Difficulty: Introductory | Prerequisites: Basic chemistry (atoms, electrons, bonding).
This topic sits at the very start of introductory biology: understanding how compounds are classified, and why carbon is the backbone of living chemistry. It bridges general chemistry (atomic structure, bonding) with the biological macromolecules you will study for the rest of the course. If you are comfortable with atoms, electrons and covalent bonds, you are ready. If not, review basic atomic structure first.
Compounds split into two camps: inorganic (water, minerals, CO₂) and organic (carbon-based, the stuff of life). Carbon's ability to form four bonds lets it build chains, branches and rings, which are the scaffolding for the four major macromolecules: carbohydrates, lipids, proteins and nucleic acids. Those macromolecules are assembled by condensation reactions and taken apart by hydrolysis.
Inorganic compound
A compound that generally does not contain carbon. Water and mineral salts are the classic examples. A handful of carbon-containing molecules (CO₂, carbonates) are still classed as inorganic.
In simple terms: the non-living-chemistry side of the ledger.
Organic compound
A carbon-containing compound, typically with C–H bonds, found widely in living systems. Organic molecules are responsible for the colours, tastes and odours of life.
Think of it as: any molecule built on a carbon skeleton.
Valence
The number of bonds an atom can form, determined by its outermost (valence) electrons. Carbon has a valence of four, meaning it can bond with up to four other atoms.
In simple terms: how many "hands" an atom has for holding on to neighbours.
Hydrocarbon
A molecule made entirely of carbon and hydrogen atoms. Hydrocarbons can be straight chains, branched chains, or rings.
Think of it as: the simplest possible organic molecule, just C and H.
Macromolecule
A very large molecule built from smaller subunits. In biology, the four classes are carbohydrates, lipids, proteins and nucleic acids.
In simple terms: the big molecules that do the heavy lifting in cells.
Polymer
A long chain of repeating molecular subunits (monomers) linked together. Carbohydrates, proteins and nucleic acids are all polymers.
Think of it as: a train, where each carriage is a monomer.
Monomer
The individual repeating unit that links together to form a polymer. Amino acids, nucleotides and monosaccharides are all monomers.
In simple terms: the single building block.
Condensation reaction (dehydration synthesis)
A chemical reaction that joins two monomers by removing a water molecule. This is how cells build polymers.
Think of it as: snapping two Lego bricks together, with a drop of water popping out.
Hydrolysis
A chemical reaction that breaks a bond between monomers by adding a water molecule. This is how cells break down polymers.
In simple terms: the reverse of condensation, water goes in, the chain splits.
Include water, mineral salts and gases such as CO₂
Generally do not contain carbon, though a few carbon-containing molecules (CO₂, carbonates, bicarbonates) are still classified as inorganic
Essential to life but are not built on carbon skeletons
Contain carbon, almost always bonded to hydrogen and often to oxygen, nitrogen, sulphur or phosphorus
Responsible for the structural and functional diversity of living systems
Contribute to the colours, tastes and odours of biological organisms
Carbon has four valence electrons, giving it a valence of four
It can form single bonds (one shared pair), double bonds (two shared pairs) and triple bonds (three shared pairs)
It bonds readily with other carbon atoms, enabling long chains, branched chains and ring structures
In structural diagrams, single bonds are drawn as one line, double bonds as two lines, triple bonds as three lines
Straight chains: carbons linked end to end (e.g. methane CH₄, butane C₄H₁₀)
Branched chains: a main chain with side groups attached (e.g. isobutane, 2-methylbutane)
Hydrocarbons with double bonds: at least one C=C double bond in the chain (e.g. ethylene C₂H₄, propylene C₃H₆)
Circular (ring) chains: carbon atoms form a closed loop (e.g. methylbenzene, ethylbenzene)
Carbohydrates: energy storage and structural support (e.g. glucose, starch, cellulose)
Lipids: energy storage, cell membrane structure, signalling (e.g. fats, phospholipids, steroids). Note: lipids are not polymers
Proteins: catalysis (enzymes), structural support, transport, immune defence (e.g. haemoglobin, collagen, antibodies)
Nucleic acids: storage and transmission of genetic information (DNA and RNA)
Three of the four classes (carbohydrates, proteins, nucleic acids) are polymers built from repeating monomer units
Lipids are the exception: they are macromolecules but not true polymers
Biological processes depend on both building and breaking down these polymers
Two monomers join together
A molecule of water (H₂O) is released as a byproduct
This is how cells assemble polymers from their monomers
A water molecule is consumed to break a bond between two monomers
This is how cells disassemble polymers back into monomers
Enables recycling of molecular building blocks
Carbon's bonding versatility is why pharmaceutical companies can design millions of distinct drug molecules: each rearrangement of the carbon skeleton changes how the drug interacts with the body.
