Chapter 10  Photosynthesis

Student Guided Notes

Overview: The Process That Feeds the Biosphere

  • The chloroplasts of plants use a process called ______ to capture light energy from the sun and convert it to chemical energy stored in sugars and other organic molecules.
  • ______ produce organic molecules from CO2 and other inorganic raw materials obtained from the environment.
  • ______ live on organic compounds produced by other organisms.

Concept 10.1 Photosynthesis converts light energy to the chemical energy of food

______are the sites of photosynthesis in plants.

  • Chloroplasts are found mainly in cells of ______, the tissue in the interior of the leaf.
  • ______exits and ______enters the leaf through microscopic pores called ______ in the leaf.
  • ______ deliver water from the roots and carry sugar from mesophyll cells to roots and other nonphotosynthetic areas of the plant.
  • Each chloroplast has two membranes around a dense fluid called the ______.
  • Suspended within the stroma is an internal membrane system of sacs, the ______.

○The interior of the thylakoids forms another compartment, the ______(or lumen).

○Thylakoids may be stacked in columns called ______.

  • ______, the green pigment in the chloroplasts, is located in the ______.

Powered by light, photosynthesis produces organic compounds and O2 from CO2 and H2O.

  • The simplified equation describing the process of photosynthesis is

Evidence that chloroplasts split water molecules enabled researchers to track atoms through photosynthesis.

  • One of the first clues to the mechanism of photosynthesis came from the discovery that the O2 given off by plants comes from ______, not ______.
  • Stanford University’s van Niel (1930’s) hypothesized that plants split water as a source of electrons from hydrogen atoms, releasing ______as a by-product.
  • Twenty years later, scientists confirmed van Niel’s hypothesis.

○Researchers used ______, a heavy isotope, as a tracer to follow the fate of oxygen atoms during photosynthesis.

○When they labeled either ______or______, they found that the ______label appeared in the oxygen produced in photosynthesis only when ______was the source of the tracer.

Photosynthesis is a redox reaction.

  • Both photosynthesis and aerobic respiration involve ______reactions.
  • Photosynthesis reverses the direction of electron flow.
  • Water is split and electrons are transferred with H+ from water to CO2, reducing it to ______.

A preview of the two stages of photosynthesis.

  • Photosynthesis is two processes, each with multiple steps: ______and the ______cycle.

○The light reactions(photo) ______.

○The Calvin cycle(synthesis) ______(named for Melvin Calvin ~1940’s.)

  • In the light reactions, ______is split, providing a source of electrons and protons (H+ ions) and giving off ______as a by-product.
  • Light absorbed by chlorophyll drives the transfer of ______and ______from water to NADP+ (nicotinamide adenine dinucleotide phosphate), forming ______.
  • The light reactions also generate ATP using ______, in a process called ______(comparable to oxidative phosphorylation in cellular respiration.)
  • The Calvin cycle begins with the incorporation of ______into ______, a process known as ______.
  • The fixed carbon is reduced with electrons provided by ______.
  • ______from the light reactions also powers parts of the Calvin cycle.
  • Thus, it is the Calvin cycle that makes ______, but only with the help of ______and ______from the ______.

Concept 10.2 The light reactions convert ______energy to the ______energy of ATP and NADPH

  • Light is a form of ______.
  • Light travels in ______and the distance between crests of electromagnetic waves is called the ______.
  • The entire range of electromagnetic radiation is the ______.
  • The most important segment of the electromagnetic spectrum for life is a narrow band between ______and ______nm, the band of ______light detected as colors by______.
  • Although light travels as a wave, many of its properties are those of a discrete particle, a ______.

Photosynthetic pigments are light receptors.

  • When light meets matter, it may be ______, ______, or______.
  • Substances that ______are known as pigments.

○A leaf looks green because chlorophyll, the dominant pigment, absorbs ______and ______light while transmitting and reflecting ______light.

  • A spectrophotometer measures ______.

○An______ plots a pigment’s light absorption versus wavelength.

  • The light reactions can perform work with wavelengths of light that are absorbed.

○Chlorophyll a, which participates directly in the light reactions, absorbs best in the ______and ______wavelengths and absorbs least in the ______.

○Accessory pigments include ______ and a group of molecules called ______.

  • An overall action spectrum for photosynthesis profiles the relative effectiveness of different wavelengths of light in driving the process.
  • The action spectrum of photosynthesis was first demonstrated in 1883 in a clever experiment performed by ______.

○Different segments of a filamentous ______were exposed to different wavelengths of light.

○Areas receiving wavelengths favorable to photosynthesis produced ______.

○Engelmann used ______as a measure of O2 production.

  • Only ______participates directly in the light reactions, but accessory photosynthetic pigments absorb light and transfer energy to chlorophyll a.

