Photosynthesis uses light energy to synthesise organic molecules

• Photosynthesis


  The overall equation is 6CO2 + 6H2O → C6H12O6 + 6O2
• Photosynthesis


  1. Light-dependent stage

  2. Light-independent stage
• Light-dependent stage


  • Converts light energy to chemical energy
• Light-dependent stage


  1. Photolysis of water releases electrons and protons

  2. Energy carried by electrons establishes a proton gradient across the thylakoid membrane

  3. Used to phosphorylate ADP, generating ATP (photophosphorylation)

  4. Protons and electrons reduce NADP
• Light-independent stage


  ATP and reduced NADP reduce carbon dioxide and produce energy-containing glucose
• Photophosphorylation


  An endergonic reaction bonding a phosphate ion to a molecule of ADP using energy from light, making ATP
• Chloroplasts


  • Surrounded by a double membrane
  • Inner membrane folds inwards to make thylakoid lamellae
  • Thylakoid lamellae combine in stacks of up to 100 disc-shaped structures, forming grana
  • Photosynthetic pigments are located in the grana
  • Light-dependent reactions take place in the grana
  • Stroma is the fluid-filled interior bathing the thylakoids and grana, where the light-independent reactions take place
  • Starch grains appear white due to the stain used for electron micrographs
• Chloroplasts are descended from cyanobacteria
• Leaf structure


  • Palisade cells have a large surface area and are arranged perpendicular to the leaf surface
  • Spongy mesophyll cells have large air spaces to allow carbon dioxide diffusion
  • Chloroplasts are found mainly in the palisade and spongy mesophyll cells, not the epidermis
• Chloroplasts are energy transducers, converting light energy into chemical energy
• Engelmann's experiment demonstrated that blue and red light are most effective for photosynthesis
• Photosynthetic pigments


  Molecules that absorb specific wavelengths of light
• Chlorophylls and carotenoids


  • Chlorophylls absorb red and blue-violet light, reflecting green light
  • Carotenoids absorb blue-green light
• Absorption spectrum


  Graph showing how much light a pigment absorbs at different wavelengths
• Action spectrum


  Graph showing the rate of photosynthesis at different wavelengths of light
• Photosystems


  • Lie in the thylakoid membrane
  • Contain an antenna complex of pigment molecules
  • Contain a reaction centre with two chlorophyll a molecules
• Antenna complex


  Array of proteins and pigment molecules in the thylakoid membrane that transfer energy from a range of wavelengths to the chlorophyll a in the reaction centre
• Light-dependent stage


  1. Photolysis of water releases electrons and protons

  2. Electrons are passed through an electron transport chain, establishing a proton gradient

  3. Proton gradient drives ATP synthesis (photophosphorylation)

  4. Protons and electrons reduce NADP
• Cyclic photophosphorylation


  ATP can be produced by electrons cycling back into the chlorophyll a of the reaction centre
• Non-cyclic photophosphorylation


  ATP and NADPH are produced by electrons passing through both photosystems
• The light-independent stage (Calvin cycle) uses the ATP and NADPH produced in the light-dependent stage to reduce carbon dioxide and synthesise glucose
• Nat-cycle


  photphophorylation ATP can be produced by elections fr tea le paway dough and PNADE which they wure
• The two stages of photosynthesis

  1. Photnthesis includes a sequence of reactions that take place on the thylakoid membranes, using light as an energy source and using water. This sequence is the dependent stage and it produce ATP, which provides the chemical energy transduced from light energy, to synthesi energy-rich molecules such as glucose

  2. The reactions using ATP and reduced NADP making molecules such as glucose, accur solution in the stroma. They can happen in the light but do not require it. These reactions const the light-independent stage, and include a cycle of reactions called the Calvin cycle, named after one of their discowarers.
• Photophosphorylation


  Phosphorylation is the addison of a phosphate ion to ADP. The term photophosphorylation imples that the energy for this reaction comes from light.
• Cyclic photophosphorylation


  1. PSI absorbs photons, which excites electrons in the chlorophyll a molecules in its reaction catre

  2. These are ented and picked up by an electron acceptor, which passes them down a chain of electron camers back to PSI. The energy released as electrons pass through the electron transport chain phosphorylates ADP to ATP.

  3. Electrons have flowed from PSI to the electron acceptor, back to PSI so this phosphorylation is described as cyclic photophosphorylation
• Non-cyclic photophosphorylation


  1. In an demetive pathway, electrons are transferred from the electron acceptor to oxidised NADP in the stroma, which, with protons from the photolysis of water, is reduced:

  2. The electrons have not been returned to PSI so its chlorophyll is left with a positive charge

  3. The positive charge is neutralised by electrons from PSII. They have been excited to a higher energy level by light absorption, picked up by an electron acceptor and passed down the electron transport chain to PSI
• Photolysis


  The splitting of water molecules by light, producing hydrogen ions, electrons and oxygen
• As electrons pass through a proton pump in the thylakoid membrane


  They provide enegy to pump protons from the stroma into the thylakoid space
• The protons join H ions from the photolysis of water and accumulate


  They generate an electrochemical gradient since there are more inside the thylakoid space than there are outside, in the stroma. This gradient is a source of potential energy
• Chemiosmosis


  The H ions diffuse down their electrochemical gradient, the energy derived from ight and camed by the electrons. As they pass through an ATP theme in the thylakoid membrane, into the stroma This makes anbe theme ADP is phosphorylated to ATP.
• Once in the stroma, Hans are passed to owded NADP


  Reducing NADP+ to reduced NADP. This removal of H ions, in conjunction with the proton pump, contributes to maintaining the proton gradient across the thylakoid membranes
• Light-dependent reactions


  The reactions that take place on the thylakoid membranes, using light as an energy source and using water
• Light-independent stage


  The reactions that use the products of the light-dependent stage, occurring in the stroma of the chloroplast
• The Calvin cycle


  1. A five-carbon acceptor molecule, ribulbee bisphosphate (RuBP) combines with carbon dioxide, catalysed by the enzyme nbulosa bisphosphate carboxylase

  2. An unstabla six-carbon compound is formed

  3. The six-carbon compound immediately splits into two molecules of a three-carbon compound, glycerate-3-phosphate (GP)

  4. GP is reduced to triose phosphate by reduced NADP. Raducing a molecule requires energy and in this case, the energy is provided by the ATP made in the light-dependent stage. Triose phosphate is the first carbohydrate made in photosynthesis.

  5. Some of the triose phosphate is converted to glucose phosphate, and then into starch by condensation

  6. Most of the triose phosphate goes through a series of reactions which regenerates RuBP so the cycle can continue. ATP made in the light-dependent stage provides the energy for this to happen.
• Rubisco is the most abundant protein in the biosphere, and the high concentration reflects its importance
• Plants need a suitable environment to be efficient at photosynthesis. They need:
• If any of these factors is lacking, photosynthesis cannot take place. Each factor has an optimum value of which the rate of photosynthesis is at a sub-optimal level, the rate of photosynthesis is controlled by that factor. It is the limiting factor, because it is limiting or controlling, the rate of photosynthesis
• As the carbon dioxide concentration increases

  The rate of the light-independent reactions increases, and so the rate of photosynthesis increases, showing that carbon dioxide concentration is a limiting factor
• When the carbon dioxide concentration is increased above about 0.5%, the rate of photosynthesis remains constant, implying that carbon dioxide concentration is not affecting the use of photosynthesis and is therefore not a limiting factor at these concentrations
• Limiting factor


  A factor that is limiting the rate of a physical process by being in short supply. An increase in the limiting factor will increase the rate of the process