IB Biology SLTopic 2 — Carbohydrates & LipidsPaper 1 & 2Structure and function~11 min read
Carbohydrates: Function & Examples
Starch, glycogen and cellulose are all built from glucose. Same monomer, near enough. Yet one feeds a potato, one feeds your muscles, and one is so tough that cows need bacteria to get through it. The difference comes down to a single hydroxyl group pointing up instead of down. This page is about how that tiny change ends up mattering so much.
📘 What you need to know
Monosaccharides are the monomers. General formula CnH2nOn, where n is the number of carbons.
Named by carbon count: triose (3, glyceraldehyde), pentose (5, ribose), hexose (6, glucose).
Glucose is C6H12O6 — soluble, stable, easy to transport, and the main substrate for respiration.
Glucose has two isomers: α-glucose (OH on carbon 1 below the ring) and β-glucose (OH above the ring).
Two monosaccharides join by condensation to give a disaccharide and a glycosidic bond. Many give a polysaccharide.
Starch (plants) and glycogen (animals and fungi) are made of α-glucose and store energy. Cellulose is made of β-glucose and gives structure.
Good storage molecules are compact and insoluble, so they do not affect the water potential of the cell.
Monosaccharides: the small ones
These are the single sugar units. They are colourless crystalline solids, they dissolve well in water, and they taste sweet. Their formula follows a neat pattern: for every carbon there is one oxygen and two hydrogens.
Monosaccharides only
CnH2nOn → a 6-carbon sugar is C6H12O6
Careful. That formula works for monosaccharides and nothing else. Maltose is two glucoses minus a water, so it is C12H22O11, not C12H24O12.
Type
Carbons
Example
Where you meet it
Triose
3
Glyceraldehyde
An intermediate in respiration
Pentose
5
Ribose, deoxyribose
The sugar in RNA and DNA
Hexose
6
Glucose, fructose, galactose
Respiration and food sugars
In an exam diagram, count the corners of the ring. A five-cornered ring is a pentose such as ribose. A six-cornered ring is a hexose such as glucose. Do not forget that one of those corners is an oxygen, not a carbon.
Glucose, and the two versions of it
In water, the straight chain of glucose curls round and its ends join, forming a ring. Depending on which way the ring closes, the hydroxyl group on carbon 1 ends up either below the ring or above it. That gives you the two isomers.
Only the group on carbon 1 moves. Every other atom is in the same place, which is exactly why students miss it in exam diagrams — check carbon 1 first, then stop looking.
What makes glucose so useful
Soluble in water, because of all those hydroxyl groups — so blood can carry it.
Stable, because its covalent bonds are strong and do not fall apart on their own.
Energy rich, because breaking those bonds in respiration releases energy to make ATP.
Small, so it crosses membranes through transport proteins and reaches every cell.
Joining sugars up
Two hydroxyl groups, one on each monosaccharide, react. Water is removed and a glycosidic bond forms between them. Between carbon 1 of one sugar and carbon 4 of the next you get a 1,4 glycosidic bond; a branch uses carbon 6 instead, giving a 1,6 glycosidic bond.
A cell that has spare glucose has a problem. Leaving it as glucose would be a disaster, so it gets packed away as a polysaccharide instead.
🤔 Why storage molecules must be insoluble
Dissolved glucose lowers the water potential inside the cell. Water then moves in by osmosis, and the cell swells — an animal cell with no wall would eventually burst. Lock those thousands of glucose molecules into one big insoluble starch grain and the water potential barely changes at all. Insoluble also means it cannot drift out of the cell, so the store stays where it is put.
Starch
The storage polysaccharide of plants, kept as grains inside chloroplasts. It is made of α-glucose and is really two molecules in one:
Amylose (about 10–30% of starch) — unbranched, joined only by 1,4 bonds. The chain coils into a helix, which makes it compact and slower to digest.
Amylopectin (about 70–90%) — has 1,4 bonds along the chain plus 1,6 bonds at branch points. Branches mean many free ends, so glucose can be removed from lots of places at once.
Glycogen
The animal and fungal version, stored as visible granules in liver and muscle cells. Also α-glucose, also 1,4 and 1,6 bonds — but more branched than amylopectin.
Follow the logic. More branches → more free ends → more places for enzymes to work at once → glucose released faster. Animals move, so they need energy quickly. Plants sit still, so they can afford a slower store. The structure fits the lifestyle.
Cellulose
Cellulose is made of β-glucose, and that changes everything. Because the OH on carbon 1 now points up, every other glucose has to flip upside down before a glycosidic bond can form. The result is a perfectly straight, unbranched chain rather than a coil.
