In this section you’ll learn the very basics and fundamental principles of biology - that is the basic biochemical concepts that underpin just about everything you’re going to learn about in your A-level biology. Whilst of course Biochemistry is a vast sub-discipline of biology in itself (covering many complex concepts such as photosynthesis and cellular respiration) the basic principles outlined here are intrinsic to all Biology. Your a-level biology begins by fully understanding these concepts and being able to apply this knowledge throughout.


Firstly you’ll be introduced to the “Biochemical Basis of Life” a lesson that outlines the 4 macromolecules you’ll consider in detail in subsequent lessons - i.e. Carbohydrates, Proteins, Lipids, and Nucleic Acids (which you’ll find in the biomolecules section). The “Biochemical Basis of Life” lesson also highlights how the small (that is to say the biochemical concepts of biology) are fundamentally linked to the bigger concepts you’ll cover later in your a-level (i.e. evolution, ecology and environmental biology). In fact, in addition to outlining the 4 macromolecules you have to know all about, this lesson also outlines how biochemical similarities are important to trace the evolutionary histories of organisms and how these similarities can help create phylogenetic trees to visualise and analyse the relatedness of species - an idea you’ll encounter again when learning about classification and phylogeny.


Next in this section you’ll learn about condensation and hydrolysis reactions (which you’ll apply time and time again throughout your entire a-level biology!). The “Making and Breaking of Polymers” lesson explains exactly what you must know about hydrolysis and condensation reactions before revisiting these foundational biochemical reactions in the biomolecules section - where you learn how the 4 macromolecules are built from their monomers. Condensation reactions involve the removal of water molecules from monomers in order to form specific bonds that link monomers together forming polymers. Hydrolysis (literally Hydro (meaning water) and Lysis (meaning to “split”) is the process of breaking polymers down into their constituent monomers by having the water molecules return to where they where removed from when the polymers where built… you’ll see. But that brings us to the next important biochemistry lessons - “The Structure of Water and "The Important Properties of Water”. 


The last lesson in this basic biochemistry section is also an incredibly important one - “The structure and important properties of ATP” here you’ll learn that ATP (adenosine triphosphate) is the universal energy currency, and that ATP is a nucleotide which has many important properties in living organisms.


To get the most out of this section, download the accompanying lesson PDF’s so you can easily follow along with each of the lessons and test your knowledge after completion of them.


A Level Biology: - The Biochemical Basis of Life.

Check Where this Lesson fits into your Exam Specification!

In This A-Level Biology Lesson “The biochemical basis of life” you will be introduced to Phylogenetic Trees, the idea that biochemical similarities support the theory of evolution and the 4 biological molecules you must know. Make sure you download the accompanying revision notes available as an A3 Poster (PDF). The notes are also available in the A4 PDF, so you can easily follow along with the video. Once you’re confident you know this content complete the accompanying biochemical basis of life knowledge check questions… you can check your answers with the properly written answers in the back of the work book (no vague mark scheme here! you see exactly how you should be writing your answers to gain maximum marks!

A-Level biology the biochemical basis of life revision notes
The biochemical Basis of Life A Level Biology Revision Notes with Knowledge check questions and answers pdf

A Level biology: The Biochemical Basis of Life.


Despite the variety of life on Earth being extensive, ALL organisms share the same basic biochemistry. 

Specifically, Carbon-based compounds such as: -

  • Nucleotides, which form DNA and RNA

  • Amino Acids, which form Polypeptides (proteins)

  • Monosaccharides, which form Polysaccharides.


You must be able to write the definition of a monomer: -


“The single unit from which a polymer is made. Monomers join together to form long complex polymers, such as polysaccharides, polypeptides and nucleic acids”.

You must also be able to write the definition of a polymer: -


“A long complex molecule composed of many repeating monomers”

The 4 Biological Molecules you have to know are: -

  • Nucleic acids: - (DNA & RNA) Which are made up from Nucleotides.

  • Proteins (polypeptides): - Which are made up from Amino Acids.

  • Carbohydrates: Which are made up from Monosaccharides.

  • Lipids: Which are NOT polymers, since they are made up from Glycerol + Fatty Acids (not repeating monomers!)

Shared common ancestry can be depicted by means of a phylogenetic tree - a diagram that represents the relatedness and evolutionary history of a given species. The branch points on the diagram relate to a shared common ancestor between species.


A Level Biology: - Making and Breaking Polymers

Check Where this Lesson fits into your Exam Specification!

In This A-Level Biology Lesson “Making and Breaking Polymers” you will be introduced to important core biological concept of condensation reactions and hydrolysis reactions. It is essential that you know this topic inside-out as it always crops up in exams. So, Make sure you download the accompanying revision notes available as an A3 Poster (PDF).

The notes are also available in the A4 PDF, so you can easily follow along with the video.

