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- Herausgeber: John Wiley & Sons
- Kategorie: Wissenschaft und neue Technologien
- Sprache: Englisch
From genetics to functional anatomy, cell biology to the equine digestive system, Equine Science, Third Edition covers all the essential scientific knowledge you need for your equine programme. Thoroughly updated, this new edition features a clear, systematic presentation, stunning full-colour photographs and illustrations, chapter summary points and self-assessment questions throughout. * Describes the structure and function of the various body systems of the horse * Explains the scientific rationale behind modern equine training practices * Features new chapters on exercise physiology and the evolution of the horse * Reflects the latest scientific advances and changes in the student curriculum * Includes new information on circadian rhythms and sleep patterns, the immune system, and hindgut microbiology. A powerful teaching and learning aid, Equine Science, Third Edition is an essential text for students on higher education equine studies and equine science programmes, as well as those studying for BHS qualifications up to BHSII Stage 4 Horse Knowledge and Care.
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Table of Contents
Cover
Title Page
Copyright
Dedication
Preface
Acknowledgement
Chapter 1: The Biochemical Nature of Cells
Metabolism
Water
Proteins
Carbohydrates
Lipids/Fats
Nucleic Acids
Protein Synthesis
The Genetic Code
Enzymes
Summary Points
Q + A
Chapter 2: Cells, Tissues and Organs
Cells – Building Blocks of Life
Tissues and Organs
Summary Points
Q + A
Chapter 3: Equine Support and Movement
Points of the Horse
The Skeletal System
Bone
The Skeleton
The Mechanics of Movement
Muscles
The Importance of Muscle Fibres in Equine Performance
Fatigue
Summary Points
Q + A
Chapter 4: The Lower Limb
Tendons and Ligaments of the Lower Limb
Blood Supply to the Lower Leg
The Hoof
The Balanced Foot
Summary Points
Q + A
Chapter 5: The Digestive System
Foregut
Hindgut
Equine Microbiota
The Gut and the Immune System
Summary Points
Q + A
Chapter 6: The Respiratory System
Anatomy
Physiology of Respiration
Respiratory–Locomotor Coupling
External Respiration or Pulmonary Gas Exchange
Internal Respiration or Systemic Gas Exchange
Cellular Respiration
Summary Points
Q + A
Chapter 7: The Circulatory System
Foetal Circulation
The Heart
Blood Vessels
Heart Evaluation and Examination
Blood
The Lymphatic or Lymph System
The Spleen
Summary Points
Q + A
Chapter 8: The Nervous System
Nerves and Neurons
Neuroglia or Glial Cells
Organisation of the Nervous System
Action Potential
Resting Membrane Potential
Synapses
Neurotransmitters
Endorphins and Enkephalins
Neuromuscular Junctions
The Brain
Spinal Cord
Reflex Actions or Arcs
Summary Points
Q + A
Chapter 9: The Endocrine System
Hypothalamus
Pituitary Gland (Hypophysis)
Thyroid Gland
Parathyroid Glands
Adrenal Glands
Pancreas
Thymus
Ovaries
Testes
Pineal Gland
The Neuroendocrine System
Circadian Rhythms in Horses
Sleep Patterns in Horses
Summary Points
Q + A
Chapter 10: The Skin
Structure of the Skin
Sensation
Melanin
Sudoriferous Glands (Sweat Glands)
Sebaceous Glands
Hair
Thermoregulation
Skin and Coat Colour
Summary Points
Q + A
Chapter 11: The Senses
Transduction
Adaptation
Somatic Receptors
Thermal Sensations
Pain Sensations
Tactile Sensations
Itch Sensation
Proprioceptor Sensation
Special Senses
Summary Points
Q + A
Chapter 12: Reproduction
Reproductive Anatomy of the Mare
The Oestrus Cycle
Reproductive Anatomy of the Stallion
Spermatogenesis
Acrosome Reaction
Endocrine Pathways in the Male
Fertilisation
Pregnancy Diagnosis
Foetal Sexing
Twins
Endocrine Maintenance of Gestation
Equine Chorionic Gonadotropin
Progesterone
Oestrogens
Relaxin
Gestation
Implantation and Placentation
Embryology
The Foetal Endocrine System
Preparation for Parturition (Birth)
Induction
Lactation
Applied Reproductive Technologies
Summary Points
Q + A
Chapter 13: Genetics
The Genetic Code or Genome
Chromosomes
Gene Expression
Mitochondrial DNA
The Y Chromosome
Alleles
Dominance
Sex Cells
Hybrids
Heredity
Sex Determination
Genotype and Phenotype
Polygenic or Multiple Gene Traits
Multiple Alleles
Sex Linkage
Lethal Genes
Congenital Curly Coat Syndrome
Epigenetics
Muscle Disorders
Nuclear Transfer (Cloning)
Parental Similarity of Clones
Mutation
Single Nucleotide Polymorphisms
The Myostatin Gene and Performance
Coat Colour and Genetics
Melanomas in Grey Horses
Exercise-Induced Pulmonary Haemorrhage (EIPH) or Epistaxis
Summary Points
Q+A
Chapter 14: The Urinary System
Kidneys
Regulation of Water
Regulation of Permeability of the Collecting Ducts by ADH
Aldosterone
Acid–Base Balance
Micturition or Urination
Summary Points
Q + A
Chapter 15: The Immune System
Health Versus Disease
Microbes
Disease Transmission Routes
Infection
Biofilms
Symptoms of Disease
Diagnosis
Pathogenic Organisms
Protection from Disease
Antigens and Antibodies
Summary Points
Q + A
Chapter 16: Exercise Physiology, Functional Anatomy and Conformation
Exercise Physiology
Functional Anatomy
Conformation
Summary Points
Q + A
Chapter 17: Teeth and Ageing
Equine Teeth
Wear and Tear
Ageing
Care of Equine Teeth
Summary Points
Q + A
Chapter 18: Evolution, Classification and Behaviour of the Horse
Evolutionary Time Period
Classification
Evolutionary Development
The Evolutionary Family Tree
Domestic Breeds and Types of Horses
Donkeys
Przewalski's Horse
Mustangs
Behaviour of the Modern Horse
Summary Points
Q + A
Appendix A: Anatomical Terms Based on the Median Plane
Appendix B: Haematology and Plasma Biochemistry Tests
Appendix C: Functions, Sources and Deficiencies of Vitamins and Minerals in Horses
Index
End User License Agreement
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Guide
Cover
Table of Contents
Preface
Begin Reading
List of Illustrations
Chapter 1: The Biochemical Nature of Cells
Figure 1.1 The biochemical nature of cells.
Figure 1.2 Structure of water.
