#106 Summary of Photosynthesis

1 In photosynthesis, light energy is absorbed by chlorophyll pigments and converted to chemical energy, which is used to produce complex organic molecules.

 2 In the light-dependent reactions, water is split by photolysis to give hydrogen ions, electrons and oxygen. The hydrogen ions and electrons are used to reduce the carrier molecule, NADP, and the oxygen is given off as a waste product.








 3 ATP is synthesised in the light-dependent reactions of cyclic and non-cyclic photophosphorylation. During these reactions the photosynthetic pigments of the chloroplast absorb light energy and give out excited electrons. Energy from the electrons is used to synthesise ATP.

 4 ATP and reduced NADP are the two main products of the light-dependent reactions of photosynthesis, and they then pass to the light-independent reactions.

 5 In the light-independent reactions, carbon dioxide is trapped by combination with a 5C compound, RuBP, which acts as an acceptor molecule. This reaction is catalysed by the enzyme ribulose bisphosphate carboxylase (rubisco), which is the most common enzyme in the world. The resulting 6C compound splits to give two molecules of a 3C compound, GP (also known as PGA). GP is reduced to carbohydrate, using ATP and reduced NADP from the light-dependent reactions. This carbohydrate can be converted into other carbohydrates, amino acids and lipids or used to regenerate RuBP. This sequence of light-independent events is called the Calvin cycle.

 6 Chloroplasts, palisade mesophyll cells and whole leaves are all adapted for the efficient absorption of light for the process of photosynthesis.

 7 When a process is affected by more than one factor, the rate of the process will be limited by the factor closest to its lowest value. The rate of photosynthesis is subject to various such limiting factors, including light intensity and wavelength, carbon dioxide concentration and temperature.

8 A graph of the particular wavelengths of light that are absorbed by a photosynthetic pigment is called an absorption spectrum, and a graph of the rate of photosynthesis at different wavelengths of light is called an action spectrum.

9 The different pigments present in a chloroplast can be separated by paper chromatography.

1. End-of-chapter questions

1  What   are the  products    of the  light-dependent      reactions   of photosynthesis?

A   ATp,  RuBP   and  reduced   NAD
B  ATp,  oxygen  and  reduced   NADP
C   GP, oxygen  and  reduced   NAD
D   GP, reduced   NADP    and  RuBP

2   Where   in  the  chloroplast  are  the  products of photophosphorylation  used?
A   envelope
B   granum
C   stroma
D   thylakoid


3   In separate   experiments,     an  actively  photosynthesising      plant   was  supplied   with  one  of two  labelled   reactants:

•    water   containing     the  ¹⁸O

•    carbon    dioxide    containing     the   ¹⁷O

In which   products    of photosynthesis     would   these  isotopes   be found?

4   Copy and complete  the table to  how the adaptations  of a dicotyledonous  leaf for photosynthesis.  The first row has been competed  for you.

5   Copy and complete  the table to show the adaptations  of a palisade mesophyll  cell for photosynthesis.

6   Rearrange the following statements  to make a flow diagram  of the mechanism  of opening  a stoma.

1.  volume  of guard  cell increases
2.  H+ transported   out  of guard  cells
3. water  enters  guard  cells by osmosis
4.  K+  diffuses into  guard  cells
5.  guard  cells curve  to open  stoma
6. water  potential   of guard  cells falls
7.  K+ channels  open

7   a    Explain   how  the  inner   membrane    system  of a chloroplast    makes  it well  adapted   for photosynthesis.
     b   Copy   the  table  below  and  insert   ticks  or  crosses  to show  which   structural    features   are shared   by a plant chloroplast    and  a typical   prokaryotic    cell.

8  a    When   isolated   chloroplasts    are placed   in  buffer  solution   with  a blue  dye  such  as DCPIP    or methylene    blue and  illuminated,     the  blue  colour   disappears.    Explain   this  observation.      

