Botany 2104 - Plant Form and Function

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Photosynthesis 

Photosynthesis and (Aerobic) Respiration
These two processes have many things in common.
1. occur in organelles that seem to be descended from bacteria (endosymbiont theory): chloroplasts and mitochondria
2. The organelles where these processes occur have complex internal membrane systems that are essential to the processes.
3. These processes rely on existing molecules in cells to carry out the energy conversion reactions: electron holders (NAD⁺, NADP⁺, FAD), ADP and ATP, miscellaneous sugars, etc.
4. Photosynthesis and respiration are essentially the reverse of each other. Photosynthesis starts with CO₂ and reduces it to sugar; reduction requires energy, which is obtained from light. Respiration starts with sugar and oxidizes it to CO₂; oxidation releases energy which is collected as ATP.
                                ------ energy input from light ----->
6 CO₂ + 12 H₂O ------------------------------------------ C₆H₁₂O₆ + 6 O₂ + 6 H₂O
                                <---- energy output as ATP -------

Photosynthesis as a Chloroplast Event
Inside the chloroplasts are stacks of membrane sacks. Each sack is a thylakoid. The photosystems are located in the thylakoid membranes. The water compartment inside the thylakoid membrane is called the lumen. The water area outside of the thylakoid, but still within the chloroplast, is the stroma.
Photosynthesis occurs in two sets of reactions that are linked by electron carrier molecules (NADP⁺/NADPH) and ADP/ATP. The two reactions go by several names. I'll be sticking to light reactions and Calvin cycle. (Alternate names you might have heard for the light reactions: photochemical reactions, light-dependent reactions. Alternate names you might have heard for the Calvin cycle: biochemical reactions, C3 cycle, carbon reduction cycle, and carbon fixation. There are two very misleading names floating around for the Calvin cycle: light-independent reactions and dark reactions. At least five of the enzymes of the Calvin cycle, including the big deal one that starts the whole ride, are light activated.)
 

1. Light Reactions

Light = radiant energy we can see
Visible light from 400-700 nm = PAR = photosynthetically active radiation
Action spectrum:  shine light of specific λ on the target and measure the physiological response.  Photosynthesis shows peak activity in the blue light and red light portions of the visible light spectrum.  These activity peaks correspond to the absorption spectra maxima of the chlorophylls (peaks in red and blue light) and the carotenoids (blue light).
increase λ, decrease E

Need pigments to absorb visible light:  chl a, chl b, carotenoids (carotenes, xanthophylls)
There are two photosystems: PSII and PSI. Each consists of a reaction center containing 2 chl a molecules  and an antenna complex of accessory pigments and more chl a. (accessory pigments: chl b, carotenoids [carotenes and xanthophylls]) The photosystems are connected to each other via a chain of electron carrier molecules. The photosystems + electron carriers = Z-scheme. The Z-scheme is located in the thylakoid membrane.  Two key electron carriers:  cyt b₆/f and plastoquinone (PQ).  (Figure 1)
Light energy is used to excite an electron in one of the reaction center chl a molecules in PSII. The excited electron leaves PSII and travels to PSI via the electron carriers. At PSI, the electron comes to rest at the PSI reaction center. The electron gets excited by more light energy, leaves PSI, and travels to NADP⁺. Once two electrons reach NADP⁺, it is reduced to NADPH. NADP⁺/NADPH are located in the stroma.
How does PSII replace the electrons that keep leaving? By splitting water in a process called photolysis:
2 H₂O ---------> O₂ + 4 H⁺ + 4 e-
Photolysis occurs on the lumen side of the thylakoid membrane. The protons (H⁺) that get released from water are trapped in the small lumen space by the thylakoid membrane. As the electrons move from PSII to PSI, more protons  are moved from the stroma to the lumen.  The movement of H+ from the stroma to the lumen occurs at PQ.   Eventually, you get a pretty sizeable H⁺ gradient between the lumen and the stroma. However, photolysis and noncyclic electron flow do not generate enough of a pH gradient.  Cyclic electron flow is needed to increase the number of protons in the lumen.  In cyclic electron flow, the electrons excited at PSI go back to cytb6/f instead of to NADP⁺.  From cytb₆/f, the electrons go to PQ and bring more H+ into the lumen as they move on to PSI again.  Photolysis, noncyclic electron flow, and cyclic electron flow build up 1000-fold difference in H+ concentration across the thylakoid membrane.  The lumen pH drops to 5, while the stroma is at pH 8.  An ATPase in the thylakoid membrane enables the chloroplasts harvest the energy of the gradient as ATP.  (photophosphorylation)  The ATP is in the stroma. (Figure 1)

Photolysis of two water molecules in the light reactions provides enough reductant and ATP
(4 e-, 4 H⁺, 3 ATP) to reduce one molecule of CO₂ to [CH₂O], producing one water molecule as a byproduct in the Calvin cycle.

