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AQA a-level biology 🧬🔬 > A01 questions > Flashcards

A01 questions Flashcards

all topics, longer markers, just have to learn them

1
Q

Describe the mechanism for the absorption of amino acids into the ileum (4 marks)

A
  • Facilitated diffusion of amino acids
  • Co transport
  • Sodium ions actively transported from cell to blood
  • Creating sodium ion concentration gradient
  • Facilitated diffusion of amino acid into blood
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2
Q

Describe how the process of meiosis results in haploid cells (4 marks)

A
  • DNA replication
  • 2 divisions
  • Separation of homologous chromosomes
  • Separation of sister chromatids in second division
  • Produces 4 haploid cells
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3
Q

Explain five properties that make water important for organisms (5 marks)

A
  • a metabolite in condensation/hydrolysis/photosynthesis/respiration
  • a solvent so metabolic reactions can occur or a solvent so allowing transport of substances
  • high specific heat capacity so buffers change in temperature
  • large latent heat of vaporisation so provides a cooling effect (through evaporation)
  • cohesion (between water molecules) so supports columns of water in plants
  • cohesion (between water molecule) so produces surface tension supporting small organisms
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4
Q

Describe the process of semi-conservative replication of DNA (5 marks)

A
  • DNA helicase breaks hydrogen bonds
  • 2 strands of DNA are separated
  • Both strands act as templates
  • Free DNA nucleotides (attracted to exposed bases) line up in complementary base pairs/A-T,C-G
  • DNA polymerase joins nucleotides
  • Forming phosphodiester bonds between adjacent nucleotides
  • Each new DNA molecule consists of one old strand and one new strand
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5
Q

Compare competitive and non-competitive inhibition (4 marks)

A

Similarities:
- Both reduce rate of reaction
- Both prevent substrate from binding to active site
Differences:
- Non competitive binds to allosteric site
- Competitive blocks active site
- Competitive can still achieve maximum rate of reaction
- Competitive is more likely to be reversible

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6
Q

Describe how the structure of glycogen is related to its functions (5 marks)

A
  • Helix/coiled/branched so compact
  • Polymer of glucose so easily hydrolysed
  • Branched so more ends for faster hydrolysis
  • Glucose (polymer) so provides respiratory substrate for energy (release)
  • Insoluble so not (easily) lost (from cell) or insoluble so does not affect water potential/osmosis
  • Mention of alpha glucose and 1,4 and 1,6 glycosidic bonds
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7
Q

Compare the structure of DNA and RNA (5 marks)

A
  • DNA has deoxyribose//mRNA has ribose
  • DNA has thymine//mRNA has uracil
  • DNA is long//mRNA is short
  • DNA is double stranded//mRNA is single stranded
  • DNA has hydrogen bonds//mRNA has no hydrogen bonds
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8
Q

Describe the appearance and behaviour of chromosomes during mitosis (5 marks)

A
  • During prophase, chromosomes condense into chromatin
  • Sister chromatids held together at centromeres
  • During metaphase, chromosomes line up on equator
  • Spindle fibres attach to centromere
  • During anaphase, spindle fibres divide centromere
  • Sister chromatids separated to opposite poles
  • During telophase, chromatin uncoils
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9
Q

Describe and explain the processes that occur during meiosis that increase genetic variation (4 marks)

A
  • Independent segregation
  • New combinations of alleles produces
  • Maternal + paternal chromosomes are reshuffled in any combination
  • Crossing over
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10
Q

Describe how mRNA is formed by transcription in eukaryotes (5 marks)

A
  • DNA helicase breaks hydrogen bonds between complementary pairs
  • Free RNA nucleotides are then attracted to exposed bases on template strand
  • RNA polymerase catalyses formation of phosphodiester bonds between adjacent RNA nucleotides
  • Pre mRNA is spliced to remove introns
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11
Q

Compare diffusion and active transport (4 marks)

A
  • Diffusion is passive, no ATP required//Active transport requires ATP
  • Active transport requires carrier proteins//Facilitated diffusion requires channel,carrier proteins
  • Carrier proteins in AT are specific to substrate
  • Carrier proteins undergo shape change to move molecules across membrane
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12
Q

Other than simple diffusion, describe the ways in which substances can cross the cell surface membrane (5 marks)

A
  • Osmosis; high-low water potential via aquaporins
  • Facilitated diffusion; high-low concentration via channel/carrier proteins
  • Active transport; against concentration gradient, requires ATP and carrier proteins
  • Exocytosis; fusion of vesicle with cell surface membrane
  • Endocytosis; entering cell
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13
Q

Describe how one amino acid is added to a polypeptide that is formed at a ribosome during translation (5 marks)

A
  • tRNA with complementary anticodon binds to codon on mRNA
  • tRNA brings specific amino acid to ribosome
  • (peptidyl transferase) catalyses formation of peptide bonds between amino acids
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14
Q

Explain appearance of cell A which is in hypotonic solution and cell B which is in hypertonic solution (5 marks)

A
  • cells in A are turgid
  • higher water potential outside the cell
  • hypotonic solution
  • water moved by osmosis
  • cells in B are plasmolysed
  • higher water potential inside cell
    hypertonic solution
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15
Q

