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LabBench Activity Plant Pigments and Photosynthesis

There is an even more important consequence with respect to water use (Figure ). The leaf surface is covered by waxes, making it rather impermeable to gases and water. However, photosynthesis obviously requires entry of carbon dioxide into the plant. This occurs via small regulated apertures on the leaf surface called stomata. Stomata typically open in the light and close in the dark, when no photosynthesis occurs . Carbon dioxide enters by diffusion through the stomata. The rate of entry therefore depends on the concentration gradient between the external atmosphere and the air spaces in the leaf (ΔC) and the conductance (g) of the stomata. Stomatal conductance increases when stomata open. However, other gases and water vapour will also move through the small hole that is conveniently provided by the stomata. Water moves out of the leaf, where the air spaces contain water-saturated air, into the atmosphere. Returning to RubisCO, one of the consequences of the competing side-reaction with oxygen is that a higher carbon dioxide concentration is required inside the leaf to support a given rate of photosynthesis. A higher internal carbon dioxide concentration can only be achieved by increasing g, i.e., opening the stomata further. The option to increase the external carbon dioxide is not available unless the plants are fortunate enough to be growing in a greenhouse with a supply of additional carbon dioxide! Increased opening of the stomata will, in turn, lead to an increased loss of water. In technical terms, the water use efficiency of photosynthesis is decreased, i.e., more molecules of water are evaporated per molecule of carbon dioxide fixed.

It is therefore required for photosynthesis and respiration .

4.6 Carbon dioxide concentration mechanisms. Algae and plants have evolved different mechanisms to concentrate CO2 in the vicinity of RubisCO, and hence depress the side reaction of RubisCO with oxygen and allow higher rates of photosynthesis at a given external CO2 concentration. (A) Pumping of bicarbonate and release of CO2 via a carbonic anhydrase reaction located in the immediate vicinity of RubisCO in subcellular structures (carboxysomes in prokaryotic cyanobacteria, or pyrenoids in eukaryotic algae). (B) C4 Photosynthesis. CO2 is pumped via a cycle involving the synthesis of 4-carbon acids like malate in external (mesophyll) cells in the leaf and their movement to internal cells (bundle sheath) where they are decarboxylated and the CO2 is fixed via the Calvin-Benson cycle. (C) CAM Photosynthesis. In the dark, stomata are opened and CO2 is used for the synthesis of 4-carbon acids like malate which are accumulated in the vacuole. In the light, stomata are closed and malate is decarboxylated to release CO2 that is fixed via the Calvin-Benson cycle.

to stimulate plant growth, photosynthesis, ..

4.6 Carbon dioxide concentration mechanisms. Algae and plants have evolved different mechanisms to concentrate CO2 in the vicinity of RubisCO, and hence depress the side reaction of RubisCO with oxygen and allow higher rates of photosynthesis at a given external CO2 concentration. (A) Pumping of bicarbonate and release of CO2 via a carbonic anhydrase reaction located in the immediate vicinity of RubisCO in subcellular structures (carboxysomes in prokaryotic cyanobacteria, or pyrenoids in eukaryotic algae). (B) C4 Photosynthesis. CO2 is pumped via a cycle involving the synthesis of 4-carbon acids like malate in external (mesophyll) cells in the leaf and their movement to internal cells (bundle sheath) where they are decarboxylated and the CO2 is fixed via the Calvin-Benson cycle. (C) CAM Photosynthesis. In the dark, stomata are opened and CO2 is used for the synthesis of 4-carbon acids like malate which are accumulated in the vacuole. In the light, stomata are closed and malate is decarboxylated to release CO2 that is fixed via the Calvin-Benson cycle.

Higher plants use a different strategy to concentrate carbon dioxide. So-called C4 plants initially incorporate carbon dioxide (actually bicarbonate) into 4-carbon organic acids like malate via a reaction that is catalyzed by phosphenolpyruvate carboxylase. This reaction occurs in cells in the outer part of the leaf. The malate diffuses into specially thickened cells in the middle of the leaf, where it is decarboxylated to release carbon dioxide, which is assimilated via RubisCO and the Calvin-Benson cycle. Because phosphenolpyruvate carboxylase has a very high affinity for bicarbonate and no side reaction with oxygen, photosynthesis can operate with much lower carbon dioxide concentrations in the internal air spaces in the leaf that are in direct contact with the stomata. This means that the stomata do not need to open so wide and that water loss is decreased. In Crassulacean Acid Metabolism (CAM) plants, phosphenolpyruvate carboxylase incorporates carbon dioxide into malate in the dark. Their cells have an extremely large central water-filled vacuole and can store concentrations of up to 1 molar malate. In the light, the malate is decarboxylated to release carbon dioxide, which is assimilated via RubisCO and the Calvin-Benson cycle. CAM allows water loss to be decreased because the stomata remain closed in the daytime and instead open at night when lower temperatures decrease evaporative water loss.