Hydrolysis is what your digestive system does to the food you eat: enzymes break polymers in bread (starch) and meat (protein) into monomers your cells can absorb.
Plastics are synthetic organic polymers, built on the same carbon-chain principles as biological polymers, which is why they are so durable and so difficult to biodegrade.
Students often think "contains carbon = organic." It does not. CO₂ and carbonates contain carbon but are classified as inorganic.
Students sometimes assume all four macromolecule classes are polymers. Lipids are macromolecules but are not built from repeating monomer chains.
A double bond does not mean the molecule is twice as strong. A C=C double bond is stronger than a single bond, but it also makes the molecule more rigid and more chemically reactive at that site.
Hydrolysis and condensation are often confused. Remember: hydrolysis adds water to break bonds ("hydro" = water, "lysis" = splitting). Condensation removes water to form bonds.
⚠️ Be able to explain why carbon is central to organic chemistry (four valence electrons, can form four bonds, bonds to itself)
⚠️ Know the difference between organic and inorganic compounds, including the exceptions (CO₂ is inorganic despite containing carbon)
⚠️ Be able to name and distinguish the four classes of macromolecules and state their primary biological roles
⚠️ Understand the relationship between monomers and polymers, and know which macromolecule class is not a polymer (lipids)
⚠️ Be able to compare condensation reactions and hydrolysis: what goes in, what comes out, what is built or broken
True or false: CO₂ is an organic compound because it contains carbon. (False. CO₂ is classified as inorganic.)
Fill in the blank: Carbon has ______ valence electrons and can form up to ______ covalent bonds. (Four; four.)
True or false: Lipids are polymers made of repeating monomer units. (False. Lipids are macromolecules but not polymers.)
Fill in the blank: A reaction that joins two monomers by releasing water is called a ______ reaction. (Condensation, or dehydration synthesis.)
True or false: Hydrolysis breaks bonds by removing water from a molecule. (False. Hydrolysis breaks bonds by adding water.)
Q: What property of carbon makes it uniquely suited to forming the backbone of organic molecules?
A: Carbon has four valence electrons, allowing it to form up to four stable covalent bonds with other atoms, including other carbon atoms. This enables the formation of long chains, branched structures and rings.
Q: Name the four classes of biological macromolecules and give one function of each.
A: Carbohydrates (energy storage and structural support), lipids (cell membrane structure and energy storage), proteins (catalysis, transport, structural support), nucleic acids (storage and transmission of genetic information).
Q: Explain the difference between a condensation reaction and hydrolysis.
A: A condensation reaction joins two monomers together and releases a water molecule. Hydrolysis does the opposite: it uses a water molecule to break the bond between two monomers. Condensation builds polymers; hydrolysis breaks them down.
Q: Why is CO₂ classified as inorganic even though it contains carbon?
A: By convention, certain simple carbon-containing molecules (CO₂, carbonates, bicarbonates) are classified as inorganic because they lack the C–H bonds and complex carbon skeletons characteristic of organic compounds.
Q: Which of the four macromolecule classes is not a true polymer, and why?
A: Lipids. Unlike carbohydrates, proteins and nucleic acids, lipids are not assembled from repeating monomer subunits linked in long chains. They are large molecules but do not fit the polymer definition.
This material connects directly to cell biology: once you understand the four macromolecule classes, you can follow how cell membranes are built (lipid bilayers), how enzymes catalyse reactions (proteins), and how genetic information flows (nucleic acids).
It also links back to general chemistry: covalent bonding, electronegativity and molecular geometry all underpin why carbon behaves the way it does.
Later units on metabolism (cellular respiration, photosynthesis) depend heavily on understanding condensation and hydrolysis, since these reactions are how energy is stored in and released from macromolecules.
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