AQA A Level Biology

Topic 1
Biological molecules
Model answer notes by @biologywitholivia

Topic Understand Memorise Practise

1.1 Monomers & polymers

1.2 Carbohydrates

1.3 Lipids

1.4.1 General properties of proteins

1.4.2 Many proteins are enzymes

Required practical 1

1.5.1 Structure of DNA and RNA

1.5.2 DNA replication

1.6 ATP

1.7 Water

1.8 Inorganic ions

AQA A Level Biology Topic 1 Biological molecules stan.store/biologywitholivia

1.1 Monomers and polymers

What are monomers and polymers?
● Monomers - smaller / repeating molecules from which larger molecules / polymers are made
● Polymer - molecule made up of many identical / similar molecules / monomers

What happens in condensation and hydrolysis reactions?

Condensation ● 2 molecules join together Hydrolysis ● 2 molecules separated

reaction ● Forming a chemical bond reaction ● Breaking a chemical bond
● Releasing a water molecule ● Using a water molecule

Give examples of polymers and the monomers from which they’re made

Exam insight: common mistakes ❌

Mistake Explanation

*Mixing up hydrolysis and ‘C’ for condensation; ‘C’ for connecting molecules. Imagine
condensation reactions.* condensation on a window to remember water is released.

*Forgetting to include H2O in diagrams of 1 H2O molecule is released for every condensation reaction
condensation and hydrolysis reactions.* and 1 H2O molecule is used for every hydrolysis reaction.

“Lipids are polymers.” Lipids are not made from repeating monomers.

“A polymer is made of two ‘Poly’ means many. Two monomers joined

or more monomers.” together is a dimer.

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1.2 Carbohydrates
What are monosaccharides? Give 3 common examples

● Monomers from which larger carbohydrates are made

● Glucose, fructose, galactose

Describe the structure of α-glucose

● Left - full structure, carbon

atoms labelled
● Right - simplified structure
as in the specification to
be memorised for exam

Describe the difference between the structure of α-glucose and β-glucose

OH group is below carbon 1 in α-glucose Alpha & beta glucose are isomers →
but above carbon 1 in β-glucose same molecular formula, differently arranged atoms

What are disaccharides and how are they formed?

● Two monosaccharides joined together with a glycosidic bond

● Formed by a condensation reaction, releasing a water molecule

List 3 common disaccharides & monosaccharides from which they’re made

Disaccharide Monosaccharides

Maltose Glucose + glucose

Sucrose Glucose + fructose

Lactose Glucose + galactose

Draw a diagram to show how two monosaccharides are joined together

What are polysaccharides and how are they formed?

● Many monosaccharides joined together with glycosidic bonds
● Formed by many condensation reactions, releasing water molecules
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Describe the basic function and structure of starch and glycogen

Starch Energy store in ● Polysaccharide of α-glucose

plant cells ● Amylose - 1,4-glycosidic bonds → unbranched
● Amylopectin - 1,4- and 1,6-glycosidic bonds → branched

Glycogen Energy store in ● Polysaccharide made of α-glucose

animal cells ● 1,4- and 1,6-glycosidic bonds → branched

Explain how the structures of starch and glycogen relate to their functions

Starch ● Helical → compact for storage in cell

(amylose) ● Large, insoluble polysaccharide molecule → can’t leave cell / cross cell membrane
● Insoluble in water → water potential of cell not affected (no osmotic effect)

Glycogen ● Branched → compact / fit more molecules in small area

(and starch ● Branched → more ends for faster hydrolysis → release glucose for respiration to
amylopectin) make ATP for energy release
● Large, insoluble polysaccharide molecule → can’t leave cell / cross cell membrane
● Insoluble in water → water potential of cell not affected (no osmotic effect)

Describe the basic function and structure of cellulose

Function ● Provides strength and structural support to plant / algal cell walls

Structure ● Polysaccharide of β-glucose

● 1,4-glycosidic bond → straight, unbranched chains
● Chains linked in parallel by hydrogen bonds forming microfibrils

Explain how the structure of cellulose relates to its function

● Every other β-glucose molecule is inverted in a

long, straight, unbranched chain
● Many hydrogen bonds link parallel strands
(crosslinks) to form microfibrils (strong fibres)
● Hydrogen bonds are strong in high numbers
● So provides strength to plant cell walls

