Chapter 6:  How Cells Aquire Energy

Sunlight and Survival

           A.   For all 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.

           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.


   I.     Photosynthesis—An Overview

           A.   Where the Reactions Take Place

                  1.   Both stages of photosynthesis occur in the chloroplast.

                          a.   The semifluid interior (stroma) is the site for the second series of photosynthesis reactions.

                          b.   Flattened sacs, thylakoids, interconnected by channels weave through the stroma; the first reactions occur here.

                  2.   There are two series of reactions:

                          a.   The light-dependent reactions convert light energy to chemical energy, which is stored in ATP and NADPH; water is split.

                          b.   The light-independent reactions assemble sugars and other organic molecules using ATP and NADPH as energy sources.

                          c.   Overall, for glucose formation:


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

           B.   But Things Don't Really End With Glucose

                  1.   The newly formed glucose in the chloroplast has a phosphate group attached.

                  2.   The glucose phosphate molecules react to form sucrose, starch, and cellulose.


II.     Sunlight As an Energy Source

           A.   Properties of Light

                  1.   Organisms use only a small range of wavelengths for photosynthesis, vision, and other processes.

                  2.   Most of these wavelengths are the ones we see as visible colors.

                  3.   Light energy is packaged as photons, which vary in energy as a function of wave­length.

           B.   Pigments—The Rainbow Catchers

                  1.   Pigment molecules on the thylakoid membranes absorb photons.

                  2.   Chlorophyll pigments absorb blue and red but transmit green (leaves).

                  3.   Carotenoid pigments absorb violet and blue but transmit yellow, orange, and red.

                  4.   Phycobilins are red and blue pigments found in red algae and cyanobacteria.

           C.   Where Are Photosynthetic Pigments Located?

                  1.   Cyanobacteria use chlorophyll a  and other pigments in some internal foldings of their plasma membrane.

                  2.   In thylakoid membranes, pigments are organized in clusters called photosystems, each consisting of 200-300 pigment molecules.


III.     The Light-Dependent Reactions

           A.   Three events are involved:

                  1.   Pigments absorb light energy and give up electrons.

                  2.   Water molecules are split; ATP and NADPH form; oxygen is released.

                  3.   Electrons are replaced in the pigment molecules that first gave them up.

           B.   What Happens to the Absorbed Energy?

                  1.   The pigments “harvest” 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.

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

                  2.   Electrons expelled from a chlorophyll molecule go through one or two electron trans­port systems, resulting in formation of ATP and NADPH.

           C.   Cyclic and Noncyclic Electron Flow

                  1.   In the cyclic pathway of ATP formation, electrons are first excited, pass through an electron transport system, and then return to the orig­inal photosystem.

                          a.   This photosystem is characterized by the presence of chlorophyll P700.

                          b.   The cyclic pathway is an ancient way to make ATP from ADP; it was used by early bacteria.

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

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

                                1)   Boosted electron moves through a transport system, which 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 by-product.

                          b.   The other pathway begins when chlorophyll P700 in photosystem I absorbs energy.

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

                                2)   Energy hole is filled by electron from P680.

           D.   The Legacy—A New Atmosphere

                  1.   Oxygen is a by-product of photosynthesis

                  2.   Since about 2 billion years ago, oxygen has been accumulating in the atmosphere making aerobic respiration in animals possible.


IV.     Case Study: A Controlled Release of Energy

           A.   Electron flow causes H+ to accumulate inside the thylakoid compartments.

           B.   When the H+ flow out to the stroma through the channel proteins, ATP synthase causes ADP to gain a phosphate to form ATP.


V.     The Light-Independent Reactions

           A.   These reactions constitute a pathway known as the Calvin-Benson cycle.

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

                          a.   ATP, which provides energy.

                          b.   NADPH, which provides hydrogen atoms and electrons.

                          c.   Atmospheric air, which provides the carbon and oxygen from carbon dioxide.

                  2.   The reactions take place in the stroma of chloroplasts and are not dependent on sunlight.

           B.   How Do Plants Capture Carbon?

                  1.   Carbon dioxide diffuses into a leaf, across the plasma membrane of a photosynthetic cell, and into the stroma of a chloroplast.

                  2.   Rubisco joins carbon dioxide to RuBP to produce an unstable intermediate that splits to form two molecules of PGA.

           C.   How Do Plants Build Glucose?

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

                  2.   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.


VI.     Fixing Carbon—So Near, Yet So Far

           A.   C4 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 unproductive process is called photorespiration.

                  3.   To overcome this fate, crabgrass, sugarcane, corn, and other plants fix carbon twice to produce oxaloacetate, a four-carbon, compound which can then donate the carbon dioxide to the Calvin-Benson cycle. These plants are called C4 plants.

           B.   CAM Plants

                  1.   In desert plants opening the stomata in the daytime would allow too much water to escape.

                  2.   Instead, they open the stomata at night and fix CO2 in the form of crassulacean acid for use the next day in carbohydrate synthesis.

                  3.   These plants are known as CAM plants.


VII.   Focus on Science:  Light in the Deep Dark Sea


VIII.  Focus on the Environment:  Autotrophs, Humans, and the Biosphere