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Digestion
Animals and other heterotrophs need to take in food sources in order to get all the nutrients they need to survive. This process of extracting nutrients from food is digestion!
Step 1: Ingestion
- Ingestion = the process of taking the food source into the body of an organism
- In many vertebrates (including mammals) this process is through the mouth
- Mastication (chewing) breaks down large food pieces into smaller ones
- Teeth are important in mastication
- Enzymes found in saliva begin chemically breaking down food
- Many animals have specialized structures and behaviours to aid in ingestion
- Examples:
- Filter feeding - blue whales, clams, oysters, mussels, ducks, flamingos
- Fluid feeding - mosquitoes, leeches, tapeworms, spiders
- Grasping - humans, elephants
Step 2: Digestion
- Digestion = physical and chemical breakdown of food inside the body of an organism
- In many vertebrates digestion occurs in the stomach
- Other organs are also required in many animals including the gall bladder
- Hydrochloric Acid is the main chemical found in animal stomachs
Step 3: Nutrient Absorption
- Nutrient Absorption = moving proteins, carbohydrates, fats, vitamins and essential ions out of the digestive tract and into cells
- In many vertebrates this occurs in the small intestine
- Other organs, like the pancreas, are also involved
- Bicarbonate and enzymes help further breakdown food into basic building blocks
Step 4: Elimination (Egestion)
- Elimination = expelling any undigested food and waste from the digestive tract
- In many vertebrates this occurs through the colon (large intestine) and out through the anus
- The colon is vital in water re-absorption
- Essential bacteria live in the colon and are required for final digestion processes
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Examples of Digestion in Non-human Organisms
Not every animal has a digestive tract similar to humans, but they still go through the same processes of ingestion, digestion and elimination.
Protozoa
- Simplest form of digestion, as protozoa are single celled
- Ingestion: Amoeba recognizes a food source, extends cellular structures called pseudopods and engulfs the food source into the cell
- Digestion: The ingestion of the food source creates a vacuole that fuses with the lysosome to breakdown the food source
- The lysosome contains hydrolytic enzymes (enzymes that use water to break chemical bonds)
- Elimination: little to no elimination is required, but would be released from the cell in vacuoles if needed
Hydra
- Hydra and multi-cellular pods with little cell specification
- Ingestion: Hydra takes food source into its gastrovascular cavity
- The gastrovascular cavity is an unspecialized "stomach" that is open to the external environment
- Digestion: Enzymes are secreted into the cavity to breakdown the food source, cells lining the gastrovascular cavity engulf the smaller food particles and continue the breakdown in their cytoplasm
- Elimination: All undigested food is released from the same opening used for ingestion
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Worms
- Worms, like humans, have a single direction digestive system
- Having a single direction, with a mouth for ingestion and anus for elimination is an alimentary canal
- Ingestion: Muscles in the pharynx (mouth) pull food in
- Digestion:
- After moving through the esophagus food is housed in the crop (stomach) before moving to the gizzard for mechanical digestion
- The gizzard is present in animals that do not have teeth to help with mechanical digestion
- Then food moves to the intestine for chemical digestion
- Nutrients are trafficked across the intestinal cells into the blood
- Elimination: Any unabsorbed nutrients and food particles are eliminated through the anus
Birds
- Birds have a more sophisticated digestive system including specified small intestinal areas, liver and a primitive large intestine
- Ingestion: Food is taken in through the mouth and travels down the esophagus to the crop (stomach)
- Digestion:
- The food travels from the crop to the gizzard for mechanical digestion
- Food travels from the gizzard to the small intestine for chemical digestion and nutrient absorption
- Elimination: Food travels to the ceca (primitive large intestine) for collection and is eliminated through the anus
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Example: Basic Digestion
You discover a new species of multicellular organism that through observation you discover is a heterotroph. It is difficult to visually observe how this organism is acting as a heterotroph, but given that it lives by eating other organisms what components of a digestive system must this organism posses?
Ingestion: there needs to be a way for the animal to take in food, with specialized muscles (like in the worm) or with tentacles (like in hydra)
Digestion: there needs to be a cavity where food is both mechanically and chemically broken down and nutrients are absorbed, in a gastrovascular cavity (like in hydra) or through an alimentary canal (like in higher organisms)
Elimination: there needs to be a mechanism to expel undigested food, through the same opening (like in hydra) or through a specialized anus (like in higher organisms)
Practice: Hydra Digestion
Hydra are a simple (mutlicellular or unicellular) organism that has a primitive digestive system.
Hydra uses their tentacles to help catch and bring prey into their cavity, through the process of .
There, food is broken down by the secretion of where small food particles are engulfed by cells lining the cavity.
Undigested food particles are eliminated through the (same or different) opening that it was taken in.
Practice: Feeder Type
Which of the following are descriptions of fluid feeding?
Practice: Whales
Many whales including the blue whale, humpback and grey whales use filter feeding through their baleen to eat. Other whales, like killer whales, are exclusively carnivores that hunt their prey. What would you expect to find in the mouth of a killer whale that a baleen whale does not have?