C3.5 - Digestive System

Obtaining and Processing Food

• All organisms must have a way of obtaining essential nutrients
• heterotroph: organisms that depend on food made by others
• autotroph: organisms that can make their own food
• Food must be broken down to a size that can be passed through the cell membrane

Four Stages of Food Processing

STEP I: Ingestion

ingestion: act of eating / drinking

STEP II: Digestion

• digestion: process of breaking food down into molecules that are small enough for the body to absorb
• mechanical digestion: processing food physically (i.e. chewing, chopping) to increase surface area
• chemical digestion: breaking down chemical bonds in nutrients of food into smaller molecules
  • occurs during hydrolysis
  • hydrolysis: process that breaks bonds into food molecules by adding water in presence of specific enzymes

STEP III: Absorption

• absorption: process where cells absorbs small molecules like amino acids and simple sugars
• occurs in small intestine
• molecules enter blood then transported to body
• joined to make bigger molecules or broken down for energy
• excess molecules converted to fat for storage

STEP IV: Elimination

elimination: process of removing any undigested materials from digestive tract

feces: eliminated undigested material (commonly known as poop / shit)

Feeding Behaviours in Animals

• filter feeding: food is filtered from water by straining it

  • i.e. whales (baleen), molluscs (gills)

  • Baleen:

    
• fluid feeding: mouthparts adapted for piercing and sucking
  • some also use digestive enzymes to break down their prey outside
  • i.e. spiders, tapeworms, mosquitos
• bulk feeding: ingestion of large particles periodically
  • requires development of more complex digestive system
  • i.e. mammals

Digestion

Intracellular Digestion	Extracellular Digestion
Engulfing of food by pirio/phagocytosis	Common in animals
Digestion occurs inside food vacuole	Open tube arrangement: mouth and anus (i.e. earthworms)
	Closed tube has one opening serving both functions (i.e. sea anemone)

• alimentary canal: digestive tract with two openings (mouth and anus)
  • canal a.k.a. gastrointestinal (GI) tract (gastro = stomach)
  • herbivores / omnivores have longer GI tract
  • carnivores have shorter tract
  • vegetation more difficult to digest than meat due to cell walls
  • GI tract much longer in humans and many other animals than dist. between mouth and anus

phagocytosis: process where cell engulfs another cell to digest it

Extracellular Digestion