Figure 1.3 Importance of proteins in the horse's body.
Figure 1.4 Structure of an amino acid.
Figure 1.5 R groups give different chemical properties to amino acids.
Figure 1.6 Formation of a dipeptide.
Figure 1.7 Bonding examples in the tertiary structure of proteins.
Figure 1.8 Model of haemoglobin.
Figure 1.9 Structure of collagen.
Figure 1.10 Alpha and beta structure of glucose.
Figure 1.11 Structure of maltose from two alpha glucose molecules.
Figure 1.12 Structure of cellulose.
Figure 1.13 Structure of glycogen.
Figure 1.14 Structure of triglycerides.
Figure 1.15 Phospholipids are the main component of all biological membranes, including cells and their organelles, and are similar to triglycerides but consist of a glycerol molecule with two fatty acid chains instead of three and a phosphate group attached (a). The phosphate group at one end is attracted to water (hydrophilic), whereas the fatty acid tail is repelled by water (hydrophobic). The hydrophobic end therefore turns inward in the phospholipid bilayer (b) and the hydrophilic phosphate end turns outward.
Figure 1.16 Structure of a nucleotide.
Figure 1.17 Symbolic structure of DNA and RNA.
Figure 1.18 Semi-conservative replication of DNA.
Figure 1.19 Transcription.
Figure 1.20 Information in a gene on the coding strand is read in the direction from the 5′ end to the 3′ end.
Figure 1.21 Transfer of the genetic code from DNA to RNA.
Figure 1.22 Translation takes place on the ribosomes.
Figure 1.23 Induced fit model of enzyme action.
Chapter 2: Cells, Tissues and Organs
Figure 2.1 Structure of a cell.
Figure 2.2 Cytoskeleton of the cell.
Figure 2.3 The fluid mosaic model of cell membranes.
Figure 2.4 Phospholipid bilayer arrangement.
Figure 2.5 Three types of endocytosis.
Figure 2.6 Exocytosis.
Figure 2.7 Mitochondrion showing location of different enzymes involved in cellular respiration.
Figure 2.8 Four stages of the cell cycle.
Figure 2.9 (a) A G-banded metaphase and (b) karyotype of a normal female horse (2n = 64XX).
Figure 2.10 Cell division: mitosis and cytokinesis.
Figure 2.11 Diagram following a pair of homologous chromosomes through one mitotic division.
Figure 2.12 Meiosis.
Figure 2.13 Cell signalling pathways.
Figure 2.14 Production of blood cells from stem cells.
Figure 2.15 Classification of epithelium.
Figure 2.16 Columnar epithelium.
Figure 2.17 Types of connective tissue.
Figure 2.18 A fresh equine wound.
Figure 2.19 Inflammatory response to injury.
Figure 2.20 A colony of embryonic stem cells, from the H9 cell line (NIH code: WA09). Viewed at 10× magnification with a Carl Zeiss Axiovert scope. (The cells in the background are mouse fibroblast cells. Only the colony in the centre is human embryonic stem cells.)
Chapter 3: Equine Support and Movement
Figure 3.1 Points of the horse.
Figure 3.2 Haversian system or osteon structure.
Figure 3.3 Gross structure of a long bone.
Figure 3.4 Cross-section through a long bone.
Figure 3.5 A splint.
Figure 3.6 Different types of bone cell.
Figure 3.7 Growth and development of a long bone. (a) How cartilage is turned into bone. (b) How the bone enlarges in length and width as the young horse grows.
Figure 3.8 Weanling with physitis of the forelimb fetlock.
Figure 3.9 The equine skeleton.
Figure 3.10 Vertebrae of the horse.
Figure 3.11 The spine is a series of curves.
Figure 3.12 Dipped spine in a mare.
Figure 3.13 Forelimb and knee.
Figure 3.14 Hindlimb and hock.
Figure 3.15 The pelvis.
Figure 3.16 Basic structure of a joint.
Figure 3.17 Cardiac muscle.
Figure 3.18 Smooth muscle.
Figure 3.19 A skeletal muscle fibre.
Figure 3.20 (a) Contractile units in a myofibril in human muscle.
Figure 3.21 Superficial muscles.
Figure 3.22 Deep muscles.
Chapter 4: The Lower Limb
Figure 4.1 The modern horse stands on the equivalent of the human middle finger.
Figure 4.2 Tendons and ligaments of the lower limb.
Figure 4.3 Tendon structure.
Figure 4.4 Arterial supply to the lower limb. a, Artery; v, vein; n, nerve.
Figure 4.5 Medial (a) and lateral (b) aspects of the distal metacarpus, fetlock and digit, showing distribution of major nerves of the lower limb. a, Artery; v, vein; n, nerve.
Figure 4.6 Internal hoof structures.
Figure 4.7 Anticoncussive structures of the hoof.
Figure 4.8 External structures of the hoof. (a) Front view; (b) lateral view.
Figure 4.9 (a) Part removal of hoof wall, showing epidermal laminae underneath. (b) Hoof following complete removal of hoof wall, showing the corium. (c) Photomicrograph cross-section of the equine hoof.
Figure 4.10 Structures of the underside of the external hoof.
Figure 4.11 Internal surface of the sole and frog.
Figure 4.12 This Pacinian corpuscle has a connective tissue receptor surrounding the terminus of the primary afferent neuron.
Figure 4.13 A vertical line drawn through the centre of the cannon bone and down through the hoof should bisect the hoof equally.
Figure 4.14 A line running along the top of the coronary band should be horizontal, that is, the same distance from the ground on both sides of the hoof.
Figure 4.15 Foot balance (solar view).
Figure 4.16 The hoof pastern axis (HPA) should always be aligned.
Figure 4.17 An unbalanced foot trimmed to remove flare and shod to support the underrun side.
Chapter 5: The Digestive System
Figure 5.1 Digestive system of the horse compared to ruminants.
Figure 5.2 Structure of the equine digestive system.
Figure 5.3 Structure of the equine digestive system as seen from beneath.
Figure 5.4 Horses can graze pastures closely using their incisors.
Figure 5.5 Position of the three pairs of salivary glands.
Figure 5.6 Structure of the equine stomach.
Figure 5.7 Gastric pit showing origins of stomach secretions.
Figure 5.8 Post-mortem image of a spontaneous gastric rupture secondary to gastric impaction in an 18-year-old Quarter-horse mare. The site of rupture is along the greater curvature, with marked reddening and haemorrhage evident from the site of perforation.
Figure 5.9 Internal anatomy of the jejunum.
Figure 5.10 Post-mortem image of a small intestinal volvulus from a 2-month-old colt. Note the extensive jejunal dilation and poor viability resulting from this strangulating obstruction.
