IB Biology SLTopic 3 — Energy & MatterPaper 1 & 2Core idea~9 min read
Obtaining Carbon Compounds
Every living thing needs carbon compounds — to burn for energy and to build its body out of. There are only two ways to get them: make your own, or eat something that already did. That single fork in the road splits life into autotrophs and heterotrophs.
📘 What you need to know
An organism’s mode of nutrition is how it obtains its carbon compounds.
Autotrophs make their own carbon compounds from simple inorganic substances. They are the producers.
Photoautotrophs use light energy; chemoautotrophs use energy from oxidising inorganic chemicals.
Heterotrophs obtain carbon compounds by ingesting them from other organisms.
Heterotrophs include consumers, detritivores and saprotrophs.
Both groups release the energy in those carbon compounds by oxidising them in respiration.
The two modes of nutrition
The word roots do the work here. Auto means self and hetero means other; troph means feeding. So autotrophs feed themselves and heterotrophs feed on others.
Two routes to the same destination: a body made of carbon compounds.
Autotrophs in more detail
An autotroph builds complex organic molecules out of simple inorganic ones. Carbon dioxide goes in; glucose comes out. Reactions that build bigger molecules from smaller ones are described as anabolic.
From that glucose the autotroph can go on to make everything else it needs — add nitrogen from the soil and you get amino acids, then proteins. Because they do all this without eating anything, autotrophs are the producers of an ecosystem.
Not just plants. Green plants are the obvious example, but algae, seaweeds and photosynthetic bacteria such as cyanobacteria are autotrophs too. Marine algae carry out a huge share of the planet’s photosynthesis.
Photoautotrophs and chemoautotrophs
Both types build their own carbon compounds. The difference is where they get the energy, and specifically where they get the electrons needed to make ATP.
Different energy source, same end product: carbon compounds built from carbon dioxide.
Chemoautotrophs matter because they are the producers in places sunlight cannot reach. Around deep-sea hydrothermal vents, and in some cave systems, whole communities are supported by bacteria oxidising chemicals such as hydrogen sulfide or iron(II) ions.
If a question mentions “no light available” and then asks what the producers are, the answer is almost always chemoautotrophic bacteria. It is a favourite because it tests whether you think producer means green plant.
Heterotrophs
Heterotrophs cannot fix carbon dioxide, so they take carbon compounds ready-made from other organisms. The sequence is always the same: ingest, digest, absorb, rebuild.
The rebuilding step is called assimilation. The organism takes the small molecules it absorbed and constructs its own proteins, lipids and nucleic acids from them. Once assimilated, those carbon compounds are available to the next trophic level.
Type of heterotroph
Source of carbon compounds
Where digestion happens
Consumer
Living organisms it eats
Inside the body
Detritivore
Dead organic matter it ingests
Inside the body
Saprotroph
Dead matter and waste
Outside the body, then absorbed
Watch the odd one out. Saprotrophs digest externally — they secrete enzymes onto the dead material and then absorb the products. Fungi are the classic example.
Oxidation: how the energy gets released
Whichever way an organism obtained its carbon compounds, it releases the stored energy the same way — by oxidising them in respiration.
Aerobic respiration
glucose + oxygen → carbon dioxide + water (energy transferred to ATP, heat released)
Glucose is the usual fuel. Other carbon compounds, such as lipids, can be converted and respired too. The ATP produced then pays for the functions of life:
Metabolism — the enzyme-controlled reactions inside cells
Reproduction — producing offspring
Homeostasis — keeping internal conditions steady
Growth — increasing in size, which is where new biomass comes from
Response — sensing and reacting to the environment
Excretion — getting rid of metabolic waste
Nutrition — obtaining energy and nutrients
Growth is the one that matters for the rest of this topic. Carbon compounds used for growth are stored in the tissues, and that stored energy is exactly what the next trophic level can eat.
WORKED EXAMPLE
Distinguish between the way an autotroph and a heterotroph obtain their carbon compounds, and explain why both must carry out respiration. [4]
Point 1: autotrophsynthesises its own carbon compounds from inorganic substances such as carbon dioxidePoint 2: heterotrophingests carbon compounds already made by other organisms, then digests and assimilates themPoint 3: what respiration doesrespiration oxidises carbon compounds to transfer energy to ATPPoint 4: why both need itall organisms need ATP to carry out the functions of life, so both groups respirecommon error: saying plants only photosynthesise and do not respire
💡 Exam tip
Autotroph means it synthesises carbon compounds, not that it “makes food” or “makes energy”.
Always add from inorganic substances to your autotroph definition — it is often the marking point.
Remember both autotrophs and heterotrophs respire, day and night.
Use oxidation when describing how energy is released from carbon compounds.
Learn one named chemoautotroph example, e.g. iron-oxidising bacteria at deep-sea vents.
Detritivore versus saprotroph: the difference is internal versus external digestion.
⚠ Common mix-up
Autotroph is not a synonym for plant. Algae and some bacteria are autotrophs too.
Producer is not defined by being green. Chemoautotrophic bacteria are producers with no chlorophyll at all.
Chemoautotroph is not the same as heterotroph. Chemoautotrophs still build their own carbon compounds.
Fungi are not producers. They are heterotrophs that digest externally.
Photosynthesis does not “produce energy”. It converts light energy into chemical energy.
Assimilation is not digestion. Digestion breaks molecules down; assimilation builds new ones up.
Up next: Trophic Levels — you know how organisms get their carbon compounds. Now we number the steps so we can talk precisely about how far along the chain any organism sits.
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