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Describe the test for reducing sugars

Reducing sugars = monosaccharides, maltose, lactose

1. Add Benedict’s solution (blue) to sample

2. Heat in a boiling water bath
3. Positive result = green / yellow / orange / red precipitate

Describe the test for non-reducing sugars

Non-reducing sugars = sucrose

1. Do Benedict’s test and stays blue / negative

2. Heat in a boiling water bath with acid (to hydrolyse into reducing sugars)
3. Neutralise with alkali (eg. sodium bicarbonate)
4. Heat in a boiling water bath with Benedict’s solution
5. Positive result = green / yellow / orange / red precipitate

Suggest a method to measure the quantity of sugar in a solution

● Carry out Benedict’s test as above, then filter and dry precipitate
● Find mass / weight

Suggest another method to measure the quantity of sugar in a solution

1. Make sugar solutions of known concentrations

(eg. dilution series)
2. Heat a set volume of each sample with a set
volume of Benedict’s solution for same time
3. Use colorimeter to measure absorbance (of
light) of each known concentration
4. Plot calibration curve - concentration on x axis,
absorbance on y axis and draw line of best fit
5. Repeat Benedict’s test with unknown sample and
measure absorbance
6. Read off calibration curve to find concentration
associated with unknown sample’s absorbance

Describe the biochemical test for starch

1. Add iodine dissolved in potassium iodide (orange / brown) and shake / stir
2. Positive result = blue-black

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Exam insight: common mistakes

Mistake Explanation

*Referring to alpha glucose This is not an equivalent of writing ‘alpha glucose’ or

as ‘a glucose’.* ‘α-glucose’ and will usually be rejected.

“Glycogen and starch are energy stores as Glycogen and starch are energy stores as they are made of
hydrolysis of glycosidic bonds releases α-glucose, which is the substrate for respiration in cells. This
energy.” produces ATP for energy release.

*Comparing and contrasting This will achieve no marks. In these questions, each statement
polysaccharides by making a list about needs to make a clear comparison. Aim to use a word like
each.* ‘whereas’ to ensure you’re covering both sides.

When chains of beta glucose are linked by Myofibrils are found in muscle fibres and are completely
hydrogen bonds, myofibrils form.” different. Microfibrils are found in cellulose cell walls.

“Cellulose is strong because of Hydrogen bonds are weak individually, but strong in high
hydrogen bonds.” numbers. You need to say that there are many hydrogen bonds.

“Use the same amount of Benedict’s Amount is too vague. You need to use the
solution on each sample when comparing term volume to get the mark.
the quantity of reducing sugar.”

1.3 Lipids
Name two groups of lipid
Triglycerides and phospholipids

Describe the structure of a fatty acid (RCOOH)

● Variable R-group - hydrocarbon chain (saturated or unsaturated)
● -COOH = carboxyl group

Describe the difference between saturated and unsaturated fatty acids

● Saturated: no C=C double bonds in hydrocarbon chain; all carbons fully saturated with hydrogen
● Unsaturated: one or more C=C double bond in hydrocarbon chain (creating bend / kink)

Describe how triglycerides form

● 1 glycerol molecule and 3 fatty acids

● Condensation reaction
● Removing 3 water molecules
● Forming 3 ester bonds

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Explain how the properties of triglycerides are related to their structure

Function: energy storage

● High ratio of C-H bonds to carbon atoms in hydrocarbon chain

○ So used in respiration to release more energy than same mass of carbohydrates
● Hydrophobic / non-polar fatty acids so insoluble in water (clump together as droplets)
○ So no effect on water potential of cell (or can be used for waterproofing)

Describe the difference between the structure

of triglycerides and phospholipids

One of the fatty acids of a triglyceride is substituted

by a phosphate-containing group

Describe how the properties of

phospholipids relate to their structure
Function: form a bilayer in cell membrane, allowing diffusion of lipid-soluble (non-polar) or very small
substances and restricting movement of water-soluble (polar) or larger substances

● Phosphate heads are hydrophilic

○ Attracted to water so point to water (aqueous environment) either side of membrane
● Fatty acid tails are hydrophobic
○ Repelled by water so point away from water / to interior of membrane

Describe the test for lipids

1. Add ethanol, shake (to dissolve lipids), then add water

2. Positive = milky white emulsion

Exam insight: common mistakes ❌

Mistake Explanation

“Phospholipids have a phosphorus group.” Phospholipids have a phosphate group (PO43-).

