Water molecule hydrogen bond. Water molecules stick to each other because of the hydrogen bonds that form between them.

Transcription

1 3 Wate,TH.E CHEMSTRY..OF LFE POLARTY OF WATER Wate molecules consist of two hydogen atoms bonded to an oxygen atom. The hydogen atoms have a slight positive chage and the oxygen atom has a slight negative chage. So, wate molecules have two poles a positive hydogen pole and a negative oxygen pole (below). This featue of a molecule is called polaity. HYDROGEN BONDNG N WATER A bond can fom between the positive pole of one wate molecule and the negative pole of anothe. This is called a hydogen bond. n liquid wate many of these bonds fom, giving wate popeties that make it a vey useful substance fo living oganisms. The diagam (below) shows a hydogen bond between two wate molecules. Wate molecule hydogen bond Hydogen pole is } slightly positive Oxygen pole is slightly negative THE PROPERTES OF WATER Name of the popety Cohesion Solvent popeties Themal popeties: heat capacity Themal popeties: boiling point Themal popeties: the cooling effect of evapoation Outline of the popeties of wate Wate molecules stick to each othe because of the hydogen bonds that fom between them. Many diffeent substances dissolve in wate because of its polaity (below). noganic paticles with positive o negative chages dissolve, fo example sodium ions. Oganic substances with pola molecules dissolve, fo example glucose. Enzymes also dissolve in wate. Wate has a lage heat capacity -lage amounts of enegy ae needed to aise its tempeatue. The enegy is needed to beak some of the hydogen bonds. The boiling point of wate (100 1 '0 is high, because to change it fom a liquid to a gas all of the hydogen bonds between the wate molecules have to be boken. Wate can evapoate at tempeatues below boiling point. Hydogen bonds have to be boken to do this. The heat enegy needed to beak the bonds is taken fom the liquid wate, cooling it down. ons with positive o negative chages dissolve as they ae attacted to the negative o positive poles of wate molecules. l] Relationship between the popeties of wate and its uses in living oganisms Stong pulling foces can be exeted to suck columns of wate up to the tops of the tallest tees in thei tanspot systems. These columns of wate aely beak. Wate is used as a tanspot medium in the xylem of plants. Most chemical eactions in living oganisms take place with all of the substances involved in the eactions dissolved in wate. Wate is the medium fo metabolic eactions. The solvent popeties of wate allow many substances to be caied dissolved in wate in the blood of animals and the sap of plants. Wate can be used as a tanspot medium. Blood, which is mainly composed of wate, can cay heat fom wame pats of the body to coole pats. Blood is used as a tanspot medium fo heat. Wate is below boiling point almost eveywhee on Eath, and in most aeas it is above feezing point. As a liquid, athe than a solid o a gas, it can act as the medium fo metabolic eactions. Evapoation of wate fom plant leaves (tanspiation) and fom the human skin (sweat) has useful cooling effects. Wate can be used as a coolant. -::- cs: ', g, Many molecules ae pola 50 ae attacted to wate molecules and dissolve.

