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BIOLOGY  
 
F214:  Communication,  Homeostasis  and  Energy      
1) Photosynthesis  
a) Define  the  terms  autotroph  and  heterotroph  
i) Autotroph  –  organisms  that  can  synthesise  complex  organic  molecules  and  self-­‐feed,  eg
...
 nitrifying  bacteria  or  hydrothermal  vent  bacteria  
(2) Photoautotrophs  –  use  light  usually  from  the  sun  as  a  source  of  energy,  and  inorganic  
molecules,  to  drive  photosynthesis  eg
...
 algae  
ii) Heterotroph  –  organisms  that  ingest  and  digest  complex,  organic  molecules  into  simpler  
soluble   ones,   from   which   they   synthesise   complex   molecules   eg
...
 Animals,  fungi  and  bacteria  
 
b) State  that  light  energy  is  used  during  photosynthesis  to  produce  complex  organic  molecules  
i) Photosynthesis   –   the   process   whereby   light   energy   from   the   Sun   is   transformed   to  
chemical   energy   and   used   to   synthesise   large   organic   molecules   from   inorganic  
substances  
ii) Chemical  potential  energy  is  then  available  to  consumers  and  decomposers  
 
c) Explain  how  respiration  in  plants  and  animals  depends  upon  the  products  of  photosynthesis  
i) The  products  of  photosynthesis  are  the  reactants  of  aerobic  respiration  ie…  
ii) 6CO2  +  6H2O  à  C6H12O6  +  6O2  
iii) Therefore  all  aerobes  rely  on  photosynthesis  for  respiration  
iv) Once   the   Earth’s   atmosphere   contained   free   oxygen   from   photosynthesis,   organisms  
evolved  that  could  use  the  oxygen  for  aerobic  respiration,  releasing  CO2  and  H2O  
 
d) State  that  in  plants  photosynthesis  is  a  two-­‐  stage  process  taking  place  in  chloroplasts  
i) The   process   of   plant   photosynthesis   can   be   split   into   the   light-­‐dependent   and   light-­‐
independent  stages  
ii) Photosynthesis  takes  place  in  the  chloroplasts  of  the  plant  cell  
 
e) Explain,   with   the   aid   of   diagrams   and   electron   micrographs,   how   the   structure   of  
chloroplasts  enables  them  to  carry  out  their  functions  
i) Plant  adaptations  for  photosynthesis  
(1) Roots  anchor  the  plant  to  the  ground  and  allow  water  +  minerals  to  enter  the  plant  
(2) Plants  grow  upwards  towards  the  light  
(3) Greater  wind  circulation  as  plants  are  off  the  ground  

