Edexcel Biology Unit 4 Model Answers

 

Topic 5

 

2. Describe the structure of chloroplasts in relation to their role in photosynthesis. •

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Grana-- Arranged in stacks with discs known as thylakoids Grana thylakoids Thylakoids- Site of photosynthesis, Thylakoidsphotosynthesis, site of green pigmentchlorophyll Chlorophyll- Captures energy from light, converts it to ATP ChlorophyllStroma-- Where ATP is used to convert carbon dioxide into Stroma glucose, contains enzymes which can be used in cellular respiration, glucose converted to starch

 

3 Describe the overall reaction of photosynthesi photosynthesiss as requiring energy from light to split apart the strong bonds in water molecules, storing the hydrogen in a fuel (glucose) by combining it with carbon dioxide and releasing oxyge oxygen n into the atmosphere. •

6CO2 + 6H20 + Energy C6H12O6 (glucose) + 6O2 

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Includes the light dependent reaction (including hydrolysis hydrolysis using light energy) and the light independent reaction (production of glucose by carbon fixation in the Calvin cycle).

 

4 Describe the light-dependent reactions of photosynthesis including how light energy is trapped by exciting exciting electrons in chlorophyll and the role of these electrons in generating ATP, and reducing NADP in photophosphorylation and producing oxygen through photolysis of water. •

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Light dependent reaction takes place in thylakoid membrane of chloroplast Light energy excites electrons from PSII along the electron transfer chain (ETC) to PSI Photolysis of water makes H+ ions, electrons and oxygen Energy lost as the electron moves along the ETC is used to pump H+ ions to one side of the thylakoid membrane. H+ ions then travel back to the stroma via the enzyme ATP synthase. The enzyme uses energy from this process to make ATP from ADP + inorganic phosphate.

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Light energy excites electrons in PSI to even higher energy levels. The electrons

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and H+ ions are transferred to NADP to make reduced NADP. Cyclic photophosphorylation makes ATP only.

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Non-cyclic photophosphorylation makes ATP and reduced NADP.

 

5 Describe how phosphorylation of ADP requires energy and how hydrolysis of ATP provides an immediate supply of energy for biological processes. Hydrolysis of ATP : Loss of phosphate molecule to release energy which is used by the cell. ATP is converted into ADP. • •

Photophosphorylation of ADP: Condensation reaction needing energy which comes from a catabolic reaction. Energy is needed to form the bond as energy was released when the bond was broken. • •

ATP – Adenosine triphosphate. ADP – Adenosine diphosphate. 

 

Jan 12

 

6. Describe the light-independent reactions as reduction of carbon dioxide using the products of the light-dependent reactions (carbon fixation in the Calvin cycle, the role of GP, GALP, RuBP and RUBISCO) and describe the products as simple sugars that are used by plants, animals and other organisms in respiration and the synthesis of new biological molecules (including polysaccharides, polysaccharid es, amino acids, lipids and nucleic acids). •

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Also known as the Calvin cycle. Does not require light energy, (directly) however it does rely on products from the light dependent reaction. (ATP and Reduced NADP) Takes place in Stroma of the chloroplast.

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CO2 enters leaf through stomata diffuses in to stroma. Combined with RuBP, Produces 2 molecules GP

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RUBISCO catalyses the reaction between CO2 and RuBP.

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ATP and Reduced NAPD reduce GP to GALP, which is then converted into glucose and other organic compounds. 5/6 of TP molec molecules ules u used sed to regenerate RuBP,using RuBP,using ATP. TP.

 

7. Carry out calculations of net primary productivity and explain the relationship between gross primary productivity productivity,, net primary productivity and plant respiration. respiration. •

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NPP = GPP – plant respiration The GPP of the grass is 19,000kJm -2 yr-1. The grass loses 8000kJm-2 yr-1 from respiration. NPP= 19,000 – 8000 = 11,000kJm-2 yr-1. Gross primary production (GPP) is the total amount of energy produced by vegetation; vegetation; some of that energy is used for cellular respiration i.e. for the growth and development of the plant. What is left over is called net primary production production (NPP) and that represents the total available energy in an ecosystem the form of dry plant biomass.

 

8 Calculate the efficiency of energy transfers transf ers between ttrophic rophic levels. •

% efficiency of energy transfer between trophic levels

=

Net productivity of a level

Net productivity of previous level

x 100

 

9. Discuss how understanding the carbon cycle can lead to methods to reduce atmospheric levels of carbon dioxide (including the use of biofuels and reforestation).

