Carbon is versatile because of covalent bonding

A covalent bond is a chemical bond where adjacent atoms share one or more pairs of electrons. The shared electrons are attracted to the nuclei of both atoms, which helps covalent bonds form stable molecules. In biology, that matters: living organisms need molecules that stay together long enough to build cells, store energy and carry information.

A carbon atom has four outer-shell electrons, so it can form up to four covalent bonds. Sometimes these are four single bonds. In other cases, carbon forms a combination of single and double bonds. A double carbon-carbon bond, written $C=C$, becomes especially important later in this topic when we compare saturated and unsaturated fatty acids.

Carbon can bond to other carbon atoms, making chains and rings. It can also bond to atoms of other non-metallic elements such as hydrogen, oxygen, nitrogen and phosphorus. So carbon chemistry is not just “lots of carbon”: it is carbon plus different atoms, different bond types and different shapes.

Carbon compounds may be unbranched chains, branched chains, single-ring molecules or molecules with multiple rings. These differences in form change properties such as solubility, melting point, rigidity and ability to pass through membranes — the big idea of this topic.

Scientific conventions: SI prefixes

A scientific convention is an agreed rule or system that lets scientists communicate measurements consistently. The International System of Units is agreed internationally, so a measurement written in one country can be understood in another without translation into a local system.

For this topic, the useful metric prefixes are:

times the base unit

times the base unit

times the base unit

times the base unit and

times the base unit. A nanometre is a billionth of a metre; a micrometre is a millionth of a metre. Biology works at these small scales, so the prefixes are not decoration — they are part of the language.

Building large molecules from small subunits

A macromolecule is a very large molecule made from many atoms, usually by joining smaller subunits together. A monomer is a small molecular subunit that can be covalently linked to other similar subunits. A polymer is a large molecule made of many monomers joined in a chain or network.

Keep three biological examples in mind: polysaccharides are polymers of monosaccharides; polypeptides are polymers of amino acids; nucleic acids are polymers of nucleotides.

A condensation reaction is a chemical reaction where two molecules join covalently and release a small molecule. In the biological polymer-building reactions in this topic, that small molecule is water. One molecule provides an –OH group and the other provides an –H; these combine to form water, while a new covalent bond joins the larger pieces.

In carbohydrates, monosaccharides join by glycosidic bonds, which are covalent C–O–C links formed by condensation. A disaccharide contains two monosaccharides; a polysaccharide contains many. Glucose monomers can form starch, glycogen or cellulose, but the exact form of glucose and the bonding pattern determine the final function.

Breaking polymers by adding water

Hydrolysis is a chemical reaction where water breaks a covalent bond. It works as the reverse of condensation: water is not removed to build a bond; it is split to break one.

During hydrolysis, a water molecule splits into –H and –OH groups. These groups attach to the two products formed when the polymer bond breaks. The name fits neatly: hydrolysis literally involves splitting with water.

Digestion uses hydrolysis to turn polymers into monomers: polysaccharides into monosaccharides, polypeptides into amino acids, and nucleic acids into nucleotides. This can happen inside cells, in digestive systems, or outside decomposer organisms after enzymes are released into the surroundings. Each time, the aim is the same: to make small soluble molecules that can be absorbed, transported, reused or oxidized for energy.

Recognising monosaccharides

A monosaccharide is a carbohydrate monomer made of a single sugar unit. In molecular diagrams, many biologically important monosaccharides appear as rings rather than straight chains.

A pentose is a monosaccharide with five carbon atoms. A hexose is a monosaccharide with six carbon atoms. When you read a ring diagram, count the carbon atoms carefully. One atom in the ring may be oxygen, so don’t treat every corner as carbon.

Glucose is a hexose and one of the central molecules in energy metabolism. Its form suits its function well. It is small and soluble enough to move through aqueous fluids such as blood plasma, but chemically stable enough not to fall apart before it is needed. Cells can also oxidize it during cell respiration, releasing energy in a controlled way.

Oxidation is a chemical process in which a substance loses electrons, often with loss of hydrogen or gain of oxygen in biological molecules. Glucose oxidation is one of the main ways cells release usable energy. So oxidation here is not just “burning”; in cells, enzymes control the transfer of electrons from energy-rich molecules such as glucose.