2. The Calvin Cycle
There are three phases to the Calvin cycle: carboxylation, reduction, and regeneration  (Figure 2)
A. Carboxylation
Carbon dioxide and RuBP (a C5 sugar) are combined to give two molecules of PGA (a C3 acid).
The enzyme that catalyzes this reaction is abbreviated rubisco.
B. Reduction
This phase uses the 2/3 of the ATP and all of the NADPH produced during the light reactions.
Each PGA molecule is reduced to PGald (a C3 sugar).
C. Regeneration
This phase uses the last of the ATP to turn a bunch of PGald molecules into a bunch of RuBP molecules to regenerate the cycle.
D. what the abbreviations stand for:
rubisco = RuBP carboxylase and oxygenase
RuBP = ribulose-1,5-bisphosphate
PGA = 3-phosphoglyceric acid (3-phosphoglycerate)
PGald = 3-phosphoglyceraldehyde (also known as glyceraldehyde-3-phosphate)
ATP = adenosine triphosphate
ADP = adenosine diphosphate
NADP+/NADPH = nicotinamide adenine dinucleotide phosphate (oxidized/reduced)
Once you have lots of PGald, it can be used to make glucose, fructose, sucrose, and starch.
(The reactions of respiration, besides providing a means to change the stored calories of sugars into useable energy, let a cell start the process of converting carbons from carbohydrates into a variety of molecules: amino acids, nucleotides, pigments, hormones, etc.)

1950s:  Melvin Calvin and associates:  used ¹⁴C (available for labeling experiments after WWII) and 2-D paper chromatography (developed as a technique in the 1940s) to identify the carbon fixation steps of photosynthesis.  The first stable product to contain the label was PGA.  At later stages, the label appeared in triose-P and then the various sugars of the regeneration steps (reductive pentose phosphate pathway).  They used Chlorella, a unicellular green alga, as their experimental organism.  With Chlorella, they were able to take small samples at various time intervals and rapidly kill all the cells simultaneously to stop the reactions.  


Photosynthesis as a Leaf Event

Besides looking at photosynthesis as a chloroplast event, you need to remember that it is also a leaf event.
Inside of a leaf are three tissues (functional collections of cells):
1. epidermis
holes for gas exchange called stomata (guard cells open and close the holes)
covered by a wax layer called cuticle
2. vascular tissue
xylem + phloem together in a vascular bundle (vein)
3. mesophyll (ground tissue)
tightly packed layer of cells = palisade mesophyll
loose cell layer with lots of air spaces = spongy mesophyll
Most plants open their stomata during the day (light) so CO₂ enters the leaf for photosynthesis. Downside: water evaporates out of the stomata whenever they are open. Evaporation is fastest when the temperatures are highest, which would also be during the day. The stomata close at night when photosynthesis is not going on (no need to let in CO₂).
Some plants have a system that lets them open their stomata at night to collect and store CO₂. During the day, they can close their stomata to conserve water, but still do photosynthesis. These plants are known as CAM plants. CAM == Crassulacean acid metabolism. CAM was first discovered in members of the Crassulaceae family. CAM has since been found in many angiosperm families (both monocots and dicots), a seedless vascular plant, and a gymnosperm.
CAM plants grow in arid habitats: deserts, alpine regions, as epiphytes. CAM plants have at least some succulence (water storing). Two CAM plants are important from the money end of things: pineapple and orchids.
PEP = phosphoenolpyruvate, a C3 acid.
CO₂ can be attached to PEP by the enzyme PEP carboxylase.
At night, the stomata are open. Starch is broken down to produce PEP. PEP combines with CO₂ to form a C4 acid. This C4 acid is stored in the vacuole. During the day, the stomata close. The C4 acid is broken down to release CO₂ and a C3 acid. The C3 acid is converted back to starch. The CO₂ enters the Calvin cycle. (Figure 3)
CAM is estimated to occur in ~ 10% of plant species. C3 photosynthesis (where the only carbon reactions are the Calvin cycle ones) occurs in ~ 89% of species. (C3 plants include wheat, rice, daisies, petunias, roses, fruit trees, and conifers.)  The remaining ~1% do C4 photosynthesis. Although C4 accounts for only a fraction of the photosynthesis it attracts a lot of study because (1) it is a highly efficient form of photosynthesis and (2) it accounts for the high productivity of such major crops as corn, sugar cane, sorghum, and millet.
rubisco = RuBP carboxylase and oxygenase
O2 + RuBP ------> PGA + phosphoglycolyate (C2 acid )
In a series of reactions that occur in the chloroplast, peroxisome, and mitochondria, 2 molecules of  phosphoglycolate are converted to PGA + CO₂.  (Figure 4)  The CO₂ that is released by this process because of the oxygenase reaction if rubisco is photorespiration:  extra CO2 released in the light compared to plants under the same conditions in the dark.   Photorespiration is highest under conditions of high temperature, high light intensity, and low water. Under these conditions, a C3 plant might lose 50% of its carbon via photorespiration.
How can you decrease photorespiration? Keep rubisco away from O₂. Some plants do this by engaging in C4 photosynthesis.
C4 plants have a distinctive leaf anatomy. There is a prominent ring of cells around the vascular bundles = the bundle sheath. The mesophyll cells form a ring that is tightly appressed to the bundle sheath cells. Kranz anatomy.
In a mesophyll cell, CO₂ and PEP combine to form a C4 acid. The C4 acid is sent to a bundle sheath cell. In the bundle sheath cell, the CO₂ is released from the C4 acid and enters the Calvin cycle. The C3 acid that remains goes back to the mesophyll cell, is made back into PEP, and is ready to carry more CO₂. (Rubisco is located only in the bundle sheath cells.) So, a CO₂ shuttle system delivers CO₂ to rubisco; the leaf anatomy keeps O₂ away from the bundle sheaths. Result ==> no photorespiration.  (Figure 5)
Downside to C4: the CO₂ shuttle is not a free ride. It adds 2 ATP to the standard 3 ATP (for the Calvin cycle) needed per CO₂. So C4 is only cost effective for plants in an environment where photorespiration could be so great as to be detrimental.

