Photosynthesis and the LDR

• Photosynthesis takes place in the chloroplasts of plant cells.
• Chloroplasts: small, flattened organelles surrounded by a double membrane. Thylakoids (fluid-filled sacs) are stacked up in the chloroplast into structures called grana (singular = granum). The grana are linked together by bits of thylakoid membrane called lamellae (singular = lamella)
• Chloroplasts contain photosynthetic pigments e.g. chlorophyll a, chlorophyll b and carotene. These are coloured substances that absorb the light energy needed for photosynthesis
• The pigments are found in the thylakoid membranes and are attached to proteins. The protein and pigment is called a photosystem
• There are 2 photosystems used by plants to capture light:

  1. Photosystem I (PSI) - absorbs light best at a wavelength of 700 nm
  2. photosystem II (PSI) absorbs light best at 680 nm
• Contained within the inner membrane of the chloroplast and surrounding the thylakoids is a gel-like substance called the stroma. It contains enzymes, sugars and organic acids. Carbohydrates produced by photosynthesis and not used straight away are stored as starch grains in the stroma
• REDOX reactions: reactions that involve oxidation and reduction
• Redox reactions occur in both photosynthesis and respiration
• If something is reduced it has gained electrons (e-), and may have gained hydrogen or lost oxygen
• If something is oxidised it has lost electrons, and may have lost hydrogen or gained oxygen
• Oxidation of one molecule always involves the reduction of another molecule
• Coenzymes: a coenzyme is a molecule that ads the function of an enzyme.
• Coenzymes work by transferring a chemical group from one molecule to another
• A coenzyme used in photosynthesis is NADP. NADP transfers hydrogen from one molecule to another - this means it can reduce (give hydrogen to) or oxidise (take hydrogen from) a molecule
• The LDR needs light energy - it takes place in the thylakoid membrane of the cholorplasts
• 1. light energy is absorbed by chlorophyll and other photosynthetic pigments in the photosystems
  2. The light energy excites the electrons in the chlorophyll, giving them more energy - eventually causes them to be released from the chlorophyll molecule - photoionisation
  3. chlorophyll molecule now a positively charged ion
  4. some of the energy from the released electrons is used to add a phosphate group to ATP to form ATP, and some is used to reduce NADP to form NADPH.
  5. ATP transfers energy and NADPH to the LIR
  6. During the process H20 is oxidised to O2
• The LIR: this reaction does not use light energy directly - but it relies on the products of the LDR. Takes place in the stroma of the cholorplasts. Here the ATP and NADPH from the LDR reaction supply the energy and hydrogen to make glucose from CO2
• In the LDR, the energy resulting from the photoionisation of chlorophyll is used for three things:

  1. making ATP from ADP and inorganic phosphate - photophosphorylation (the process of adding phosphate to a molecule using light
  2. making reduced NADP from NADP
  3. Splitting water into protons (H+ ions), electrons and oxygen - photolysis (the splitting (lysis) of a molecule using light (photo) energy)
• The LDR reaction actually takes includes 2 types of phosphorylation - non-cyclic and cyclic - the processes have different products
• Non-cyclic photophosphorylation produces ATP, NADPH and oxygen
• Photosystems in the thylakoid membranes are linked by electron carriers - proteins that transfer electrons
• The photosystems and electron carriers form an electron transport chain - a chain of proteins through which excited electrons flow
• There are several processes going on at once in non-cyclic phosphorylation
• 1: light energy excites electrons in chlorophyll - light energy is absorbed by PSII and the light energy excites electrons in chlorophyll. The electrons move to a higher energy level (i.e. they have more energy). These high-energy electrons are released from the chlorophyll and move down the electron transport chain to PSI
• 2: Photolysis of water produces protons, electrons and oxygen. - As the excited electrons from chlorophyll leave PSII to move down the electron transport chain, they must be replaced. Light energy splits water into protons (H+ ions), electrons and oxygen - photolysis

  H2O = 2H + 1/2O2
• 3: Energy from the excited electrons makes ATP - the excited electrons lose energy as they move down the electron transport chain. This energy is used to transport protons (H+ ions) into the thylakoid so that the thylakoid has a higher concentration of protons than the stroma. This forms a proton gradient across the thylakoid membrane. Protons move down their concentration gradient into the stroma, via the enzyme ATP synthase, which is embedded into the thylakoid membrane. The energy from this movement combines ADP and inorganic phosphate Pi to form ATP
• 4: Energy from the excited electrons generates reduced NADP - light energy is absorbed by PSI, which excites the electrons again to an even higher energy level. Finally the electrons are transferred to NADP, along with a proton (H+ ion) from the stroma, to form reduced NADP
• Chemiosmotic theory - the process of electrons flowing down the electron transport chain and creating a proton gradient across the membrane to drive ATP synthesis is called chemiosmosis
• Cyclic phosphorylation: produces ATP and only used PSI. It is called cyclic because the electrons from the chlorophyll molecule aren't passed onto NADP, but are passed back to PSI via electron carriers. This means that the electrons are recycled and can repeatedly flow through PSI. This process doesn't produce any reduced NADP or oxygen - it only produces small amounts of ATP