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Photosynthesis / Calvin Side / CAM Plants


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{NEWFLAG} Baxter

Mon Mar 09 2009, 03:55AM

OkComputer

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Joined: Tue Jan 13 2009, 06:26PM
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Concepts:

Photosynthesis: the bonding together of CO2 (carbon dioxide) with H2O (water) to make CH2O (sugar) and O2 (oxygen), using the sun's energy. The sugar contains the stored energy and serves as the raw material from which other compounds are made. Respiration is the opposite of photosynthesis -- the stored energy in the sugar is released in the presence of oxygen, and this reaction releases the CO2 and H2O originally jammed together by the sun's energy.
Stomata: the "pores" in leaves (and stems) through which CO2 is taken in and O2 is released during photosynthesis. Plants control when stomata are open or closed and the width of the opening (formed by two guard cells that expand and contract to open and close the space between them).
Transpiration: the water that evaporates out of stomata when they are open. This pulls more water and nutrients up to the top of the plant, but causes the plant to lose water and potentially dehydrate.
Water Use Efficiency (WUE): How good the plant is at bringing in carbon dioxide for photosynthesis without losing much water out of its stomata. More specifically, it is the ratio of carbon dioxide intake to water lost through transpiration.
Photorespiration: Under high light and high heat, the enzyme (RUBISCO) that grabs carbon dioxide for photosynthesis may grab oxygen instead, causing repiration to occur instead of photosynthesis, thus causing a slowing of the production of sugars from photosynthesis..

The three types of photosynthesis are C3, C4, and CAM. C3 photosynthesis is the typical photosynthesis tha most plants use and that everyone learns about in school (it was all we knew about until a few decades ago). C4 and CAM photosynthesis are both adaptations to arid conditions because they result in better water use efficiency. In addition, CAM plants can "idle," saving precious energy and water during harsh times, and C4 plants can photosynthesize faster under the desert's high heat and light conditions than C3 plants because they use an extra biochemical pathway and special anatomy to reduce photorespiration. Below are the details. C3 Photosynthesis : C3 plants.

Called C3 because the CO2 is first incorporated into a 3-carbon compound.
Stomata are open during the day.
RUBISCO, the enzyme involved in photosynthesis, is also the enzyme involved in the uptake of CO2.
Photosynthesis takes place throughout the leaf.
Adaptive Value: more efficient than C4 and CAM plants under cool and moist conditions and under normal light because requires less machinery (fewer enzymes and no specialized anatomy)..
Most plants are C3.

C4 Photosynthesis : C4 plants.

Called C4 because the CO2 is first incorporated into a 4-carbon compound.
Stomata are open during the day.
Uses PEP Carboxylase for the enzyme involved in the uptake of CO2. This enzyme allows CO2 to be taken into the plant very quickly, and then it "delivers" the CO2 directly to RUBISCO for photsynthesis.
Photosynthesis takes place in inner cells (requires special anatomy called Kranz Anatomy)
Adaptive Value:
- Photosynthesizes faster than C3 plants under high light intensity and high temperatures because the CO2 is delivered directly to RUBISCO, not allowing it to grab oxygen and undergo photorespiration.
- Has better Water Use Efficiency because PEP Carboxylase brings in CO2 faster and so does not need to keep stomata open as much (less water lost by transpiration) for the same amount of CO2 gain for photosynthesis.
C4 plants include several thousand species in at least 19 plant families. Example: fourwing saltbush pictured here, corn, and many of our summer annual plants.

CAM Photosynthesis : CAM plants. CAM stands for Crassulacean Acid Metabolism

Called CAM after the plant family in which it was first found (Crassulaceae) and because the CO2 is stored in the form of an acid before use in photosynthesis.
Stomata open at night (when evaporation rates are usually lower) and are usually closed during the day. The CO2 is converted to an acid and stored during the night. During the day, the acid is broken down and the CO2 is released to RUBISCO for photosynthesis
Adaptive Value:
- Better Water Use Efficiency than C3 plants under arid conditions due to opening stomata at night when transpiration rates are lower (no sunlight, lower temperatures, lower wind speeds, etc.).
- May CAM-idle. When conditions are extremely arid, CAM plants can just leave their stomata closed night and day. Oxygen given off in photosynthesis is used for respiration and CO2 given off in respiration is used for photosynthesis. This is a little like a perpetual energy machine, but there are costs associated with running the machinery for respiration and photosynthesis so the plant cannot CAM-idle forever. But CAM-idling does allow the plant to survive dry spells, and it allows the plant to recover very quickly when water is available again (unlike plants that drop their leaves and twigs and go dormant during dry spells).
CAM plants include many succulents such as cactuses and agaves and also some orchids and bromeliads

{NEWFLAG} Baxter

Mon Mar 09 2009, 04:08AM

OkComputer

Registered Member #1

Joined: Tue Jan 13 2009, 06:26PM
Posts: 29

C3, C4 and CAM. Regulation of The Activity of Photosynthesis

C4

A number of plants display an increased and more efficient net photosynthesis during strong light intensities. A prime example are the Gramineae of warmer regions like maize or sugar-cane.

