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AS level biology question, Summaries of Biology

Biology

AS level biology question and answer

Typology: Summaries

2025/2026

Uploaded on 04/15/2026

prasanna-lakshmi-3 🇮🇳

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Temperature affects the kinetic energy that molecules possess. References to

diffusion pathways (thickness of the membrane) will be ignored because all cell

membranes are essentially the same distance across - approx 7-8 nm. A fourth

mark will be awarded for referring to the ability of a molecule to cross the bilayer /

penetrate the hydrophobic centre, either directly or via available channel/carrier

proteins. Active transport is not within the scope of this question because it is

effectively the opposite of diffusion; diffusion is always passive and occurs down a

concentration gradient, unlike active transport.

The differences between channel and carrier proteins are…

Anyfour(of the following

Channel proteins are an open ‘pore’ that can allow polar/hydrophilic

molecules to pass through e.g. aquaporins;([1 mark]

Carrier proteins change shape/conformation, channel proteins do not;([1

mark]

Active transport requires carrier proteins - these have a site of ATP

hydrolysis;([1 mark]

Carrier proteins can be active or passive / channel proteins are always

passive;([1 mark]

Carrier proteins can transport up or down the concentration gradient /

channel proteins always transport down the concentration gradient;([1

mark]

[Total: 4 marks]

It’s a part of having good exam technique to realise that the command word

‘distinguish’ means, ‘highlight the differences. So try and avoid listing features

that both types of protein have in common, such as, ‘both are transmembrane /

intrinsic proteins’ as this would not be a relevant difference between the two types

of protein

(

Water potential is...

The tendency of water to diffuse across a partially permeable membrane;([1

mark]

kPa is a unit of pressure, because moving water

creates(pressure(changes;([1 mark]

[Total: 2 marks]

A discussion of why water potential (ψ) is a negative scale is not required for this

question. Water has the highest water potential, at zero kPa, whereas aqueous

solutions all have negative water potential values. This is because pure water will

always be drawn towards a solution, creating a negative pressure on the water

side of the membrane.

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Temperature affects the kinetic energy that molecules possess. References to diffusion pathways (thickness of the membrane) will be ignored because all cell membranes are essentially the same distance across - approx 7-8 nm. A fourth mark will be awarded for referring to the ability of a molecule to cross the bilayer / penetrate the hydrophobic centre, either directly or via available channel/carrier proteins. Active transport is not within the scope of this question because it is effectively the opposite of diffusion; diffusion is always passive and occurs down a concentration gradient, unlike active transport. The differences between channel and carrier proteins are… Any four of the following  Channel proteins are an open ‘pore’ that can allow polar/hydrophilic molecules to pass through e.g. aquaporins; [1 mark]  Carrier proteins change shape/conformation, channel proteins do not; [ mark]  Active transport requires carrier proteins - these have a site of ATP hydrolysis; [1 mark]  Carrier proteins can be active or passive / channel proteins are always passive; [1 mark]  Carrier proteins can transport up or down the concentration gradient / channel proteins always transport down the concentration gradient; [ mark] [Total: 4 marks] It’s a part of having good exam technique to realise that the command word ‘distinguish’ means, ‘highlight the differences. So try and avoid listing features that both types of protein have in common, such as, ‘both are transmembrane / intrinsic proteins’ as this would not be a relevant difference between the two types of protein Water potential is...  The tendency of water to diffuse across a partially permeable membrane; [ mark]  kPa is a unit of pressure, because moving water creates pressure changes; [1 mark] [Total: 2 marks] A discussion of why water potential (ψ) is a negative scale is not required for this question. Water has the highest water potential, at zero kPa, whereas aqueous solutions all have negative water potential values. This is because pure water will always be drawn towards a solution, creating a negative pressure on the water side of the membrane.

in aqueous solution has a lower water potential than pure water because...  Solute molecules in the solution occupy / immobilise water molecules; [ mark]  ...therefore fewer water molecules are free to diffuse freely across a membrane; [1 mark] OR  Pure water has no solute molecules; [1 mark]  ...therefore, all water molecules are free to diffuse freely across a membrane; [1 mark] [Total: 2 marks] The key point for this question is relating solute particles to the ‘bodyguard’ of water molecules that surround each solute molecule. The ‘bodyguard’ water molecules are stuck in that role, surrounding each solute molecule and keeping it separate from the other solute molecules. This means that those water molecules are occupied and therefore not free to move independently, so they cannot cross the partially permeable membrane. Only the free water molecules can diffuse, and by definition, there are fewer of those in a solution than in pure water. he mode of transport of ions that this protein employs is...  Facilitated diffusion; [1 mark] Because...  Ions are polar molecules / need a channel to pass through the bilayer; [ mark]  (Not active transport because..) ions diffuse down their concentration gradient / process is passive / no indication of ATP hydrolysis to provide energy; [1 mark] [Total: 3 marks] We can rule out simple diffusion and osmosis; simple diffusion passes straight through the phospholipid bilayer and osmosis is the transport of water. That leaves facilitated diffusion and active transport. We can rule out active transport because the diagram shows no evidence of ATP hydrolysis / the input of energy. Ions are polar / hydrophilic (by virtue of their electronic charge) so require a channel. Be careful not to confuse the opening of the gates by changes in voltage with the supply of energy from ATP hydrolysis; these are very different. An advantage to medicine of studying the permeability of the cell surface membranes of bacteria is...  To cause (pathogenic / disease-causing) bacteria to become permeable to antibiotics; [1 mark]