Identify 2 features that adapt the epithelial cell for its function and explain how each feature helps (4 marks)

A
  • villi/microvilli for increased surface area
  • for absorption of glucose
  • many mitochondria
  • site of aerobic respiration as active transport requires ATP
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16
Q

Discuss the importance of shape of molecules to organisms (4 marks)

A

AO1 (up to 3 marks):
- Tertiary structure of protein (intermolecular forces between R groups) determines shape of enzyme (globular protein);
- Active site is complementary shape to substrates;
- Substrates form intermolecular forces with R groups of amino acids in active site;
- Which induces shape change within enzyme, straining bonds in substrates;
AO2 (up to 2 marks):
- Catalyse biological reactions – lower activation energy by providing alternative pathway;
- Named example from A Level spec with function, e.g. DNA polymerase, DNA ligase;;

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17
Q

Compare and contrast the structure and propertiesof triglycerides and phospholipids (3 marks)

A
  • Both contain ester bonds (between glycerol and fatty acid);
  • Both contain glycerol;
  • Fatty acids on both may be saturated or unsaturated;
  • Both are insoluble in water;
  • Both contain C, H and O but phospholipids also contain P;
  • Triglyceride has three fatty acids and phospholipid has two fatty acids plus phosphate group;
  • Triglycerides are hydrophobic/non-polar and phospholipids have hydrophilic and hydrophobic region;
  • Phospholipids form monolayer (on surface)/micelle/bilayer (in water) but triglycerides don’t;
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18
Q

Succession occurs in natural ecosystems. Describe and explain how succession occurs. (4 marks)

A
  • Colonisation by pioneer species
  • Pioneer species changes the environment
  • Environment becomes more suitable for other species/ enables other species to colonise
  • Increasing biodiversity
  • stability increases/ less hostile environment
  • Until reaches climax community
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19
Q

Describe how the structure of the insect gas exchange system:
- provides cells with sufficient oxygen
- limits water loss.
Explain your answers. (5 marks)

A
  • Spiracles to trachea to tracheoles
  • Open spiracles allow diffusion of oxygen from air
  • Tracheoles highly branched so large surface area
  • Tracheole walls thin so short diffusion distance
  • Tracheoles walls are permeable to oxygen
  • Cuticle/chitin in trachea impermeable so reduces water
    Spiracles closed preventing water loss
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20
Q

Describe how humans breathe in and out. (5 marks)

A
  • Diaphragm muscles contract and diaphragm flattens
  • External intercostal muscles contract and ribcage pull up and out
  • Causing volume increase and pressure decrease in thoracic cavity to below atmospheric pressure
  • Diaphragm muscles relax and internal intercostal muscles contract
  • Causes volume decrease and pressure increase in the thoracic cavity to above atmospheric pressure
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21
Q

Haemoglobins are chemically similar molecules found in many different species. Differences in the primary structure of haemoglobin molecules can provide evidence of phylogenetic (evolutionary) relationships between species.
Explain how. (5 marks)

A
  • Mutations change base sequence
  • Causing change in amino acid sequence
  • Mutations build over time
  • Fewer mutations in closely related species
  • Closely related species have earlier common ancestor
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22
Q

Compare and contrast the DNA in eukaryotic cells with the DNA in prokaryotic cells. (5 marks)

A
  • Nucleotide structure is identical
  • Nucleotides joined by phosphodiester bonds
  • DNA in mitochondria
  • Eukaryotic DNA is longer
  • Eukaryotic DNA contains introns, prokaryotic DNA does not
  • Eukaryotic DNA is linear, prokaryotic DNA is circular
  • Eukaryotic DNA is associated with histone proteins, prokaryotic DNA is not
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23
Q

Describe and explain how cell fractionation and ultracentrifugation can be used to isolate mitochondria from a suspension of animal cells. (5 marks)

A
  • Cell homogenisation to break open cells
  • Filter to remove debris
  • Use isotonic solution to prevent damage to mitochondria
  • Keep cold to reduce damage by enzymes
  • Centrifuge to separate cell fragments
  • Re spin the supernatant at higher speed to get mitochondria in pellet
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24
Q

ATP is useful in many biological processes. Explain why. (4 marks)

A
  • Releases energy in small/manageable amounts
  • Releases energy instantaneously
  • Broken down in one step
  • Phosphorylates other compounds lowers activation energy and making them more reactive
  • Reformed (rapidly resynthesized)
  • is not lost from/does not leave cells
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25
Q

Describe how a heartbeat is initiated (5 marks)

A
  • Sinoatrial node generates waves of excitation causing atria to contract
  • Non conductive tissue between atria and ventricles prevents ventricular contraction
  • AVN delays impulse to ventricles to allow atria to fully empty and the ventricles to fill with blood
  • Impulse travels to bundle of His and purkinje fibres
  • Ventricles contract from the base upwards
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26
Q

Explain how the heart muscle and and the heart valves maintain a one way flow of blood from the left atrium to the aorta (5 marks)

A
  • When pressure is higher in the left atrium>left ventricle, AV valve opens
  • Blood flows into the the ventricles and once pressure in the ventricles, AV valve closes to prevent backflow
  • Pressure building in the ventricles causes semi-lunar valves to open
  • Blood flows through aorta and once pressure is greater in the aorta than ventricles, semi-lunar valves close
  • Contraction of muscles causes changes in pressure
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27
Q