Figure 5.11 Section of pancreatic tissue.
Figure 5.12 A liver lobule.
Figure 5.13 (a) Schematic illustration identifying portions of the gastrointestinal tract accessible from the ventral midline incision. DF, diaphragmatic flexure; LDC, left dorsal colon; LVC, left ventral colon; MES, mesentery of the jejunum with arcuate vessels; PF, pelvic flexure; RVC, right ventral colon. (b) Photograph of exteriorised intestine. The surgeon is standing on the left side of the intestine.
Figure 5.14 (a) Schematic illustration depicting caecal orientation. Lighter-colored parts of the intestine are able to be exteriorised. CCL, Caecocolic ligament; CCO, caecocolic orifice; ICF, ileocaecal fold; ICO, Ileocaecal orifice. (b) Photograph of exteriorised caecum showing the apex, body and caecal bands. Note that the caecum has been reflected backwards towards the caudal aspect of the horse and the ventral caecal band is not visible.
Chapter 6: The Respiratory System
Figure 6.1 Airways of the head.
Figure 6.2 Function of the pharynx. (a) Swallowing and (b) breathing.
Figure 6.3 Relationship between the stomach, diaphragm and lungs.
Figure 6.4 Structure of the lungs.
Figure 6.5 Lung air volumes and capacities.
Figure 6.6 Position of the lungs externally.
Figure 6.7 Synchronisation of stride and breathing.
Figure 6.8 Composition of gases in inhaled and exhaled air.
Figure 6.9 Summary of erythrocyte chemistry related to carriage of respiratory gases.
Figure 6.10 Oxygen dissociation curve.
Figure 6.11 Structure of ATP.
Figure 6.12 Hydrolysis of ATP.
Figure 6.13 Summary of aerobic respiration of glucose.
Figure 6.14 The ten steps of glycolysis.
Figure 6.15 The link reaction.
Figure 6.16 The Krebs, TCA or citric acid cycle.
Figure 6.17 Summary of ATP production during cellular respiration.
Figure 6.18 Aerobic respiration using lipids, glycogen and protein.
Chapter 7: The Circulatory System
Figure 7.1 The heart and circulation of the horse.
Figure 7.2 External structure of the heart.
Figure 7.3 Toxic mucous membranes as demonstrated by hyperaemia, with increased discolouration noted at the gum line.
Figure 7.4 Vertical section through the equine heart, showing the flow of blood through it. LA, Left atrium; LV, left ventricle; RA, right atrium; RV, right ventricle.
Figure 7.5 Movement of blood through the circulatory system.
Figure 7.6 The natural pacemaker of the heart.
Figure 7.7 The cardiac cycle.
Figure 7.8 Cross-section of a human artery.
Figure 7.9 Photomicrograph showing red blood cells squeezing through a blood capillary.
Figure 7.10 Capillary network and direction of blood through it.
Figure 7.11 Photomicrograph of a venule with a vein valve in cross-section.
Figure 7.12 Relationship of pressure in the ventricles to the ECG during the cardiac cycle.
Figure 7.13 (a) Summary of blood constituents. (b) Origin of blood cells.
Figure 7.14 Red blood cells.
Source
: Blaus, https://commons.wikimedia.org/wiki/File:Blausen_0761_RedBloodCells.png. CC BY 3.0.
Figure 7.15 Summary of the transport of oxygen and carbon dioxide.
Figure 7.16 White blood cells.
Source
: Blaus, https://commons.wikimedia.org/wiki/File:Blausen_0425_Formed_Elements.png. CC BY 3.0.
Figure 7.17 Blood clot formation.
Figure 7.18 Simplified drawing of a lymph node.
Figure 7.19 Pony with strangles, viewed from the side before abscesses burst.
Chapter 8: The Nervous System
Figure 8.1 Cranial nerves of the horse.
Figure 8.2 Major nerves of the horse.
Figure 8.3 Summary of actions of the nervous system.
Figure 8.4 (a) Structure of a motor neuron. (b) Structure of a sensory neuron.
Figure 8.5 Types of neuron.
Figure 8.6 Transverse section through the spinal cord.
Figure 8.7 Organisation of the nervous system.
Figure 8.8 The sodium–potassium pump.
Figure 8.9 Propagation of an action potential along a myelinated nerve fibre.
Figure 8.10 Basic structure of a synapse.
Figure 8.11 Structure of the equine brain.
Figure 8.12 Reflex arc.
Chapter 9: The Endocrine System
Figure 9.1 The endocrine glands.
Figure 9.2 The anterior and posterior pituitary gland.
Source
: Southwood 2013. Reproduced with permission of John Wiley & Sons.
Figure 9.3 Delayed/partial shedding of winter coat is a symptom of PPID (Cushing's syndrome).
Figure 9.4 Section through the thyroid glad.
Figure 9.5 Thyroid follicle cells.
Figure 9.6 Diagrammatic section through the adrenal gland.
Figure 9.7 Section through the medulla of an adrenal gland (human). The pointer is indicating the medulla.
Source
: Jpogi, https://commons.wikimedia.org/wiki/File:Adrenal_gland_(medulla).JPG.
Figure 9.8 Effects of negative feedback on the hypothalamus and pituitary gland.
Figure 9.9 Negative feedback of glucose production by glucagon.
Figure 9.10 Flehmen response shown by an Andalusian stallion.
Figure 9.11 The Equilume™ Light Mask is an innovative method of providing light to horses. The automated mask provides the optimum level of blue light to a single eye.
Figure 9.12 Foals in (a) lateral recumbency, (b) sternal recumbency and (c) paradoxical sleep.
Source
: Courtesy of Harthill Stud.
Chapter 10: The Skin
Figure 10.1 Structure of the skin.
Figure 10.2 Layers of the epidermis – photomicrograph of a portion of skin.
Figure 10.3 Horse sweating freely.
Figure 10.4 A donkey shedding its winter coat, revealing the shorter summer coat underneath.
Figure 10.5 Heat exchange between horses and their environment.
Figure 10.6 The hypothalamus controls thermoregulation (homeostasis).
Chapter 11: The Senses
Figure 11.1 The horse's tongue contains a high number of chemoreceptors.
Figure 11.2 Sensory hairs on the muzzle.
Figure 11.3 Pacinian corpuscle.
Figure 11.4 The horse's field of vision.
Figure 11.5 The eye of the horse.
Figure 11.6 Longitudinal section through the human eye.
Figure 11.7 Structure of rods and cones.
Figure 11.8 Retina under high magnification.
Figure 11.9 Structure of the retina.
Figure 11.10 Horse listening to distant sounds.
Figure 11.11 Anatomy of the horse's ear.
