Year 13 AQA BIOLOGY 2022

Question 1

Describe the light dependent reaction

Answer 1

• Chlorophyll absorbs light energy & Excites electrons
• Chlorophyll loses electrons (Oxidation of chlorophyll) via photoionisation;
• Electrons move along carriers/electron transport chain releasing energy (Series of REDOX reactions)
• Energy released (by electrons) used to form proton gradient;
• H+ ions move through ATP synthase;
• providing energy to join ADP and Pi to form ATP;
• Photolysis of water produces 2 protons, 2 electrons and ½ oxygen;
• NADP reduced by electrons / electrons and protons / hydrogen;

Question 2

Describe the light independent reaction (Calvin Cycle)

Answer 2

• Carbon dioxide combines with ribulose bisphosphate/RuBP;
• Produces two glycerate (3-)phosphate/GP;
• GP reduced to triose phosphate;
• Using reduced NADP;
• Using energy from ATP;
• Triose phosphate converted to glucose/RuBP/ other named organic substance;

Question 3

ATP is produced in the light dependant reaction, suggest why this is not their (plants) only source of ATP.

Answer 3

• Plants don’t photosynthesis in the dark;
• Not all the parts (e.g. roots) of the plants photosynthesise;
• Plants require more ATP than is produced in the light dependant reaction;
• ATP used in Active Transport (accept other named processes)

Question 4

Describe the effect of introducing a herbicide/inhibitor on the electron transport chain

Answer 4

• Reduced transfer of protons across thylakoid membrane
OR
• Reduced chemiosmotic gradient/proton gradient across thylakoid membrane;
• (So) less ATP produced;
• (So) less reduced NADP produced;
• (So) light-independent reaction rate decreases / slows / stops
OR
• Less reduction of GP to triose phosphate;

Question 5

Describe what happens during PHOTOIONISATION in the light dependent reaction.

Answer 5

• Chlorophyll absorbs light OR Light excites/moves electrons in chlorophyll;
• Electron/s are lost OR (Chlorophyll) becomes positively charged OR Chlorophyll is OXIDISED

Accept electrons go to electron transport/carrier chain for ‘electrons lost’.

Question 6

When producing a chromatogram explain why the origin is marked using a pencil rather than ink.

Answer 6

• Ink and (leaf) pigments would mix OR (With ink) origin/line in different position
OR
• (With pencil) origin/line in same position OR (With pencil) origin/line still visible;

Question 7

While making a chromatogram, describe the method used to separate the pigments after the solution of pigment had been applied to the origin.

Answer 7

• Level of solvent below origin/line;
• Remove/stop before (solvent) reaches top/end;

Question 8

Suggest and explain the advantage to plants of having different colour pigments in leaves.

Answer 8

• Absorb different/more wavelengths of light for faster rates of photosynthesis;

Question 9

Describe the need for plants to both photosynthesise AND respire

Answer 9

• In the dark no ATP production in photosynthesis;
• Some tissues (e.g. roots) 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;
• ATP for synthesis (of named substance);

Question 10

Describe the process of succession

Answer 10

• (Colonisation by) pioneer species;
• Pioneers cause change in environmental abiotic / biotic factors(give an example);
• Pioneers make the environment less hostile for new species;
• New species change/make conditions less suitable for previous species;
• Change/increase in diversity/biodiversity;
• Stability increases [population/richness/abiotic factors];
• Climax community;

Question 11

Explain how succession results in a wide variety of fish living on coral reefs.

Answer 11

• Increase in variety/diversity of species/plants/animals; OR Increase in number of species/populations; OR Increase in species richness / biodiversity
• Provides more/different habitats/niches OR Provides greater variety/types of food OR becomes less hostile;

Question 12

Describe random sampling
[estimation of population density]

Answer 12

• Use a grid / split area into squares/sections;
• Method of obtaining random coordinates / numbers, e.g. calculator/computer/random numbers table/random number generator;
• Count number/frequency of plants in a quadrat;
• Large sample (20+ quadrats) AND Calculate mean/average number (per quadrat/section);
• Valid method of calculating total number of ……… e.g. mean number of plants per quadrat/section/m2 multiplied by number of quadrats/sections/m2 in wood;

Question 13

Describe systematic sampling

Answer 13

• Transect/lay line/tape measure (from one side of the dune to the other);
• Place quadrats at regular intervals along the line;
• Count plants/percentage cover/abundance scale (in quadrats) OR Count plants and record where they touch line/transect;

Question 14

Describe how you would determine the mean percentage cover for beach grass on a sand dune.

Answer 14

• Method of randomly determining position (of quadrats) e.g. random numbers table/generator;
• Large number/sample of quadrats; (min 20)
• Divide total percentage by number of quadrats/samples/readings;

Question 15

Describe a method that could be used to determine the mean percentage cover of algae on a coral reef.

Answer 15

• Method of randomly determining position (of quadrats) e.g. random numbers table/generator;
• Large number/sample of quadrats; (>20)
• Divide total percentage by number of quadrats/samples/readings;

Question 16

Describe the mark, release, recapture technique

Answer 16

• Capture sample, mark and release;
• Appropriate method of marking suggested / method of marking does not harm fish;
• Take second sample and count marked organisms;
• No in No in Population = [No in sample1 × No in sample2] / Number marked in sample2;

Question 17

The mark-release-recapture method can be used to estimate the size of a fish population.

Explain how.

Answer 17

• Capture/collect sample, mark and release;
• Ensure marking is not harmful (to fish) OR Ensure marking does not affect survival (of fish);
• Allow (time for) fish to (randomly) distribute before collecting a second sample;
• (Population =) number in first sample × number in second sample divided by number of marked fish in second sample/number recaptured;

Question 18

Suggest why the mark-release-recapture method can produce unreliable results in very large lakes

Answer 18

• Less chance of recapturing fish OR Unlikely fish distribute randomly/evenly;
• Fish may remain in one area OR fish may congregate

Question 19

Describe how you would determine how many quadrats to use when investigating a habitat.

Answer 19

• Calculate running mean/description of running mean;
• When enough quadrats, this shows little change/levels out (if plotted as a graph);
• Enough to carry out a statistical test;
• A large number to make sure results are reliable;
• Need to make sure work can be carried out in the time available;

Question 20

HSW:

Valid conclusions

Answer 20

• Large sample size SO representative
• Long study SO can allow…. / see effect of X
• Control used SO comparison can be made.
• Mean & SD SO significant differences can be determined.

Question 21

Limitations:

You can only use the information provided.

Answer 21

• Small sample size, not representative;
• Only sampled males, females may respond differently to treatment;
• Only tested on …….. in a lab, in the wild, may obtain different pattern in data;
• Only tested on …….. species, might not be true for all species;
• No STATS test so differences could be due to chance;
• Data is SUBJECTIVE, some people may have lied.