Describe the cohesion tension theory of water transport in xylem

A
  • Water lost from leaf because of transpiration
  • Lowers water potential of the leaf
  • Pulling water up the xylem creating tension
  • Water molecules stick together by hydrogen bonds cohesion
  • Adhesion of water molecules to lignin xylem walls
  • Forming continuous water column
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28
Q

Describe the processes involved in transport of sugars in plant stem (5 marks)

A
  • Sucrose actively transported into phloem (cell);
  • (By) companion/transfer cells;
  • Lowers water potential (in phloem) and water enters (from xylem) by osmosis;
  • (Produces) high(er) (hydrostatic) pressure;
  • Mass flow to respiring cells
  • Unloaded/removed (from phloem) by active transport;
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29
Q

Describe the light independent reaction of photosynthesis

A
  • CO2 reacts with RuBP
  • Produces 2x GP using Rubisco
  • GP reduced to triose phosphate
  • Using NADP
  • Using energy from ATP
  • Triose phosphate converted to glucose
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30
Q

Describe the chemical reactions involved in the conversion of polymers to monomers and monomers to polymers.
Give two named examples of polymers and their associated monomers to illustrate your answer.

A
  • A condensation reaction joins monomers together and forms a (chemical) bond and releases water;
  • A hydrolysis reaction breaks a (chemical) bond between monomers and uses water;
  • A suitable example of polymers and the monomers from which they are made;
  • Polymers must contain many monomers.
  • suitable examples include
    amino acid and polypeptide, protein, enzyme, antibody or specific/ nucleotide and polynucleotide, DNA or RNA
  • Reference to a correct bond within a named polymer-
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31
Q

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

A

Lipid
- Add ethanol/alcohol then add water and shake/mix
- White/milky emulsion
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;

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32
Q

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

A
  • Both polysaccharides;
  • Both contain glycosidic bonds (between monomers);
  • Both contain carbon, hydrogen and oxygen/C, H and O;
  • Starch made of α-glucose and cellulose made of 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;-
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33
Q

Mucus also contains glycoproteins. One of these glycoproteins is a polypeptide with the sugar, lactose, attached.
Describe how lactose is formed and where in the cell it would be attached to a polypeptide to form a glycoprotein.

A
  • Glucose and galactose;
  • Joined by condensation (reaction);
  • Joined by glycosidic bond;
  • Added to polypeptide in Golgi (apparatus);;
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34
Q

Describe how the structures of starch and cellulose molecules are related to their functions

A

Starch (max 3)
- Helical/ spiral shape so compact;
- Large (molecule)/insoluble so osmotically inactive;
- Branched so glucose is (easily) released for respiration;
- Large (molecule) so cannot leave cell/cross cell-surface membrane;

Cellulose (max 3)
- Long, straight/unbranched chains of β glucose;
- Joined by hydrogen bonding;
- To form (micro/macro)fibrils;
- Provides rigidity/strength;-

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35
Q

Describe how the structure of a protein depends on the amino acids it contains

A
  • Structure is determined by (relative) position of amino acid/R group/interactions;
  • Primary structure is sequence/order of amino acids;
  • Secondary structure formed by hydrogen bonding (between amino acids);
  • Tertiary structure formed by interactions (between R groups);
  • Creates active site in enzymes
  • Quaternary structure contains >1 polypeptide chain
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36
Q

Describe the structure of proteins

A
  • Polymer of amino acids;
  • Joined by peptide bonds;
  • Formed by condensation;
  • Primary structure is order of amino acids;
  • Secondary structure is folding of polypeptide chain due to hydrogen bonding
  • Tertiary structure is 3-D folding due to hydrogen bonding and ionic / disulfide bonds;
  • Quaternary structure is two or more polypeptide chains.
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37
Q

In humans, the enzyme maltase breaks down maltose to glucose. This takes place at normal body temperature.
Explain why maltase:
- only breaks down maltose
- allows this reaction to take place at normal body temperature

A
  • Tertiary structure / 3D shape of enzyme (means);
  • Active site complementary to maltose / substrate / maltose fits into active site / active site and substrate fit like a lock and key;
  • Description of induced fit;
  • Enzyme is a catalyst / lowers activation energy / energy required for reaction;
  • By forming enzyme-substrate complex;
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38
Q

Describe the role of iron ions, sodium ions, and phosphate ions in cells

A

Iron ions
- Haemoglobin binds/associates with oxygen
Sodium ions
- Co-transport of glucose/amino acids (into cells);
- (Because) sodium moved out by active transport/Na – K pump;
- Creates a sodium concentration/diffusion gradient;
- Affects osmosis/water potential;
Phosphate ions
- Affects osmosis/water potential;
- Joins nucleotides/in phosphodiester bond/in backbone of DNA/RNA/in nucleotides;
- Used in/to produce ATP;
- Phosphorylates other compounds (usually) making them more reactive;
- Hydrophilic/water soluble part of phospholipid bilayer/membrane;

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39
Q

The movement of substances across cell membrane is affected by membrane structure. Describe how.