“Phospholipids don’t contain glycerol.” Both phospholipids and triglycerides contain glycerol.

“To test for lipids, add water then ethanol.” Ethanol is added first and this is crucial to pick up the mark.

“A positive test for lipids is cloudy.” This is too vague. You need to use the term ‘emulsion’.

“Phospholipids have phosphodiester bonds.” Phospholipids have ester bonds. Phosphodiester bonds are
found in nucleic acids such as DNA and RNA.

“In the test for lipid, ethanol is heated.” Only Benedict's solution (for sugars) requires heating.

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1.4 Proteins
1.4.1 General properties of proteins
Describe / draw the general
structure of an amino acid

● COOH = carboxyl group

● R = variable side chain / group
● H2N = amine group

How many amino acids are common in all organisms? How do they vary?
The 20 amino acids that are common in all organisms differ only in their side group (R).

Describe how amino acids join together

● Condensation reaction
● Removing a water molecule
● Between carboxyl / COOH group of one
and amine / NH2 group of another
● Forming a peptide bond

What are dipeptides and polypeptides?

● Dipeptide - 2 amino acids joined together

● Polypeptide - many amino acids joined together

A functional protein may contain one or more polypeptides.

Describe the primary structure of a protein

Sequence of amino acids in a polypeptide chain, joined by peptide bonds

Describe the secondary structure of a protein

● Folding (repeating patterns) of polypeptide chain eg.

alpha helix / beta pleated sheets
● Due to hydrogen bonding between amino acids
● Between NH (group of one amino acid) and C=O (group)

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Describe the tertiary structure of a protein

● 3D folding of polypeptide chain

● Due to interactions between amino acid R groups
(dependent on sequence of amino acids)
● Forming hydrogen bonds, ionic bonds and disulfide bridges

Describe the quaternary structure of a protein

● More than one polypeptide chain

● Formed by interactions between polypeptides
(hydrogen bonds, ionic bonds, disulfide bridges)

Describe the test for proteins

1. Add biuret reagent (sodium hydroxide + copper (II) sulphate)

2. Positive result = purple / lilac colour (negative stays blue) → indicates presence of peptide bonds

Proteins have a variety of functions within all living organisms. You need to be able to relate the structure of
proteins to properties of proteins named throughout the specification eg. enzymes / antibodies.

Exam insight: common mistakes ❌

Mistake Explanation

“Amino acids contain DNA triplets.” A DNA triplet codes for a specific amino acid, but these are
completely separate structures.

“A dipeptide has a primary structure.” A dipeptide is not a protein so doesn’t have a primary structure.

“All hydrogen bonds are In the secondary structure, hydrogen bonds are
between R groups.” between NH and C=O groups.

“All proteins have a quaternary structure.” Only proteins with more than one polypeptide chain possess a
quaternary structure. Examples include haemoglobin and
antibodies.

“Quaternary structure is made of four Not all quaternary structure proteins are made of four
polypeptides.” polypeptides, but they do consist of more than one polypeptide.

“Quaternary structure is multiple tertiary Each polypeptide in the quaternary structure of a protein has its
structures.” own primary, secondary, and tertiary structure. However, by
definition the quaternary structure is more than one
polypeptide chain and so this won’t achieve a mark.

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1.2.1 Many proteins are enzymes

How do enzymes act as
biological catalysts?

● Each enzyme lowers activation

energy of reaction it catalyses
● To speed up rate of reaction

Enzymes catalyse a wide range of

intracellular and extracellular reactions
that determine structures and functions
from cellular to whole-organism level.