 

10 Explain that the numbers and distribution of organisms in a habitat are controlled by biotic and abiotic factors. Abiotic factors •

E.g. temperature, light availability, and water/space availability.

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Ideal abiotic factors allow organisms to grow fast and reproduce successfully

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Therefore, ideal abiotic factors cause increased population size and wide

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distribution. Abiotic factors that aren’t ideal can cause an organism to be unable to live successfully in an area

Biotic factors •

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E.g. predation, interspecific competition, and intraspecific competition If an organism is out-competed, it has to move away from the better-adapted organism. This affects distribution. Ideal biotic factors allow organisms to grow fast and reproduce successfully

 

11 Describe how to carry out a study on the ecology of a habitat to produce v valid alid and reliable data (including the use of quadrats and transects to assess abundance and distribution of organisms organisms and the measurement of abiotic factors, eg solar energy input, climate, topography, oxygen availability and edaphic factors).

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Chose a location eg. sea shore, search the habitat and compose a hypothesis.

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Mark out an area tosystematic collect dataor(transect). Use either random, stratified sampling. Measure abiotic factors factors at each 1m point along the line. Sample organism along transect using chosen sampling method with a 1m2 quadrat and collect required data.

 

12. Explain how the concept of niche accounts for distribution and abundance of organisms in a habitat. •

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Niche- The role an organism play playss within its environment. Abundance -T -Two wo organisms cannot occ occupy upy the same niche, if this does occur the species will compete until only one remains. Distribution – Organisms can only exist in habitats

where all the conditions that make make up their role exist.

  13. Describe the concept of succession to a climax community.

 

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Succession is the process by which communities colonise an ecosystem and are then replaced over time by other communities after being out-competed. Primary succession starts when pioneer species (e.g. lichen) colonise new land surface. Abiotic conditions are harsh and there is no soil. These pioneer species die and the decomposed material forms a basic soil. New organisms (e.g. grass) move in and the process repeats making the soil develop. Secondary succession begins with a soil layer and the pioneer species are larger plants (e.g. shrubs, small trees). Competition occurs and dominant species out-compete less adapted plants (e.g. grass). More species move in and create species diversity. The final stage – climax community – eco system supports largest, complex community of plants – unlikely to change: steady state.

 

14 Outline the causes of global warming – including the role of greenhouse gases (carbon dioxide and methane, CH4) in the greenhouse effect. •

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Causes of global warming: human activity, increase of greenhouse gases (causes the greenhouse effect), burning of fossil fuels, farming, deforestation, carbon sinks, Greenhouse effect- incoming short infrared wavelength, earth radiates longer infrared wavelength, excess amount greenhouse gases cause large amounts re-radiated infrared wavelength to be reflected back- warming of earth surface Co2- concentration increasing- human activity and carbon sinks Methane- concentration increasing- produced by extraction of fossil fuels and more decaying waste and more cows, thawing of frozen ground

 

15 Describe the eff effects ects of global warming (rising temperatur temperature, e, changing rainfall rainfall patterns and seasonal cycles) on plants and animals (distribution of species, development and life cycles). •

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Rising Temperature affects metabolism because of enzyme optimum temperatures = affects affects distribution and decrease in rate of growth. Changing Rainfall Patterns Patterns = aff affects ects life cycles and distribution Seasonal Cycles = affects affects life cycles and distribution (because of availability of food and reproduction).

 

16 Explain the effect effect of increasing temperature temperature on the rate of enzyme activity in plants, animals and micro-organisms. micro-organisms.

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Higher temperature temperature = higher high er kinetic energy

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Higher energy = more collisions

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More collisions = more efficient chemical reactions in enzymes

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Temperature too high = enzymes denature and chemical reactions cannot occur efficiently Optimum temperature =   Temperature too low – enzymes do not work and chemical reactions cannot occur efficiently Therefore, increase in temperature Therefore, temperature increases growth in plants, animals and microoragnisms. They are  

 

17 Describe how to investigate the effects of temperature on the development of organisms (eg seedling growth rate, brine shrimp hatch rates).