2 Elements and compounds in living oganisms ELEMENTS N LVNG ORGANSMS Living oganisms contain many chemical elements, some in lage quantities and some in vey small amounts. The fou commonest chemical elements of life ae cabon, hydogen, oxygen and nitogen. They ae pat of all the main oganic compounds in living oganisms. Examples of othe elements that ae needed ae shown in the table opposite. ORGANC AND NORGANC COMPOUNDS Living oganisms contain many chemical compounds. Some of them ae oganic and some ae inoganic. Oganic compounds ae defined as compounds containing cabon that ae found in living oganisms. Thee ae a few cabon compounds that ae inoganic even though they can be found in living oganisms. These ae all simple cabon compounds that ae also widely found in the envionment. Cabon dioxide, cabonates and hydogen cabonates ae thee examples of inoganic cabon compounds. All compounds that contain no cabon ae inoganic. Thee types of oganic compound ae found in lage amounts in living oganismscabohydates, lipids and poteins. -UB-UNTS OF ORGANC MACROMOLECULE] The molecules of many oganic compounds ae lage and so ae called macomolecules. They ae built up using small and elatively simple subunits. Some impotant subunits ae shown below Subunits of poteins, cabohydates and lipids CHEMCAL ELEMENTS AND THER ROLES Element and symbol Sulphu S Calcium Ca Phosphous P on Fe Sodium Na Role in plants, animals and pokayotes Needed to make two of the twenty amino acids that poteins contain Ads as a messenge, binding to calmodulin and othe poteins that egulate pocesses inside cells, including tansciption Pat of the phosphate goups in ATP and DNA molecules Needed to make cytochomes -poteins used fo electon tanspot duing aeobic cell espiation Pumped into the cytoplasm to aise the solute concentation and cause wate to ente by osmosis These elements have othe specific oles in some oganisms. Fo example, ion is needed to make hemoglobin in many animals and calcium is needed to make the mineals that stengthen bones and teeth CH 2 0H 0 H / C 1\ H H C H \1 /oh C-C 1 OH OH ibose (a monosacchaide) H R 0 \ / N-C-C H \OH H amino acids (each of the twenty amino acids in poteins has a diffeent R goup) H CH 2 0H c--o H!/ \! C H C \OH OH H OH glucose (a monosacchaide) OH (CH 3 ) -- (CH ) -.- / 2n -C \ OH fatty acids (geneal stuctue) o O,\/OH H fatty acid (numbe of cabon atoms and bonding between cabon atoms vaies)

3 Building macomolecules CONDENSATON REACTONS n a condensation eaction two molecules ae joined togethe to fom a lage molecule. Wate is also fomed in the eaction. Fo example, two amino acids can be joined togethe to fom a dipeptide by a condensation eaction. The new bond fomed is a peptide linkage. Condensation of two amino acids to fom a dipeptide and wate o R R 0 R 0 H H / /0 H / N-C C N-C-C -+- N-C-C -N-C-C +H 0 / / H/ 2 H H OH H H OH H H H OH Futhe condensation eactions can link amino acids to eithe end of the dipeptide, eventually foming a chain of many amino acids. This is called a polypeptide. n a simila way, condensation eactions can be used to build up cabohydates and lipids. The basic subunits of cabohydates ae monosacchaides. Two monosacchaides can be linked to fom a disacchaide and moe monosacchaides can be linked to a disacchaide to fom a lage molecule called a polysacchaide. Fatty acids can be linked to glyceol by condensation eactions to poduce lipids called glyceides. A maximum of thee fatty acids can be linked to each glyceol, poducing a tiglyceide. HYDROL YSS REACTONS lage molecules such as polypeptides, polysacchaides and tiglyceides can be boken down into smalle molecules by hydolysis eactions. Wate molecules ae used up in hydolysis eactions. Hydolysis eactions ae the evese of condensation eactions. polypeptides + wate --.. dipeptides o amino acids polysacchaides + wate glyceides + wate --.. disacchaides o