10.1 Photosynthesis Harnesses Sunlight to Make Carbohydrates from CO2

• Photosynthetic organisms cannot store the electromagnetic energy of light unless it is first converted into another form.

• The reactions that reduce carbon dioxide and produce sugar came to be known as the Calvin cycle.

• Once later experiments established that photosynthesis takes place only in the green portions of plants, biologists focused on the bright green organelles called chloroplasts

• The interior of the organelle is dominated by flattened, membranous sac-like structures called thylakoids, which then occur in interconnected stacks called grana.

• The space inside a thylakoid is its lumen.

• The fluid-filled space between thylakoids and the inner membrane is the stroma.

• Pigments are molecules that absorb only certain wavelengths of light-other wavelengths are either reflected or transmitted.

• Pigments appear colored because people see only the wavelengths that are not absorbed.

10.2 How Do Pigments Capture Light Energy?

• Electromagnetic radiation is characterized by its wavelength-the distance between two successive wave crests or troughs.

• The range of wavelengths of electromagnetic radiation is the electromagnetic spectrum.

• To emphasize the particle-like nature of light, physicists point out that it exists in discrete packets called photons.

• Chlorophylls, designated chlorophyll αand chlorophyll b, absorb strongly in the blue and red regions of the visible spectrum.

• Carotenoids absorb wavelengths in the blue and green parts of the visible spectrum.

• Engelmann concluded that they defined the action spectrum for photosynthesis-the wavelengths that drive the light-capturing reactions.

• An absorption spectrum measures how the wavelength of photons influences the amount of light absorbed by a pigment.

• When the electron energy produces light, it is called fluorescence.

• In the chloroplast thylakoid membrane, 200-300 chlorophyll molecules and accessory pigments are organized by associated proteins to form large complexes called photosystems.

• When antenna pigments absorb photons, the energy-but not the electron itselι is passed to a nearby pigment molecule, where another electron is excited in response. This phenomenon is known as resonance energy transfer.

• The energy released from these electrons can

  • Be emitted in the form of light via fluorescence

  • Be given off as heat

  • Excite an electron in a nearby pigment and induce resonance

  • Be transferred to an electron acceptor in a redox reaction

10.3 The Discovery of Photosystems I and II

• In photosystem II, the action often begins when a mobile accessory UCture called the light-harvesting complex transmits resonance energy to an antenna pigment inside the photosystem.

• Pheophytin is identical to chlorophyll except that pheophytin lacks a magnesium atom in its head region. Functionally, the two molecules are very different.

• Plastoquinone (PQ) is a quinone similar to ubiquinone in the ETC of cellular respiration

• Since the synthesis of ATP in chloroplasts is initiated by the energy from light, it is called photophosphorylation.

• Photosynthetic organisms that oxidize water will generate oxygen (02) as a by-product, and thus perform oxygenic (“oxygen-producing”) photosynthesis.

• Electrons from photosystem I are used to produce NADPH, which is a reducing agent similar in function to the NADH and FADH2 produced by the citric acid cycle

• Electrons from photosystem II, in contrast, are used to produce a proton-motive force that drives the synthesis of ATP.

• In combination, then, photosystems II and I produce chemical energy stored in ATP and NADPH.

• When electrons reach the end of the cytochrome complex, they are passed to a small diffusible protein called plastocyanin (PC).

• The electrons that pass from water to NADP+ move through a chain of redox reactions in a linear fashion, referred to as noncyclic electron flow.

10.4 How Do Cells Capture Carbon Dioxide?

• The surface of a leaf is dotted with openings bordered by two distinctively shaped cells called guard cells

• The opening between these paired cells is called a pore, and the entire structure is a stoma

• Carbon fixation is the addition of carbon atoms from inorganic carbon dioxide to an organic compound.

• The reaction sequence resembles respiration, because it consumes oxygen and produces carbon dioxide. As a result, it is called photorespiration.

• PEP carboxylase is common in mesophyll cells near the surface of leaves, while rubisco is found in bundle sheath cells that surround the vascular tissue in the interior of the leaf

10.5 Captured Carbon Dioxide Is Reduced to Make Sugar

• The complete Calvin Cycle has three phases

  • Fixation phase

  • Reduction phase

  • Regeneration phase

• All three phases take place in the stroma of chloroplasts.

• The most important of these reaction sequences uses G3P to produce the monosaccharide glucose, a process called gluconeogenesis.

• This glucose is then combined with fructose, which is also made from G3P, to form the disaccharide (“two-sugar”) sucrose.