Overview: The Need to Feed
All animals eat other organisms—dead or alive, whole or by the piece (including parasites).
· In general, animals fit into one of three dietary categories.
1. Herbivores, such as gorillas, cows, hares, and many snails, eat mainly autotrophs (plants and algae).
2. Carnivores, such as sharks, hawks, spiders, and snakes, eat other animals.
3. Omnivores, such as cockroaches, bears, raccoons, and humans, consume animal and plant or algal matter.
o Humans evolved as hunters, scavengers, and gatherers.
· While the terms herbivore, carnivore, and omnivore represent the kinds of food that an animal usually eats, most animals are opportunistic, eating foods that are outside their main dietary category when these foods are available.
o For example, cattle and deer, which are herbivores, may occasionally eat small animals or bird eggs.
o Most carnivores obtain some nutrients from plant materials that remain in the digestive tract of the prey that they eat.
o All animals consume bacteria along with other types of food.
For any animal, a nutritionally adequate diet must satisfy three nutritional needs:
- A balanced diet must provide fuel for cellular work.
- It must supply the organic raw materials needed to construct organic molecules.
- Essential nutrients that the animal cannot make from raw materials must be provided in its food.
Concept 41.1 Homeostatic mechanisms manage an animal’s energy budget
· The flow of food energy into and out of an animal can be viewed as a “budget,” with the production of ATP accounting for the largest fraction by far of the energy budget of most animals.
o ATP powers basal or resting metabolism, as well as activity and, in endothermic animals, thermoregulation.
· Nearly all ATP generation is based on the oxidation of organic fuel molecules—carbohydrates, proteins, and fats—in cellular respiration.
o The monomers of any of these substances can be used as fuel.
o Fats are especially rich in energy, liberating about twice the energy liberated from an equal amount of carbohydrate or protein during oxidation.
· When an animal takes in more calories than it needs to produce ATP, the excess can be used for biosynthesis.
o This biosynthesis can be used to grow in size or for reproduction, or it can be stored in energy depots.
o In humans, the liver and muscle cells store energy as glycogen, a polymer made up of many glucose units.
Concept 41.2 An animal’s diet must supply carbon skeletons and essential nutrients
In addition to fuel for ATP production, an animal’s diet must supply all the raw materials for biosynthesis (the formation of a chemical compound by a living organism).
o Given a source of organic carbon (such as sugar) and a source of organic nitrogen (usually in amino acids from the digestion of proteins), animals can fabricate a great variety of organic molecules—carbohydrates, proteins, and lipids.
An animal whose diet is missing one or more essential nutrients is said to be malnourished.
Animal nutrition: Nutritional Requirements
An adequate diet supplies three things: fuel in the form of chemical energy, the organics raw materials for biosynthesis, and essential nutrients.
The essential nutrients required by an animal are those that must be obtained in pre-assembled organic for because the animal cannot produce them.
About half of the 20 essential amino acids must be obtained from food. There are also essential fatty acids which animals can not make and must ingest. Nonessential can be made form other substances in the body.
Absorbed nutrients are in the form of monomers.
Vitamins are organic molecules that are required in the diet in small amounts. They are used as co-factors in enzyme-controlled biochemical reactions.
**Minerals, such as calcium and phosphorus, are simple inorganic nutrients that are also required in the diet in small amounts**
· There are four classes of essential nutrients: essential amino acids, essential fatty acids, vitamins, and minerals.
· Animals require 20 amino acids to make proteins.
o Most animals can synthesize half of these if their diet includes organic nitrogen.
· Because the body cannot easily store amino acids, a diet with all essential amino acids must be eaten each day, or protein synthesis is retarded.
· Some animals have special adaptations that get them through periods where their bodies demand extraordinary amounts of protein.
o For example, penguins use muscle proteins as a source of amino acids to make new proteins during molting.
While animals can synthesize most of the fatty acids they need, they cannot synthesize essential fatty acids.
- These are certain unsaturated fatty acids, including linoleic acids, which are required by humans.
- Most diets furnish ample quantities of essential fatty acids, and thus deficiencies are rare.
Vitamins are organic molecules required in the diet in quantities that are quite small compared with the relatively large quantities of essential amino acids and fatty acids animals need.
