Lecture Outline

                      Sun, Rain, and Survival

           A.   For life based on organic compounds, two questions can be raised:

                  1.   Where does the carbon come from?

                  2.   Where does the energy come from to link carbon and other atoms into organic compounds?

           B.   Autotrophs are “self-nourishing.”

                  1.   They obtain carbon from carbon dioxide.

                  2.   Photosynthetic autotrophs (plant, protistan, and bacterial members) harness light energy.

                  3.   Chemosynthetic autotrophs (a few bacteria) extract energy from chemical reactions involving inorganic substances (such as sulfur compounds).

           C.   Heterotrophs feed on autotrophs, each other, and organic wastes.

                  1.   Heterotrophs acquire carbon and energy from autotrophs.

                  2.   Heterotrophs include animals, protistans, bacteria, and fungi.

           D.   Carbon and energy enter the web of life by photosynthesis and in turn are released by glycolysis and aerobic respiration.

 

          7.1      Photosynthesis–An Overview

           A.   Where the Reactions Take Place

                  1.   The two stages of photosynthesis take place in the chloroplast.

                  2.   Light-dependent reactions occur in the thylakoid membrane system.

                          a.   The thylakoids are folded into grana (stacks of disks) and channels.

                         b.   The interior spaces of the thylakoid disks and channels are continuous and are filled with H+ needed during ATP synthesis.

                  3.   Carbohydrate formation occurs in the stroma (semifluid) area that surrounds the grana.

           B.   But Things Don't Really End With Glucose

                  1.   The light-dependent reactions convert light energy to chemical energy (which is then stored in ATP); the liberated electrons are picked up by NADPH.

                  2.   The light-independent reactions assemble sugars and other organic molecules using ATP, NADPH, and CO2.

                  3.   Overall, the equation for glucose formation is written:

                                                         sunlight

                        12H2O + 6CO2   —————>  6O2 + C6H12O6 + 6H2O

 

7.2       Sunlight as an Energy Source

           A. Properties of Light

                  1.   Energy from the sun radiates through space in wavelengths ranging from gamma rays to radio waves (the electromagnetic spectrum).

                  2.   Photoautotrophs use only a small range (400–750 nm) of wavelengths for photosynthesis; these wavelengths are the range of visible light.

                  3.   Light energy is packaged as photons, which vary in energy as a function of wavelength (most energetic in blue-violet; least energetic in red light).

           B.   Pigments–Molecular Bridge From Sunlight to Photosynthesis

                  1.   Pigments are the molecular bridge between sunlight and photosynthetic activity.

                  2.   Chlorophyll pigments absorb blue and red but reflect green (the color of leaves).

 

          7.3      The Rainbow Catchers

           A.   The Chemical Basis of Color

                  1.   Electrons in pigments absorb photons of specific energies, which correspond to specific colors of light.

                  2.   If the quantity of energy of an incoming photon matches the energy level required to boost an electron to a higher energy level, that wavelength will be absorbed; photons that are a mis-match will be transmitted (reflected) in the color visible to an observer.

           B.   On the Variety of Photosynthetic Pigments

                  1.   Chlorophylls are the main pigments in all but one group of photoautotrophs.

                          a.   Chlorophyll a  (green) is the main pigment inside chloroplasts.

                         b.   Chlorophyll b (bluish-green) occurs in plants, green algae, and photoautotrophic bacteria.

                  2.   Carotenoid pigments absorb blue-violet wavelengths but reflect yellow, orange, and red.

                  3.   Anthocyanins are pigments in many flowers.

                  4.   Phycobilins are the red and blue pigments of the red algae and cyanobacteria.

           C.   Where Are Photosynthetic Pigments Located?

                  1.   Photosynthetic pigments are found in bacteria where they are located on the plasma membrane.

                  2.   In the thylakoid membrane systems of chloroplasts the pigments are organized in clusters called photosystems consisting of 200 to 300 pigment molecules.

 

          7.4      The Light-Dependent Reactions

           A.   Three events occur:

                  1.   Pigments absorb sunlight energy and give up excited electrons.

                  2.   Electron and hydrogen transfers lead to ATP and NADPH formation.

                  3.   The pigments that gave up the electrons in the first place get electron replacements.

           B.   What Happens to the Absorbed Energy?

                  1.   The pigments “harvest” photon energy from sunlight.

                          a.   Absorbed photons of energy boost electrons to a higher level.

                         b.   The electrons quickly return to the lower level and release energy.

                          c.   Released energy is trapped by chlorophylls located in the photosystem's reaction center.

                         d.   The trapped energy is then used to transfer a chlorophyll electron to an acceptor molecule.

                  2.   Electron transport systems accept electrons which are then processed through a step-by-step array to produce ATP and NADPH.

           C.   Cyclic and Noncyclic Electron Flow

                  1.   The cyclic pathway is probably the oldest means of ATP production, being used by early bacteria.

                          a.   In the cyclic pathway of ATP formation, excited electrons leave the P700 reaction center, pass through an electron transport system, and then return to the original photosystem I.

