Photosynthesis

• The stroma contains enzymes for the light independent reaction
• The granum is a stack of thylakoids, maximising surface area
• Thylakoids contain photosynthetic pigments such as chlorophyll which absorb light at a variety of wavelengths
• Starch vacuoles in the stroma store excess carbohydrates from photosynthesis
• The light dependent reaction occurs in the thylakoid membrane
• The light independent reaction occurs in the stroma
• Photosynthetic pigments include chlorophyll a and b, as well as carotenoids (accessory pigments)
• Carotenoids reflect red, orange and yellow light and absorb green light
• Chlorophyll A has an associated magnesium on the polar head
• Photosystems are clusters of chlorophyll and accessory pigments, accessory pigments pass photons to chlorophyll a in the reaction centre
• By having carotenoids which absorb green light, plants can photosynthesise more efficiently as all wavelengths are absorbed
• Carotenoids include carotenes and xanthopylls
• Photosystem 1 has an absorption peak of 700nm
• Photosystem 2 has an absorption peak of 680nm
• Rf value - distance travelled by pigment / distance travelled by solvent
• Rf values can be used to identify unknown pigments on a chromatogram
• An absorption spectra shows the wavelength of light absorbed by different chlorophyll pigments
• An action spectra shows the rate of photosynthesis at each wavelength
• Photosynthesis increases as light absorbed increases
• Reduction is where a molecule gains an electron or hydrogen ion or loses an oxygen
• Oxidation is where a molecule gains an oxygen or loses an electron or hydrogen ion
• The light dependent reaction produces ATP and NADPH for the light independent reaction
• NADP is a coenzyme that transfers hydrogen
• When light energy hits PS2, its electrons are excited and travel to an electron acceptor, oxidising the chlorophyll
• Photolysis is where light energy is used to break water into hydrogen ions and oxygen, the electrons from the hydrogen ions reduce the chlorophyll molecule
• The electron acceptor next to PS2 will transfer electrons down the electron transport chain to PS1s electron acceptor
• The electron transport chain causes H+ ions to be pumped by a proton pump into the thylakoid lumen, increasing the concentration
• Electrons reach PS1 where they are excited by light, causing them to travel to the electron acceptor
• The electrons in the PS1 electron acceptor reduce NADP to NADPH for the calvin cycle
• H+ ions in the thylakoid lumen will diffuse by facilitated diffusion through ATP synthase down an electrochemical gradient, causing ATP to be produced by ADP + Pi by chemiosmosis for the calvin cycle
• The light independent reaction begins with the enzyme RUBISCO, which catalyses the fixation of CO2 to RUBP from CO2 forming an intermediate 6 carbon compound
• Calvins lollipop experiment involved using photosynthetic products with associated radioactive carbon in chromatography before using x-ray to identify products based on when they were created
• This intermediate 6 carbon compound is then split into 2 GP molecules (3C)
• The GP molecules are then reduced via oxidation of 2 NADPH to 2 NADP using energy released from 2 ATP to 2 ADP + 2 Pi
• The 2 GP are reduced into 2 triose phosphates (3C)
• 1/6 of the triose phosphate molecules made in calvin cycles go towards producing hexose sugars
• 5/6 of the triose phosphates go towards regeneration of RUBP using 1 ATP to 1 ADP + 1 Pi
• The regenerated RUBP (5C) returns to the calvin cycle by action of RUBISCO
• Decreasing the concentration of carbon dioxide increases the volume of RUBP as it can no longer be combined with CO2, it decreases the volume of GP and triose phosphate as they are used up to regenerate RUBP
• Decreasing the light intensity will increase volumes of GP as less ATP and NADPH will be made in the light independent reaction so less triose phosphate and RUBP can be made