4.3 PHOTOSYNTHESIS USES LIGHT ENERGY TO SYNTHESISE ORGANIC MOLECULES
1. Candidates should be able to define the term ‘photosynthesis’.
The process by which plants produce energy and glucose from sunlight.
2. The main site of photosynthesis in the leaf is palisade tissue.
3. The main photosynthetic pigments found in plants are chlorophyll a and b, carotene and xanthophyll.
Chlorophylls (a and b) – Red and Blue Violet region
Carotenoids (carotene and xanthophyll)– Blue Violet region
Carotenoids are thought to be the ACCESSORY PIGMENTS
4. The function of these pigments is to absorb light energy.
Their function is to absorb light energy and convert it to chemical energy.
5. The principles of chromatography as used in the separation of leaf pigments.
Different pigments separate up in a CHROMATOGRAM
Different pigments can be identified by the distance they’ve travelled over the distance the solvent has travelled; this is called the Rf value – RETARDATION FACTOR.
6. Candidates should understand the terms ‘absorption spectra’ and action spectra’ and describe the relationship of the two when considering leaf pigments.
Absorption Spectra – a graph that indicates how much light a particular pigment absorbs at each wavelength.
Action Spectra – A graph that shows the RATE of PHOTOSYNTHESIS at different wavelengths of light.
- Both Action and Absorption have a similar shape ( Peaks and Troughs)
- This shows that pigments absorb light that is then used in Photosynthesis.
7. Photosynthesis is a series of chemical steps which are arranged in two main stages, the light-dependent and light-independent stages.
8. Candidates should be able to describe the arrangement and function of the leaf pigments, the antennae complex and the reaction centres of photosystems I and II and their positions within the thylakoid membrane.
9. Chlorophyll a molecules, in the reaction centre, are excited and lose electrons when they absorb light energy.
Light enters PHOTOSYSTEM II where it is trapped by CHLOROPHYLL A in the REACTION CENTRE. Electrons are then raised to a higher energy level to be picked up by an ELECTRON ACCEPTOR .
10. These high energy electrons are transferred between acceptors (names not required) and used in the manufacture of ATP through cyclic and non-cyclic photophosphorylation (Z scheme).
Pairs of electrons are passed DOWNHILL to PS I along the ELECTRON TRANSPORT CHAIN and this generates energy. This energy is used generate a PROTON GRADIENT which drives the synthesis of ATP from ADP and Pi. This occurs in both CYCLIC and NON CYCLIC PHOTOPHOSPHORYLATION.
11. The loss of electrons is a form of oxidation.
Oxidation is LOSS of Hydrogen Ions or Electrons
Reduction is GAIN of Hydrogen Ions or Electrons
12. The hydroxide ion from the splitting of water, is a source of electrons for photosystem II.
13. Molecules of water are split into hydrogen ions, electrons and oxygen and the electrons are removed to replace those lost in the chlorophyll in the photosystems. Light is responsible only indirectly for splitting water.
Electrons lost by PS II are replaced by electrons which become free due to the splitting of WATER by PHOTOLYSIS.
14. The splitting of water is referred to as photolysis. It occurs in the thylakoid space, therefore raising the concentration of protons there.
Photolysis – Splitting of a Compound by means of LIGHT ENERGY/PHOTONS
15. The reduction of NADP is brought about by the addition of electrons and hydrogen ions (protons). Since it takes place in the stroma, the concentration of protons is lowered there, further contributing to the maintenance of the electrochemical gradient.
Light is absorbed by PS I and is passed to the Reaction centre when an electron is passed up to the second electron accepter. These electrons are passed out to the stroma where they join with protons to reduce NADP to NADPH2. This contributes to the maintenance of the electrochemical gradient. This stage does not occur in CYCLIC PHOTOPHOSPHORYLATION.
In cyclic photophosphorylation, electrons which are not passed out to the stroma are returned to PS I via a chain of Electron Carriers, which generate sufficient energy to make ATP.
16. The light dependent stage produces reduced NADP, ATP and oxygen. (Reduced NADP may be represented by NADPH2 or NADPH + H+).
Products of the LIGHT DEPENDENT REACTIONS
- ATP
-NADPH
-O2 – as a BYPRODUCT
17. The following steps occur in the light-independent stage (Calvin Cycle): uptake of carbon dioxide by 5C ribulose bisphosphate (Rubisco) to form 2 x 3C glycerate-3-phosphates; utilisation of ATP and reduced NADP from the light-dependent stage to reduce glycerate- 3-phosphate to the 3C carbohydrate, triose phosphate and the consequent regeneration of ribulose bisphosphate which requires ATP.
CO2 is fixed to RuBP 5C (Catalysed by Rubisco). The unstable 6C intermolecule is HYDRATED and SPLIT to form 2 x 3C molecules – Glycerate3Phosphate. GP is then PHOSPHORYLATED and REDUCED by ATP and NADPH2 to form TRIOSE PHOSPHATE 3C. Triose Phosphate is then used to REGENERATE RuBP via a series of reactions driven by ATP
18. Glucose, lipids and amino acids (with the addition of nitrogen obtained from nitrates) may be manufactured from triose phosphate (no details of chemistry required).
From Triose Phosphate, Glucose Lipids and Amino acids may be manufactured. This will need the addition of Nitrogen for Amino Acids and addition of Phosphate for Phosopholipids.
19. Various inorganic nutrients are needed by plants and may be limiting factors to metabolism if in short supply.
Mineral Ions are required for the synthesis of compounds needed for the growth of the plant. Macro nutrients, e.g. Potassium, Sodium, Magnesium, Calcium, Nitrate and Phosphate are required in substantial amounts but the micronutrients, Manganese and Copper are needed in much smaller amounts.
20. Nitrogen is used for the synthesis of proteins and nucleic acids so nitrogen deficiency will cause stunted growth because lack of nucleic acids will hinder cell division. Magnesium is required for chlorophyll therefore deficiency leads to chlorosis and death.
Nitrogen is needed for the formation of Nucleic Acids. It is taken up by the roots as nitrates (4.5) It is used in the synthesis of Nucleic acids and Amino Acids/Proteins. Nitrogen deficiency can lead to reduced growth of all organs, including Chlorophyll and so a YELLOWING of leaves occurs – This is known as CHLOROSIS.
Magnesium is absorbed as Mg2+ and it’s function is in Chlorophyll production and activation of ATPase. Mg forms part of the chlorophyll molecule (similar to Iron in Hb) Chlorosis is also a symptom of Mg deficiency.