(4) Arrangement   of   the   leaves   ensures   the   largest   surface   are   possible   exists   for   the  
absorption  of  light  
(5) Leaves  don’t  overlap  so  there  is  no  shade  effect  
(6) Leaves  orient  themselves  to  face  the  sun  
(7) Leaves  are  thin  and  flat  
ii) Leaf  adaptations  for  photosynthesis  
(1) Waxy  cuticle  –  transparent  outer  layer  which  is  of  a  waxy  nature  
(a) Cuticle  protects  and  reduces  evaporation  of  water  from  the  surface  of  the  leaf  
(b) Transparent  –  light  is  allowed  through  to  the  palisade  layer  
(2) Upper  epidermis  –  layer  covered  by  the  waxy  cuticle  
(a) Protects  and  reduces  evaporation  of  water  from  the  cell  
(b) Transparent  –  light  is  allowed  through  to  palisade  mesophyll  
(c) Layer  one  cell  thick  
(d) Flattened  cells  
(e) No  chloroplasts  
(f) Usually  no  or  very  few  stomata  present  –  prevents  gases  from  leaving/entering  at  
a  rate  too  fast  to  handle  
(3) Palisade  mesophyll  –  most  photosynthesis  occurs  here  
(a) Densely  packed  layer  –  high  concentration  of  chloroplasts  for  photosynthesis  
(b) Column   shaped   cells   –   enables   cells   to   be   densely   packed   to   avoid   light   passing  
and  absorb  more  incident  light  
(c) Thin  cell  walls  –  greater  light  penetration  and  shorter  diffusion  pathway  
(d) Large  number  of  chloroplasts  –  lots  of  chlorophyll  
(e) Chloroplasts   on   the   periphery   of   the   cell  –   to   absorb   light   and   to   produce   a   short  
diffusion  path  for  CO2  
(f) Cytoskeleton   in   cell   –   chloroplasts   can   move   within   the   cell   to   absorb   as   much  
light  as  possible  and  to  prevent  damage  in  high  light  intensity  
(g) Large  vacuoles  in  cells  –  pushes  chloroplasts  to  the  edge  of  the  cell  
(h) Turgid  cells  –  provide  support  to  the  layer  
(4) Spongy  mesophyll  –  irregular  shaped  cells  
(a) Loose  network  –  important  for  the  diffusion  of  gasses  
(b) Large  air  spaces  –  connect  up  with  stomata  
(c) Contains  fewer  chloroplasts  –  some  photosynthesis  
(d) Turgid  cells  –  contributes  to  support  
(5) Vascular  tissue  in  midrib  –  xylem  and  phloem  tissues  support  the  leaf  and  carry  the  
transport  tissues  
(a) Meristem   cells   eg
...
 sucrose,  hormones  and  amino  acids  
(i) Small  cells  that  elongate  and  line  up  end-­‐to-­‐end  to  form  a  long  tube  
(ii) Sieve   plates   between   cells   –   ends   of   cells   do   not   break   down   completely,  
allowing  the  movement  of  materials  up  or  down  the  tubes  
(iii) Metabolically  active  companion  cells  next  to  each  sieve  tube  
(6) Lower  epidermis  –  one  cell  thick  layer  covered  by  a  cuticle  with  cells  similar  to  the  
upper  epidermis  
(a) High  stomatal  density  –  allow  gas  exchange  in  and  out  of  the  leaf  
(b) Each  stoma  has  two  guard  cells  –  swell  when  turgid  to  open  the  pore  
(i) Contain  chloroplasts    
(ii) Spiral   thickenings   of   cellulose   of   in   their   inner   cells   walls   that   restrict   their  
stretching  
(iii) Become  turgid  when  water  moves  in  and  only  the  outer  walls  stretch  
(iv) Two  guard  cells  bulge  at  both  ends  so  pore  opens  between  them  –  stomata    
iii) Chloroplast  adaptations  for  photosynthesis  

(1) Basic  facts  
(a) Vary  in  shape  and  size  –  most  are  disc  shaped  
(b) Between  2-­‐10  micrometres  long  
(2) Envelope  –  double  membrane  surrounding  each  chloroplast  
(3) Intermembrane  space  –  10-­‐20  nm  wide  between  the  inner  and  outer  membrane  
(a) Outer  membrane  –  permeable  to  many  small  ions  
(b) Inner  thylakoid  membrane  –  less  permeable  with  embedded  transport  proteins  to  
control   entry   and   exit   of   substances   between   the   cytoplasm   and   stroma   inside  
the  chloroplast,  folded  into…  
(4) Thylakoids   –   flattened   membrane   compartments   which   are   the   site   of   light  
dependent   reaction   containing   chlorophyll,   accessory   pigments,   electron   transport  
systems  and  enzymes    
(5) Grana  –  stacks  of  up  to  100  thylakoid  membranes  

(a) Give  a  large  surface  area  for  photosynthetic  pigments,  electron  carriers  and  ATP  
synthase  enzymes  (involved  in  the  light  dependent  reaction)  
(b) Surrounded  by  stroma  –  products  of  light  dependent  reaction  can  pass  easily  to  
stroma  for  light  independent  reaction    
(6) Intergranal  lamellae  –  between  the  grana  
(7) Stroma   –   fluid   filled   matrix   (cytoplasm)   containing   the   necessary   enzymes   that  
catalyse  the  light  independent  reaction  eg
...
 acetone  
(c) Micropipette  to  paper  (silica  gel  plate)  repeatedly  to  make  a  concentrated  spot  
(d) Allow  to  dry  each  time  before  repeating  to  make  a  concentrated  spot  
(3) Precautions  
(a) Do  not  allow  the  solvent  to  touch  the  loading/origin  line  
(b) Use  a  pencil  to  draw  line  (not  ink)  
(c) Avoid  contamination  of  paper  with  fingers  
(d) Seal  container  and  allow  time  for  a  saturated  atmosphere  to  be  achieved  inside  
(4) Rf  value  –  physical  constant  for  a  specific  solute  in  a  specific  solvent  
(a) =  the  distance  moved  by  the  spot  (measured  from  the  origin  to  the  centre  of  the  
spot)  divided  by  the  distance  moved  by  the  solvent  (measured  from  the  origin  to  
the  solvent  front)    
 
