Life Processes · Lesson 4 of 13
Heterotrophic Nutrition
“Not every organism cooks its own dinner.”
• Explain why heterotrophic strategies vary among organisms. • Distinguish external digestion, internal digestion and parasitic nutrition. • Relate feeding structures to the nature and location of food. • Classify organisms by their method of heterotrophic nutrition. • Explain why heterotrophs remain dependent on autotrophs.
A mushroom growing on bread, a cow grazing and a tick feeding on an animal all obtain organic food from other organisms, yet they do so in very different ways. The form, mobility and accessibility of food determine how an organism captures, digests and absorbs it.
Different Strategies Of Heterotrophic Nutrition
Fungi such as bread mould, yeast and mushrooms release digestive substances outside their bodies and absorb the soluble products. Many animals ingest food and digest it internally. Parasites obtain nutrients from a living host without immediately killing it. Each method depends on body design and on whether the food is stationary, mobile, solid, liquid or protected inside another organism.
Saprotrophic nutrition is a heterotrophic method in which an organism digests dead or decaying organic material outside its body and absorbs the soluble products.
Parasitic nutrition is a method in which an organism obtains food from a living host, usually harming the host without immediately killing it.
| Strategy | How food is processed | Examples |
|---|---|---|
| External digestion and absorption | Enzymes act outside the body; soluble products are absorbed | Bread mould, yeast, mushrooms |
| Ingestion and internal digestion | Food enters the body and is broken down inside | Cow, lion, human |
| Parasitic nutrition | Nutrients are drawn from a living host | Cuscuta, ticks, lice, leeches, tapeworms |
Food Source And Body Design
A grazing animal obtains stationary plant material and needs structures suited to cropping and grinding it. A predator must detect, pursue and capture mobile prey. A tapeworm lives in an intestine where digested food is already available, so it absorbs nutrients through its body surface. Thus, feeding apparatus cannot be understood separately from habitat and food source.
Problem
Bread mould spreads over moist bread and does not swallow pieces of it. How does it obtain nutrition?
- 1.The mould grows on dead organic material.
- 2.It releases digestive enzymes onto the bread.
- 3.Complex substances are changed into small soluble molecules outside the body.
- 4.These molecules are absorbed by the mould.
- 5.The method is saprotrophic heterotrophic nutrition.
Problem
Cuscuta coils around a living plant and draws prepared food from it. Identify its nutritional strategy.
- 1.The food comes from another living organism.
- 2.Cuscuta remains attached to the host and removes nutrients.
- 3.The host is harmed but is not immediately killed for food.
- 4.Therefore, the strategy is parasitic nutrition.
External digestion is not the same as external feeding. A fungus may grow over its food, but the defining feature is that chemical digestion occurs outside its cells before absorption.
Quiz
Which organism commonly digests food outside its body?
Which is an example of parasitic nutrition?
Why do a cow and a lion have different feeding adaptations?
What happens before absorption in saprotrophic nutrition?
Which statement applies to every heterotroph?
Practice Problems
- Compare saprotrophic and parasitic nutrition.
- Explain why a tapeworm can have a reduced digestive system.
- Classify yeast growing in sugar solution and justify your answer.
- Relate the feeding structures of a predator to the mobility of its food.
- Explain how a parasite differs from a predator even though both obtain food from other organisms.
Key Takeaways
• Heterotrophs obtain ready-made organic food. • Feeding strategy depends on food type, availability and body design. • Fungi commonly digest material outside the body and absorb products. • Animals commonly ingest and digest food internally. • Parasites draw nutrients from living hosts. • All heterotrophs depend directly or indirectly on autotrophs.