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Chapter 10 Photosynthesis. Photosynthesis transforms solar light energy trapped by chloroplasts into chemical bond energy stored in sugar and other organic molecules. This process: Synthesizes energy-rich organic molecules from the energy-poor molecules, CO 2 and H 2 O.
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Photosynthesis transforms solar light energy trapped by chloroplasts into chemical bond energy stored in sugar and other organic molecules. This process: • Synthesizes energy-rich organic molecules from the energy-poor molecules, CO2 and H2O. • Uses CO2 as a carbon source and light energy as the energy source. • Directly or indirectly supplies energy to most living organisms.
Organisms acquire organic molecules used for energy and carbon skeletons by one of two nutritional modes: 1) Autotrophic Nutrition or 2) Heterotrophic Nutrition. • Autotrophic nutrition = (Auto = self; trophos = feed) Nutritional mode of synthesizing organic molecules from inorganic raw materials. • Examples of autotrophic organisms are plants, which require only CO2, H2O and minerals as nutrients. • Because autotrophic organisms produce organic molecules that enter an ecosystem’s food store, autotrophs are also known as producers. • Autotrophic organisms require an energy source to synthesize organic molecules. That energy source may be from light (photoautotrophic) or from the oxidation of inorganic substances (chemoautotrophic). • Photoautotrophs = Autotrophic organisms that use light as an energy source to synthesize organic molecules. Examples are photosynthetic organisms such as plants, algae and some prokaryotes. • Chemoautotrophs = Autotrophic organisms that use the oxidation of inorganic substances, such as sulfur or ammonia, as an energy source to synthesize organic molecules. Unique to some bacteria, this is a rarer form of autotrophic nutrition.
Heterotrophic nutrition = (Heteros = other; trophos = feed) Nutritional mode of acquiring organic molecules from compounds produced by other organisms; heterotrophs are unable to synthesize organic molecules from inorganic raw materials. • Heterotrophs are also known as consumers. • Examples are animals that eat plants or other animals. • Examples also include decomposers, heterotrophs that decompose and feed on organic litter. Most fungi and many bacteria are decomposers. • Most heterotrophs depend on photoautotrophs for food and oxygen (a by-product of photosynthesis).
Chloroplasts are the sites of photosynthesis in plants • Although all green plant parts have chloroplasts, leaves are the major organs of photosynthesis in most plants. • Chlorophyll is the green pigment in chloroplasts hat gives a leaf its color and that absorbs the light energy used to drive photosynthesis • Chloroplasts are primarily in cells of mesophyll, green tissue in the leaf's interior. • CO2 enters and O2 exits the leaf through microscopic pores called stomata.
Some steps in photosynthesis are not yet understood, but the following summary equation has been known since the early 1800's: 6 CO2 + 6 H2O + light energy C6H12O6 + 6 O2 • Photosynthesis is an endergonic redox process; energy is required to reduce carbon dioxide. • Light is the energy source that boosts potential energy of electrons as they are moved from water to sugar. • When water is split, electrons are transferred from the water to carbon dioxide, reducing it to sugar.
Photosynthesis occurs in two stages: the light reactions and the Calvin cycle. • Light reactions = In photosynthesis, the reactions that convert light energy to chemical bond energy in ATP and NADPH. These reactions: • Occur in the thylakoid membranes of chloroplasts. • Reduce NADP+ to NADPH. • Light absorbed by chlorophyll provides the energy to reduce NADP+ to NADPH, which temporarily stores the energized electrons transferred from water. • NADP+ (nicotinamide adenine dinucleotide phosphate), a coenzyme similar to NAD+ in respiration, is reduced by adding a pair of electrons along with a hydrogen nucleus, or H+. • Give off O2 as a by-product from the splitting of water. • Generate ATP. The light reactions power the addition of a phosphate group to ADP in a process called photophosphorylation. • Calvin cycle = In photosynthesis, the carbon-fixation reactions that assimilate atmospheric CO2 and then reduce it to a carbohydrate; named for Melvin Calvin. These reactions: • Occur in the stroma of the chloroplast. • First incorporate atmospheric CO2 into existing organic molecules by a process called carbon fixation, and then reduce fixed carbon to carbohydrate. • Carbon fixation = The process of incorporating CO2 into organic molecules.
Interactions of light with matter in a chloroplast. The pigments of chloroplasts absorb blue and red light, the colors most effective in photosynthesis. The pigments reflect or transmit green light, which is why leaves appear green.
Absorption and action spectra for photosynthesis. (a) A comparison of the absorption spectra for chlorophyll a and accessory pigments extracted from chloroplasts. (b) An action spectrum, profiling the effectiveness of different wavelengths of light in driving photosynthesis. Compared to the peaks in the absorption spectrum for chlorophyll a (blue-green line in graph at top), the peaks in the action spectrum are broader, and the valley is narrower and not as deep. This is partly due to the absorption of light by accessory pigments, which broaden the spectrum of colors that can be used for photosynthesis. (c) An elegant experiment demonstrating the action spectrum for photosynthesis was first performed in 1883 by Thomas Engelmann, a German botanist. He illuminated a filamentous alga with light that had been passed through a prism, thus exposing different segments of the alga to different wavelengths of light. Engelmann used aerobic bacteria, which concentrate near an oxygen source, to determine which segments of the alga were releasing the most O2. Bacteria congregated in greatest numbers around the parts of the alga illuminated with red or blue light. Notice the close match of the bacterial distribution to the action spectrum in part b.
Structure of chlorophyll: the primary photosynthetic pigment Chlorophyll a, the pigment that participates directly in the light reactions of photosynthesis, has a "head," called a porphyrin ring, with a magnesium atom at its center. Attached to the porphyrin is a hydrocarbon tail, which interacts with hydrophobic regions of proteins in the thylakoid membrane.
Cyclic electron flow occurs occasionally during the light reactions. This results in more ATP production, but no NADPH.
The logistics of chemiosmosis in mitochondria and chloroplasts. The inner membrane of the mitochondrion pumps protons (H+) from the matrix into the intermembrane space (darker brown). ATP is made on the matrix side of the membrane as hydrogen ions diffuse through ATP synthase complexes. In chloroplasts, the thylakoid membrane pumps protons from the stroma into the thylakoid space (lumen). As the hydrogen ions leak back across the membrane through the ATP synthase, phosphorylation of ADP occurs on the stroma side of the membrane.
The Calvin cycle: sugar production in the stroma This diagram tracks carbon atoms (gray balls) through the cycle. The three phases of the cycle correspond to the phases discussed in the text. For every three molecules of CO2 that enter the cycle, the net output is one molecule of glyceraldehyde 3-phosphate (G3P), a three-carbon sugar. For each G3P synthesized, the cycle spends nine molecules of ATP and six molecules of NADPH. The light reactions sustain the Calvin cycle by regenerating ATP and NADPH.
The C4 anatomy and pathway: a different approach to carbon fixation (a) Leaves of C4 plants contain two types of photosynthetic cells: a cylinder of bundle-sheath cells surrounding the vein, and mesophyll cells located outside the bundle sheath. (b) Carbon dioxide is fixed in mesophyll cells by the enzyme PEP carboxylase. A four-carbon compound--malate, in this case--conveys the atoms of the CO2 into a bundle-sheath cell, via plasmodesmata. There CO2 is released and enters the Calvin cycle. This adaptation maintains a CO2 concentration in the bundle sheath that favors photosynthesis over photorespiration.