g) State  that  the  light-­‐dependent  stage  takes  place  in  thylakoid  membranes  and  that  the  light-­‐
independent  stage  takes  place  in  the  stroma  
i) Light-­‐dependent  stage  

(1) Takes   place   on   the   thylakoid   membranes   of   the   chloroplasts   –   where   the  
photosystems  with  photosynthetic  pigments  are  embedded  
(2) Involves  the  capture  of  light  whose  energy  is  used  for  two  purposes  
(a) Add  an  inorganic  phosphate  molecule  to  ADP  –  phosphorylation  to  make  ATP  
(b) Split  water  into  hydrogen  ions  and  hydroxide  ions  –  photolysis  
(i) 2H2O  à  4H+  +  O2  +  4e-­‐  
ii) Light-­‐independent  stage  
(1) Takes  place  in  the  stroma  
(2) Products   of   the   light   dependent   reaction   (ATP   and   reduced   NADP)   are   used   to  
reduce  CO2  to  form  glucose  and  other  organic  molecules  
       
h) Outline   how   light   energy   is   converted   to   chemical   energy   (ATP   and   reduced   NADP)   in   the  
light-­‐dependent   stage   (reference   should   be   made   to   cyclic   and   non-­‐cyclic  
photophosphorylation,  but  no  biochemical  detail  is  required)  
i) Photophosphorylation  –  making  of  ATP  from  ADP  and  Pi  in  the  presence  of  light  

(1) Cyclic  –  produces  ATP  
(a) Only  uses  photosystem  I  (P700)  –  occurs  mainly  on  the  intergranal  lamellae  
(b) Electrons  from  the  central  magnesium  atom  in  the  chlorophyll  of  photosystem  I  
are  excited  by  a  photon  of  light  
(c) The   electron   pass   to   an   electron   acceptor   and   back   to   the   chlorophyll   reaction  
centre  from  which  they  were  lost  
(i) Electron  acceptor  –  chemicals  that  accept  electrons  from  another  compound  
(reduced  as  they  act  as  oxidising  agents)  
(d) There  is  no  photolysis  of  water  and  no  generation  of  reduced  NADP  
(e) Small  amounts  of  ATP  are  made  –  may  be  used  in  the  light  independent  reaction  
(i) Or  for  guard  cells  to  actively  pump  potassium  ions  

(ii) Water  potential  is  lowered  inside  the  guard  cells  
(iii) Water  moves  in  by  osmosis  and  makes  the  guard  cells  turgid  
(iv) Stomata  open  as  guard  cells  swell    
(2) Non-­‐cyclic  –  produces  reduced  NADP,  ATP  and  O2  
(a) Involves  both  photosystems  I  (P700)  and  II  (P680)  
(b) A   photon   of   light   strikes   photosystem   II   –   occurs   almost   exclusively   on   the   granal  
lamellae  
(c) A   pair   of   electrons   are   excited   from   the   central   magnesium   atom   of   the  
chlorophyll  in  the  primary  pigment  reaction  centre  
(d) The  electrons  leave  the  chlorophyll  from  the  primary  reaction  centre  –  captured  
by  electron  acceptors  
(e) The  electrons  pass  along  a  chain  of  electron  carriers  (such  as  ferredoxin)  –  NB:  a  
lack  of  iron  could  lead  to  a  lack  of  these  carriers  so  less  LDR  would  take  place  
(i) Electron  carriers  –  molecules  that  transfer  electrons  
(f) A  small  amount  of  energy  is  released  and  is  used  to  synthesise  ATP  (by  pumping  
hydrogen  ions  into  the  thylakoid  space  to  produce  a  proton  gradient)  
(g) Protons  from  photolysed  water  also  take  part  in  chemiosmosis  –  ATP  is  made    
(h) A  photon  of  light  also  strikes  photosystem  I  
(i) A   pair   of   electrons   along   with   the   pair   of   protons   (produced   at   photosystem   II   by  
photolysis  of  water)  join  oxidised  NADP  to  form  reduced  NADP  –  to  be  used  in  the  
light  independent  stage  
(j) The   electrons   from   the   oxidised   photosystem   II   replace   the   electrons   lost   from  
photosystem  I  
(k) The  electrons  from  the  oxidised  photosystem  II  are  replaced  by  a  pair  of  electrons  
also  formed  from  the  photolysis  of  water  
 
