B1.1 Carbohydrates and lipids

Carbohydrates and lipids connect carbon chemistry, condensation and hydrolysis reactions, molecular structure, solubility, membrane formation, and energy storage functions in organisms.

Learning objectives

B1.1.1Carbon atom properties• Carbon forms four strong covalent bonds with C, H, O, N, S, and P• Carbon skeletons form chains, branches, rings, and single or double bonds• Functional groups give carbon compounds distinctive chemical properties B1.1.2Macromolecules by condensation• Condensation links monomers with covalent bonds and releases water• Polysaccharides, polypeptides, and nucleic acids are polymers made this way• Glycosidic, peptide, and phosphodiester bonds are key examples B1.1.3Digestion by hydrolysis• Hydrolysis breaks covalent bonds in polymers using water• Water provides -H and -OH groups to form monomers• Amylases, proteases, and nucleases catalyse hydrolysis of major biomolecules B1.1.4Form and function of monosaccharides• Pentoses such as ribose and hexoses such as glucose are monosaccharides• Glucose is soluble, transportable, chemically stable, and a direct respiratory substrate• Alpha- and beta-glucose differ at carbon 1 and form different polysaccharides B1.1.5Polysaccharides as energy storage• Starch stores energy in plants as amylose and amylopectin• Glycogen stores energy in animals and fungi and is more highly branched• Insolubility, compactness, and easy hydrolysis make both effective glucose stores B1.1.6Cellulose structure and function• Cellulose is made from beta-glucose joined by 1,4 glycosidic bonds• Alternating glucose orientation makes straight, unbranched chains• Hydrogen bonds form fibrils and fibres that strengthen plant cell walls B1.1.7Glycoproteins in cell-cell recognition• Glycoproteins and glycolipids form the external glycocalyx of membranes• Cell-surface carbohydrates enable self/non-self recognition, adhesion, and signalling• ABO blood group antigens show how surface sugars affect immune compatibility B1.1.8Hydrophobic properties of lipids• Lipids are hydrophobic, sparingly soluble in water, and soluble in non-polar solvents• Fats, oils, waxes, phospholipids, and steroids are lipid examples• Lipids are not true polymers because they are not built from repeating identical monomers B1.1.9Formation of triglycerides and phospholipids• Condensation forms ester bonds between glycerol and fatty acids• Triglycerides contain glycerol plus three fatty acids• Phospholipids contain glycerol, two fatty acids, and an ionized phosphate group B1.1.10Fatty acids• Saturated fatty acids have no carbon-carbon double bonds• Monounsaturated and polyunsaturated fatty acids have one or multiple double bonds• Double bonds create kinks, lower melting points, and affect membrane and storage properties B1.1.11Triglycerides functions• Triglycerides in adipose tissue store concentrated long-term energy• Insoluble fat stores avoid osmotic effects and can provide insulation, protection, and buoyancy• Fat oxidation releases more energy and metabolic water than carbohydrate of the same mass B1.1.12Phospholipid bilayers• Phospholipids are amphipathic with hydrophilic phosphate heads and hydrophobic tails• In water they form monolayers or bilayers with tails away from water• Bilayers are stable barriers and the basic structure of cell membranes B1.1.13Non-polar steroids• Steroids are mostly non-polar lipids with four fused carbon rings• Non-polar steroids pass through the hydrophobic core of phospholipid bilayers• Oestradiol and testosterone are cholesterol-derived steroid hormone examples

Carbon has four electrons available for bonding and can form four strong covalent bonds. It bonds to carbon and to elements such as H, O, N, S and P, so a stable skeleton can carry many different groups.

A carbon atom making four covalent bonds and carbon skeletons arranged as a chain, branch, ring and double-bonded structure.

Structural choice	New possibility
chain, branch or ring	different overall shapes
single or double bond	different geometry and reactivity
attached functional group	characteristic chemical behaviour

The carbon skeleton provides a stable framework; the arrangement of bonds and functional groups determines how the molecule behaves.

In a condensation reaction, an –OH from one molecule and an –H from another leave as H₂O. A new covalent bond joins the remaining parts.

Two molecules lose OH and H as water while a new covalent bond forms between them.

Subunits joined	Larger molecule	Linkage formed
monosaccharides	polysaccharide	glycosidic
amino acids	polypeptide	peptide
nucleotides	nucleic acid	phosphodiester

Each joining event releases one water molecule. Repeating the reaction builds a macromolecule; the covalent linkage, not the released water, holds its subunits together.

During hydrolysis:

water is a reactant
a covalent linkage breaks
water supplies –H to one product and –OH to the other
enzyme-catalysed repetitions release small, absorbable molecules

Enzyme class	Bond-containing substrate	Main small products
amylase	starch	shorter sugars, then glucose
protease	polypeptide	peptides and amino acids
nuclease	nucleic acid	nucleotides

Condensation produces water while forming a bond. Hydrolysis consumes water while breaking one. The reactions are reverse in chemistry but are controlled by different enzymes in cells.