Figure 11.12 (a) Inner ear and (b) cochlea straightened out.
Figure 11.13 Structure of a taste bud.
Figure 11.14 The olfactory system. The view is of the equine head in sagittal section. Inset shows a microscopic view of the olfactory epithelium, which covers the ethmoid bone within the caudodorsal nasal cavity. a, Nasal cavity; b, palate; c, oral cavity; d, larynx; e, pharynx; f, vomeronasal organ; g, olfactory bulb of the brain.
Chapter 12: Reproduction
Figure 12.1 The artificial breeding season of TB mares.
Figure 12.2 The reproductive system of the mare.
Figure 12.3 External genitalia of the mare.
Figure 12.4 Anatomy of the equine uterus.
Source
: Frandson 2009. Reproduced with permission of john Wiley & Sons.
Figure 12.5 Section through an ovary.
Figure 12.6 The oestrus cycle of the mare.
Figure 12.7 Diagrammatic representation of the oestrus cycle.
Figure 12.8 Gametogenesis – spermatogenesis and oogenesis.
Figure 12.9 Mare in season.
Figure 12.10 Teasing the mare.
Figure 12.11 The Equilume™ Light Mask is an innovative method of providing light to horses to encourage the early onset of oestrus, following the winter anoestrus period.
Figure 12.12 Stallion genitalia. This Figure was produced with kind permission of the copyright holder Wiley-Blackwell from Angus O. McKinnon, Edwards L. Squires, Wendy E. Vaala and Dickson D. Varner (2011) Equine Reproduction, Vols 1 & 2, 2nd edn. Oxford: Wiley-Blackwell.
Figure 12.13 Cross-section through a testis.
Figure 12.14 (a) Section through a seminiferous tubule. (b) Light microscopic view of a cross-section of an equine seminiferous tubule, showing the nuclei of some spermatogonia.
Figure 12.15 A spermatozoon.
Figure 12.16 Events immediately following fertilisation.
Figure 12.17 Day 14 sonogram of an equine embryo.
Figure 12.18 A 27.5-day-old embryo.
Figure 12.19 A 34-day-old equine foetus – forelimb and hindlimb buds protruding.
Figure 12.20 Mare giving birth: presentation of the forefeet. In a normal presentation the first visible sign is a transparent bluish-white amnion which surrounds the foal, then come the forefeet with one slightly extended in front of the other for easier passage of the shoulders.
Figure 12.21 Lactation curve.
Figure 12.22 The stallion is encouraged to mount a phantom or dummy mare.
Chapter 13: Genetics
Figure 13.1 Thoroughbred mare ‘Twilight’ – the DNA donor for the sequencing project.
Figure 13.2 A karyotype of a mare with 63XO (Turners Syndrome) resulting in infertility.
Figure 13.3 Terms used to describe genes and chromosomes.
Figure 13.4 Grévy's zebra.
Figure 13.5 A typical ‘curly coated’ yearling.
Figure 13.6 The four basic types of chromosome mutation.
Figure 13.7 (a) Grey mare with ‘bay’ foal with grey around the eyes (b).
Figure 13.8 Deaf American Paint Horse with frame splashed with white overo phenotype.
Figure 13.9 Palomino foal with deep pink skin.
Figure 13.10 Buckskin Quarter-horse.
Chapter 14: The Urinary System
Figure 14.1 Excretory products of the horse.
Figure 14.2 Position of the urinary organs.
Figure 14.3 Dorsal view of the urinary system.
Figure 14.4 Section through a kidney.
Figure 14.5 Renal papillae fused into the renal crest.
Figure 14.6 Structure of a nephron.
Figure 14.7 Malpighian corpuscle showing blood passing from glomerular capillaries to the tubules.
Figure 14.8 Glomerulum of mouse kidney under a scanning electron microscope, magnification 1000×.
Figure 14.9 Control of water balance.
Figure 14.10 The loop of Henle countercurrent multiplier system.
Chapter 15: The Immune System
Figure 15.1 A generalised bacteria cell.
Figure 15.2 Bacterial cell division. Colourised electron micrograph of a
Salmonella
bacterium dividing.
Figure 15.3 West Nile virus transmission cycle.
Figure 15.4 Structure of a simple virus.
Figure 15.5 Viral replication within the host cell.
Figure 15.6 Summary of the equine immune system.
Figure 15.7 Summary of physical and chemical barriers against harmful organisms.
Figure 15.8 Comparison of innate and acquired immune responses.
Figure 15.9 Summary of inflammation.
Figure 15.10 Interferon interferes with replication of viruses.
Figure 15.11 A 1975 transmission electron micrograph (TEM) revealing the presence of a number of eastern equine encephalitis (EEE) virus virions in a specimen of central nervous system tissue.
Figure 15.12 Training of T cells in the thymus.
Chapter 16: Exercise Physiology, Functional Anatomy and Conformation
Figure 16.1 Racehorse at full gallop.
Figure 16.2 The ATP/CP system. (The energy continuum.)
Figure 16.3 Use of glucose in a working muscle cell.
Figure 16.4 Fate of lactate build-up in working muscles.
Figure 16.5 Training causes increased capillarisation of muscle fibres. PPAR, Peroxisome proliferator-activated receptor gamma; NRF-1, nuclear respiratory factor 1.
Figure 16.6 Horses involved in endurance competitions need extensive training.
Figure 16.7 Training the racehorse.
Figure 16.8 The horse's centre of gravity.
Figure 16.9 The horse's body is slung in a cradle of muscle known as the serratus ventralis.
Figure 16.10 The stay apparatus.
Figure 16.11 Reciprocal apparatus or mechanism of the hindlimb.
Figure 16.12 There should be room at the poll and jaw for flexion.
Figure 16.13 Conformation of the neck.
Figure 16.14 Angulation of the scapula and humerus.
Figure 16.15 Forelimbs viewed from the front (above) and side (below).
Figure 16.16 Poor conformation/farriery can cause horses to be unsound and more prone to injuries such as broken knees and bowed tendons.
Figure 16.17 Using the head as a standard measurement.
Figure 16.18 Hindquarters and leg viewed from behind (above) and side (below).
Chapter 17: Teeth and Ageing
Figure 17.1 Dentition of the horse.
Figure 17.2 Incisors or biting teeth of a Mediterranean Miniature donkey.
Figure 17.3 Hypsodont teeth.
Figure 17.4 Vertical section through a molar tooth.
Figure 17.5 Incisor with part of the cementum removed.
Figure 17.6 Sections through an incisor tooth. As the tooth wears down, the pattern on the tooth table changes.
Figure 17.7 Implantation and continual eruption of the teeth.
Figure 17.8 (a) Upper and (b) lower jaws, showing the wearing surface of molars.
