Lecture Outline - Chapter 8

8.1. Radiant Energy

1. Living organisms ultimately depend upon food produced by photosynthesis. (Fig. 8.1)
2. Autotrophs have ability to synthesize organic molecule from raw material; heterotrophs must take in reformed organic molecules.
3. Bodies of plants also become fossil fuels used for energy to drive modern machinery and heat buildings.
4. Sunlight (Fig. 8.2)
   • a. Radiant energy is described by its wavelength; gamma rays are shortest and radio waves are longest.
   • b. The visible spectrum is a narrow band ranging from violet (shortest) to red (longest).
   • c. Energy content is also highest for shorter violet and lowest for longer red light.
5. About 42% of total solar radiation that hits the atmosphere reaches through to the surface; higher energy wavelengths are screened out by ozone, lower energy wavelengths are screened out by water vapor and carbon dioxide.
6. Life is adapted in vision and photosynthesis to the middle wavelengths.
7. Chlorophylls and carotenoids are pigments capable of absorbing portions of the visible light spectrum.
8. Chlorophyll absorbs far less green light; thus green is reflected and leaves appear green.
9. Carotenoids absorb violet-blue-green and reflect yellow-orange; when chlorophyll breaks down in fall, these pigments remain to give some leaves their fall color.

1. Chloroplasts are organelles found in plant cells that carry on photosynthesis.
2. Water is both utilized and produced by photosynthesis.
3. A generalized carbohydrate (CH₂O) is also produced.
4. The oxygen in the O₂ produced by photosynthesis comes from the input of water; this as shown by experiment where heavy oxygen (¹⁸C) in water turns up as the total oxygen produced. The oxygen from CO₂ therefore becomes part of the carbohydrate. (Fig. 8.3)
5. Photosynthesis can also be represented as the reverse of cellular respiration; water molecules are oxidized and CO₂ is reduced.
6. Anatomy of Chloroplasts
   • a. Most chloroplasts are in leaves. (Fig. 8.4)
   • b. Mesophyll cells receive water from vessels extending up from roots.
   • c. CO₂ enters and O₂ exits a leaf through small pores.
   • d. Chloroplasts are bounded by a double membrane.
   • e. Inside these membranes is a large space called the stroma; the stroma contains an energy-rich solution that reduces carbon dioxide (CO₂), converting it to an organic compound.
   • f. Grana are stacks of flattened sacs called thylakoids that contain the pigment chlorophyll.
   • g. Chlorophyll and other pigments in the membranes absorb solar energy which energizes electrons before reducing CO₂ in the stroma.

1. Photosynthesis involves two sets of reactions: the light-dependent reactions that require light be present, and the light-independent reactions that can take place in the dark. (Fig. 8.5)
2. Generally, the light-dependent reactions remove low energy electrons from water when chlorophyll absorbs energy; these electrons move down an electron transport system to produce ATP from ADP and (P); energized electrons are also taken up by NADP⁺¹, which temporarily holds energy to fuel upcoming CO₂ reduction.
3. Generally, the light-independent reactions use ATP and NADPH formed in thylakoids to reduce CO₂ in the stroma; the CO₂ from the air is fixed by a substrate of the Calvin cycle to produce CH₂O.
4. The Calvin cycle is named for Melvin Calvin who used radioactive carbon-14 to label the CO₂ to discover the light-independent reactions.
5. Thylakoid membranes contain two light-gathering units, Photosystem I (PS I) and Photosystem II (PS II), named in the order of their discovery.
6. Pigment Complex Molecules
   • a. Include chlorophyll a and chlorophyll b, and carotenoids.
   • b. Serve as an "antenna" to gather sunlight energy until concentrated in the reaction-center chlorophyll.
   • c. Electrons become so excited that they escape to a nearby electron acceptor molecule.
7. Cyclic Electron Pathway (Fig. 8.6)
   • a. This pathway begins after the PS I pigment complex absorbs solar energy.
   • b. Energized electrons leave the reaction-center chlorophyll of PS I, pass through the electron transport system (cytochrome system), release energy used to produce ATP, and then return to the reaction-center again.
   • c. This pathway only produces ATP.
   • d. Some photosynthetic bacteria only utilize this pathway; it therefore may have evolved early in the history of life.
8. Noncyclic Electron Pathway (Fig. 8.7)
   • a. Noncyclic electron pathway results in both ATP and NADPH.
   • b. Pigment complex of PS II absorbs solar energy.
   • c. Excited electrons leave reaction-center chlorophyll a molecule.
   • d. PS II takes replacement electrons from water, which splits releasing oxygen.
   • e. High energy electrons that leave PS II are captured by an acceptor molecule.
   • f. Electrons pass from one carrier to the next in the electron transport system as the released energy is used for ATP production.
   • g. When PS I pigment complex absorbs solar energy, excited electrons leave the reaction-center chlorophyll a and are captured by an acceptor molecule that passes electrons on to NADP⁺, and NADP⁺ picks up H⁺ from stroma to form NADPH.
   • h. Results of noncyclic electron flow: water is split, yielding electrons, oxygen and hydrogen ions; ATP is produced; and NADP⁺ becomes NADPH.
9. Chemiosmotic ATP Synthesis
   • a. The membranous system within the stroma forms flattened sacs called thylakoids, in some places they are stacked to form grana.
   • b. The thylakoid space within acts as a reservoir for hydrogen ions.
   • c. Each time a water is split, two H⁺ remain in the thylakoid space.
   • d. Compared to the stroma, the large number of hydrogen ions in the thylakoid space create an electrochemical gradient.
   • e. The flow of H⁺ from high to low concentration across the thylakoid membrane provides energy that allows ATP synthase enzyme to produce ATP from ADP + (P).
10. The Thylakoid Membrane is Organized (Fig. 8.8)
    • a. Biochemical and structural studies show intact complexes in the thylakoid membrane.
    • b. PS II is the light-gathering complex that splits water and produces oxygen.
    • c. The cytochrome complex transports electrons between PS II and PS I.
    • d. PS I is a light-gathering pigment associated with the enzyme that reduces NADP⁺ to NADPH.
    • e. ATP synthase complex has a H⁺ channel and a protruding ATP synthase; H⁺ flows down this channel and ATP is produced from ADP + (P).