1. Place Place equal equal volume volumess of of salt salt wate waterr int into o 6 cont contain ainer erss 2. Place Place each each cont contain ainer er in in diff differen erentt tempe temperrature ature wate waterr baths, ranging from 0-60 degrees 3. Contr Control ol othe otherr var varia iabl bles es (eg (eg.. Lig Light ht int inten ensi sity ty)) 4. Leave Leave cont contain ainer erss in in w wat ater er baths baths unt until il wat water er has reached appropriate temperature. 5. container. Add Add equa equall quan quanti titi ties es of of brin brine e shri shrimp mp eggs eggs (40) (40) to to each each 6. Chec Check k ever every y 10 hou hourrs over over a 72 72 hour hour per perio iods ds for for the the presence of tiny swimming larvae. 7. Recor ecord d tim time e at at whi which ch ha hattchin ching g beg begin ins. s.

 

18. Analyse and interpret different types of evidence for global warming and its causes (including records of carbon dioxide levels, temperature records, pollen in peat bogs and dendrochronology) recognising correlations and causal relationships. • •

Temperature The general trend of increasing global temperature over the last century is evidence for global warming.

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Dendrochronology A method for figuring out the age of a tree, the thickness of the ring depends on the climate when ring is formed, when its warmer the rings are thicker. Pollen in Peat Bogs Pollen is preserved in peat bogs, they accumulate in layers so the age of preserved pollen increases with depth. More pollen show an increase in temp.

 

19. Describe that data can be extrapolated to make predictions, that these are used in models of future global warming, and that these models have limitations. •

Data on greenhouse gases can be extrapolated and to be used in models to make predictions about willlong happen temperature future what and the termtoeffects of the in the environment.

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Limitations: Impossible to predict the exact impacts of CO2 or on aspects a spects of the world climate. Extrapolations from past data cannot take into account unknown factors in the future.

 

20 Discuss the way in which scientific conclusions about controver controversial sial issues, such as what actions a ctions should be take taken n to reduce global warming or the t he degree to which humans are affecting global warming, can sometimes depend on who is reaching the conclusions. •

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Depends on what company the person is working for Bias

 

21 Describe how evolution (a change in the allele frequency) can come about through gene mutation and natural selection. •

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Allele frequency – the proportion of an allele in the gene pool of a population. There are different alleles due to mutations some of these are

advantageous and others aren’t.  advantageous Therefore different individuals are better adapted to their environmentt so are more likely to survive and reproduce to environmen pass on their genes (including the beneficial allele). This also then increases the offspring’s offspring’s chances of survival sur vival and reproduction. Overtime this increases allele frequency frequency.. This leads to evolution.

 

22 Explain how reproductive isolation can lead to speciation. •

No crossover of gene pool

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Adaptations differ because of different circumstances/environments

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Different adaptations lead to difficulties in reproduction to produce fertile Different offspring

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= Different species

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Boom! Speciation.

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Can be caused by geographical barriers (allopatric speciation) and sympatric speciation

 

23 Describe the role of the scientific community in validating new evidence (including molecular biology, biology, eg DNA, proteomics) proteomics) supporting the accepted scientific theory of evolution (scientific journals, the peer review process, scientific conferences). conferences). 

 

Topic 6

 

2.Explain the nature of the genetic code (triplet code, non-overlapping and degenerate) degenerate) •

A codon(or triplet code) is 3 bases, bas es, that code for an amino acid.

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These Codons are non-overlapping DegenerateDegenera te- there are more possible combinations of triplets then there are

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available amino acids. available acid s.  (20 amino acids but 64 possible combinations)

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  3. Explain the process of protein synthesis and explain the roles of the template (antisense) DNA strand in transcription, codons on messenger RNA, anticodons on transfer RNA. •

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Transcription : RNA polymerase attaches to the DNA at the beginning of a gene causing the hydrogen bonds to break and the DNA strands to separate. The anti-sense (template) strand is used to make mRNA RNA nucleotides line up alongside the template strand strand and pair up with their complementary bases. Joined together to form mRNA. mRNA leaves the nucleus through a nuclear pore.

 

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mRNA modification

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During transcription (in the nucleus), introns

(sections of DNA that don’t code for amino acid) are removed by splicing and exons (the sections that do code) are joined to form mRNA strands. strands. •

Exons can be joined in different orders to form different differ ent mRNA strands, meaning mo more re than one amino acid sequence and so more than one protein can be produced from one gene.

 

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Translation – mRNA attaches to a ribosome and tRNA carry carr y amino acids to the ribosome. The anti-codon on tRNA that’s complementary to the firstbase codon on mRNA mRN A attaches itself to mRNA by complementary pairing.

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A second tRNA attaches itself to the next codon on mRNA. The amino acids are joined by a peptide bond. The first tRNA

molecule moves away leaving it’ it ’s amino acid behind.  This process continues until there is a stop codon on the mRNA molecule and there is a chain of amino acids (polypeptide chain).