Feature in a ring diagram	What it identifies
5 carbon atoms in the molecule	a pentose such as ribose
6 carbon atoms in the molecule	a hexose such as glucose
carbon-1 –OH below the α-glucose ring	α-glucose
carbon-1 –OH above the β-glucose ring	β-glucose

α- and β-glucose have the same molecular formula but differ in the position of the –OH attached to carbon 1. Enzymes can distinguish this small geometric difference.

That one orientation controls the polymer route: α-glucose builds starch and glycogen, whereas β-glucose builds cellulose.

Property of glucose	Biological consequence
many polar –OH groups	dissolves in aqueous cytoplasm and blood plasma
small molecule	can be transported and taken up by cells
stable covalent ring	can circulate without reacting at random
energy-rich C–H bonds	acts directly as a respiratory substrate
functional groups for condensation	can be assembled into larger carbohydrates

Photosynthesis supplies glucose; transport brings it to cells; respiration transfers some of its chemical energy to ATP. When immediate fuel is not needed, condensation can divert glucose into storage or structural polysaccharides.

Solubility makes glucose useful in transit but unsuitable for storing large amounts inside a cell: many dissolved glucose molecules would lower water potential and draw in water.

Starch and glycogen are compact, insoluble stores of α-glucose. Their large molecules do not diffuse away and have far less osmotic effect than the same number of separate glucose molecules.

Store	Main structure	What the structure changes
amylose in plant starch	mostly unbranched α-1,4 chain, coiled	compact but relatively few chain ends
amylopectin in plant starch	α-1,4 chains with α-1,6 branches	more ends available for enzyme action
glycogen in animals and fungi	more frequent α-1,6 branching	many ends allow rapid glucose release

Branching does not add more energy to each glucose unit. It increases the number of chain ends that enzymes can hydrolyse at the same time, so stored fuel can be mobilised faster.

β-glucose monomers join by β-1,4 glycosidic bonds.
Alternate monomers are rotated 180°, producing a straight, unbranched chain.
Parallel chains form many hydrogen bonds.
Chains bundle into microfibrils; microfibrils combine into fibres.

Straight beta-glucose chains joined by many hydrogen bonds, bundled into a microfibril within a plant cell wall.

Each hydrogen bond is weak, but many acting in parallel give high tensile strength. Cellulose therefore resists stretching and helps a water-filled plant cell maintain its shape without bursting.

Short carbohydrate chains attached to membrane proteins and lipids project from the outer membrane surface. Together they form the glycocalyx: a chemically varied cell-surface coat.

The exposed carbohydrate patterns act as:

identity markers for self/non-self recognition
binding sites in cell adhesion
parts of receptors used in cell signalling

ABO antigens differ in their terminal surface sugars. A recipient's antibodies can bind an unfamiliar antigen and agglutinate donor red cells, showing that small carbohydrate differences can have whole-organism consequences.

Red blood cells of groups A, B, AB and O showing the presence or absence of A and B surface antigens.

Starting form	Linkage or arrangement	Emergent form	Main job
α-glucose	α-1,4 with optional α-1,6 branches	amylose, amylopectin, glycogen	energy storage
β-glucose	alternating β-1,4 chain; hydrogen-bonded bundles	cellulose fibres	tensile support
short surface sugars	attached to protein or lipid; exposed outside	glycocalyx markers	recognition, adhesion, signalling

Condensation builds a covalent linkage and releases water; hydrolysis consumes water and breaks that linkage. Which enzyme acts depends on the bond and substrate.

For a structure–function explanation:

name the monomer or functional group
identify the linkage and three-dimensional arrangement
state the resulting property
connect that property to the biological job

Long hydrocarbon regions contain mostly non-polar C–C and C–H bonds. They cannot replace the hydrogen-bonding interactions that water molecules make with one another, so water excludes them and lipid molecules aggregate.

Observation	Structural explanation
poor solubility in water	large non-polar region has little attraction to water
solubility in non-polar solvents	similar intermolecular interactions are compatible
fat droplets coalesce	hydrophobic regions minimise contact with water

Fats, oils, waxes, phospholipids and steroids are grouped as lipids by physical behaviour, not by one repeating monomer. Lipids are therefore not true polymers.

A fatty-acid carboxyl group reacts with a glycerol hydroxyl group. Condensation releases water and forms an ester bond. Glycerol has three hydroxyl groups, so it provides three attachment positions.

A triglyceride with glycerol and three fatty acids beside a phospholipid with glycerol, two fatty acids and a phosphate head.

Product	Components attached to glycerol	Overall character
triglyceride	3 fatty acids	almost entirely hydrophobic
phospholipid	2 fatty acids + ionised phosphate group	amphipathic: polar head, non-polar tails

Glycerol is an alcohol, not a fatty acid. Replacing one fatty acid with phosphate changes not only the parts list but the molecule's behaviour in water.

