AccelePrep for the ACT Test 2nd Edition Student Text

P OWER U P ! T HE E XCLUSIVE C AMBRIDGE S AMPLE E XAM • 283

GO ON TO THE NEXT PAGE. unique leaf structure known as Kranz anatomy ( C 4 plants). In Kranz plants, the bundle-sheath cells have numerous chloroplasts (other plants usually do not), and the mesophyll cells are clustered in a ring-like arrangement around the bundle sheath. These plants can carry out photosynthesis under conditions of high temperature and concentrated light, when loss of water induces closure of the stomata. When the stomata close, the concentration of CO 2 in the air spaces inside the leaf falls, and the concentration of O 2 rises. Under these conditions most plants ( C 3 ) would experience a net loss of CO 2 because of photorespiration. Kranz plants ( C 4 ) do not because level. After being passed from molecule to molecule, the electron may return to the chlorophyll from which it started. Some of the energy released as the electron is passed down the energy gradient is used to synthesize the compound ATP from ADP and inorganic phosphate. ATP is a universal energy packet used by cells to do work. ATP is synthesized from ADP and inorganic phosphate in a process called phosphorylation. Phosphorylation is a very high energy-demanding process. Cyclic photophosphorylation occurs when the energy used for ATP synthesis comes from light-energized electrons as they are returned to the chlorophyll molecules from which they originated. Another process that occurs in green plants is noncyclic photophosphorylation. In this reaction, some electrons are passed from the chlorophyll to a different type of acceptor molecule called NAPD ox , which retains the electron and is therefore reduced to become NAPD re . The ATP and NAPD re produced in the light reaction are used to reduce carbon dioxide to carbohydrate in a series of reactions called the ƒŽ˜‹…›…Ž‡ ȋ†ƒ” ”‡ƒ…–‹‘ȌǤ ƒ•‹…ƒŽŽ›ǡ ƒ ϐ‹˜‡Ǧ…ƒ”„‘ sugar, ribulose diphosphate (RuDP), is combined with CO 2 . This process is called carboxylation. The products are then phosphorylated by ATP and reduced by NAPD re to form PGAL, a three-carbon sugar. Under certain conditions, the very same enzyme that under more agreeable conditions would facilitate its carboxylation oxidizes RuDP. This process, called photorespiration, is seemingly a wasteful process since no ATP is created. Photorespiration predominates over photosynthesis when CO 2 levels are low and O 2 levels are high. Some tropical angiosperm plants have a

PASSAGE IV NATURAL SCIENCE: This passage explains how energy becomes usable through photosynthesis. Every living cell must acquire energy in a usable form. According to the First Law of Thermodynamics, energy, which is the capacity for doing work, can be converted from one form into another without any net gain or loss. An organism must have an outside source of usable energy. The Second Law of Thermodynamics states that every energy transformation reduces the free (usable) energy of the system. Living cells primarily use chemical energy derived from complex organic compounds. Photosynthesis is the process by which green plants transform sunlight into a usable energy source. Green plants utilize the energy of light to combine carbon dioxide with water to form organic material (sugar) and oxygen. 6 12 6 CO H O light O C H O 6H O 2 2 chlorophyll 2 6 12 6 2 & Photosynthesis is a reduction reaction. Reduction is the addition of one or more electrons to an atom or molecule. Oxidation is the removal of electrons from an atom or molecule. Reduction stores energy, while oxidation releases it. Biological systems rely on the addition or removal of an electron from hydrogen. Photosynthesis is based on two key processes. Light energy is trapped and stored, and hydrogen atoms are transformed from water to carbon dioxide to form carbohydrate. Photosynthesis takes place within the chloroplasts. The pigments within the chloroplasts are precisely arranged within the membranes of ϐŽƒ––‡‡† •ƒ…•…ƒŽŽ‡† –Š›Žƒ‘‹†•Ǥ Š›Žƒ‘‹†• ‘ˆ–‡ lie close together in sacks called grana. The light reactions of photosynthesis take place within the thylakoid membranes, while the dark reactions take place in the colorless matrix (stroma) surrounding the thylakoids. Different wavelengths of light, especially red and blue light, are trapped by various pigment molecules contained within chloroplasts. When a photon of light strikes a pigment molecule and is absorbed, the energy is transferred to an electron, which is raised to a high-energy state. A specialized form of chlorophyll passes the energized electron –‘ ƒ ƒ……‡’–‘” ‘Ž‡…—Ž‡ǡ ǡ ™Š‹…Š Šƒ• ƒ Š‹‰Š ƒˆϐ‹‹–› for electrons. X passes the electron to a series of acceptor molecules, each at a slightly lower energy

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