monosacchaides --.. fatty acids + glyceol EXAMPLES OF CARBOHYDRATES Examples Example of use in animals Example of use in plants Monosacchaides glucose Glucose is caied by the blood to Fuctose is used to make fuits galactose tanspot enegy to cells thoughout sweet-tasting, attacting animals fuctose the body to dispese seeds in the fuit Disacchaides maltose lactose is the suga in milk, that Sucose is caied by phloem to lactose povides enegy to young mammals tanspot enegy to cells thoughout sucose until they ae weaned the plant Polysacchaides stach Glycogen is used as a shot-tem Cellulose is used to make stong glycogen enegy stoe in live and in fibes that ae used to constuct cellulose muscles the plant cell wall FUNCTONS OF LPDS Enegy stoage in the fom offat in humans and oil in plants Heat insulation - a laye of fat unde the skin educes heat loss Buoyancy lipids ae less dense than wate so help animals to float J CARBOHYDRATES AND LPDS N ENERGY STORAGE Both lipids and cabohydates have advantages as enegy stoage compounds in living oganisms. Cabohydates ae usually used fo enegy stoage ove shot peiods and lipids fo long-tem stoage. Advantages of lipids 1. Lipids contain moe enegy pe gam than cabohydates so stoes of lipid ae lighte than stoes of cabohydate that contain the same amount of enegy 2. Lipids ae insoluble in wate, so they do not cause poblems with osmosis in cells Advantages of cabohydates 1. Cabohydates ae moe easily digested than lipids so the enegy stoed by them can be eleased moe apidly 2. Cabohydates ae soluble in wate so ae easie to tanspot to and fom the stoe

4 -"- ntoducing DNA THE NUCLEOTDE SUBUNTS OF DNA Although DNA is the genetic mateial of living oganisms and is theefoe of immense impotance, it is made of elatively simple subunits. These ae called nucleotides. Each nucleotide consists of thee pats - a suga (called deoxyibose), a phosphate goup and a base. n diagams of DNA stuctue these ae usually shown as pentagons, cicles and ectangles, espectively. The figue (below) shows how the suga, the phosphate and the base ae linked up in a nucleotide. pbo;;;;': suga b, DNA nucfeotides do not all have the same base. Fou diffeent bases ae found adenine, cytosine, guanine and thymine. These ae usually simply efeed to as A, C, G and T. -{j:j::[ A BULDNG DNA MOLECULES Two DNA nucfeotides can be linked togethe by a covalent bond between the suga of one nucfeotide and the phosphate goup of the othe. Moe nucleotides can be added in a simila way to fom a stand of nucfeotides. DNA molecules consist of two stands of nucfeotides wound togethe into a double helix. Hydogen bonds link the two stands togethe. These fom between the bases of the two stands. Howeve, adenine only foms hydogen bonds with thymine and cytosine only foms hydogen bonds with guanine. This is called complementay base paiing. DNA REPLCATON DNA eplication is a way of copying DNA to poduce new molecules with the same base sequence. t is semi-consevative - each molecule fomed by eplication consists 01 one new stand and one old stand conseved fom the paent DNA molecule. Stage 1 The DNA double helix is unwound and sepaated into stands by beaking the hydogen bonds. Helicase is the main enzyme involved. \ Stage 2 The single stands act as templates fo new stands. Fee nucleotides ae pesent in lage numbes aound the eplication fok. The bases of these nucleotides fom hydogen bonds with the bases on the paent stand. The nucleotides ae linked up to fom the new stand. DNA polymease is the main enzyme involved. The two daughte DNA molecules ae identical in base sequence to each othe and to the paent molecule, because of complementay base paiing (A pais with T and C with G). Each of the new stands is complementay to the template on which it was made and identical to the othe template. Stage 3 The daughte DNA molecules each ewind into a double helix J...

