28 August 2015

#100 Chloroplasts

Photosynthesis takes place inside chloroplasts.

These are organelles surrounded by 2 membranes, called an envelope.

Chloroplasts are found in mesophyll cells in leaves:

Palisade mesophyll cells contain most chloroplasts.
Spongy mesophyll cells and Guard cells also contain chloroplasts.

Lamellae and light-dependent reactions 

The membranes inside a chloroplast are called lamellae, and it is here that the light-dependent
reactions take place. The membranes contain chlorophyl molecules, arranged in groups called photosystems. There are two kinds of photosysterns, PSI and PSII, each of which contains slightly different kinds of chlorophyll.

There are enclosed spaces between pairs of membranes, forming fluid-filled sacs called thylakoids. These are involved in photophosphorylation - the formation of ATP using energy from light. Thylakoids are often arranged in stacks called grana (singular: granum),

Stroma and light-independent reactions 

The 'background material' of the chloroplast is called the stroma, and this is where the light-independent reactions take place.

Chloroplasts often contain starch grains and lipid droplets. These are stores of energy-containing substances that have been made in the chloroplast but are not immediately needed by the cell or by other parts of the plant.

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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