1965:  Kortschack, Hartt, and Burr repeated the experiments done by Calvin’s group, using sugar cane as the experimental organism.  The results indicated that sugar cane made malate (C4 acid) as the first stable product, not PGA (C3 acid).  Soon afterwards, Hatch and Slack confirmed these results and showed that this form of photosynthesis was found in a number of plants, both monocots and dicots.

Photosynthesis as a Whole Plant Event
One final thing to remember about photosynthesis is that it is a whole plant event. The roots need to take in essential elements from the soil. Many of the elements that plants require have some role in photosynthesis: sulfur, magnesium, iron, manganese, chlorine, nitrogen, copper, phosphorus. Potassium is needed to open the stomata to let in CO2. There needs to be an adequate water supply coming in from the roots to keep the stomata open. Under conditions of water stress, like can occur on a July afternoon, the stomata will close (at least partly if not completely).

Review

Photosynthesis and Respiration
What do photosynthesis and respiration have in common? In what way are they essentially the reverse of each other?

Photosynthesis
    action spectrum vs. absorption spectrum
    relationship between wavelength and energy of radiation
    result of radiation being absorbed by a target molecule
    photosystems
    cytochrome b6/f
    chloroplast structure and its relation to the reactions of photosynthesis
    light reactions:  Z-scheme, cyclic and non-cyclic electron flow, photolysis, photophosphorylation
    Calvin cycle:  rubisco, RuBP, PGA, PGald (triose-P); carboxylation, reduction, and regeneration
    ATP/ADP; NADP+/NADPH:  links between the light reactions and the Calvin cycle
    C3 vs. C4 photosynthesis
    CAM photosynthesis; implication for water conservation
    photorespiration:  organelles involved, causes and contributing factors, consequences

What is the sugar that is produced as a result of the carbon reduction process? What are the possible fates of the sugar once a plant has made a lot of it?

What are CAM plants? How do they manage to do photosynthesis if their stomata are closed during the day for water conservation? In what habitats do you find CAM plants?
CAM is estimated to occur in ~ 10% of plant species. C3 photosynthesis (where the only carbon reactions are the Calvin cycle ones) occurs in ~ 89% of species. The remaining ~1% do C4 photosynthesis. Why do C4 plants attract so much attention? What is photorespiration? What is rubisco? How is it involved in photosynthesis? How is it involved in photorespiration?
C4 plants have Kranz anatomy. How does Kranz anatomy relate to C4 photosynthesis?
Be able to give at least two examples each of C3 plants, C4 plants, and CAM plants.

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30 October 2012