Ch' 2 Photosynthesis Ps and Light

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Ch. 2 Photosynthesis (Ps) and Light
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Photosynthesis

• Look at leaf anatomy
• Look at chemical reactions
• 3 major photosynthesis patterns
• C3 photosynthesis
• C4 photosynthesis
• CAM photosynthesis

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Leaf anatomy C3 plant

• Note mesophyll in 2 layers (cells have
  chloroplasts)
• Palisade mesophyll
• Spongy mesophyll
• Note also that vein has bundle sheath of small
  cells (brown cells), usually have chloroplasts

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Overview of C3 photosynthesis

• Process has 2 major stages
• 1) Light reactions convert light energy to
  chemical energy
• 2) Calvin cycle (or dark reactions) convert CO2
  to sugar

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Overview of C3 photosynthesis

• 1) Light reactions
• Convert light energy into chemical energy
• Make molecules with high-energy electrons in
  their covalent bonds
• ATP
• NADPH a high-energy electron carrier.
• Involve photolysis splitting water to make
  oxygen gas.

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Overview of C3 photosynthesis
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Overview of C3 photosynthesis

• 2) Calvin Cycle
• Puts chemical energy from ATP and NADPH into
  carbon of CO2
• Makes C-H bonds (reduces carbon)
• Produces sugar (glucose) from CO2
• 2 main stages fixing CO2 (C goes from inorganic
  to organic form), C fixation step
• Energizing Carbon (making C-H bonds)

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Overview of C3 photosynthesis

• 2) Calvin Cycle
• Melvin Calvin

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Overview of C3 photosynthesis

• Starts with 5-C molecule (RuBP)
• Using Rubisco (enzyme), adds 1 C to make 2 3-C
  pieces (PGA)
• Why called C3 photosynthesis?

Fig. 2.3
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Overview of C3 photosynthesis

• Why called C3 photosynthesis? 3-C compound is
  first product
• Carbon in pieces is reduced (energized) using ATP
  and NADPH
• Eventually, 6 CO2 are converted to 1 glucose.

Fig. 2.3
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Calvin cycle

• Summary
• Fixes CO2 using Rubisco to make 3-C molecules
• These used to make sugar by rest of Calvin cycle

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C3 Photosynthesis

• Overall
• Occurs in chloroplasts of mesophyll cells
• Carbon uptake and Ps done in a mesophyll cells
  chloroplasts during the day (light available)
• During night no light, no C uptake or Ps.
  Stomata closed to conserve water

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Fatal Flaws of Fotosynthesis!

• 1) Water loss/CO2 gain conundrum (dilemma)
• Air inside leaf 100 humidity, air outside less
• Air inside leaf low CO2, more CO2 outside
• Gases will diffuse from high to low concentration
• To gain C must open stomata, will lose water!

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Fatal Flaws of Fotosynthesis!

• 2) Photorespiration
• Not same as respiration in mitochondria (aerobic
  respiration)
• This begins in chloroplasts
• Is a reaction between RuBP and O2, catalyzed by
  Rubisco!

Box 2A
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Fatal Flaws of Fotosynthesis!

• 2) Photorespiration
• Makes 1 PGA (3 C) and a glycolate (2 C)
• Glycolate further processed by peroxisome and
  mitochondrion 1 C to amino acids, the other to
  CO2

Glycolate
Box 2A
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Fatal Flaws of Fotosynthesis!

• 2) Photorespiration
• Net result is LOSS of Carbon, rather than C gain

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Fatal Flaws of Fotosynthesis!

• 2) Photorespiration
• What drives photorespiration?
• Increases as light intensity increases
• Increases as leaf temperature increases
• Increases as ratio of O2CO2 increases
• More CO2, lots photosynthesis
• Less CO2, less photosynthesis
• Recall atmosphere is 0.04 CO2, about 21 O2

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C4 Photosynthesis

• Chemical pathway that minimizes the FFF (fatal
  flaws of fotosynthesis)

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C4 leaf anatomy

• Kranz anatomy (wreath in German)
• Bundle sheath cells of veins greatly enlarged

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C4 leaf anatomy

• Bundle sheath cells with large chloroplasts
• Mesophyll cells also with chloroplasts, have many
  cellular connections to bundle sheath cells

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C4 pathway

• Mesophyll cells fix CO2
• Use new enzyme (PEP carboxylase)

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C4 pathway

• PEP carboxylase uses 3 C compound (PEP) and adds
  one CO2, making a 4 C organic acid (oxaloacetate)
• Organic acid transported to bundle sheath cells

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C4 pathway

• Organic acid transported to bundle sheath cells
• Reaction reversed to yield CO2 and pyruvate (3 C)
• CO2 used for Calvin Cycle
• Pyruvate transported back to mesophyll, converted
  to PEP

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C4 pathway

• Carbon fixation and photosynthesis separated in
  space (occur in different cells)

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C4 advantages

• Much more complicated. Note that bundle sheath
  cells use Calvin Cycle to actually do
  photosynthesis
• However
• 1) PEP Carboxylase used to fix C
• This beneficial because it has high affinity for
  CO2 (reacts readily)
• Doesnt react with oxygen

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C4 advantages

• However
• 2) Shuttle system concentrates CO2 in bundle
  sheath chloroplasts (to 10 X that in atmosphere)
• Keeps O2CO2 ratio low, minimizes
  photorespiration
• If photorespiration does occur, escaping CO2 can
  be recaptured by mesophyll cells before it can
  leave leaf

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C4 advantages

• However
• 3) Can decrease water loss from leaf. Greater
  affinity of PEP carboxylase for CO2 allows
  stomata to be partly open for same CO2 intake
• Sucks CO2 well

Stoma
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C4 advantages

• However
• 4) Can decrease Rubisco concentration of leaf.
  Because enzymes contain N, C4 plants have high
  NUE (nitrogen use efficiency). Do more Ps with
  less N than C3 plants can do

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CAM photosynthesis

• Also minimizes FFF (fatal flaws of fotosynthesis)
• CAM Crassulacean Acid Metabolism
• Crassulaceae family of succulent plants first
  known for this pathway
• Acid Early scientists noted the leaves of these
  plants become acidic at night

Burros tail (Sedum morganianum)
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Leaf anatomy

• Succulent (fleshy) leaves (or photosynthetic
  stems)
• Mesophyll not in layers
• Cells large, with large central vacuoles
• Veins with small vascular bundle sheath cells

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CAM pathway

• Uses same enzymes and similar chemical reactions
  to C4 pathway
• Separates C fixation and photosynthesis in time
  rather than space

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CAM pathway

• Night Stomata open, CO2 taken up, fixed using
  PEP carboxylase to make 4 C acid (often malic
  acid).

Malic acid
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CAM pathway

• Night Stomata open, CO2 taken up, fixed using
  PEP carboxylase to make 4 C acid (often malic
  acid).
• Acid stored in large central vacuole of leaf cell
• Leaf pH goes down during night (acid drop)

Acid drop skateboarding
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CAM pathway

• Day Stomata close. 4 C acid removed from vacuole
  to chloroplast, reaction reversed to release CO2
• Photosynthesis by Calvin Cycle during day

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CAM advantages

• 1) Uses PEP carboxylase to fix C no
  photorespiration, higher affinity for CO2
• 2) Stomata open at night temperature lower,
  humidity higher. Less water lost
• 3) Do Calvin cycle during day with stomata
  closed. Seals CO2 in leaf, gives low O2CO2
  ratio, minimizes photorespiration