HL Bio: Unit 3.6 - Light Dependent Reaction of Photosynthesis

Light as a Photon

• The particle unit that has certain quantities of energy aligning with their wavelength→ Longer wavelengths = less energy 

• Absorbed by pigment molecules to get excited

  • Higher energy levels are unstable making them want to move back down and release energy as they do that

Spectrums:

X-axis is the wavelength of light

• Absorption Spectrums: percentage of light absorbed at each wavelength by a pigment 

  • Y-axis: Absorption of light 

  • Chlorophyll a and b have high absoption of light in the violte blue and red light wave lenght

  • Other pigments aren’t as good

• Action Spectrums: Shows the rate of photosynthesis (%) at each light wavelength

  • Y-axis: measure of the relative amount of photosynthesis

  • Chlorophyll a and b have high efficiency

  • Other pigments aren’t as good

Pigments

• Chemical substances that absorb specific wavelengths and reflect others (the colour we see is the light reflected)

  • Note: White and transparent substances aren’t pigments 

  • Note: Black absorbs all wavelengths and convert the energy to heat

Chlorophylls

• Main photosynthetic pigment having ranging colours of green

  • Red and blue wavelength photons are absorbed and excite the electrons within 

Photosystems

• The pigment-protein complexes located in the thylakoids membrane within the chloroplast - Goal is to absorb light and form excited electrons to then emit 

• Composed of 100 chlorophyll molecules with 30 other (accessory) pigments 

Accessory Pigments

• The variation between photosystems, like carotene and xanthophyll 

• This allows for more light to be absorbed by the great number of pigments but also the different lights that they can absorb 

  • Needs to be really closely knit to actually transfer the energy

Main types of Photosystems that work in tandem

1. Photosystem 1: Reaction center is only activated by light with wavelengths of 700nm

   • Reaction Center P700

   • Locate: Stroma Lamellae

     • in thylakoid membranes between grana

2. Photosystem 2: Reaction center is only activated by light with a wavelength of 680 nm  

   • Reaction center P680: Higher energy requirement (shorter wavelength)

   • Located: Grana

     • Cylindrical stacks of thylakoids

Photosystem 2:

• First in the loop and takes excited electrons from Photosystem II and passed along a chain of electron carries to photosystem I

• Replacement Electrons: Received through the photolysis of water

  • Done in the Oxygen-Evolving Complex (OEC)

    • Binds to two and splits them 

  • The hydrogens released will also reduce NADP

  • Oxygen is then diffused out through the stomata as a waste product

Photosystem 1

Takes exctied electrons from photosystem 2

• Excited Electron transfer: To NADP reductase enzymes 

  • These enzymes use the electrons to reduce themselves to NADP

  • The electrons in the chlorophyll molecule is excited and emitted from the reaction centre

    • Passing along a short electron carrier chain to the enzyme NADP reductase

    • Reducing tit to Reduced NADP in the stroma

• Replacement electrons: From the electron carreir plastocyanin

NADP (Nicotinamide adenine dinucleotide phosphate)

• It has an extra phosphate group

• Requires to electrons 

Cyclic Photophosphorylation

• When all NADP is reduced in the chloroplast 

  • Excited electrons from PS1 instead are given to plastoquinone carriers instead of the NADP 

  • Electrons will then flow back to PS1 via plastocyanin 

    • Causes proton pumping??

  • Continues to produce ATP when reduced NADP production isn’t possible

Parts of a Photosystem

• Light Harvesting ANtenna Complex: Located on both sides of the protein 

• Core Complex: Contains a pair of chlorophyll molecules within the reaction centre

Light Harvesting ANtenna Complex

• It takes light energy that causes the electrons in the atoms of pigments to become excited and jump to higher energy levels (only a very specific wavelength: Wavelength increase = Energy down) 

• Excitation Energy Transfer: When energy drops to it’s og level energy emitted is re-absorbed by electrons in the adjacent pigment 

  • It just keeps passing along until it reaches the main complex

Fluorescence

Light energy being re-emitted as light when the electrons drop down

• Interestingly it’s more effective in low light intensities because then everything is absorbed, higher light intensities has tons of remission by fluorescence

• this is because the carbon fixation step isn’t working fast enough

Core Complex

• Contains a pair of chlorophyll molecules within the reaction centre

  • Donate pairs of excited electrons to electron acceptors to be emitted 

  • The removal of negative electrons leaves a positive charge which is quickly replaced to continue photosynthesis

Chemiosmosis: ATP Production

1. Excited electrons are generated by the photosystem II and passed to photosystem I

2. The excited electrons are passed to plastoquinone → Accepts two electrons and protons from the forming plastoquinol

3. Plastoquinol movies through the thylakoid membrane to the cytochrome b₆f complex and gives it two protons 

   • Converting the plastoquinol back to plastoquinone which returns to the PS1

4. Proton gradient is formed across the thylakoid membrane 

5. Cytochrome b₆f contains electron transport chains taking the electron from plastoquinol to plastocyanin

   • Plastocyanin is water-soluble unlike plastoquinone and can freely move in the fluid space of thylakoid

6. Plastocyanin transports the electrons to photosystem I through the fluid 

   • Electrons going to PS1 have less energy 

   • The energy was used to pump protons from the stroma to the thylakoid space

   • fromign a concentration gradient/ store of potential energy

     • Not a lot of protons required to make a high concentration

7. The proton concentration gradient generates ATP by passing protons across the membrane via ATP synthase protein 

   • The energy released by the passage of protons is used to make ATP from ADP and inorganic phosphates 

   • And releases it to the stroma

plastoquinone

• That is an electron carrier in the thylakoid membrane