Describe the induced-fit model of enzyme action

1. Substrate binds to (not completely complementary) active site of enzyme

2. Causing active site to change shape (slightly) so it is complementary to substrate
3. So enzyme-substrate complex forms
4. Causing bonds in substrate to bend / distort, lowering activation energy

Describe how models of enzyme action have changed over time

● Initially lock and key model (now outdated)
○ Active site a fixed shape, complementary to one substrate
● Now induced-fit model

Explain the specificity of enzymes

● Specific tertiary structure determines shape of active site

○ Dependent on sequence of amino acids (primary structure)
● Active site is complementary to a specific substrate
● Only this substrate can bind to active site, inducing fit and forming an enzyme-substrate complex

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Describe and explain the effect of enzyme concentration on

the rate of enzyme-controlled reactions

● As enzyme conc. increases, rate of reaction increases

○ Enzyme conc. = limiting factor (excess substrate)
○ More enzymes so more available active sites
○ So more enzyme-substrate (E-S) complexes form
● At a certain point, rate of reaction stops increasing / levels off
○ Substrate conc. = limiting factor (all substrates in use)

Describe and explain the effect of substrate concentration on

the rate of enzyme-controlled reactions

● As substrate conc. increases, rate of reaction increases

○ Substrate conc. = limiting factor (too few enzyme
molecules to occupy all active sites)
○ More E-S complexes form
● At a certain point, rate of reaction stops increasing / levels off
○ Enzyme conc. = limiting factor
○ As all active sites saturated / occupied (at a given time)

Describe and explain the effect of temperature on

the rate of enzyme-controlled reactions

● As temp. increases up to optimum, rate of reaction increases

○ More kinetic energy
○ So more E-S complexes form
● As temp. increases above optimum, rate of reaction decreases
○ Enzymes denature - tertiary structure and active site
change shape
○ As hydrogen / ionic bonds break
○ So active site no longer complementary
○ So fewer E-S complexes form

Describe and explain the effect of pH on

the rate of enzyme-controlled reactions

● As pH increases / decreases above / below an optimum, rate of

reaction decreases
○ Enzymes denature - tertiary structure and active site
change shape
○ As hydrogen / ionic bonds break
○ So active site no longer complementary
○ So fewer E-S complexes form

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Describe and explain the effect of concentration of competitive inhibitors on

the rate of enzyme-controlled reactions

● As concentration of competitive inhibitor increases, rate of

reaction decreases
○ Similar shape to substrate
○ Competes for / binds to / blocks active site
○ So substrates can’t bind and fewer E-S complexes form
● Increasing substrate conc. reduces effect of inhibitors
(dependent on relative concentrations of substrate and inhibitor)

Describe and explain the effect of concentration of

non-competitive inhibitors on the rate of enzyme-controlled reactions

● As concentration of non-competitive inhibitor increases, rate of

reaction decreases
○ Binds to site other than the active site (allosteric site)
○ Changes enzyme tertiary structure / active site shape
○ So active site no longer complementary to substrate
○ So substrates can’t bind so fewer E-S complexes form
● Increasing substrate conc. has no effect on rate of reaction as
change to active site is permanent

Exam insight: common mistakes ❌

Mistake Explanation

“E-S complexes form.” This abbreviation isn’t listed in the spec, so is often not
accepted unless the full name is given elsewhere in the answer.

“In the induced fit model, the active site is The active site is not fully complementary to start with, but
complementary to the substrate.” changes shape slightly when the substrate binds.

“The substrate changes shape in the It is the enzyme’s active site that changes shape slightly to
induced fit model of enzyme action.” become complementary to the substrate.

“High / low pH or temperature change the This is too vague. They cause the tertiary structure to change,
shape of the enzyme.” which causes the active site to change shape.

“Bonds break when an enzyme denature.” Be specific - hydrogen and ionic bonds.

“A competitive inhibitor has the same shape A competitive inhibitor has a similar shape to the substrate,
as the substrate.” but is not identical.

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Required practical 1
Investigation into the effect of a named variable on the rate of an enzyme-controlled reaction.

Give examples of variables that could affect the rate of an enzyme-

controlled reaction

● Enzyme concentration / volume Any one of these can be the independent variable
● Substrate concentration / volume and need to be varied (eg. by preparing a dilution
● Temperature of solution series of varying concentrations). All others (except
● pH of solution inhibitors) would be control variables so would
● Inhibitor concentration need to be kept constant.