5 ,-----,--,-, ,- - '1 Tansciption and tanslation GENES AND POLYPEPTDES polypeptides ae long chains of amino acids_ Thee ae twenty diffeent amino acids that can! fom pat of a polypeptide, To make one paticula polypeptide, amino acids must be linked up in a pecise sequence. Genes stoe the infomation needed fo making polypeptides. The infomation is stoed in a coded fom. The sequence of bases in a gene codes fo the sequence of amino acids in a polypeptide. The infomation in the gene is decoded duing the making of the polypeptide. Thee ae two stages in this pocess: tansciption and tanslation. DFFERENCES BETWEEN DNA AND RNA DNA and RNA both consist of chains of nucleotides, each composed of a suga, a base and a phosphate. Thee ae thee diffeences between them. Featue Numbe of stands in the molecule Type of suga in each nucleotide Types of bases contained DNA Two stands foming a double helix Deoxyibose RNA One stand only Ribose A, C, G and T A, C, G and U Uacil eplaces thymine TRANSCRPTON nstead of the DNA of genes being used diectly to diect the synthesis of polypeptides, a copy is made. The copy is RNA. t caies the infomation needed to make a polypeptide out into the cytoplasm, so is called mrna (messenge RNA). The copying of the base sequence of a gene by making an RNA molecule is called tansciption. n tansciption, the same ules of complementay base paiing ae followed as in eplication, except that uacil pais with adenine, as RNA does not contain thymine. The RNA molecule poduced theefoe has a base sequence that is complementay to the tanscibed stand and identical to the othe DNA stand except that U eplaces T. 1. The DNA double helix uncoils and the two stands sepaate. Tansciption moves along in this diection. \,-.'/ --< 2, Fee RNA nucleotides ae assembled using one of the two DNA stands as the template (the tanscibed stand). ' 1<"......,. T Y The RNA nucleotides ae linked up to fom a stand of RNA. The mrna sepaates fom the DNA. Stages 1! 2 and 3 ae all caied out by the enzyme RNA polymease. TRANSLATON. Tanslation is caied out by ibosomes, using mrna and trna. t is the genetic code that is being tanslated. The genetic code is a tiplet code thee bases code fo one amino acid. A goup of thee bases is called a codon. 1. Messenge RNA binds to the small subunit of the ibosome. The mrna contains a seies of codons, each of which codes fo one amino acid. 2. Tansfe RNA molecules ae pesent aound the anticodon ls Each trna has a sped al. tiplet of bases called an / anticodon and caies the amino acid coesponding to this anticodon. \ \ amino acid lage subunit ) of ibosome mrna small subunit of ibosome L ' 3. trna n cules bind to the ibosome. Two can bind at once. trna can only bind if it has the anticodon that is complementay to the codon on the mrna. The bases on the codon and anticodon link togethe by foming hydogen bonds, following the same ules of complementay base paiing as in eplication and tansciption. 4. The two amino acids caied by the trna molecules ae bonded togethe by a peptide linkage. A dipeptide is fomed, attached to the trna on the ight. The trna on the left detaches. The ibosome moves along the mrna to the next codon. Anothe trna caying an amino acid binds. A chain of thee amino acids is fomed. These stages ae epeated until a polypeptide is fomed... J-"--'--'--'---'-- diection of movement of ibosome

6 Genes, polypeptides and enzymes ONE GENE-ONE POLYPEPTDE HYPOTHESS Genes detemine the amino acid sequence of poteins. Howeve, some poteins contain moe than one type of polypeptide. Hemoglobin is an example of this it contains two diffeent types of polypeptide. t was found that a diffeent gene is needed to make each polypeptide. Futhe eseach has shown that thee is almost always a single gene to code fo a polypeptide, which does not code fo any othe polypeptide. This discovey led to an impotant hypothesis in molecula biology - the one gene-one polypeptide hypothesis. Thee ae some exceptions to this geneal ule: Some genes code fo tansfe RNA o messenge RNA, not fo polypeptides. Some DNA sequences act as egulatos of gene expession and ae not tanslated into polypeptides. n lymphocytes, pieces of DNA fom diffeent pats of the genome ae spliced togethe and tanscibed and tanslated to poduce antibodies. Diffeent lymphocytes poduce diffeent antibodies by splicing togethe DNA inheited fom paents, in diffeent ways. Stages in enzyme catalysis substate Substate molecules ae in continual andom motion. f one collides with the active site it can bind to it. enzyme-substate complex, NTRODUCNG ENZYMES Catalysts speed up chemical eactions without being changed themselves. Living oganisms make biological catalysts called enzymes. Enzymes ae globula poteins which act as catalysts of chemical eactions. Without enzymes to catalyse them, many chemical pocesses happen at a vey slow ate in living oganisms. By making some enzymes and not othes, cells can contol what chemical