A
  • Phospholipid (bilayer) allows movement/diffusion of non-polar/lipid-soluble substances;
  • Phospholipid (bilayer) prevents movement/diffusion of polar/ charged/lipid-insoluble substances
  • Carrier proteins allow active transport;
  • Channel/carrier proteins allow facilitated diffusion/co-transport;
  • Shape/charge of channel / carrier determines which substances move;
  • Number of channels/carriers determines how much movement;
  • Membrane surface area determines how much diffusion/movement;
  • Cholesterol affects fluidity/rigidity/permeability-
40
Q

The epithelial cells that line the small intestine are adapted for the absorption of glucose. Explain how.

A
  • Microvilli provide a large / increased surface area;
  • Many mitochondria produce ATP / release or provide energy (for active transport);
  • Carrier proteins for active transport;
  • Channel / carrier proteins for facilitated diffusion;
  • Co-transport of sodium (ions) and glucose or symport / carrier protein for sodium (ions) and glucose;
  • Membrane-bound enzymes digest disaccharides / produce glucose0
41
Q

Describe the processes involved in the absorption of the products of starch digestion.

A
  • Glucose moves in with sodium (into epithelial cell);
  • Via (carrier / channel) protein / symport;
  • Sodium removed (from epithelial cell) by active transport / sodium- potassium pump;
  • Into blood;
  • Maintaining low concentration of sodium (in epithelial cell) / maintaining sodium concentration gradient (between lumen and epithelial cell);
  • Glucose moves into blood;
  • By (facilitated) diffusion;
42
Q

Describe how the human immunodeficiency virus (HIV) is replicated once inside helper T cells (TH cells). (4 marks)

A
  • RNA converted into DNA using reverse transcriptase;
  • DNA incorporated/inserted into (helper T cell) DNA/chromosome/genome/nucleus;
  • DNA transcribed into (HIV m)RNA;
  • (HIV mRNA) translated into (new) HIV/viral proteins (for assembly into viral particles);
43
Q

Explain how HIV affects the production of antibodies when AIDS develops in a person. (3 marks)

A
  • Less/no antibody produced;
  • (Because HIV) destroys helper T cells;
  • (So) few/no B cells activated / stimulate
44
Q

Bacterial meningitis is a potentially fatal disease affecting the membranes around the brain. Neisseria meningitidis (Nm) is a leading cause of bacterial meningitis.
In the UK, children are vaccinated against this disease. Describe how vaccination can lead to protection against bacterial meningitis. (6 marks)

A
  • Antigen / epitope on surface of N. meninigitidis / bacterium binds to surface protein / surface receptor on a (specific / single) B cell.
  • (Activated) B cell divides by mitosis / produces clone;
  • (Division) stimulated by cytokines / by T cells;
  • B cells / plasma cells release antibodies;
  • (Some) B cells become memory cells;
  • Memory cells produce plasma / antibodies faster
45
Q

When a vaccine is given to a person, it leads to the production of antibodies against a disease-causing organism. Describe how. (5 marks)

A
  • Vaccine contains antigen from pathogen;
  • Macrophage presents antigen on its surface;
  • T cell with complementary receptor protein binds to antigen;
  • T cell stimulates B cell;
  • (With) complementary antibody on its surface;
  • B cell secretes large amounts of antibody;
  • B cell divides to form clone all secreting / producing same antibody.
46
Q

Describe the difference between active and passive immunity.

A
  • Active involves memory cells, passive does not;
  • Active involves production of antibody by plasma cells / memory cells;
  • Passive involves antibody introduced into body from outside / named source;
  • Active long term, because antibody produced in response to antigen;
  • Passive short term, because antibody (given) is broken down;
  • Active (can) take time to develop / work, passive fast acting
47
Q

Some white blood cells are phagocytic. Describe how these phagocytic white blood cells destroy bacteria.

A
  • Phagocyte attracted to bacteria by chemicals / recognise antigens on bacteria as foreign;
  • Engulf / ingest bacteria;
  • Bacteria in vacuole / vesicle;
  • Lysosome fuses with / empties enzymes into vacuole;
  • Bacteria digested / hydrolysed-
48
Q

Describe and explain how the lungs are adapted to allow rapid exchange of oxygen between air in the alveoli and blood in the capillaries around them. (5 marks)

A
  • Many alveoli / alveoli walls folded provide a large surface area;
  • Many capillaries provide a large surface area;
  • (So) fast diffusion;
  • Alveoli or capillary walls / epithelium / lining are thin / short distance between alveoli and - - - Flattened / squamous epithelium;
  • (So) short diffusion distance / pathway;
  • (So) fast diffusion;
  • Ventilation / circulation;
  • Maintains a diffusion / concentration gradient;
  • (So) fast diffusion;
49
Q

Describe and explain how fish maintain a flow of water over their gills (4 marks)

A
  • mouth opens, operculum / opercular valve shuts;
  • floor of mouth lowered;
  • water enters due to decreased pressure / increased volume;
  • mouth closes, operculum / opercular valve opens;
  • floor raised results in increased pressure / decreased volume;
  • high / increased pressure forces / pushes water over gills;
50
Q

Describe and explain how the structure of the mammalian breathing system enables efficient uptake of oxygen into the blood. (6 marks)