Question 22

HSW:

Null hypothesis

Answer 22

• [x] will have no effect on [y]
• E.g. Temperature will have no effect on rate of reaction
• There will be no correlation between age and weight
• There will be no difference between the observed and expected ratio of 3:1 brown eyes to blue eyes

Question 23

Name the two products of the light-dependent reaction that are required for the light-independent reaction.

Answer 23

1. ATP;
2. Reduced NADP;

Question 24

Where precisely is rubisco found in a cell?

Answer 24

Stroma

Question 25

Explain why scientists measure the rate of production of oxygen in this investigation. (Rate of photosynthesis)

Answer 25

1. Oxygen produced in light-dependent reaction; 2. The faster (oxygen) is produced, the faster the light-dependent reaction.

Question 26

Explain why plants that have more chlorophyll will grow faster than plants with less chlorophyll.

Answer 26

1. Have faster production of ATP and reduced NADP; 2. (So) have faster / more light-independent reaction; 3. (So) produce more sugars that can be used in respiration; 4. (So) have more energy for growth; 5. Have faster / more synthesis of new organic materials.

Question 27

Explain the relationship between stomatal opening and photosynthesis.

Answer 27

1. Stomata allow uptake of carbon dioxide; 2. Carbon dioxide used in / required for photosynthesis;

Question 28

Describe what happens during **photoionisation** in the light dependent reaction.

Answer 28

* Chlorophyll absorbs light **OR** Light excites/moves electrons in chlorophyll; * Electron/s are lost **OR** (Chlorophyll) becomes positively charged; OR Chlorophyll is oxidised; *Accept* electrons go to electron transport/carrier chain for ‘electrons lost’.

Question 29

**Describe the process of glycolysis.**

Answer 29

* **Phosphorylation of glucose using (2) ATP;** * **Oxidation of Triose phosphate to Pyruvate;** * **Net gain of ATP;** * **NAD is reduced;**

Question 30

Name two uses of ATP in a cell

Answer 30

* Phosphorylation of (named substance) to make more reactive / Lowers activation energy; * **_Releases_** small manageable amounts of energy for (named process);

Question 31

Describe how oxidation takes place in glycolysis and in the Krebs cycle.

Answer 31

* Removal of hydrogen/dehydrogenation; * by enzymes/dehydrogenases; * H accepted by NAD/reduced NAD formed; * In Krebs cycle, FAD (used as well);

Question 32

Water is a waste product of aerobic respiration. Describe how water is formed at the end of aerobic respiration.

Answer 32

* Oxygen is terminal/final electron acceptor; * Combines with _electrons and protons_ (to form water);

Question 33

Explain how the amount of ATP is increased by reactions occurring inside a mitochondrion.

Answer 33

This is **because**: * oxidation of/removal of electrons and H+ * from pyruvate * acetyl CoA / 6 carbon compound; (credit oxidative decarboxylation) * substrate level production of ATP / ATP produced in Krebs cycle; * production of reduced NAD / FAD (allow they take up hydrogen); * in matrix of mitochondria; * electrons fed into electron transport chain / used in oxidative phosphorlation * (Electrons) pass along carriers/through electron transport chain/through series of redox reactions; * Energy is released; * Protons move into intermembrane space; * ADP/ADP + Pi; * ATP synthase;

Question 34

Describe the events of oxidative phosphorylation.

Answer 34

* NAD/FAD reduced / hydrogen attached to NAD/FAD; * H+ ions/electrons transferred from coenzyme to coenzyme/carrier to carrier ***(ETC on cristae of inner membrane)*** * Energy released (from electrons) through series of redox reactions; * Energy released used to pump H+/ protons into intermembrane space forming an electro-chemical gradient (of protons); * H+/ protons flow back through ATP synthase to produce ATP from ADP and phosphate.

Question 35

Describe the roles of the coenzymes and carrier proteins in the synthesis of ATP.

Answer 35

* NAD/FAD reduced / hydrogen attached to NAD/FAD; * H+ ions/electrons transferred from coenzyme to coenzyme/carrier to carrier ***(ETC on cristae of inner membrane)*** * Energy released (from electrons) through series of redox reactions; * Energy released used to pump H+/ protons into intermembrane space forming an electro-chemical gradient (of protons); * H+/ protons flow back through ATP synthase to produce ATP from ADP and phosphate

Question 36

In many mammals, ‘uncoupling proteins’ help to maintain a constant body temperature during hibernation. Suggest and explain how.

Answer 36

1. Allow passage of protons/H+; 2. (Energy) **released** as heat;

Question 37

The mitochondria in muscles contain many cristae. Explain the advantage of this.

Answer 37

* larger surface area for electron carrier system / MORE oxidative phosphorylation; * provide **_MORE**_ ATP / energy _**for contraction_**;

Question 38

Give two reasons why the respirometer was left for 10 minutes when it was first placed in the water bath**.**

Answer 38

* Equilibrium reached; * Allow for expansion (gases/liquids) /pressure change in apparatus; * Allow respiration **_rate_** of seeds to stabilise;

Question 39

Explain why a log scale is used to record the number of cells/bacteria.

Answer 39

* Large **_range**_/difference/_**increase_** in numbers (of cells/bacteria); * Accept reference to exponential (increase)

Question 40

Explain why converting pyruvate to lactate allows the continued production of ATP by anaerobic respiration.

Answer 40

* **_Regenerates NAD_** / Oxidises reduced NAD; * (So) glycolysis **_continues_**;

Question 41

Malonate inhibits a reaction in the Krebs cycle. Explain why malonate would decrease the uptake of oxygen in a respiring cell.

Answer 41

* Less/no reduced NAD/coenzymes OR Fewer/no hydrogens/electrons removed (and passed to electron transfer chain); * Oxygen is the final/terminal (electron) acceptor;

Question 42

# Respirometer Question The student found that the coloured liquid moved 1.5 cm in 24 hours. The diameter of the lumen (hole) of the capillary tubing was 1 mm. The volume of a capillary tubing is given by *π*r²*l*, where *π* is 3.14 and *l* = length. Calculate the volume of gas produced in cm³ hour^–1. Show your working. Answer = ____________________ cm³ hour^–1

Answer 42

1. Correct answer in range of 4. 9 × 10^–4 to 4.91 × 10^–4 = **2 marks**;; * Accept any equivalent mathematical representation of this answer* 2. Incorrect but shows division by 24 = **1 mark** **OR** 1175 to 1178 = **1 mark**; **OR** 49 = **1 mark**;

Question 43

Define Biomass

Answer 43

* Mass of carbon (organic compounds) * Dry mass of tissue per given area

Question 44

Suggest what you should do to ensure all water is removed from a tissue / sample.