Figure 17.9 Incisor teeth sloping in an older horse.
Figure 17.10 Ageing the horse.
Figure 17.11 The upper jaw is slightly wider than the lower jaw and sharp edges may result.
Figure 17.12 Rasping the teeth with manual instruments.
Chapter 18: Evolution, Classification and Behaviour of the Horse
Figure 18.1 Przewalski's wild horse from Colwyn Bay Zoo.
Figure 18.2 Herd of Przewalski's horses reintroduced into the wild in Mongolia.
Source
: Karamollaoglu, https://www.flickr.com/photos/okaramollaoglu/8367788369/. CC BY 2.0.
Figure 18.4 Grevy's zebra.
Figure 18.5 Evolutionary pathway of the horse.
Figure 18.6 Equine evolution. Composed from skeletons of Staatliches Museum für Naturkunde Karlsruhe, Germany. From left to right: size development, biometrical changes in the cranium, reduction of toes (left forefoot). Highest point of the withers:
Equus
1.5 m;
Pliohippus
1.2 m;
Merychippus
0.8 m;
Mesohippus
0.5 m.
Figure 18.7 Changes to the forelimb through evolution.
Figure 18.8 Mediterranean Miniature donkey.
Figure 18.9 Poitou donkeys in the nature reserve of Olfen, Nordrhein Westfalen, Germany. Dreadlocks are a characteristic of this breed.
Figure 18.10 Youngstock turned out in groups are happier than on their own.
List of Tables
Chapter 1: The Biochemical Nature of Cells
Table 1.1 Examples of organic and inorganic compounds in the horse's body
Table 1.2 Essential and non-essential amino acids
Table 1.3 Structural classification of proteins
Table 1.4 Common carbohydrates – disaccharides, monosaccharides and polysaccharides
Table 1.5 Some common fatty acids
Table 1.6 Comparison of RNA and DNA
Table 1.7 The genetic code – mRNA/amino acids (64 codons)
Table 1.8 Amino acids and their codons
Chapter 2: Cells, Tissues and Organs
Table 2.1 Summary of the similarities and differences between eukaryotic and prokaryotic cells
Table 2.2 Classification of lining epithelia
Table 2.3 Functions of connective tissue
Chapter 3: Equine Support and Movement
Table 3.1 Muscles of the neck and shoulder.
Table 3.2 Muscles of the trunk
Table 3.3 Muscles of the upper hindlimb
Table 3.4 Slow twitch versus fast twitch muscle
Table 3.5 Slow twitch fibres in different breeds
Chapter 5: The Digestive System
Table 5.1 Digestive enzymes: origins, substrates and end products.
Table 5.2 Distribution of bacteria and protozoa in the horse's gut
Table 5.3 Microbiota – examples of functions of some hindgut bacteria
Chapter 6: The Respiratory System
Table 6.1 Respiratory rate or respiratory frequency (fR) (breaths per minute) of various domestic animals and man
Table 6.2 Some factors affecting the lung volumes of equines
Table 6.3 Total ATP produced from oxidative phosphorylation in aerobic respiration
Chapter 7: The Circulatory System
Table 7.1 Components of plasma
Chapter 8: The Nervous System
Table 8.1 Examples of parasympathetic and sympathetic functions
Chapter 9: The Endocrine System
Table 9.1 Classification of major hormones
Table 9.2 Summary of major hormones in the horse's body, their sites of production and effects.
Chapter 10: The Skin
Table 10.1 Composition of sweat
Table 10.2 Heat loss ability at different effective temperatures
Chapter 12: Reproduction
Table 12.1 Main derivatives of embryonic germ layers
Table 12.2 Composition (%) of milk from various species
Chapter 13: Genetics
Table 13.1 Genetic disease and allelic variants identified by DNA testing
Table 13.2 Common coat colours and dilutions with their genetic formulae
Chapter 15: The Immune System
Table 15.1 Species of bacteria found in the horse's normal flora
Table 15.2 Some equine bacterial infectious diseases
Table 15.3 Bacterial species that may be found in equine wounds
Table 15.4 Common virus families and examples of diseases they cause
Table 15.5 Influenza virus A – equine influenza
Table 15.6 Common mycotoxins in equine feeds
Table 15.7 Summary of adaptive/acquired immunity in horses
Table 15.8 Class I and II MHC glycoproteins have different properties
Chapter 16: Exercise Physiology, Functional Anatomy and Conformation
Table 16.1 Energy production systems available for horses and their timings
Table 16.2 Description of conformation
Chapter 17: Teeth and Ageing
Table 17.1 Eruption of the horse's teeth
Chapter 18: Evolution, Classification and Behaviour of the Horse
Table 18.1 Timeline of the appearance of different life forms
Equine Science
Zoe Davies
Third Edition
This edition first published 2018
© Zoe Davies and Sarah Pilliner
Edition History
This edition first published 2018. © Zoe Davies and Sarah Pilliner
First edition published 1996 by Blackwell Publishing Ltd.
Second edition published 2004 by Blackwell Publishing Ltd.
All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means, electronic, mechanical, photocopying, recording or otherwise, except as permitted by law. Advice on how to obtain permission to reuse material from this title is available at http://www.wiley.com/go/permissions.
The right of Zoe Davies and Sarah Pilliner to be identified as the authors of Equine Science, 3ed has been asserted in accordance with law.
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Library of Congress Cataloging-in-Publication Data
Names: Davies, Zoe, author.
Title: Equine science / Zoe Davies.
Description: Third edition. | Hoboken, NJ : John Wiley & Sons, 2017. | Includes index. |
Identifiers: LCCN 2017020437 (print) | LCCN 2017035568 (ebook) | ISBN 9781118741177 (pdf) | ISBN 9781118741160 (epub) | ISBN 9781118741184 (pbk.)
Subjects: LCSH: Horses. | Horses-Health.
Classification: LCC SF285.3 (ebook) | LCC SF285.3 .P54 2017 (print) | DDC 636.1-dc23
LC record available at https://lccn.loc.gov/2017020437
Cover image: Courtesy of Eva-Maria Broomer
Cover design by Wiley
This book is dedicated to my lovely family, Ian, Sophie and Katie, and to all the horses I have owned or helped as part of my work that have inspired me to write it.
Preface
Horses are integral to our culture, and they continue to be used for work, leisure and competition throughout the world. Knowledge of this magnificent animal is continually improving as science delves deeper, uncovering new information that helps us to understand how the horse functions.
This edition of Equine Science has been rewritten and expanded to provide more detailed and up-to-date information for those studying equine courses at higher academic levels or more knowledgeable horse owners keen to understand the inner workings of horses in their care. The systems of the horse are covered extensively with new and expanded information, with full colour artwork and photographs to assist the reader in understanding its scientific content.
