Photosynthesis

Concepts:

Chemical Equation:

  • 6H2O + 6CO2 (light → energy) C6H12O6 + 6CO2

Location of Photosynthesis

  • Mesophyll Cells in the Chloroplast

Structure of a Chloroplast

Double Membrane Envelope:

  • Outer Membrane - Covers the entire chloroplast. The barrier between the chloroplast and the cell

  • Inner Membrane - inside the outer membrane.

  • Intermembrane Space - the space in between them

Inside

  • Stroma - the aqueous environment within the inner membrane. Enzymes that catalyze the Calvin Cycle are here. The Calvin Cycle happens here.
    Thylakoid - the discs in the chloroplast. Contains Chlorophyll + other pigments

  • Granum - stacked thylakoids

  • Stroma Lamella - connects the granum

Thylakoid Membranes

  • Pigments: Absorb photons from sunlight energy to use for the LIGHT DEPENDENT RXNS

  • Light reactions happen here

  • Thylakoid Lumen: Similar to the STROMA but inside the thylakoid

Light Energy + bits of the Photosystems

  • Light gets absorbed by the pigments within the thylakoid. These pigments absorb different wavelengths.

  • When they absorb light, the ELECTRONS within the chloroplasts from the MAGNESIUM 2+ in the middle of the structure either

    1. Get Excited but then return to their ground state and energy is lost as thermal energy

    2. Transfers the electrons to the neighbouring pigment molecule

    3. Transfers the electrons to the PRIMARY ELECTRON ACCEPTOR

  • Chlorophyll A gets oxidized as it loses its electrons to the Primary Electron Acceptor.

  • Primary Electron Acceptor gets reduced as it accepts Chlorophyll a’s electrons


  • Pigments do NOT absorb all wavelengths. The ones that are not absorbed either pass through the pigment or are reflected

  • Red and Blue wavelengths are absorbed the most by chlorophyll as it is the most common pigment and it reflects green, making plants appear green. During fall chlorophyll is reduced and other colours are reflected instead (yellow, orange)

  • Chlorophyll a STRONGLY absorbs red and blue lights but not green or yellow lights which get reflected instead. Anything not absorbed by Chlorophyll A that is red/blue gets absorbed by Chlorophyll B???

Pathways of Photosynthesis

Photosystem II / PSII / PS680

  • Contains P680 molecules that CONTAIN CHLOROPHYLL A.

  • Photon → Antenna Complex → sends the photon to P680 which gets it excited and goes from ground state into high energy

  • When P680 is excited, it gives it’s 2 electrons to the Primary Electron Acceptor. It turns it into P680+ and becomes very electronegative (the most in biology)

  • Because of it’s electronegativity, it can split up H2O. Through the Water Complex Enzyme, the electronegative pull from P680+ splits H2O → 2H+ and ½ O2 and transfers its 2 electrons through P680+ which neutralizes it → P680 again and it sends the electrons to the Primary Electron Acceptor

  • In this process P680 is OXIDIZED (Loses Electrons) and the P.E.A is REDUCED (gains electrons)

PQ

  • An Electron Transporter

  • Transports Electrons and H+ ions

  • Gets Electrons from P.E.A and transfers them to Cytochrome COmplex

  • Gets H+ from Stroma (REDUCED) and transfers them to the Lumen (OXIDIZED)

Photosystem 1 / PSI / P700

  • Re-energizes the electrons that went through PSII → PQ → Cyt. Comp. → PC → PSI

Synthesis of ATP & Reduction of NADP to NADPH

  • PQ carries H+ from STROMA to LUMEN = proton gradient. H+ high in lumen lower in stroma

  • H+ in LUMEN diffuse through ATP Synthase = ATP and transports them to the stroma

  • (2) H+ in the STROMA combined with NADP + using the electron transported to enzyme NADP reductase, NADP → NADPH

  • NADP + 2H+ (through NADP reductase and electron sent to it) → NADPH + H+

Cyclic ET

  • PSI is not dependent on PSII anymore

  • PSI absorbs a photon → passes through the pigments to PS700 → PS700 gets reduced when it gets the electron and gets excited → gets oxidized when sends it to the P.E.A and PEA gets reduced → PEA sends it to Fd → Fd sends e- to PQ which also receives H+ → PQ sends it to Cytochrome Complex and releases H+ → Cytochrome Complex sends it back to PSI and cycle repeats