                         b.   Energy associated with the electron flow drives the formation of ATP from ADP.

                  2.   The noncyclic pathway of ATP formation transfers electrons through two photosystems and two electron transport systems (ETS) in the thylakoid membranes.

                          a.   The pathway begins when chlorophyll P680 in photosystem II absorbs energy.

                                1)    Boosted electron moves through a transport system that releases energy for ADP + Pi  ——> ATP.

                                2)    Electron fills “hole” left by electron boost in P700 of photosystem I.

                                3)    Electron from photolysis of water fills “electron hole” left in P680 and produces oxygen byproduct.

                         b.   Pathway continues when chlorophyll P700 in photosystem I absorbs energy.

                                1)    Energy hole is filled by electron from P680.

                                2)    Boosted electron from P700 passes to acceptor, then ETS; it finally joins NADP to form NADPH (which along with ATP can be used in synthesis of organic compounds).

           D.   The Legacy—A New Atmosphere

                  1.   Oxygen is a by-product of the noncyclic pathway.

                  2.   Beginning about 1.5 billion years ago, large amounts of oxygen began accumulating in the atmosphere, which up to that time had been oxygen-free.

 

          7.5      Case Study: A Controlled Release of Energy

           A.   Hydrogen ions from photolysis of water accumulate inside the thylakoid compartment of chloroplasts to set up concentration and electric gradients.

                  1.   Oxygen atoms from photolysis combine to form O2 which is released into the atmosphere.

                  2.   As the hydrogen ions flow out through channels into the stroma, enzyme action links Pi  to ADP to form ATP.

           B.   This mechanism is called the chemiosmotic model of ATP formation.

 

          7.6      Light-Independent Reactions

           A.   These reactions (Calvin-Benson cycle) are the "synthesis" of photosynthesis.

                  1.   The participants and their roles in the synthesis of carbohydrate are:

                          a.   ATP, which provides energy;

                         b.   NADPH, which provides hydrogen atoms and electrons;

                          c.   Atmospheric air, which provides carbon dioxide (the source of carbon and oxygen).

                  2.   The reactions are not dependent on sunlight directly.

           B.   How Do Plants Capture Carbon?

                  1.   Carbon dioxide diffuses from the air, across the plasma membrane of the plant cell and into the stroma.

                  2.   Carbon fixation occurs when the carbon atom of CO2 becomes attached to ribulose bisphosphate (RuBP) to form a six-carbon intermediate; the enzyme responsible is rubisco.

                  3.   The six-carbon intermediate splits at once to form two PGA (phosphoglycerate) molecules.

           C.   How Do Plants Build Glucose?

                  1.   Each PGA then receives a phosphate group from ATP plus H+ and electrons from NADPH to form PGAL (phosphoglyceraldehyde).

                          a.   Most of the PGAL molecules continue in the cycle to fix more carbon dioxide, but two PGAL join to form a sugar phosphate, which will be modified to sucrose, starch, and cellulose.

                         b.   Final tally: 12H2O + 6CO2 + 18ATP + 12NADPH ———> C6H12O6 + 18ADP + 18Pi + 12NADP+ + 6H2O + 12H+

                  2.   Sugar phosphates are used as cellular fuel and as building blocks in synthesis of sucrose or starch.

                          a.   Sucrose is the most easily transportable.

                         b.   Starch is the main storage form, but it can be converted back to sucrose for distribution to leaves, stems, and roots.

                  3.   Photosynthesis also yields intermediates and products that can be used in lipid and amino acid synthesis.

 

          7.7      Fixing Carbon—So Near, Yet So Far

           A.   C4 Versus C3 Plants

                  1.   Plants in hot, dry environments close their stomata to conserve water but in so doing retard carbon dioxide entry and permit oxygen buildup inside the leaves.

                  2.   Thus, oxygen—not carbon dioxide—becomes attached to RuBP to yield one PGA (instead of two) and one phosphoglycolate (not useful); this nonproductive process is called photorespiration.

                  3.   To overcome this fate, crabgrass, sugarcane, corn, and other plants fix carbon twice (in mesophyll cells then in bundle-sheath cells) to produce oxaloacetate (a four-carbon, hence C4) compound, which can then donate the carbon dioxide to the Calvin-Benson cycle.

           B.   CAM Plants

                  1.   Succulents, such as cacti, open their stomata and fix CO2 only at night, storing the intermediate product for use in photosynthesis the next day.

                  2.   These plants are known as CAM plants because, unlike C4 species, they do not fix carbon in separate cells but at different times in the same cell.

 

          7.8      Autotrophs, Humans, and the Biosphere

           A.   The oceans are host to vast numbers of photoautotrophic prokaryotic cells and protistans.

                  1.   They shaped global climate by dealing with staggering numbers of reactant and product molecules.

                  2.   Without them, global warming may occur with serious consequences for us all.

           B.   Chemoautotrophs at the hydrothermal vents on the ocean floor affect the cycling of certain elements.