i) Explain  the  role  of  water  in  the  light-­‐dependent  stage  
i) Photosystem  II  has  an  enzyme  that  can  split  water  in  the  presence  of  light  into  protons,  
electrons  and  oxygen  –  photolysis  
ii) 2H2O  à  4H+  +  O2  +  4e-­‐  
iii) Therefore,  water  is  a  source  of…  
(1) Oxygen   –   some   is   used   for   aerobic   respiration   but   most   diffuses   out   of   the   leaves  
through  stomata  and  into  the  air  
(2) Hydrogen  ions  
(a) Used  in  chemiosmosis  (cyclic  photophosphorylation)  to  produce  ATP  
(i) An  accumulation  of  hydrogen  ions  will  produce  a  proton  gradient  
(ii) Protons   will   move   from   the   thylakoid   space   back   out   to   the   stroma   via   ATP  
synthases,  through  the  thylakoid  membrane,  down  a  proton  gradient  
(iii) A  proton  motive  force  is  produced  
(iv) The  proton  motive  force  will  generate  ATP  from  ADP  and  Pi  (phosphorylation)  
–  chemiosmosis  

(v) ATP   can   be   used   for   light   independent   reaction   or   for   guard   cells   to   actively  
pump  potassium  ions  into  them  so  they  can  swell  and  the  stomata  can  open  
(b) Protons  are  accepted  by  the  coenzyme  NADP  to  form  reduced  NADPs  –  reduced  
NADPs  are  then  used  in  the  light  independent  stage  to  reduce  CO2  and  produce  
organic  molecules  
(3) Electrons  –  replace  those  lost  by  oxidised  chlorophyll  at  the  centre  of  photosystem  II  
iv) Water  also  keeps  palisade  vacuoles  of  cells  turgid,  so  pushes  chloroplasts  to  the  outer  
edges   of   the   cells   where   they   can   move   to   readily   trap   light   and   create   a   short   diffusion  
pathway  for  CO2  
 
j) Outline   how   the   products   of   the   light-­‐dependent   stage   are   used   in   the   light-­‐   independent  
stage  (Calvin  cycle)  to  produce  triose  phosphate  (TP)  (reference  should  be  made  to  ribulose  
bisphosphate   (RuBP),   ribulose   bisphosphate   carboxylase   (rubisco)   and   glycerate   3-­‐
phosphate  (GP),  but  no  other  biochemical  detail  is  required)  
k) Explain  the  role  of  carbon  dioxide  in  the  light-­‐independent  stage  (Calvin  cycle)  

i) Light   independent   stage   –   where   carbon   dioxide   is   fixed   and   used   to   build   complex,  
organic  molecules  which  takes  place  in  the  stroma  of  chloroplasts  
ii) Role  of  carbon  dioxide  
(1) Source  of  carbon  and  oxygen  for  the  production  of  all  large  inorganic  molecules  
(2) Molecules  are  used  as  structures,  act  as  energy  stores  or  are  sources  for  all  carbon-­‐
based  life  forms  on  the  planet  
iii) CO2   diffuses   from   the   air   through   the   stomata,   the   spongy   mesophyll   layer,   the   palisade  
cells,   thin   cellulose   cell   walls,   cell   surface   membrane,   cytoplasm,   chloroplast   envelope  
and  then  into  the  stroma  