Knowledge of equine science for all involved with horses results in higher standards of management and welfare, particularly for those working or competing, no matter what the discipline.
Zoe Davies, MSc., R.Nutr.
Acknowledgement
I would like to thank Sarah Pilliner for her important contributions to some chapters of this new edition and Julie Musk for her editing skills and support during the production process. I would also like to thank Harthill Stud and all the other contributors for supplying many of the photographs.
Chapter 1The Biochemical Nature of Cells
Biochemistry is the study of chemicals within biology. A knowledge of biological molecules and their structure and function is essential for a full understanding of the nature, performance and behaviour of horses. There are approximately one hundred elements that exist on Earth; of these, 16 are essential for life and only four make up 95% of all living matter, namely carbon, hydrogen, oxygen and nitrogen.
The combination of carbon with other elements creates a huge variety of organic molecules, and all organic compounds therefore contain a carbon backbone. The four main classes of organic molecules are proteins, carbohydrates, lipids and nucleic acids. In addition, a relatively small number of inorganic ions such as sodium and potassium are essential for life, as components of larger molecules or extracellular fluids. Table 1.1 shows examples of organic and inorganic molecules.
Table 1.1 Examples of organic and inorganic compounds in the horse's body
Organic compounds
Inorganic compounds
Glucose (C
6
H
12
O
6
)
Water (H
2
O)
Ethane (C
2
H
6
)
Ammonia (NH
3
)
Glycine (amino acid) (C
2
H
5
NO
2
)
Carbon dioxide (CO
2
)
Cytosine (nucleotide base) (C
4
H
5
N
3
O)
Nitrate ion (NO
3–
)
Horses therefore contain water plus a huge number of macromolecules which have been built up from smaller simpler ones. These molecules are also involved in the basic structure and function of all cells (Figure 1.1). These simple building blocks are similar in all organisms, suggesting a common origin for all life forms.
Figure 1.1 The biochemical nature of cells.
Metabolism
All chemical reactions that take place within the horse are collectively known as ‘metabolism’. Metabolic reactions can be anabolic (building up large molecules from smaller ones) or catabolic (breaking down larger molecules into smaller ones). Anabolic reactions usually involve removal of water molecules and are known as condensation reactions, such as when glycogen is built up from glucose molecules. Catabolic reactions are the reverse and usually involve larger molecules being split when reacting with water. These are known as hydrolysis reactions, for example digestion of proteins in the digestive system.
Horses must have a supply of energy to fuel energy-requiring processes, such as sustaining life and movement. This is obtained from cellular respiration – the oxidation of organic molecules such as glucose into simpler molecules namely carbon dioxide and water.
Water
All life on Earth began in water and water is the main component of all organisms, including horses, providing an environment in which metabolic reactions can occur. Approximately 65–70% of the bodyweight (bwt) of the horse on a fat-free basis is made up of water and newborn foals may contain as much as 90% water. Male horses contain slightly more water than females.
Fluids are present in the body in two main compartments: inside cells and outside cells. Approximately two-thirds of water is found within cells and this is known as intracellular fluid (ICF). The remainder (one-third) is outside cells and is called extracellular fluid (ECF). From this, roughly 80% of ECF is found in interstitial fluid, that is, bathing and surrounding cells in tissues, and about 20% in blood plasma. Interstitial fluid encompasses lymph, cerebrospinal fluid, synovial fluid, and pleural, pericardial and peritoneal fluids, to name but a few.
The horse's body also naturally generates metabolic water as a result of breaking down protein, carbohydrates and fat, mostly from condensation reactions. This does not provide a large amount of water, but does contribute to the daily water balance and may change the horse's need for water. Diet will also affect water requirements. Horses grazing on pasture which has a low dry matter will sometimes drink little or no additional water compared to those on a mostly dry forage diet such as hay. Voluntary water intake by resting horses in a moderate temperature environment is roughly 25–70 ml/kg bwt/day. For a 500-kg horse this equates to 12.5–35 litres per day. Obviously this depends upon water intake such as from feed and drinking and water losses. High-protein feeds such as alfalfa will need more water to help remove excess nitrogen.
Water intake is regulated by the thirst centre in the hypothalamus. Water loss greater than gain (from intake and metabolic water) leads to dehydration. This leads to an increased osmotic pressure of body fluids and decrease in volume leading to stimulation of thirst.
Water is liquid at room temperature and many substances dissolve in it due to its weak polar nature. This means that water molecules have a weak attraction for each other, and other inorganic ions which form large numbers of weak hydrogen bonds. This gives water its unique properties, including acting as a universal solvent and a low viscosity material (Figure 1.2).
Figure 1.2 Structure of water.
The chemical behaviour of water is a result of its dipolar nature having a small negative charge on the oxygen and two small positive charges on the hydrogen atoms. This means water molecules tend to stick together, allowing water to flow, and this is ideal for transport of substances around the body.
To change state from solid to liquid to gas, water requires a substantial amount of energy, which makes water a thermally stable compound. Water evaporates when the hydrogen bonds in water are broken, allowing the surface water molecules to escape as gas and evaporate. It takes a large amount of heat energy to break the hydrogen bonds and this uses up a substantial amount of energy; water therefore has a high latent heat of evaporation. When 1 g of water changes from liquid to vapour, it takes up 580 calories of heat. To put this in perspective, heating 1 g of water from freezing to boiling requires only 117 calories. Sweating causes rapid cooling as water carries away heat energy when it evaporates from the horse's skin.
Important Properties of Water
•
High specific heat capacity – prevents rapid temperature changes and therefore creates a stable chemical environment for the horse's body.
•
High latent heat of evaporation – creates rapid cooling.
•
Good solvent – can take up minerals into the body and transport.
•
Cohesive – water molecules stick together.
•
Lubricant properties – synovial fluid in joints, pleural fluid in lungs and mucus.
•
Support – amniotic fluid supporting and protecting the growing foetus.
Proteins
Proteins play a vital role in virtually all biological processes in horses (Figure 1.3). They make up more than 50% of the dry weight of equine cells. It is also estimated that mammals have the ability to generate approximately two million different types of proteins, coded by genes, each with a specific function and shape.
Figure 1.3 Importance of proteins in the horse's body.
Proteins consist of basic units or amino acids. Plants make all the amino acids they need from smaller molecules, whereas horses (as all animals) must obtain many amino acids from their diet. These are called essential amino acids as they cannot be made in the horse's body. Others can be made in the body and are called non-essential amino acids; however, the division is not clear, as some amino acids can be used to make others and some can be converted to others via the urea cycle. Table 1.2 lists the essential and non-essential amino acids.
