2017 + 2019 + 2021 + 2022 + 2020 + 2023 + 2024 Paper 1 A level

Describe the structure and function of the nucleus

Structure

• Nuclear envelope and pores OR Double membrane and pores

• Chromosomes/chromatin OR DNA with histones

• Nucleolus/nucleoli

Function

• (Holds/stores) genetic information/material for polypeptides (production) OR (Is) code for polypeptides

• DNA replication (occurs)

• Production of mRNA/tRNA OR Transcription (occurs)

• Production of rRNA/ribosomes;

Polymer that forms a fungal cell wall

chitin

Explain how the use of antibiotics has led to antibiotic-resistant strains of bacteria becoming a common cause of infection acquired when in hospital.

• (Some bacteria have) alleles for resistance

• (Exposure to) antibiotics is the selection pressure OR Non-resistant bacteria die OR Resistant bacteria survive/reproduce

• More antibiotics used in hospital (compared with elsewhere) OR Patients have weakened immune systems OR (So) high frequency of resistance allele (in bacterial population)

Describe how the scientists could use aseptic techniques to transfer 0.3 cm3 of C. difficile in liquid culture from a bottle onto an agar plate.

• Wash hands with soap OR Disinfect surfaces

• Use sterile pipette/syringe (to transfer bacteria)

• (Remove bottle lid and) flame neck of bottle

• Lift lid of (agar) plate at an angle

• Work close to upward air movement

• Use sterile spreader

• Place pipette/spreader into disinfectant (immediately after use)

Use Figure 2 to evaluate whether more trehalose in the diet could be a factor in the increased number of antibiotic-resistant C. difficile infections

For

• Resistant bacteria grow faster with trehalose

• (So) resistant bacteria (likely to) increase in frequency in the population/people

• Resistant bacteria (likely to) outcompete non-resistant bacteria;

Against

• In laboratory not in people

• Other disaccharides (in the diet) might affect bacteria

• Other bacterial species (in the body) might affect bacteria

• No stats test to see if difference/increase is significant

• No data for both resistant and non-resistant bacteria growing together

• No data for different concentrations of trehalose;

Explain how two enzymes with different amino acid sequences can catalyse the same reaction.

• (Both) active sites have similar/identical tertiary structures OR (Both) active sites have identical amino acid sequences

• (So) form enzyme-substrate complexes (with the same substrate);

Describe how the scientists would remove large organelles from this suspension of cell contents.

• Use centrifuge/centrifugation at slow/low/increasing (sequence of) speed(s)

• Large/dense organelles (removed) in (first/early) pellet OR Less dense organelles (removed) in supernatant

Compare and contrast the structure of starch and the structure of cellulose

• Both polysaccharides OR Both are glucose polymers OR Both are made of glucose monomers

• Both contain glycosidic bonds (between monomers)

• Both contain carbon, hydrogen and oxygen/C, H and O

• Starch has α-glucose and cellulose has β-glucose

• Starch (molecule) is helical/coiled and cellulose (molecule) is straight

• Starch (molecule) is branched and cellulose is not/unbranched

• Cellulose has (micro/macro) fibrils and starch does not;

Explain the advantage for larger animals of having a specialised system that facilitates oxygen uptake.

• Large(r) organisms have a small(er) surface area:volume (ratio); OR Small(er) organisms have a large(r) surface area:volume (ratio)

• Overcomes long diffusion pathway OR Faster diffusion;

Describe how a sample of chloroplasts could be isolated from leaves

• Break open cells/tissue and filter OR Grind/blend cells/tissue/leaves and filter

• In cold, same water potential/concentration, pH controlled solution

• Centrifuge/spin and remove nuclei/cell debris

• (Centrifuge/spin) at high(er) speed, chloroplasts settle out;

Describe how a triglyceride molecule is formed.

• One glycerol and three fatty acids

• Condensation (reactions) and removal of three molecules of water

• Ester bond(s) (formed);

What elements are found in a phospholipid?

carbon, oxygen, phosphorous

What elements are found in RNA?

carbon, nitrogen, oxygen, phosphorous

Describe the structure of DNA.