Answer 44

* Regularly weigh **and** _Heat_ (less than 100 °C) * Until mass is _constant_

Question 45

Define Gross Primary Productivity

Answer 45

* Chemical energy store in plant biomass, in a given area or volume. (Rate of photosynthesis)

Question 46

Define Net Primary Production

Answer 46

* Chemical energy store in plant biomass after respiratory losses to the environment have been taken into account. NPP= GPP -R * NPP is available for new plant growth and reproduction OR available for other trophic levels in the ecosystem, such as herbivores and saprobionts. * **kJ ha^–1 year^–1** OR **kJ km^–2 year^–1** OR **kJ** **km^–3 year^–1**

Question 47

The percentage of the light energy trapped by the producers is very low. Give two reasons why.

Answer 47

* Reflected / absorbed by water vapour; * Reflected from producers / wrong wavelength; * Transmitted / passes between chloroplasts/ between plants / too few chloroplasts;

Question 48

In natural ecosystems, most of the light falling on producers is not used in photosynthesis. Suggest two reasons why.

Answer 48

1. (Light is) reflected; Light is not absorbed on its own is not enough. 2. (Light is) wrong wavelength; Accept frequency for wavelength. Accept reference to absorbing specified wavelengths/frequencies. 3. (Light) misses chlorophyll/ chloroplasts/photosynthetic tissue; 4. CO2 concentration or temperature is a limiting factor.

Question 49

The biomass of primary consumers is less than the biomass of producers. Explain why.

Answer 49

* Loss of energy/heat / use of energy / less energy to be passed on; * In respiration; * In **excreta** / excretion / urine / carbon dioxide; * Inedible parts / indigestible parts / **egesta** / egestion / to decomposers;

Question 50

Describe how and explain why the efficiency of energy transfer is different at different stages in the transfer.

Answer 50

* Some light energy fails to strike/is reflected/not of appropriate wavelength; * Efficiency of photosynthesis in plants is low/approximately * 2% efficient; * Respiratory loss / excretion / faeces / not eaten; * Loss as heat; * Efficiency of transfer to consumers greater than transfer to producers/approximately 10%; * Efficiency lower in older animals/herbivores/ primary consumers/warm blooded animals; * Carnivores use more of their food than herbivores;

Question 51

Explain how the intensive rearing of domestic livestock increases net productivity

Answer 51

* Slaughtered when still growing/before maturity/while young so more energy transferred to biomass/tissue/production; * Fed on concentrate /controlled diet /controlled conditions/so higher proportion of (digested) food absorbed/lower proportion lost in faeces / valid reason for addition; * Movement restricted so less respiratory loss / less energy used; * Kept inside/heating/shelter / confined so less heat loss / no predators; * Genetically selected for high productivity;

Question 52

Describe the need for plants to both photosynthesise AND respire

Answer 52

* In the dark no ATP production in photosynthesis; * 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; * ATP for synthesis (of named substance);

Question 53

Describe the **role** of saprobionts in the nitrogen cycle

Answer 53

* (use enzymes to) decompose proteins / DNA / RNA / urea; * Producing / releasing ammonia / ammonium ions;

Question 54

Explain how carbon-containing compounds present in the pine leaves that fall from the trees are absorbed and used for growth by saprobionts/fungi that live in the soil.

Answer 54

* extracellular digestion; * by secretion of enzymes; * absorption of digested/soluble products; * synthesis of structural compounds/named compound;. * respiration provide energy for growth

Question 55

Nitrogen compounds in the plants are made available for the main crop after ploughing in spring. Describe the role of microorganisms in this process.

Answer 55

* proteins/amino acids broken down; * deamination/ammonification/ release of ammonium compounds; * By saprophytes/saprobionts/decomposers; * conversion to nitrates via nitrites; * by nitrifying bacteria/named bacterium; * nitrates absorbed into roots via active transport

Question 56

Give **two** examples of biological molecules containing nitrogen that would be removed when a crop is harvested.

Answer 56

1. Amino acid/protein/ polypeptide/peptide; 2. nucleic acid/nucleotide/base; 3. DNA; 4. RNA / pre-mRNA / mRNA / rRNA / tRNA 5. ATP/ADP; 6. NAD/NADP (reduced or not); 7. Cyclic AMP/cAMP; 8. Chlorophyll;

Question 57

Plants absorb a number of other nutrients from the soil including phosphates. Describe why phosphates are needed by a growing plant.

Answer 57

* production of phospholipids; * in cell membranes; * synthesis of ATP; * production of DNA; * production of RNA; * production of NADP

Question 58

Describe how a lack of phosphates in the soil surrounding a plant can affect its growth

Answer 58

* (Required to) make ATP/glucose phosphate, **soless** respiration/less energy for growth; * (Required to) make nucleotides, **so less** DNA/mRNA/tRNA for cell division/production of protein (for growth); * (Required to) make RuBP/NADP, **so less** CO2 fixed/reduced into sugar; * (Required to) make phospholipids **for** membranes;

Question 59

Outline the advantages of having Mycorrhizae growth near plants

Answer 59

* Mycorrhizae help plants to defend themselves (causing an increase in crop yield); * Mycorrhizae help plants to take up nitrates/phosphates (causing an increase in crop yield);

Question 60

You are given samples of water from three different rivers. Describe how you would obtain a quantitative measurement of their cloudiness.

Answer 60

* Use of colorimeter; * Measure the absorbance/transmission (of light); * Example of how method can be standardised eg same volume of water, zeroing colorimeter, same wavelength of light, shaking the sample;

Question 61

Describe the process of eutrophication.

Answer 61

* Nitrates / Phosphates / Ammonium ions flushed/leached into waterway * Increased algal bloom * Light blocked out * Submerged aquatic plant unable to photosynthesise and die * Increase in saprobionts so increases rate of **aerobic** respiration * Saprobionts / aero.resp organismsn die as lack of oxygen (anoxic) * Increase in anaerobic microorganisms * Production of toxins

Question 62

State **three** causes of genetic variation

Answer 62

* Mutation * Crossing over * Independent segregation / assortment (of homologous chromosomes) * Random _fusion of gametes_ / fertilisation / mating

Question 63

What is meant by a genome?

Answer 63

* ‘(all) the ‘genes’/alleles’ ‘genetic material/code’ in a cell/organism/ person’ * ‘the total number of DNA bases in a cell/organism’ * the full base sequence of all the genetic material of an organism

Question 64

What is a ‘gene pool’?

Answer 64

All the alleles in a population. ***Reject: Community;***

Question 65

How do multiple alleles of a gene arise?

Answer 65

* mutations; * which are different / at different positions within the same gene;

Question 66

In genetic crosses, the observed phenotypic ratios obtained in the offspring are often **not** the same as the expected ratios. Suggest **two** reasons why.

Answer 66

* Small sample size; * Fusion/fertilisation of gametes is random; * Linked Genes; Sex-linkage / crossing over; * Epistasis; * Lethal genotypes;

Question 67

What is meant by a recessive allele?