Table 1.2 Essential and non-essential amino acids
Essential amino acids
Non-essential amino acids
Lysine
Alanine
Methionine (contains sulphur)
Arginine
Tryptophan
Asparagine
Leucine
Aspartic acid
Isoleucine
Cysteine (contains sulphur)
Phenylalanine
Glutamine
Threonine
Glutamic acid
Valine
Glycine
Histidine
Proline
Serine
Tyrosine
Selenocysteine
Basic Structure of Amino Acids
Although there are more than 150 amino acids found in cells, only 20 commonly occur in proteins. The remaining non-protein amino acids have roles in metabolic reactions or as hormones and neurotransmitters. A new 21st amino acid has been found, named selenocysteine (SeC), which can be used to make proteins. It has properties making it very suitable in proteins that are involved in antioxidant activity. It is not universal in all organisms. Again, unlike the other amino acids, no free pool of selenocysteine exists in cells because its high reactivity would cause damage. Instead, cells store selenium in the less reactive selenide form.
All amino acids have a common structure (Figure 1.4). The R groups are varied and give the amino acids different chemical properties (Figure 1.5). Disulphide bridges in the amino acid cysteine allow cross-linkages with other cysteine molecules in a polypeptide chain. Amino acids are linked together in long chains, sometimes up to several hundred amino acid units long, to form polypeptides. Some polypeptides will be functional themselves, whereas others will be joined to other polypeptide chains before they become functional.
Figure 1.4 Structure of an amino acid.
Figure 1.5 R groups give different chemical properties to amino acids.
The simplest amino acid is glycine. All the rest are optical isomers. This means that they can result in two different arrangements of the four available bonding sites on the carbon atoms. This gives the ‘D’ and ‘L’ forms, but only the L forms (remember L for living) are found in living organisms such as horses.
Polypeptides
Polypeptide chains are chains of amino acids joined together by peptide bonds. A typical polypeptide contains 100–300 amino acids. The building of long polypeptide chains uses condensation reactions, that is, removal of water to join two amino acids together to form a peptide bond. The splitting or removal of amino acids from a polypeptide, such as occurs during digestion, uses hydrolysis, that is, the addition of water via a hydrogen (H) and hydroxyl (OH) group. Figure 1.6 shows the formation of a dipeptide.
Figure 1.6 Formation of a dipeptide.
Levels of Protein Structure
Because proteins are such large complex molecules their structures are often described by considering four different levels: primary, secondary, tertiary and quaternary.
Primary Structure of Proteins (Amino Acid Sequence)
The primary structure refers to the sequence of amino acids in the protein chain. A simple primary structure of a small protein could be shown as:
The hormone insulin, for example, consists of a chain of 51 amino acids. The code for the primary structure of all proteins is contained in the gene or genes that code for that specific protein and this determines the precise order that the chain is built or assembled within the cell. The order also determines the way the chain will twist and turn to make up its three-dimensional (3D) shape which allows it to carry out its specific function within the horse's body.
Secondary Structure of Proteins (Alpha Helix or Beta Pleated Sheet)
The secondary structure refers to the first level of 3D twisting. These twists occur as the amino acids twist to find the most stable arrangement of their hydrogen bonds within the chain. The main secondary structures are the alpha helix spiral or beta pleated sheet. The structure depends upon the amino acids that make up the chain. The alpha helix spiral is the most common, where amino acids twist on their axes, each forming a hydrogen bond with another amino acid four units along. This stabilises the alpha helix shape.
The beta pleated sheet is a flat structure consisting of two or more amino acid chains running parallel, linked together by hydrogen bonds. Some amino acids tend to produce sharp bends in the chain, allowing the structure to bend backwards upon itself.
Tertiary Structure of Proteins (3D Shape/Folding)
The tertiary structure refers to the overall 3D shape and depends upon several factors: first, the exact sequence of amino acids that produces either the alpha helix or beta pleated shapes at particular points on the long amino acid chain; and second, the hydrophobic side chains on globular proteins (which are surrounded by water) that tend to point inwards, and also any weak ionic bonds or hydrogen bonds (Figure 1.7). The strongest links are found when neighbouring cysteine amino acids form disulphide bridges. These bridges occur most commonly in structural proteins, contributing to their strength. Some proteins are complete and function with a tertiary structure only, such as enzymes.
Figure 1.7 Bonding examples in the tertiary structure of proteins.
Quaternary Structure of Proteins (Aggregations of Polypeptide Chains)
Many complex proteins are made up of one or more than one polypeptide chain and sometimes have additional non-protein prosthetic groups usually vital to the function of that protein, for example, the ‘haem’ group in haemoglobin (Hb). The arrangement of these polypeptide chains is known as the quaternary structure. Hb is a globular protein composed of four polypeptide units joined together: two identical alpha chains and two identical beta chains (Figure 1.8).
Figure 1.8 Model of haemoglobin.
Source: Zephryis, https://en.wikipedia.org/wiki/Hemoglobin#/media/File:1GZX_Haemoglobin.png. CC-BY-SA 3.0.
Each has the haem group at the centre of the chain which binds oxygen, that is, the Hb molecule contains just four Fe2+ (iron) atoms. When one molecule of oxygen binds to Hb, it changes its shape, encouraging more oxygen molecules to bind. This is called cooperative binding. The chemical formula of Haemoglobin is C2952H4664O832N812S8Fe4. Proteins containing non-protein parts are known as conjugated proteins.
Classification of Proteins
Table 1.3 shows the structural classification of proteins. The final 3D structure results in two main classes of protein:
•
Globular – large individual molecules which are round and compact in shape with complex tertiary and quaternary structures, mostly soluble in water, with mostly biochemical functions.
•
Fibrous – tough and rope shaped, containing polypeptides bound together in long sheets or fibres.
Table 1.3 Structural classification of proteins
Globular proteins
Fibrous proteins
Soluble in water, easily transported in fluids
Insoluble in water
Tertiary structure determines function of protein
Tough structurally, may be elastic
Folded into compact spherical or globular shape
Parallel polypeptide chains forming sheets or long fibres
Examples
Examples
Enzymes
Collagen
Haemoglobin
Cartilage
Antibodies
Tendons
Hormones
BonesWalls of blood vessels
Structure of Collagen
Collagen is an important fibrous protein which provides structure and support to tissues. It is strong and flexible. Collagen is made up of three polypeptide chains that are tightly coiled into a triple helix formation (Figure 1.9). The polypeptide chains are connected by strong interlinks formed from covalent bonds. Minerals may be added to the triple helix to increase its rigidity, such as found in bone.
Figure 1.9 Structure of collagen.