A
  • alveoli provide a large surface area;
  • walls of alveoli thin to provide a short diffusion pathway;
  • walls of capillary thin / close to alveoli provides a short diffusion pathway;
  • walls (of capillaries / alveoli) have flattened cells;
  • cell membrane permeable to gases;
  • many blood capillaries provide a large surface area;
  • intercostal / chest muscles / diaphragm muscles / to ventilate lungs /maintain a diffusion / concentration gradient;
  • wide trachea / branching of bronchi / bronchioles for efficient flow of air;
  • cartilage rings keep airways open;
51
Q

Explain how the ventilation mechanism of a fish and the structure of its gills result in the efficient uptake of oxygen from water. (5 marks)

A
  • filaments/ lamellae - large sa
  • gill plates or secondary lamellae
  • large number of capillaries - to remove oxygen to maintain a gradient
  • thin epithelium - short diffusion pathway
  • pressure changes - to bring in more water, to maintain gradient
  • counter current flow - exchange/diffusion along whole length therefore concentration gradient maintained
52
Q

Describe the complete digestion of starch by a mammal (5 marks)

A
  • Hydrolysis;
  • (Of) glycosidic bonds;
  • (Starch) to maltose by amylase;
  • (Maltose) to glucose by disaccharidase/maltase;
  • Membrane-bound (disaccharidase/maltase);
53
Q

Describe the processes involved in the absorption and transport of digested lipid molecules from the ileum into lymph vessels (5 marks)

A
  • Micelles contain bile salts and fatty acids/monoglycerides;
  • Make fatty acids/monoglycerides (more) soluble (in water)/ Maintain high(er) concentration of fatty acids/monoglycerides to cell/lining (of the ileum);
  • Fatty acids/monoglycerides absorbed by diffusion;
  • Triglycerides (re)formed (in cells);
  • Vesicles move to cell membrane;
54
Q

Describe how proteins are digested in the human gut (4 marks)

A
  • Hydrolysis of peptide bonds;
  • Endopeptidases break polypeptides into smaller peptide chains;
  • Exopeptidases remove terminal amino acids;
  • Dipeptidases hydrolyse / break down dipeptides into amino acids.
55
Q

Explain how water from tissue fluid is returned to the circulatory system (4 marks)

A
  • (Plasma) proteins remain;
  • (Creates) water potential gradient
  • Water moves (to blood) by osmosis;
  • Returns (to blood) by lymphatic system;
56
Q

Describe and explain four ways in which the structure of a capillary adapts it for the exchange of substances between blood and surrounding tissue (4 marks)

A
  • permeable capillary wall / membrane;
  • single cell thick / thin walls, reduces diffusion distance;
  • flattened (endothelial) cells, reduces diffusion distance;
  • fenestrations, allows large molecules through;
  • small diameter / narrow, gives a large surface area to volume / short diffusion distance;
  • narrow lumen, reduces flow rate giving more time for diffusion;
  • red blood cells in contact with wall / pass singly, gives short diffusion distance / more time for diffusion;
57
Q

Describe and explain three ways in which the leaves of xerophytic plants may be adapted to reduce water loss (3 marks)

A
  • waxy layer, impermeable
  • sunken stomata, saturated layer of air still outside
  • hairy, saturated layer of air still outside
  • small leaves, reduced surface area for water loss
  • leaves roll up in dry weather, less surface area for water loss/stomata covered/saturated region of still air
  • reduced number of stomata, reduced surface area for water loss
  • CAM, stomata closed in light, only open in dark
58
Q

Use knowledge of the cohesion tension theory to explain why the diameter of the trunk is smallest at midday. (5 marks)

A
  • Diameter of trunk minimal at warmest / brightest time of day / midday = warmest /
    brightest;
  • Stomata open in light → more water loss;
  • Water evaporates more when warm / more heat energy for water evaporation;
  • Hydrogen-bonding between water molecules / cohesion ( / described) between water molecules;
  • Adhesion (described) between water molecules and walls of xylem vessels;
  • (Xylem) pulled inwards by faster flow of water / pulled in by tension;
59
Q

Mutation can result in an increase in genetic variation within a species. Describe and explain the other processes the results in increases in genetic variation within a species. (4 marks)

A
  • Independent segregation of homologous chromosomes
  • Crossing over between homologous pairs
  • Random fertilisation of gametes
  • Therefore new combinations of alleles
60
Q

Penicillin has been the antibiotic of choice for the treatment of bacterial meningitis. Since the year 2000, strains of Neisseria meningitidis that are resistant to penicillin, sulfonamides and rifampin have been discovered in the UK.
Describe how a population of Neisseria meningitidis (Nm) can become resistant to these antibiotics (4 marks)

A
  • mutation
  • results in NM cell with allele for resistance to one antibiotic
  • this cell survives and passes the allele for resistance to offspring
  • process repeated with different genes conferring resistance to each of the other two antibiotics
61
Q

Describe what happens to chromosomes in meiosis. (6 marks)

A
  • chromosomes shorten/thicken/condense
  • chromosomes associate in homologous pairs/formation of bivalents
  • crossing over/ chiasmata form
  • join to spindle fibres
  • at equator/middle of cell
  • join via centromere
  • homologous chromosomes move to opposite poles/ chromosomes separate
  • pairs of chromatids separated in 2nd division
62
Q