Common questions

Describe how temperature can be controlled. ● Use a thermostatically controlled water bath
● Monitor using a thermometer at regular intervals and
add hot / cold water if temperature fluctuates

Describe how pH can be controlled. ● Use a buffer solution

● Monitor using a pH meter at regular intervals

Why were the enzyme & substrate solutions left ● So solutions equilibrate / reach the temperature of
in the water bath for 10 mins before mixing? the water bath

Describe a control experiment. ● Use denatured enzymes (eg. by boiling)

● Everything else same as experiment, eg. same conc. /
volume of substrate (at start) and enzyme, same
type / volume of buffer solution, same temperature

Describe how the rate of an enzyme-controlled reaction can be measured

● Measure time taken for reaction to reach a set point,

eg. concentration / volume / mass / colour of
substrate or product
○ Rate of reaction = 1 / time; example units = s-1
● Measure concentration / volume / mass / colour of
substrate or product at regular intervals (or using a
continuous data logger) throughout reaction
○ Plot on a graph with time on the x axis and
whatever is being measured on the y axis
○ Draw a tangent at t = 0 (or any other time for
rate at a particular point)
○ Initial rate of reaction = change in y / change
in x; example units = cm3 s-1

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Common questions

Suggest a safety risk and explain how ● Handling enzymes may cause an allergic reaction
to reduce this risk. ● Avoid contact with skin by wearing gloves and eye protection

Explain why using a colorimeter to ● Not subjective

measure colour change is better than ● More accurate
comparison to colour standards.

Explain a procedure that could be ● Boil / add strong acid / alkali → denature enzyme
used to stop each reaction. ● Put in ice → lower kinetic energy so no E-S complexes form
● Add high concentration of inhibitor → no E-S complexes form

Describe how processed data can be presented as a graph

● Independent variable on x axis, rate of reaction on y axis, including units

● Linear number sequence on axis, appropriate scale (graph should cover at least half of grid)
● Plot coordinates accurately as crosses
● Join point to point with straight lines if cannot be certain of intermediate values OR draw a
smooth curve but do not extrapolate

Explain why the rate of reaction decreases over time throughout each
experiment

● Initial rate is highest as substrate concentration not limiting / many E-S complexes form
● Reaction slows as substrate used up and often stops as there is no substrate left

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1.5 Nucleic acids are important

information-carrying molecules
1.5.1 Structure of DNA
Describe the basic functions of DNA and RNA in all living cells

DNA Holds genetic information which codes for polypeptides (proteins)

RNA Transfers genetic information from DNA to ribosomes

There are different types of RNA - these are covered in the relevant topics.

Name the two types of molecule from which a ribosome is made

RNA and proteins

Draw and label a DNA nucleotide and an RNA nucleotide

Describe the differences between a DNA nucleotide and an RNA nucleotide

DNA nucleotide RNA nucleotide

Pentose sugar is deoxyribose Pentose sugar is ribose

Base can be thymine Base can be uracil

Describe how nucleotides join together to form polynucleotides

● Condensation reactions, removing water molecules

● Between phosphate group of one nucleotide and deoxyribose/ribose of another
● Forming phosphodiester bonds

Why did many scientists initially doubt that DNA carried the genetic code?
The relative simplicity of DNA - chemically simple molecule with few components.
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Describe the structure of DNA

● Polymer of nucleotides (polynucleotide)

● Each nucleotide formed from
deoxyribose, a phosphate group and a
nitrogen-containing organic base
● Phosphodiester bonds join adjacent
nucleotides
● 2 polynucleotide chains held together by
hydrogen bonds
● Between specific complementary base
pairs - adenine / thymine and
cytosine / guanine
● Double helix

Describe the structure of (messenger) RNA

● Polymer of nucleotides (polynucleotide)

● Each nucleotide formed from ribose, a phosphate
group and a nitrogen-containing organic base
● Bases - uracil, adenine, cytosine, guanine
● Phosphodiester bonds join adjacent nucleotides
● Single helix

Compare and contrast the structure of DNA and (messenger) RNA

DNA nucleotide RNA nucleotide

Pentose sugar is deoxyribose Pentose sugar is ribose

Has the base thymine Has the base uracil

Double stranded / double helix Single stranded / single helix

Long (many nucleotides) Shorter (fewer nucleotides)