1. The light-independent reactions are the second stage of photosynthesis.
   • a. NADPH and ATP from the light-dependent reactions reduce carbon dioxide to form a carbohydrate.
   • b. Reduction of CO₂ within the stroma of the chloroplast occurs by the Calvin cycle.
2. The Calvin Cycle (Fig. 8.9)
   • a. Summary: carbon dioxide combines with a five-carbon sugar; the six-carbon molecule breaks down to form two PGA (three-carbon) molecules; PGA is reduced to PGAL by using NADPH and ATP; regenerating the five carbon sugar. PGAL, the end product of the Calvin cycle, is converted to many other molecules (glucose phosphate, fatty acids, amino acids, etc.) especially by algae and plants.
   • b. Details of the Calvin cycle can be divided into CO₂ fixation, CO₂ reduction, and regeneration of RuBP.
   • c. Fixing Carbon Dioxide (Fig. 8.10)
     • i. Carbon dioxide combines with RuBP, a five-carbon molecule due to the enzyme RuBP carboxylase.
     • ii. This protein makes up 20-50% of protein content of chloroplasts.
   • d. Reducing Carbon Dioxide
     • i. The six-carbon molecule from CO₂ fixation immediately breaks down to form two PGA (phosphoglycerate) three-carbon molecules.
     • ii. PGA is reduced to PGAL (phosphoglyceraldehyde) by using NADPH and ATP from the light- dependent reaction in two steps.
   • e. Regenerating RuBP
     • i. For every three turns of the Calvin cycle, five molecules of PGAL are used to re-form three molecules of RuBP.
     • ii. The net gain of three turns of the Calvin cycle is one PGAL molecule.
     • iii. This reaction utilizes some ATP produced by the light-dependent reactions.

1. C₃ Versus C₄ Photosynthesis
   • a. C₃ photosynthesis is described as above because a C₃ molecule is detected immediately following CO₂ fixation.
   • b. In C₄ photosynthesis, a C₃ molecule is detected following CO₂ fixation.
   • c. C₃ plants:
     -include wheat, rice, oats and Kentucky blue grass
     -have mesophyll cells with well-formed chloroplasts, cells are arranged in parallel layers
   • d. C₄ plants:
     -include sugarcane, corn and crabgrass
     -CO₂ is taken up in mesophyll cells and then a C₄ molecule (oxaloacetate) is pumped into the bundle sheath cells where it releases CO₂ to the Calvin cycle
     -have chlorophyll in both the bundle sheath cells and the mesophyll cells that are arranged around the sheath.

     v. C₃ plants have an advantage in moderate climates, C₄ plants are more sheltered from drying in hot and dry climates because oxygen accumulates when stomates close to protect the plant.
2. CAM Photosynthesis
   • a. CAM plants fix some CO₂ at night, forming C₄ molecule.
   • b. CAM from "crassulacean-acid metabolism"; Crassulaceae is family of warm, arid region flowers.
   • c. With stomates closed in daytime, these plants conserve water.

1. Both plants and animals carry out respiration, only plants carry on photosynthesis.
2. Cell organelle for aerobic respiration is the mitochondrion; the organelle for photosynthesis is the chloroplast.
3. Overall equation for aerobic cellular respiration is the opposite of that for photosynthesis:
   ATP energy + 6 CO₂ + 6 H₂O --> C₆H₁₂O₆ + 6 O₂ (cellular respiration)
   solar energy + 6 CO₂ + 6 H₂O <-- C₆H₁₂O₆ + 6 O₂ (photosynthesis)

   Cellular respiration requires oxygen, breaks down carbon dioxide, and occurs in both plants and animals, day or night.

   Photosynthesis requires carbon dioxide, releases oxygen, involves reduction, and stores energy.

   Both photosynthesis and cellular respiration are metabolic pathways.

   Both make use of an electron transport system located in a membrane to produce ATP.

   Both use a hydrogen carrier (NAD⁺ in respiration, NADP⁺ in photosynthesis).

   Photosynthesis occurs only during daytime in plants; during daylight hours, the rate of photosynthesis exceeds the rate of aerobic cellular respiration.