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Topic 1 : Photosynthesis
• Definition: An anabolic biological process, through which light energy is converted
into chemical energy, in the form of carbohydrate.
• Balanced Equation to summarise Photosynthesis::
sunlight
6CO2 + 6H2O
C6H1206 + 6O2
chlorophyll
Ultrastructure of chloroplast
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Cyclic Light Stage
Non-Cyclic Light Stage
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Dark Stage
Biochemistry of photosynthesis
A. Outline account of light stage:
• Also called the ‘light-dependent’ stage.
• Occurs in the grana of the chloroplast.
• Products = Oxygen, NADPH and ATP.
• Involves the flow of electrons through two different pathways as outlined below:
Pathway 1 --- Cyclic light stage
º Light is absorbed by a reaction centre chlorophyll.
º Chlorophyll emits an energised electron ( e-)
º This electron releases its energy forming ATP, by the phosphorylation of ADP
º The same electron returns to the reaction centre chlorophyll, hence the term ‘cyclic light stage’.
º Product of cyclic light stage = ATP.
Pathway 2 --- Non-cyclic light stage
º Two energised electrons (2e-) are picked up by the electron acceptor NADP+, forming NADP -.
º Photolysis of water (‘splitting of water’) occurs, producing protons (H+), electrons
and oxygen.
º NADP- picks up a H+ forming NADPH
Oxygen is used in plant respiration and excess leaves through the leaf stomata
Electrons from photolysis of water return to the reaction centre chlorophyll
• Products of non-cyclic light stage = ATP, NADPH and Oxygen.
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B. Outline account of Dark Stage:
• Also called the ‘light-independent’ stage
• Temperature-dependent as enzyme controlled, hence the need for ribosomes in
the chloroplast, as enzymes are proteins and are formed at the ribosomes.
• Occurs in the stroma of the chloroplast
• Product = Glucose ( a carbohydrate )
º CO2 is the reactant of the dark stage and supplies carbon for the production of
glucose.
º NADPH from the light stage supplies the necessary electrons and hydrogen to ‘reduce’ CO2 to the carbohydrate glucose.
º The necessary energy comes from ATP, produced during the light stage.
º Following the breakdown of ATP and NADPH, products not needed such as ADP
and NADP+ return to the light stage and are re-used.
2011 Q.11(B)
The second stage of photosynthesis is called the dark stage or light-independent
stage.
(i) Why is the dark stage given the alternative name of the light-independent stage?
(ii) Name a gas that is essential for the dark stage.
(iii) Two products of the light stage are vital for the dark stage. Name each of them.
(iv) State the precise role in the dark stage of each of the substances that you named in
(iii).
(v) To what group of biomolecules do the main products of the dark stage belong?
Answers
(i)
(ii)
(iii)
(iv)
•
•
(v)
Light not required.
CO2
ATP and NADPH
Role in dark stage:
ATP supplies energy
NADPH supplied hydrogen and electrons for the reduction of CO2 to glucose
Carbohydrates
2011 Q14 (A)
Name the openings in the leaf which allow the entry of carbon dioxide for photosynthesis.
State a factor which influences the diameter of these openings.
(ii) During photosynthesis oxygen is produced.
1. From what substance is oxygen produced?
2. In which stage of photosynthesis is oxygen produced?
3. Give two possible fates of oxygen following its production.
(iii) Give an account of the role of each of the following in photosynthesis:
1. ATP, 2. NADP.
exam b ief Biology
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Answers
(i) Stomata
CO2
(ii) 1.
Water
2.
Light stage
3.
Plant respiration
Diffuses out through stomata into the atmosphere.
(iii)
ATP
• Provides energy for the dark stage
• to reduce CO2 to glucose
NADP
• An electron acceptor and hydrogen carrier
• Used in the dark stage for glucose formation
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