At the beginning of the sixties observed H. KORTSCHAK (Hawaiian Sugar Planter's Association) that the first product of photosynthesis in sugar-cane is not the C3 unit 3-phosphoglycerate but a unit with four C-atoms. The Australian plant physiologist M. D. HATCH and his English colleague C. R. SLACK confirmed this result and identified the compound as oxaloacetate (OAA). It is produced by the addition of one molecule of carbon dioxide to phosphoenolpyruvate (PEP). The cycle is also known as the HATCH- SLACK-cycle or the C4 cycle. Plants with this cycle are called C4-plants (and CAM plants, respectively) in contrast to C3 plants where the carbon dioxide is directly fed into the CALVIN cycle. The oxaloacetate is usually converted into malate of which the carbon dioxide is split off again with the help of an enzyme.

This carbon dioxide is now bound by ribulose-1,5-diphosphate and assimilated via the CALVIN cycle. :

Some species use malate instead of aspartate

oxaloacetate + L-glutamate > aspartate + alpha-ketoglutarate.

The reversible binding of carbon dioxide has the function to accumulate and store CO2. The process consumes energy, so that it could also be spoken of a carbon dioxide pump. It should be mentioned that the HATCH-SLACK cycle requires two molecules of ATP are per fixed carbon dioxide.

Photosynthesis of C4 plants. CO2 is bound to phosphoenolpyruvate (PEP) in mesophyll cells. The product is oxaloacetate. The next step generates malate. In the cells of the vascular bundle sheath, the 'Kranz' cells, is carbon dioxide split off the malate and fed into the CALVIN cycle. The pyruvate is transported back into the mesophyll cells (active transport) and is with the help of additional ATP phosphorylated to PEP.

The anatomy of C4 leaves with so-called 'Kranz' cells differs fundamentally from that of C3 plants. The chloroplasts of C3 plants are of homogeneous structure, while two types of chloroplasts occur in C4 plants. The mesophyll cells contain normal chloroplasts, that of the vascular bundle sheath have chloroplasts without grana , i.e. they are partially impaired in function. This peculiarity does not affect the CALVIN cycle, it concerns only the light reactions of photosynthesis. The first binding of carbon dioxide (the HATCH-SLACK reaction) occurs in the mesophyll cells, the incorporation into carbohydrates (the CALVIN cycle) in the cells of the vascular bundle sheath. Both processes of photosynthesis are spatially separated.

The Crassulacean Acid Metabolism (CAM)

CAM is the abbreviation of Crassulacean acid metabolism. The name points at the fact that this pathway occurs mainly in Crassulacean species (and other succulent plants). The chemical reaction of the carbon dioxide accumulation is similar to that of C4 plants but here are carbon dioxide fixation and its assimilation not separated spatially but in time. CAM plants occur mainly in arid regions. The opening of the stomata to take up carbon dioxide is always connected with large losses of water. To inhibit this loss during intense sun (the transpiration via the cuticle remains intact) has a mechanism developed that allows the uptake of carbon dioxide during the night. The prefixed carbon dioxide is stored in the vacuoles as malate (and isocitrate) and is used during the daytime for photosynthesis.

Which Metabolism Goes With Which Conditions?

The enzyme that catalyzes the primary carbon dioxide fixation of C4 and CAM plants is phosphoenolpyruvate carboxylase (PEPC). Its affinity for carbon dioxide is by far higher than that of Rubisco, the first enzyme of the CALVIN cycle. As a consequence are C4 plants able to use even trace amounts of carbon dioxide. PEPC occurs in small amounts (roughly 2 - 3 %) also in C3 plants, where it, too, has a key position in the metabolic regulation.

Carbon dioxide yield of C4 and C3 plants of open grasslands in different parts of the world. In temperate regions is the rather low light intensity decisive for the disadvantage of C4 plants. C3 plants have an advantage due to their low rate of photorespiration and because they need no energy for the previous fixation of CO2. (J. R. EHRLICHER, 1978).

In growing roots (of maize seedlings, for example) does PEPC help to supply the lipid synthesis with NADH + H+. The following reactions take place:

1. phosphoenolpyruvate + HCO3- > oxaloacetate + Pi
2. oxaloacetate > malate. - During this step is NADH + H+ oxidized to NAD+.
3. malate > pyruvate + CO2.

During the last reaction is NAD reduced to NADH + H+.