○______, with a slightly different structure than chlorophyll a, has a slightly different absorption spectrum and funnels the energy from these wavelengths to chlorophyll a.

○______ can funnel the energy from other wavelengths to chlorophyll a and also participate in ______against excessive light.

When chlorophyll and other pigments absorb light, an electron is boosted to an excited state.

  • When a molecule absorbs a photon, one of the molecule’s electrons is elevated to an orbital with more potential energy. The electron moves from its ground state to an excited state, where they are unstable.
  • In the thylakoid membrane, chlorophyll is organized along with proteins and smaller organic molecules into ______.
  • A photosystem is composed of a ______surrounded by several light-harvesting complexes.
  • Each light-harvesting complex consists of ______(which may include chlorophyll a, chlorophyll b, and carotenoids) bound to ______.
  • When a pigment molecule absorbs a photon, the energy is transferred from pigment molecule to pigment molecule until it is funneled into the reaction-center complex.
  • At the reaction center is a ______, which accepts an excited electron from the ______. A series of redox reactions follows.

There are two types of photosystems in the thylakoid membrane.

  • Photosystem II (PS II or P680) and ______(PS I or ______) each have a characteristic reaction-center complex.

○The two photosystems were named ______, but they function sequentially, with photosystem II functioning first.

  • Photosystem II (P680), has an absorption peak at ______nm.
  • Photosystem I (P700) has an absorption peak at ______nm.

During the light reactions, there are two possible routes for electron flow: ______and ______.

  • Linear electron flow (aka non- cyclic) drives the synthesis of ______and ______

1.Photosystem II absorbs a ______. One of the electrons of P680 is excited to a higher energy state.

2.This electron is captured by the ______, leaving P680 oxidized (P680+).

3.An enzyme extracts ______from water and supplies them to the oxidized P680+ pair. This reaction splits water into two ______ions and an______atom that combines with another oxygen atom to form ______. The H+ are released into the ______.

4.Each photoexcited electron passes from the primary electron acceptor of PS II to PS I via an electron transport chain. The electron transport chain between PS II and PS I is made up of the electron carrier ______, a______complex, and a protein called ______(Pc).

5.As these electrons “fall” to a lower energy level, their energy is harnessed to produce ______. As electrons pass through the cytochrome complex, H+ are pumped into the ______, contributing to the ______that is subsequently used in chemiosmosis.

6.Meanwhile, light energy has excited an electron of PS I’s P700 reaction center. The photoexcited electron was captured by PS I’s primary electron acceptor, creating an electron “hole” in P700 (to produce P700+). This hole is filled by an______that reaches the bottom of the electron transport chain from______.

7.Photoexcited electrons are passed in a series of ______reactions from PS I’s primary electron acceptor down a second electron transport chain through the protein ______(___).

8.The enzyme ______ catalyzes the electrons from Fd to NADP+. Two electrons are required for NADP+’s reduction to NADPH. NADPH will carry the reducing power of these high-energy electrons to the ______.

  • The light reactions use the ______power of ______absorbed by PS II and PS I to provide ______and ______to the ______.
  • Under certain conditions, photoexcited electrons from photosystem ____, but not photosystem ____, can take an alternative pathway, a short circuit called ______.

○The electrons cycle back from ______to the cytochrome complex and from there continue on to a P700 chlorophyll in the PS I reaction-center complex.

○There is no production of ______and no release of ______. Cyclic flow does, however, generate ______.

Chloroplasts and mitochondria generate ATP by the same mechanism: ______.

  • In both chloroplasts and mitochondria, an electron transport chain pumps protons across a membrane as electrons are passed along a series of increasingly ______carriers; this creates an______across the membrane.
  • ______molecules harness the proton-motive force to generate ATP as H+ diffuses back across the membrane.
  • The inner membrane of the mitochondrion pumps protons from ______out to the ______.

The thylakoid membrane of the chloroplast pumps protons from ______into the ______inside the thylakoid.

  • To summarize the light reactions:

○Electron flow pushes electrons from ______, where they have low potential energy, to ______, where they have high potential energy.

○The light-driven electron current also generates ______.

○This process also produces ______as a by-product.

Concept 10.3 The Calvin cycle uses the chemical energy of ATP and NADPH to reduce CO2 to sugar

  • The Calvin cycle regenerates its starting material after molecules enter and leave the cycle.
  • Carbon enters the cycle as ______and leaves as ______.
  • The cycle spends the energy of ATP and the reducing power of ______carried by ______to make ______.
  • The actual sugar product of the Calvin cycle is not glucose but a three-carbon sugar, ______.
  • For the net synthesis of one G3P molecule, the cycle must take place ______times, fixing ______molecules of ______.
  • To make one glucose molecule requires ______cycles and the fixation of ______molecules.