Fatty acid	C=C double bonds	Typical tail shape
saturated	0	relatively straight
monounsaturated	1	one cis kink
polyunsaturated	2 or more	several kinks

Straight saturated and kinked unsaturated fatty-acid structures with space-filling models and tightly versus loosely packed triglycerides.

More cis double bonds prevent neighbouring tails from approaching closely. Intermolecular attractions are weaker, so less thermal energy is needed to separate the molecules and the melting point falls.

At the same temperature, tightly packed saturated lipids are more likely to be solid; more unsaturated lipids are more likely to remain fluid.

Storage property	Triglyceride consequence
highly reduced C–H-rich molecules	more than twice the energy per unit mass of carbohydrate
insoluble in water	stored without lowering water potential
oxidised completely	yields ATP, carbon dioxide and substantial metabolic water

Glycogen and triglyceride entering respiration, comparing rapid carbohydrate use with higher energy yield and metabolic water from fat.

Adipose triglyceride also provides:

thermal insulation beneath the skin
cushioning around organs
buoyancy when stored as low-density blubber

Triglyceride is compact long-term fuel, but it is mobilised less rapidly than branched glycogen and its oxidation requires more oxygen. Storage suitability depends on the biological timescale and demand.

A phospholipid is amphipathic: its ionised phosphate head is hydrophilic, while its two hydrocarbon tails are hydrophobic.

In water, phospholipids arrange so that:

heads remain in contact with water
tails turn away from water
a surface favours a monolayer
water on both sides favours a bilayer with tails enclosed inside

A labelled phospholipid and its arrangement as a monolayer at a water surface and a bilayer when surrounded by water.

Self-assembly lowers the exposure of non-polar tails to water. The resulting stable bilayer has a hydrophobic core that restricts many ions and polar solutes, forming the basic boundary of a cell.

A steroid has a characteristic core of four fused carbon rings. Cholesterol, oestradiol and testosterone share this framework even though their functional groups differ.

For a non-polar steroid hormone:

the molecule leaves the aqueous phase at the membrane surface
it dissolves in the bilayer's non-polar interior
random molecular motion carries it across
it re-enters the aqueous phase on the other side

Molecule property	Passage through the bilayer core
small and mostly non-polar	favoured
ionised or strongly polar	strongly restricted without a protein route

Crossing the membrane is not the same as travelling freely through blood. Hydrophobic steroids often require carrier proteins in aqueous plasma, then diffuse through target-cell membranes.

Structure seen	Property predicted	Main biological outcome
3 fatty-acid tails on glycerol	hydrophobic, highly reduced	dense long-term energy store
more cis C=C bonds in tails	poorer packing, lower melting point	greater fluidity at a given temperature
phosphate head + 2 tails	amphipathic	self-assembled bilayer barrier
4 fused carbon rings, mostly non-polar	lipid-soluble	diffusion through bilayer core

Ester condensation joins fatty acids to glycerol. The components retained in the product determine whether it behaves as a hydrophobic store or an amphipathic membrane molecule.

Across carbohydrates and lipids, use the same reasoning chain:

atoms and functional groups → bonds → molecular shape and polarity → interactions with water and other molecules → biological function

4 marks

Outline the chemical properties of carbon that allow it to form diverse compounds.

5 marks

Outline the production of a dipeptide by a condensation reaction, showing the structure of a generalized dipeptide.

6 marks

Describe the importance of hydrolysis in digestion.

5 marks

Outline how the properties of glucose are linked to their uses in organisms.

4 marks

Outline how and where energy is stored in plants.

3 marks

Describe how cellulose is formed from monosaccharides.

1 mark

Which substance must be transported in the blood by lipoprotein complexes?

1 mark

Identify the molecule that was used to form part Y of the triglyceride.

4 marks

Distinguish between the structures of the different types of fatty acids in food.

4 marks

Outline the use of lipids to store energy in humans.

2 marks

Annotate the diagram to illustrate the amphipathic nature of phospholipids.

1 mark

Testosterone is a hormone that is important for male reproductive development.

To which group of compounds does testosterone belong?

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—B1.2 ProteinsRead

B1.1 Carbohydrates and lipids

Learning objectives

Four bonds let carbon vary without losing stability

Condensation stores subunits in a covalent bond

Hydrolysis uses water to release monomers

Read a sugar ring before naming its role

Glucose is mobile fuel and a stable building block

Branching controls access to stored glucose

Cellulose scales weak attractions into strong fibres

Surface sugars make cells distinguishable

Carbohydrate summary: follow monomer to biological job

Hydrocarbon regions make lipids avoid water

One ester reaction builds two different lipid types

Double bonds loosen fatty-acid packing

Triglycerides are dense, long-term energy stores

Phospholipids hide their tails to make a barrier

A steroid crosses a bilayer through its non-polar core

Lipid summary: predict behaviour from molecular regions

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