Answer 67

* **Only expressed (in the phenotype) when homozygous** * Not expressed in the heterozygote * Not expressed if dominant present

Question 68

Define dominant allele

Answer 68

Always expressed within the phenotype

Question 69

What does Hardy Weinberg’s equation predict? [3]

Answer 69

* The **frequency** of alleles (of a particular gene); * Will **stay constant** from one generation to the next/over generations / no genetic change over time; * (3 of the following) Providing no mutation/no selection / population large / population genetically isolated / mating at random / no migration;

Question 70

Define gene linkage

Answer 70

(Genes/loci) on same chromosome

Question 71

Define epistasis

Answer 71

* The allele of one gene affects the expression of another in the phenotype;

Question 72

Define codominance

Answer 72

Both alleles (equally) expressed in the phenotype;

Question 73

Rules for Dominant alleles

Answer 73

* Affected offspring **_MUST_** have at least one affected parent. * Unaffected parents **_ONLY_** have unaffected offspring. * **If both** parents are **affected** and have an **unaffected** offspring, both parents must be **Heterozygous**

Question 74

Rules for recessive alleles

Answer 74

* **Unaffected parents** can have an **affected** offspring (if they are **Heterozygous**)

Question 75

Male offspring (XY) are more likely than females (XX) to show recessive sex-linked characteristics. Explain why.

Answer 75

* (Recessive) allele is always expressed in males / males have one (recessive) allele; * Females need two recessive alleles / females need to be homozygous recessive / females could have dominant and recessive alleles / be heterozygous;

Question 76

Expected offspring phenotype ratio from heterozygous parents: Monohybrid

Answer 76

3 : 1

Question 77

Expected offspring phenotype ratio from heterozygous parents: Dihybrid

Answer 77

9 : 3 : 3 : 1

Question 78

Expected offspring phenotype ratio from heterozygous parents: Epistasis

Answer 78

9 : 4 : 3 **or** 15 : 1 (Ratio adds to 16 but is not 9 : 3 : 3 : 1) **or** 9 : 7

Question 79

Expected offspring phenotype ratio from heterozygous parents: Autosomal linkage

Answer 79

3:1 (if no crossing over) (no other pattern other than 4 phenotypes with recombination of alleles)

Question 80

What is meant by the term phenotype

Answer 80

* (**Expression** / appearance / characteristic due to) genetic constitution / **genotype** / allele(s); * (Expression / appearance / characteristic due to) **interaction with environment;**

Question 81

Explain how a single base substitution causes a change in the structure of a polypeptide.

Answer 81

* Change in **sequence of amino acid(s)** / primary structure (of polypeptide); * Change in (position of) **hydrogen, ionic, disulfide** bonds; * Alters **tertiary** structure;

Question 82

Describe the process of crossing over and explain how it increases genetic diversity.

Answer 82

1. **Homologous pairs of chromosomes** associate / form a **bivalent**; 2. **Chiasma(ta)** form; 3. (Equal) **lengths of (non-sister) chromatids** / alleles are exchanged; 4. Producing **new combinations of alleles;** **THIS IS 'RARE' = Unequal chance of recombinant alleles forming.** Less recombinant gametes form.

Question 83

Give two differences between mitosis and meiosis.

Answer 83

**Mitosis given first** 1. One division, two divisions in meiosis; 2. (Daughter) cells genetically identical, daughter cells genetically different in meiosis; 3. Two cells produced, (usually) four cells produced in meiosis; 4. Diploid to diploid/haploid to haploid, diploid to haploid in meiosis; 5. Separation of homologous chromosomes only in meiosis; 6. Crossing over only in meiosis; 7. Independent segregation only in meiosis;

Question 84

A population of fruit flies contained 64% grey-bodied flies. Body colour is controlled by gene G which has 2 alleles. **G** produces Grey and is dominant to **g,** which produces black. Use the Hardy–Weinberg equation to calculate the percentage of flies **heterozygous** for gene **G**. [2]

Answer 84

**48% [2]** ## Footnote q²=0.36 or (36/100) so, q=0.6 and P=0.4 Therefore, 2 x 0.4 x 0.6 x [100] = 48% 2Pq [1] 0.48 [1]

Question 85

There were 850 fruit flies in one population. In this population, 510 fruit flies had the genotype W^NW^N, 255 had the genotype W^NW^V and 85 had the genotype W^VW^V. Calculate the **actual** frequency of the allele **W^V**. **Do not** use the Hardy-Weinberg equation in your calculation. [1]

Answer 85

0.25; [1] W^NW^V = 1x 255 = 255 W^NW^V = 2 x 85 = 170 Therefore, (255 + 170) divided by (1700) = **_0.25_** **_Frequency_** is always shown as a proportion of 1.

Question 86

In a population, the frequency of the allele for tongue-rolling, **T**, is 0.4. Use the Hardy-Weinberg equation to calculate the percentage of people in this population that are heterozygous for tongue-rolling.

Answer 86

48% [2] P=0.4 so, q=0.6 2Pq [1] 0.48 [1] = 2 x 0.4 x 0.6 x [100] = 48% [2]

Question 87

In a species of snail, shell colour is controlled by a gene with three alleles. The shell may be brown, pink or yellow. The allele for brown, **C^B**, is dominant to the other two alleles. The allele for pink, **C^P**, is dominant to the allele for yellow, **C^Y**. Give **all** the genotypes which would result in a brown-shelled snail.

Answer 87

C^BC^B, C^BC^P and C^BC^Y;

Question 88

What are the most common LIMITATIONS to a conclusion?

Answer 88

* **No STATISTICAL TEST**: Differences could be due to CHANCE * Data is for **only ONE** (named) **species** * Data only collected in a lab / *In vitro* * **Only tested on** males / females. Observations may differ in females / males * Data is **subjective** * Data only collected for ………….days / weeks / years * Long term **side effects** are not known * At no point is the ……… **100% effective**.

Question 89

Explain what is meant by the terms totipotent and pluripotent?

Answer 89

* Totipotent cells can give rise to **all** cell types; * Pluripotent can only give **some** cell types;

Question 90

Explain how cells produced from stem cells can have the same genes yet be of different types.

Answer 90

* {not all / different} genes are switched {on / off} /active / activated ; * correct and appropriate reference to factors / mechanisms for gene switching ; e.g. reference to promoters / transcription factors

Question 91

Describe the mechanism by which a signal protein causes the synthesis of mRNA.

Answer 91

* signal protein {binds to / joins to / interacts with / activates} * receptor on surface membrane; * messenger molecule moves from cytoplasm and enters nucleus; * {produces / activates} transcription factor; * binds to promoter region; * RNA polymerase transcribes target gene;

Question 92

Explain how oestrogen enables RNA polymerase to transcribe its target gene.