• Polymer of nucleotides

• Each nucleotide formed from deoxyribose, a phosphate (group) and an organic/nitrogenous base

• Phosphodiester bonds (between nucleotides)

• Double helix/2 strands held by hydrogen bonds

• (Hydrogen bonds/pairing) between adenine, thymine and cytosine, guanine;

Name and describe five ways substances can move across the cell-surface membrane into a cell.

• (Simple) diffusion of small/non-polar molecules down a concentration gradient

• Facilitated diffusion down a concentration gradient via protein carrier/channel

• Osmosis of water down a water potential gradient

• Active transport against a concentration gradient via protein carrier using ATP

• Co-transport of 2 different substances using a carrier protein;

Contrast the structure of the two cells visible in the electron micrographs shown in Figure 14. (eu vs pro)

• Magnification (figures) show A is bigger than B

• A has a nucleus whereas B has free DNA

• A has mitochondria whereas B does not

• A has Golgi body/endoplasmic reticulum whereas B does not

• A has no cell wall whereas B has a murein/glycoprotein cell wall

• A has no capsule whereas B has a capsule

• A has DNA is bound to histones/proteins whereas B has DNA not associated with histones/proteins OR A has linear DNA whereas B has circular DNA

• A has larger ribosomes

The action of the carrier protein X in Figure 1 is linked to a membrane-bound ATP hydrolase enzyme. Explain the function of this ATP hydrolase (epithelial cells)

• Releases energy

• (energy) allows ions to be moved against a concentration gradient OR (energy) allows active transport of ions

The movement of Na+ out of the cell allows the absorption of glucose into the cell lining the ileum. Explain how

• (Maintains/generates) a concentration/diffusion gradient for Na+ (from ileum into cell)

• Na+ moving (in) by facilitated diffusion, brings glucose with it OR Na+ moving (in) by co-transport, brings glucose with it

Describe and explain two features you would expect to find in a cell specialised for absorption.

• Folded membrane/microvilli so large surface area (for absorption)

• Large number of co-transport/carrier/channel proteins so fast rate (of absorption) OR Large number of co-transport/carrier proteins for active transport OR Large number of co-transport/carrier/channel proteins for facilitated diffusion

• Large number of mitochondria so make (more) ATP (by respiration) OR Large number of mitochondria for aerobic respiration OR Large number of mitochondria to release energy for active transport

• Membrane-bound (digestive) enzymes so maintains concentration gradient (for fast absorption);

Describe how amino acids join to form a polypeptide so there is always NH2 at one end and COOH at the other end.

• One amine/NH2 group joins to a carboxyl/COOH group to form a peptide bond

• (So in chain) there is a free amine/NH2 group at one end and a free carboxyl/COOH group at the other OR Each amino acid is orientated in the same direction in the chain;

After collecting the samples, the scientist immediately heated them to 70 °C for 10 minutes. Explain why.

• To denature the enzymes/lipase

• So no further digestion/hydrolysis/catalysis occurred;

what do phospholipids do in ethanol?

dissolve

Describe how an enzyme can be phosphorylated.

• Attachment/association of (inorganic) phosphate (to the enzyme)

• (Released from) hydrolysis of ATP OR (Released from) ATP to ADP + Pi

Complete Table 2 to show three differences between DNA in the nucleus of a plant cell and DNA in a prokaryotic cell.

• (Associated with) histones/proteins v no histones/proteins

• Linear v circular

• No plasmids v plasmids

• Introns v no introns

• Long(er) v short(er)

Define ‘non-coding base sequences’ and describe where the non-coding multiple repeats are positioned in the genome.

• DNA that does not code for protein/polypeptides OR DNA that does not code for (sequences of) amino acids OR DNA that does not code for tRNA/rRNA

• (Positioned) between genes;

Define ‘gene mutation’ and explain how a gene mutation can have:

• no effect on an individual • a positive effect on an individual. (4 marks)

(Definition of gene mutation)

• Change in the base/nucleotide (sequence of chromosomes/DNA)

• Results in the formation of new allele

(Has no effect because)

• Genetic code is degenerate (so amino acid sequence may not change); OR Mutation is in an intron (so amino acid sequence may not change)

• Does change amino acid but no effect on tertiary structure

• (New allele) is recessive so does not influence phenotype

(Has positive effect because)

• Results in change in polypeptide that positively changes the properties (of the protein) OR Results in change in polypeptide that positively changes a named protein

• May result in increased reproductive success OR May result in increased survival (chances);

No large lipid droplets are visible with the optical microscope in the samples from suspension A. Explain why. (contains bile salts)

• Emulsification

• (Cannot be seen) due to resolution (of optical microscope);

what are algae, prokaryotic and fungi cell walls made of respectively?

cellulose, murein, chitin

evaluate a conclusion questions

correlation not causation, little evidence, no statistical test, other named factors, not long duration

how can replating hedges help farmers?