(Shade plant) has lower (rate of) respiration / respiratory losses / less CO released at 0 light intensity / in dark;
Greater (net) productivity / less sugars / glucose used / more sugars / glucose available; Neutral: any references to rate of photosynthesis. On islands in the Caribbean, there are almost 150 species of lizards belonging to the genus Anolis. Scientists believe that these species evolved from two species found on mainland USA. Explain how the Caribbean species could have evolved.
Geographic(al) isolation;
Separate gene pools / no interbreeding / gene flow (between populations);
Variation due to mutation;
Different selection pressures / different abiotic / biotic conditions / environments / habitats;
Different(ial) reproductive success / selected organisms (survive and) reproduce;
Leads to change / increase in allele frequency. Anolis sagrei is a species of lizard that is found on some of the smallest Caribbean islands. Describe how you could use the mark-release- recapture method to estimate the number of Anolis sagrei on one of these islands.
Capture / collect sample, mark and release;
Method of marking does not harm lizard / make it more visible to predators;
Leave sufficient time for lizards to (randomly) distribute (on island) before collecting a second sample;
(Population =) number in first sample × number in second sample divided by number of marked lizards in second sample / number recaptured. Large areas of tropical forest are still found on some Caribbean islands. The concentration of carbon dioxide in the air of these forests changes over a period of 24 hours and at different heights above ground. Use your knowledge of photosynthesis and respiration to describe and explain how the concentration of carbon dioxide in the air changes:

**- over a period of 24 hours

at different heights above ground.**

High concentration of / increase in carbon dioxide linked with respiration at night / in darkness;
No photosynthesis in dark / night / photosynthesis only in light / day;
In light net uptake of carbon dioxide / use more carbon dioxide than produced / (rate of) photosynthesis greater than rate of respiration;
Decrease in carbon dioxide concentration with height;
(At ground level) less photosynthesis / less photosynthesising tissue / more respiration / more micro-organisms / micro-organisms produce carbon dioxide. The scientists suggested that mutant plants producing more chlorophyll b would grow faster than normal plants in all light intensities. Explain how these data support this suggestion. At all light intensities, chloroplasts from mutant plants:
Have faster production of ATP and reduced NADP;
(So) have faster / more light-independent reaction;
(So) produce more sugars that can be used in respiration;
(So) have more energy for growth;
Have faster / more synthesis of new organic materials. Explain the advantage of stomata to the plant.
(Because) water is lost through stomata;
(Closure) prevents / reduces water loss;
Maintain water content of cells. In photosynthesis, which chemicals are needed for the light-dependent reaction? ☑ NADP, ADP, Pi and water; Describe what happens during photoionisation in the light-dependent reaction. 1.Chlorophyll absorbs light/Light excites/moves electrons in chlorophyll;
Electron/s are lost/(Chlorophyll) becomes positively charged;

OR

Less reduction of GP to triose phosphate. When treated with Atrazine, weeds have been shown to give off small amounts of heat. Suggest an explanation for this observation. Because energy is released from high energy / excited electron/s (that were lost from chlorophyll) If SDs overlap means =no significant difference If No SDs overlap means = significant difference in growth/mass No statistical test, so do not know if differences are significant OR No statistical test, so differences could be due to chance; A teacher studying these data with her students told her class that no definite conclusions could be drawn when comparing the mean values in the graph. Suggest why the teacher said this. No error bars / SD; No SD / statistical test to determine significance

**1. Net productivity = gross productivity minus respiratory loss;

Decrease in gross productivity / photosynthesis / increase in respiration Suggest three reasons why the apparatus was left for 10 minutes.** 1 Accept equilibrate
Allow for expansion / pressure change in apparatus. Equilibrium reached.
Allow respiration rate of seeds to stabilise. Reject: production of energy, but accept production of energy in the form of ATP.

Explain the line between P and Q.

