Editorial Note: Photosynthesis

The International Law Reports endeavour to provide within a single series of volumes comprehensive access in English to judicial materials bearing on public international law. On certain topics it is not always easy to draw a clear line between cases which are essentially ones of public international law interest and those which are primarily applications of special domestic rules. For example, in relation to extradition, the Reports will include cases which bear on the exception of “political offences” or the rule of double criminality, but will restrict the number of cases dealing with purely procedural aspects of extradition. Similarly, while the general rules relating to the admission and exclusion of aliens, especially of refugees, are of international legal interest, cases on the procedure of admission usually are not. In such borderline areas, and sometimes also where there is a series of domestic decisions all dealing with a single point in essentially the same manner, only one illustrative decision will be printed and references to the remainder will be given in an accompanying note.


Photosynthetic structures: the dicot leaf
• The leaf is the main photosynthetic organ of a plant and it is adapted to facilitate the efficient uptake and absorption of the materials required for photosynthesis • Large surface area for light absorption • Leaves are arranged to minimise overlapping • Thin lamina -short diffusion distance • Transparent cuticle/epidermis -light can pass through to photosynthetic mesophyll cells • The leaf possess stomata which open to permit the entry of carbon dioxide into the leaf which then diffuses into the mesophyl cells where photosynthesis occurs • The leaf possess veins which contain the xylem that brings water from the root and stem. The water moves out of the xylem into neighbouring mesophyl cells • The veins also contain phloem which carries sucrose and other assimilates away from the leaf • Long narrow upper mesophylls packed with chloroplasts • Air spaces in lower mesophyll to allow diffusion of oxygen and carbon dioxide describe the structure of a dicotyledonous leaf, a palisade cell and a chloroplast and relate their structures to their roles in photosynthesis Photosynthetic structures: the palisade cell • The cells are closely packed together so as to enable them absorb more incident light • They are arranged near or close to the upper surface of leaf so as to maximize light interception ; • They are arranged at right angles to leaf surface --to reduce number of light absorbing walls ; • They are cylindrical in shape and this allows for air spaces between cells; these air spaces act as reservoir of carbon dioxide ; • The cells have a large surface area for gas exchange ; • The cell walls thin and thus it creates short diffusion pathway for gases entering into the cell • They possess large vacuole which pushes chloroplasts to edge of cell; thus the chloroplasts are on the periphery of the cell and so they can to absorb light more efficiently ; • They possess a large number of chloroplasts so as to maximise light absorption ; • The chloroplasts can move within cells towards light and thus maximize light absorption • The chloroplasts can move away from high light intensity so as to to avoid damage Photosystems are two types: PSI and PSII. • A photosystem is a complex assembly/collection of hundreds of accessory pigment molecules surrounding a primary pigment molecule. The light energy absorbed by the different accessory pigments is passed on to the primary pigment.
•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 •describe the uses of ATP and reduced NADP in the light-independent stage of photosynthesis;

Role of pigments
• Primary pigments: chlorophyl a and chlorophyl b • The pigments absorb light of different wavelength, but they absorb violet, blue and red best. • Most green wavelength is reflected back, thus leaves appear green • Chlorophyl a is the reaction centre of PS II, and it absorbs light at 680nm • Chlorophyl b is the reaction centre of PS I, and it absorbs light at 700nm • Accessory pigments: carotene and xanthophyl • Absorption spectrum is a graph that shows the absorbance of a pigment at different wavelength of light • Action spectrum is a graph that shows the rate of photosynthesis at different wavelength of light •interpret absorption and action spectra of chloroplast pigments •discuss the role of chloroplast pigments in absorption and action spectra, and separate them using chromatography • The absorption spectra is a graph that shows the degree/quantity of light energy absorbed by various pigments in the chloroplast at different wavelength of light • The action spectra is a graph that shows the rate of photosynthesis at different wavelength of light. • The absorption spectra shows that chlorophyl a has an absorbance that increases as wavelength of light increases from 420 until it peaks at around 450 -460nm after which it reduces sharply. The absorbance remains very low between 500nm -580nm after which it begins to increase again and peaks at 680nm and then it reduces again as wavelength increases beyond 680nm. The peak at 450nm is greater than the peak at 680 • Thee absorption spectra of chlorophyl b… • The absorption spectra of the carotenoids… • The action spectra shows that…

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 outline the three main stages of the Calvin cycle:

Conversion of Calvin Cycle intermediates
• Carbon dioxide diffuses into leaf through stomata on underside of leaf and enters stomata • Combines with 5-carbon ribulose bisphosphate (RuBP) in a reaction catalysed by the enzyme rubisco • Two 3-carbon glycerate 3-phosphates (GP) created • GP is reduced and phosphorylated by NADP and ATP to make triose phosphate (TP) • Triose phosphate goes on to make sugars e.g. glucose • RuBP is reformed when TP combines with ATP describe, in outline, the conversion of Calvin cycle intermediates to carbohydrates, lipids and amino acids and their uses in the plant cell

Conversion of Calvin cycle intermediates
• Some of the triose phosphate produced at the end of Calvin Cycle will be used to regenrate RuBP • Some will condense to form hexose phosphate which will then be used to produce starch, sucrose or cellulose • Some will be converted to glycerol and fatty acid which are used to produce lipids for cell membrane synthesis • Some will be converted to acetyl coenzyme A and be used for respiration • Some will be used for synthesis of amino acids describe, in outline, the conversion of Calvin cycle intermediates to carbohydrates, lipids and amino acids and their uses in the plant cell

Limiting Factors
• A limiting factor is one of the many factors that affect rate of photosynthesis when it is near its lowest value and every other factor is at it's optimum. • The rate of photosynthesis is affected by light intensity, carbon dioxide concentration and temperature; and all three factors work together to influence the rate. • At every point in time, one of those three factors is always the limiting factor. This means that when the value of the other two factors are changed, the rate of photosynthesis will not be altered (since they are not the limiting factor). But when the value of limiting factor is changed, the rate of photosynthesis will be altered.
explain the term limiting factor in relation to photosynthesis explain the effects of changes in light intensity, carbon dioxide concentration and temperature on the rate of photosynthesis

Effect of light intensity
• As light intensity increases, the rate of photosynthesis increases • Light is essential for the light dependent stage of photosynthesis • The greater the light intensity, the greater the rate of formation of ATP during photophosphorylation (cyclic and non cyclic), the greater the rate of formation of reduced NADP. • And more of these are available for the light independent reactions • However at higher light intensity, the rate no longer increases, because at such point, light ceases to be a limiting factor explain the term limiting factor in relation to photosynthesis explain the effects of changes in light intensity, carbon dioxide concentration and temperature on the rate of photosynthesis