Food can be defined as "substances providing a source of energy, and components that are necessary for growth and maintenance of life". These components are usually referred to as nutrients.

Major nutrients are water, carbohydrates,, fats and lipids,  protein,  vitamins and minerals

NUTRIENTS

Water.

Constitutes 50% + of body mass of adult animals, higher amount in newborns. Functions of water are 2 fold:

A key role in body metabolism since all biochemical reactions in the body require water. Water acts as a solvent, a transport medium for dissolved substances in blood , tissue fluids etc. Is a component in many chemical reactions such as hydrolysis and oxidation.

Maintaining body temperature through evaporative cooling , blood flow etc.

Water is supplied as:
Drinking water
Water in feeds
Water from metabolic reactions
Water associated with fat or protein breakdown (negative energy balance).

Carbohydrates.

Composed of carbon, hydrogen and oxygen. Often known as sugars.
Classified into various groups based upon number of sugar molecules that are linked together.

Monosaccharides
(one molecule)
Pentoses ( 5 carbons) C₅H₁₀O₅ - arabinose, xylose, ribose.
Hexoses (6 carbons) C₆H₁₂O₆ - glucose, fructose, galactose

While we represent ribose as C₅H₁₀O₅ or glucose   C₆H₁₂O₆ . In actuality they are ring structures as shown in the diagram below

©1998 by Alberts, Bray, Johnson, Lewis, Raff, Roberts, Walter.   http://www.essentialcellbiology.com . Published by Garland Publishing, a member of the Taylor & Francis Group.

Disaccaharides
( 2 monosaccharides linked together)

C₁₂H₂₂O₁₁ (note loss of one H₂O)
Sucrose (glucose+ fructose)
Lactose (glucose + galactose)
Cellobiose (glucose+ glucose)

Trisaccharides

C₁₈H₃₂O₁₆ (note loss of one more H2O)
 

Raffinose (glucose+fructose+ galactose)

Polysaccharides

Many pentoses or hexoses linked together
 
Xylan (xylose--------xylose)
Starch (glucose-----------glucose) alpha linked
Cellulose (glucose --------glucose) beta linked

Some examples of the basic  types of sugars molecules found in cells are shown in the following diagram.
 
 

©1998 by Alberts, Bray, Johnson, Lewis, Raff, Roberts, Walter.   http://www.essentialcellbiology.com . Published by Garland Publishing, a member of the Taylor & Francis Group.
 

Lipids

Lipids are relatively insoluble in water and consist primarily, as for carbohydrates of C,H,O and often contain phosphorus or nitrogen.  Basic component units are fatty acids and glycerol.
Simplest fatty acid is a 2 carbon structure - acetic acid CH3. COOH.

Longest is 24 carbons in length.

CH₃. CH₂.CH₂----------COOH.

Fatty acids may be:

saturated (no double bonds) -C-C-C-C
monunsaturated (one double bond) -C-C=C-
or polyunsaturated (more than one double bond) -C=C=C-C

Glycerol structure is:

HOCH₂
       l
HOCH
       l

HOCH₂

Mono, di and tri glycerides are esters of fatty acids and glycerol

Monoglyceride

                        HOCH₂
                               l
                        HOCH
                               l
 Fatty acid-------OCH₂

 Diglyceride

                        HOCH₂
                              l
 Fatty acid-------OCH
                              l
 Fatty acid-------OCH₂

etc.

Some example structures are shown in the 2 diagrams below

DIAGRAM 1

DIAGRAM 2
 

Lipids have 5 major functions:

Supply of energy
Source of essential fatty acids (linoleic and linolenic)
Carry fat soluble vitamins
Component of membranes
Components and precursors of hormones and vitamin D.

Proteins

Proteins consist of amino-acids linked together in chains through peptide bonds. The sequence of  aminoa cids and the chain length determines protein structure and properties.
 

DIAGRAM  1 of protein primary structure

DIAGRAM 2 of linkage of amino acids in a protein molecule.

Amino acids have a carboxyl group -C00H and an amino group -NH₂.