**1. Aerobic respiration (uses oxygen); Accept ‘oxidative phosphorylation / electron transfer takes place’.

Oxygen is terminal / electron acceptor;
(oxygen combines with) protons / H+ and electrons / eto form water / H2O; Some gene mutations affect the amino acid sequence. Some mutations do not.** Has no effect / same amino acid (sequence) / same Reject same amino acid formed or produced. how mutation could lead to the formation of a non-functional enzyme. Explain how this mutation leads to the formation of non-functioning PAH

Change in tertiary structure / active site; Neutral: change in 3D shape / structure
(So) faulty / non-functional protein / enzyme; Explain how the structure of DNA is related to its functions. Must be a direct link / obvious to get the mark Neutral: reference to histones 1.Sugar-phosphate (backbone) / double stranded / helix so provides strength / stability / protects bases / protects hydrogen bonds;
large molecule so can store lots of information;
Helix / coiled so compact; Accept: can store in a small amount of space for ‘compact’
base sequence codesfor amino acids / protein;
Double stranded so replication can occur semi-conservatively / strands can act as templates / complementary base pairing / A-T and G-C so accurate replication / identical copies can be made;
(Weak) hydrogen bonds for replication / unzipping / strand separation /many hydrogen bonds so stable / strong; Accept: 'H-bonds' for ‘hydrogen bonds’ More cancer cells could be destroyed if the drug was given more frequently. Suggest why the drug was not given more frequently. ........................................................................................................................
More / too many healthy cells killed;
(So) will take time to replace / increase in number;
Person may die / have side effects; What is a substitution mutation? Replacement of a base by a different base in DNA

Scientists can use protein structure to investigate the evolutionary relationships between different species. Explain why.

Closer the (amino acid) sequence the closer the relationship;
(Protein structure) related to (DNA) base / triplet sequence; Amino acid sequence is related to (DNA) base / triplet sequence = two marks; Comparing the base sequence of genes provides more evolutionary information than comparing the structure of proteins. Explain why. Different (base) triplets code for same amino acids = 2marks; In photosynthesis, which chemicals are needed for the light- dependent reaction? ☑ NADP, ADP, Pi and water; **Explain why the scientists measured the rate of production of oxygen in this investigation.
Oxygen produced in light-dependent reaction;
The faster (oxygen) is produced, the faster the light-dependent reaction. What measurements should the student have taken to determine the rate of photosynthesis?** Oxygen production and time. temperature and pH, give two factors which should be kept constant during this investigation.
Intensity of light;
Amount / number / mass / species of algae / photosynthesising cells;
Carbon dioxide (concentration / partial pressure);
Time.

Correct answers are often shown as: net productivity = (gross) photosynthesis – (minus) respiration. Use the information in the figure to explain how the shade plant is better adapted than the sun plant to growing at low light intensities.

(Shade plant) has lower (rate of) respiration / respiratory losses / less CO released at 0 light intensity / in dark;
Greater (net) productivity / less sugars / glucose used / more sugars / glucose available; Neutral: any references to rate of photosynthesis. On islands in the Caribbean, there are almost 150 species of lizards belonging to the genus Anolis. Scientists believe that these species evolved from two species found on mainland USA. Explain how the Caribbean species could have evolved.
Geographic(al) isolation;
Separate gene pools / no interbreeding / gene flow (between populations);
Variation due to mutation;
Different selection pressures / different abiotic / biotic conditions / environments / habitats;
Different(ial) reproductive success / selected organisms (survive and) reproduce;
Leads to change / increase in allele frequency. Anolis sagrei is a species of lizard that is found on some of the smallest Caribbean islands. Describe how you could use the mark-release- recapture method to estimate the number of Anolis sagrei on one of these islands.
Capture / collect sample, mark and release;
Method of marking does not harm lizard / make it more visible to predators;
Leave sufficient time for lizards to (randomly) distribute (on island) before collecting a second sample;
(Population =) number in first sample × number in second sample divided by number of marked lizards in second sample / number recaptured. Large areas of tropical forest are still found on some Caribbean islands. The concentration of carbon dioxide in the air of these forests changes over a period of 24 hours and at different heights above ground. Use your knowledge of photosynthesis and respiration to describe and explain how

the concentration of carbon dioxide in the air changes:

**- over a period of 24 hours

at different heights above ground.**

High concentration of / increase in carbon dioxide linked with respiration at night / in darkness;
No photosynthesis in dark / night / photosynthesis only in light / day;
In light net uptake of carbon dioxide / use more carbon dioxide than produced / (rate of) photosynthesis greater than rate of respiration;
Decrease in carbon dioxide concentration with height;
(At ground level) less photosynthesis / less photosynthesising tissue / more respiration / more micro-organisms / micro-organisms produce carbon dioxide. The scientists suggested that mutant plants producing more chlorophyll b would grow faster than normal plants in all light intensities. Explain how these data support this suggestion. At all light intensities, chloroplasts from mutant plants:
Have faster production of ATP and reduced NADP;
(So) have faster / more light-independent reaction;
(So) produce more sugars that can be used in respiration;
(So) have more energy for growth;
Have faster / more synthesis of new organic materials. Explain the advantage of stomata to the plant.
(Because) water is lost through stomata;
(Closure) prevents / reduces water loss;
Maintain water content of cells. In photosynthesis, which chemicals are needed for the light-dependent reaction? ☑ NADP, ADP, Pi and water; Describe what happens during photoionisation in the light-dependent reaction. 1.Chlorophyll absorbs light/Light excites/moves electrons in chlorophyll;
Electron/s are lost/(Chlorophyll) becomes positively charged; Heat stress decreases the light-dependent reaction of photosynthesis.