Once you’re confident you know this content complete the accompanying making and breaking polymers knowledge check questions you can check your answers with the properly written answers in the back of the work book (no vague mark schemes here! You'll see exactly how you should be writing your answers so you gain maximum marks!

A Level Biology Making and Breaking Polymers. Hydrolysis and Condensation reactions Revision notes and knowledge check Q and A PDF
A Level Biology Making and Breaking Polymers. Hydrolysis and Condensation reactions Revision notes Poster PDF

A-Level Biology: Making and Breaking polymers.


Carbohydrates (e.g. Starch and Glycogen), Proteins and Nucleic Acids (e.g. DNA and RNA) are Polymers – "Long complex molecules made up from many repeating units known as monomers", which are linked together by specific covalent bonds. For example, the covalent bond that links monosaccharides together is known as a glycosidic bond, whereas the covalent bond that links amino acids together to form proteins is called a peptide bond.

Monomers Join together via condensation reactions.


Condensation reactions involve the removal of a water molecule (H2O) [a Hydrogen is removed from one monomer whilst a Hydrogen and Oxygen [OH - a Hydroxyl group]  is removed from the other monomer] resulting in the formation of a covalent bond between the monomers being joined together.

Polymers are broken down (Hydrolysed) in a process called Hydrolysis.

Hydrolysis reactions are the exact opposite of condensation reactions, i.e. a water molecule (H2O) is utilised to “split” the covalent bond linking the monomers. The Hydrogen and the OH (Hydroxyl group) are added back to their respective monomers… (they are put back to where they were taken from!)


A Level Biology: The Structure and Properties of Water

Check Where this Lesson fits into your Exam Specification!

Brian Cox: How does H + O = Water?


A Level Biology: The Structure of Water Revision Notes

Check Where this Lesson fits into your Exam Specification!

00:00 Learning Outcomes

00:25 Water is a Polar Molecule

00:50 Water has a Simple Molecular Structure

01:16 Water Forms Hydrogen bonds

02:06 Cohesive Properties of Water

Water molecules are 'charged' meaning they are polar molecules, with the oxygen atom being slightly negative (delta -) (δ-) and the hydrogen atoms being slightly positive (delta +) (δ+). It is these opposite charges on water molecules that attract each other, forming weak hydrogen bonds. In-fact it is mostly due to hydrogen bonds in water that give this essential molecule its many  important properties essential for life. 

A Level Biology: The Structure of Water Revision Notes with Knowledge Check Questions and Answers PDF
The Structure of a Water Molecules (A-Level Biology)

Life on Earth evolved in the water and all life depends on water for survival. just think, around 80% of the mass of living organisms is water - and almost all biochemical reactions that take place do so in aqueous solution (e.g. the cytosol of cells). Do you recall  the other chemicals that make up living things? The organic macromolecules - belonging to the four groups: proteins, nucleic acids, carbohydrates and lipids. Between these four groups 93% of the dry mass of living organisms is made up, the remaining 7% is made up from small organic molecules such as inorganic ions and vitamins. Download and complete PDF Table to show the four macromolecules and their monomers and  the relative & dry weight in living organisms. 

Table to show the four macromolecules (proteins, nucleic acids, carbohydrates and  lipids) the monomers, polymers and % dry weight in living organisms - A Level Biology Revision -

A Level Biology: The Properties of Water "podcast"

Describe several biologically important properties of water


The biological significance of water: Water has Several key biologically important properties.


Water is the universal solvent: a major constituent of cells (e.g. cytosol) and has many substances dissolved in it (i.e. forming the cytoplasm). Dissolved substances, such as Sodium and Chloride ions are components of the extracellular fluid in multicellular organisms. Consequently, water has many biologically significant roles in a cell being the medium in which metabolic reactions take place. Not only is water the medium for biochemical reactions to occur, but is also directly involved in the breakdown of larger molecules. Water is also an important by-product of many biochemical reactions. Beyond the biochemically important properties of water, it is also essential for structural support and turgor. Making up around two thirds of any organism and three fourths of the Earth, water is extremely abundant – and super important for life as we know it. In fact, life evolved in water for approximately 2 billion years before migrating to land!


Water is liquid at room temperature, and a lot of energy is required for it to change state. It is this property that makes water thermally stable: and as a result water benefits life as it stabilises the temperature of an organism and the environment. So, aquatic environments are thermally stable, for example ice is less dense than water, which is why ice floats. This benefits the environment as the ice insulates the water beneath it – maintaining a relatively stable and valuable habitat. Terrestrial organisms on the other hand, take advantage of evaporative coolingsweating and transpiration for example, to maintain core temperatures.


