Revise Carbohydrates and lipids for IB Biology (B1.1)

Carbohydrates are made of carbon, hydrogen and oxygen, with hydrogen and oxygen roughly in the 2:1 ratio of water, giving the general formula $C_n(H_2O)_n$. The monomer is the monosaccharide, or single sugar. Glucose ($C_6H_{12}O_6$) is the central example: a six-carbon sugar, soluble in water, sweet, and the primary respiratory substrate of nearly every cell. Other monosaccharides you should know are fructose (fruit sugar) and ribose (a five-carbon sugar, $C_5H_{10}O_5$, that forms part of RNA). In solution glucose closes into a ring, and that ring can form in two ways.

The two ring forms of glucose differ only in where the hydroxyl (–OH) group on carbon 1 sits. In α-glucose it points below the ring; in β-glucose it points above. This single difference has enormous consequences: α-glucose polymers become compact energy stores (starch and glycogen), while β-glucose polymers become strong structural fibres (cellulose). It is the clearest example in the topic of a tiny change in form producing a completely different function.

Monosaccharides are linked into larger carbohydrates by condensation reactions. In a condensation reaction two subunits join and a molecule of water is removed; the new covalent link between two sugars is a glycosidic bond. Two monosaccharides give a disaccharide — for example glucose + glucose → maltose, glucose + fructose → sucrose, and glucose + galactose → lactose. Add many monosaccharides in this way and you build a polysaccharide. Crucially, every bond formed releases exactly one water molecule, so joining $n$ sugars into a chain releases $n-1$ waters.

The reverse process is hydrolysis (hydro = water, lysis = splitting). Here a water molecule is added across a bond to break it, regenerating the smaller subunits. Hydrolysis is how digestion works: amylase hydrolyses starch to maltose, and maltase hydrolyses maltose to glucose, each step using water to cut a glycosidic bond. The same condensation/hydrolysis logic applies to every biological polymer, including the ester bonds in lipids, so it is worth learning the pattern once and applying it everywhere.

Three polysaccharides show, better than anything else in the course, how structure is matched to function. Starch is the glucose store of plants. It is a mixture of amylose, an unbranched chain of α-glucose joined by 1,4-glycosidic bonds that coils into a compact helix, and amylopectin, which adds 1,6 branch points. Being a large, insoluble molecule, starch does not dissolve to alter the cell's water potential and cannot diffuse out of the cell — ideal properties for a store you want to stay put and stay inert until needed.

Glycogen is the equivalent store in animals, held in liver and muscle. Like starch it is a polymer of α-glucose, but it is more highly branched. Each branch ends in a free end where enzymes can attach, so a highly branched molecule can be hydrolysed at many points at once, releasing glucose rapidly. Animals, with their high and variable metabolic rate, need exactly that fast release — so the extra branching is function-driven. Cellulose is different in kind: a structural polysaccharide of plant cell walls built from β-glucose. Because β-glucose has its carbon-1 –OH pointing up, alternate units must rotate 180° to bond, producing long, straight, unbranched chains. Parallel chains hydrogen-bond to one another to form microfibrils of great tensile strength, which resist the turgor pressure pushing outward on the cell wall. Straight and strong for support; branched and compact for storage — same glucose building block, opposite jobs.

Lipids are a chemically varied group united by one property: they are hydrophobic and do not dissolve in water. The most important for energy storage is the triglyceride. A triglyceride forms when one molecule of glycerol condenses with three fatty acids. Each fatty acid's carboxyl (–COOH) group reacts with one of glycerol's three hydroxyl groups, forming an ester bond and releasing one water molecule — so making a triglyceride releases three waters in total. A fatty acid is a long hydrocarbon chain ending in a carboxyl group; the long, energy-rich, water-repelling tails are what make triglycerides such a dense, waterproof store.

A phospholipid is a close relative with a decisive twist: one of the three fatty acids is replaced by a phosphate group. This gives the molecule two personalities — a hydrophilic (water-loving) phosphate head and two hydrophobic (water-hating) fatty-acid tails. When surrounded by water, phospholipids spontaneously arrange into a bilayer, heads outward toward the water and tails inward away from it. This bilayer is the fundamental structure of every cell membrane, so the same lipid chemistry that stores energy in one form builds the boundary of the cell in another.

Fatty-acid tails vary in one important way: whether their carbon chain contains any carbon–carbon double bonds. A saturated fatty acid has no C=C double bonds — every carbon is 'saturated' with the maximum number of hydrogen atoms. Its straight chains pack closely together, so saturated fats (like butter and animal fat) are usually solid at room temperature. An unsaturated fatty acid has one or more C=C double bonds; monounsaturated has one, polyunsaturated has several. A cis double bond puts a permanent kink in the chain, so unsaturated fatty acids cannot pack tightly, and unsaturated fats (like olive oil and other plant oils) are usually liquid at room temperature. The degree of saturation therefore controls whether a triglyceride is a solid 'fat' or a liquid 'oil' — a direct structure-to-property link — and it also matters biologically, because unsaturated tails keep membranes fluid.

Lipids do far more than store energy. Their major functions are long-term energy storage (triglycerides packed into adipose tissue), thermal insulation (a subcutaneous fat layer slows heat loss, vital in mammals and especially in the blubber of marine mammals), physical protection of organs, buoyancy, and — through phospholipids — forming the membranes of every cell. Some lipids, such as steroid hormones, even act as chemical signals. As an energy store specifically, lipids and carbohydrates play complementary roles, and understanding the trade-off between them is a favourite exam theme.

The headline difference is energy density. Lipids release about 38 kJ per gram when respired, roughly double the 17 kJ per gram from carbohydrate. This is because fatty-acid tails are highly reduced — they are rich in energy-releasing C–H bonds and contain little oxygen — so their oxidation liberates more energy. Lipids are also hydrophobic and stored anhydrous (with no associated water), whereas glycogen is stored hydrated, adding mass. Together these make lipids a lighter, more compact reserve — an advantage for animals that must carry their store around, and the reason migrating birds and hibernating mammals rely on fat. Carbohydrate, however, is not obsolete: glycogen is more rapidly mobilised, is water-soluble and easy to transport, and can be respired anaerobically for short bursts. So organisms keep a small, fast carbohydrate store for immediate demand and a large, energy-dense lipid store for the long term.

'Compare and contrast' questions are marked analytically: each distinct valid comparison point is worth one mark. The engine is looking for COMPARATIVE statements — sentences that mention both carbohydrate and lipid and say how they are alike or how they differ — not two separate lists. A correct point earns the answer mark (A); error-carried-forward means an earlier slip does not sink later independent points; and equivalents are accepted as long as the comparison is genuine. Study how each mark below is tied to a specific comparison rather than to loose phrasing.

The ideas here run right through the course. Condensation and hydrolysis reappear for proteins and nucleic acids in later topics, so the water-in/water-out rule you learn on sugars is a template for every macromolecule. The phospholipid bilayer is the foundation of membrane structure and transport, and the energy in triglycerides feeds directly into cell respiration. Above all, keep the habit this topic teaches — read a molecule's structure and predict its function — because it is the single most powerful move in the whole of IB Biology.