Answer 92

* Oestrogen is **lipid soluble** so diffuses through the Oestrogen diffuses through the **phospholipid** cell membrane; * attaches to ERα receptor; * ERα receptor changes shape / Tertiary structure; * ERα receptor leaves protein complex which inhibited it’s action; * oestrogen receptor binds to promoter region; * enables RNA polymerase to transcribe target gene.

Question 93

Compare the structure of dsRNA and DNA.

Answer 93

* Similarities; 2 max * Polynucleotides/polymer of nucleotides; * Contain Adenine, Guanine, Cytosine; * Have pentose sugar/5 carbon sugar; * Double stranded/hydrogen bonds/base pairs. * Differences; 2 max * dsRNA contains uracil, DNA contains thymine; * dsRNA contains ribose DNA contains Deoxyribose; * dsRNA is Shorter than DNA; fewer base pairs in length;

Question 94

Explain how the methylation of tumour suppressor genes can lead to cancer.

Answer 94

* Methylation prevents / inhibits transcription of (named) gene; * Protein not produced that prevents cell division / causes cell death / apoptosis; * No control of mitosis.

Question 95

Describe how alterations to tumour suppressor genes can lead to the development of tumours.

Answer 95

* (Increased) methylation (of tumour suppressor genes); * Mutation (in tumour suppressor genes); * Tumour suppressor genes are not transcribed/expressed **OR** * Amino acid sequence/primary/ tertiary structure altered; * (Results in) rapid/uncontrollable cell division;

Question 96

Describe what is meant by a malignant tumour.

Answer 96

* mass of undifferentiated / unspecialised / totipotent cells; * uncontrolled cell division * metastasis / (cells break off and) form new tumours / spread to other parts of body;

Question 97

Describe how altered DNA may lead to cancer.

Answer 97

* (DNA altered by) mutation; * (mutation) changes base sequence; * of gene controlling cell growth / oncogene / that monitors cell division; * of tumour suppressor gene; * change protein structure / non-functional protein / protein not formed; * (tumour suppressor genes) produce proteins that inhibit cell division; * mitosis; * uncontrolled / rapid / abnormal (cell division); * malignant tumour;

Question 98

Describe how alterations to tumour suppressor genes can lead to the development of tumours.

Answer 98

* (Increased) methylation (of tumour suppressor genes); * Mutation (in tumour suppressor genes); * Tumour suppressor genes are not transcribed / expressed **OR** Amino acid sequence / primary structure altered; * (Results in) rapid/uncontrollable cell division;

Question 99

Define epigenetics

Answer 99

* Heritable phenotype changes (gene function) that do not involve alterations in the DNA sequence/mutation.

Question 100

Explain how carbon-containing compounds present in the pine leaves that fall from the trees are absorbed and used for growth by saprobionts/fungi that live in the soil.

Answer 100

* extracellular digestion; * by secretion of enzymes; * absorption of digested/soluble products; * synthesis of structural compounds/named compound;. * respiration provide energy for growth

Question 101

Describe the process of eutrophication.

Answer 101

* Nitrates / Phosphates / Ammonium ions flushed into waterway * Increased algal bloom light blocked out * Submerged aquatic plant unable to photosynthesise and die * Increase in saprobionts so increases rate of aerobic respiration * Saprobionts / aero.resp organismsn die as lack of oxygen (anoxic) * Increase in anaerobic microorganisms * Production of toxins

Question 102

Explain how a single base substitution causes a change in the structure of a polypeptide. Do **not** include details of transcription and translation in your answer.

Answer 102

1.    Change in (sequence of) amino acid(s)/primary structure; * Reject amino acids are formed.* * Reject amino acids code.* 2.    Change in hydrogen & ionic & disulfide bonds; 3.    Alters _tertiary/3_⁰ structure;

Question 103

There are different types of gene mutation. Which statement describes **_incorrectly_** the effect of the mutation in an exon of a gene. A substitution may not result in a change to the encoded amino acid. An inversion will result in a change in the number of DNA bases. A deletion will result in a frame shift. An addition will result in a frame shift.

Answer 103

**An inversion will result in a change in the number of DNA bases.**

Question 104

A type of malignant tumour cell divides every 8 hours. Starting with one of these cells, how many tumour cells will be present after 4 weeks? Assume none of these cells will die. Give your answer in standard form.

Answer 104

**1.9/1.93 × 10****²⁵**

Question 105

What name is used for the non-coding sections of a gene?

Answer 105

Introns

Question 106

Explain why fragments of DNA from cancer cells may be present in blood plasma.

Answer 106

cancer cells die / break open releasing DNA;

Question 107

Describe how **alterations** to tumour suppressor genes can lead to the development of tumours.

Answer 107

1. (Increased) methylation (of tumour suppressor genes); * Accept abnormal methylation or hypermethylation* 2. Mutation (in tumour suppressor genes); 3. Tumour suppressor genes are not transcribed/expressed **OR** Amino acid sequence/primary structure altered; 4. (Results in) rapid/uncontrollable cell division;

Question 108

The time required for a cell to complete the cell cycle was 4 hours 18 minutes. Calculate the time required in minutes for this cell to multiply to produce eight cells.

Answer 108

774 minutes / 12 hours 54mins

Question 109

Describe how DNA is replicated in a cell.

Answer 109

* DNA strands separate / hydrogen bonds broken; * Parent strand acts as a template / copied / semi-conservative replication; * Nucleotides line up by complementary base pairing; (Adenine & Thymine etc) * Role of DNA polymerase: joins adjacent nucleotides on the developing strand via condensation and formation of phosphodiester bond; * 5’ to 3’ direction * Each new DNA molecule has 1 template and 1 new strand * Formed by semi-conservative replication.

Question 110

Why is the DNA heat to 95°C during PCR?

Answer 110

* Produce single stranded DNA * Breaks WEAK hydrogen bonds between strands

Question 111

Why do you add primers during PCR?

Answer 111

* Attaches to / complementary to start of the gene / end of fragment; * Replication of base sequence from here; * Prevents strands annealing

Question 112

Explain why ‘base-pairs’ is a suitable unit for measuring the length of a piece of DNA.

Answer 112

* DNA = 2 chains / joined by linking of 2 bases / A with T and G with C/ purine pairs with pyrimidine; * Bases are a constant distance apart / nucleotides occupy constant distance/each base-pair is same length / sugar-phosphate is a constant distance;

Question 113

Name one mutagenic agent.

Answer 113

* high energy radiation /ionising particles e.g. named particles/α, β, γ & X-rays; * benzene; * x rays/cosmic rays; * uv (light); * carcinogen / named carcinogen; * mustard gas / phenols / tar (qualified);

Question 114

A deletion mutation occurs in gene 1. Describe how a deletion mutation alters the structure of a gene.

Answer 114

* removal of one or more bases/nucleotide; * frameshift/(from point of mutation) base sequence change;

Question 115

Describe the main stages in the copying, cutting and separation of the DNA (fragments).