Simplest structure is glycine    NH_2.CH₂.COOH
Next would be alanine CH₃.NH₂.CH.COOH

There are 20 + amino acids and can be divided into essential and non-essential amino acids.  Non-essential amino acids we can make but essential amino acids must be part of our dietary intake. These differ somewhat between animal species but can be classified as below:
 

ESSENTIAL	NON-ESSENTIAL
Arginine	Alanine
Histidine	Aspartic Acid
Isoleucine	Citrulline
Leucine	Cystine
Lysine	Glutamic acid
Methionine	Glycine
Phenylalanine	Proline
Threonine	Hydroxyproline
Tryptophan	Serine
Valine	Tyrosine

Proteins play key role as:

Body components (blood, muscle etc.)
Catalysts of chemical reactions (enzymes)
Direct biochemical effects on metabolism, synthesis and immunity -  peptides, hormones, antibodies
Can be an energy source

Vitamins
Required in small amounts for metabolism and health. Can be classified as fat soluble and water soluble.

Fat soluble - vitamins A,D,E and K

Water soluble - B1, B2, B6, B12, niacin, C. etc.

Examples of functions of vitamins.
A- vision, bone growth, epithelial growth
D- bone growth (we can manufacture vitamin D if exposed to sunlight)
E - membrane structure, antioxidant, immune functions
K- blood clotting
B1- carbohydrate and fat metabolism
B2- energy metabolism and  normal growth
Niacin - co-enyme for energy metabolism

Minerals

Some mineral are required in large amounts macrominerals (Ca,P,Na,K,Mg,S,Cl). These usually have major structural or electrolytic roles or metabolism.
Others in low amounts microminerals (B,Co,Cr,F,Fe,Mn, SE,Si  etc.) involved as co-enzymes, components of proteins.
 

THE DIGESTIVE SYSTEM

Animals have systems that are designed for:

Digestion: the chemical and physical processes by which food is broken down, releasing the components that can then be used for energy generation and tissue building

Absorbtion: the process by which nutrients are taken from the gastrointestinal tract into the blood system so that they can be utilized

Different domestic species utilize foods with differing chemical compositions with differing efficiencies. For example the horse and cow are herbivores relying on plant foods for their nutrition. The cat is a carnivore which eats meat while the pig and chicken are omnivores i.e. they eat meat and plant material. Most domestic herbivores are ruminants (cow, sheep, goat etc.). The horse is a non-ruminant herbivore.

Herbivores

A basic difference between herbivores and the other animals is that herbivores have efficient mechanisms to digest cellulose. Cellulose is a major component of plant cells. It consists of a polymer in which glucose units are linked together. However, there are no animal enzymes that breakdown cellulose. The enzymes that breakdown cellulose are called cellulases and are produced by micro-organism such as bacteria, protozoa, and fungi. The herbivores have found a way to utilize these micro-organisms to develop an essential ecological niche. By having specialized parts of their gut containing large populations of micro-organisms they can use the microbial cellulase enzymes to digest the cellulose very efficiently.

Why can animals digest starch but not cellulose?

Both cellulose and starches consist of glucose units linked together. However the linkages between the glucose units are different. In starch there is an alpha 1-4 linkage that an enzyme called amylase can split to the component sugars. In cellulose a beta 1-4 linkage is present because the alternate sugar molecules are rotated 180 degrees. Animal enzymes cannot break this linkage. As mentioned above herbivores have developed systems to be particularly efficient at digesting plants but all animals have micro-organisms with cellulases in their gastrointestinal tract so have some capacity to digest cellulose.
 
 

Digestive systems of different domestic animals have basically similar components. Four examples are discussed below - a pig, avian system, ruminant and horse. All, except the ruminant are single stomached or monogastric.
 
 

MONOGASTRIC DIGESTIVE SYSTEM e.g. the pig

 Mouth - teeth are used to reduce sizes of food particles, salivary glands to provide lubrication and aid swallowing. Saliva also contains an enzyme amylase that digests starch to sugar (evolutionary function to help identify useful and avoid dangerous foods?). Food then passes down esophagus into

 Stomach - a hydrochloric acid bath. The acid kills bacteria and aids the digestion of food. The stomach has a unique mucous layer to protect it from self digestion by the acid. The stomach also contains a proteinase (pepsin) which begins digestion of proteins to polypeptides etc. Leaves stomach and enters duodenum. In young mammals another enzyme, rennin, is also present which aids clotting and digestion of milk proteins.

Duodenum - first part of small intestine into which pancreatic juice and bile are secreted. Bile helps emulsify fat (break it into smaller droplets) making its digestion easier. Pancreatic juices contains several enzymes trypsin, chymotrypsin which acts on proteins and polypeptides, amylase which breaks down starch, steapsin which breaks down fats and collagenase which digests collagen.

