aqa a level biology unit 2 notes

7/24/2019 AQA A level Biology Unit 2 Notes

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BIOLOGY UNIT 2 NOTES BY Maaz Chati

DNA is a double stranded molecule, with the two twisted strands joint by Hydrogen bonds. Eachtwisted strand is a polynucleotide which is made up of lots of single nucleotides via condensation.

This is a single nucleotideEvery Nucleotide has ! components

"ugar called Deo#yribose$ deo#yribose is a carbohydrate

A %hosphate &roup

A Nitrogenous 'rganic (ase )contains nitrogen*

"ingle +ing (ases ytosine - Thymine

Double +ing (ases Adenine - &uanineA T with / H0(onds$ & 1 with ! H0(onds. Also this meansthere will be the same percentage of Adenine as there is

Thymine and the same percentage of &uanine as there is ofytosine. This form of DNA found in all eu2aryotes.However, in di3erent cells, there will be di3erent percentages of each pair of bases as each cellproduces D455E+ENT %+'TE4N" and has D455E+ENT &ENE" and has D455E+ENT DNA (A"E"E67ENE". DNA is also a polymer as it is made of monomers )nucleotides*. DNA in eu2aryotes,'N89 E:4"T" 4N N78E7", this is why red blood cells do not have DNA, as they have no nucleus5unction of DNA "tore information

5eature How it helps 5eature How it helps

"ugar0%hosphatebac2bone %rovides strength Double heli# %rotects wea2 H0(onds

oiling &ives compact shape Hydrogen (onds Allows chain to split fortranslation

"e;uence of bases Allows info to be stored triplet codewhilst other amino acids have /0? triplet codes. 4.e. /0? di3erent triplets would all code for the sameamino acid.ells may be genetically identical, but di3erent in appearance, this is because

>* Di3erent genes are e#pressed in each/* Therefore producing di3erent en@ymes

How di3erent se;uences of bases code for di3erent proteins>* %roteins are made of chains of amino acids

/* Each amino acid has its own triplet code

Therefore in applications such as if you 2now the amino acid se;uenceof a substance, you could ma2e an arti=cial gene coded for thatsubstance as ! bases code for one amino acid, therefore a correct basese;uence can be wor2ed out.4n Eu2aryotic ells, there are many base se;uences that do not code foran amino acid, they are 2nown as 4ntrons, also there are many non0coding strandsand addition of bases by mutations, therefore for thesereasons, the number of bases will be more than the number of amino

acids multiplied by ! as the triplet code suggests.4n %ro2aryotic ells>* DNA molecules are circular /* DNA molecules are not associated with proteins4n Eu2aryotic ells, DNA molecules are associated with proteins and coil up to form a structure calledH+'


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Number*, but when they combine with another "e# ell the Diploid Number )? chromosomes* isrestored.AlleleDi3erent form of the same gene positioned in the same locus, one allele is inherited from eachparent, sometimes they are the same, e.g. Eye colour from dad is blue and from mom is also blue,sometimes they are di3erent, and however one will be the dominant one, e.g. Eye colour from dad isblue but from mom it is brown.


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The di3erence between se# cells and normal body cells>* "e# ells are haploid

/* (ody ells are diploid4f an organism has an odd diploid number )e.g. !!*

4t cannot undergo * auses Cariation in phenotype/* "election of di3erent phenotypes in di3erent environment

eventually leads to species change

The greater the number of di3erent alleles that all members of aspecies posses, the greater the genetic diversity, the greater thegenetic diversity, the more li2ely that a species will adapt toenvironmental change.