                                                                                                                                                    [4]

 b   Name   the  compound,     normally   present   in photosynthesis,      that  is replaced   by the  blue  dye  in  this  investigation.     [1]

[Total:  5]

9   Distinguish    between:

   a    cyclic  and  non-cyclic   photophosphorylation                                      [2]

   b   photophosphorylation  and  oxidative   phosphorylation                         [2]

   c    the  roles  of NAD  and  NADP   in  a plant.                                          [2]

 [Total:  6]

10  a Draw  a simple   flow  diagram   of the  Calvin   cycle  to show  the  relative  positions    in  the  cycle  of the  following molecules:

•   CO2   (IC)

•    GP/PGA     (3C)

•    triose   phosphate     (3C)

•    RuBP   (5C).                                                                                                                                                                                         [4]

b   Show  the  point   in  the  cycle  at which   the  enzyme   rubisco   is active.                                 [1]

[Total:  5]

11 a   Explain   what   is meant   by a limiting    factor.

     b   List four  factors   that  may  be rate-limiting     in photosynthesis.

     c   At low  light  intensities,    increasing    the  temperature     has  little  effect  on  the  rate  of                  photosynthesis.

          At high  light  intensities,    increasing    the  temperature     increases   the  rate  of                                photosynthesis.

Explain   these  observations.                                                                                                                                                                      [5]

[Total:   10]

12 a    Distinguish    between   an absorption     spectrum    and  an action   spectrum.               [4]                                                                   

  b   Pondweed   was  exposed   to  each  of three  different   wavelengths    of light  for  the  same  length   of time.   For  each wavelength,    the  number    of bubbles   produced    from  the  cut  ends  of the  pondweed    were  counted    and  are shown   in  the  table.                                  

Explain   these  results.                                                                                                              [4]

 [Total:   8]

2. End-of-chapter answers

1 B
 2 C
 3 A

6 2, 7, 4, 6, 3, 1, 5
 All correct = 4, subtract marks for mistakes.

7 a allows chlorophyll and other pigments to be arranged into photosystems;
     provides large surface area for pigments;
    increases effi ciency of light harvesting; allows electron carriers to be arranged appropriately;             provides structure for proton gradient for chemiosmosis; anchors ATP synthase;

Exam-style questions

8 a photolysis of water occurs in light;
    H+ released; accepted by DCPIP/methylene blue;
    colourless when reduced;
    shows ‘reducing power’ of chloroplasts;                                                                     [max. 4]

 b NADP;                                                                                                                           [1]                                                                                                                 

  [Total: 5]

9 a cyclic photophosphorylation: electron emitted by chlorophyll of photosystem I returns to    chlorophyll by a series of carriers;
  non-cyclic photophosphorylation: electron emitted by chlorophyll of photosystem II does not return    to that chlorophyll (but is absorbed by photosystem I and electron emitted by photosystem I is            absorbed by NADP);                                                                                                         [2]

b photophosphorylation: synthesis of ATP using light energy in photosynthesis in a chloroplast;       oxidative phosphorylation: synthesis of ATP using energy released from the electron transport chain in aerobic respiration in a mitochondrion;                                                                           [2]

c NAD: hydrogen carrier in respiration; NADP: hydrogen carrier in photosynthesis;     [2] 

 [Total: 6]

11 a limiting factor: one factor, of many aff ecting a process, that is nearest its lowest value and hence is rate-limiting;                                                                                                         [1] 

 b light intensity; light wavelength; concentration of carbon dioxide; temperature;           [4] 

 c shows that there are two sets of reaction in photosynthesis;

    a light-dependent photochemical stage; 

    a light-independent temperature-dependent stage; 

    photochemical reactions are not aff ected by temperature; at low light intensities, light intensity is    the ratelimiting factor;

    at high light intensities and low temperatures, temperature is the rate-limiting factor; [max. 5]

 [Total: 10] 

 12 a absorption spectrum: a graph of the absorbance of diff erent wavelengths of light by a compound; action spectrum: a graph of the rate of a process, e.g. photosynthesis at diff erent wavelengths of light; [4] b number of bubbles shows rate of photosynthesis; rate similar at 450 nm (blue) and 650 nm (red); these are wavelengths that are absorbed by chlorophyll; rate, much lower/refer to fi gures, at 550 nm (green); very little absorbed by any pigment;                     [max. 4] 

                [Total: 8]