Glucose has one drawback as a storage molecule. Because it is small and soluble, large amounts would strongly affect a cell’s osmotic concentration. For that reason, organisms usually convert excess glucose into larger, less soluble storage polysaccharides such as starch or glycogen.

Starch and glycogen store glucose compactly

A polysaccharide is a carbohydrate polymer made of many monosaccharide monomers joined by glycosidic bonds. Starch is the main glucose storage polysaccharide in plants. Glycogen is the main glucose storage polysaccharide in animals.

Both starch and glycogen are made from alpha-glucose monomers. In starch, amylose forms coiled, unbranched chains; amylopectin is branched. Glycogen has even more branching. Because these polymers coil and branch, they pack into a compact shape, letting many glucose units fit into a small volume.

Starch and glycogen are large molecules, so they are relatively insoluble compared with glucose. That helps storage. A cell can keep many glucose units without filling the cytoplasm with thousands of small solute particles, which would draw in water by osmosis.

Cells add glucose monomers to storage polysaccharides by condensation and remove them by hydrolysis. Branching creates many chain ends, giving enzymes plenty of sites where glucose can be added or removed. This makes glycogen especially useful in animals, where glucose demand can change quickly.

Carbohydrate and lipid stores both contain carbon compounds synthesized by living organisms. When these compounds build up faster than they are broken down or respired, their carbon can stay stored in biomass or in longer-term deposits. This is the link to carbon sinks: biological synthesis can hold carbon in organic compounds temporarily or, under some conditions, for very long periods, instead of returning it immediately to carbon dioxide.

Beta-glucose gives straight chains

Cellulose is a structural polysaccharide in plant cell walls made from $\beta$-glucose monomers. Like starch and glycogen, it is built from glucose units, but using a different form of glucose produces a very different molecule.

In cellulose, $\beta$-glucose monomers join by $1\to4$ glycosidic bonds. The key detail is their alternating orientation: each $\beta$-glucose monomer is rotated relative to the next. Because of this, the chain extends straight instead of coiling.

Straight cellulose chains can lie side by side in parallel bundles. Many hydroxyl groups along neighbouring chains form hydrogen bonds between the chains. A hydrogen bond is an intermolecular attraction in which a hydrogen atom covalently bonded to an electronegative atom is attracted to another electronegative atom nearby.

Together, many long cellulose chains cross-linked by many hydrogen bonds form a cellulose microfibril. One hydrogen bond is weak by itself, but hundreds of them together give high tensile strength. This strength helps plant cell walls resist stretching and prevents cells from bursting when water enters by osmosis.

Carbohydrate chains act like cell identity labels

A glycoprotein is a membrane protein that has a carbohydrate chain covalently attached to it. In animal cell membranes, this carbohydrate part usually sticks out from the cell surface, so other cells can detect it.

Cell-cell recognition occurs when molecules on the surface of one cell bind specifically to molecules on another cell surface. Glycoproteins help cells spot members of the same tissue, identify abnormal or infected cells, and tell self from non-self.

ABO antigens are the required example. An antigen is a molecule or molecular pattern recognized by the immune system. Red blood cells display ABO glycoproteins with different terminal carbohydrate arrangements. A person’s immune system tolerates the ABO antigens it produces, but it may react against unfamiliar A or B antigens on transfused red blood cells.

Lipids are grouped by solubility, not by one repeated structure

A lipid is a substance in living organisms that dissolves in non-polar solvents but is only sparingly soluble in aqueous solvents. The definition depends on solubility, so lipids form a chemically varied group rather than one tidy polymer family.

A non-polar solvent has molecules with no strong separation of charge, so it dissolves non-polar substances well. An aqueous solvent is water-based. Lipids are called hydrophobic, meaning they mix poorly with water because they interact more favourably with non-polar substances than with water.

Fats, oils, waxes and steroids are all lipids. Fats and oils are often triglycerides; fats tend to be solid at room temperature, while oils tend to be liquid at room temperature. Waxes are strongly hydrophobic, which makes them useful as water-resistant coatings. Steroids have a distinctive fused-ring structure, which you meet again at the end of the topic.

Glycerol links to fatty acids by condensation

Glycerol is a three-carbon alcohol with three hydroxyl groups. A fatty acid has a carboxyl group at one end; the rest of the molecule is a hydrocarbon chain.

