ATP

ATP is the molecule cells use to carry and transfer energy. It is built from three parts: a sugar called ribose, a base called adenine, and three phosphate groups.

Biological molecules

ATP

An enzyme called ATP hydrolase breaks ATP apart using water. It splits ATP into ADP and a free phosphate group, releasing energy the cell can use.

Biological molecules

ATP

When ATP breaks down, it releases energy. Cells link this release directly to processes that need energy, such as muscle contraction or building proteins.

Biological molecules

ATP

When ATP breaks down, it releases a phosphate group. That phosphate can attach to other molecules, making them more chemically reactive and easier for cells to use.

Biological molecules

ATP

Cells rebuild ATP by joining ADP and inorganic phosphate (Pi) together. The enzyme ATP synthase catalyses this reaction during photosynthesis and respiration.

Biological molecules

ATP

Adenosine triphosphate (ATP) is the universal energy currency of cells — a small nucleotide derivative built from ribose, adenine and three phosphate groups, whose structure allows it to store and release energy on demand. When a cell needs energy, the enzyme ATP hydrolase breaks one phosphate bond in a hydrolysis reaction (splitting using water), releasing inorganic phosphate (Pi) and converting ATP into ADP; this released energy can directly power energy-requiring processes such as muscle contraction, active transport and biosynthesis. Understanding ATP is essential because it bridges the energy-releasing reactions of respiration and photosynthesis — where ATP synthase rebuilds ATP from ADP and Pi — with virtually every energy-consuming process in a living organism.

Biological molecules

Carbohydrates

Monosaccharides are the smallest sugar units in biology. Glucose, galactose, and fructose are three common examples — they act as building blocks for larger carbohydrates.

Biological molecules

Carbohydrates

When two sugar units join together, they release a water molecule. This reaction creates a covalent link called a glycosidic bond.

Biological molecules

Carbohydrates

Two simple sugars (monosaccharides) can join together to form a double sugar called a disaccharide. Maltose, sucrose, and lactose are the three disaccharides you need to know.

Biological molecules

Carbohydrates

Glucose exists in two slightly different forms, called α-glucose and β-glucose. They have the same chemical formula but differ in the position of one hydroxyl group.

Biological molecules

Carbohydrates

A polysaccharide is a large carbohydrate molecule. Many glucose units join together through condensation reactions, each releasing a water molecule, to build it.

Biological molecules

Carbohydrates

Both starch and glycogen are large energy-storage molecules. Cells build them by linking many α-glucose sugar units together, releasing water each time two units join.

Biological molecules

Carbohydrates

Cellulose is a large carbohydrate made by joining many β-glucose molecules together. Each join releases a water molecule in a condensation reaction.

Biological molecules

Carbohydrates

Glycogen, starch, and cellulose are all large carbohydrates made from glucose. Their different structures make them suited to very different jobs — energy storage or physical support.

Biological molecules

Carbohydrates

The shape and bonding of a polysaccharide directly determines what job it does in a cell. Glycogen and starch store energy; cellulose builds rigid plant cell walls.

Biological molecules

Carbohydrates

Two chemical tests identify carbohydrates in a sample. Benedict's solution detects sugars, and iodine solution detects starch — each test produces a visible colour change as its positive result.

Biological molecules

Carbohydrates

Carbohydrates are built from monosaccharides — simple sugar units such as glucose — which join together through condensation reactions (reactions that release water) to form increasingly complex molecules, from two-unit disaccharides through to large polysaccharides. The precise structure of a carbohydrate determines its function: small differences, such as whether glucose exists in its α or β form, dictate whether the resulting polysaccharide acts as a compact energy store like starch and glycogen, or a rigid structural material like cellulose. Understanding this structure-to-function relationship is central to explaining how both animal and plant cells are organised and fuelled.

Biological molecules

Inorganic ions

Organisms contain charged mineral particles called inorganic ions, dissolved in their fluids. Some ions are present in large amounts, while others exist in tiny traces.

Biological molecules

Inorganic ions

Every inorganic ion does a particular job in the body. Its chemical properties — such as its charge or size — determine exactly what that job is.

Biological molecules

Inorganic ions

Each of the four ions below has a distinct structural or functional role: **Hydrogen ions (H⁺) and pH** Hydrogen ions are simply protons released when acids dissolve in water. The more H⁺ present, the lower the pH. pH controls enzyme activity by altering the shape of the active site — a small change in H⁺ concentration can denature an enzyme entirely. **Iron ions (Fe²⁺) and haemoglobin** Haemogl

Biological molecules

Inorganic ions

Inorganic ions — charged mineral particles dissolved in the cytoplasm and body fluids of organisms — are present in varying concentrations and each plays a precise role determined by its chemical properties. For example, iron ions are essential components of haemoglobin (the oxygen-carrying protein in red blood cells), phosphate ions form the backbone of DNA and the energy-carrying molecule ATP, and sodium ions drive the co-transport of glucose and amino acids across cell membranes. Understanding these roles ties together several biological molecules you have already studied and shows how small, simple ions underpin large-scale physiological processes.

Biological molecules

Lipids

Lipids are a group of biological molecules that includes fats and oils. Triglycerides and phospholipids are the two main types you need to know.

Biological molecules

Lipids

A triglyceride forms when one glycerol molecule joins to three fatty acid molecules. Each join releases a water molecule — this type of reaction is called condensation.

Biological molecules

Lipids

When glycerol and a fatty acid join together, they release a water molecule. This reaction creates a new chemical link called an ester bond.

