Carbon is versatile because of covalent bonding

A covalent bond is a chemical bond where adjacent atoms share one or more pairs of electrons. Those shared electrons are attracted to the nuclei of both atoms, which helps covalent bonds form stable molecules. In biology, that stability 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. It may form four single bonds, or a mixture 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, forming 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 with 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 converting it into a local system.

For this topic, the useful metric prefixes are: kilo means 10³ times the base unit, centi means 10⁻² times the base unit, milli means 10⁻³ times the base unit, micro means 10⁻⁶ times the base unit and nano means 10⁻⁹ times the base unit. A nanometre is a billionth of a metre; a micrometre is a millionth of a metre. Biology works at those 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 linking smaller subunits together. A monomer is a small molecular subunit that can be covalently linked to other similar subunits. When many monomers join in a chain or network, the large molecule formed is a polymer.

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 in which two molecules are covalently joined and a small molecule is released. For 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 has two monosaccharides; a polysaccharide has many. Glucose monomers can join to make starch, glycogen or cellulose, but the exact form of glucose and the bonding pattern decide the final function.

Breaking polymers by adding water

Hydrolysis is a chemical reaction where water breaks a covalent bond. It works in the opposite way to condensation: condensation removes water to make a bond, while hydrolysis splits water to break one.

During hydrolysis, a water molecule separates into –H and –OH groups. These groups attach to the two products made 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 may happen inside cells, in digestive systems, or outside decomposer organisms after enzymes are released into the surroundings. In each case, the aim is the same: produce small soluble molecules that can be absorbed, transported, reused or oxidized for energy.

Recognising monosaccharides

A monosaccharide is a carbohydrate monomer made from one sugar unit. In molecular diagrams, many biologically important monosaccharides appear as rings, not straight chains.

A pentose is a monosaccharide with five carbon atoms. A hexose is a monosaccharide with six carbon atoms. When you look at ring diagrams, 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 structure fits its role well. It is small and soluble enough to move through aqueous fluids such as blood plasma, yet chemically stable enough to stay intact until 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. Oxidizing glucose is one of the main ways cells release usable energy. So, for the linking question in this topic, oxidation is not simply “burning”; in cells, enzymes control the transfer of electrons from energy-rich molecules such as glucose.

Glucose has a drawback as a storage molecule. Because it is small and soluble, large amounts would strongly affect the osmotic concentration of a cell. Organisms therefore 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. In animals, the main glucose storage polysaccharide is glycogen.

Both starch and glycogen are made from alpha-glucose monomers. In starch, amylose forms coiled unbranched chains, while amylopectin has branches. Glycogen branches even more. Those coils and branches pack the polymers tightly, so a small volume can hold many glucose units.

Because starch and glycogen are large molecules, they are relatively insoluble compared with glucose. For storage, that matters. A cell can keep many glucose units without loading its 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 many sites where glucose can be added or removed. That 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 accumulate faster than they are broken down or respired, their carbon can stay stored in biomass or in longer-term deposits. This links to carbon sinks: biological synthesis can hold carbon in organic compounds temporarily, or under some conditions for very long periods, rather than returning it immediately to carbon dioxide.

Beta-glucose gives straight chains

Cellulose is a structural polysaccharide found in plant cell walls, built from beta-glucose monomers. It uses the same basic glucose building block as starch and glycogen, but because the glucose is in a different form, the final molecule behaves very differently.

In cellulose, beta-glucose monomers join through 1→4 glycosidic bonds. The key detail is their alternating orientation: each beta-glucose monomer is rotated relative to the next one. Because of this, the chain extends straight instead of coiling.

Straight cellulose chains can pack side by side in parallel bundles. Along neighbouring chains, many hydroxyl groups 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.

This arrangement forms a cellulose microfibril: many long chains cross-linked by many hydrogen bonds. One hydrogen bond is weak, but large numbers of them together give high tensile strength. That strength lets plant cell walls resist stretching and stops 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 let cells recognize members of the same tissue, identify abnormal or infected cells, and distinguish self from non-self.

ABO antigens are the required example. An antigen is a molecule or molecular pattern that the immune system can recognize. Red blood cells display ABO glycoproteins with different terminal carbohydrate arrangements. A person’s immune system tolerates the ABO antigens they produce, 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. So the definition comes from solubility, not from a shared repeating unit; that’s why lipids form a chemically mixed group rather than a tidy polymer family.

A non-polar solvent is a solvent whose molecules have 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 are 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 links covalently 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 change matters in biology: a triglyceride is fully hydrophobic, but 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. Because saturated chains are relatively straight, they can pack closely together, which gives them a higher melting point.

A monounsaturated fatty acid is a fatty acid with one C=C double bond in its hydrocarbon chain. A polyunsaturated fatty acid is a fatty acid with more than one C=C double bond in its hydrocarbon chain. In many naturally occurring unsaturated fatty acids, these double bonds put bends into the chain. The molecules then pack less tightly and have lower melting points.

You see the pattern in stored lipids. 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, making them more likely to be solid at room temperature while still being usable at body temperature.

No molecule “knows” whether it is in a plant or an animal. It comes down to 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 connective tissue adapted to store 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 collect as 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 makes them useful when mass matters, such as in active animals.

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

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 for 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.