IB Biology HLCarbohydrates & LipidsPaper 1 & 2~10 min read
Properties of Carbon
Life on Earth is described as “carbon based” for one simple reason: carbon has four electrons in its outer shell. That single fact is what lets one element build sugars, fats, proteins and DNA.
📚 What you need to know
A covalent bond forms when a pair of electrons is shared between two atoms. A single bond is drawn as a short straight line.
Carbon has four electrons in its outer shell, so each atom can form four covalent bonds.
Carbon is present in all four major categories of biological molecule: carbohydrates, lipids, proteins and nucleic acids.
Carbon forms millions of different covalently bonded compounds, mainly with hydrogen and oxygen.
It can build straight chains, branched chains, single rings and multiple rings, and produces a tetrahedral arrangement giving different 3-D shapes.
It forms up to four single bonds, or a combination of double and single bonds. Double and triple bonds between carbons allow unsaturated compounds.
Learn four functional groups: hydroxyl, carboxyl, amino and phosphate.
Carbon forms covalent bonds
Definition
A covalent bond forms when a pair of electrons is shared between two atoms
Electrons are shared between atoms to generate strong bonds within compounds. On a diagram, a single covalent bond is shown as a short straight line between the two atoms, for example H–H.
Carbon has four electrons in its outer shell. To fill that shell it needs four more, so each carbon atom can form four covalent bonds. That is the property everything else on this page follows from — four bonds is enough to hold a molecule together and still have spare connections for building outwards.
Count the lines around each carbon in any exam diagram. If there are not exactly four, the structure has been drawn wrong.
Carbon in biological molecules
Carbon is present in all four major categories of biological molecule, which is why life on Earth is often described as carbon based.
Category
Example
Carbohydrates
Glucose, starch, cellulose
Lipids
Triglycerides, phospholipids
Proteins
Enzymes, haemoglobin
Nucleic acids
DNA and RNA
The shapes carbon can build
Because it can bond to other carbon atoms as well as to hydrogen, nitrogen, oxygen and sulfur, carbon can arrange itself into a huge variety of compounds.
Long branched chains, such as glycogen.
Long straight chains, such as cellulose.
Molecules containing cyclic single rings, such as the pyrimidines — thymine, uracil and cytosine.
Molecules with multiple rings, including starches and the purines, adenine and guanine.
A tetrahedral structure, which allows carbon compounds with different 3-D shapes and therefore different biological properties.
Carbon atoms can form up to four single covalent bonds, or a combination of double and single bonds. Carbon dioxide contains two double bonds; methane contains four single ones. Double and triple bonds can also form with an adjacent carbon atom, which is what allows unsaturated compounds to exist — a point that comes back on the fatty acids page.
The tetrahedral shape matters more than it looks. Because four bonds point into three dimensions rather than lying flat, two molecules with the same atoms can have genuinely different shapes — and in biology, shape is function.
Functional groups
Carbon atoms also form part of many different functional groups: small clusters of atoms that give an organic compound its individual properties. Four are worth knowing by sight.
You will meet all four again: hydroxyl groups make glycosidic bonds, carboxyl groups make peptide and ester bonds, and phosphate makes the head of a phospholipid.
NOS: scientific conventions are agreed internationally
The professional scientific community is global. Scientists all over the world may work on the same research and need to communicate clearly with each other, so conventions are agreed upon and used internationally.
Drawing a single covalent bond as a short straight line is one such convention. SI unit prefixes are another.
Prefix
Multiplier
kilo
103
centi
10–2
milli
10–3
micro
10–6
nano
10–9
Why it matters for you. SI stands for système international. Cell measurements come up constantly, so these prefixes are worth knowing cold — a cell is measured in micrometres, a membrane in nanometres.
Worked examples
WE 1
Why carbon
Explain why carbon is able to form such a large number of different compounds. (3 marks)
Point 1: the electrons
Carbon has four electrons in its outer shell, so each atom can form four covalent bonds.
Point 2: what that allows
It can bond to other carbon atoms as well as to hydrogen, nitrogen, oxygen and sulfur, forming straight chains, branched chains and rings.
Point 3: shape
The tetrahedral arrangement of the bonds gives compounds different 3-D shapes, and therefore different biological properties.
Four bonds, many partners, three dimensionssay “covalent” explicitly — the mark scheme usually wants the bond type named
WE 2
Reading a structure
A student draws a carbon atom with three lines coming from it. State what is wrong and explain why. (2 marks)
Point 1: the error
Carbon must show four bonds, not three.
Point 2: the reason
It has four outer electrons, so it forms four covalent bonds — either four single bonds, or a combination including a double bond, which counts as two.
Four lines around every carbon, alwaysa double bond is drawn as two lines but is one bond using two shared pairs
💡 Exam tips
Define a covalent bond as a shared pair of electrons, not just “atoms joined together”.
Always link carbon’s versatility back to its four outer electrons.
Name all four categories of biological molecule if asked where carbon is found.
Have an example ready for each shape: glycogen branched, cellulose straight, pyrimidines single ring, purines multiple rings.
Learn the four functional groups by their formulas, not just their names.
Know the SI prefixes down to nano.
⚠ Common mistakes
Saying electrons are transferred. That is ionic bonding. Covalent bonding is sharing.
Drawing carbon with the wrong number of bonds. It is always four.
Counting a double bond as one line. It uses two of carbon’s four bonds.
Confusing carboxyl and hydroxyl. Carboxyl is –COOH and is acidic; hydroxyl is just –OH.
Writing “carbon based” without explaining it. The explanation is that carbon appears in all four molecule categories.
Up next: Macromolecules — how those carbon compounds get joined into polymers, and the two reactions that build and break every one of them.
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