B1.1 Carbohydrates and lipids

Carbon is central to biological molecules because one carbon atom can form four covalent bonds, allowing stable chains, branches and rings.

Its four outer-shell electrons can be shared with carbon, hydrogen, oxygen, nitrogen or sulfur. Changing the carbon skeleton or attached groups changes shape and chemical behaviour, so structure can produce different biological functions.

Useful consequences:

• carbon atoms link into chains, branches and rings
• single and double bonds change shape and reactivity
• large molecules can contain many different carbon arrangements

Glycogen and cellulose both contain glucose units, but their bonding arrangement gives glycogen a compact storage form and cellulose strong fibres.

Four bonds explain carbon's versatility; they do not make every carbon compound chemically identical. Always connect the particular structure to the claimed function.

A condensation reaction links monomers with a new covalent bond while an H and an OH are removed to form water.

Repeating condensation builds macromolecules: monosaccharides form polysaccharides through glycosidic bonds, amino acids form polypeptides through peptide bonds, and nucleotides form nucleic acids through phosphodiester bonds.

For each step identify the two reacting groups, the new covalent bond and the water released. Polymerization repeats the same bond-forming logic many times, although not every biological macromolecule is a polymer.

When two monosaccharides condense, one supplies H and the other OH; water is released and the remaining atoms are joined by a glycosidic bond.

Condensation is not merely mixing monomers. A covalent bond must form and water must be produced; hydrolysis is the reverse bond-breaking reaction.

Hydrolysis breaks a covalent bond in a larger biological molecule by using water.

The water molecule separates into H and OH, which attach to the two products. This reverses the bond-forming logic of condensation and lets digestive enzymes release absorbable smaller molecules.

Trace the reaction as:

• water enters the reaction
• a polymer bond is cleaved
• H and OH cap the two products

Hydrolysing a disaccharide produces two monosaccharides: one receives H and the other OH. The products are smaller than the starting molecule and can be transported for metabolism.

Hydrolysis is not the same as physically dissolving a food. It changes a covalent bond; water alone may be present, but enzymes usually control the biological rate.

Monosaccharides are single sugar units. Pentoses such as ribose have five carbon atoms, whereas hexoses such as glucose have six; both can be recognized in ring-form molecular diagrams.

Glucose is polar enough to dissolve in water, so it can be transported in blood or plant sap. It is chemically stable enough for transport yet can be oxidized directly in respiration to release usable energy.

Alpha- and beta-glucose have the same formula but differ in the orientation of the hydroxyl group at carbon 1. That small structural difference determines which glycosidic bonds and polysaccharide shapes they can form.

A glucose molecule can move in an aqueous transport fluid and then enter respiratory pathways; many alpha-glucose units can also be condensed into starch or glycogen for storage.

Do not identify a monosaccharide from the word 'sugar' alone. Use carbon number and ring structure, and do not assume molecules with the same formula have the same arrangement or role.

Starch stores alpha-glucose in plants, while glycogen stores alpha-glucose in animals and fungi. Their large, compact molecules are relatively insoluble and therefore have little osmotic effect.

Starch contains coiled amylose and branched amylopectin; glycogen is more highly branched. Coiling and branching make the stores compact, and branch ends provide many sites where glucose can be added by condensation or removed by hydrolysis.

Between meals, enzymes can hydrolyze glucose units from many glycogen branch ends at once, allowing rapid mobilization without storing a large pool of osmotically active free glucose.

Cellulose is also a glucose polymer but uses beta-glucose and forms structural fibres; monomer identity alone does not determine function.

Cellulose is a structural polysaccharide made from beta-glucose joined by beta-1,4 glycosidic bonds. Alternate monomers are inverted, producing straight, unbranched chains.

Straight chains align in parallel. Many hydrogen bonds form between hydroxyl groups on neighbouring chains, cross-linking them into microfibrils and larger fibres with high tensile strength.

Structure-to-function chain: beta-glucose → alternating orientation → straight chains → parallel bundles → many interchain hydrogen bonds → strong plant cell walls.

When water enters a plant cell, the cellulose wall resists stretching, helping prevent excessive expansion while still allowing the cell to remain turgid.

The chains themselves contain covalent glycosidic bonds, but neighbouring cellulose chains are not covalently joined; their collective strength comes largely from many hydrogen bonds.

Glycoproteins are membrane proteins with exposed carbohydrate chains whose shapes act as cell-surface identity and recognition markers.

A receptor, antibody or neighbouring cell can bind a matching carbohydrate pattern. This supports self/non-self recognition, cell adhesion and signalling; the membrane anchor presents the marker in the correct location.