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>* ancer ells often have damaged DNA/* Therefore when this cell containing faulty DNA divides it divides uncontrollably and +A%4D89

producing cancer!* The time cancer cells spend at each phase during mitosis is much less as they are completing

cycles rapidlyA 2ey factor that a3ects cancer is age. Also chemotherapy only 2ills some cancer cells$ hence theothers continue to divide.4n cancer, medical screening is advised so that cancer can be diagnosed early.ancerous tumours result in a mass of cells with an inade;uate blood supply and therefore a low'#ygen environment."ome cancer treatments reduce blood Jow to cancer cells, this helps because

8ess '#ygenB&lucose to cancer cells Therefore less carcinogens

Therefore reduction in spread of cancer cells

4mportance of * DNA has two strands )polynucleotides*, which are joined through bases via Hydrogen (onding/* H0(onding is wea2, therefore DNA HE84A"E )en@yme* brea2s bond and separates strands!* These strands act as template and other free N78E'T4DE" )see above* attach themselves to

their complimentary bases* Adenine joins with Thymine and ytosine joins with &uanine* 'nce activated DNA %'89* &>&rowth %hase hen proteins are produced

/* " "ynthesis %hase hen DNA is replicated$ )&ene


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!* &/"econd &rowth %hase hen organelles grow and divide and energy stores areincreased

/* Nuclear Division N78E7" divides either into two pieces )* %revent (acteria from forming a cell wall/* 4nhibit production of speci=c peptide cross lin2ages

ea2en cell wall so as to allow water to enterfrom high water potential to low waterpotential, therefore cell wall is not strongenough to resist outward pressure, causing'smotic 8ysis.

!* (inds to ribosomes, therefore inhibitstranslation and protein synthesis

* 4nhibits nucleic acid, therefore preventstranscription

Antibiotics do not wor2 against viruses as they have a di3erent covering from bacteria.Antibiotics do not harm


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* * Hori@ontal &ene Transmission/* Cia conjugation!* %lasmid is replicated and then plasmid is transferred

How &ene Transmission and selection have increased diculty of treating bacteria with antibiotics>* Antibiotic resistant gene is produced/* Therefore vertical transmission ta2es place and resistant bacteria reproduce!* This increases fre;uency of resistant gene

* Thus hori@ontal gene transmission ta2es place wherein the resistant plasmid is passed toanother species of bacteria* This is done via conjugation

/ ays in which (acteria becomes resistant to antibiotics>* * * 4t 2eeps changing shape/* Therefore en@yme of bacteria no longer complimentary to new shape of %enicillin!* Therefore %enicillin not bro2en down

"ome farmers add antibiotics into food of cattle, this is illegal, but it is bene=cial for farmers because>* Antibiotics 2ill pathogenic bacteria/* This reduces spread of infection!* Therefore faster growth of animal as less energy wasted =ghting disease

hy this action is illegal>* 4t favours antibiotic resistance/* Therefore resistant bacteria may be formed!* This may cause harm to humans

OVERALL SUMMARY OF STRUCTURE OF 'NA AN' )O+ SE,UENCE OF 'NA IS REPLICATE':>* There are nucleotides present, which contain Deo#yribose, %hosphate and a Nitrogenous (ase/* There are nitrogenous bases Adenine, Thymine, ytosine, &uanine!* Hence DNA has a Msugar0phosphate bac2boneL and contains two polynucleotide strands each

connected with speci=c base pairing* (etween base pairs there are Hydrogen0(onds which can easily be bro2en* 4n DNA +eplication there are MuncoilingL of these strands via the en@yme DNA Helicase


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?* The replication is semi0conservative as original DNA strands can act as a template for newcomplimentary strands, together they form speci=c base pairing

I* Nucleotides of new strand joined together by DNA %olymeraseK* There are now two molecules of DNA, each containing a parent strand and a new standO* DNA is then inserted into NucleusB%lasmids, which in the case of plasmids are self0replicating


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ell Di3erentiation %rocess by which cells become specialisedB cells become adapted for di3erentfunctionsE:A* %artial %ressure of o#ygen is high in lungs/* '/binds to Haemoglobin!* 5orms '#yhaemoglobin* 4n red blood cells, Haemoglobin has a high anity to '#ygen* Dissociation occurs when partial pressure of '/is low?* Dissociation is increased by high '/pressure

