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Option 9.9 Biochemistry: 4. The light reaction

Syllabus reference (October 2002 version)
4. By the middle of the 20th century, a description of the light dependent reaction was developing

Students learn to:

Students:

Extract from Biology Stage 6 Syllabus (Amended October 2002) © Board of Studies, NSW.
[Edit 18 June 09]


Prior learning: Stage 4, Structures and Systems 4.8.2 (c), 4.8.4 (d) Interactions 4.10 (c). Preliminary module 8.2 (subsection 2) module 8.3 (subsection 4), module 8.4 (subsections 2 and 3).


Background: By the middle of the 20th century, a description of the light dependent reaction was developing.

 

describe and discuss the importance of Van Niel’s hypothesis that water was the source of oxygen given out in photosynthesis

CO2 + 2H2S → CH2O + H2O + 2S

CO2 + 2H2O → CH2O + H2O + O2

The importance of van Niel’s hypothesis is that it gave one of the first clues to the mechanism of photosynthesis. Prior to Van Niel’s work, scientists had thought that carbon dioxide was split, the oxygen released and the C added to water to make glucose. Van Niel’s hypothesis that water is split to produce oxygen was later proved using radioactive oxygen – 18. Water labeled with 18O produced oxygen gas labeled with 18O. When CO2 labeled with 18O was used, the radioactivity was detected in the glucose and water, not the oxygen gas produced.

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present information from secondary sources to describe the repetitive flash technique used for the first time by Emerson and Wahl and outline its subsequent importance in the study of photosynthesis

Graph of Emerson's and Arnold's results (external website) University of Illinois, Illinois, USA.

Milestones in photosynthesis (external website) University of Illinois.

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outline the classic experiments of Emerson and Arnold and their interpretation by Gaffron and Wahl that led to the hypothesis of a photosynthetic unit consisting of chlorophyll and photoenzyme molecules

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identify the light dependent reaction as that which traps light energy and coverts it to chemical energy stored in ATP

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identify the role of chlorophylls in the light reactions

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explain the significance of the difference in function of photosystems I and II

2H2O + 4 photons of light → 4H+ + 4e- + O2

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identify the role of the coenzymes ADP and NADP in the light reactions

Animation of the light reaction (external website) Cornell University, USA. [Flash required]

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gather and process information from secondary sources to trace the light dependent reaction of photosynthesis on a suitable biochemical pathways flow chart

Photosystem I and II and the Light Reaction (external website) McDaniel College, Westminster, Maryland, USA

Don’t try to memorise this information but use it to interpret as many diagrams of the light reaction as you can find! You may be interested to know the electrons removed from the water move to fill holes created by light in chlorophyll molecules in the reaction centre of PS II.

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