Has hydrogen bonds / base pairing Does not

Suggest how the structure of DNA relates to its functions

● Two strands → both can act as templates for semi-conservative replication
● Hydrogen bonds between bases are weak → strands can be separated for replication
● Complementary base pairing → accurate replication
● Many hydrogen bonds between bases → stable / strong molecule
● Double helix with sugar phosphate backbone → protects bases / hydrogen bonds
● Long molecule → store lots of genetic information (that codes for polypeptides)
● Double helix (coiled) → compact

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Suggest how you can use incomplete information about the frequency of
bases on DNA strands to find the frequency of other bases

1. % of adenine in strand 1 = % of thymine in strand 2 (and vice versa)

2. % of guanine in strand 1 = % of cytosine in strand 2 (and vice versa)
Because of specific complementary base pairing between 2 strands

Exam insight: common mistakes ❌

Mistake Explanation

*Using letters instead of naming bases.* Bases should be written in full, as they appear in the specification.

*Not naming the pentose sugar when DNA nucleotides have the pentose sugar deoxyribose, while RNA
describing DNA or RNA nucleotides.* nucleotides have the pentose sugar ribose.

“DNA is strong because of Hydrogen bonds are weak individually, but strong in high numbers.
hydrogen bonds.” You need to say that there are many hydrogen bonds.

1.5.2 DNA replication

Why is semi-conservative replication important?
Ensures genetic continuity between generations of cells.

Describe the process of semi-conservative DNA replication

1. DNA helicase breaks hydrogen bonds between complementary bases, unwinding the double helix
2. Both strands act as templates
3. Free DNA nucleotides attracted to exposed bases and join by specific complementary base pairing
4. Hydrogen bonds form between adenine-thymine and guanine-cytosine
5. DNA polymerase joins adjacent nucleotides on new strand by condensation reactions
6. Forming phosphodiester bonds

Semi-conservative - each new DNA molecule consists of one original / template strand and one new strand

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Use your knowledge of enzyme action to suggest why DNA polymerase

moves in opposite directions along DNA strands

● DNA has antiparallel strands

● So shapes / arrangements of nucleotides on two ends are different
● DNA polymerase is an enzyme with a specific shaped active site
● So can only bind to substrate with complementary shape (phosphate end of developing strand)

Name the two scientists who proposed models of the chemical structure of
DNA and of DNA replication

Watson and Crick

Describe the work of Meselson and Stahl in validating the Watson-Crick

model of semi-conservative DNA replication

1. Bacteria grown in medium containing heavy nitrogen (15N) and

nitrogen is incorporated into DNA bases
● DNA extracted & centrifuged → settles near bottom, as all
DNA molecules contain 2 ‘heavy’ strands
2. Bacteria transferred to medium containing light nitrogen (14N)
and allowed to divide once
● DNA extracted & centrifuged → settles in middle, as all DNA
molecules contain 1 original ‘heavy’ and 1 new ‘light’ strand
3. Bacteria in light nitrogen (14N) allowed to divide again
● DNA extracted & centrifuged → half settles in middle, as
contains 1 original ‘heavy’ and 1 new ‘light’ strand; half
settles near top, as contains 2 ‘light’ strands

Other models (eg. dispersive, conservative) not supported - bands would be in different places.

Exam insight: common mistakes ❌

Mistake Explanation

“DNA polymerase forms hydrogen bonds DNA polymerase joins adjacent (next to, not opposite)
/ joins together complementary bases.” nucleotides, forming phosphodiester bonds.

“Hydrogen bonds are hydrolysed.” Breaking of hydrogen bonds is not a hydrolysis reaction.

“Helicase unzips the double helix.” This is too vague. DNA helicase breaks hydrogen bonds.

“DNA polymerase forms phosphodiester DNA polymerase forms phosphodiester bonds between the
bonds between bases.” phosphate group of one nucleotide & the deoxyribose of another.

“Free bases attach to exposed bases.” Free nucleotides attach to exposed nucleotide bases via
complementary base pairing, but not the bases on their own.