- While vitamins are required in tiny amounts—from about 0.01 mg to 100 mg per day—depending on the vitamin, vitamin deficiency (or overdose in some cases) can cause serious problems.
So far, 13 vitamins essential to humans have been identified.
- These can be grouped into water-soluble vitamins and fat-soluble vitamins, with extremely diverse physiological functions.
The water-soluble vitamins include the B complex, which consists of several compounds that function as coenzymes in key metabolic processes.
- Vitamin C, also water soluble, is required for the production of connective tissue.
- Excessive amounts of water-soluble vitamins are excreted in urine, and moderate overdoses are probably harmless.
The fat-soluble vitamins are A, D, E, and K.
- They have a wide variety of functions.
- Vitamin A is incorporated in the visual pigments of the eye.
- Vitamin D aids in calcium absorption and bone formation.
- Vitamin E seems to protect membrane phospholipids from oxidation.
- Vitamin K is required for blood clotting.
- Excess amounts of fat-soluble vitamins are not excreted but are deposited in body fat.
- Overconsumption may lead to toxic accumulations of these compounds.
Minerals are simple inorganic nutrients, usually required in small amounts—from less than 1 mg to about 2,500 mg per day.
- Mineral requirements vary with animal species.
- Humans and other vertebrates require relatively large quantities of calcium and phosphorus for the construction and maintenance of bone.
- Calcium is also necessary for the normal functioning of nerves and muscles.
- Phosphorus is a component of the cytochromes that function in cellular respiration.
Sodium, potassium, and chloride are important in nerve function and have a major influence on the osmotic balance between cells and the interstitial fluids.
Concept 41.3 The main stages of food processing are ingestion, digestion, absorption, and elimination
Overview: Food Processing
1. Ingestion is the act of taking in food, and it is the first stage in the processing of food. Ingestion, the act of eating, is only the first stage of food processing.
· Food is “packaged” in bulk form and contains very complex arrays of molecules, including large polymers and various substances that may be difficult to process or even toxic.
2. Animals cannot use macromolecules like proteins, fats, and carbohydrates in the form of starch or other polysaccharides.
· First, polymers are too large to pass through membranes and enter the cells of the animal.
· Second, the macromolecules that make up an animal are not identical to those of its food.
i. In building their macromolecules, however, all organisms use common monomers.
ii. For example, soybeans, fruit flies, and humans all assemble their proteins from the same 20 amino acids.
3. Digestion is the second stage of the processing of food. It is the breakdown of food into small molecules capable of being absorbed by the cells of the body.
- Digestion cleaves fats into glycerol and fatty acids, catalyzed by enzymes, cleaves nucleic acids into nucleotides, polysaccharides into simple sugars.
- Digestion, the second stage of food processing, is the process of breaking food down into molecules small enough for the body to absorb.
- Digestion cleaves macromolecules into their component monomers, which the animal then uses to make its own molecules or as fuel for ATP production.
- Polysaccharides and disaccharides are split into simple sugars.
- Fats are digested to glycerol and fatty acids.
- Proteins are broken down into amino acids.
- Nucleic acids are cleaved into nucleotides.
- Digestion reverses the process that a cell uses to link together monomers to form macromolecules.
- Rather than removing a molecule of water for each new covalent bond formed, digestion breaks bonds with the addition of water via enzymatic hydrolysis.
- A variety of hydrolytic enzymes catalyze the digestion of each of the classes of macromolecules found in food.
4. Enzymatic hydrolysis is the reaction by which macromolecules are broken up. It involves the addition of water. It obviously includes enzymes.
· Chemical digestion is usually preceded by mechanical fragmentation of the food—by chewing, for instance.
i. Breaking food into smaller pieces increases the surface area exposed to digestive juices containing hydrolytic enzymes.
5. Absorption is the stage in food processing when the body’s cells take up small molecules from the digestive tract. Cross the cell membrane with transport mechanisms.
· After the food is digested, the animal’s cells take up small molecules such as amino acids and simple sugars from the digestive compartment, a process called absorption.
6. Elimination occurs when the undigested material passes out of the digestive tract.
· During elimination, undigested material passes out of the digestive compartment.