eactions happen in thei cytoplasm. The stuctue of enzymes is quite delicate and can be damaged by vaious substances and conditions. This is called denatuation. Denatuation is changing the stuctue of an enzyme (o othe potein) so that it can no longe cay out its function. Denatuation is usually pemanent. n chemical eactions, one o moe eactants ae conveted into one o moe poducts. n eactions catalysed by enzymes, the eactants ae called substates. The substate fits the active site. f othe molecules collide with the active site they do not fit and fail to bind. ENZYME-SUBSTRATE SPECFCTY Most enzymes ae specific - they catalyse vey few diffeent eactions. They theefoe only have a vey small numbe of possible substates. This is called enzyme-substate specificity. The substates bind to a special egion on the suface of the enzyme called the active site. An active site is a egion on the suface of an enzyme to which substates bind and which catalyses a chemical eaction involving the substates. The active site of an enzyme has a vey inticate and pecise shape. t also has distinctive chemical popeties. Active sites match the shape and chemical popeties of thei substates. Molecules of substate fit the active site and ae chemically attacted to it (ight). Othe molecules eithe do not fit o ae not chemically attacted. They do not theefoe bind to the active site. This is how enzymes ae substate-specific. The way in which the enzyme and substate fit togethe is simila to the way in which a key fits a lock. The enzyme is like the lock and the substate is like the key that fits it. The active site catalyses a chemical eaction. The substates ae tuned into poducts. enzyme _.-J.. 1'. *,. '" 1_... "'''. (.,...'.'''_ "",0....,',.., 1#-, ' " , '.\..\.1;''''' " f:l.,. ;.;J!, i t;'...>';.,,,...(... ' """.,,! '.., f. ":'1... t." flit,. "..:," "..!...., '. "." t....._... '.('i, -1_, 't,. -' ' ".'"'' t..., ',... «;.. t., )\,/,..,/':l.'.'...,.,' t.,...!,- "''. " J..., !','" 9"...',' ". ". -', s....,t..,... V!'.\."",. t". -" -. ""'-.... poducts The poducts detach fom the active site, leaving it fee fo moe substate to bind..,

7 '1 Enzymes in action FACTORS AFFECTNG ENZYME ACTVTY /' Wheeve enzymes ae used, it is impotant that they have the conditions that they need to wok effectively. TempeatUje,' ph and substate concentation all affect the ate at which enzymes catalyse chemical eactions. The figues (below and ight) show the elationships between enzyme activity and substate concentation, tempeatue and ph. L.. EFFECT OF SUBSTRATE CONCENTRATON At low substate concentations, enzyme activity inceases steeply as substate concentation inceases. This is because andom collisions between substate and active site happen moe fequently with highe substate concentations. EFFECT OF TEMPERATURE Enzyme activity inceases as tempeatue inceases, often doubling with evey 10 DC ise. This is because collisions between substate and active site happen moe fequently at highe tempeatues due to faste molecula motion. c : u o E'" >.. N c: LU At high substate concentations most of the active sites ae occupied, so aising the substate concentation has little effect on enzyme activity. 1 c : '" N c: LU Tempeatue EFFECT OF ph Optimum ph at which enzyme activity is fastest (ph 7 is optimum fo most enzymes). At high tempeatues enzymes ae denatued and stop woking. This is because heat causes vibations inside enzymes which beak bonds needed to maintain the stuctue of the enzyme. Substate concentation _ LACTASE AND LACTOSE FREE MLK Lactose is the suga that is natually pesent in be conveted into glucose and galactose by the enzyme lactase. lactase lactose )10 glucose + galactose Lactase is obtained fom Kluveomyces lactis, a type of yeast that gows natually in milk. Biotechnology companies cultue the yeast, extact the lactase fom the yeast and puify it, fo sale to food manufactuing companies. Thee ae seveal easons fo using lactase in food pocessing: Some people ae lactose intoleant and cannot dink moe than about 250ml of milk pe day unless it is lactoseeduced. Galactose and glucose ae sweete than lactose, so less suga needs to be added to sweet foods containing milk, such as milk shakes o fuit yoghut. 1 c : u o v N c: UJ ph )0 As ph inceases o deceases fom the optimum, enzyme activity is educed. Both acids and alkalis can denatue enzymes. Lactose tends to cystallize duing poduction of ice ceam, giving a gitty textue. Because glucose and galactose ae moe soluble than lactose they emain dissolved, giving a smoothe textue. Bacteia fement glucose and galactose moe quickly than lactose, so the poduction of yoghut and cottage cheese is faste. Lactase is used in two ways duing food pocessing: 1. t can be added to milk. The final poduct contains the enzyme. 2. t can be immobilized on a suface o in beads of a poous mateial. The milk is then allowed to flow past the immobilised lactase. This avoids contamination of the poduct with lactase.

