An action spectrum plots the rate of photosynthesis at various wavelengths of visible light, and it shows that blue light with a wavelength of about nm is effective in driving photosynthesis. Based on this information and the absorption spectra shown at left, what role may chlorophyll b and carotenoids play in photosynthesis?
These pigments are able to absorb more wavelengths of light and thus more energy than chlorophyll a alone can absorb. As part of light-harvesting complexes in photosystems, they broaden the range of light that can be used in the light reactions.
Photophosphorylation The excitation of chlorophyll by light energy initiates a chain of events that leads to ATP production. True or false? The chemiosmotic hypothesis states that the synthesis of ATP generates a proton gradient that leads to electron flow through an electron transport chain. According to the chemiosmotic hypothesis, what provides the energy that directly drives ATP synthesis?
Chloroplast membrane vesicles are equilibrated in a simple solution of pH 5. The solution is then adjusted to pH 8. Which of the following conclusions can be drawn from these experimental conditions?
Which wavelengths of light drive the highest rates of photosynthesis? Select the two best answers. Some organisms grow underwater where light intensity and quality decrease and change with depth.
Other organisms grow in competition for light. Plants on the rainforest floor must be able to absorb any light that comes through because the taller trees absorb most of the sunlight and scatter the remaining solar radiation. When studying a photosynthetic organism, scientists can determine the types of pigments present by using a spectrophotometer.
These instruments can differentiate which wavelengths of light a substance can absorb. Spectrophotometers measure transmitted light and compute its absorption. By extracting pigments from leaves and placing these samples into a spectrophotometer, scientists can identify which wavelengths of light an organism can absorb. Learning Objectives Differentiate between chlorophyll and carotenoids. Key Points Plant pigment molecules absorb only light in the wavelength range of nm to nm; this range is referred to as photosynthetically-active radiation.
Violet and blue have the shortest wavelengths and the most energy, whereas red has the longest wavelengths and carries the least amount of energy. Chorophylls and carotenoids are the major pigments in plants; while there are dozens of carotenoids, there are only five important chorophylls: a , b , c , d, and bacteriochlorophyll. Chlorophyll a absorbs light in the blue-violet region, chlorophyll b absorbs red-blue light, and both a and b reflect green light which is why chlorophyll appears green.
Carotenoids absorb light in the blue-green and violet region and reflect the longer yellow, red, and orange wavelengths; these pigments also dispose excess energy out of the cell. Key Terms chlorophyll : Any of a group of green pigments that are found in the chloroplasts of plants and in other photosynthetic organisms such as cyanobacteria.
But in fact, it makes great sense. When too much light strikes a leaf, that energy has to be dissipated. When just enough light reaches a leaf, it is used to move electrons and protons so the leaf can make sugars during photosynthesis.
But if too much energy comes in, the electron transport chain ETC , which is responsible for moving the electrons, gets overloaded. A note here: the ETC is not really a chain in the literal sense. Rather, it is a series of molecules that alternately accept or donate electrons, all the while moving them in a single direction. Think of it as a bucket brigade at a fire, where each person transfers a bucket of water to the next person, except that instead of moving water, the molecules move electrons.
When the ETC gets overloaded, bad things can happen. In one case, electrons that are energized when light strikes the chlorophyll molecule can be dumped onto oxygen molecules, creating a type of new type of oxygen known as superoxide O2-, essentially oxygen with an extra negative charge. This is a very dangerous molecule, because it can react with membranes and proteins in the chloroplast, causing severe damage and eventual death.
In another case, an excited chlorophyll may cause the formation of singlet oxygen, which is also highly reactive but not charged. This molecule too can wreak havoc within the leaf. When this happens, anti-oxidants are brought in which render these compounds called reactive oxygen species, or ROS inert.
In leaves, carotenoids can accept the energy from an excited chlorophyll molecule and dissipate that energy as heat. This happens because the carotenoid is especially good at vibrating when it absorbs this excess energy, and that results in the loss of heat just like if you rapidly bend a piece of metal—it gets hot and the energy of bending is given off as heat.
If the excess light energy is released as heat, it is no longer available to make those nasty ROS, and the leaf is protected from photodamage. What is particularly interesting is that leaves can regulate their potential to dissipate excess energy depending on the environmental conditions.
Consider a leaf growing in deep shade. At intervals throughout the day, direct light may reach that leaf by finding a way through a hole in the canopy above. We call this a sunfleck. Although most sunflecks are of short duration, some can last for minutes to hours, and can greatly stress a shade leaf, especially one that is physiologically adapted to low light.
When this bright light hits the leaf, it causes an imbalance between energy capture the conversion of light energy into chemical energy and energy utilization the making of sugars in photosynthesis. In turn, this causes certain spaces inside a chloroplast to become acidic which means that protons accumulate.
Protons are simply hydrogen atoms missing their one electron. The more protons, the more acidic is a solution, and the lower the pH. Figure 5. The xanthophyll cycle. As you go from zeaxanthin to violaxanthin, notice how the ring structures each gain an oxygen. This is known as de-epoxidation. Violaxanthin can dissipate excess light energy as heat by vibrating rapidly, whereas zeaxanthin cannot. A low pH acts as a signal to start de-epoxidation.
Antheraxanthin is an intermediate compound in this cycle. When stress conditions are relieved, pH rises, and the violaxanthin is converted back to zeaxanthin, in a process known as epoxidation. Acidic conditions stimulate the production of enzymes that convert a special xanthophyll known as zeaxanthin which is yellow into a new compound known as violaxanthin which is orange through the intermediate compound antheraxanthin. Figure 5 shows this conversion scheme.
Note that the conversion involves adding an oxygen molecule to each of the two six-sided rings on either side of the zeaxanthin, a process known as de-epoxidation. Violaxanthin is the compound that dissipates the excess light energy as heat. As long as the leaf is stressed, this compound is retained in the chloroplast, and energy is diverted away from chlorophyll to be lost as heat.
When the sunfleck passes, and the leaf returns to low light conditions, the acidity that built up decays away, the cycle reverses, and the violaxanthin converts back to zeaxanthin via an epoxidation reaction. This way, the leaf does not divert energy away from photosynthesis when light is limiting, but does when it is in excess.
Thus, this system acts like a pressure release valve, except that instead of steam being released, it is the energy of the photons.
As chlorophyll degrades in the fall, light energy impinging on the leaf can cause injury to the internal biochemical machinery, especially the parts responsible for withdrawing nutrients back into the leaf. The presence of the carotenoids may help the leaf dissipate this excess energy via the xanthophyll cycle, or, they may physically shield the proteins and membranes by acting as a light screen, which may assist the leaf in withdrawing nutrients back into the twigs so that the tree can reuse them next season when it forms new leaves.
So as we have seen, the beauty of fall color is not just an arbitrary act for our visual pleasure.
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