B1.1 Carbohydrates and Lipids

Why carbon supports molecular diversity?

Covalent Bond Definition

A covalent bond forms when atoms share one or more pairs of electrons. Carbon has four valence electrons and can form up to four covalent bonds. These can be four single bonds or combinations of single and double bonds. Carbon also bonds to other carbon atoms, creating stable carbon skeletons.

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Catenation: carbon-carbon bonding permits long unbranched ends.

Branching: side chains change shape, compactness and the number of accessible ends.

Rings: carbon skeletons can form single rings, as in glucose, or several fused rings, as in steroids.

Functional groups: oxygen, nitrogen- and phosphorus-containing groups change polarity and chemical reactivity.

Common trap: Do not say that carbon 'has four bonds'. An isolated carbon atom does not permanently possess four bonds; it can form up to four covalent bonds.

Polysaccharides

Monosaccharides linked by glycosidic bonds.

Polypeptides

Amino acids linked by peptide bonds.

Nucleic Acids

Nucleotides linked by phosphodiester bonds.

Triglycerides

One glycerol linked to three fatty acids by ester bonds.

Monomer Definition

Definition: A small molecular subunit that can be joined to other subunits.

Example: Glucose in a polysaccharide.

Polymer Definition

Definition: A large molecule made of many repeating monomer units linked covalenty.

Example: Starch, glycogen or cellulose.

Macromolecule Definition

Definition: A very large biological molecule made from many polymers.

Example: A polysaccharide; the term also includes proteins and nucleic acids.

Lipid

Definition: A hydrophobic biological substance, not necessarily a polymer.

Example: Triglycerides and phospholipids are assembled by condensation but are not chains of repeating identical monomers.

Condensation Definition

A condensation reaction joins molecules by forming a new covalent bond while releasing a water molecule.

The -OH comes from one reacting group and the -H from another. Enzymes control these reactions in cells.

Hydrolysis Definition

A hydrolysis reaction breaks a covalent bond by adding water. The water molecule is split; -H is added to

one product and -OH to the other. Hydrolysis is therefore not merely 'adding water around a molecule': the

atoms of water become part of the products.

What is a monosaccharide (Reducing Sugars)?

A monosaccharide is a single sugar unit. In aqueous biological conditions, many monosaccharides form rings.

You should recognise pentoses and hexoses from molecular diagrams rather than rely only on a formula.

Different Examples of Monosaccharides (Reducing Sugars)

Pentose: Has 5 carbon atoms. In a common ring form, 4 carbon atoms are in the ring and one is outside.

Hexose: Has 6 carbon atoms. In a common ring form, 5 carbon atoms are in the ring and one is outside.

Ring Oxygen: 1 position in the ring is often oxygen, so do not count every corner as a carbon atom.

Examples: Ribose is a pentose while glucose is a hexose.

**Common trap: A hexose ring is usually drawn as a six-membered ring, but one member is oxygen. Count

carbon labels and external carbon groups, not polygon sides (Cannot count oxygen).**

Glucose Properties Linked to Use (Solubility and Transportability; Energy Transport Media)

Glucose contains several polar hydroxyl (-OH) groups. These can form hydrogen bonds with water,

making glucose soluble.

A small soluble molecule can be carried in blood plasma, tissue fluid, phloem

sap and cytosol.

Membrane proteins are still required for movement through the hydrophobic core of

most cell membranes.

Glucose Properties Linked to Use (Chemical Stability and Energy Yield)

Glucose has stable covalent bonds (does not break easily), so it does not spontaneously release all of its energy.

Enzyme-controlled oxidation during cell respiration transfers energy in manageable steps.

Its many C-H bonds make it a useful fuel while its solubility allows rapid delivery to cells.

The Formation of a Glycosidic Bond

A glycosidic bond is the covalent bond formed between monosaccharides by condensation.

Alpha 1,4 Glycosidic Bond

Where it occurs?

Amylose and the main chains of amylopectin and glycogen.

Structural effect?

Permits coiling; forms storage chains.

Alpha 1,6 Glycosidic Bond

Where it occurs?

Branch points in amylopectin and glycogen.

Structural effect?

Creates branches and additional terminal ends.

Beta 1,4 Glycosidic Bond

Where it occurs?

Cellulose.

Structural effect?

Alternating monomer orientation producing straight chains (makes it stronger).

Disaccharides Examples (Maltose)

Monosaccharides: Glucose + Glucose

Maltose Use:

Product of starch digestion, useful in condensation.

Disaccharides Examples (Sucrose)

Monosaccharides: Glucose + Fructose

Sucrose Use:

Transport sugar in many plants

Disaccharides Examples (Lactose)

Monosaccharides: Glucose + Galactose

Lactose Use:

Sugar in milk.

Why polymerise glucose?

Compactness: coiling and branching pack many glucose units into a small volume.

Relative insolubility: large molecules do not dissolve readily and are retained in cells.

Low osmotic effect: many glucose units are stored as one molecule rather than many separate solute

particles.

Reversible mobilisation: condensation adds alpha-glucose; hydrolysis removes it when energy is required.

Many ends: branching provides terminal points where enzymes can act.

Starch and Glycogen

Starch -> mixture of amylose and amylopectin and is stored in plant plastids, including amyloplasts and

chloroplasts. The helical and branched forms allow dense storage without greatly affecting water potential

Glycogen -> glycogen is more highly branched than amylopectin. Many non-reducing terminal ends can be

acted upon simultaneously by enzymes, supporting rapid release of glucose in animals with changing

energy demands.

Lipid Definition

Lipids are substances in living organisms that dissolve in non-polar solvents but are only sparingly soluble in

aqueous solvents. The category includes fats, oils, waxes, phospholipids and steroids. Lipids are chemically

diverse; their shared feature is substantial non-polar character.

Why hydrophobic?

Long hydrocarbon regions contain mostly non-polar C-C and C-H bonds. They cannot form favourable

hydrogen bonds with water. Water molecules therefore remain more strongly associated with one another and

non-polar molecules cluster away from water. (Water proof).

Why cis double bonds matter?

• A C=C bond restricts rotation. In a cis arrangement, the carbon chain bends. Bent chains have less surface

contact with neighbouring chains, so collective London dispersion interactions are weaker and less thermal

energy is needed to separate the molecules.