In Africa today, most of the human population are resistant to malaria caused by P.vivax. Use your knowledge of natural selection to explain why this resistance is so common in Africa. (4 marks)

A
  • mutations produced allele
  • those will allele/resistance less likely to/do not get malaria
  • so more likely to reproduce and pass on the allele
  • over generations allele frequency increases
63
Q

Define gene mutation and explain how a gene mutation can have
- no effect of an individual
- a positive effect on an individual
(5 marks)

A
  • change in the base/nucleotides sequence of DNA
  • results in the formation of new allele
  • genetic code is degenerate so amino acid sequence may not change
  • does change amino acid sequence but no effect on tertiary structure
  • new allele is recessive so does not influence phenotype
  • results in change in polypeptide that positively changes the properties of the protein
  • may result in increased reproductive success
64
Q

The sundew is a small flowering plant, growing in wet habitats such as bogs and marshes. The soil in bogs and marshes is acidic and has very low concentrations of some nutrients. The sundew can trap and digest insects.
Describe how you could estimate the size of a population of sundews in a small marsh (5 marks)

A
  • use a grid
  • method of obtaining random coordinates e.g. random number calculator
  • count number/frequency in a quadrat
  • large sample and calculate mean/average number
  • valid method of calculating total number of sundews
65
Q

Haemoglobins are chemically similar molecules found in many different species.
Differences in the primary structure of haemoglobin molecules can provide evidence of phylogenetic (evolutionary) relationships between species.
Explain how. (4 marks)

A
  • mutations change base/nucleotide
  • causing change in amino acid sequence
  • mutations build up over time
  • more mutations/more difference in amino acids sequence between distantly related species
  • distantly related species have earlier common ancestor
66
Q

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. (5 marks)

A
  • Use random sample of seeds (from each population);
  • Use (large enough) sample to be representative of whole population;
  • Indication of what size was measured e.g. 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;
67
Q

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.
(5 marks)

A
  • 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; More carbon dioxide absorbed higher up
  • (At ground level) less photosynthesis / less photosynthesising tissue / more respiration / more micro-organisms / micro-organisms produce carbon dioxide.
68
Q

Plants produce ATP in their chloroplasts during photosynthesis. They also produce ATP during respiration. Explain why it is important for plants to produce ATP during respiration in addition to during photosynthesis. (5 marks)

A
  • In the dark no ATP production in photosynthesis;
  • In context of in photosynthetic tissue / leaves
  • Some tissues unable to photosynthesise / produce ATP;
  • ATP cannot be moved from cell to cell / stored;
  • Plant uses more ATP than produced in photosynthesis;
  • ATP for active transport / synthesis (of named substance);
69
Q

During photosynthesis, oil-palm trees convert carbon dioxide into organic substances. Describe how. (6 marks)

A
  • Carbon dioxide combines with ribulose bisphosphate / RuBP;
  • Produces two molecules of glycerate (3-)phosphate / GP;
  • Reduced to triose phosphate / TP;
  • Using reduced NADP;
  • Using energy from ATP;
  • Triose phosphate converted to other organic substances / named organic substances / ribulose bisphosphate;
  • In light independent reaction / Calvin cycle
70
Q

Describe the light-independent reactions of photosynthesis and explain how they allow the continued synthesis of hexose sugars. (6 marks)

A
  • 5C / RuBP combines with CO2;
  • to form 3C compound / TP / GP;
  • using ATP;
  • and reduced NADP / eq;
  • 2 molecules of 3C compound / TP / GP form hexose;
  • all RuBP is regenerated;
  • 10 molecules of 3C / TP / GP form 6 molecules of 5C / RuBP;
71
Q

Describe the role of electron transport chains in the light-dependent reactions of photosynthesis. (6 marks)

A
  • electron transport chain accepts excited electrons;
  • from chlorophyll / photosystem;
  • electrons lose energy along chain;
  • ATP produced;
  • from ADP and Pi;
  • reduced NADP formed;
  • when electrons (from transport chain) and H+ combine with NADP;
  • H+ from photolysis;
72
Q

Explain why the increase in the dry mass of a plant over twelve months is less than the mass of hexose produced over the same period. (3 marks)

A
  • some hexose / biomass / eq. used in respiration;
  • CO2 produced (is lost to air);
  • some parts of the plant are eaten / some parts lost to decomposer
73
Q

Describe how ATP is made in mitochondria. (6 marks)

A
  • Substrate level phosphorylation / ATP produced in Krebs cycle;
  • Krebs cycle / link reaction produces reduced coenzyme / reduced NAD / reduced FAD;
  • Electrons released from reduced / coenzymes / NAD / FAD;
  • (Electrons) pass along carriers / through electron transport chain / through series of redox reactions;
  • Energy released;
  • ADP / ADP + Pi;
  • Protons move into intermembrane space;
  • ATP synthase;
74
Q

After harvesting, the remains of crop plants are often ploughed into the soil.
Explain how microorganisms in the soil produce a source of nitrates from these remains. (5 marks)

