Digestion Chapter 1

The First Stage of Digestion

• Mechanical digestion occurs in the oral cavity.
• Salivary glands deliver saliva to lubricate food and initiate breakdown of glucose polymers.
• Saliva contains mucus, a viscous mixture of water, salts, cells, and glycoproteins.
• Teeth chew food into smaller particles, and the tongue shapes food into a bolus and provides help with swallowing.
• The throat, or pharynx, is the junction that opens to both the esophagus and the trachea.
• The esophagus conducts food from the pharynx down to the stomach by peristalsis.

Digestion in the Stomach

• Gastric juice has a low pH of about 2, which kills bacteria and denatures proteins.
• Gastric juice is made up of hydrochloric acid (HCl) and pepsin.
• Pepsin is a protease, or protein-digesting enzyme, that cleaves proteins into smaller peptides.
• Parietal cells secrete hydrogen and chloride ions separately into the lumen of the stomach.
• Chief cells secrete inactive pepsinogen, which is activated to pepsin when mixed with hydrochloric acid in the stomach.
• Mucus protects the stomach lining from gastric juice.
• Coordinated contraction and relaxation of stomach muscle churn the stomach's contents.

Stages of Feeding

• Ingestion: taking in food.
• Digestion: breaking food into small pieces for absorption.
• Absorption: nutrient uptake by cells.
• Elimination: expulsion of undigested material.

Digestion Forms

• Intracellular digestion: food particles are engulfed by phagocytosis, and food vacuoles fuse with lysosomes containing hydrolytic enzymes.
• Extracellular digestion: breakdown of food particles outside of cells, occurring in compartments that are continuous with the outside of the animal's body.

Mammalian Digestive System

• The mammalian digestive system consists of an alimentary canal and accessory glands that secrete digestive juices through ducts.
• Accessory glands are the salivary glands, the pancreas, the liver, and the gallbladder.

Animal Nutrition

• Macronutrients: carbohydrates, lipids, and proteins.
• Essential nutrients: those the animal body cannot synthesize, including essential amino acids, essential fatty acids, vitamins, and minerals.
• Undernutrition: results when a diet does not provide enough chemical energy, leading to protein deficiency, loss of muscle mass, and potentially death or irreversible damage.

Digestive Strategies in Animals

• There are four stages of feeding: ingestion, digestion, absorption, and elimination.

Sponges

• Sponges are filter feeders, benefiting from small particles of food in the water that comes into their bodies through incurrent canals.
• Coanocytes capture food particles through phagocytosis and digestion occurs when enzymes in vesicles fuse with food vacuoles, breaking down food into smaller molecules.
• Absorption occurs when these molecules are used by the cell or packaged into vesicles for sharing with other cells.
• Wastes are eliminated through exocytosis.

Cnidarians

• Cnidarians, such as jellyfish and sea anemones, have extracellular digestion in the gastrovascular cavity.
• Cells produce enzymes in vesicles, which are released through exocytosis for digestion.
• Absorption occurs in cells lining the gastrovascular cavity.
• Wastes are eliminated through the mouth, which also serves as the anus.

Bilaterally Symmetrical Animals

• Bilaterally symmetrical animals, such as segmented worms and arthropods, have a complete digestive system with a mouth and anus.
• Ingestion occurs through the mouth, followed by mechanical digestion through chewing mandibles or a gizzard.
• Digestion occurs in the stomach, aided by digestive enzymes.
• Absorption occurs along the intestine.
• Wastes are eliminated through the anus.

Birds

• Birds have a crop for storing food, which is then broken down in the gizzard with the help of digestive enzymes and mechanical grinding.
• Absorption occurs in the intestine.
• Wastes are eliminated through the cloaca, which also serves as the reproductive and excretory system.

Mammals

• Mammals have a complex digestive system with well-developed teeth for mechanical digestion.
• Ingestion occurs through the mouth, followed by chemical digestion with enzymes in the stomach.
• Absorption occurs in the small intestine, and water is recycled in the large intestine.
• Wastes are eliminated through the anus.

Energy Sources for Animals

• Carbohydrates, lipids, and proteins are macronutrients that provide energy for animals.
• Fruits, vegetables, and meat are examples of food sources that contain large amounts of macronutrients.

Importance of Energy for Animals

• Energy is necessary for daily activities, growth, and maintenance of bodily functions.
• Animals require energy to move, find food, and respond to their environment.

Macronutrients and their Functions

• Carbohydrates provide energy for the body and are found in fruits, vegetables, and grains.
• Lipids provide energy, are found in fatty foods, and are essential for brain function.
• Proteins build and repair tissues, and are found in meat, fish, and eggs.

