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Fig. 10.1. 3 Stages of Photosynthesis. Capturing light energy Using this energy to make ATP to split H2O molecules and use (H+) to reduce NADP+ to NADPH Both 1&2 need light and are known as the LIGHT REACTIONS

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3 stages of photosynthesis
3 Stages of Photosynthesis
  • Capturing light energy
  • Using this energy to make ATP to split H2O molecules and use (H+) to reduce NADP+ to NADPH

Both 1&2 need light and are known as the LIGHT REACTIONS

3. The power of ATP and NADPH is used to synthesize organic molecules (glucose) from CO2 in the air (carbon fixation)

Carbon fixation can happen without light and is called THE CALVIN CYCLE

  • Photosystems: clusters of pigment of the thylakoids

- each pigment molecule of a photosystem is capable of capturing PHOTONS (packets of electromagnetic energy) that boost the pigment’s atoms to a higher energy level.

- a protein lattice holds the pigments in close contact with one another and the energy gained in each pigment molecule is transferred from one chlorophyll molecule to another in the thylakoid until it reaches a key protein that transfers the excited electrons of energy to a series of proteins that make NADPH+ and ATP to make glucose in the stroma.

- leaves absorb mostly blue and red light and the green if reflected (that is why we SEE it)

photosystem ii
Photosystem II
  • Photons strike a chloroplast and the reactions center’s chlorophyll a electrons are boosted
  • Reaction Center: transmembrane protein-pigment complex where electrons are excited by electromagnetic energy of sun and light energy is converted to chemical energy
  • 2. The excited electrons leave the chlorophyll molecule and jump to a nearby thylakoid membrane protein in the reaction center where conversion of light to chemical energy occurs
  • Each excited electron is pumped through a series of membrane protein channels (electron transport chain) and some of the energy is used to make ADP into ATP, which is transferred to the stroma.
  • The remaining electrons are passed to Photosystem I in a process known as non-cyclic photophosphorylation
photosystem i
Photosystem I
  • The remaining electrons from II are absorbed by photosystem I
  • Photosystem I also absorbs light and the chlorophyll B and carotenoid pigment molecule (slightly shorter light wavelengths but more energy) electrons are boosted
  • Remaining energy from II is used to catabolize H2O (electron donor) into H and the O is released
  • The H atoms and the energy from photosystem I are used to reduce NADP+ (electron acceptor) to NADPH (energy carrying molecule)
  • The NADPH is released into the stroma of the thylakoid where it combines with the enzyme rubisco (synthesized in photosynthetic cells) to power the making of sugars in the Calvin Cycle, in which the carbon atoms from CO2 are synthesized into complex carbohydrates (carbon fixation)
calvin cycle
Calvin Cycle

Phase #1: Carbon Fixation

CO2 is incorporated in to a five-carbon molecule called ribulose biphosphate (RuBP) catalyzed by the enzyme rubisco (RuBP carboxylase). The product of this reaction is six carbon molecule which is very unstable and splits in ½ to make two 3-carbon molecules of 3-phosphoglycerate.

calvin cycle1
Calvin Cycle

Phase #2: Reduction

ATP and NADPH (light reactions) are used to convert the 3 carbon molecules into glyceraldehyde 3-phosphate (G3P) which is a 3-carbon precursor to glucose.

Phase #3: Regeneration

More ATP is used to convert some of the product of #2 back to RuBP (acceptor of CO2).

Net Output: 3 molecules of CO2 yields one molecule of G3P

  • The enzyme rubisco has a second enzyme activity (oxidation of ribulose) that can interfere with the Calvin Cycle
  • Both reactions are catalyzed on the same active site on rubisco and compete with each other.
  • The second enzyme activity actually REALEASES CO2, undoing the reduction of CO2 to carbohydrates.
  • Approx. 20% of the fixed carbon of photosynthesis is lost to photorespiration and warmer temps increase this loss (tropical plants) in C3 plants.
  • C4 plants use C4 respiration in which the CO2 encounters a different enzyme (PEP carboxylate) first that has no oxidation activity (so no photorespiration) where ther CO2 is modified before it is passed on to the rubisco and then enters the Calvin Cycle. (Ex. crab grass, corn, sugar cane, sorghum)
other c4 and cam pathways
Other C4 and CAM Pathways
  • Some C3 plants have evolved other modifications (C4 pathways) to the typical photosynthesis process that helps them to avoid the oxidation of ribulose.
  • Ex. The light reactions still occur in the chloroplasts but the Calvin Cycle occurs in the bundle sheath that surrounds the vascular bundle. The cells there are impermeable to CO2 release so no photorespiration occurs.
other c4 and cam pathways1
Other C4 and CAM Pathways
  • CAM Pathway (cossulaeum acid metabolism)

Used by some tropical plants to cut down on photorespiration %.

These plants switch the time of day when stomata are open. Open at NIGHT when cooler to avoid heat of day that normally increases CO2 loss.