60194. Energy and Materials for the Reactions

Photosynthesis is an ancient pathway, and it evolved in distinct ways in different organisms.

The pathway consists of two stages, each with its own set of reactions. In the light-dependent reactions, energy from sunlight is absorbed and converted to ATP energy. Water molecules are split, and the coenzyme NADP⁺ picks up the liberated hydrogen and electrons, thereby becoming NADPH.

In the light-independent reactions, ATP donates energy to sites where glucose is put together from carbon, hydrogen, and oxygen. Carbon dioxide provides the carbon and oxygen. Water provides the hydrogen, as delivered by NADPH.

Photosynthesis is often summarized in this manner:

. 12H₂0 + 6CO₂ -------> 60₂ + C₆H₁₂O₆ + 6H₂0

This summary equation shows glucose as an end product in order to keep the chemical bookkeeping simple. The reactions don’t really stop with glucose. Glucose and other simple sugars combine at once to form sucrose, starch, and other carbohydrates--the true end products of photosynthesis.

60195. Where the Reactions Take Place

a. The two stages of photosynthesis proceed at different sites inside the chloroplast. Only the photosynthetic cells of plants and a few protistans contain this type of organelle. Each chloroplast has two outer membranes wrapped around its interior, the stroma. An inner membrane weaves through the stroma, which is largely fluid. Often the membrane has the form of flattened channels and disks, which are arranged in stacks called grana. The first stage of photosynthesis proceeds at this inner membrane, which is the thlyakoid membrane system. The spaces inside the disks and channels connect as a single compartment for hydrogen ions, which are used in ATP production. The second stage of photosynthesis--the reactions by which sugars are assembled--takes place in the stroma.

60196. Pigments are molecules that can absorb light. In animals, melanin and other pigments have roles in vision, coloration of body surfaces, and other functions. In photosynthetic organisms, a variety of pigment molecules trap photons from the sun.

60197. Photons are packets of light energy that travel through space in undulating motion, rather like ocean waves. A photon’s wavelength (the distance from one wave peak to the next) is related to its energy. The most energetic photons travel as short wavelengths, and the least energetic as long wavelengths. Humans and some other animals can perceive part of the spectrum of different wavelengths as different colors of light.

60198. In the thylakoid membranes of chloroplasts, clusters of pigments trap light of certain wavelengths. Chlorophylls absorb violet-to-blue as well as red wavelengths. They are key photosynthetic pigments in green algae and plants. They transmit green wavelengths, so that is why plant parts having an abundance of chlorophyll appear green to us. A variety of carotenoids absorb violet and blue wavelengths but transmit red, orange, and yellow.

60199. In all plants, chlorophyll a is the main pigment of photosynthesis. Chlorophyll b, carotenoids, and other pigments absorb energy of different wavelengths. They don’t use the energy themselves; they transfer it to the main pigment and so enhance its effectiveness.

60200. In green leaves, the carotenoids are far less abundant than the chlorophylls. Often they become visible in autumn, when many plants stop producing chlorophyll. Several other pigments contribute to the distinctive coloration of various organisms. Among these are the red and blue phycobilins, the signature pigments of red algae and cyanobacteria.

60201. Three events unfold during the light-dependent reactions, the first stage of photosynthesis. First, pigments absorb sunlight energy and give up electrons. Second, electron and hydrogen transfers lead to ATP and NADPH formation. Third, the pigments that gave up electrons in the first place get electron replacements.

60202. Photosystems

a. Photosystems are light-trapping clusters embedded in thylakoid membranes. There may be many thousands of them, each includes 200 to 300 pigment molecules. Most of the pigments "harvest" sunlight. When they absorb a photon, one of their electrons gets boosted to a higher energy level. When the electron returns to a lower level, it quickly gives up the added energy . The energy bounces among pigments, and a bit is lost at each bounce (as heat). Soon the energy remaining corresponds to a certain wavelength that only a few special chlorophylls can trap. Only these chlorophylls give up the electrons used in photosynthesis. They will transfer them to an electron-accepting molecule poised at the start of a neighboring transport system.

60203. ATP and NADPH: Loading Up Energy, Hydrogen, and Electrons

a. Recall that electron transport systems are organized sequences of enzymes and other proteins bound in a cell membrane. When excited electrons are transferred through these systems, they release their extra energy, some of which is harnessed to drive specific reactions. In thylakoid membranes, two kinds of photosystems give up electrons to different transport systems. They allow plants to make ATP by two different pathways, one cyclic and the other noncyclic.