I* %artial %ressure of '#ygen is low in respiring tissueK* Therefore Haemoglobin has low anity to '#ygenO* Dissociation curve shifts to the right

Throughout our whole body this graph will be moving from left to right shift constantly.An easy way to remember this is 8E5T08'AD4N&087N&", +4&HT0+E8EA"4N&0+E"%4+AT4'N'nce all the Haemoglobin groups are attached to '#ygen, increasing the partial pressure of '#ygenwill have no e3ect.During respiration

urve moves to right as concentration of '/is high whilst '/concentration is low

Advantage of having curve to the left in people who live at high altitudes>* &reater anity of Haemoglobin to '#ygen at lower partial pressure of '#ygen

/* Higher saturation of Haemoglobin with '#ygen as high altitudes have low partial pressure of'#ygen in lungs%eople who live at high altitudes have more red blood cells, the advantage of this is


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>* 5oetus has higher o#ygen anity at lower partial pressure of '#ygen/* At partial pressures when adult haemoglobin dissociates from '#ygen, foetus haemoglobin

ta2es up '#ygen from motherLs bloodDisadvantage of having graph too much to the left

>* Dicult to dissociate/* Therefore cells su3er

Advantage of having curve to the right during e#ercise>* Haemoglobin has a lower anity for '#ygen/* Therefore more '/for respiration

How having a lot of Haemoglobin can help an athlete>* * %artial pressure of '#ygen very low in muscle cells/*


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The %alisade plant cell as show is adapted so as to allowma#imum photosynthesis to ta2e place

>* . Thin Jat shape to provide large surface area

/. * "horter stem$ less energy used in stem growth/* Thic2er root$ storage!* 8arger leaves$ %hotosynthesis

4f a plant grows ;uic2er, then this is bene=cial for a farmer as it will produce its fruits ;uic2er.


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'verall "tructure of ellulose>* lin2/* Alternate molecules MJip overL

Any time scientists wish to see if a cell has any of the above in it, they will cut a thin section of atissue.

4t has to be a TH4N "ET4'N because>* 'nly a single layer of cells is needed /* 8ight must be ableto pass throughays in which ellulose is similar to "tarch>* (oth are polymers$ made of monomers/* (oth have monomers joined by condensation!* (oth have >0 lin2s* (oth contain arbon, Hydrogen and '#ygen

"TA+H &89'&EN E8878'"E


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* (oth are made up of &lucose?* (oth are insolubleI* (oth contain &lycosidic (ondsays in which ellulose is di3erent to "tarch

>* "tarch made up of 0glucose molecules, whereas ellulose made of 0glucose molecules/* "tarch is heli#Bcompact, whereas ellulose is straight!* "tarch forms no micro =brils, whereas ellulose does form micro =brils


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'ne way by which di3erent species may be distinguished 4nter0breed to produce infertile o3springEvery species has a (4N'


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4t is still better to compare the base se;uences of &ENE" rather than comparing structure ofproteinsBamino acid se;uence, this is becauseDE&ENE+A9 '5 T+4%8ET 'DE in each species can becompared.Another way that scientists compare two species is through 4


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>* Have a small surface area volume ratio, i.e. their surface area is ;uite small compared to theirvolume.

/* Therefore they have specially adapted gas e#change surfaces!* Also, the di3usion is slow as di3usion depends on surface area* The cells of a large organism are far away, therefore they must be supplied to by a transport

system* 8arge animals that are more active have high metabolic rate, therefore they +espire * Air enters through open spiracles/* Then through trachea where there is a di3usion gradient!* The trachea is closely associated with cells* '#ygen di3uses into cells of muscle =bre* Centilation replaces air in trachea

ATE+ 7"7A889 E"A%E" HEN THE "%4+A8E" '%EN T' +E8EA"E '/"piracles in an insect open

hen concentration of '/increases

oncentration of '#ygen falls in tracheaBtracheoles because

'#ygen is used in respiration, therefore it di3uses from trachea into tissues

How an insect limits water loss>* (ody covered with waterproof wa#y layer/* "piracles are able to close, they will close in dry conditions to maintain water and prevent

dehydration!* "un2en spiracles to trap moist air* Trachea cuticle lined so as to only lose water through tracheoles