Answer 115

* heat DNA to 95°C / 90°C; * strands separate; * cool so that primers bind to DNA; * add DNA polymerase/nucleotides; * use of restriction enzymes to cut DNA at specific base sequence/ breaks phosphodiester bonds * use of electric current and agar/gel; * shorter fragments move further;

Question 116

Describe the polymerase chain reaction.

Answer 116

* Heat DNA (95 degrees C); * Breaks hydrogen bonds/separates strands; * Add primers; * Add nucleotides; * Cool to (50 d.C); * (to allow) binding of nucleotides/primers; * Heat to 70-75 degrees C to activate Taq DNA polymerase; * Role of (DNA) polymerase; * Repeat cycle many times;

Question 117

Describe a plasmid

Answer 117

* circular DNA; * separate from main bacterial DNA; * contains only a few genes;

Question 118

Suggest **one** reason why DNA replication stops in the polymerase chain reaction.

Answer 118

* Limited number of primers/nucleotides; / Primers / nucleotides ‘used up’. * DNA polymerase (eventually)denatures

Question 119

Suggest why a restriction enzyme has cut the human DNA in many places but has cut the plasmid DNA only once.

Answer 119

* enzymes only cut DNA at specific base sequence/recognition site/specific point; * sequence of bases/recognition site/specific point (on which enzyme acts) * occurs once in plasmid and many times in human DNA; * (max 1 if no reference to base sequence or recognition site)

Question 120

Describe how the bacteria containing the insulin gene are used to obtain sufficient insulin for commercial use.

Answer 120

* use of fermenters; * provides nutrients plus suitable conditions for optimum growth/named * environmental factor; * reproduction of bacteria; * insulin accumulates and is extracted;

Question 121

Explain what is meant by a vector.

Answer 121

* Carrier; * DNA/gene; (context of foreign DNA) * Into cell/other organism/host;

Question 122

Explain how modified plasmids are made by genetic engineering and how the use of markers enable bacteria containing these plasmids to be detected.

Answer 122

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

Question 123

mRNA may be described as a polymer. Explain why.

Answer 123

* Made up of many (similar) molecules/monomers/nucleotides/units;

Question 124

What is a DNA probe?

Answer 124

* (Short) single strand of DNA; * Bases complementary (with DNA/allele/gene)

Question 125

Name **three** techniques used by scientists to compare DNA sequences.

Answer 125

* Polymerase Chain Reaction * DNA fingerprinting * Gel electrophoresis

Question 126

What is meant by a genome?

Answer 126

* (All) the DNA in a cell/organism; * *‘(all) the ‘genes’/alleles’ ‘genetic material/code’ in a cell/organism/ person’* * *the total number of DNA bases in a cell/organism’* ***Reject all the DNA/ genes within a _species_***

Question 127

Describe the roles of **two** named types of enzymes used to insert DNA fragments into plasmids.

Answer 127

1. Restriction (endonuclease/enzyme) to cut plasmid/vector; 2. Ligase joins gene/DNA to plasmid/vector;

Question 128

Describe how enzymes could be used to insert the *GH* gene into a plasmid.

Answer 128

1. Restriction endonucleases/enzymes cuts plasmid; **OR** Restriction endonucleases/enzymes produces ‘sticky ends’; * Reject restriction enzymes **cuts** the **gene**.* 2. Ligase joins gene/DNA and plasmid **OR** Ligase joins ‘sticky ends’;

Question 129

After obtaining copies of the *HGH* gene, the geneticist will attempt to insert them into plasmid vectors. Describe how the geneticist would attempt to insert copies of the *HGH* gene into these plasmids.

Answer 129

1. Cut the plasmid with a restriction endonuclease; * Allow ‘add base sequences to blunt ends of plasmid and HGH gene’* 2. (So that) both have complementary / sticky ends; 3. (Mix together) and add ligase to join the complementary / sticky ends;

Question 130

Suggest why the plasmids were injected into the _eggs_ of **silkworms**, rather than into the **silkworms**.

Answer 130

1. (If injected into egg), gene gets into all / most of cells of silkworm; 2. So gets into cells that make silk.

Question 131

Name the type of enzyme used to produce the cDNA.

Answer 131

Reverse transcriptase;

Question 132

Describe what is meant by speciation (allopatric)

Answer 132

* Geographical isolation; * Separate gene pools / no interbreeding (between populations); * Variation due to mutation; * Different environmental/abiotic/biotic conditions / selection pressures; * Selection for different/advantageous, features/characteristics/mutation/ /allele; * Differential reproductive success / (selected) organisms survive and reproduce; * Leads to change in allelic frequencies; * Cannot breed/mate to produce fertile offspring.

Question 133

Describe what is meant by speciation (sympatric)

Answer 133

* NOT Geographical isolation; * Leads to reproductive isolation * Separate gene pools / no interbreeding (between populations); * Selection for different/advantageous, features/characteristics/mutation/ /allele; * Differential reproductive success / (selected) organisms survive and reproduce; * Leads to change in allelic frequencies; Cannot breed/mate to produce fertile offspring.

Question 134

Describe how bacteria can become resistant to antibiotics

Answer 134

* Variation/variety; * Mutation; * Some bacteria have allele to survive/grow/live in high concentration of named antibiotic * adapted bacteria survive and replicate by binary fission; * Increase in frequency of allele;

Question 135

Describe the process of succession

Answer 135

* (Colonisation by) pioneer species; * Pioneers cause change in environmental abiotic / biotic factors(give an example); * Pioneers make the environment less hostile for new species; * New species change/make conditions less suitable for previous species; * Change/increase in diversity/biodiversity; * Stability increases [population/richness/abiotic factors]; * Climax community;

Question 136

Describe the effect of IAA on root and shoot growth.

Answer 136

* IAA synthesised in shoots tips. * IAA diffuses into growing (cell elongating) region. * Light causes movement of IAA from light side to shaded side. * Proportionally more IAA on shaded side. * Causes directional growth as there is greater cell elongation on shaded side. * Shaded side grows faster and causes shoot to bend towards the light. * IAA synthesised in root tips. * IAA diffuses into growing (cell elongating) region. * IAA moves towards underside/shaded side. * Inhibits cell elongation. * Uneven growth (more elongation on side with less IAA). * Causes directional growth away from light and downwards towards gravity.

Question 137

Describe what is meant by the term taxis

Answer 137

* Moves towards or away from **directional** stimulus

Question 138

Describe what is meant by the term kinesis

Answer 138

* Movement is random/non-directional response OR * Organism does not move **towards** a particular stimulus;

Question 139

Compare & Contrast taxis and tropism

Answer 139

* **Both** are directional responses to stimuli * Taxis involves the movement of the entire organism **whereas** tropism is the movement of part of an organism

Question 140

Explain the advantages of simple reflex arcs

Answer 140

* Rapid; * Protect against damage to body tissues; * Do not have to be learnt; * Help escape from predators; * Enable homeostatic control;

Question 141

As a neurone transmits an impulse, its rate of oxygen consumption increases. Explain why.