60204. Cyclic Pathway

a. One pathway starts at an electron-donating chlorophyll of a "type I" photosystem. In the cyclic pathway of ATP formation, electrons "cycle" from chlorophyll P700, through a transport system, then back to P700. Energy linked with the electron flow drives the formation of ATP from ADP and unbound phosphate

b. The cyclic pathway is probably the oldest means of ATP production. The first cells to use it were as tiny as existing bacteria, so their body-building programs were scarcely enormous. ATP alone would have provided enough energy to build their organic compounds. Building larger organisms requires far more organic compounds--and vast amounts of hydrogen atoms and electrons. Long ago, in the forerunners of multicelled plants, the cyclic pathway’s machinery underwent expansion and became the basis of a more efficient ATP-forming pathway.

60205. Noncyclic Pathway

a. Today, the cyclic pathway still operates in trees, weeds, and other leafy members of the plant kingdom. But the noncyclic pathway of ATP formation dominates. Electrons are not cycled through this pathway. They depart(in NADPH), and electrons from water molecules replace them.

b. The pathway starts when the sun’s rays bombard a "type II" photosystem. Photon energy makes this photosystem’s special chlorophyll (P680) give up electrons. It also triggers photolysis, a reaction sequence in which water molecules split into oxygen, hydrogen ions, and electrons. P680 attacks the rather unexcited electrons as replacements for the excited ones that got away.

c. Meanwhile, the excited electrons are transferred through a transport system--then to chlorophyll P700 of photosystem I. Electrons arriving at P700 haven’t lost all of their extra energy. Because photons are also bombarding P700, the incoming energy boosts electrons to a higher energy level that allows them to enter a second transport system. One enzyme of this system has a helper- NADP⁺. This coenzyme picks up two electrons and a hydrogen ion, and so becomes NADPH. It carts hydrogen and electrons to sites where organic compounds are built.

60206. The Legacy--A New Atmosphere

a. Oxygen is a by-product of the noncyclic pathway of photosynthesis. This pathway may have evolved more than 2 billion years ago. At first, the oxygen simply dissolved in seas, lakes, wet mud, and other bacterial habitats. By about 1.5 billion years ago, however, large amounts of dissolved oxygen were escaping into what had been an oxygen-free atmosphere. Its accumulation changed the atmosphere forever. And it made possible aerobic respiration the most efficient pathway for extracting energy from organic compounds.

60207. A Closer Look at ATP Formation in Chloroplasts

a. How, exactly, does ATP form during the noncyclic (and cyclic) pathways?

b. When electrons flow through the membrane-bound transport systems, they pick up hydrogen ions (H⁺) outside the membrane and dump them into the thylakoid compartment. This sets up H⁺ concentration and electric gradients across the membrane. Hydrogen ions that were split away from water molecules increase the gradients. The ions respond by flowing out through the interior of channel proteins that span the membrane. Energy associated with the flow drives the binding of unbound phosphate to ADP, the result being ATP.

60208. The light-independent reactions are the "synthesis" part of photosynthesis. ATP molecules deliver the required energy for the reactions. NADPH molecules deliver the required hydrogen and electrons. Carbon dioxide in the air around photosynthetic cells provides the carbon and oxygen.

a. The reactions are light-independent because they don’t depend directly on sunlight. They can proceed even in the dark, as long as ATP and NADPH are available.

60209. Capturing Carbon

a. A carbon molecule that diffuses into the air spaces inside a sow thistle leaf and ends up next to a photosynthetic cell. From there, it diffuses across the plasma membrane and on into the stroma of a chloroplast. The light-independent reactions start when the carbon atom of the CO₂ becomes attached to RuBP (ribulose biphosphate), a molecule with a backbone of five carbon atoms. This is called carbon dioxide fixation.

60210. Building the Glucose Subunits

a. Attaching carbon to RuBP is the first step of a cyclic pathway that produces a sugar phosphate molecule and regenerates the RuBP. The pathway s named the Calvin-Benson cycle, in honor of its discoverers. A specific enzyme catalyzes each step.

b. The attachment of a carbon atom to RuBP produces an unstable six-carbon intermediate. This splits into two molecules of PGA (phosphoglycerate), each with a three-carbon backbone. ATP donates a phosphate group to each PGA. NADPH donates hydrogen and electrons to the resulting intermediate, thereby forming PGAL (phosphoglyceraldehyde).

c. The reaction steps just outlined proceed not once but six times. In other words, six CO₂ molecules are fixed, and twelve PGAL molecules are produced. Most of the PGAL becomes rearranged to form new RuBP molecules, which can be used to fix more carbon. But two of the PGAL combine, forming a six-carbon sugar phosphate. When a sugar has a phosphate group attached, it is primed for further reaction.

d. The Calvin-Benson cycle produces enough RuBP molecules to replace the ones used in carbon dioxide fixation. The ADP, NADP+, and phosphate leftovers diffuse back to sites of the light-dependent reactions and can be converted once more to NADPH and ATP. The sugar phosphate formed in the cycle can serve as a building block for sucrose, starch, or cellulose--the plant’s main carbohydrate. Synthesis of these large organic compounds by other pathways marks the conclusion of the light-independent reactions.

