Photosynthesis 🪴

• Calculating stomatal density

  1. Apply clear nail polish to the lower epidermis of a leaf between the veins and let it dry

  2. Use forceps to peel off the nail polish from the leaf it'll produce a replica of the lower epidermis

  3. Place it on a microscope and count the number of stomata using microscope
• Photosynthetic pigments

  Found within the thylakoid membrane
• During photosynthesis different wavelength of light strike the leaf
• Role of photosynthetic pigments

  Absorbs and converts light energy from sun into chemical energy
• Accessory pigments

  • they are Important as they absorb photons of light that are not absorbed by primary pigments to ensure a wider range of light is absorbed increasing efficiency of photosynthesis
• Chlorophyll b

  Absorbs blue and red wavelength of light
• Carotenoids

  Absorb violet or blue light
• Chlorophyll a

  Absorbs blue and red wavelength of light, Mg needed to produce it
• Chromatography (to separate photosynthetic pigments)

  1. Grind teared up leaves with acetone which dissolves the phospholipid membrane to extract the pigment

  2. Use capillary tube to spot the pigment on chromatography paper

  3. Place paper into solvent of petroleum or acetone to dissolve and carry the pigment up

  4. Remove paper and observe different distances pigments have travelled

  5. More soluble substances move further

  6. Calculate RF value = distance travelled by pigment/ distance travelled by solvent

  7. Compare rf values
• Action spectrum

  Shows the rate of photosynthesis at different wavelengths
• Absorption spectrum

  Shows amount of light absorbed by each pigment at each wavelength
• Action spectrum and absorption spectrum

  There is a close correlation between the 2 spectra as peaks on absorption spectra are at similar wavelength to peaks on action spectrum this suggests that the light absorbed by pigments is used in photosynthesis
• Photosystems


  1. Light energy is absorbed and passed on to chlorophyll a in reaction centre, this energy causes pair of electrons in chlorophyll a to move to higher energy level (excited) these electrons leave chlorophyll a (photo ionisation)

  2. Absorbs photons of light energy and transfer light energy to high energy electron this energy is used to make ATP and fuel proton pumps

  3. Chlorophyll a and its associated protein= reaction centre which is surrounded by accessory pigments and proteins= light harvesting system/antenna complex

  4. Light harvesting system and reaction centre= photosystem

  5. Photosystem 1 = 700nm

  6. Photosystem 2 = 680nm
• Cyclic photophosphorylation

  Light energy is absorbed by photosystem 1, 2 electrons are excited and passed across the ETC generating ATP, 2 electrons return back to photosystem 1
• Non-cyclic photophosphorylation

  Light energy absorbed by PS2, 2 electrons excited and passed along ETC by series of oxidation and reduction reactions, electrons are then passed to PS1, as electrons pass their energy is used to produce ATP by chemiosmosis, light energy is absorbed by PS1 and 2 electrons are excited and passed through ETC generating ATP by chemiosmosis, at the end 2 electrons are added to NADP in stroma to form reduced NADP, photolysis occurs to replace the 2 electrons that are lost= enzyme splits water into O, 2H and 2e (energy from light is used for this)
• Calvin cycle (light-independent reaction)

  RuBP(5c) reacts with CO2 (reaction catalysed by enzyme rubisco)forming two glycerate-3-phosphate, each molecule of G3P is reduced by accepting H of reduced NADP to form 2x triose phosphate, energy is used from ATP (ATP and NADPH comes from light-dependent reaction), some TP is removed from cycle to make glucose or lipids or amino acids, regeneration= ATP is used to regenerate RuBP allowing cycle to continue
• 6CO2 + 6H2O -> C6H12O6 + 6O2
• LDR (non-cyclic)