27 August 2015

#99 Photosynthesis overview

Photosynthesis is a series of reactions in which energy transferred as light is transformed to chemical energy.

Energy from light is trapped by chlorophyll, and this energy is then used to

• split apart the strong bonds in water molecules to release hydrogen
• produce ATP
• reduce a substance called NADP.

NADP stands for nicotinamide adenine dinucleotide phosphate, which - like NAD - is a coenzyme.
The ATP and reduced NADP are then used to add hydrogen to carbon dioxide, to produce carbohydrate molecules such as glucose. These complex organic molecules contain some of the energy that was originally in the light. The oxygen from the split water molecules is a waste product, and is released into the air.

There are three basic steps in photosynthesis:

1. Light-dependent reactions - energy capture
    chemiosmosis generation of ATP (adenosine triphosphate) from harvested sunlight

2. Light-independent reactions - fixation of carbon
    enzyme catalyzed reactions using the energy formed (ATP) in the light reactions to fix          atmospherically derived carbon (CO2) into sugars (CH2O)

3. Pigment Regeneration - electron replacement
   from the splitting of H2O in oxygenic photosynthesis.

The 2 main stages are light-dependent reactions and light-independent reactions (Calvin cycle).

Syllabus 2016-2018 

13.1  Photosynthesis as an energy transfer process

Light energy absorbed by chloroplast pigments in the light dependent stage of photosynthesis is used to drive reactions of the light independent stage that produce complex organic compounds.

Chromatography is used to identify chloroplast pigments and was  also used to identify the intermediates in the Calvin cycle.

a)   explain that  energy transferred as ATP and reduced NADP from the light dependent stage is used during the light independent stage (Calvin cycle) of photosynthesis to produce complex organic molecules

b)   state the sites of the light dependent and the light independent stages in the chloroplast

c)   describe the role of chloroplast pigments (chlorophyll a, chlorophyll b, carotene and xanthophyll) in light absorption in the grana

d)   interpret absorption and action spectra of chloroplast pigments 

e)   use  chromatography to separate and identify chloroplast pigments and carry out an investigation to compare the chloroplast pigments in different plants  (reference should  be made to Rf  values  in identification)

f) describe the light dependent stage as the photoactivation of chlorophyll resulting in the photolysis of water and the transfer of energy to ATP and reduced NADP (cyclic and non-cyclic photophosphorylation should  be described in outline  only)

g)   outline  the three main stages of the Calvin cycle:

fixation of carbon  dioxide by combination with ribulose bisphosphate (RuBP), a 5C compound, to yield two molecules of GP (PGA), a 3C compound
•   the reduction of GP to triose  phosphate (TP) involving ATP and reduced NADP
•   the regeneration of ribulose  bisphosphate (RuBP) using ATP

h)   describe, in outline,  the conversion of Calvin cycle intermediates to carbohydrates, lipids and amino  acids  and their uses in the plant cell

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