8 ,. Cell espiation and enegy ENERGY AND CELLS All living cells need a continual supply of enegy. This enegy is used fo a wide ange of pocesses including active tanspot and potein synthesis. Most of these pocesses equie enegy in the fom of ATP (adenosine tiphosphate). ATP is a chemical substance that can diffuse to any pat of the cell and elease enegy. Evey cell poduces its own ATP, by a pocess called cell espiation. n cell espiation, oganic compounds such as glucose o fat ae caefully boken down. Enegy fom them is used to make ATP. Cell espiation is defined as contolled elease of enegy, in the fom ofa TP, fom oganic compounds in cells. Cell espiation can be aeobic o anaeobic. Aeobic cell espiation involves the use of oxygen and anaeobic cell espiation does not THE USE OF GLUCOSE N RESPRATON Glucose is often the oganic compound that is used in cell espiation. Chemical eactions in the cytoplasm beak down glucose into a simple oganic compound called pyuvate. n these eactions a small amount of ATP is made using enegy eleased fom glucose. Glucose """'.. 'y""., Small amount of A TP ANAEROBC CELL RESPRATON f no oxygen is available, the pyuvate emains in the cytoplasm and is conveted into a waste poduct that can be emoved fom the cell. No ATP is poduced in these eactions. n humans the waste poduct is lactate (lactic acid). n yeast the poducts ae ethanol and cabon dioxide. Humans Pyuvate i... lactate < Yeast Pyuvate c Ethanol Cabon dioxide, AEROBC CELL RESPRATON f oxygen is available, the pyuvate is absobed by the mitochondion. nside the mitochondion the pyuvate is boken down into cabon dioxide and wate. A lage amount of ATP is poduced as a esult of these eactions. Aeobic cell espiation theefoe has a much highe yield of ATP pe gam of glucose than anaeobic cell espiation. Pyuvate Cabon dioxide Wate lage amount of ATP

9 Photosynthesis NTRODUCNG PHOTOSYNTHESS Photosynthesis is the pocess used by plants and some othe oganisms to poduce all thei own oganic substances (food), using only light enegy and simple inoganic substances. t involves many stages and some complex chemical eactions, but it can be outl i ned ina seies of statements. Photosynthesis involves an enegy convesion. Light enegy, usually sunlight, is conveted into chemical enegy. Sunlight is called white light, but it is actually made up of a wide ange of wavelengths, including ed, geen and blue. Some substances called pigments can absob light. The main pigment used to absob light in photosynthesis is chloophyll. The stuctue of chloophyll allows it to absob some colous o wavelengths of light bette than othes. Red and blue light ae absobed moe than geen. The geen light that chloophyll cannot absob is eflected. This makes chloophyll and theefoe chlooplasts and plant leaves look geen. Some of the enegy absobed by chloophyll is used to poduce ATP. Some of the enegy absobed by chloophyll is used to split wate molecules. This is called photolysis of wate. Photolysis of wate esults in the fomation of oxygen and hydogen. The oxygen is eleased as a waste poduct. Cabon dioxide is absobed fo use in photosynthesis. The cabon fom it is used to make a wide ange of oganic substances. The convesion of cabon in a gas to cabon in solid compounds is called cabon fixation. Cabon fixation involves the use of hydogen fom photolysis and enegy fom ATP. MEASURNG RATES OF PHOTOSYNTHESs-l Photosynthesis involves the poduction of oxygen, the utake of cabon dioxide and an incease in biomass. Any of these can be used as a measue of the ate of photosynthesis. Effect of light intensity on photosynthesis