 

4 Explain how one gene can give rise to more than one protein through post-transcriptional post-transcriptional changes to messenger RNA.

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During transcription introns and exons are copied into mRNA Introns- sections of gene which don’t code for amino acids, exons do code for amino acids

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Depending on what intronsstrand are removed remov ed by splicing, splicing , and exons are  joined to form new mRNA new str and this can lead to different differ entwhich prot proteins eins being formed as exons can be joined in different orders •

More than one amino acid sequence and so more than one protein can be produced from one gene

 

5 Describe how DNA profiling is used for identification and determining genetic relationships between organisms (plants and animals). •

The more bands on 2 DNA profiles that match, the more closely related those two people are

 

6 Describe how DNA can be amplified a mplified using the polymerase chain reaction (PCR)

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Mix DNA sample, free nucleotides, primers and DNA polymerase Primers = short pieces of DNA complementary complementary to the bases at the start of the DNA fragment desired

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Heat to 95°C to break hydrogen hydrogen bonds between DNA strands Cool to 50-65°C so the primers anneal (bind) ( bind) to the separated strands

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Heat to 72°C so DNA polymerase can work

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DNA polymerase lines up free nucleotides along the DNA template template strands to form complementary strands

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Repeat

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Each PCR cycle doubles the amount of DNA present, amplifying the DNA.

 

7 Describe how gel electrophoresis can be used to separate DNA fragments of different length 

 

8. Distinguish between the structure of bacteria and viruses. Bacteria 

Virus

Ribosomes

Protein Coat(Caspid)

Cell Wall Flagellum

Nucleic Acid(either DNA or RNA)

Slime Capsule

Smaller than Bacteria

Plasmids Plasma Membrane

Not cells

Nucleic Acid

 

9. Describe the role of micro-org micro-organisms anisms in the decomposition of organic matter matter and the recycling of carbon.

 

10 Describe the major routes pathogens may take when entering the body and explain the role of barriers in protecting the body from infection, including the roles of skin, stomach acid, gut and skin flora. f lora. •

Pathogens Pathog ens can enter the body:  –

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Cuts in the skin Through digestive system via contaminated food food and drink

Through respiratory system Mucosal surfaces Barriers . Stomach acid . Skin (physical barrier) . Gut and Skin flora (compete with pathogens) . Lysozyme Lysozyme (enzyme in mucosal layers, kills bacteria by damaging their cell walls)

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11 Explain how bacterial and viral infectious diseases have a sequence of symptoms sympt oms that may result in death, including the diseases caused by Mycobacterium tuberculosis tuberculosis (TB) and Human Immunodeficiency Virus (HIV). •

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Pathogens are viruses, some fungi, some bacteria (anything that causes ca uses disease) As infectious diseases develop, they cause a sequence of symptoms, which could lead your ultimate annihilation. HIV infects and destroys immune system cells. Eventually progresses to AIDS when immune system deteriorates and fails.  –

Initial AIDS symptoms: reduction in no. of T-helper cells (with CD4 receptors), minor infections of mucus membranes, recurring respiratory infections

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As AIDS progresses: no. of T-helper cells decreases further, susceptible to

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more serious infections Late stages: very low no. of T-helper cells, serious infections that can kill.

 

12 Describe the non-specific responses of the body to infection, including inflammation, lysozyme action, interferon and phagocytosis PHAGOCYTOSIS BY PHAGOCYTES e.g. macrophages. •

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During phagocytosis, a pathogen is engulfed A lysosome full of lysozyme enzym enzymes es fuses with the phagocytic vacuole holding the pathogen The lysozyme break down the pathogen

INTERFERONS •

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Produced by cells infected with viruses Prevent viruses spreading to uninfected cells by inhibiting the production of viral proteins

INFLAMMATION AT SITE OF INFECTION (s.o.i.) •

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Vasodilation increases blood flow to s.o.i. which increases immune system cell concentration at s.o.i. Increases permeability of blood vessels at s.o.i. which allows immune system cells to move out of the blood and to the affected areas to destroy the pathogens.

 

13 Explain the roles of antigens and antibodies in the body’ body ’s immune response including the involvement of plasma cells, macrophages and antigen-presenting cells. •

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Antigen – Antigen   – A protei protein n or glycoprotein glycoprotein on invading pathogens that triggers trig gers a B-cell to engulf it. Plasma cell – B cells are stimulated stimulated by cy cytokines tokines to become B effector effector cells which differentiate differentiate into plasmas cells to produce antibodies.

