Every single molecule in your body that does anything interesting is built around carbon. Not because carbon is rare or special-looking, but because of one plain fact: it can make four bonds at once. That one fact is where the whole of biochemistry comes from, and it is the thing examiners want you to explain.
📘 What you need to know
A covalent bond is a shared pair of electrons. We draw it as a short straight line, like H—H.
Carbon has four electrons in its outer shell, so each carbon atom forms four covalent bonds.
Those four bonds point to the corners of a tetrahedron, so carbon molecules have real 3D shapes.
Carbon bonds to other carbons and to hydrogen, oxygen, nitrogen and sulfur, giving chains, branches and rings.
Carbon can form single or double bonds (methane has four singles, carbon dioxide has two doubles).
All four groups of biological molecules — carbohydrates, lipids, proteins and nucleic acids — are carbon based.
Functional groups (hydroxyl, carboxyl, amino, phosphate) are the bits that give a molecule its behaviour.
Why four bonds changes everything
Atoms are stable when their outer shell is full. Carbon has four electrons out of the eight it needs, which puts it in an unusual position: it is exactly halfway. It cannot easily give four electrons away, and it cannot easily take four in. So it does the only thing left — it shares. Four times over.
Sharing a pair of electrons with another atom is a covalent bond, and covalent bonds are strong. That is the second half of the story. Carbon does not just make lots of bonds; it makes lots of strong bonds, which is why a molecule of DNA or starch can be enormous and still hold together.
Methane is drawn flat here to keep it simple, but the four bonds actually spread out into 3D. That shape is why two molecules with the same atoms can behave completely differently.
If a question asks why carbon is so important, do not just say “it makes four bonds” and stop. Say what the four bonds let it do — long chains, branches, rings, and stable giant molecules. That is where the marks are.
Chains, branches and rings
Because carbon can bond to other carbon atoms, you can join them end to end almost forever. That gives you a skeleton, and everything else hangs off it. Change the shape of the skeleton and you change what the molecule does.
Long straight chains — cellulose in a plant cell wall.
Long branched chains — glycogen, your own glucose store.
Single rings — glucose in solution, and the bases thymine, uracil and cytosine.
Several rings joined together — starch bases like adenine and guanine, and steroids such as cholesterol.
Chemists leave out the C and H labels on skeletons like these. Every corner is a carbon, and enough hydrogens are assumed to fill up its four bonds.
Single and double bonds
Carbon does not have to spread its four bonds between four different partners. It can use two of them on the same neighbour, which gives a double bond.
Methane has four single bonds, one to each hydrogen.
Carbon dioxide has two double bonds, one to each oxygen.
A molecule with a C=C double bond is called unsaturated. You will meet that word again with fatty acids.
Count to four. Whatever you draw, every carbon atom must end up with exactly four lines coming out of it. A double bond counts as two. If your carbon has three or five, the structure is wrong — and that is an easy mark to lose in a drawing question.
Functional groups: the working parts
A long carbon and hydrogen skeleton is not very reactive on its own. The interesting chemistry comes from small clusters of atoms attached to it, called functional groups. Learn these four and a lot of Biology stops looking like a memory test.
These groups keep coming back all year. Hydroxyl groups join sugars together, carboxyl groups join to glycerol in fats, and phosphate groups build the backbone of DNA.
Group
Written as
Where you meet it
Hydroxyl
—OH
Glucose, glycerol; makes molecules soluble in water
Carboxyl
—COOH
Fatty acids and amino acids; slightly acidic
Amino
—NH2
Amino acids, so every protein you own
Phosphate
—PO4
Phospholipids, DNA, RNA and ATP; carries a negative charge
🤔 Why does this matter for the rest of the topic?
Almost every reaction you will learn in this topic is two functional groups meeting and joining up. Two hydroxyl groups make a glycosidic bond in a sugar. A carboxyl group and a hydroxyl group make an ester bond in a fat. An amino group and a carboxyl group make a peptide bond in a protein. Same idea three times over.
A quick word on units
Biological molecules are small and their measurements come with SI prefixes. Scientists worldwide agreed these so that a result from a lab in Lahore means exactly the same thing in Lisbon.
Scale check. A cell is measured in micrometres, a membrane in nanometres. A phospholipid bilayer is about 7–10 nm thick, which is roughly a ten-thousandth of the width of a hair.
Worked examples
WORKED EXAMPLE
Explain why carbon can form such a large variety of molecules. [3]
Point 1
Carbon has 4 electrons in its outer shell, so it forms 4 covalent bonds.
Point 2
It can bond to other carbon atoms, so it builds long chains, branched chains and rings.
Point 3
It can also form double bonds and bond to H, O, N and S, giving many different shapes and functional groups.
3 marks: four bonds + joins to itself + variety of shapesOne mark per separate idea. Do not write the same idea three ways.
WORKED EXAMPLE
A carbon atom in the middle of a hydrocarbon chain is bonded to two neighbouring carbons by single bonds. How many hydrogen atoms are attached to it? [1]
Step 1: total bonds available
Every carbon makes 4 bonds. No exceptions.
Step 2: subtract the bonds already used4 − 2 = 2 bonds left over.
2 hydrogen atomsThis is exactly how you check a fatty acid is saturated.
WORKED EXAMPLE
A molecule contains both an amino group and a carboxyl group. Identify the type of molecule and the polymer it forms. [2]
Step 1: read the functional groups
—NH₂ is the amino group and —COOH is the carboxyl group.
Step 2: match them to a family
Only one monomer carries both.
An amino acid, which joins up to form a polypeptide (protein)Functional groups are the fingerprint of the molecule family.
💡 Exam tip
Learn one clean sentence: carbon has four outer electrons, so it forms four covalent bonds. Almost every carbon question starts there.
When you draw a structure, count the lines around every carbon. Four, always.
“Explain” means give a reason. Fact plus consequence: four bonds, so it can form long stable chains.
Learn the four functional groups by their formula, not just their name. Papers usually show the formula.
Remember the four molecule families in one breath: carbohydrates, lipids, proteins, nucleic acids.
If a question mentions a double bond, the word the examiner is fishing for is usually unsaturated.
⚠ Common mix-up
“Carbon has four bonds” is not the same as “carbon has four electrons”. It has four outer electrons, and that is why it makes four bonds. Say both.
Covalent is not ionic. Covalent means atoms share electrons. Nothing is given away, so nothing becomes a charged ion.
Molecules are not flat. We draw them flat on paper, but the four bonds spread into 3D. Shape questions depend on this.
A double bond is two bonds, not one. Count it as two when you check your drawing.
Do not mix up hydroxyl (—OH) and carboxyl (—COOH). Carboxyl contains a hydroxyl, plus a C=O as well.
Not every carbon compound is organic in a living sense. Carbon dioxide contains carbon but is not a biological macromolecule.
Up next: Macromolecules — how these small carbon molecules get joined into giant ones, and how your gut takes them apart again.
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