Digestion and Absorption

• Digestion breaks down macronutrients into smaller molecules.
• The digestive system absorbs these molecules, which are then used by the body for energy and growth.

Importance of Vitamins and Minerals

• Vitamins and minerals are essential nutrients that help the body function properly.
• They are found in small amounts in food sources and are crucial for maintaining health.

Energy Efficiency

• Animals use energy efficiently to conserve energy.
• Energy efficiency is vital for survival, especially in environments where food is scarce.

Nutrient Requirements

• Different animals have unique nutrient requirements based on their size, age, and environment.
• A balanced diet meeting an animal's nutrient needs is necessary for maintaining health and function.

Characteristics of Mammalian Digestive System

• Has fewer compartments compared to other vertebrates
• Lacks crop and gizzard

Mouth and Saliva

• Initial digestion occurs in the mouth
• Saliva contains amylase to break down polysaccharides into smaller polysaccharides and disaccharides
• Saliva also contains mucous, water, salts, and glycoproteins to aid food passage through the esophagus
• Formation of bolus occurs during chewing
• Epiglottis prevents food from entering the respiratory tract

Stomach

• Stomach pH is 2 due to hydrochloric acid
• Gastric pits contain:
  • Parietal cells that secrete hydrogen and chloride ions
  • Chief cells that secrete pepsinogen, which is converted to pepsin by hydrochloric acid
  • Mucous cells that produce mucus to protect the stomach lining
• Stomach muscles mix food with digestive enzymes through churning action

Small Intestine

• Main site of chemical digestion
• Divided into three regions: duodenum, jejunum, and ileum
• Pancreas secretes various enzymes, such as trypsin, chymotrypsin, and amylase, into the duodenum
• Bile from liver and gallbladder aids in fat digestion and absorption
• Epithelial cells lining the intestine produce digestive enzymes, such as amylase and carboxypeptidase
• Villi increase the surface area for nutrient absorption
• Fats are broken down into fatty acids and glycerol, then formed into chylomicrons with phospholipids, cholesterol, and proteins for transport to the blood and distribution to the body

Particle Feeding

• Particle feeding involves feeding on fine particles in aquatic environments, such as plankton.

Types of Plankton

• Phytoplankton are plant-like plankton, including microscopic algae like diatoms, found in freshwater and oceans.
• Zooplankton are animal-like plankton, including larvae of animals and protists, found in aquatic environments.

Suspension Feeding

• Suspension feeding involves generating water currents to move water towards the mouth and trapping food particles in mucus.
• Two conditions required for suspension feeding are:
  • A mechanism for guiding water with suspended food particles towards the mouth.
  • The ability to secrete mucus to trap food particles.
• Examples of animals that use suspension feeding include clams and tunicates.

Filter Feeding

• Filter feeding involves using a straining mechanism to trap small particles from the water.
• Animals that use filter feeding must have a straining mechanism to mechanically pick up food particles from the water.
• Examples of animals that use filter feeding include baleen whales, spoonbills, flamingos, and ducks.

Deposit Feeding

• Deposit feeding involves feeding on depositors or larvae that settle to the bottom of a lake or ocean.
• Animals that use deposit feeding must wait for food particles to settle to the bottom before collecting them.
• Examples of animals that use deposit feeding include worms, snails, limpets, and clams.

Feeding Strategies

• Particle feeding involves collecting small particles, including dead organisms, with minimal effort, but may not be satisfying.
• Filter feeders and deposit feeders are examples of particle feeding.

Bulk Feeding

• Bulk feeding involves consuming large items, such as prey, which requires more effort and investment.
• It provides a lot of energy at once, allows selection of food items, but needs greater effort to find, capture, and consume prey.

Symbiotic Relationships

• Mutualism involves both organisms benefiting, such as aphids and ants, where aphids feed on plant sap and ants provide protection in exchange for honeydew.
• Aphids benefit from protection, and ants benefit from honeydew.

Traps and Deception

• Some animals set up traps, like spider webs or larval ant lions, to catch prey.
• Deception involves using fake or misleading signals, such as the angler fish's lure or spider's pheromone imitation, to attract prey.
• Lantern fly, spider, and angler fish are examples of traps and deception.

Tool Use

• Some animals use tools, like chimps using sticks to extract termites or Egyptian vultures using rocks to crack open ostrich eggs, to obtain food.
• Chimpanzees and Egyptian vultures are examples of tool use.

Group Feeding

• Group feeding increases food capture efficiency, reduces search area, and lowers predation risk.
• Examples of group feeding include lions, hyenas, wild dogs, dolphins, grey whales, and Harris Hawks.
• Strategies include coursing, stalking, ambushing prey, and using group hunting tactics like bubble nets or driving prey towards the shore.