C1.3 Photosynthesis

• photosynthesis is the transformation of light energy into chemical energy in the form of carbon compounds
• the glucose produced by photosynthesis is used to synthesize the macromolecules of life
• photosynthesis supplies most of the energy needed for life processes in eco-systems
• photosynthesis produces glucose which is then used to produce other organic compounds
• photolysis is the use of light energy (which is absorbed by chlorophyll) to split water into hydrogen ions and oxygen gas
• photosynthesis is carried out by cyanobacteria, algae and plants (all 3 produce oxygen gas as a byproduct due to the splitting of water)
• chlorophyll is the main light-absorbing pigment in leaves. It absorbs blue and red light best and reflects green.
• leaves contain other accessory pigments that absorb different wavelengths of light than chlorophyll
• accessory pigments include;

  carotene - has an orange/red color

  xanthophyll - has a deep yellow color
• chromatography can be used to separate pigments in leaves
• calculating rf values from leaf samples helps us distinguish the different pigments present (chlorophyll, Xanthophyll, etc)
• photosynthetic pigments absorb light which excite electrons within the pigment. this excited electron leaves the pigment and provides energy for the synthesis of organic compounds from carbon dioxide and water
• the molecular structure of a pigment determines which wavelengths are absorbed and which are reflected
• a spectrophotometer can be used to determine the absorption spectrum for photosynthetic pigments
• a limiting factor is anything in short supply that prevents photosynthesis from occurring at a maximum rate
• there are three limiting factors in photosynthesis;

  light intensity (increases till it plateaus)

  level of CO2 (increases till something else is limiting it)

  temperature (rises initially then drops as it exceeds optimum temperature)
• scientists have used enclosed greenhouses and free-air carbon dioxide enrichment experiments (FACE) to investigate the effects of carbon dioxide on plants to a larger scale
• FACE experiments are examples of field experiments carried out in natural ecosystems
• photosystems are an array of chlorophyll and accessory pigments with a chlorophyll molecule as the reaction center (excited electrons are emitted from there)
• in photosystems excited electrons pass along an electron transport chain which then provides energy for chemiosmosis
• photosystems are located in membranes of photosynthetic organisms (cyanobacteria) and chloroplasts of photosynthetic eukaryotes
• photosynthesis uses 2 photosystems (PS II and PS I). Both are located in the thylakoid membrane of chloroplasts.
• a single molecule of chlorophyll or any other pigment would not be able to perform any part of photosynthesis
• photolysis uses light to split water into protons (H+) and electrons. oxygen gas is a waste product
• photolysis of water

  photoactivation of photosystem II - light is shined and absorbed by PS II electrons which escape from chlorophyll. to replace electrons lost the positively charged photosystem II takes electrons from water.
• prebiotic earth did not include oxygen gas. this was developed by cyanobacteria which evolved to carry out photolysis during photosynthesis
• oxygen gas released from cyanobacteria dissolved in oceans which led to aerobic respiration in organisms. once the ocean was saturated it diffused back into the atmosphere creating O3 (also known as ozone).

  this ozone prevents ionizing radiation from the sun to reach earth.
• prebiotic oceans contained Fe2+ (iron ions) which were soluble in water, as cyanobacteria started producing oxygen that iron was oxidized to Fe3+ (an insoluble compound in water).

  this new iron formed precipitates (substance becoming a solid) sediments on the ocean floor.
• light-independent reactions of photosynthesis require photosystems to be photoactivated by light. these reactions include;

  non-cyclic photophosphorylation (where electrons do not move in a circle)

  cyclic photophosphorylation (electrons move in a cycle starting and ending at PS I)
• phosphorylation is the addition of a phosphate to a molecule
• photophosphorylation adds the phosphate to ADP using light energy
• non-cyclic photophosphorylation includes;

  photoactivation of PS II (excited electrons escape chlorophyll and enter electron transport chain)

  photolysis (water replaces electrons lost by photoactivation)

  excited electrons move to PS I (producing ATP by chemiosmosis)

  PS I is photoactivated and excited electrons are used to produce NADPH
• cyclic photophosphorylation;

  photoactivation of PS I (excited electrons escape chlorophyll and enter electron transport chain)

  electrons return to PS I from the electron transport chain (producing ATP by chemiosmosis)
• electrons released from PS II do not return to PS II during non-cyclic photophosphorylation
• electrons moving along the electron transport chain provide the energy to actively transport protons (H+) from the stroma to the thylakoid space (where a high conc of protons build up in that space)
• protons are charged particles and cannot move directly through the thylakoid membrane. chemiosmosis occurs and move these protons by ATP synthase (the kinetic energy of the protons provide the activation energy needed for ATP synthase to convert ADP and an inorganic phosphate to ATP)
• cyclic photophosphorylation occurs when there's a limited supply of NADP+. (This process only produces ATP not NADPH)
• NAPD is reduced by two electrons that have come from PS I
• "NAPD and reduced NAPD" as well as "NAPD+ and NAPDH" should be paired consistently
• the thylakoid membrane is a system for performing light-independent reactions of photosynthesis. photolysis occurs within the thylakoid space.