  • NADPH is not produced but more ATP is produced (the H+ released by PQ gets diffused in ATP synthase)

Calvin Cycle

Carbon Fixation

  1. 3 CO2 molecules enter the cycle

  2. 3CO2 + RuBP (using enzyme Rubisco) → 6 3PGA

CO2 (C) + RuBP (P-C-C-C-C-C-P) → 2 GPA (C-C-C-P)

Reduction

  1. The 6 3PGA (through ATP → ATP + Pi) → 6BiPGA (gains an extra phosphate from the Pi)

PGA (C-C-C-P) + Pi → BiPGA (P-C-C-C-P)

  1. The 6 BiPGA → 6 G3P through (6NADPH getting reduced to 6NADP+, losing e- and H+)

BiPGA (P-C-C-C-P) → G3P(C-C-C-P)

  • 1 of the G3P molecules will be used to make sugar (2 is needed to make glucose)

Regeneration

  1. The 5 other G3P turns into 3 RuBP (using 3 Pi from ATP → ADP + Pi)

5 G3P (C-C-C-P) → 3RuBP(P-C-C-C-C-C-P)

2 G3P are needed to make 1 Glucose; Calvin cycle needs to occur twice to make 1 glucose; requires double the molecules needed,

1 Cycle: 3CO2, 6 + 3 ATP, 3NADPH

2 Cycles: 6CO2 12 + 6 ATP, 6 NADPH


C4 Plants

  • CO2 enters the Mesophyll cells and immediately gets fixed into a 4-carbon molecule



Comparing C3/C4/CAM

C3: normal one

  • Occurs in the mesophyll cells

  • CO2 → directly to the Calvin Cycle to be fixed into a CO3 molecule

C4: alters the location of photosynthesis

  • Occurs in the mesophyll cells and the bundle sheath cells

  • CO2 → mesophyll cells, get fixed into a 4-carbon molecule (oxaloacetate) → bundle sheath cells to protect it from the O2 accumulating → Calvin Cycle

CAM: alters the time of photosynthesis

  • Stomata opens during the day to let CO2 in




Terms:

Stomata: small openings within a plant cell which takes in CO2 and releases O2

Photolysis: the process of breaking down a compound into smaller pieces through light

Photophosphorylation: the process of using light energy to convert ADP into ATP for plant

  • Light Dependent Rxns

Primary Electron Acceptor - A molecule that is capable of accepting electrons and becoming reduced in photosynthesis.

Reaction Centre - a bunch of proteins and pigments. Contains the Primary Electron Acceptor and Chlorophyll A

Photosystem II (PS680): a collection of pigment proteins that includes P680 Chlorophyll a molecules and absorbs light the best at a wavelength of 680nm

Photosystem I (PS700): a collection of pigment proteins including P700 Chlorophyll a molecules and absorb light the best at a wavelength of 700nm.

Antenna Complex - Transfers light to the chlorophyll a molecule in the photosystem

PQ - an electron carrier (Primary Electron Acceptor → Cytochrome Complex)

  • Calvin Cycle

Rubisco - an enzyme that is responsible for the fixation of CO2 in the Calvin Cycle. Acts as a catalyst to reduce CO2.

RuBP - Rubisco BiPhosphate (P-C-C-C-C-C-P) 5 carbon molecule

PGA - Phosphoglycerate (C-C-C-P) 3 carbon molecule

CO2 fixation - turning CO2 (an inorganic carbon molecule) → ORGANIC molecules

  • Other

Photorespiration - The catalysis of O2 instead of CO2 by Rubisco into RuBP which slows the Calvin cycle, consumes ATP, and releases Carbon

C4 - 4 carbon compound

Plasmodesmata

Mesophyll Cells - DO NOT contain Rubisco (around the sheath cells)

Bundle Sheath Cells - Contain Rubisco (around the veins)

Transpiration - loss of water vapour from plants

incandescent - emitting light as a result of being heated.

fluorescent - emitting light normally from like photons