The many small branching tracheoles provide a 8A+&E "7+5AE A+EA.During this process you also have muscles which

reate large movement of gases in and out of trachea

This speed up e#change of respiratory gases

Also, the gas e#change system has a large surface area as well as the tracheoles are (+ANHED,inside the muscle =bre.5eatures of an insectLs gas e#change system that allows ecient o#ygen supply

>* 8ots of tracheoles give large surface area, loss of water from tracheoles also helps '/di3usefrom tracheoles

/* "hort distance between tracheoles gives short di3usion pathway!* Di3usion through


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'ur lung alveoli have a moist surface$ this is because Alveolar epithelium is permeable to smallmolecules such as water.

The diagram shows how there is '7NTE+07++ENT 58' system in gas e#change in =sh.The structure of gills allows ecient '#ygen upta2e because>* * less energy needed /*continuous Jow of waterHow a =sh maintains a 58' '5 ATE+ over its gills

>* * ater too dense to move in and out of lungs/* 8ow o#ygen concentration in water!* High metabolic rate in terms of activity in mammals

How a oncentration gradient is maintained between a =sh gill and surrounding water>* irculatory system brings in blood

with low concentration of '#ygen dueto respiration etc./* There is also a counter0current system!* Also the water surrounding the =sh is

highly o#ygenated* Also the =sh has a ventilation system

to replace waterHow gills of a =sh are adapted for ecientgas e#change

Adaptation How it helps

8arge surface area provided bylamella

4ncreases di3usion

Thin epithelium between water andblood

"horter di3usion pathway

ater and blood Jow in oppositedirection


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"ome =shes have lungs and gills$ this means that if they perform gas e#change through lungs a lot aswell, then it is li2ely that they can survive even after water dries up$ this is because gills do notfunction as eciently in air as they do in water.5eatures of Transport "ystem

>* A suitable transport medium/* 5orm of mass transport in which transport medium is moved!* losed system of tubular vessels that contain transport medium* * Thin alls$ because it does not have to withstand high pressure as there is littledi3erence in a vein between the ma#imum pressure and minimum pressure

/* Calves throughout$ prevent bac2Jow when muscle contract and increase pressure,valves close when pressure at one side of the valve is higher than pressure at theother side of the valve

!* * Thin wall$ allows "H'+T D4557"4'N %ATHA9/* Numerous and highly branched$ provides large surface area for di3usion!* Narrow lumen$ reduces di3usion distance* 'nly endothelium present$ short di3usion pathway


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* Narrow diameter$ permeate tissues, i.e. ensures that no cell is far from capillary?* (lood Jow in capillaries is slow because

&ives substances time to pass through wall of capillary

Therate ofbloodJow

decreases from Aorta to apillaries, this is because 8arge increase in total cross0sectional area.+emember Arteries will contain '#ygen, &lucose, Amino Acids, 8ipids, etc. Ceins will not.(oth capillaries and arteries are similar in the way that they both have an endothelium./ di3erences between veins and arteries

>* Arteries have thic2er muscular wall

/* Arteries have more elastic tissue!* Cein have relatively larger lumen* Ceins have valves

hy total number of Arterioles is more than total number of main arteries

Arteries divide to form arterioles

hy the total number of main Arteries is same as total number of ain veins

(lood goes to an organ along an artery and leaves by a vein

The more active a muscle, the more blood Jow there is to the capillaries of that muscle, this helpsbecause

>*


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The above picture shows how tissue Juid at the Arterial end containing '/and ater is passed alongthe capillary at the Cenule end,where it now contains '/andwaste material.