of light on plant growth during photosynthesis

By the Carboniferous, all plants used RubisCO for the net carboxylation step of photosynthesis, and RubisCO was well integrated into the primary metabolism of the plant. Because of this integration, the likelihood of evolutionarily solving the photorespiratory problem within the context of C3 photosynthesis [photosynthesis using the Calvin-Benson cycle] was probably nil. Even if a novel carboxylase could be produced, it would probably be useless because the plant would lack the metabolic pathways to regenerate acceptor molecules and process the carboxylation products.

C4 and CAM plants have evolved multiple times in different taxa and families . This probably occurred in response to intensive selective pressure during periods in the last 20 million years, when the atmospheric carbon dioxide concentration was often lower than today, sometimes falling to below 200 ppm . The most recent “low carbon dioxide periods” were as recent as 10–12,000 years ago. These low carbon dioxide levels would have strongly restricted photosynthesis by C3 plants and provided strong selective pressure for plants with C4 or CAM photosynthesis.

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and allow plants to carry out photosynthesis

glycolate . Oxygen is a competing substrate to carbon dioxide; as the oxygen concentration increases, the rate of the reaction with oxygen will rise and the rate of the reaction with carbon dioxide will fall. Under current atmospheric conditions (21% oxygen, 78 % nitrogen, 0.038% carbon dioxide), every third reaction uses oxygen instead of carbon dioxide, resulting in the rapid formation of 2-phosphoglycolate. 2-Phosphoglycolate is recycled by a complex metabolic pathway, termed photorespiration, which leads to the release of carbon dioxide and further energy consumption . The oxygenase reaction of RubisCO and the salvaging of 2-phosphoglycolate lead to a decrease in the rate of photosynthesis of approximately 20–40% and to a 40–50% decrease in the efficiency of energy conversion . This can be seen in a simple experiment, in which the side reaction is suppressed by decreasing the oxygen concentration from 21 to 2%. This leads to an immediate increase in the rate of photosynthesis. The energy loss due to photorespiration increases as the temperature rises because high temperatures favor the reaction of RubisCO with oxygen as compared to carbon dioxide.

Plant photosynthesis occurs in leaves and green ..

The other reason deals with the way biological systems evolve. Oxygenic photosynthesis evolved in an atmosphere that contained high carbon dioxide and very little oxygen, under which conditions the side reaction of RubisCO with oxygen was quantitatively negligible . This “construction mistake” was not revealed until the gradually falling carbon dioxide and rising oxygen concentrations in the atmosphere led to the side reaction with oxygen becoming quantitatively important. This probably did not occur until the last 400 million years. It is thought that atmospheric conditions will only have favored significant levels of photorespiration during the Carboniferous period (280–340 million years ago) and in the past 35 million years. However, by that time it was too late to change the complex dark reactions that had evolved around RubisCO and to develop an oxygen-insensitive pathway for carbon dioxide fixation instead.

to suspect sunlight as the major contributor to a plant’s growth

Energy dissipation occurs via a regulated mechanism, which is turned off at low light intensities and is activated when photosynthesis becomes light saturated. As discussed by Zhu et al. , it is important that energy dissipation is switched on and off at the correct light intensities for a given leaf and condition. If energy dissipation is turned on at light intensities that are not yet saturating, the ongoing rate of photosynthesis will be decreased. If it is switched on too late, the un-dissipated light energy results in damage to the photosynthetic apparatus, which will also lead to an inhibition of photosynthesis and requires repair of the apparatus, which is itself an energy-consuming process. This raises the questions if this and other regulatory processes are optimally regulated, if it is possible for a plant to do this across a wide range of different environmental conditions, and if this is a possible target for plant breeding.

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