7. Intracellular digestion occurs within a cell enclosed by a protective membrane. Sponges digest their food this way.
8. Extracellular digestion is carried out by most animals; in this type of digestion, food is broken down outside of cells.
· Many simple animals have a gastrovascular cavity, where digestion takes place. These simple animals have a single opening through which food enters and waste is eliminated.
More complex animals have complete digestive tracts (alimentary canals) which are one-way digestive tubes that begin with the mouth at one end of the terminate in the anus at the other.
Concept 41.4 Each organ of the mammalian digestive system has specialized food-processing functions
· The general principles of food processing are similar for a diversity of animals, including the mammalian system that we will use as a representative example.
· The mammalian digestive system consists of the alimentary canal and various accessory glands that secrete digestive juices into the canal through ducts.
o Peristalsis, rhythmic waves of contraction by smooth muscles in the walls of the canal, pushes food along.
o Sphincters, muscular ring-like valves, regulate the passage of material between specialized chambers of the canal.
o The accessory glands include the salivary glands, the pancreas, the liver, and the gallbladder.
· After chewing and swallowing, it takes 5 to 10 seconds for food to pass down the esophagus to the stomach, where it spends 2 to 6 hours being partially digested.
· Final digestion and nutrient absorption occur in the small intestine over a period of 5 to 6 hours.
· In 12 to 24 hours, any undigested material passes through the large intestine, and feces are expelled through the anus.
The oral cavity, pharynx, and esophagus initiate food processing.
Digestion occurs in specialized compartments.
To avoid digesting their own cells and tissues, most organisms conduct digestion in specialized compartments.
· The simplest digestive compartments are food vacuoles, organelles in which hydrolytic enzymes break down food without digesting the cell’s own cytoplasm, a process termed intracellular digestion.
· This process begins after a cell has engulfed food by phagocytosis or pinocytosis.
· Newly formed food vacuoles fuse with lysosomes, which are organelles containing hydrolytic enzymes.
· These tubes are called complete digestive tracts or alimentary canals.
o Because food moves in one direction, the tube can be organized into specialized regions that carry out digestion and nutrient absorption in a stepwise fashion.o In addition, animals with alimentary canals can eat more food before the earlier meal is completely digested.
Step by Step of the Human Digestive System
When food is in the mouth, or oral cavity, a nervous reflex occur which causes saliva to be secreted into the mouth. Saliva lubricates the food and contains the enzyme salivary amylase, which hydrolyzes starch and glycogen into smaller polysaccharides and the disaccharide maltose. This is typically the first type of macromolecules to be enzymatically attacked.
**Know what type of digestion (chemical, etc.) is occurring. Know also where the polysaccharide/monosaccharide is being digested and what is happening**
During chewing, food is shaped into a ball called a bolus. After being swallowed, the bolus enters the pharynx – a junction that opens to the esophagus and the trachea. During swallowing, the epiglottis (a flap made of cartilage) moves to the cover the trachea. This will divert the food to go down the esophagus (also called the alimentary canal).
The esophagus moves food from the pharynx down to the stomach through peristalsis – rhythmic waves of contraction by smooth muscle in the walls of the esophagus (also called the alimentary canal).
The stomach is in the upper abdominal cavity, and its functions include storing food and secreting gastric juice. Gastric juice contains hydrochloric acid, which is very acidic (pH of about 2). Gastric juice breaks down the extra cellular matrix of meat and plant materials, and it also kills most of the bacteria ingested with the food.
Gastrin is a hormone produced by the epithelial lining of the stomach.
Pepsin is an enzyme in gastric juice that begins to hydrolyze proteins into smaller polypeptides. Pepsin is secreted in an inactive form called pepsinogen, which is activated by the hydrochloric acid in the stomach.
The result of digestion in the stomach is a substance called acid chyme. The acid chime is shunted from the end of the stomach into the beginning of the small intestine via the pyloric sphincter.
1. Which of the following organs is incorrectly paired with its function?
a. stomach - protein digestion
b. oral cavity - starch digestion
c. large intestine - bile production
d. small intestine - nutrient absorption
e. pancreas - enzyme production
2.The esophagus conducts food from the pharynx down to the stomach by
3.Chemical digestion is usually preceded by __________ fragmentation of the food—by chewing, for instance.
1. c 2. a 3. b
1. c 2. a 3. b