The reason of net loss of aterfrom the capillary at the arterialend

Hydrostatic

%ressureB(lood %ressuregreater than pressure ofwater potential gradient

The reason as to why there is ahigh Hydrostatic %ressureB(lood%ressure at the arterial end

ontraction of leftventricle in heart

hy Hydrostatic %ressuredecreases as blood moves alongcapillary

5rictional force from contact with wall

8oss of Juid

Total volume of Juid going from capillary is greater than volume coming bac2 in, this is because

"ome Juid is removed by the 8ymphatic Node, which returns the Juid to the bloodThe blood Jow in capillaries is slow, this is because

To allow more time for di3usion of substances

Tissue Juid accumulates in the tissues of people who do not eat enough protein, this is because

4n an unhealthy person, less protein in blood

Therefore water potential gradient lower at venule end

8ess water reabsorbed into capillaries via osmosis

How tissue Juid is formed>* High hydrostatic pressure at Aortic end pushes Juid out/* 8arge proteins however, remain in blood as they cannot pass through capillary wall

How tissue Juid is returned to capillary

>* Hydrostatic pressure lower in capillary at venule end/* ater returns via osmosis from high water potential to low water potential!* 8ow water potential in capillary caused by proteins in blood, i.e. proteins more concentrated

due to less or no water* Tissue Juid also returns to the circulatory system via lymph vessels

Also remember that (lood %lasma AND Tissue 5luid contain glucose and sodium ions$ however Tissue5luid contains less '#ygen as '#ygen is used by respiring cells.hy water potential is of (lood %lasma is lower than (lood %lasma at venule end

>* %lasma proteins too large to leave capillary/* ater lost, i.e. plasma proteins more concentrated

The lymph vessel has valves$ these valves assist the return of lymph to blood plasma>* "top bac2Jow

/* No pumping actionThe function of the 8ymphatic vessels

>* Drains e#cess Juid/* %revents accumulation of Juid!* Transports fats

'smosis Di3usion through a partially permeable from less

negative water potential


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to a more negative waterpotential

:ylemTransports water and ions up the root, and then up the stem, it has ligni=ed walls to providesupport

%hloemTransports soluble food molecules up the plant such as sucrose$ it does this using ActiveTransport and thereforere;uires AT%

How an increase in rate of transpiration a3ects water potential of leaves>* Transpiration involves evaporation of water through the stomata/* +educed water content lowers water potential

How a decrease in water content of soil a3ects the rate of transpiration

+educes transpiration as less water upta2e, therefore water potential gradient across leaves and

atmosphere reducedHow water enters root hair cells

>* Active transport of mineral ions by the root hair cells reduce water potential gradient/* Therefore water enters down the water potential gradient via osmosis

H' ATE+ &ET" T' THE :98E* ater moves through intra0cellular cellulose of E88 A88 =bres of ortical ells/* 4t is caused by transpiration pull and water moves down a concentration gradient!* ater movement is by cohesive properties of water* ater moves in continuous column until it reaches endodermal layer* Endodermal cells contain caparian strip, which is a water proof substance, at this point water

cannot pass?* ater Jips to the "9* hole process ta2es place through cytoplasm )N'T CA7'8E* of cells/* ater passes from cell to cell through plasmodesmata from high water potential to low water

potential via osmosis!* Therefore a continuous column of cytoplasm present from root hair cell to #ylem

hy all water passes through the END'DE+


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>* Endodermis has waterproof strip so water must cross cell membrane go via "ymplastic %athway/* * * The stomata are covered/* The thic2 substance gives longer di3usion pathway!* Therefore less transpirationHow rate of Jow of water in #ylem is highest at midday>* "tomata are open/* Transpiration highest around midday as middle of day is lighter and warmer!* This increases the tension as more water is pulled up* Therefore, there is cohesion between water moleculeshy :ylem diameter is least during midday

>* ater pulled up trun2

/* ater column under tension!* Adhesion between water molecules and #ylem* :ylem pulled in

The values of pressure in :ylem are negative because>* %ressure inside :ylem lower than atmospheric pressure/* ater is under tension


