A Level Biology Year 1 Extended Answers

Describe how the structures of starch and cellulose molecules are related to their functions (5)

Starch
Coiled for compact storage (amylose)
Branched for quick hydrolysis (amylopectin)
Insoluble so has no affect on water potential

Cellulose
Straight unbranched chain of β glucose
Joined by hydrogen bonding
Provides strength and rigidity for structure

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Describe the processes involved in the transport of sugars in plant stems. (5)

At source sucrose is actively transported into the phloem
By companion cells
Lowers water potential in phloem so high hydrostatic pressure
Mass flow towards sink
Sucrose unloaded at sink

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Describe how the structure of glycogen is related to its function. (4)

Coiled so compact for storage
Highly branched for quick hydrolysis (high metabolic rate of animals)
Polymer of glucose so easily hydrolysed
Insoluble so does not affect water potential

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

Less dense as solid than liquid so provides habitats
High specific heat capacity so buffers change in temperature
Cohesion for water columns in plants
Large latent heat of vaporization so provides cooling effect
Solvent for transport of substances

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Describe the biochemical tests you would use to confirm the presence of lipid,
non-reducing sugar and amylase in a sample. (5)

Lipid
Add ethanol then water and mix
Emulsion test turns white
Non-Reducing sugar
Negative result after benedicts (blue)
Boil in acid then neutralise with alkali
Heat with benedict's to produce brick red colour
Add biuret reagent to produce purple colour

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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. (5)

Monomer to polymer = condensation
Removal of water to form glycosidic bond
Polymer to monomer = hydrolysis
Addition of water causes glycosidic bond to break
E.g. Alpha glucose to glycogen for storage and glycogen hydrolysed to alpha glucose for use as res

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Contrast how an optical microscope and a transmission electron microscope work and
contrast the limitations of their use when studying cells. (6)

Optical uses visible light TEM uses electrons
TEM: higher resolution images
TEM: dead subjects, Optical: live
TEM: B&W Optical: Colour

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In an investigation into the relationship between pollution and seed size, describe how data should be collected and processed from seeds to investigate whether
there is a difference in seed size between populations of trees in the city and countryside.
(

Random sampling to remove bias
Large samples to represent whole population
Indicate what was measured (mass, height)
Calculate mean and standard deviation
T-test
Analyse if significant difference (no overlap)

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Describe the structure of DNA. (5)

Polymer of nucleotides
Nucleotides formed of deoxyribose sugar, phosphate and nitrogenous base
Joined by phosphodiester bond between deoxyribose of 1 and phosphate of other
Double helix structure
Complementary bases hydrogen bonded
A and T
G and C

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Name and describe five ways substances can move across the cell-surface membrane
into a cell. (5)

Simple Diffusion - Small non-polar mol. down conc. gradient
Facilitated Diffusion - Down conc. gradient through protein channels
Osmosis - Water down water pot. gradient
Active transport - Against conc. gradient using ATP
Cotransport - 2 substances using

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

P: Circular E: Linear
P: No associated proteins E: Associated with proteins
P: No introns E: Introns
Nucleotides identical
Phosphodiester bonds join nucleotides

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Differences in the primary structure of haemoglobin molecules can provide evidence of
phylogenetic (evolutionary) relationships between species.
Explain how. (5)

Mutations change nucleotide sequence
Changing amino acid sequence
Build up over time
More mutations between distantly related species
Presence of common ancestor

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

Cold - Slow enzyme activity
Buffered - Prevent pH changes causing damage
Isotonic - Prevent change in water pot. causing lysis
Homogeniser used to break open cells
Filter to remove debris
Spin filtrate on low speed to form pellet of large organelle
Filter

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Describe the principles and the limitations of using a transmission electron microscope to
investigate cell structure. (5)

Electrons pass through sample
Desner parts absorb more electrons
So appear darker on image

Can only examine dead samples
Specimen must be thin

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Describe and explain how the structure of DNA results in accurate replication. (4)

Two strands allows for semi-conservative replication
Hydrogen bonds hold base parings together
H bonds easily broken to separate strands
2 strands formed with one parent and one new strand each

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Describe the behaviour of chromosomes during mitosis and explain how this results in the
production of two genetically identical cells. (7)

DNA replication leads to homologous chromosomes containing 2 copies of the chromatids
Chromatids move to equator
Attach to spindle fibres
Spindle fibres contract
Chromosomes separate
Each pole receives all genetic information
Nuclear envelope forms around

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Some substances can cross the cell-surface membrane of a cell by simple diffusion through the
phospholipid bilayer. Describe other ways by which substances cross this membrane. (5)

Osmosis from high to low water pot. - through aquaporins
Fac. diffusion, down conc. gradient - through channel protein
Active trans. against conc. - proton pumps

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Glucose is absorbed from the lumen of the small intestine into epithelial cells.
Explain how the transport of sodium ions is involved in the absorption of glucose by
epithelial cells. (5)