60211. Pg. 116, figure 7.10 summarizes the key reactants, intermediates, and products of both the light-dependent and light-independent reactions of photosynthesis.

60212. During daylight hours, photosynthetic cells convert the newly formed sugar phosphates to sucrose or starch. Of all plant carbohydrates, sucrose is the most easily transportable, and starch is the most common storage form. The cells convert excess PGAL to starch also. They briefly store the starch as grains in the stroma. After the sun goes down, the cells convert their starch to sucrose, for export to the living cells in leaves, stems, and roots. Ultimately, the products and intermediates of photosynthesis end up as energy sources and building block for all the lipids, amino acids, and other organic compounds required for plant growth, survival, and reproduction.

60213. If light intensity, air temperature, rainfall, and soil composition were uniform all year long, everywhere in the world, then maybe the photosynthetic reactions would proceed in exactly the same way in all plants. However, environments do differ, and so do the details of photosynthesis.

60214. Fixing Carbon Twice, In Two Cell Types

a. A Kentucky bluegrass plant, as with all other plants, depends on CO₂ uptake for growth. Although CO2 is plentiful in the air, it is not always abundantly available to photosynthetic cells inside the leaves. Leaves have a waxy cover that restricts water loss. Water escapes mainly through the stomata (sing. stoma), which are tiny openings across the leaf surface. Most CO₂ diffuses into the leaves, and most O₂ diffuses out through these openings.

b. On hot, dry days, all plants close their stomata and so conserve water. But when they do, CO₂ can’t diffuse into their leaves. Meanwhile, photosynthetic cells are busy, so oxygen builds up in each leaf. The stage is set for a wasteful process called "photorespiration." By this process, oxygen instead of CO₂ becomes attached to the RuBP used in the Calvin-Benson cycle, with different results:

i. Normal Conditions: High CO₂/Low O₂ --- CO₂ + RuBP = two PGA

ii. Hot, Dry Conditions: Low CO₂/High O₂ --- O₂ + RuBP = one PGA, one phosphoglycolate

c. Formation of sugar phosphates depends on PGA. When photorespiration wins out, less PGA forms--and the bluegrass plant’s capacity for growth suffers.

d. Kentucky bluegrass and many other species are known as C3 plants, because the three-carbon PGA is the first intermediate formed by carbon fixation. By contrast, when corn, crabgrass, sugarcane, and man other plants fix carbon, the resulting intermediate is not PGA. It is the four-carbon oxaloacetate. Hence the name, C4 plants.

e. C4 plants maintain adequate amounts of CO₂ inside the leaves even though they too close stomata on hot, dry days. They fix CO₂ not once but twice, in two different types of photosynthetic cells. Mesophyll cells have first crack at the CO₂ in the leaf and use it to form oxaloacetate. It is a temporary fix. The oxaloacetate is quickly transferred to bundle-sheath cells that wrap around every vein in C4 leaves. In those cells, CO₂ is released and fixed again--in the Calvin-Benson cycle.

f. With their more efficient carbon-fixing mechanism, C4 species get by with tinier stomata--and so lose less water--than C3 species. C4 plants are usually better adapted where temperatures are highest during the growing season. In Florida, 80 percent of the plants that evolved are C4 plants--compared to 0 percent in Manitoba, Canada. C3 plants have an advantage where temperatures are below 25 _C; they are less sensitive to cold.

60215. Fixing and Storing Carbon by Night, Using It by Day

a. Different carbon-fixing adaptation is found in deserts and other dry environments. Succulents--plants with juicy, water-storing tissues and very thick surface layers that restrict water loss. They include cacti. Like many flowering plants, they cannot open their stomata during the day without losing precious water. They open them and fix carbon dioxide only at night. Their cells store the resulting intermediate in the central vacuole, then use it the next day--when the stomata are closed.

b. These are called CAM plants. Unlike C4 species, CAM plants do not fix carbon in separate cells. They fix it at different times.

c. Sometimes during droughts, CAM plants survive by closing their stomata at night. They get carbon by repeatedly fixing the carbon dioxide that forms during aerobic respiration. Not that much forms, but it is enough to allow these plants to maintain very low rates of metabolism.

d. CAM plants grow slowly.

60216. Bacteria obtain energy not from sunlight but rather by pulling away hydrogen and electrons from ammonium ions, iron or sulfur compounds, and other inorganic substances. Many influence the global cycling of nitrogen and other vital elements through the biosphere.