Modification of Proteins
After production by ribosomes, many proteins may be passed to the inside of rough endoplasmic reticulum (ER) where they are modified by the addition of carbohydrates to produce glycoproteins. These appear to be markers to determine the destination of the protein. Proteins made by free ribosomes in the cytosol do not have added carbohydrate. Some proteins may have fatty acids added, producing lipoproteins that help transport lipids round the horse's body.
Denaturation of Proteins
Denaturation results in the breakdown of the 3D structure of proteins and therefore, most often, their biological function. Denaturation occurs due to breakdown of the bonds holding the 3D structure in place, while the amino acid sequence usually remains unchanged. Denaturation can be caused by heat (such as raised body temperature), heavy metals, strong acids and alkalis, some solvents and detergents.
Carbohydrates
Carbohydrates are required to provide energy for fuel for cellular metabolism, that is, for life. Carbohydrates are a group of molecules consisting of carbon, hydrogen and oxygen with the general formula (CH2O)x. One of the most common of all carbohydrates is glucose. Glucose is essential for nerve cells including the brain, as the brain cannot use any other form of fuel.
Monosaccharides (one) and disaccharides (two) are classed as sugars and usually end in -ose (e.g. glucose, lactose). All monosaccharides are reducing sugars in that they can be used in reduction reactions. Glucose is a very important monosaccharide (i.e. a single sugar molecule) as it is required for energy for all plants and animals. It is soluble and so easily transported. The chemical bonds within glucose contain a large amount of energy that can be released in the mitochondria via cellular respiration. Glucose is a six-carbon sugar known as a hexose sugar and these sugars occur most frequently, whereas five-carbon sugars are known as pentose sugars. Three-carbon sugars are termed triose sugars. Lactose is the primary carbohydrate source for suckling foals. Common carbohydrates are shown in Table 1.4.
Table 1.4 Common carbohydrates – disaccharides, monosaccharides and polysaccharides
Common disaccharides (double sugars)
Constituent monosaccharides
Sucrose (table sugar)
Alpha glucose + beta fructose (found in sap of plants)
Maltose
Alpha glucose + alpha glucose (found in germinating seeds)
Lactose (milk sugar)
Beta glucose + beta galactose (found in milk)
No. of carbon atoms in monosaccharide
Type of sugar and examples
3
Triose – glyceraldehyde
5
Pentose – ribose, deoxyribose
6
Hexose – glucose, fructose (fruit sugar), galactose (milk sugar)
Common polysaccharides
Constituent monosaccharide
Starch (plants – storage)
Glucose
Glycogen (animals – storage)
Glucose
Cellulose (plant cell walls)
Glucose
Carbohydrates also exist as isomers, having the same chemical formula with a different structural formula. This results in different properties such as taste and digestibility. Glucose exists as alpha and beta glucose (Figure 1.10). Starch and glycogen are polymers consisting of long chains of alpha glucose, whereas cellulose is a polymer of beta glucose.
Figure 1.10 Alpha and beta structure of glucose.
Monosaccharides are joined together by glycosidic bonds to form disaccharides and polysaccharides. This is a condensation reaction, with the resultant removal of a water molecule (similar to polypeptides). When polysaccharides are broken down, the glycosidic bonds are broken by hydrolysis, that is, by the addition of a water molecule. The disaccharide maltose, for example, is built up from two alpha glucose molecules by a condensation reaction (Figure 1.11).
Figure 1.11 Structure of maltose from two alpha glucose molecules.
Fructans
Fructans are polysaccharides consisting mainly of fructose, with sometimes glucose present too. Fructans are important for storage of carbohydrates in the stems of many species of grasses and confer a degree of freezing tolerance and possibly drought tolerance as well. Fructans may be very large 3D-structured molecules, of which there are many different ones. They have a highly complicated nomenclature based on the type and site of chemical linkages. Fructans in grass are called levans or phleins, with 2,6 linkages, whereas fructans from broad-leaved plants are mainly inulin with 2,1 linkages. Not all grasses make fructans and some have their own individual type. Fructans are important in equine nutrition as horses do not possess the enzymes to break them down in the small intestine and they therefore pass undigested into the hindgut where they become a substrate for microbes.
Starch
Starch is the main energy storage material of plants including grasses and cereals. It is also the major component of cereal grains traditionally used to feed horses, such as oats, barley and maize, which therefore contain high levels of starch. Plant cells make glucose via photosynthesis and store excess glucose primarily as starch. Starch is insoluble in water so it does not affect osmosis. Because there are few ‘ends’ within the starch molecule, there are fewer points for enzymic breakdown, and therefore starch is an excellent long-term storage compound for plants. Starch is a mixture of two polysaccharides of alpha glucose, namely:
•
Amylose – consisting of a long unbranched chain with a coiled structure for easy storage. Makes up 25–30% of the starch molecule, linked by alpha 1,4 glycosidic bonds.
•
Amylopectin – consisting of a long branched chain containing up to 1500 glucose units in which alpha 1,4 chains are cross-linked by alpha 1,6 glycosidic bonds, giving side branches for easy enzymic breakdown to glucose. Makes up 70–75% of the starch molecule, with alpha 1,6 glycosidic bonds every 24–30 glucose units.
Cellulose
Cellulose is the most common molecule on earth. It is the major structural component of plant cell walls, made up of long straight unbranched chains of beta glucose held together by 1,4 glycosidic bonds. Cellulose gives plants structure and strength. As many as 10,000 glucose molecules may be linked together in the chain. The parallel cellulose chains are cross-linked together by hydrogen bonds which form strong fibres known as microfibrils. This provides the plant cells with firm structural support for the cells and plants as a whole. The cross-linking prevents access by water, so that cellulose is very resistant to hydrolysis and is therefore an excellent structural molecule for plants (Figure 1.12).
Figure 1.12 Structure of cellulose.
Glycogen
Glycogen is the main energy storage form of all animals including horses and is similar to starch. It is a branched chain polysaccharide like amylopectin, being composed of alpha glucose molecules but with more alpha 1,6 glycosidic bonds mixed with alpha 1,4 glycosidic bonds, that is, more branches (Figure 1.13). This means it can be broken down quickly by enzymic hydrolysis when glucose is required for energy. Glycogen is also more water soluble than starch. Glycogen is mainly found within the horse's muscle and liver cells.
Figure 1.13 Structure of glycogen.
Lipids/Fats
Lipids are not the same as carbohydrates and proteins as they are not made up of long chains of monomer units. Lipids include fats and oils and have an oily or fatty consistency; they are insoluble in water but soluble in alcohol and ether, except for phospholipids which have polar heads, that is, they are hydrophilic or ‘water loving’.
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