Biological molecules

Lipids

Every fatty acid has a long hydrocarbon tail called the R-group. This tail is either saturated (contains only single bonds) or unsaturated (contains one or more double bonds between carbon atoms).

Biological molecules

Lipids

A phospholipid looks like a triglyceride, but with one fatty acid swapped out for a phosphate-containing group. This swap gives the molecule a water-attracting head and two water-repelling tails.

Biological molecules

Lipids

Triglycerides and phospholipids have different structures, so they behave differently and do different jobs. Triglycerides store energy; phospholipids form the membranes that surround every cell.

Biological molecules

Lipids

The emulsion test detects lipids in a sample. You dissolve the sample in ethanol, then add water — a cloudy white emulsion forms if lipids are present.

Biological molecules

Lipids

Lipids are a group of biological molecules that includes triglycerides — energy-storage molecules built from one glycerol and three fatty acids joined by ester bonds (formed during condensation reactions) — and phospholipids, which have a similar structure but with one fatty acid replaced by a phosphate-containing group. This structural difference is what makes phospholipids ideal for forming cell membranes, while triglycerides excel at storing energy and insulating the body. Understanding how structure determines function here is a key principle that runs throughout A-level Biology.

Biological molecules

Monomers and polymers

A monomer is a small molecule that acts as a building block. Many monomers join together to build much larger molecules inside living organisms.

Biological molecules

Monomers and polymers

A polymer is a large molecule built by joining many smaller units called monomers together. Think of it like a long chain made from many identical links.

Biological molecules

Monomers and polymers

Some molecules act as building blocks that join together to form larger biological molecules. Monosaccharides, amino acids and nucleotides are the three key building blocks you need to know.

Biological molecules

Monomers and polymers

When two monomers (the small repeating units that build larger molecules) join together, they undergo a condensation reaction. The reaction works in three steps: 1. Two monomers come close together, each carrying a hydroxyl group (–OH) or an amino group (–NH₂) at the point where they will bond. 2. A new covalent bond forms between the two monomers, linking them into a single, longer molecule. 3.

Biological molecules

Monomers and polymers

The word hydrolysis comes from the Greek for water (hydro) and splitting (lysis) — so the name itself tells you what happens. When a polymer needs to be broken down into its monomers, the body uses hydrolysis to split the bonds holding them together. Here is how the reaction works: 1. A water molecule (H₂O) targets the chemical bond between two monomers. 2. The water molecule splits: one hydroge

Biological molecules

Monomers and polymers

Many of the large molecules found in living organisms are polymers — long chains built by joining many smaller repeating units called monomers together. This linking happens through condensation reactions, where a chemical bond forms between two monomers and a molecule of water is released as a by-product; the reverse process, hydrolysis, breaks those bonds by consuming water. Understanding these two reactions is the foundation for everything that follows in this section, because carbohydrates, proteins and nucleic acids are all built and broken down in exactly this way.

Biological molecules

Nucleic acids are important information-carrying molecules

DNA and RNA are nucleic acids — polymers (long chains) built from smaller units called nucleotides — and their primary role is to store and transfer the genetic information that instructs cells how to build proteins. Understanding their structure, including how nucleotides bond together and how the two strands of DNA pair up, explains how that information is copied accurately and passed on during cell division. This subtopic also introduces how RNA differs from DNA in structure and function, which is essential groundwork for understanding gene expression later in the course.

Biological molecules

Proteins

Proteins are polymers — long chains built from smaller monomer units called amino acids, joined together by peptide bonds through condensation reactions. Each amino acid has a variable region called an R-group, and it is the sequence and variety of these R-groups that determines how a protein folds into its precise three-dimensional shape. Because a protein's shape dictates its function, understanding protein structure explains how molecules like haemoglobin carry oxygen and how enzymes catalyse reactions — making this subtopic essential for almost every biological process you will study at A-level.

Biological molecules

Water

Water makes up the majority of every living cell. Most animal cells are roughly 70–80% water by mass.

Biological molecules

Water

Water takes part directly in chemical reactions inside cells. Cells consume water in some reactions and produce it in others.

Biological molecules

Water

Water dissolves substances inside cells. This allows the chemical reactions that keep organisms alive to take place.

Biological molecules

Water

Water resists changes in temperature better than most liquids. This keeps cells and aquatic environments stable when heat is gained or lost.

Biological molecules

Water

Latent heat of vaporisation is the energy required to convert a liquid into a vapour without changing its temperature. Water has an unusually large value for this property. This is because water molecules form hydrogen bonds with each other — weak attractions between the slightly positive hydrogen atoms of one molecule and the slightly negative oxygen atom of another. Before a water molecule can e

Biological molecules

Water

Cohesion is the attraction between molecules of the same substance. In water, each molecule carries a slight positive charge on its hydrogen atoms and a slight negative charge on its oxygen atom. These opposite charges attract neighbouring water molecules, forming hydrogen bonds (weak electrostatic attractions between a δ+ hydrogen on one molecule and the δ− oxygen on another). This produces two

Biological molecules

Water

Despite being a simple molecule, water is essential to life — it makes up the majority of every cell and is directly involved in metabolic reactions (the chemical reactions that keep organisms alive), acting as both a metabolite (a molecule consumed or produced in those reactions) and a solvent (the liquid in which other substances dissolve so reactions can take place). Its unique physical properties — including a high heat capacity (resistance to temperature change), a large latent heat of vaporisation (energy needed to convert liquid water to vapour), and strong cohesion (attraction between water molecules) — make it ideally suited to maintaining the stable internal conditions that living organisms depend on.

Biological molecules