When treated with Atrazine, weeds have been shown to give off small amounts of heat. Suggest an explanation for this observation. Because energy is released from high energy / excited electron/s (that were lost from chlorophyll) Respiration produces more ATP per molecule of glucose in the presence of oxygen than it does when oxygen is absent. Explain why.

Oxygen is final / terminal (electron) acceptor / oxygen combines with electrons and protons;
Oxidative phosphorylation / electron transport chain provides (most) ATP / only glycolysis occurs without oxygen / no Krebs / no link reaction; Describe how acetylcoenzyme A is formed in the link reaction.
Oxidation of / hydrogen removed from pyruvate and carbon dioxide released;
Addition of coenzyme A. Explain how oxaloacetate enables the acetylcoenzyme A to then bind to the enzyme. ........................................................................................................... ....
Change (in shape) of active site / active site moulds around the substrate;
(Substrate / active site) now complementary. Another substance in the Krebs cycle is called succinyl coenzyme A. This substance has a very similar shape to acetylcoenzyme A. Suggest how production of succinyl coenzyme A could control the rate of the reaction catalysed by citrate synthase. ............................................................................................................ ...

Is a competitive inhibitor / attaches to active site;
Reduces / prevents enzyme-substrate / E-S complex forming.

In muscles, pyruvate is converted

to lactate during anaerobic

respiration. Explain why

converting pyruvate to lactate

allows the continued production

of

ATP during anaerobic

respiration.

1. Regenerates / produces NAD /

oxidises reduced NAD;

2. (NAD used) in glycolysis.

Explain why a log scale is used to record the number of cells. Large range/difference/increase in numbers; Accept reference to exponential (increase) Many yeast cells die during the death phase. Suggest one reason why.

Decrease/no glucose/substrate OR Increase in ethanol/carbon dioxide/acidity; (a) Describe the advantage of the Bohr effect during intense exercise.

Increases dissociation of oxygen; Accept unloading/ release/reduced affinity for dissociation
For aerobic respiration at the tissues/muscles/cells OR Anaerobic respiration delayed at the tissues/muscles/cells OR Less lactate at the tissues/muscles/cells; GW1516 is a performance-enhancing drug. GW1516 activates AMPK and develops slow muscle fibres at rest. Use diagram above to justify why professional athletes are not allowed to take GW1516.
More acetylcoenzyme A would enter the Krebs cycle;
(So) the Krebs cycle generates (more) reduced coenzymes OR (So more) reduced coenzymes pass their electrons to the electron transfer chain;
(So more) ATP would be produced; idea for more is required once
Athletes could build (slow) muscle (fibres) without exercising;
(Having more) slow muscle (fibres) would increase endurance; A heart attack is caused by a lack of glucose and oxygen being delivered to cardiac muscle via the coronary arteries. The overuse of EPO can increase

the risk of a heart attack. Suggest how.

(EPO) causes blood to thicken; Accept descriptions of thickening, eg more viscous
(The thickened blood) could block the coronary arteries Suggest an explanation for the effect of temperature on the rate of carbon dioxide release. Spiracle
Enzymes / metabolism faster;
Higher rate of respiration and carbon dioxide production / release;
Spiracles open more often / remain open to excrete / get rid of carbon dioxide / get more oxygen; Nitrate from fertiliser applied to crops may enter ponds and lakes. Explain how nitrate may cause the death of fish in fresh water. Nitrates and phosphates are two of these nutrients. They provide a source of nitrogen and phosphorus for cells. Give a biological molecule that contains:
nitrogen .......... Amino acid / protein / enzyme / urea / nucleic acid
phosphorus........... DNA / RNA / nucleic acid / ATP / ADP / AMP / NADP / TP / GP

AS level biology question, Summaries of Biology

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Partial preview of the text

Download AS level biology question and more Summaries Biology in PDF only on Docsity!

OR

In muscles, pyruvate is converted

to lactate during anaerobic

respiration. Explain why

converting pyruvate to lactate

allows the continued production

of

ATP during anaerobic

respiration.

1. Regenerates / produces NAD /

oxidises reduced NAD;

2. (NAD used) in glycolysis.