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1.6 ATP
What is ATP?
Adenosine triphosphate

Describe the structure of ATP

● Ribose bound to a molecule of adenine

(base) and 3 phosphate groups
● Nucleotide derivative (modified nucleotide)

Describe how ATP is broken down

● ATP (+ water) → ADP (adenosine diphosphate) + Pi (inorganic phosphate)

● Hydrolysis reaction, using a water molecule
● Catalysed by ATP hydrolase (enzyme)

Give two ways in which the hydrolysis of ATP is used in cells

● Coupled to energy requiring reactions within cells (releases / provides energy)

○ Eg. active transport, protein synthesis
● Inorganic phosphate released can be used to phosphorylate (add phosphate
to) other compounds, making them more reactive

Describe how ATP is resynthesised in cells

● ADP + Pi → ATP (+ water)

● Condensation reaction, removing a water molecule
● Catalysed by ATP synthase (enzyme)
● During respiration and photosynthesis

Suggest how the properties of ATP make it a suitable immediate source of

energy for cells

● Releases energy in (relatively) small amounts / little energy lost as heat

● Single reaction / one bond hydrolysed to release energy (so immediate release)
● Cannot pass out of cell

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Exam insight: common mistakes

Mistake Explanation

“ATP is adenine diphosphate.” ATP is adenosine triphosphate.

“ATP has 3 phosphorus groups.” ATP has 3 phosphate groups (PO43-).

“ATP hydrolysis creates energy.” Energy cannot be created - only transferred / released.

“ATP can be stored.” ATP is too unstable to be stored within cells.

“The only use of ATP is energy release.” The inorganic phosphate can be used to phosphorylate other
compounds, making them more reactive.

1.7 Water
Water is a major component of cells.

Explain how hydrogen bonds occur between water molecules

● Water is polar molecule

● Slightly negatively charged oxygen atoms attract slightly positively
charged hydrogen atoms of other water molecules

Explain 5 properties of water that are important in biology

Property Importance in biology

Metabolite Used in condensation / hydrolysis / photosynthesis / respiration

Solvent (can 1. Allows metabolic reactions to occur (faster in solution)

dissolve solutes) 2. Allows transport of substances eg. nitrates in xylem, urea in blood

High specific heat ● Buffers changes in temperature

capacity ● As can gain / lose a lot of heat / energy without changing temperature
1. Good habitat for aquatic organisms as temperature more stable than land
2. Helps organisms maintain a constant internal body temperature

High latent heat of ● Allows effective cooling via evaporation of a small volume (eg. sweat)
vaporisation ● So helps organisms maintain a constant internal body temperature

Strong cohesion 1. Supports columns of water eg. transpiration stream through xylem in plants
between water 2. Produces surface tension, supporting small organisms (to walk on water)
molecules

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Exam insight: common mistakes

Mistake Explanation

“Water has a high latent heat of evaporation.” Water has a high latent heat of vaporisation.

“Water is a solute.” Water is a solvent that dissolves other substances (solutes).

“Water is cohesive which aids transpiration.” Transpiration is the loss of water vapour from leaves. The
transpiration stream is the constant movement of water
through the plant. Cohesion aids the transpiration stream.

*Mixing up specific heat capacity and These are completely different properties -
latent heat of vaporisation.* make sure you know the difference.

1.8 Inorganic ions

Where are inorganic ions found in the body?
In solution in cytoplasm and body fluid, some in high concentrations and others in very low concentrations.

Describe the role of hydrogen, iron, sodium and phosphate ions

Hydrogen ● Maintain pH levels in the body → high conc. = acidic / low pH

+
ions (H ) ● Affects enzyme rate of reaction as can cause enzymes to denature (topic 1.4.2)

Iron ions ● Component of haem group of haemoglobin

2+
(Fe ) ● Allowing oxygen to bind / associate for transport as oxyhaemoglobin (topic 3.4.1)

Sodium 1. Involved in co-transport of glucose / amino acids into cells (topic 2.3 / 3.3)
+
ions (Na ) 2. Involved in action potentials in neurons (topic 6.2)
3. Affects water potential of cells / osmosis (topic 2.3)

Phosphate 1. Component of nucleotides, allowing phosphodiester bonds to form in DNA / RNA

3-
ions (PO 4 ) 2. Component of ATP, allowing energy release
3. Phosphorylates other compounds making them more reactive (topic 1.6)
4. Hydrophilic part of phospholipids, allowing a bilayer to form (topic 1.3 / 2.3)

Exam insight: common mistakes ❌

Mistake Explanation

*Naming an ion as an element.* For example, you need to say ‘iron ions’ and not just ‘iron’.

“ATP hydrolysis creates energy.” Energy cannot be created - only transferred / released.

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