• Greater (bio)diversity so increase in predators of pests OR

• Increase in predators of pests so more yield/income/less pesticides/less damage to crops OR

• Increase in pollinators so more yield/income OR

• May attract more tourists/subsidies to their farm so more income (from diversification);

Use Figure 5 to explain how human mass at birth is affected by stabilising selection

• (Most likely to be) transferred to a special care unit are those under 2800 g OR (Most likely to be) transferred to a special care unit are those over 4200 g

• Extreme mass babies least likely to survive (to reproduce) and so less likely to pass on their alleles (for extreme mass at birth)

• Extreme mass at birth decreases in frequency (in the population) OR Alleles (for extreme mass at birth) decrease in frequency (in the population);

Use the data in Table 3 and your knowledge of the immune response to suggest why HIV controllers do not develop symptoms of AIDS. (less viral particles and more cd4 t cells)

• (All) have more T helper/CD4 cells

• Lower viral load to infect/destroy helper T/CD4 cells

• (So more/continued) activation of B cells/cytotoxic T cells/phagocytes

• (With B cells more/continued) production of plasma cells/antibodies OR (With cytotoxic T cells more/continued) ability to kill virus infected cells

• (More able to) destroy other microbes/pathogens OR (More able to) destroy mutated/cancer cells;

Unlike plants, Ulva lactuca does not have xylem tissue. Suggest how Ulva lactuca is able to survive without xylem tissue. (covered in sea water)

• Short diffusion pathway (to cells)

  OR

• It has a surface permeable (to water/ions into cells);

what may be the case for xerophytes in growth compared to normal?

• Growth may continue at lower water potentials

• (Due to) adaptations in enzymes involved in photosynthesis/metabolic reactions;

Suggest how these differences allow the mouse to have a higher metabolic rate than the horse. (difference in oxygen dissociation curve)

• Mouse haemoglobin/Hb has a lower affinity for oxygen OR For the same pO2 the mouse haemoglobin/Hb is less saturated OR At oxygen concentrations found in tissue mouse haemoglobin/Hb is less saturated

• More oxygen can be dissociated/released/unloaded (for metabolic reactions/respiration);

Use your knowledge of surface area to volume ratio to explain the higher metabolic rate of a mouse compared to a horse

• (Smaller so) larger surface area to volume ratio

• More/faster heat loss (per gram/in relation to body size)

• (Faster rate of) respiration/metabolism releases heat;

Describe the biochemical tests you would use to confirm the presence of lipid, non-reducing sugar and amylase in a sample

Lipid

• Add ethanol/alcohol then add water and shake/mix OR Add ethanol/alcohol and shake/mix then pour into/add water

• White/milky emulsion OR emulsion test turns white/milky;

  Non-reducing sugar

• Do Benedict’s test and stays blue/negative

• Boil with acid then neutralise with alkali

• Heat with Benedict’s and becomes red/orange (precipitate)

  Amylase

• Add biuret (reagent) and becomes purple/violet/mauve/lilac

• Add starch, (leave for a time), test for reducing sugar/absence of starch;

Explain why viruses are described as acellular and non-living.

• (Acellular) no cell(-surface) membrane

  OR

  Not made of cells

• (Non-living) have no metabolism/metabolic reactions;

  OR

  Cannot (independently) move/respire/replicate/ excrete

  OR

  (Have) no nutrition;

Give one reason why antibiotics are not effective against viruses.

• Do not have bacterial structures/enzymes

  OR

  Do not have metabolic processes

  OR

  Do not have a cell wall/murein;

Explain the importance of the xylem being kept open as a continuous tube.