In the root nodules of leguminosae is nitrogen fixed. Enough carbon bodies have to be supplied in order to incorporate the ammonia produced by the bacteria. the CO2-binding via PEPC-reaction thus is an important supplement

Furthermore produces the PEPC intermediates of the citric acid cycle (oxaloacetate and / or malate), a back-up in case of shortages. The activity of PEPC is controlled by extern factors, the day length is decisive. In some cases have different isoenzymes be found in different tissues. Their production is controlled by different triggers.

C3 plants can loose up to 20 percent of the carbon fixed in the CALVIN cycle at intense radiation. Under strong light is the photorespiration 1.5 - 3.5 times as high as the usual respiration in darkness. In C4 plants becomes the photorespiration drastically reduced, it may even not be detectable any more. In other words:

The net rate of photosynthesis (and consequently also the net production of biomass) of C4 plants is far larger at high light intensities than that of C3 plants. The optimal temperature of photosynthesis is below that of the respiration in darkness. As a consequence are losses caused by respiration larger at high than at low temperatures. Where light is a limiting factor and temperatures are low (i.e. in temperate climatic zones) have C3 plants the advantage, C4 plant do hardly occur (one of the exceptions is Spartina townsendii). C4 plants, nearly always herbs or shrubs, are more successful in the open country of warmer zones.

It has to be mentioned that two molecules of ATP are consumed in the HATCH-SLACK cycle

C4 plants belong to numerous, phylogenetically not related monocotyledonous and dicotyledonous families. Moreover have C4 activities also been detected in the blue-green alga Anacystis nidulans as well as in some dinoflagellates.

Since the alternative C3 or C4 is accompanied by considerable changes of the leaf anatomy has it to be assumed that the genetic potential for both pathways is quite common in the plant kingdom and that, depending on the ecological needs, one way is chosen by a species while a related species may choose the other one.

A well-studied example is the genus Atriplex, where both ways are realized. The C3 plants belong to one phylogenetic group, the C4 plants to another. In some cases can hybrids of C3 and C4 species be generated.

Influence of different parameters on the efficiency of the carbon dioxide uptake (ordinate) of a C3 plant (Atriplex patula, yellow line) and a C4 plant (Atriplex rosea, green line). Measured parameters (from left to right): light intensity, leaf temperature and concentration of carbon dioxide within the intercellular space (according to O. BJÖRKMAN and J. BERRY, 1973).

In several plant species of the genera Zea, Mollugo, Moricandia, Flaveria, etc. occur both types of CO2 fixation within one plant. In younger plants is usually the C3-, in older ones the C4 pathway taken. The amount of C4 is controlled by environmental factors.

CAM: Advantages and Disadvantages. CAM has been detected in more than 1000 angiosperms of 17 different families. It is usually accompanied by succulence, though not all Crassulaceae, for example, display CAM and succulence is no precondition of CAM. Tillandsia usneoides of the bromelia family is not succulent, but uses CAM. Mesemryanthemum crystallinum (a plant with succulent leaves) can use the C3 pathway but switches to CAM when growing in saline soils. Under experimental conditions can the shift be achieved by increasing the NaCl concentration of the nutrient medium (K. WINTER and D. J. von WILLERT, 1972). While the advantage of C4 plants comes in useful under high light intensities, is the degree of the CAM influence in CAM plants regulated mainly by temperature, atmospheric humidity and salinity. Both strong and weak CAM plants are known. In weak CAM plants becomes CAM only apparent at certain differences between day and night temperature. CAM plants that store a lot of malate and due to the thus high osmotic value also a lot of water, are usually less frost resistant than C3 plants. Because of the high concentration of acid are they less heat resistant, too. Species of arid regions are therefore forced to break their pool of malate down during the daytime (R. LÖSCH and H. KAPPEN, Universität Kiel, 1985). Usually do the C4 pathway and CAM exclude each other. An exception is the succulent C4 dicotyledon Portulaca oleracea that is able to choose the optimal pathway. under natural conditions

{NEWFLAG} Oxygen Plants

Sun Jul 11 2010, 02:28AM

Registered Member #24

Joined: Sun Jul 11 2010, 01:23AM
Posts: 1

hi
Crassulacean acid metabolism, also known as CAM photosynthesis, is a carbon fixation pathway present in some plants. These plants fix carbon dioxide (CO2) during the night, storing it as the four-carbon acid malate. The CO2 is released during the day, where it is concentrated around the enzyme RuBisCO, increasing the efficiency of photosynthesis. The CAM pathway allows stomata to remain shut during the day, reducing evapotranspiration; therefore it is especially common in plants adapted to arid conditions.
oxygen plant | oxygen plant

Oxygen Plant | Oxygen Plant


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