (including cheese and ice cream) are the best source of calcium; beans, eggs, and meats are
among the best sources of iron.
DIGESTION IN ANIMALS:
Unicellular organisms ingest food by phagocytosis.
Most digestion by
lower animals is intracellular, but extracellular digestion (both
mechanical and chemical) predominates among higher animals.
"Assembly line" (mouth-to-anus) digestion allows regional specialization
to evolve, so that digestion happens in stages.
Intracellular digestion predominates in lower animals.
In phagocytosis, folds of the cell membrane engulf food material
originally outside the cell, forming a vacuole. Chemical digestion then
follows when these vacuoles fuse with lysosomes.
Extracellular digestion predominates in higher animals.
- Mechanical digestion: Food is minced or crushed, exposing
more surface area.
- Chemical digestion: Food is broken down chemically with the
help of enzymes.
"Cul-de-sac" versus "assembly line" digestion:
- Among Cnidaria, digestive wastes can only go out the mouth. New food that is taken in
will therefore include many wastes recently discarded (an inefficient process). Also, every portion
of the digestive tract must carry out all phases of digestion, so there is no possibility
of regional speciailzation (for different nutrients) or of sequential processing.
- Among animals with a separate mouth and anus, digestion proceeds in stepwise fashion, as in
an assembly line. Different regions can be adapted to handle different nutrients, or different steps
in the sequential processing of complex nutrients like proteins.
TISSUE ORGANIZATION and the division of labor:
In most multicellular organisms, cells are specialized to do different things (the "division of labor").
Cells of the same type are generally organized into tissues, defined as groups of similar cells (and their
extracellular products) built together ("structurally integrated") and working together ("functionally integrated").
For example, the several tissues that line the gut (the digestive tube) are shown in one of the accompanying illustrations.
Illustrations: digestion
VERTEBRATE DIGESTIVE SYSTEMS:
Mouth: Major site of mechanical digestion (using teeth, etc.).
Chemical digestion begins with amylase enzyme in saliva that breaks
down starches. Food passes from mouth to stomach via the esophagus.
Stomach: Major site of protein digestion. Pepsin
breaks proteins into small peptides. Hydrochloric acid (HCl) acidifies
stomach contents; this helps pepsin, which works best at acidic pH (near
2.0). Mechanical digestion also occurs by contraction of 3 muscle layers,
kneading food. Some species (like birds) have a large storage crop,
followed by a strong muscular gizzard, specialized for mechanical
digestion.
Liver: Secretes bile, containing bile pigments (derived from
hemoglobin) and bile salts, which emulsify fats. Bile is stored in the
gall bladder until needed.
Bile salts are soap-like ("amphipathic") molecules with a long nonpolar hydrocarbon chain
and a polar (water-soluble) "head" at one end. Emulsification is a process in which these molecules coat each fat droplet
with the polar "heads" while the nonpolar "tails" dissolve in the fat. Without emulsification, small fat droplets bump into one another
and fuse to form bigger and bigger droplets. Emulsified droplets bounce off one another and remain small, without fusing. This
increases their surface area, and thus the area over which they can be attacked by fat-digesting enzymes.
Small intestine: Long and coiled, with large surface area.
- Duodenum: Digestion of lipids, carbohydrates, and
proteins, using enzymes secreted by the duodenum itself and by the
pancreas. Lipids are emulsified by bile salts secreted
by the liver.
- Jejunum: Chemical digestion of most nutrients is completed here.
- Ileum: Site of most absorption of digestion
end-products.
Large (large-diameter) intestine: Includes:
- Caecum
(size varies), site of microbial digestion of cellulose in herbivores
Humans and carnivores have small caeca. Rodents, rabbits, horses, and other herbivores have
large caeca that contain many millions of symbiotic bacteria that help digest plant cell walls, producing sugars. Many of the
sugars are also absorbed by the lining of the caecum. Cows and other ruminant Artiodactyla are an exception: instead of
a large caecum, they have a partitioned stomach, with a different bacterial flora in each of the four chambers.
- Colon site of much water resorption
Anus: Undigested wastes are eliminated.
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