Autotrophs and Heterotrophs

HSC Biology | Study Notes

This topic matters because Module 2 specifically asks students to compare the nutrient and gas requirements of autotrophs and heterotrophs, and to link these requirements to the structures organisms use for photosynthesis, digestion and gas exchange.

In this lesson

• the definitions of autotrophs and heterotrophs

• the main nutrient requirements of each

• the main gas requirements of each

• clear examples of autotrophs and heterotrophs

What is an autotroph?

An autotroph is an organism that can make its own organic food molecules from simple inorganic substances.

In most cases at this level, autotrophs are organisms that carry out photosynthesis.

Main idea

Autotrophs use:

• light energy

• carbon dioxide

• water

...to produce glucose.

Why this matters

Because autotrophs can make their own food, they do not need to consume other organisms to get organic nutrients.

What is a heterotroph?

A heterotroph is an organism that cannot make its own food and must obtain organic nutrients by consuming other organisms or their products

Main idea

Heterotrophs depend on other organisms for food.

They obtain nutrients by:

• eating plants

• eating animals

• eating both

• absorbing nutrients from organic matter in some cases

Nutrient requirements of autotrophs

Autotrophs need:

• water

• carbon dioxide

• mineral ions from the environment

• light energy for photosynthesis

Why these are needed

• water is a reactant in photosynthesis

• carbon dioxide is a reactant in photosynthesis

• mineral ions are needed for growth and normal cell function

• light energy is used to drive photosynthesis

Autotrophs then produce glucose, which can be used in respiration and to build larger molecules.

Nutrient requirements of heterotrophs

Heterotrophs need:

• organic nutrients such as carbohydrates, proteins and lipids

• water

• mineral ions

Why these are needed

Heterotrophs cannot make glucose from carbon dioxide and water alone, so they must obtain ready-made organic molecules from food.

After digestion, they absorb simple nutrients such as:

• glucose

• amino acids

• fatty acids and glycerol

The Module 2 syllabus also includes tracing digestion in mammals, including the absorption of nutrients, minerals and water, which links directly to heterotroph requirements.  

Gas requirements of autotrophs

Autotrophs need both carbon dioxide and oxygen, but for different processes.

Carbon dioxide

Autotrophs need carbon dioxide for photosynthesis.

Oxygen

Autotrophs also need oxygen for cellular respiration.

Important point

Plants do not only need carbon dioxide. They also respire, so they need oxygen as well.

Gas requirements of heterotrophs

Heterotrophs mainly need oxygen for cellular respiration.

They produce carbon dioxide as a waste product of respiration.

Important point

At this level, the key comparison is:

• autotrophs use carbon dioxide in photosynthesis and oxygen in respiration

• heterotrophs mainly use oxygen in respiration and release carbon dioxide

Comparing autotrophs and heterotrophs

Similarities

Both autotrophs and heterotrophs:

• need water

• need mineral ions

• carry out cellular respiration

• need gas exchange

• require transport systems or surfaces to move materials

Differences

The main difference is how they obtain organic nutrients.

• Autotrophs make organic molecules themselves, usually by photosynthesis.

• Heterotrophs obtain organic molecules by consuming other organisms.

Summary table

Feature	Autotrophs	Heterotrophs
Definition	Make their own organic food	Obtain organic food from other organisms
Main nutrient source	Carbon dioxide, water, mineral ions, light energy	Organic food, water, mineral ions
Main gas needs	Carbon dioxide for photosynthesis, oxygen for respiration	Oxygen for respiration
Example	Flowering plant	Human

Examples of autotrophs

Common examples include:

• flowering plants

• grasses

• trees

• algae

Examples of heterotrophs

Common examples include:

• humans

• cows

• fish

• insects

• fungi

Why this topic matters in Module 2

This topic leads into the rest of Module 2 because nutrient and gas requirements help explain why organisms need particular structures.

For example:

• autotrophs need structures for photosynthesis and gas exchange

• heterotrophs need structures for digestion, absorption and gas exchange

That is why the syllabus links this comparison to plant structures, gas exchange structures, and digestion in mammals.

Worked example

Exam-style question

Compare the nutrient requirements of autotrophs and heterotrophs.

Worked answer

Autotrophs need simple inorganic substances such as carbon dioxide, water and mineral ions, and they use light energy to make glucose by photosynthesis. Heterotrophs cannot make their own food, so they need to obtain organic nutrients such as carbohydrates, proteins and lipids from other organisms.

Why this works

This answer:

• compares both groups directly

• includes the key nutrient differences

• uses the term photosynthesis accurately

Common mistakes

• Saying plants only need carbon dioxide and not oxygen.

• Saying heterotrophs do not need water or ions.

• Confusing nutrients with gases.

• Saying autotrophs do not respire.

• Giving examples without explaining the difference in how food is obtained.

Quick quiz

1. What is an autotroph?

2. What is a heterotroph?

3. Which gas do autotrophs use in photosynthesis?

4. Why do heterotrophs need organic nutrients?

5. Give two examples of autotrophs and two examples of heterotrophs

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