WHAT IS REQUIRED FOR PHOTOSYNTHESIS?
| INPUTS | REQUIRED SYSTEMS |
| LIght | Absorptive |
| Carbon Dioxide | Absorptive |
| Water | Absorptive and Transport (Xylem) |
| INPUTS | PHOTOSYNTHETIC PROCESS | OUTPUTS |
| Light | Light Dependent Reactions | Chemical Energy |
| Carbon Dioxide | Light Independent Reactions | Fixed carbon (glucose) for growth or input into Storage Systems via Phloem component of Transport System |
| Water | Photolysis | Oxygen and protons |
I. Review of Light Dependent Reaction and Photolysis
A. Photosystem I & II = harvest light energy
1. Thousands of pigment molecules in thylakoid membranes
2. Absorption of photons raises energy level of pigment molecules
3. Pigment molecule returns to ground state when energy is released in the form of heat and electrons.
B. The "Z" Scheme or Electron transport system
1. Coupled with PS I & II
2. Releases energy in steps
3. Photolysis = Water molecule is split to release 2 H+ + O-+ e-
4. e- => PSII + light energy => higher energy state
5. electron transport => lower energy state + H+ pumped
6. e- => PSI + light energy => higher energy state
7. electron transport energy reduces NADP- to NADPH
8. ATP synthesized via ATPsynthetase as H+ diffuses down gradient
II. Review of Light Independent Reaction
B. 6 CO2 + 12 NADPH +18 ATP -> 1 C6H2O6 + 12 NADP + 18 ADP + 18 Pi + 6 H2O
C. One complete cycle fixes one molecule of CO2
1. Six complete cycles fixes enough CO2
to make one molecule of glucose
2. RuBP = Ribulose Bisphosphate
3. PGA = Phosphoglycerate
3. PGAL = Glyceraldehyde phosphate
4. RuMP = Ribulose Monophosphate
III. Calvin Benson Cycle (C3 Photosynthesis)
A. Ribulose bisphosphate Carboxylase (RuBP Carboxylase) "Rubisco"
1. High CO2 -> 2 PGA
a. High O2 -> 1 PGA + phosphoglycolate
b. phosphoglycolate -> PGA + CO2
c. 20-30 % of Carbon fixed is lost
IV. Anatomical variation within C3 Photosynthetic System
A. Chlorenchyma is generally located near surface of Aerial Parts of plants for maximum interception of light
1. Translucent epidermis and light conducting cellulose facilitates light interception
2. Lens shaped curvature of outer epidermal walls can focus light into interior of plant
B. Chlorenchyma of temperate dicot leaf mesophyll often subdivided into two specialized geometries
1. Palisade
a. Stake like cells with long axis orthogonal to surface layer
b. Less prominent intercellular air space, but more larger free surface area exposed to intercellular air (1.6 - 3.5 X that of spongy)
c. Have more chloroplasts per cell
d. Maximizes light interception via efficient packing of chloroplasts per unit volume of leaf
e. Number of paradermal layers of palisade is typically amplified in reponse to exposure of developing leaf primordia to high light intensities -> "Sun Leaves"
2. Spongy
a. Irregularly shaped cells
b. Larger intercellular air space
c. Typically located on abaxial side, where stomata are located
d. Contributes to uptake of CO2 (and loss of H20)
3. Distinction between these two layers varies
a. In plants with multilayered palisade, the interface layer between palisade and spongy is transitional in form
4. Degree of development of palisade and spongy layers related to
habitat
| HABITAT | PALISADE | SPONGY | Internal/External Surface Area Ratio* |
| Temperate Mesomorphic | Adaxial | Abaxial | 6.8 -9.9 (Shade Leaf)
11.6 - 19.2 (Sun Leaf) |
| Xeromorphic | Adaxial and Abaxial | Central, much reduced | 17.2 - 31.3 |
| Hydromorphic | Reduced or lacking all together | Throughout |
C. Temperate monocot leaves typically
1. Have one type of cell, or
2. Shorter "U" shaped palisade cells
3. Leaves typically vertically oriented
4. Stomata on both sides of leaves
V. Anatomical variation associated with Hatch & Slack (C4) Photosynthesis
A. Spatial separation of initial CO2 fixation and C3 cycle often reflected in Kranz Anatomy
B. Mesophyll Cells
1. "normal" looking chloroplasts
2. CO2 + pyruvate + NADPH + H+ + 2 ATP -> malate + NADP+ + 2 ADP + 2 Pi
3. PEP carboxylate is insensitive to O2
C.
Malate transported to
D. Bundle Sheath Cells
1. chloroplasts lack grana
2. malate + NADP+ -> pyruvate + NADPH + H+ + CO2
3. Calvin Cycle proceeds in CO2 enriched environment
E. Pyruvate transported to Mesophyll Cells
F. More efficient under conditions of high light and high temperature
1. PEP carboxylase more efficient than RuBP under these conditions
2. CO2 concentrated at site of Calvin Cycle effectively eliminates photorespiration
IV. Anatomical variation associated with Crassulacean Acid Metabolism = CAM
A. Temporal separation of initial CO2 fixation and C3 Cycle
B. Night
1. Stomata open = rate of water loss reduced
2. CO2 + pyruvate + NADPH + H+ + 2 ATP -> malate + NADP+ + 2 ADP + 2 Pi
3. Malate stored in vacuole = pH decreases
C. Day
1. Stomata closed = minimum water loss
2. Light reaction provide energy
3. Malate retrieved from vacuole and converted = pH increases
4. malate +NADP+ -> pyruvate + NADPH+ + H+ +CO2
5. Calvin cycle
D. Adapted to conditions of high light, high temperature, and extreme dryness
1. Water conserved
2. Succulent plants
3. Slow growth
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| RATE OF CO2 FIXED
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| TRANSPIRATION RATIO
mg H20 lost / mg CO2 gained |
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