Water molecules are Cohesive. Hydrogen bonds hold water molecules together, for example this is important in transpiration, as water is ‘pulled’ upwards in a plant from the root system. The cohesiveness of water molecules is also responsible for providing high surface tension – (again, due to the hydrogen bonding between water molecules). It is this property that allows small insects, such as the water strider, to literally walk on water! Additionally, due to the polarity of water, water is adhesive. This can be demonstrated with capillary action – which causes water molecules to rise against the force of gravity within narrow tubes (again think about transpiration and the way water moves up the xylem of plants). Here links should also be made to the importance of surface area: as the narrower the tube – the greater the surface area for the adhesion of water molecules – resulting in water rising higher in the “tube”.


To summarise: water has many biologically important properties and in this overview several have been discussed. Cohesion (hydrogen bonds holding water molecules together) and subsequent properties such as adhesion and surface tension.


High specific heat capacity - hydrogen bonds absorb heat when they break but release heat when they are formed – resulting in the Thermal stability of water.


Lower density of Iceresulting in stable aquatic environments, i.e. lakes don’t freeze completely solid, therefore fish and other aquatic life are better able to survive winter.


Finally water is a universal solvent – thus many molecules move freely in cells allowing for a many biochemical reactions to take place.


Water is essential to life and of course understanding water and its properties is essential to the study of biology.

The important properties of water.

1. Universal Solvent: Water is a 'charged' molecule which makes it a very good solvent. Why? Well, 'charged' or polar molecules such as salts, sugars and amino acids dissolve readily in water and so are called hydrophilic ("water loving").

On the other hand we have 'uncharged' or non-polar molecules such as lipids which do not dissolve particularly well in water and are hydrophobic ("water hating").

2. Specific heat capacity: Water has a specific heat capacity of 4.2 J g-1 °C-1, which simply means that it takes 4.2 joules of energy to heat 1 g of water by 1°C.

This is actually quite high and means that water does not change temperature very easily. So, because of the specific heat capacity of water, fluctuations of temperature inside cells are minimised. It also means that sea/ocean temperatures are kept remarkably constant too.

3. Latent heat of vaporisation:  As we know, Water has a specific heat capacity and as such water requires significant amount of energy to change state from a liquid into a gas. It is precisely this property "the Latent heat of vaporisation" that is made use of as a cooling mechanism in animals, specifically "sweating" and "panting". This is also advantageous for plants too, just think about transpiration of water.


So, in a nutshell... you simply need to know that the Latent heat of vaporisation is an essential property of water, because... "as water evaporates it extracts heat from its surrounding environment, cooling the organism down as it evaporates".

4. Latent heat of fusion. Wen know also know that water requires a lot of heat energy to change state from a solid to a liquid, and must loose a lot of heat to change state from a liquid to a solid. Why is this important? Well, this essential property of water means it is actually quite difficult to freeze water (meaning ice crystals are much less likely to form inside cells - and killing the organism).

5. Density: Water is unique in that when its in its solid state (ice) it is less dense than when its in its liquid state - so, thats why ice floats on water. This is super important for aquatic life. Why? Because as air temperatures cool, bodies of water freeze from the top down - so, the surface of the water freezes first, forming a layer of ice with liquid water beneath it. It is the property that allows aquatic ecosystems to survive even flourish in 'arctic' conditions.

6. Cohesion. Water molecules have the ability to "stick together" because of their hydrogen bonds. Meaning water has high cohesion properties. It is becasue of this property of water that long columns of water can be 'sucked up' giant trees by mean of transpiration - without breaking.


It is also due the high cohesion properties of water that explains surface tension - allows small animals to "walk on water".

7. Ionisation: Many salts are readily dissolved in water and as they dissolve they ionise into discrete positive and negative ions. For example: Sodium chloride (NaCldissolves readily in water and ionises into to  Sodium ions Na++  and chloride ions Cl-.


Many important biological molecules are weak acids, which also ionise in solution. The names of the acid and ionised forms (acetic acid and acetate for example) are often used interchangeably, (which can be a bit confusing!). You will come across many examples of two names which are actually referring to the same thing in biology... like: -

  • Phosphoric acid and phosphate,

  • Lactic acid and lactate,

  • Citric acid and citrate,

  • Pyruvic acid and pyruvate


Remember that it is the ionised form which is the one found in living cells!

8. pH: Water is partly ionised (remember that it is slightly negative (δ-) and  slightly  positive  (δ+) (H+  OH-).  This means that  water  is a source of  protons (H+ ions).


Now, you need to know that many biochemical reactions are sensitive to changes in pH and importantly,  pure water cannot buffer changes in pH (H+ concentration), so,  water is not a buffer and because of this can easily be any pH (0 - 14).


Why is this important?  Well think about the cytoplasm of cells and tissue fluids of living organisms - which are typically well buffered at have a neutral pH of around 7 - 8.  So, when you get that super common exam question asking why a buffer solution was added to... 'whatever' - the answer is always to be  "to resist changes in pH / keep the pH constant.


A Level Biology: The Structure of ATP, The ATP-ADP Cycle and the Important Properties of ATP