Photosynthesis

• energy is needed for active transport, anabolic reactions and movement
• energy cannot be created or destroyed
• light energy from the sun is captured by chlorophyll and is used to drive the synthesis of glucose from CO2 and H2O
• energy is trapped in the bonds between organic molecules e.g. glucose
• energy is needed to break bonds and is released when bonds form
• atoms in small inorganic molecules are joined by strong bonds, and release a lot of energy when broken
• organic molecules have weaker bonds which release less energy but also require less energy to form
• low levels of energy are required to break the bonds between C-H atoms in organic molecules. the C and H then form strong bonds with oxygen atoms, which releases large quantities of energy.
• the bond energy in ATP is used to drive essential metabolic processes
• Chemiosmosis - the synthesis of ATP driven by the flow of protons across a membrane from a region of high conc. to one of low conc.

  • the flow of protons releases energy that combines an inorganic phosphate with ADP to generate ATP
  • to ensure chemiosmosis can occur, a proton conc. gradient must be created
• • electrons can become excited two ways:
  • absorbing photons (in chlorophyll)
  • released when chemical bonds break
  • they can pass into a transport chain and set up a protein gradient
• Electron transport chain:

  • the ETC is made up of a series of electron carriers which accept and release electrons
  • each electron carrier has a slightly lower energy level
  • as high energy electrons move from one carrier to the next, energy is released
  • this energy is used to pump protons across a membrane, setting up a conc. gradient
  • the protons can only move back through via hydrophilic channels attached to ATP synthase
  • the flow of protons through the channel provides energy to synthesis ATP from ADP + Pi
• Chloroplasts:

  • double membrane
  • intermembrane space
  • outer membrane is permeable to many small ions
  • inner membrane is less permeable and has transport proteins embedded in it
  • inner membrane folds to form Thylakoids which are stacked into Grana
  • contain DNA and ribosomes to synthesise proteins needed for photosynthesis
• Grana:

  • made of fluid filled sacks called thylakoids
  • site of light absorption and ATP synthesis
  • light dependent stage of photosynthesis takes place here
  • photosynthetic pigments embedded in membrances
  • they absorb energy from different wavelengths of light
• a network of proteins in the grana hold the photosynthetic pigments in a precise manner, forming special units called a light harvesting system (antennae complex) which is located around the reaction centre. All together this is known as the photosystem. The antennae complex absorbs different wavelengths of light and pass the energy onto the reaction centre
• Stroma:

  • fluid contains all enzymes needed to carry out light independent reaction
  • surrounds the grana so the products of the LDR don't have to travel far before being used in the LIR
• Photosystems:

  • when sunlight is absorbed by pigments, it behaves as a particle or photon
  • each photon contains a fixed amount of energy called a quantum
  • the size of the quantum varies with the wavelength of light; shorter wavelength = larger quantum
  • when chlorophyll absorbs a photon, a pair of magnesium electrons become excited
  • the chlorophyll is raised to an excited state
• PSII:

  • photosystem 2
  • primary pigment P680; a type of chlorophyll a
• PSI:

  • photosystem 1
  • primary pigment P700
• light-dependent stage:

  • occurs on thylakoid membranes
  • light energy absorbed by chlorophyll
  • energy used to make ATP
  • water molecules split to make H+, O2 and electrons
  • H+ and electrons picked up by NADP reductase to form reduced NADP
  • oxygen excreted from chloroplast
• light-independent stage:

  • ATP and reduced NADP used
  • takes place in stroma
  • stroma contains RuBP to form triose phosphate
  • Calvin cycle
• Photophosphorylation:

  • ADP + phosphate --> ATP using energy from sunlight
  • when a photon hits a chlorophyll molecule (PSII), the energy is transferred to 2 electrons which become excited
  • electrons are captured by primary proton acceptor and is passed along a series of electron carriers which contain iron
  • energy is released as the electrons travel
  • this pumps protons across the thylakoid membrane, creating a proton gradient
  • protons flow down gradient through channels associated with ATP synthase
• Cyclic phosphorylation:

  • involves only PSI
  • produces ATP not reduced NADP
  • once the electron has lost all energy, it returns to PSI
  • thus, PSI can produce ATP without electrons from PSII
• Non-cyclic phosphorylation Stage 1:

  • PSII absorbs light and excites a pair of electrons at the reaction centre; they are captured by the primary electron carrier
  • the oxidised chlorophyll becomes a strong oxidising agent
  • an enzyme in PSII catalyses the breakdown of water into H+, e- and O2 using energy from the sun. The 2e- replace the lost electrons from PSII
• Non-cyclic phosphorylation Stage 2:

  • protons are released into the lumen of the thylakoids, increasing the proton conc. gradient
  • as they move back, ATP is generate
  • the H+ ions then combine with NADP and electrons from PSI
  • this also removes H+ from the stroma, maintaining the proton gradient
• Non-cyclic phosphorylation Stage 3:

  • the electrons from stage 2 reach the bottom of the transport chain and fill the hole left by the electrons from PSI
• Light-independent reaction:

  • occurs in stroma
  • known as Calvin cycle
  • products of LDR used
• Calvin cycle Stage 1:

  • CO2 diffuses from air into open stomata
  • diffuses into spongy and palisade mesophyll
  • diffuses into cytoplasm and then into the stroma
• Calvin cycle Stage 2:

  • CO2 combines with RuBP
  • the reaction is catalysed by Rubisco
  • the RuBP is carboxylated
• Calvin cycle Stage 3:

  • the unstable six-carbon compound formed in stage 2 splits into 2 molecules of glycerate-3-phosphate with 3 carbons
• Calvin cycle Stage 4:

  • GP is converted into a triose phosphate (TP) using reduced NADP
  • TP is a 3 carbon sugar which can be used to synthesise many complex biological molecules or recycled to make RuBP
• Calvin cycle Stage 5:

  • 1/6th of the total TP is used to make glucose which can be converted into other carbs, amino acids, fatty acids, glycerol
  • the remaining 5/6ths is converted back into 3 molecules of RuBP
• For one glucose molecule to be produced, 6 carbon dioxide molecules must enter the Calvin cycle, resulting in 6 full turns of the cycle. This results in the production of 12 TP molecules, two of which are removed to make a glucose molecule. Therefore, 10 TP molecules are recycled to make six RuBP molecules.
• higher light intensity = more ATP and reduced NADP generated
• more carbon dioxide = mroe CO2 to be fixed into GP
• higher temp = faster reactions (enzymes)