A
  • Protein/amino acids/DNA into ammonium compounds / ammonia;
  • By saprobionts;
  • Ammonium/ammonia into nitrite;
  • Nitrite into nitrate;
  • By nitrifying bacteria/microorganisms;
75
Q

Nitrate from fertiliser applied to crops may enter ponds and lakes. Explain how nitrate may cause the death of fish in fresh water. (5 marks)

A
  • Growth of algae / surface plants / algal bloom blocks light;
  • competition for light/ effect of competition
  • Reduced / no photosynthesis so (submerged) plants die;
  • death of algae, increases food supply/microorganisms
  • Saprobiotic (microorganisms / bacteria);
  • Aerobically respire / use oxygen in respiration;
  • Less oxygen for fish to respire / aerobic organisms die;
76
Q

Explain how farming practices increase the productivity of agricultural crops. (5 marks)

A
  • Fertilisers / minerals / named ion (added to soil);
  • Role of named nutrient or element e.g. nitrate / nitrogen for proteins / phosphate / phosphorus for ATP / DNA;
  • Selective breeding / genetic modification (of crops);
  • Ploughing / aeration allows nitrification / decreases denitrification;
  • Benefit of crop rotation in terms of soil nutrients / fertility / pest reduction;
77
Q

Explain how the decay of dead plants results in reduced oxygen concentration and increased nitrate production. (6 marks)

A
  • bacteria / fungi feed on dead matter saprobiotically;
  • respiration uses up oxygen;
  • converts proteins to amino acids;
  • then to ammonium compounds;
  • nitrifying bacteria convert ammonium compounds;
  • via nitrates
78
Q

Describe how the stimulation of a Pacinian corpuscle produces a generator potential. (3 marks)

A
  • There are stretch mediated sodium ion channels
  • Increased pressure deforms the sodium ion channels
  • Sodium ion channels open
  • Sodium ions diffuse in
  • Depolarisation (leading to generator potential)
79
Q

Exercise causes an increase in heart rate.
Describe the role of receptors and of the nervous system in this process. (4 marks)

A
  • Chemoreceptors detect rise in CO2 / H+ / acidity / carbonic acid / fall in pH
  • Send impulses to cardiac centre / medulla;
  • More impulses to SAN;
  • By sympathetic (nervous system for chemoreceptors / CO2)
80
Q

Describe how a heartbeat is initiated and coordinated. (5 marks)

A
  • SAN sends wave of electrical activity / impulses (across atria) causing atrial contraction;
  • Non-conducting tissue prevents immediate contraction of ventricles / prevents impulses reaching the ventricles;
  • AVN delays (impulse) whilst blood leaves atria / ventricles fill;
  • (AVN) sends wave of electrical activity / impulses down Bundle of His/ purkinje fibres
  • Causing ventricles to contract from base up;
81
Q

Explain how a rise in blood pressure results in a decrease in the rate of heartbeat. (6 marks)

A
  • pressure receptors / baroreceptors / stretch receptors in aorta / carotid arteries / carotid sinus; (reject carotid body)
  • send impulses to cardiovascular centre / medulla / cardio-inhibitory centre;
    (reject signals / messages / electronic)
  • impulses via parasympathetic nerves / vagus; (accept inhibitory nerve)
  • to SAN;
  • release of ACh / inhibits SAN / decreases impulses from SAN;
  • decreases impulses to AVN / decreased stimulation of AVN / decreases impulses from AVN;
82
Q

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.

A
  • Geographic(al) isolation/ allopatric speciation
  • Separate gene pools / no interbreeding / gene flow/ reproductive separation
  • 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.
  • Eventually different species cannot interbreed to produce fertile offspring
83
Q

Changes in ecosystems can lead to speciation. A high concentration of copper in soil is toxic to most plants. In some areas where the soil is polluted with copper, populations of grasses are found to be growing. These populations of grass belong to a species also found growing on unpolluted soils.
It has been suggested that a new species of grass may evolve on soil that has been polluted with copper. Explain how this new species might evolve.

A
  • Variation / variety;
  • Mutation;
  • Some plants have allele to survive / grow / live in high concentration of copper / polluted soils;
  • (Differential) reproductive success / adapted organisms reproduce;
  • Increase in frequency of allele;
  • No interbreeding (with other populations) / separate gene pool / gene pool differs
84
Q

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.

A
  • 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.
85
Q

A laboratory has oat plants containing the resistance gene and a supply of plasmids.
Describe how bacteria may be produced which have the resistance gene in their plasmids. (6 marks)

A
  • Cut desired gene from DNA of oat plant
  • Using restriction endonucleases/ restriction enzyme
  • Cut plasmid open
  • With same restriction endonucleases/ restriction enzyme
  • Reference to sticky ends/ unpaired bases attached
  • Use DNA ligase to join
  • Return plasmid to bacterial cells
  • Use of electric shock
86
Q

The polymerase chain reaction (PCR) can be used to produce large quantities of DNA.
Describe how the PCR is carried out. (6 marks)

A
  • DNA heated to90 to 95°C;
  • strands separate;
  • cooled / to temperature below 70°C
  • primers bind;
  • nucleotides attach;
  • by complementary base pairing;
  • temperature 70 - 75°C;
  • DNA polymerase joins nucleotides together;
  • cycle repeated;
87
Q

