Revision:Photosynthesis

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Draw the structure of the chloroplast as seen in electron micrographs

State that Photosynthesis-consists of light dependent and light-independent reactions

Explain Light-dependent reactions

Explain Light-dependent reactions including the photoactivation of photosystem II, phytolysis of water, electron transport, cyclic and non-cyclic phosphorylation, photoactivation of Photosystem I and reduction of NADP⁺.

• In the thylakoids, chlorophyll and other molecules are packed into units called photosystems.
  • Each unit contains from 250 to 400 molecules of pigment
    • Which serve as light-trapping antennae
  • Once a photon of light energy is absorbed by one of the antenna pigments, it is bounced around among the other pigment molecules of the photosytem until it reaches a special form of chlorophyll a, which is the reaction centre
  • When this pigment absorbs the energy, an electron is boosted to a higher energy level from which it is transferred to another molecule, a primary electron acceptor. The chlorophyll molecule is thus oxidized and positively charged
  • Photosystem 1 - the reactive chlorophyll a molecule is known as P700 because one of the peaks of its absorption spectrum is at 700 nanometers
    • a slightly longer wavelength than the usual chlorophyll a peak.
    • When P700 is oxidized, it bleaches
  • Photosystem 2 - the reactive chlorophyll a molecule is P680.

Explain phosphorylation in terms of chemiosmosis

• Definitions
  • Phosphorylation - the addition of a phosphoryl group to an organic compound. The process by which much of the energy in foods is conserved and made available to the cell is called oxidative phosphorylation. The process by which green plants convert light energy to chemical energy is called photophosphorylation.
  • Chemiosmosis
• Light energy enters PS2, where it is trapped by P680
• An electron from P680 is boosted to a higher energy level from which it is transferred to a primary electron transport chain to PS1
• As electrons pass along this transport chain, a proton gradient is established across the thylakoid membrane
• The potential energy of this electrochemical gradient is used to form ATP from ADP in a chemiosmotic process similar to that in the mitochondria
• This is known as photophosphorylation.

Explain the light-independent reactions

Explain the light-independent reactions including the roles of ribulose bisphospate (RuBP) carboxylase, reduction of glycerate 3-phosphate, (GP), to triose phosphate (TP or GALP), NADPH, H⁺, ATP, regeneration of RuBP and synthesis of carbohydrate and other products.

• Part two of non-cyclic- light independent
• In the stroma of the chloroplasts
  • ATP provides energy, and the NADPH energy and reducing power for biosynthesis using carbon dioxide.
  • The Calvin cycle in which the five-carbon molecule RuBP acts as the Co₂ acceptor (catalysed by RuBP carboxylase) is important (then forming two 3 carbon molecules-GP) but the intermediates can be given the initials GP and TP.
  • Note that the reduction of GP (removal of O) to TP requires H and most of the energy from the NADPH (and some from ATP) TP can be converted to glucose, sucrose, starch, fatty acids, amino acids, and other products.
  • Some of the TP is used, via intermediates, to regenerate more RuBP, a process that involves some more ATP.

Outline the differences in Carbon Dioxide fixation between C3, C4, and CAM plants noting their adaptive significance

• C3
  • Described in part two of non-cyclic light independent.
• Types of plants-rice and most temperature region crops, wheat, potatoes.
• C4 plants
  • C4 pathway uses propenoate 2-phosphate (3 carbon) to manufacture a four carbon compound.
  • Type of plants that use it
    • sugar cane
  • maize
• CAM
  • Is an adaptation where some plants living in dry areas keep their stomata closed during the day to conserve water
  • They open them at night and fix the CO₂ using PEP carboxylase to eventually form various organic acids which are later decarboxylated releasing the CO₂ for synthesis.
  • These are particularly adapted to high light, high temperature, and drought.
  • Types of plants
    • Pineapple
    • Prickly pear
    • Vanilla orchid

State one crop plant example of each of CAM, C3, C4

• C3 Rice
• C4 Sugar Cane
• CAM Pineapple

Describe how photosynthetic pigments can be separated and identified by means of chromatography

• A method of separating photosynthetic pigments.
• First the plant in question is ground up, and the liquid-pasty stuff is put into a special kind of cylinder and a special kind of chemical is put into the cylinder and then the chlorophyll drips out, first the chlorophyll b and then chlorophyll a.

Draw the action spectrum of photosynthesis

Explain the relationship between the action spectrum and the absorption spectra of photosynthetic pigments

• Relationship
• Action spectrum shows the relative effectiveness of different wavelength of light for light-requiring processes, such as photosynthesis, flowering, phototropism and vision.
• Similarity between the absorption spectrum of a pigment and the action spectrum of a process is considered evidence that the particular pigment is responsible for that particular process.

Explain the limiting factors of photosynthesis

Light intensity, temperature, and concentration of carbon dioxide. (See Ecology.)

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