Phase 1: Carbon fixation

  • In the ______ phase, each CO2 molecule is attached to a five-carbon sugar, ______.

○This reaction is catalyzed by RuBP carboxylase-oxygenase, or ______.

  • The six-carbon intermediate is unstable and splits in half to form two molecules of ______for each CO2 fixed.

Phase 2: Reduction

  • During ______, each 3-phosphoglycerate receives another ______group from ______to form ______.
  • A pair of electrons from ______reduces each 1,3-bisphosphoglycerate to ______.
  • For every three molecules of CO2 that enter the cycle, there are ______ molecules of G3P formed…but ______have to be recycled so we only get net gain of ______ G3P.
  • This molecule exits the cycle to be used by the plant cell, while the other five molecules are recycled to regenerate the______molecules of ______.

Phase 3: Regeneration of the CO2 acceptor (______)

  • In a complex series of reactions, the carbon skeletons of five molecules of G3P are rearranged by the last steps of the Calvin cycle to regenerate three molecules of RuBP.
  • To accomplish this, the cycle spends ______more molecules of ______.
  • The G3P from the Calvin cycle is the starting material for metabolic pathways that synthesize ______.

Concept 10.4 Alternative mechanisms of carbon fixation have evolved in hot, arid climates

  • One of the major problems facing terrestrial plants is ______.
  • Metabolic adaptations to reduce dehydration often require trade-offs with other metabolic processes, especially photosynthesis.
  • The ______are both the major route for ______(______in and ______out) and the main site of the ______.

Photorespiration may be an evolutionary relic.

  • In most plants (C3 plants), initial fixation of CO2 occurs via rubisco, forming a three-carbon compound, 3-phosphoglycerate.

○C3 plants include rice, wheat, and soybeans.

  • When the stomata of C3 plants partially close on a hot, dry day, ______levels drop as CO2 is consumed in the______.
  • At the same time, ______levels rise as the light reactions convert light to ______energy.
  • Although rubisco normally accepts ______, as ______becomes scarce, rubisco can add ______to ______.
  • When rubisco adds ______to ______, RuBP splits into a ______-carbon compound and a ______-carbon compound in a process called ______.
  • Photorespiration consumes ATP.
  • Photorespiration decreases______by siphoning organic material from the ______and releasing ______that would otherwise be fixed.
  • One hypothesis for the existence of photorespiration is that it is evolutionary baggage.

○When rubisco first evolved, the atmosphere had far less ______and more ______than it does today.

○The inability of the active site of rubisco to exclude ______would have made little difference.

Certain plant species have evolved alternative modes of carbon fixation to minimize ______.

  • In one strategy, ______plants first fix CO2 in a four-carbon compound.

○Plants including sugarcane and corn, use this pathway.

  • In C4 plants, the Calvin cycle is confined to the chloroplasts of the ______cells.

○______cells are arranged in tightly packed sheaths around the ______of the leaf.

  • The key enzyme, ______, adds CO2 to ______ to form the four-carbon product ______.

○______ has a very high affinity for ______and no affinity for ______.

○Therefore, PEP carboxylase can fix CO2 efficiently when rubisco cannot (that is, on hot, dry days when the stomata are closed).

  • In effect, the mesophyll cells pump ______into the ______cells, keeping CO2 levels high enough for ______to accept ______and not ______.
  • C4 photosynthesis minimizes ______and enhances ______.
  • C4 plants thrive in ______regions with intense ______.
  • A second adaptation (______) has evolved in succulent plants, cacti, pineapples, and several other plant families.
  • These plants open their stomata during the ______and close them during the ______.
  • During the night, these plants fix CO2 into a variety of ______in ______cells.
  • During the day, the light reactions supply ______and ______to the ______cycle, and ______is released from the ______to become incorporated into ______.
  • Both C4 and CAM plants add ______to organic intermediates before it enters the Calvin cycle.

○In C4 plants, carbon fixation and the Calvin cycle are structurally separated (______cells vs. ______cells)

○In CAM plants, carbon fixation and the Calvin cycle are temporally separated (______vs. ______)

A review of the importance of photosynthesis.

  • The light reactions capture ______energy and use it to make ATP and transfer electrons from ______to NADP+.
  • The Calvin cycle uses the ______and ______to produce ______from ______.
  • Carbohydrates in the form of the disaccharide ______travels via the veins to non-photosynthetic cells in the plant body.
  • Plants also store excess sugar by the synthesis of ______.
  • Heterotrophs, including humans, completely or partially consume plants for fuel and raw materials.
  • On a global scale, ______is the most important process on Earth.

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