Answer 141

* ATP required for active transport; * Na+ (actively) moved out only at nodes in myelinated / Na+ (actively) moved out along whole length of axon in non-myelinated;

Question 142

Describe how a resting potential is maintained in a neurone.

Answer 142

* active transport/pumping of sodium (ions across membrane); * out of neurone/higher concentration outside; * differential permeability to K+ and Na+; * Membrane more permeable to K+ ions;

Question 143

The potential across the membrane is reversed when an action potential is produced. Describe how.

Answer 143

* Sodium ion gates / channel (proteins) open; * Na+ (rapidly) diffuse in;

Question 144

SYNAPSE: Describe the sequence of events leading to the release of acetylcholine and its binding to the postsynaptic membrane.

Answer 144

* Depolarisation of presynaptic membrane; * Ca2+ 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) binds to receptors on the postsynaptic membrane; * Sodium ions enter (postsynaptic neurone) leading to depolarisation;

Question 145

When a nerve impulse arrives at a synapse, it causes the release of neurotransmitter from vesicles in the presynaptic knob. Describe how.

Answer 145

* (Nerve impulse/depolarisation of membrane) causes Ca2+ channel (proteins) to open; * Ca2+ enter by (facilitated) diffusion; * Causes (synaptic) vesicles to fuse with (presynaptic) membrane;

Question 146

The binding of GABA to receptors on postsynaptic membranes causes negatively charged chloride ions to enter postsynaptic neurones. Explain how this will inhibit transmission of nerve impulses by postsynaptic neurones.

Answer 146

* (Inside of postsynaptic) neurone becomes more negative/hyperpolarised; * More sodium ions required (to reach threshold) OR Not enough sodium ions enter (to reach threshold); * For depolarisation/action potential;

Question 147

Describe how the speed of the conduction could be increased in a neurone.

Answer 147

* Axon is myelinated; * So shows saltatory conduction/impulses jump from node of Ranvier to node of Ranvier; OR * Axon has a larger diameter; * So less resistance to flow of ions;

Question 148

Myelination affects the rate of conduction of a nerve impulse. Explain how.

Answer 148

* Actno potential jumps from node of Ranvier to node of Ranvier / depolarisation only at node of Ranvier; * Fewer jumps / depolarisations to travel length of axon;

Question 149

Describe how the inhibition of acetylcholinesterase affects the action of synapses.

Answer 149

* Acetylcholine not broken down / stays bound to receptor; * Na+ ions (continue to) enter / (continued) depolarisation / Na+ channels (kept) open / action potentials;

Question 150

Describe what is meant by the term refractory period

Answer 150

* (Refractory period) limits number of impulses per second/frequency of nerve impulses; * Maximum frequency of impulse transmission * Period of time between **threshold** and return to **resting membrane potential.** * When maximum frequency reached/exceeded, no further increase in information/ all (higher) stimuli seem the same;

Question 151

What is meant by a fatigued synapse?

Answer 151

When release (Fusion of vesicles with presynaptic membrane) of neurotransmitters exceeds production of vesicle bound neurtransmitters.

Question 152

Describe how synaptic transmission is unidirectional.

Answer 152

* Vesicles with neurotransmitters are only ever produced in the terminal end branch of the presynaptic neuone * Neurotransmitter receptors are only ever located on the post synaptic membrane.

Question 153

Explain how the fovea increases the detail of an image.

Answer 153

* High (visual) acuity * Each / single cone is connected to a single bipolar/neurone * Each cone sends separate impulse to the brain

Question 154

Explain how the connection of several rods to a single bipolar cell (Retinal convergence) influences visual acuity

Answer 154

* (Spatial) Summation means cannot distinguish between stimuli from different rod cells linked to same bipolar cell; * Decreases acuity;

Question 155

Explain how the connection of several rods to a single bipolar cell (Retinal convergence) influences sensitivity

Answer 155

* (Summation of) sub-threshold stimuli produces threshold stimulation; * Increases sensitivity;

Question 156

People with red-green colour blindness are unable to distinguish between red and green, and also between other colours Explain why.

Answer 156

* Green sensitive pigment/cones non-functional OR Cones that detect green light non-functional; * Three different types of pigment/cone; * Other/different colours (‘seen’) due to stimulation of more than one cone/pigment;

Question 157

Explain how pressure on the Pacinian corpuscle produces the changes in membrane potential recorded by microelectrode A.

Answer 157

* (Pressure) deforms / opens (sodium) channels; * Sodium ions enter; * Causing depolarisation; * Increased pressure opens more channels / greater sodium entry; * [Size of generator potential determines frequency of action potentials]

Question 158

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

Answer 158

1. Chemoreceptors detect rise in CO2 / H+ / acidity / carbonic acid /f all in pH OR Baro / pressure receptors detect rise in blood pressure; 2. Send impulses to cardiac centre/medulla; 3. More impulses to SAN; 4. By sympathetic (nervous system for chemoreceptors / CO2) OR By parasympathetic (nervous system for baro / pressure receptors / blood pressure);

Question 159

The rate of ATP consumption of a de-myelinated neurone is greater than that of a myelinated neurone when conducting impulses at the same frequency. Explain why.

Answer 159

* Greater entry of sodium ions / greater exit of K+ * in de-myelinated neurone; * Ref. to active transport / ref. to ion pumps;

Question 160

Explain how nervous control in a human can cause increased cardiac output during exercise.

Answer 160

* Coordination via medulla (of brain) / cardiac centre; * (Increased) impulses along sympathetic nerve; * To S.A. node; * Release of noradrenalin; * More impulses sent from S.A.N (across atria); * Increased heart rate / increased stroke volume;

Question 161

Explain why increased cardiac output is an advantage during exercise.

Answer 161

* Higher cardiac output - Increases O2 supply (to muscles); * Increases glucose supply (to muscles); * More ATP produced by oxidative phosphorylation / more energy release / more aerobic respiration / actively respiring muscles * Increases CO2 removal (from muscles) / lactate removal; * Increases heat removal (from muscles) / for cooling; * Delays the formation of lactate

Question 162

Describe how the Pacinian corpuscle propagates an action potential.

Answer 162

* (Pressure causes) membrane/lamellae to become deformed/stretched; * Sodium ion channels in membrane open and sodium ions move in; * Depolarisation leading to generator potential; * Greater pressure more channels open/sodium ions enter;

Question 163

Describe what is meant by the term threshold

Answer 163

* When threshold has been reached; * (Threshold or above) causes maximal response / all or nothing principle;

Question 164

Describe the banding pattern in striated muscle.

Answer 164

* Lightest band is I band, actin only * Darkest band is overlapping region, actin and myosin * Medium shading is H zone/band is myosin only.