At high light intensities the ate eaches a plateau. Poduction of oxygen Aquatic plants (e.g. Myiophyllum) elease bubbles of oxygen when they cay out photosynthesis. f these bubbles ae collected, thei volume can be measued. Uptake of cabon dioxide Leaves take in CO, fom the aio wate aound them, but this is difficult to measue diectly. f CO, is absobed fom wate, the ph of the wate ises. This can be monitoed with ph indicatos o with ph metes. nceases in biomass f batches of plants ae havested at a seies of times and the biomass of the batches is detemined, the ate of incease in biomass gives an indiect measue of the ate of photosynthesis in the plants. '". -5 c.9 o..c a. '0.ill & v At low to medium light intensities the ate is diectly popotional to light intensity. Light intensity Effect of CO 2 concentation on photosynthesis Effect of tempeatue on photosynthesis At vey high CO, concentations the ate eaches a plateau. As tempeatue inceases the ate inceases moe and moe steeply. Optimum tempeatue Above the optimum tempeatue the ate falls steeply. V> 'u; <l)..c E..c a. '0 2:i c: '" At low to faily high ""._._ CO 2 concentations the ate is positively coelated with CO 2 concentation. on on <l)..c E >-...c 0. '0 2:i c: '" CO 2 concentation '" Tempeatue

10 EXAM QUESTONS ON TOPC 3 The table below shows the base composition of genetic mateial fom ten souces. Souce of genetic mateial Cattle thymus gland Cattle spleen Cattle spem Pig thymus gland Salmon VVheat Yeast E. coli (bacteia) human spem influenza vius Base 'tion (%).., i"'-'....' ''''f Ade line Guanine Thymine Cytosine Uacil a} Deduce the type of genetic mateial used by (i) cattle (ii) E. coli (iii) influenza viuses. b) Suggest a eason fo the diffeence between cattle thymus gland, spleen and spem in the measuements of thei base composition. [1 J [1] [1 J [1 J c) (i) Explain the easons fo the total amount of adenine plus guanine being close to 50% in the genetic mateial of many of the species in the table. [3J (ii) dentify two othe tends in the base composition of the species that have 50% adenine and guanine. [2J d) (i) dentify a species shown in the table that does not follow the tends in base composition descibed in (c). [1 J (ii) Explain the easons fo the base composition of this species being diffeent. [2] 2 The gaph (ight) shows the esults of a data logging expeiment. Chloella, a type of alga that is often used in photosynthesis expeiments, was cultued in wate in a lage glass vessel. light intensity, tempeatue and the ph of the wate wee monitoed ove a thee-day peiod. The changes in ph wee due to changes in cabon dioxide concentation. An incease in CO 2 concentation causes a decease in ph. a) State the elationship shown in the gaph between (i) light intensity and CO 2 concentation (ii) tempeatue and CO 2 concentation. [1] b) Deduce, fom the data in the gaph, whethe the effect of light intensity o tempeatue on cabon dioxide concentation is geate. [2J c) The gaph shows both ises and falls in CO 2 concentation. Explain the causes of [1 J u 0 30 ::!! ) '" E 10.s x..:: (i) ises in CO 2 concentation [2] (ii) falls in CO 2 concentation. [2J 2 3 days 3 The diagam shows the basic stuctue of amino acids a} State what is epesented in the diagam by the lette R. R COOH C- H NH2 (1) b) Daw a simple diagam to show how two amino acids ae linked togethe. [2J c) Amino acids ae linked togethe to fom polypeptides at special sites in the cytoplasm of both pokayotic and eukayotic cells. Compae the sites whee polypeptides ae fomed in pokayotic cells with the sites in eukayotic cells. [2J