Life Processes · Lesson 2 of 13
Nutrition
“Every cell needs groceries, even when the organism is resting.”
• Explain why organisms require food even when they are resting. • Distinguish autotrophic and heterotrophic nutrition. • Relate food complexity to the need for digestion. • Explain the role of enzymes in nutrition. • Describe the dependence of heterotrophs on autotrophs.
Walking, cycling and lifting objects clearly require energy, but an apparently resting body also spends energy maintaining temperature, repairing cells and carrying out chemical reactions. Organisms additionally need matter to grow and replace worn-out structures. Nutrition is the process that meets both needs.
Nutrition is the process by which an organism obtains food and uses its nutrients for energy, growth, repair and maintenance.
How Do Living Things Get Their Food?
All organisms need energy and materials, but they obtain them in different ways. Green plants and some bacteria build energy-rich organic food from simple inorganic substances. Animals and fungi cannot do this; they obtain complex organic food made by other organisms and usually break it into smaller substances before absorption.
A mode of nutrition in which an organism synthesises organic food from simple inorganic substances using an external source of energy.
A mode of nutrition in which an organism obtains ready-made organic food from other organisms or their products.
| Feature | Autotrophic nutrition | Heterotrophic nutrition |
|---|---|---|
| Source material | Carbon dioxide, water and mineral substances | Complex organic food |
| Energy | Usually light energy | Chemical energy already stored in food |
| Examples | Green plants and some bacteria | Animals and fungi |
| Ecological role | Produces organic food | Depends directly or indirectly on producers |
Why Enzymes Are Needed
Many food molecules are too large or insoluble to cross cell membranes. They must be broken into smaller, soluble molecules. Enzymes speed up these chemical changes without being used up. Different enzymes act on different substances, which is why digestion occurs through several coordinated steps rather than one general reaction.
An enzyme is a biological catalyst that speeds up a specific chemical reaction in a living system without being permanently consumed.
Problem
A mushroom grows on decaying wood and absorbs soluble products after digesting material outside its body. Is it autotrophic or heterotrophic?
- 1.The mushroom does not manufacture food from carbon dioxide and water.
- 2.It obtains organic material originally made by other organisms.
- 3.External digestion changes complex material into absorbable substances.
- 4.Therefore, the mushroom uses heterotrophic nutrition.
Heterotrophs do not merely “eat animals.” Herbivores, fungi and parasites are also heterotrophs because their organic food ultimately comes from other organisms.
Quiz
What is the central purpose of nutrition?
Which materials are commonly used by autotrophs?
Why are enzymes important in heterotrophic nutrition?
Which organism is heterotrophic?
Why do resting organisms still need food?
Practice Problems
- Explain why growth requires both energy and raw material.
- Compare autotrophic and heterotrophic nutrition using source material and energy source.
- A bacterium makes organic food from carbon dioxide using chemical energy. Classify its nutrition and justify your answer.
- Explain why a large protein molecule generally needs digestion before absorption.
- Trace the dependence of a carnivore on autotrophs through a simple food chain.
Key Takeaways
• Nutrition supplies energy and material for growth, repair and maintenance. • Autotrophs build organic food from simple inorganic substances. • Heterotrophs obtain organic food from other organisms. • Heterotrophs depend directly or indirectly on autotrophs. • Enzymes catalyse specific reactions that make complex food usable.