• (Allows unbroken) water column OR (So) no barrier to (water) movement

• Cohesion from H bonds between (all) water (molecules) OR Cohesion from (polar) attraction between (all) water (molecules)

• Evaporation/transpiration creates tension (in column) OR Water moves from xylem (into cells) creates tension OR (To) pull up water creates tension (in xylem);

Suggest why several bacterial species have been renamed in recent years.

• DNA/mRNA/RNA base sequencing

  OR

  Amino acid sequencing

  OR

  Use of electron microscopes with greater resolution

  OR

  Use of electron microscopes and improved staining/preparation;

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

• RNA/nucleic acids

• protein/amino acids

Suggest how glycogen acts as a source of energy.

• Hydrolysed (to glucose)

• Glucose used in respiration

Suggest and explain two ways the cell-surface membranes of the cells lining the uterus may be adapted to allow rapid transport of nutrients.

• Membrane folded so increased/large surface area

  OR

  Membrane has increased/large surface area for (fast) diffusion/facilitated diffusion/active transport/co-transport

• Large number of protein channels/carriers (in membrane) for facilitated diffusion

• Large number of protein carriers (in membrane) for active transport

• Large number of protein (channels/carriers in membrane) for co-transport;

NHE3 actively transports one sodium ion into the cell in exchange for one proton (hydrogen ion) out of the cell. Use your knowledge of transport across cell membranes to suggest how NHE3 does this.

• Co-transport

• Uses (hydrolysis of) ATP

• Sodium ion and proton bind to the protein

• Protein changes shape (to move sodium ion and/or proton across the membrane)

Explain why protease activity decreases with temperature and why sharper decresae at higher temperature?

• Both denatured (by high temperature)

• Denaturation faster at 60 °C due to more (kinetic) energy

• Breaks hydrogen/ionic bonds (between amino acids/R groups)

• Change in shape of the active site/active site no longer complementary so fewer enzymesubstrate complexes formed/substrate does not fit;

Suggest one advantage to a bacterium of secreting an extracellular protease in its natural environment.

• To digest protein

• (So) they can absorb amino acids for growth/reproduction/protein synthesis/synthesis of named cell component

  OR

  (So) they can destroy a toxic substance/protein;

Why is it useful to use both an old antibiotic (which some bacteria are resistant to) and a mroe effective new one simultaeneously?

• New/old antibiotic does not kill all bacteria; OR Some bacteria are resistant to the new/old antibiotic

• Resistant bacteria will reproduce to produce (more) resistant bacteria

• (Use of both) one antibiotic will kill bacteria resistant to the other antibiotic

  OR

  Unlikely that bacteria are resistant to both the new and the old antibiotic

  OR

  Use of both antibiotics (likely to) kill all/most bacteria;

An environmental scientist investigated a possible relationship between air pollution and the size of seeds produced by one species of tree.

He was provided with a very large number of seeds collected from a population of trees in the centre of a city and also a very large number of seeds collected from a population of trees in the countryside.

Describe how he should collect and process data from these seeds to investigate whether there is a difference in seed size between these two populations of trees

• Use random sample of seeds (from each population)

• Use (large enough) sample to be representative of whole population

• Indication of what size was measured eg mass

• Calculate a mean and standard deviation (for each population)

• Use the (Student’s) t-test

• Analyse whether there is a significant difference between (the means of) the two populations;

Describe the process of facilitated diffusion and active transport

• movement of polar/charged molecules

• down a concentration gradient via carrier/channel proteins

• movement against a concentration gradient via carrier protein using ATP

what are microvilli?

highly folded cell surface membrane

what is the equation for mitotic index

number of cells in mitosis / total number of cells in field of view

why may speed of reaction vary when more phenol is present which could react with oxygen with an enzyme?

• more phenol in E

• so more enzyme substrate complexes formed

ways to make rate of reaction increase with an enzyme reaction (2 ways)?

• use enzymes higher/optimum temperature

• so more kinetic energy

• causing increased rate of reaction or more enzyme substrate complexes

OR

• use enzymes lower/optimum temperature or pH

• less denaturation

• increased rate of reaction or enzyme substrate complexes

give two structural features of an aorta wall and explain how they are related to the function of the aorta?

• (smooth) muscle absorbs/resists/withstands high blood pressure

• elastic tissue stretches and recoils maintaining/smooth blood pressure

• smooth endothelium reduces friction

• protein coat prevents artery wall splitting or resists high pressure