Straight chains lie neatly side by side. Hydroxyl groups sticking out along them form hydrogen bonds between neighbouring chains. One hydrogen bond is weak; tens of thousands of them are not. Bundles of about 60–70 cellulose molecules bind into microfibrils, which build into fibres and give the cell wall its strength.
Enzymes can only chop from a free end. Count the red dots and you can see immediately why a branched store releases glucose faster than a coiled one.
Feature
Amylose
Amylopectin
Glycogen
Cellulose
Monomer
α-glucose
α-glucose
α-glucose
β-glucose
Branched?
No
Yes, about every 20 monomers
Yes, about every 10 monomers
No
Shape
Coiled helix
Branched
Highly branched
Straight chains
Bonds
1,4
1,4 and 1,6
1,4 and 1,6
1,4 plus hydrogen bonds between chains
Found in
Plant cells
Plant cells
Animal and fungal cells
Plant cell walls
Job
Energy store
Energy store
Energy store
Structural strength
Worked examples
WORKED EXAMPLE
A monosaccharide contains 5 carbon atoms. State its molecular formula and the name given to this type of sugar. [2]
Step 1: use the general formula
CnH2nOn with n = 5Step 2: fill it inC₅H₁₀O₅C₅H₁₀O₅, a pentose (e.g. ribose)5 carbons, so 10 hydrogens and 5 oxygens. Nothing to memorise.
WORKED EXAMPLE
Calculate the relative molecular mass of maltose, given that glucose has a relative molecular mass of 180. [2]
Step 1: two glucose molecules180 × 2 = 360Step 2: one condensation, so one water leaves360 − 18 = 342Relative molecular mass of maltose = 342Same −18 trick as the dipeptide. It never changes.
WORKED EXAMPLE
Explain how the structure of starch suits its function as a storage molecule in plants. [4]
Point 1
It is insoluble, so it does not change the water potential of the cell and water does not enter by osmosis.
Point 2
It is a large molecule, so it cannot cross the membrane and leave the cell.
Point 3
Amylose coils into a helix, so a lot of glucose is packed into a small space.
Point 4
Amylopectin is branched with many free ends, so it can be hydrolysed back to glucose quickly when needed.
4 marks: insoluble, large, compact, easily hydrolysedEach point pairs a structure with a consequence. That pairing is the mark.
WORKED EXAMPLE
Cellulose and amylose are both unbranched chains of glucose. Explain why only cellulose is strong enough to support a plant cell. [3]
Point 1
Cellulose is made of β-glucose, so alternate molecules are flipped upside down.
Point 2
This gives straight chains that lie parallel to each other, unlike the coiled amylose helix.
Point 3
Hydroxyl groups on neighbouring chains form many hydrogen bonds, holding them together as microfibrils.
β-glucose → straight chains → hydrogen bonding → strengthSay “many” hydrogen bonds. One on its own is weak, and examiners want you to know that.
💡 Exam tip
Learn the α/β rule as a picture: α is below, β is above. Alphabetical order, going up.
For “explain how structure suits function”, always answer in pairs: one structural feature, then what it lets the molecule do.
Say hydrolysed, not “broken down”, when glucose is released from starch or glycogen.
Glycogen is more branched than amylopectin. If a question compares plant and animal stores, that is usually the mark.
Use “insoluble so it does not affect water potential” — that exact chain of reasoning appears in mark schemes.
The general formula CnH2nOn applies to monosaccharides only. Never use it for a disaccharide.
⚠ Common mix-up
Saying cellulose is made of glucose and leaving it there. It has to be β-glucose, and that word is the mark.
Calling starch a monomer or glucose a polymer. Glucose is the monomer; starch is the polymer.
Thinking starch is one molecule. It is two: amylose and amylopectin.
Saying glycogen is found in plants. Glycogen is animals and fungi. Plants use starch.
Confusing hydrogen bonds with glycosidic bonds in cellulose. Glycosidic bonds run along a chain; hydrogen bonds run between chains.
Writing that branching makes the molecule stronger. Branching makes it faster to hydrolyse. Strength comes from hydrogen bonding in cellulose.
Forgetting the water in disaccharide mass questions. Two monomers minus 18, not just two monomers added.
Up next: The Role of Glycoproteins — what happens when a carbohydrate gets attached to a protein, and why it decides whether a blood transfusion saves someone or kills them.
Want this explained one-to-one?
Book a free session with an experienced IB Biology tutor and get your trickiest topics made simple.