Na+ ions leave epithelial cells and enter blood
By active transport
Na+ conc. of cell is lower than in lumen
Na+ enter by facilitated diffusion
Glucose absorbed with Na+ against glucose conc. gradient

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Oxygen and chloride ions can diffuse across cell-surface membranes. The diffusion of
chloride ions involves a membrane protein. The diffusion of oxygen does not involve . Explain why. (5)

Cl- polar
Cannot cross lipid bilayer
Cl- transported by fac. diffusion
O2 non-polar
Diffuses across bilayer

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Describe the roles of iron ions, sodium ions, and phosphate ions in cells. (5)

Fe - Haemoglobin binding with O
Na - Co-transport of glucose
- Creates Na conc. gradient
PO4 - Used to produce ATP
- Hydrophilic heads of
phospholipid bilayer

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The movement of substances across cell membranes is affected by membrane structure.
Describe how. (5)

Phospholipid bilayer allows diffusion of non-polar substances
Prevents diffusion of polar substances
Carrier proteins allow active transport
Channel proteins allow fac. dif.
Cholesterol affects fluidity

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Vaccines protect people against disease. Explain how. (5)

Vaccine = dead/inactive pathogen
Memory cells form
On second exposure memory cells recognise pathogen
Rapid production of antibodies
Antibodies destroy pathogen

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

Mouth opens, operculum shuts
Water enters due to decreased pressure
Mouth shuts, operculum opens
Increased pressure forces water over gills

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

Alveoli - high SA
Thin alveoli walls for short diffusion pathway
Thin capillary walls to be close to alveoli for short diffusion path
Cell membrane permeable to gas
Intercostal muscles maintain conc. gradient
Cartilage rings keep airways open

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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. (5)

Tertiary structure
Active site complementary
Induced fit
Enzyme catalyses reaction
By forming enzyme substrate complex

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Scientists have investigated the effects of competitive and non-competitive inhibitors of the
enzyme maltase.
Describe competitive and non-competitive inhibition of an enzyme. (5)

Inhibitors prevent formation of ES complex
Competitive similar to shape of S
Binds to active site of enzyme
Non-competitive binds to allosteric site
Changes shape of active site

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Describe the role of the enzymes of the digestive system in the complete breakdown of
starch. (5)

Amylase
Starch to maltose
Maltase
Maltose to glucose
Hydrolysis
Of glycosidic bond

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Describe the processes involved in the absorption of the products of starch digestion. (5)

Glucose moves in with sodium
Via carrier protein
Sodium removed by active transport
Into blood
Glycose moves into blood
By facilitated diffusion

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Messenger RNA (mRNA) is used during translation to form polypeptides.
Describe how mRNA is produced in the nucleus of a cell. (6)

Helicase
Breaks H bonds
One strand acts as template
RNA nucleotides attach to exposed bases
According to base pair rule
RNA polymerase joins RNA nucleotides

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Describe the structure of proteins. (5)

Polymer of amino acids
1y- amino acid sequence
2y - Folding from H bonds
3y - 3-D folding due to further H bonds and ionic/disulfide bonds
4y - Multiple polypeptide chains

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

Hydrolysis of peptide bonds
Endopeptidases break polypeptides into smaller chains
Exopeptidases remove terminal amino acids
Dipeptidases hydrolyse dipeptides to amino acids

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

Micelles contain bile salts and fatty acids
Make fatty acids more soluble
Fatty acids absorbed by diffusion
Triglycerides reformed in cells
Vesicles move to cell membrane

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Describe how the structure of a protein depends on the amino acids it contains. (5)

Structure determined by position of amino acids
1y is sequence of amino acids
2y structure formed by H bonds
3y structure formed by interactions between R groups
Creates shape of active site

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In the UK, children are vaccinated against this disease. Describe how vaccination can lead to protection against bacterial meningitis. (6)

Antigen binds to surface receptor on specific B cell
B cell divides by mitosis
Stimulated by t cells
B cells release antibodies
Some B cells become memory
Memory cells produce plasma faster

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Describe how a population of bacteria may become resistant to antibiotics (4)

Mutation
Results in bacteria with allele for resistance to one antibiotic
Bacteria with allele survives and passes allele to offspring
Repetition causes allele to be passed across whole population

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Contrast the structure of a bacterial cell and the structure of a human cell. (5)

Bacteria much smaller
Bacterial DNA circular, human DNA linear
Bacteria lacks membrane bound organelle
Human DNA associated with histones, bacterial not
Human cell contains nucleus, bacterial cell not

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

Vaccine contains antigen from pathogen
Macrophage presents antigen on surface
Complimentary T cell binds
Stimulates B cell
B cell secretes antibodies

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Describe the difference between active and passive immunity. (5)

Active involves memory cells passive does not
Active can take time to develop, passive is much faster
Active long term, passive short
Active involves production of antibodies
Passive involves introduction of antibody to body

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A woman takes moderate exercise. Explain what causes her heart rate to increase while
she exercises. (6)