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How the plant uses water

reate turgor

7se as solvent for transport

7se as a medium for chemical reactions

'CE+A88, H' ATE+ ENTE+" 5+'< +''T T' :98E* Through the open stomata/* Down the water potential gradient!* (y D4557"4'N )do not mention osmosis here*

"ometimes, the "tomatal openings are remaining constant, however, the rate of transpirationincreases, this is due to 4N+EA"E 4N TE* Di3usion gradient not as steep as diameter in stomata increases/* "maller stomata have more MedgeL of area


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The diameter of a trun2 decreases as rate of Jow of water increases because>* 4ncreasing transpiration/* Transpiration creates tension!* ater molecules are cohesive, therefore adhesive forces between #ylem and water* %roduces water column

A %otometer measures rate of ATE+ 7%TAFE )cm!per minute* via the following apparatusHow the air bubble is returned to start ofcapillary tube

'pen tap and add water from reservoir

%recautions to be ta2en to obtain reliable

results>* "eal joints/* ut shoot under water and cut shoot at

slant!* Dry o3 leaves* 4nsert into apparatus under water* Note where bubble is at the start?* Ensure no other air bubbles present

* 8ight

/* Humidity

!* Air


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4f any of the above ta2es place, the Apoplastic %athway will decrease as less water will have been lostby the leaves, therefore, decreasing the water potential gradient.:erophytic %lants%lants that do not have plentiful water therefore have adaptations to limit water0loss through transpiration

The best #erophytic plant that will survive is the one with the shallowest roots, i.e. 4DE spread ofroots for rapid absorption.%opulationTotal number of organisms of a species in an area(io DiversityCariety of living organisms, the range of variety of genes, species, and habitat within aparticular regionEthical argument for preserving (iodiversity %revents e#tinction and loss of habitats

"pecies Diversity Number of di3erent species and number of individuals of each species within onecommunity&enetic DiversityDi3erences in DNA se;uenceEcosystem DiversityCariety of habitats within a particular areahat we need to wor2 out 4nde# of Diversity

Total number of organisms of all species

Number of organisms of each species

How you would =nd the number of individuals of one species in a habitat>* "uitable method of capture/*


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>* 4ncrease in variety of plants/* 4ncrease in variety of plants allows more habits!* &reater variety of food sources* 4nsect diversity will usually be proportional to plant diversity

hy using inde# of diversity is a better measure of biodiversity than counting the di3erent number ofspecies

>* * Deliberately done by humans/* %ermanent clearing of forests, then the space used for agriculture,housing!*


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"amplingTa2ing measurements of individual species from a selected population of organisms beinginvestigated+easons why sample may not represent of whole population

>* "ampling (ias investigators may ma2e unrepresentative choices/* hance Even if sampling bias has been avoided you may still get a bias sample by chance

+andom "ampling>* Divide study area into grid of numbered line/* 7sing random numbers, from a table or generated by computer, obtain a series of coordinates!* Ta2e samples at the intersection of each coordinates of number of individuals and number of

species* +epeat many times and ta2e a mean

"ometimes, you may even lay ;uadrants every >Q metres rather than use random sampling, this isbecause

>* 4t is systematic sampling/* To establish a pattern

4ncreasing +eliability

8arger sample si@e$ produces more reliable mean and ensures representative sample

+andom number generator$ removes bias

that 4sotonic "olutions can be used to dilute solutions because>* They have the same water potential/* Therefore no osmosis!* Therefore cells in solution will not burst

"tandard Deviation


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4n &raph Analysis and Drug Trials

>* hen describing a graph, mention pea2 points, increase and decrease in graph and overallincreaseBdecrease in graph

/* "ometimes the evidence from the graph may not be enough to draw a certain conclusion. H9

Any correlation does not mean there is a causal relationship

There may some other factor producing a riseBfall in both factors

"ometimes, there is no relation between both factors further along the graph

Also if a control group was not used, then e3ectiveness cannot be measured

5luctuations in a graph are a clear sign that conclusions cannot be drawn from it

4n graphs, where conclusions are being drawn regarding a disease, chec2 if it saysMreported casesL, if it does you can mention about how we donLt 2now the full amount ofdisease cases as many were not reported and THE+E5'+E we canLt draw conclusions.