ABO blood-group antigens are cell-surface carbohydrate patterns carried on glycoproteins and glycolipids. Type A and B cells display different terminal sugars; type O lacks those A/B additions, affecting antibody compatibility.

If red cells bearing an unfamiliar A or B antigen meet matching antibodies in incompatible plasma, antibody cross-linking can cause agglutination.

Recognition is specific to molecular shape and a compatible binding partner. Do not describe ABO identity as a free sugar floating outside the cell or as the protein sequence alone.

Lipids are hydrophobic substances that dissolve in non-polar solvents but are only sparingly soluble in aqueous solvents.

Their structures contain large non-polar hydrocarbon regions that cannot form favourable interactions with water, so lipids cluster away from water or form separate phases.

Fats, oils, waxes, phospholipids and steroids are lipid examples. Lipids are grouped by solubility and hydrophobic behaviour rather than as one true polymer family built from repeating identical monomers.

Oil forms a separate layer on water but dissolves in a non-polar solvent because substances mix most readily when their intermolecular interactions are compatible.

Hydrophobic does not mean 'contains no oxygen' or 'is repelled by every substance'. Judge the whole molecule's polarity and its interaction with water.

Triglycerides and phospholipids form when glycerol reacts with fatty acids in condensation reactions that create ester bonds.

Each ester bond joins a hydroxyl group of glycerol to a carboxyl group of a fatty acid and releases water. The number and type of attached groups then determine whether the molecule is mainly for storage or membrane structure.

For a synthesis diagram, count:

• glycerol backbone
• fatty-acid tails attached
• ester bonds and water molecules released

Attaching three fatty acids to glycerol forms a triglyceride and releases three water molecules; replacing one tail with a phosphate-containing group gives a phospholipid.

A phospholipid is not simply a triglyceride with fewer tails. Its phosphate-containing head changes polarity and therefore its behaviour in water.

A saturated fatty acid has no carbon-carbon double bonds, a monounsaturated fatty acid has one, and a polyunsaturated fatty acid has more than one.

Cis double bonds introduce bends that prevent close packing and weaken intermolecular attractions, generally lowering melting point. Straight saturated chains pack more tightly and tend to melt at higher temperatures.

Plants commonly store energy in oils rich in unsaturated fatty acids, which remain fluid at typical plant temperatures; endotherms can store more saturated fats while maintaining body temperature.

Double-bond number is important but not the only influence: chain length and cis/trans geometry also affect packing and melting point.

Triglycerides in adipose tissue provide concentrated long-term energy storage and thermal insulation.

Their many reduced C–H bonds yield more energy per unit mass than carbohydrate when oxidized. Hydrophobic triglycerides are stored without a large hydration shell and have little osmotic effect; oxidation also produces metabolic water.

Adipose tissue beneath the skin slows heat transfer, helping an endotherm maintain a stable body temperature. Thick fat layers are especially useful in cold or aquatic habitats and can also cushion organs or aid buoyancy.

A marine mammal can carry a compact energy reserve in blubber while the same adipose layer reduces heat loss to cold water.

Triglycerides form storage droplets, not bilayers: three hydrophobic tails and no strongly polar phosphate head make them unsuitable as the basic membrane sheet.

Phospholipids are amphipathic: a polar phosphate-containing head interacts with water while non-polar tails avoid it, so they self-assemble into bilayers.

In water, heads face the aqueous environments and tails pack away from water in the interior. This arrangement creates a flexible hydrophobic barrier that separates compartments while allowing selected molecules to cross.

A bilayer requires:

• hydrophilic heads facing water
• hydrophobic tails facing inward
• a continuous, dynamic sheet rather than a solid wall

In a cell membrane, water contacts both outer and inner head surfaces, while the tail core makes it difficult for many ions and polar molecules to pass without transport proteins.

The bilayer is not formed with tails facing water. Reversing the orientation would expose the least water-compatible part and would not create a stable membrane barrier.

Steroids have four fused carbon rings. Mostly non-polar steroids can dissolve in the hydrophobic core of a phospholipid bilayer and diffuse through it.

The fused-ring skeleton is dominated by C–C and C–H bonds, so it interacts more favourably with lipid tails than with water. Attached polar groups modify, but do not necessarily remove, this lipid solubility.

Oestradiol and testosterone are steroid hormones derived from cholesterol. Their bilayer permeability allows them to enter target cells, where their receptors are intracellular rather than exposed only on the cell surface.

A testosterone molecule can partition into the membrane's hydrophobic core, cross the bilayer and bind an intracellular receptor that changes cell activity.

Steroid identifies a four-ring structural family. Not every steroid is completely non-polar, and membrane passage does not mean diffusion is equally rapid for every attached functional group.