Rate of respiration increases
CO2 conc. increases
Chemoreceptors in aortic body
Sends impulses to medulla
Increased frequency of impulse
Along sympathetic nervous system to SA node

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Describe the gross structure of the human gas exchange system and how we breathe in
and out. (6)

Trachea, bonchi, bronchioles, alveoli
Inspiration diaphragm contracts, E-intercostal muscles contract
Increase in vol. decrease in pressure air rushes into thoracic cavity
Expiration diaphragm relaxes I-intercostal muscles contract
Decrease in vol. incre

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Compare and contrast the structure and properties of triglycerides and phospholipids. (5)

Both contain glycerol
Both contain C,H,O but phospholipids contain P
Both insoluble in water
Triglyceride 3 fatty acids phospholipids 2 plus PO4 ion
Phospholipids form bilayer triglycerides don't

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Describe how lactose is formed and where in the cell it would be attached to a polypeptide
to form a glycoprotein. (4)

Glucose + galactose
Condensation reaction
Glycosidic bond
Added to polypeptide in golgi

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

Baroreceptors
send impulses to cardiovascular centre
impulses via parasympathetic nervous system
to SA node
release of ACh inhibits SA node
Decreases impulses to AV node

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The heart controls and coordinates the regular contraction of the atria and ventricles. Describe how. (5)

SA, AV, bundle of His, Purkinje
Impulse over atria
Atria contract
Delay at AV to ensure atria empty
Ventricles contract from apex up

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

SA sends impulse across atria causing atrial contraction
Non-conducting tissue prevents immediate contraction of ventricles
AV delays impulse while atria emptied
AV sends wake of electrical activity down bundle of His
Ventricles contract from apex up

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

When atria full atria p>ventricle p
so atrioventricular valves open
When ventricles full opposite true so valves close
When ventricles full pressure>aorta so semilunar valves open
In diastole aorta pressure greater so semilunar valves close
Contraction ca

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Tissue fluid is formed from blood at the arteriole end of a capillary bed. Explain how water from tissue fluid is returned to the circulatory system. (4)

Plasma proteins remain
Creates water pot gradient
Water moves to blood by osmosis
Returns to blood by lymphatic system

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Describe the cohesion-tension theory of water transport in the xylem. (5)

Water lost through transpiration
Lower water potential in leaves
Water pulled up xylem
Cohesion between water molecules
Forming water column

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Describe how mRNA is produced in a plant cell. (5)

DNA strands separate by breaking H bonds
One strand used as template
With complimentary base rule
RNA nucleotides join by RNA polymerase
pre-mRNA formed
Splicing forms mRNA

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

Single cell thick for short diffusion distance
Narrow lumen reduces flow rate to give more time for diffusion
Permeable capillary membrane
flattened cells give short distance

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Explain how tissue fluid is formed and how it may be returned to the circulatory system. (6)

Hydrostatic pressure of blood high at arterial end
Fluid pass out
Proteins remain
Lowers water pot
Water moves back into venous end
Lymphatic system collects excess tissue fluid returns it to blood

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Describe what happens to chromosomes in meiosis. (6)

Chromosomes condense
Associate in homologous pairs
Crossing over
Join to spindle fibres
At centromere
Homologous chromosomes move to opposite poles
Pairs of chromatids separated in 2nd division

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Describe how meiosis causes variation and explain the
advantage of variation to the species. (5)

Crossing over
Independent segregation of chromosomes in meiosis 1
Independent segregation of chromatids in meiosis 2
Different adaptations
Some survive
Reproduce
Pass on gene/allele

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

H bonds break
One strand acts as template
RNA nucleotides align by base pair rule
T --> U
RNA polymerase joins nucleotides

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Describe how a polypeptide is formed by translation of mRNA. (6)

mRNA attaches to ribosomes
tRNA anticodons bind to complementary mRNA codons
tRNA brings specific amino acids
Amino acids join by peptide bonds
Amino acids join together with use of ATP
tRNA released

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Define ‘gene mutation’ and explain how a gene mutation can have:
• no effect on an individual
• a positive effect on an individual. (4)

Gene mutation - Change in base sequence resulting in form of new allele
No effect - degenerate code
Positive effect - increased survival chance

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

Chromosomes condense
Appear as sister chromatids joined at centromere
Chromosomes line up on equator
Attach to spindle fibres by centromere
Centromere splits
Chromosomes uncoil

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Describe and explain the processes that occur during meiosis that increase genetic
variation. (5)

Homologous chromosomes pair up
Independent segregation
Maternal and paternal chromosomes re-shuffles in any combination
Crossing over leads to exchange of sections of chromatids
Both create new combinations of alleles

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Describe the process of sympatric speciation (6)

Occurs in same habitat
Mutations cause changes in courtship behaviour/mating
Reproductive separation
Mutations cause alleles to be passed to offspring in separate groups
Natural selection
Different species cannot interbreed to produce fertile offspring

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A Level Biology Year 1 Extended Answers

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