Question 165

Describe the sliding filament theory

Answer 165

* Attachment / cross bridges between actin and myosin heads; * ‘Power stroke’ / movement of myosin heads / pulling of actin (over myosin); * Detachment of myosin heads (requires ATP binding); * (Energy from ATP) Myosin heads move back/to original position / ‘recovery stroke’;

Question 166

Describe the function of calcium ions in muscle contraction.

Answer 166

* Ca2+ Binding/changing shape/removing tropomyosin; * Exposes actin binding sites; * Myosin head attaches/cross-bridge formation; * Activates ATP hydrolase;

Question 167

Nerve impulses arriving at the presynaptic membrane at the neuromuscular junction result in shortening of sarcomeres. **Describe how.** [5 marks]

Answer 167

* Entry of calcium ions (presynaptic membrane); * Vesicles fuse with membrane / exocytosis /release Ach (Acetylcholine); * Neurotransmitter diffuses; * Binds to receptors, postsynaptic / membrane / muscle membrane; * Depolarisation / sodium ions enter; * Release of calcium ions (from within the muscle); * Removes tropomyosin / bind to troponin; * Exposing binding sites on the actin; * Actinomyosin cross bridge formation / myosin binds; * Myosin head moves / pulls the actin along; * Rachet mechanism / description /detach and reattach; * ATP hydrolase activated;

Question 168

Explain the importance of ATP hydrolase during muscle contraction.

Answer 168

* Hydrolysis of ATP releasing energy; * used to form / break actinomyosin cross-bridges;

Question 169

Muscle contraction requires ATP. What are the advantages of using aerobic rather than anaerobic respiration to provide ATP in a long-distance race?

Answer 169

* Aerobic respiration releases more energy / produces **more ATP**; * Little/no lactate produced / does not accumulate; * Avoids cramp / muscle fatigue; * CO2 easily removed from the body / CO2 removed by breathing;

Question 170

A muscle fibre contracts when it is stimulated by a motor neurone. Describe how transmission occurs across the synapse between a motor neurone and a muscle fibre.

Answer 170

* Ca2+ channels / gates open; * Ca2+ ions enter (pre-synaptic neurone); * Vesicles move towards / fuse with presynaptic membrane; * Release / exocytosis of transmitter substance / of acetylcholine; * Diffusion (of transmitter) across gap / cleft; * (Transmitter) binds to receptors in postsynaptic membrane; * Na+ channels open / Na+ ions enter (postsynaptic side);

Question 171

After death, cross bridges between actin and myosin remain firmly bound resulting in rigor mortis. Explain what causes the cross bridges to remain firmly bound.

Answer 171

* respiration stops; * no ATP produced; * ATP required for separation of actin and myosin/cross bridges;

Question 172

Describe the role of calcium ions in the contraction of a sarcomere.

Answer 172

* interact with/move/touch tropomyosin; (allow troponin as alternative) * to reveal binding sites on actin; (not active sites) * allowing myosin (heads) to bind/touch actin / actinomyosin formed; * activate ATP hydrolase / energy released from ATP;

Question 173

Describe slow twitch muscle fibres

Answer 173

* have lots of mitochondria/ (slow fibres) respire aerobically; * More myoglobin

Question 174

Describe fast twitch muscle fibres

Answer 174

* used for rapid/brief/powerful/strong contractions; * Phosphocreatine used up rapidly during contraction/to make ATP; * Anaerobic respiration involved; * ATP used to reform phosphocreatine; * Lots of phosphocreatine in fast twitch fibres; * No myoglobin

Question 175

Describe the role of phosphocreatine

Answer 175

* Provides (energy and) phosphate / phosphorylates; * To make ATP from ADP & Pi;

Question 176

# Homeostasis Explain how insulin reduces the blood glucose concentration.

Answer 176

* (More) insulin binds to receptors; * (Stimulates) uptake of glucose by channel/transport proteins * Activates enzymes which convert glucose to glycogen;

Question 177

Explain the action of Glucagon

Answer 177

* Attaches to receptors on target cells and activates/stimulates enzymes; * Glycogen to glucose / glycogenolysis; * Gluconeogenesis;

Question 178

Explain the effect of sweating or panting on temperature control.

Answer 178

* Evaporation (of water from lining of mouth or skin); * Heat transferred from blood;

Question 179

Describe how a change in blood pH or blood pressure can cause a change in heart rate.

Answer 179

* (Carbon dioxide) detected by chemoreceptors / (pressure) detected by baroreceptors; * Medulla/cardiac centre involved; * More impulses to SAN/along sympathetic nerve; * (Decrease) pH detected by chemoreceptors in carotid artery /aorta; * Sends (more) impulses to medulla (oblongata); * More Nerve impulses sent by sympathetic nervous system to SAN;

Question 180

Less CO2 in the blood leads to a reduction in heart rate. Describe how.

Answer 180

* (less CO2 in blood) Detected by chemoreceptors; * (Chemoreceptors) located in aorta / carotid artery; * Fewer impulses to cardiac centre / medulla (oblongata); * (More) impulses along parasympathetic nerve OR less impulses along sympathetic nerve; * (To) SAN;

Question 181

Describe the secondary messenger model

Answer 181

* **Adenylate cyclase** activated / cAMP produced / second messenger produced; * Activates enzyme(s) (in cell); * (So) glycogenolysis/ gluconeogenesis occurs / glycogenesis inhibited;

Question 182

Describe the role of glycogen formation and its role in lowering blood glucose levels.

Answer 182

* Glucose concentration in cell/liver falls; * Below that in blood (plasma)/ higher in blood; * Creates/maintains glucose concentration/diffusion gradient; * Glucose enters cell/leaves blood by facilitated diffusion/via carrier(protein)/channel (protein);

Question 183

Describe how blood glucose levels can be increased using hormones.

Answer 183

* Release of glucagon; * Leads to formation of glucose in liver (cells); * From non-carbohydrates/amino acids/fatty acids;

Question 184

Describe how ultrafiltration occurs in a glomerulus.

Answer 184

* Higher than normal High blood/hydrostatic pressure; * Two named small substances pass out eg water, glucose, ions, urea; * (Through small) gaps/pores/**fenestrations** in (capillary) **endothelium**; * (And) through (capillary) **basement membrane**;

Question 185

Explain why a thicker medulla leads to more concentrated urine.

Answer 185

* Thicker medulla means a **longer** loop of Henle; * (The longer the loop of Henle means) increase in **sodium ion** concentration (in medulla) so sodium ion gradient **maintained for longer** (in medulla); * (Therefore) **water potential gradient maintained** (for longer), so **more** water (re)absorbed (from loop and collecting duct) by osmosis;

Question 186

Describe the action of ADH in the kidney.

Answer 186

* Permeability of membrane/cells (to water) is **increased**; * **More water absorbed** from/leaves distal tubule/collecting duct; * **Smaller** volume of urine; * Urine becomes **more concentrated**;