!* "ometimes, data is given in a logarithmic form$ this e#aggerates the numbers and allows thee3ect of low numbers to be seen, therefore it increases the range of values. "ometimes, data isgiven in a ratio$ this allows comparison as it shows proportional change.

* 4n some graphs, medical cases are presented for the prevalence of a certain disease$ however,the number presented may not be the actual number of people with that disease as manypeople do not go the doctors.

* hen an investigation involves metabolism, scientists may e#periment on people of di3erentages as metabolism rate di3ers on age.

?* hen it as2s you to describe a graph, be sure to mention a pea2 point if there is one, if not,describe the correlation, i.e. whether it is negative or positive.

I* "ometimes data on a graph is given in a percentage, the advantage of this is>* Allows comparison/* "hows proportional hange as sometimes the initial weightBsi@eBvolume etc. of substances

is di3erent.K* E#periments are repeated for the following reasons

>* Allows anomalies to be identi=ed/* Q* hen as2ed to give an evaluation on a claim or statement, mention all the points that

indicate the claim or statement to be true, and then mention how it could be false.>>* 4f it as2s you why a drug may not be perfect

7n2nown 8ong0Term side e3ects

"tudy should be carried out on humans and the study should be repeated

>/* (efore carrying drug trials, scientists should consider the following regarding the drug

Dose to be given

No serious side e3ects

How e3ective

ost of drug

>!* hen ta2ing drug trial, scientists should consider the following of volunteers

Age

Health &ender

Ethnicity

&enetic 5actors

8ifestyle

(ody


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>* hen scientists wish to use an animal in a test, before deciding the number of animals touse they must consider

Ethical onsideration

Ta2e a large number to improve reliability

onsider cost and space available

>* hen e#periment involves volunteers, they must be healthy as they will have MnormallyLfunctioning bodies. Also when dividing volunteers into groups, it should be done randomly toavoid bias.

>?* 4f scientists use two drugsBvaccines and both combined have higher e3ect, then bothmust be similar i.e. they must have the same antibodies, etc.

>I* Any time doctors wish to see how well the lungs wor2, they will use the gas arbon

K* (lood Jow is given per gram of tissue as organs di3er in si@e, therefore it allowscomparison

>O* How mean can be found from a graph

Draw line of best =t

Then =nd the gradient and divide it by the distance moved/Q* Any e#periment wherein the words MwaterL and mention of the word Mpartially permeableL

come together, you should automatically realise that water potential will be involved somehow.

/>* "ometimes, in an e#periment, they e#periment on people of all the same age this is sothat a comparison can be made.

//* hen in an e#periment, people are assigned to di3erent groups +AND'


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begin to observe same results, and therefore @erochanges. E#aminers could as2 what did he observewhen the graph levelled o3 The answer is nothing, @ero.

Also be wary of cases where they present data regarding chic2enpo#, mumps and rubella as childrenwere made to catch these diseases by their parents as the symptoms of these diseases at childhoodare much less severe than they are after puberty, and these diseases ta2e place only once usuallyduring oneLs lifetime.

"TANDA+D DEC4AT4'N Degree of variation from the mean.+he" Sta".a#. 'e$iati!"s !$e#&a%( !"&0si!"s a""!t 7e ma.e as eeti$e&/ as #es0&tsma/ 7e .0e t! C)ANCE, if standard deviations do not overlap, we say the di3erence is +EA8.

hy at times "tandard Deviation is better than the mean>* +ange only shows highest and lowest values/* Also it is possible to have two very di3erent data sets with the same range!* Also range is a3ected by a single outlier* "tandard Deviation shows spread about the mean and allows statistical use

aqa a level biology unit 2 notes

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