The order of colors is determined by the wavelength of light. Visible light is one small part of the electromagnetic spectrum. The longer the wavelength of visible light, the more red the color.
Likewise the shorter wavelengths are towards the violet side of the spectrum. Wavelengths longer than red are referred to as infrared, while those shorter than violet are ultraviolet. The electromagnetic spectrum. Light behaves both as a wave and a particle.
Wave properties of light include the bending of the wave path when passing from one material medium into another i. The particle properties are demonstrated by the photoelectric effect. Zinc exposed to ultraviolet light becomes positively charged because light energy forces electrons from the zinc. These electrons can create an electrical current.
Sodium, potassium and selenium have critical wavelengths in the visible light range. The critical wavelength is the maximum wavelength of light visible or invisible that creates a photoelectric effect. The color of the pigment comes from the wavelengths of light reflected in other words, those not absorbed. Chlorophyll , the green pigment common to all photosynthetic cells, absorbs all wavelengths of visible light except green, which it reflects to be detected by our eyes.
Black pigments absorb all of the wavelengths that strike them. Pigments have their own characteristic absorption spectra, the absorption pattern of a given pigment. Absorption and transmission of different wavelengths of light by a hypothetical pigment. Chlorophyll is a complex molecule. Several modifications of chlorophyll occur among plants and other photosynthetic organisms. All photosynthetic organisms plants, certain protistans, prochlorobacteria, and cyanobacteria have chlorophyll a.
Accessory pigments absorb energy that chlorophyll a does not absorb. Accessory pigments include chlorophyll b also c, d, and e in algae and protistans , xanthophylls, and carotenoids such as beta-carotene. Chlorophyll a absorbs its energy from the Violet-Blue and Reddish orange-Red wavelengths, and little from the intermediate Green-Yellow-Orange wavelengths.
Molecular model of chlorophyll. Molecular model of carotene. Carotenoids and chlorophyll b absorb some of the energy in the green wavelength. Why not so much in the orange and yellow wavelengths?
Both chlorophylls also absorb in the orange-red end of the spectrum with longer wavelengths and lower energy. The origins of photosynthetic organisms in the sea may account for this. Shorter wavelengths with more energy do not penetrate much below 5 meters deep in sea water.
The ability to absorb some energy from the longer hence more penetrating wavelengths might have been an advantage to early photosynthetic algae that were not able to be in the upper photic zone of the sea all the time. The molecular structure of chlorophylls. The action spectrum of photosynthesis is the relative effectiveness of different wavelengths of light at generating electrons. If a pigment absorbs light energy, one of three things will occur.
Energy is dissipated as heat. The energy may be emitted immediately as a longer wavelength, a phenomenon known as fluorescence. Energy may trigger a chemical reaction, as in photosynthesis. Chlorophyll only triggers a chemical reaction when it is associated with proteins embedded in a membrane as in a chloroplast or the membrane infoldings found in photosynthetic prokaryotes such as cyanobacteria and prochlorobacteria.
Absorption spectrum of several plant pigments left and action spectrum of elodea right , a common aquarium plant used in lab experiments about photosynthesis.
Images from Purves et al. The structure of the chloroplast and photosynthetic membranes Back to Top The thylakoid is the structural unit of photosynthesis. Only eukaryotes have chloroplasts with a surrounding membrane. Thylakoids are stacked like pancakes in stacks known collectively as grana. The areas between grana are referred to as stroma. While the mitochondrion has two membrane systems, the chloroplast has three, forming three compartments.
Structure of a chloroplast. Stages of Photosynthesis Back to Top Photosynthesis is a two stage process. The first process is the Light Dependent Process Light Reactions , requires the direct energy of light to make energy carrier molecules that are used in the second process.
The Dark Reactions can usually occur in the dark, if the energy carriers from the light process are present. Similarly, the CO2 compensation point is higher in CVC-affected plants when compared to healthy ones, indicating that diseased plants need twice the amount of CO2 concentration in order to balance respiration and photorespiration when compared to heathy plants table 1. Would all these physiological disturbances encountered in CVC-affected plants be more related to the lack of water supply in the mesophyll, caused by the bacteria xylem blockage, or is it more linked with some phytotoxin produced by the bacteria?
Although primarily defended by Hopkins as a hypothesis, the presence of any phytotoxin can not be ignored. It would be of scientific interest to determine the time when all these disturbances in leaf gas exchange start, and if possible, determine the exact time of the decrease in carboxylation efficiency and stomatal closure in relation to the period between inocculation and symptoms outbreak.
Then, it could be investigated if the lower photosynthesis rates are not just related to the chlorotic areas on the leaf, or in other words, if the stomatal dysfuction is already present in a non-symptomatic leaf of an infected branch. Or would any phytotoxin be already causing these physiological disturbances before the symptoms outbreak?
Queiroz-Voltan and Paradela Filho observed that chloroplasts from chlorotic areas of leaves with CVC symptoms were almost completely destroyed. The same observation was made by Queiroz-Voltan et al.
Simpson et al. However, these same authors apparently gave more importance to the xylem blockage hypothesis. In fact, the matrix composed of extracellular polysaccharides EPSs - xanthan gum , in which X. EPS synthesis can significantly enhance the colonization of phytopathogenic bacteria. EPS-defective strains of Erwinia stewartii presented a reduced virulence partly because they spread more slowly than the wild type in the vascular system of maize plants Braun, In a similar study, Saile et al.
In a study with Pseudomonas syringae pv. Also, desiccation stress stimulated alginate synthesis in both P. Medina et al. When plants were rewatered, they reported that diseased plants recovered more slowly than healthy ones.
So, the disease coupled with water deficiency caused more physiological damages. Abscisic acid ABA is also related to stomatal physiology. However, in the case of CVC-affected plants, it seems that ABA may be more related to the decrease in soil water content when the plants are submitted to soil water deficit than to the disease itself Gomes et al.
But Goodwin et al. These results indicate that xylem blockage really occurs. From our results it could be concluded that the low net photosynthesis and transpiration rates and stomatal conductances figure 2 would be an effect of low Yw because of the xylem obliteration.
This could lead to a lower water supply for photosynthesis in the mesophyll, especially to produce the adequate turgor pressure and consequent stomatal opening.
Furthermore, we suggest that in diseased plants the stomata have a more important effect on depressing photosynthesis than in healthy plants, thus characterizing an important stomatal dysfunctioning. Although Population B has a slower photosynthetic response to increases in light intensity its Pmax is higher than that of Population A. This allows for eventual population dominance at greater light intensities. There are many determining factors influencing population success; using the PI curve to elicit predictions of rate flux to environmental changes is useful for monitoring phytoplankton bloom dynamics and ecosystem stability.
The second equation accounts for the phenomenon of photoinhibition. In the upper few meters of the ocean, phytoplankton may be subjected to irradiance levels that damage the chlorophyll-a pigment inside the cell, subsequently decreasing photosynthetic rate.
The response curve depicts photoinhibition as a decrease in photosynthetic rate at light intensities stronger than those necessary for achievement of Pmax.Only eukaryotes have chloroplasts with a surrounding membrane. This allows for eventual population dominance at greater light intensities. Unfortunately for the plant, while these gases are moving between the inside and outside of the leaf, a great deal of water is also lost. This image is copyright Dennis Kunkel at www.
While individual researchers may have their own preferences, all are readily acceptable for use in the literature. What is Photosynthesis?
The lower values of net photosynthesis and transpiration rates are probably related to the lower stomatal conductance.
They cannot cross the membrane as they are charged, and within the cytosol they turn back into CO2 very slowly without the help of carbonic anhydrase.