iv) In  the  stroma,  CO2  combines  with  5C  compound  (ribulose  bisphosphate,  RuBP)  –  which  
becomes  carboxylated  as  it  is  a  carbon  dioxide  acceptor  
(1) Catalysed  by  an  enzyme  –  ribulose  bisphosphate  carboxylase  oxygenase  (Rubisco)  
v) Unstable   6-­‐carbon   compound   immediately   breaks   down   into   2   molecules   of   3-­‐carbon  
glycerate  phosphate  (GP)  (carbon  dioxide  has  now  been  fixed)  
vi) Using   1   ATP   and   the   hydrogen   ions   from   1   reduced   NADP   from   the   LDR,   glycerate   3  
phosphate  (GP)  is  converted  to  triose  phosphate  (TP)  –  reduction  and  phosphorylation    
vii) TP   molecules   combine   in   pairs   to   form   a   6-­‐carbon   hexose   sugars   which   may   be  
polymerised  into  starch  
(1) 5   out   of   every   6   carbon   molecules   produced   are   used   to   regenerate   RuBP   through  
phosphorylation,  using  the  remainder  of  ATP  from  the  LDR  as  a  source  of  energy  
viii) Phosphorylation  to  form  ATP  from  ADP  is  used  for…  
(1) Conversion  of  GP  to  TP  
(2) Formation/regeneration  of  RuBP  
 
l) State  that  TP  can  be  used  to  make  carbohydrates,  lipids  and  amino  acids  
m) State  that  most  TP  is  recycled  to  RuBP;    
i) GP  –  used  to  make  amino  acids  or  fatty  acids  
ii) Most  TP  is  recycled  to  RuBP  or…  
iii) Pairs  of  triosphosphate  (TP)  –  combine  to  form  hexose  (6C)  sugars  eg
...
 fructose  
(2) Glucose  +  fructose  à  disaccharide  sucrose  (sugar  translocated  in  the  phloem)  
(3) Or  hexose  sugars  –  polymerised  into  other  carbohydrates  eg
...
 
enzymes  such  as  rubisco  are  below  the  optimum  temperature  for  them  to  work  
(5) Eventually   (above   25°C),   the   rate   of   photosynthesis   will   level   off   even   though   the  
temperature  continues  to  increase    
(a) Proteins  such  as  enzymes  involved  in  the  Calvin  Cycle  may  be  denatured  
(b) Oxygenase   activity   of   rubisco   increases   more   than   its   carboxylase   activity   –  
oxygen  more  successfully  competes  for  the  active  site  of  rubisco  and  prevents  it  
from  accepting  carbon  dioxide  (photorespiration  exceeds  photosynthesis)  
(c) ATP  and  reduced  NADP  from  the  LDR  are  wasted  
(d) More   water   loss   from   stomata   (increased   transpiration   rate)   leads   to   a   stress  
response  in  which  the  stomata  close  –  limiting  the  availability  of  carbon  dioxide  
 
o) Discuss   limiting   factors   in   photosynthesis   with   reference   to   carbon   dioxide   concentration,  
light  intensity  and  temperature  
i) Chloroplasts   with   chlorophyll   are   present   in   the   plant,   but   light,   carbon   dioxide   and  
water  are  all  present  in  the  environment  and  so  can  influence  the  rate  of  photosynthesis  
ii) Limiting  factor  –  the  factor  that  is  present  in  the  least  favourable  amount  
iii) Law  of  limiting  factors  –  at  any  given  moment,  the  rate  of  metabolic  process  is  limited  by  
the  factor  that  is  present  at  its  least  favourable  (lowest)  value  
iv) As  the  carbon  dioxide  concentration  increases…  
(1) The   rate   of   photosynthesis   increases   as   the   light-­‐independent   stage   occurs   more  
over  time  
(2) Up  to  a  certain  point,  the  carbon  dioxide  concentration  is  the  limiting  factor  
(3) Eventually,  the  rate  of  photosynthesis  will  level  off  even  though  the  carbon  dioxide  
concentration  continues  to  increase  
(4) There  is  another  limiting  factor  eg
...
 carbon  dioxide  concentration  (or  temperature)  
vi) As  the  temperature  increases…  

(1) The  rate  of  photosynthesis  increases  up  to  a  certain  point  as  it  is  the  limiting  factor  
(2) Eventually   (above   25°C),   the   rate   of   photosynthesis   will   level   off   even   though   the  
temperature  continues  to  increase  
(3) There  is  another  limiting  factor  eg

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