Topic 1 Biological molecules.pdf

AQA A Level Biology Topic 1 Biological molecules Model answer notes by @biologywitholivia Topic 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...

AQA A Level Biology Topic 1 Biological molecules Model answer notes by @biologywitholivia Topic 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 Understand Memorise Practise 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. 2 AQA A Level Biology Topic 1 Biological molecules stan.store/biologywitholivia 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 3 AQA A Level Biology Topic 1 Biological molecules stan.store/biologywitholivia Describe the basic function and structure of starch and glycogen Starch Glycogen Energy store in Polysaccharide of α-glucose plant cells Amylose - 1,4-glycosidic bonds → unbranched Amylopectin - 1,4- and 1,6-glycosidic bonds → branched 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 4 AQA A Level Biology Topic 1 Biological molecules stan.store/biologywitholivia 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 5 AQA A Level Biology Topic 1 Biological molecules Exam insight: common mistakes ❌ stan.store/biologywitholivia 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 6 AQA A Level Biology Topic 1 Biological molecules stan.store/biologywitholivia 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. 7 AQA A Level Biology Topic 1 Biological molecules stan.store/biologywitholivia 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) 8 AQA A Level Biology Topic 1 Biological molecules stan.store/biologywitholivia 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. 9 AQA A Level Biology Topic 1 Biological molecules stan.store/biologywitholivia 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) Now induced-fit model ○ Active site a fixed shape, complementary to one substrate Explain the specificity of enzymes Specific tertiary structure determines shape of active site Active site is complementary to a specific substrate Only this substrate can bind to active site, inducing fit and forming an enzyme-substrate complex ○ Dependent on sequence of amino acids (primary structure) 10 AQA A Level Biology Topic 1 Biological molecules stan.store/biologywitholivia 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 11 AQA A Level Biology Topic 1 Biological molecules stan.store/biologywitholivia 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. 12 AQA A Level Biology Topic 1 Biological molecules stan.store/biologywitholivia 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 enzymecontrolled 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. Why were the enzyme & substrate solutions left Use a buffer solution Monitor using a pH meter at regular intervals So solutions equilibrate / reach the temperature of in the water bath for 10 mins before mixing? Describe a control experiment. the water bath 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 13 AQA A Level Biology Topic 1 Biological molecules stan.store/biologywitholivia 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 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 comparison to colour standards. 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 14 AQA A Level Biology Topic 1 Biological molecules stan.store/biologywitholivia 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. 15 AQA A Level Biology Topic 1 Biological molecules stan.store/biologywitholivia 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 16 AQA A Level Biology Topic 1 Biological molecules stan.store/biologywitholivia 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 17 AQA A Level Biology Topic 1 Biological molecules stan.store/biologywitholivia 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 Bacteria grown in medium containing heavy nitrogen (15N) and 1. 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. 18 AQA A Level Biology Topic 1 Biological molecules stan.store/biologywitholivia 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) Inorganic phosphate released can be used to phosphorylate (add phosphate ○ Eg. active transport, protein synthesis 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 19 AQA A Level Biology Topic 1 Biological molecules Exam insight: common mistakes ❌ stan.store/biologywitholivia 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 Metabolite Importance in biology 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 20 AQA A Level Biology Topic 1 Biological molecules Exam insight: common mistakes ❌ stan.store/biologywitholivia 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+ Allowing oxygen to bind / associate for transport as oxyhaemoglobin (topic 3.4.1) 1. Involved in co-transport of glucose / amino acids into cells (topic 2.3 / 3.3) 2. Involved in action potentials in neurons (topic 6.2) 3. Affects water potential of cells / osmosis (topic 2.3) 1. Component of nucleotides, allowing phosphodiester bonds to form in DNA / RNA 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) + (Fe ) Sodium + ions (Na ) Phosphate ions (PO 34 ) 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. Created with BioRender.com 21