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Antibody – Antibody   – Binds to antigens, becoming an antitoxin antitoxin to disable the pathogens toxins chemical actions and signals to be englufed by a macrophage. Macrophage – Macrophage   – Englufs a pathogen through phagocytosis forming a phagocytic vacuole which fuses to the organelle lysosome lysosome containing the digestive enzyme lysozyme. lysozyme. This breaks down the pathogen and for the antigens to be processed.

Antigen-presenting Antigen-prese nting that dhas processed thecell antigens  – A macrophage from the surface of acell pathogen and presented presente them in its own surface

membrane for B cell recognition.  

14. Distinguish between between the roles of B cells (including B memory and B effector cells) and T cells (T helper, T killer and T memory cells) in the body’s immune response.  •

B Cells- WBC covered in proteins(antibodies)

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T-Cells WBC covered with receptors

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Plasma Cells – produce antibodies during infection. Memory Cells-produces antibodies during second infection T-killer Cells- targ target et inf infected ected cells and kill them by lysis (bursting/cutting the cells)

T-helper Cells- Activate B-cells

 

15. Explain how individuals may develop immunity (natural, artificial, active, passive).

 

16 Discuss how the theory of an ‘evolutionary ‘evolutionary race’ between pathogens and their hosts is supported by the evasion mechanisms as shown by Human Immunodeficiency Virus (HIV) and Mycobacterium tuberculosis tuberculosis (TB). • •

Antibiotics HIV  –

AS HIV kills immune cells it infects, reduces overall number which reduces the chances of it being detected

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High rate of mutation in genes that code for antigen proteins Means memory cells that recognised original strain wont recognise mutated strain

TB  –

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When engulfed by phagocytes, produce substances that prevent lysosome from fusing with phagocytic vacuole Bacteria cant be broken down and can multiply undetected Disrupts antigen presentation in infected cells which mean it can not be detected and killed

Shows pathogens have evolved batter ways to avoid detection while reproducing which means they will be able to survive without being detected and killed, spreading to other cells and so can spread throughout the body.

 

17 Distinguish between bacteriostatic bacteriostatic and bactericidal antibiotics. •

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Bacteriostatic Bacteriost atic inhibits growth of the b bacteria. acteria. Bactericidal destroys the bacteria.

 

18 Describe how to investigate the effect of different antibiotics on bacteria.

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Use an agar plate Bacteria to be tested are spread on an agar plate under aseptic conditions, like using a Bunsen burner to sterilise equipment. Place paperon discs in e. antibiotics, and one soaked in water water to act as four a control, c ontrol, thesoaked agar plate. plat Do this under aseptic conditions. Use two different antibiotics and two different different concentrations (e.g. 150mg and 250mg). So one paper disc at 150 mg of antibiotic one, one paper disc at 250 mg of antibiotic one, one paper disc at 150mg of antibiotic two t wo etc. Incubate the plate at 25-30°C for 24-36 hours, with the plate sealed in a way that allows oxygen in.

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Measure the zones of inhibition around each plate.

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The bigger the zone, the better the concentration/type of antibiotic.

 

19 Describe how an understanding of the contributory causes of hospital acquired infections have led to codes of practice relating to antibiotic prescription and hospital practice relating to infection prevention and control.  •

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MRSA – Causes boils and abscesses, regularly mutating to form antibiotic resistance. C.Diff – Causes diarrhoea. Prevention:  –

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Controlling of Antibiotics to decrease antibiotic

resistant bacteria mutating. Hygiene  to prevent transmission by washing hands Hygiene using alcohol and chlorine foam. 

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Isolation  to contain infections and reduce spreading.   Isolation

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Prevention Enter to Hospitals by screening patients

and visitors and encouraging hygiene.   

20. Describe how to determine the time of death of a mammal by examining the extent of decomposition, stage of succession, forensic entomology, entomology, body temperature temper ature and degree of muscle contraction Temperature Bodies cool at 2⁰c/phour, therefore a body at 35⁰c would have been dead for an hour hour..  •

Muscle Contraction •

Rigor mortis – 4-6 hours ho urs after to death, muscle contract and become stiff. stiff.

Entymology Appearance of insects and larvae. •

Demcoposition Enzymes and bacteria, decompose the body. •

Sucession Types of organisms found on carrion change over time. Bacteria – flies + larvae – Beetles – Nothing (No tissue) •

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