Plasmids can be modified by genetic engineering and inserted into bacteria. These bacteria can then make useful substances normally made by another organism. Explain how modified plasmids are made by genetic engineering and how the use of markers enable bacteria containing these plasmids to be detected. (6 marks)

A
  • isolate wanted gene / DNA from another organism / mRNA from cell / organism;
  • using restriction endonuclease / restriction enzyme / reverse transcriptase to get DNA and produce sticky ends;
  • use ligase to join wanted gene to plasmid;
  • also include marker gene e.g. antibiotic resistance;
  • add plasmid to bacteria to grow (colonies)then (replica) plate onto medium where the marker gene is expressed;
  • bacteria / colonies not killed have antibiotic resistance gene and (probably) the wanted gene;
88
Q

Describe the sequence of events involved in transmission across a cholinergic synapse.
Do not include details on the breakdown of acetylcholine in your answer.

A
  • Depolarisation of presynaptic membrane;
  • Calcium channels open and calcium ions enter (synaptic knob);
  • (Calcium ions cause) synaptic vesicles move to/fuse with presynaptic membrane and release acetylcholine/neurotransmitter;
  • Acetylcholine/neurotransmitter diffuses across (synaptic cleft);
  • (Acetylcholine attaches) to receptors on the postsynaptic membrane;
  • Sodium ions enter (postsynaptic neurone) leading to depolarisation;
89
Q

Describe the role of calcium ions and ATP in muscle contraction.

A
  • Calcium ions diffuse into myofibrils from (sarcoplasmic) reticulum;
  • (Calcium ions) cause movement of tropomyosin (on actin);
  • (This movement causes) exposure of the binding sites on the actin;
  • Myosin heads attach to binding sites on actin;
  • Hydrolysis of ATP (on myosin heads) causes myosin heads to bend;
  • (Bending) pulling actin molecules;
  • Attachment of a new ATP molecule to each myosin head causes myosin heads to detach (from actin sites).
90
Q

Describe how ultrafiltration produces glomerular filtrate. (5 marks)

A
  • Blood pressure / hydrostatic pressure;
  • Small molecules / named example;
  • Pass through basement membrane / basement membrane acts as filter;
  • Protein too large to go through / large so stays behind;
  • Presence of pores in capillaries / presence of podocytes
91
Q

Some people who have diabetes do not secrete insulin. Explain how a lack of insulin affects reabsorption of glucose in the kidneys of a person who does not secrete insulin. (4 marks)

A
  • High concentration of glucose in blood;
  • High concentration in tubule / in filtrate;
  • Reabsorbed by facilitated diffusion / active transport;
  • Requires proteins / carriers;
  • These are working at maximum rate / are saturated;
  • Not all glucose is reabsorbed / some is lost in urine;
92
Q

Some desert mammals have long loops of Henle and secrete large amounts of antidiuretic hormone (ADH). Explain how these two features are adaptations to living in desert conditions. (6 marks)

A
  • More water (from filtrate) reabsorbed / returned to blood / less lost in urine;
  • By osmosis;
  • From collecting duct / from end of second convoluted tubule;
  • Due to longer loop of Henle;
    For loop of Henle, maximum 2 marks:
  • Sodium / chloride ions absorbed from filtrate in ascending limb;
  • Gradient established in medulla / concentration of ions increases down medulla;
    For ADH, maximum 2 marks:
  • Acts on collecting duct / distal convoluted tubule / second convoluted tubule;
  • Makes cells more permeable / inserts aquaporins in plasma membranes;
93
Q

Humans can produce urine which is more concentrated than their blood plasma.
Explain the role of the loop of Henle in the absorption of water from the filtrate. (6 marks)

A
  • In the ascending limb sodium(ions) actively removed;
  • Ascending limb impermeable to water;
  • In descending limb sodium(ions) diffuse in;
  • Descending limb water moves out / permeable to water;
  • Low water potential / high concentration of ions in the medulla / tissue fluid;
  • The longer the loop / the deeper into medulla, the lower the water potential in medulla / tissue fluid;
  • Water leaves collecting duct / DCT;
  • By osmosis / down water potential gradient;
94
Q

The kidney plays an important part in the regulation of blood water potential. This involves control of the amount of water reabsorbed from the filtrate produced in the kidney tubules. The amount of water reabsorbed affects the volume of urine produced, the rate at which the bladder fills and how often it has to be emptied.
Explain how the loop of Henle maintains the gradient of ions which allows water to be reabsorbed from filtrate in the collecting duct. (5 marks)

A
  • (epithelial cell) of tubule cells carry out active transport;
  • transport chloride / sodium ions out (of filtrate);
  • against concentration gradient;
  • into surrounding tissue / tissue fluid;
  • creates / maintains water potential gradient for water reabsorption;
  • countercurrent multiplier;
95
Q

Explain the role of ADH in the production of concentrated urine. (4 marks)

A
  • if water potential of blood falls, detected by receptors in hypothalamus;
  • leads to ADH released from pituitary gland;
  • ADH makes cells of collecting duct / distal convoluted tubule permeable to water;
  • water leaves filtrate by osmosis;
  • smaller volume of urine produced;

AQA a-level biology 🧬🔬 (32 decks)

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