A triglyceride forms when one glycerol molecule covalently links to three fatty acid molecules. Each link is made by condensation, releasing three water molecules in total. The bond between glycerol and each fatty acid is an ester bond — a covalent bond formed by condensation between a carboxyl group and a hydroxyl group.

A phospholipid forms when one glycerol molecule links to two fatty acid molecules and one phosphate-containing group. That small structural difference matters in biology: a triglyceride is fully hydrophobic, while a phospholipid has a hydrophilic phosphate region and hydrophobic fatty acid tails.

Double bonds change shape and melting point

A saturated fatty acid is a fatty acid in which all carbon-carbon bonds in the hydrocarbon chain are single bonds, so the chain holds the maximum possible number of hydrogen atoms. These chains are fairly straight. They can pack closely together, which gives them a higher melting point.

A monounsaturated fatty acid is a fatty acid

xxxFormulaEndxxx A polyunsaturated fatty acid is a fatty acid

xxxFormulaEndxxx In many naturally occurring unsaturated fatty acids, double bonds put bends into the chain. The molecules then pack less tightly and have lower melting points.

This explains a familiar pattern. Plant oils used for energy storage often contain a high proportion of unsaturated fatty acids, so they stay liquid at ordinary temperatures. Fats used for energy storage in endotherms often contain more saturated fatty acids, so they are more likely to be solid at room temperature while still usable at body temperature.

The point is not that one molecule “knows” it is in a plant or an animal. It’s form and function: the number of $C{=}C$ bonds affects chain shape, chain shape affects packing, packing affects melting point, and melting point affects whether the stored lipid behaves as an oil or a fat.

Long-term energy storage

Adipose tissue is a connective tissue specialized for storing triglycerides in fat cells. In animals, it is found under the skin and around some organs.

Triglycerides work well for long-term energy storage because they are chemically stable, hydrophobic and energy-rich. Since they are hydrophobic, they form droplets instead of dissolving in the cytoplasm, so they have little effect on osmotic balance. When oxidized, they release more energy per gram than carbohydrates, which helps in situations where mass matters, such as in active animals.

Carbohydrate stores, especially glycogen, can be mobilized more quickly, and glucose travels more easily in aqueous fluids. Lipid stores are better suited to dense long-term storage. So carbohydrates and lipids are not “better” in the abstract; they are useful for different jobs.

Comparison of carbohydrate and triglyceride stores in animals.

Thermal insulation

A thermal insulator is a material that reduces heat transfer. Triglycerides are poor conductors of heat, so layers of adipose tissue slow heat loss from the body.

This matters most in animals that keep their body temperature above that of their habitat, especially in cold environments. Marine mammals, for example, may have thick subcutaneous adipose tissue called blubber. In warmer conditions, the same property can be a disadvantage, because insulation also slows heat loss when the animal needs to cool down.

Amphipathic molecules self-arrange in water

A hydrophilic substance interacts favourably with water. A hydrophobic substance interacts poorly with water and tends to associate with non-polar substances instead.

An amphipathic molecule has both hydrophilic and hydrophobic regions. Phospholipids are amphipathic: the phosphate-containing head is hydrophilic, while the two hydrocarbon tails are hydrophobic.

Place phospholipids in water, and the hydrophilic heads face the water while the hydrophobic tails cluster away from it. The result is a phospholipid bilayer, a double layer of phospholipids with tails pointing inward and heads facing the aqueous environments on both sides.

This arrangement forms the basis of cell membranes. The bilayer forms because the two regions of each phospholipid molecule have contrasting properties.

Recognising steroids and explaining membrane passage

A steroid is a lipid with a characteristic structure of four fused carbon rings. In molecular diagrams, the easiest clue is the ring pattern: three six-membered rings and one five-membered ring joined together. Side groups and double-bond positions differ between steroids, but that four-ring backbone gives them away.

Oestradiol and testosterone are steroids. They act very differently in the body, yet both share the same steroid core. Because they are mostly non-polar, they pass through the hydrophobic interior of the phospholipid bilayer more readily than polar or charged substances.

This links back to the guiding question: small changes in molecular form can produce large differences in biological function. Carbohydrates use rings and glycosidic bonds to make soluble fuels, compact stores, strong fibres and recognition markers. Lipids use hydrophobic regions, fatty acid saturation and fused rings to make energy stores, membranes, insulation and membrane-crossing signalling molecules.