Topic 4 - Animal Needs

Animal Nutrition and Energy Needs

Cellular Requirements and ATP Production

All animal cells require ATP to perform cellular work. ATP is generated primarily through aerobic respiration, though some cells can use lactic acid fermentation under anaerobic conditions. To sustain metabolism, cells need a continuous supply of reactants and efficient removal of wastes.

• Reactants required: Oxygen (O2), sugars (glucose), water (H2O)

• Wastes produced: Carbon dioxide (CO2), nitrogenous waste (N-waste)

• Transport mechanisms: Systems to connect collection sites (e.g., gut, respiratory surfaces) to usage and removal sites (e.g., tissues, kidneys, skin)

Exchange with the Environment

Surface Area, Volume, and Transport

Efficient exchange of materials depends on the balance between surface area (for exchange) and volume (metabolic demand). Larger animals require specialized transport systems to overcome limitations of diffusion.

• Collection sites: Gut (nutrients), respiratory surfaces (O2)

• Usage sites: Tissues (consume nutrients and O2)

• Removal sites: Skin, kidneys, respiratory surfaces (eliminate CO2, N-waste, water)

Transport Mechanisms in Animals

Connecting Collection and Usage Sites

Transport systems move essential substances from collection sites to tissues and remove wastes. The gut and respiratory surfaces collect nutrients and oxygen, while renal organs and respiratory surfaces remove nitrogenous wastes and CO2.

Circulatory Systems

Closed Circulatory Systems

In closed circulatory systems, blood is confined within vessels, allowing efficient transport between collection and usage sites. Vertebrates exhibit increasing complexity in their circulatory systems.

• Heart: Pumps blood through vessels

• Arteries: Carry blood away from the heart

• Capillaries: Sites of exchange with tissues

• Veins: Return blood to the heart

• Extracellular fluid: Bathes tissues outside vessels

Vertebrate Circulatory System Evolution

• Fish: 2-chambered heart (1 atrium, 1 ventricle), single circuit

• Amphibians: 3-chambered heart (2 atria, 1 ventricle), double circulation (pulmocutaneous and systemic circuits)

• Reptiles: 3-chambered heart with partial septum, double circulation (pulmonary and systemic circuits)

• Birds & Mammals: 4-chambered heart (complete septum), double circulation (pulmonary and systemic circuits)

Open Circulatory Systems

In open circulatory systems, the same fluid (hemolymph) bathes tissues and is pumped by the heart into open spaces. Exchange occurs directly between hemolymph and cells.

Animals Without Specialized Circulatory Systems

Some small or porous animals rely solely on diffusion for transport. Gastrovascular cavities in cnidarians and platyhelminths circulate nutrients and also serve as respiratory surfaces.

Animal Nutrition

Essential Nutrients

Animals are ingestive heterotrophs, requiring organic carbon for ATP and macromolecule synthesis. Essential nutrients include certain amino acids, fatty acids, vitamins, and minerals. Deficiencies can lead to malnutrition.

Food Processing

Food processing in animals involves four main steps: ingestion, digestion, absorption, and elimination. Digestion can be mechanical (chewing) or chemical (enzymatic hydrolysis).

Feeding Mechanisms

Animals have evolved various feeding strategies:

• Suspension feeding: Filtering small particles from water

• Substrate feeding: Living in or on food source

• Fluid feeding: Sucking nutrient-rich fluids

• Bulk feeding: Eating large pieces or whole prey

Bulk Feeding Adaptations

Bulk feeders may have expandable bodies or jaws for swallowing prey whole, or specialized teeth and appendages for tearing food into pieces.

Enzymatic Digestion

Digestion can be intracellular (within food vacuoles) or extracellular (within digestive chambers). Most animals use extracellular digestion, with specialized enzymes for breaking down carbohydrates, proteins, lipids, and nucleic acids.

Macromolecule	Enzymes & Products	Molecules Absorbed
Carbohydrates	Amylase → maltose, then maltase → glucose Sucrase → glucose + fructose Lactase → glucose + galactose	Monosaccharides
Proteins	Pepsin & trypsin → small peptides Carboxypeptidases → amino acids + di-/tripeptides Aminopeptidases → amino acids + di-/tripeptides Cytoplasmic peptidases → amino acids	Tripeptides, dipeptides, amino acids
Lipids	Lipase → fatty acids + monoglycerides	Fatty acids, monoglycerides
Nucleic acids	DNAses, RNAses → nucleotides	Nucleotides

Digestive System Types

• Gastrovascular cavity: Single opening, functions in both digestion and distribution of nutrients (e.g., cnidarians, flatworms)

• Alimentary canal: Tube with two openings (mouth and anus), allows for specialized regions and more efficient digestion

Adaptations to Diet

Mammalian teeth are adapted to diet: herbivores have grinding molars, carnivores have sharp canines and shearing teeth. Herbivores often have larger cecums for digesting cellulose, sometimes with symbiotic microorganisms or specialized stomachs (e.g., ruminants).

Respiratory Systems

Respiratory Media and Gas Exchange

Animals exchange gases with their environment using air or water as the respiratory medium. Oxygen is less soluble in water, making aquatic respiration more challenging. Gas exchange relies on diffusion across moist, large, and vascularized surfaces.

• Respiratory surfaces: Skin, gills, lungs

• Ventilation: Movement of respiratory medium over the surface (e.g., gill bailers, buccal pump, ram-jet ventilation)

Types of Respiratory Systems

• Cutaneous respiration: Gas exchange across the skin (e.g., amphibians, some annelids)

• Gills: Specialized for aquatic respiration; can be external or internal

• Lungs: Internal, highly branched structures for air breathing (e.g., mammals, birds, reptiles)

• Tracheal system: In insects, air is delivered directly to tissues via tracheae and tracheoles

Mechanics of Breathing

In mammals, breathing is governed by Boyle's Law: pressure is inversely proportional to volume ($P \propto \frac{1}{V}$). Inhalation increases thoracic volume, decreasing pressure and drawing air in; exhalation reverses this process.

Oxygen and Carbon Dioxide Transport

• Oxygen: Carried by metalloproteins (hemoglobin in vertebrates, hemocyanin in some invertebrates)

• Carbon dioxide: Highly soluble, transported as dissolved CO2, bound to hemoglobin, or as bicarbonate (HCO3-)

Carbonic anhydrase catalyzes the conversion between CO2 and HCO3-:

$\mathrm{CO_2 + H_2O \xrightarrow{carbonic\ anhydrase} H_2CO_3 \rightarrow HCO_3^- + H^+}$

Excretory Systems

Nitrogenous Waste Removal

Animals produce nitrogenous wastes from protein and nucleic acid metabolism. The main forms are ammonia (highly toxic, very soluble), urea (less toxic, moderately soluble), and uric acid (least toxic, poorly soluble).

• Filtration: Blood pressure forces plasma into excretory tubules

• Reabsorption: Useful solutes are reclaimed

• Secretion: Additional wastes are actively transported into filtrate

• Excretion: Concentrated urine is expelled

Types of Excretory Organs

• Protonephridia: Found in flatworms; network of tubules with flame bulbs

• Metanephridia: Found in annelids; tubules open to coelom and exterior

• Malpighian tubules: Found in insects; remove wastes from hemolymph

• Kidneys: Found in vertebrates; composed of nephrons for filtration and reabsorption