Small intestine - the lining is covered with small finger-like protuberances or villi which increase the surface area for nutrient absorbtion. Enzymes secreted by the intestine wall include peptidase, sucrase and maltase. Molecules of the food nutrients such as glucose, amino acids, fatty acids etc can be absorbed here and passed into blood system.

Large intestine (cecum and colon) - large populations of bacteria are present which provide some cellulase digesting activity. The cecum is a blind ending sac which is very small in most monogastrics. In humans it is referred to as the appendix. The cecum contains microorganisms which perform some digestion of plant material. The major function of the large intestine is water absorption. The microorganisms present also manufacture some essential vitamins (B vitamins and vitamin K) for use by the host animal.

 Food has what is described as a moderate speed of passage through the gut, usually 24-48hrs
 
 

AVIAN DIGESTIVE SYSTEM e.g. the chicken

 The avian gut differs from the example above by having some different structures in the fore gut. These are:

Crop - secretes mucus to lubricate and aid swallowing

Gizzard - Birds have no teeth therefore require a different means of grinding up food. The gizzard is a muscular organ in which small pieces of stone and grit pecked up by the bird are used to grind the food by muscular contractions.

Proventriculus - small stomach containing gastric juices.

 Avian gut has small volume so that there is not excess weight which would prevent flight. Consequently it has a fast passage time of a few hours.
 
 

NON RUMINANT HERBIVORE (the horse)

 The horse is an example of a non ruminant herbivore that is able to digest cellulase by using microorganisms stored in an enlarged cecum. The digestion of cellulose in the cecum is an anaerobic process (i.e. it takes place without air) similar to that described for the ruminant below. Because the cellulose digestion is occurring in the hind gut the large intestine and colon are modified so that absorption of the nutrients generated can occur. In other animals the hind gut basically absorbs moisture to reduce moisture loss in feces. This digestion process is less effective that ruminant digestion and therefore horses need generally rather high quality forages than ruminants. Horses will practice coprophagy (feces eating) because the feces formed from food passing through the gut the first time contains a large amount of undigested fiber. Conversely, because there is no rumen, they do not suffer the same problems of gut fill with poor quality forages that the ruminant does.
 
 

RUMINANT DIGESTIVE SYSTEM e.g. the cow
 
 

Ruminants have complex and very specialized digestive systems. Ruminants are polygastric ( many stomached). The cow has four stomachs. These are called the abomasum, the rumen, the reticulum and omasum. Most cellulose digestion occurs in the rumen, a large fluid filled sac, containing with micro-organisms. The rumen is anaerobic (without oxygen). The true stomach, analogous to the monogastric stomach (i.e. acid containing etc.), is the abomasum.

Forages are chewed in the mouth and then swallowed and pass into the rumen. Cellulose digestion begins in rumen but is a relatively slow process. To speed up digestion the the ruminant will regurgitate the partly digested forages back up to the mouth and chew them again. This makes the particles smaller still, increasing digestibility. This is called RUMINATION and takes place for around 8 hours daily in the cow.

In a monogastric animal energy is usually generated by the breakdown of carbohydrates into CO₂ and H₂O with the release of energy. This energy is stored in the form of ATP. This is analogous to burning wood or paper and therfore requires oxygen. The rumen contains no oxygen so this reaction cannot occur. Consequently the micro-organisms use an energetically less efficient but gradual means of energy release in which they produce volatile fatty acid (VFAs). The ruminant then uses these VFAs as energy source. There are three main VFAs produced in the rumen and used for energy by the cow. These are:

• acetic acid CH₃.COOH;
• propionic acid CH₃.CH₂.C00H;
• butyric acid CH₃.CH₂.CH₂.COOH.

It is important for the ruminant to maintain a diet with appropriate amount of fiber as forages. If the ruminant is given high energy feed, without a high fiber content, rumination stops. The readily digested high energy feed and the lack of saliva associated with less rumination will lead to rapid acid production in rumen. This is termed rumen acidosis and when it occurs the microorganisms in the rumen die and so may the ruminant. At the other extreme if the feed is too high in fiber without adequate digestibility e.g. wood it will take the rumen will take too long too digest. The rumen becomes blocked with the indigestible material and the animal starves. This is a common occurrence in wild ruminants such as deer in hard winters.

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