Chapter 6 Where It Starts–Photosynthesis

6.1 Biofuels Autotrophs make their own food Harvest energy from the environment and carbon from inorganic molecules Photosynthesis obtains energy from sunlight Plants and most autotrophs use photosynthesis to make organic molecules used as food Heterotrophs get their carbon from organic molecules made by autotrophs

6.1 Biofuels Fossil fuels (the remains of ancient forests) are a limited resource Biofuels are made from organic matter that is not fossilized – a renewable resource Produced mainly from crops such as corn, soybeans, and sugarcane in the U.S. Researchers are looking for ways to use non-food-plant matter such as switchgrass and agricultural wastes

6.2 Sunlight as an Energy Source Energy flow through nearly all ecosystems on Earth begins when photosynthesizers intercept energy from the sun Photosynthetic organisms use pigments to capture the energy of sunlight and convert it to chemical energy – the energy stored in chemical bonds

Properties of Light Visible light is part of an electromagnetic spectrum of energy radiating from the sun Travels in waves Organized into photons Wavelength The distance between the crests of two successive waves of light (nm) Shorter wavelength have greater energy

Electromagnetic Spectrum of Radiant Energy range of heat escaping from Earth’s surface shortest wavelengths (highest energy) range of most radiation reaching Earth’s surface longest wavelengths (lowest energy) visible light gamma rays ultraviolet radiation near-infrared radiation radiation infrared x-rays microwaves radio waves Figure 6.2 Properties of light. A Electromagnetic radiation moves through space in waves that we measure in nanometers (nm). Visible light makes up a very small part of this energy. raindrops or a prism can separate visible light’s different wavelengths, which we see as different colors. About 25 million nanometers are equal to 1 inch. 400 nm 500 nm 600 nm 700 nm

Pigments: The Rainbow Catchers Different wavelengths form colors of the rainbow Photosynthesis uses wavelengths of 380-750 nm Pigment An organic molecule that selectively absorbs light of specific wavelengths Chlorophyll a The most common photosynthetic pigment Absorbs violet and red light (appears green)

Photosynthetic Pigments Collectively, chlorophyll and accessory pigments absorb most wavelengths of visible light Certain electrons in pigment molecules absorb photons of light energy, boosting electrons to a higher energy level Energy is captured and used for photosynthesis

Examples of Photosynthetic Pigments H2C Produced by H3C Pigment Color Plants Protists Bacteria Archaea Chlorophyll a Other chlorophylls green green ● Phycobilins H3C N CH3 H3C O N Mg N CH3 CH3 CH3 CH3 O O N CH3 H3C O phycocyanobilin phycoerythrobilin phycoviolobilin blue red violet ● ● CH3 O Carotenoids beta-carotene orange ● ● ● lycopene lutein zeaxanthin fucoxanthin red yellow yellow brown ● ● Anthocyanins red, blue ●● Retinal violet ● H3C H3C H3C CH3 CH3 CH3 CH3 CH3 CH3 CH3 Figure 6.4 Examples of photosynthetic pigments. photosynthetic pigments can collectively absorb almost all visible light wavelengths. Left, the light-catching part of a pigment (shown in color) is the region in which single bonds alternate with double bonds. These and many other pigments (including heme, Section 5.6) are derived from evolutionary remodeling of the same compound. Animals convert dietary beta-carotene into a similar pigment (retinal) that is the basis of vision. H3C CH3 CH3 CH3 O H CH3

6.3 Exploring the Rainbow Photosynthetic pigments work together to harvest light of different wavelengths Engelmann identified colors of light that drive photosynthesis (violet and red) by using a prism to divide light into colors Algae using these wavelengths gave off the most oxygen

Photosynthesis and Wavelengths of Light bacteria Figure 6.4 Discovery that photosynthesis is driven by particular wavelengths of light. A Light micrograph of cells in a strand of Chladophora. Theodor Engelmann used this green alga in an early experiment to show that certain colors of light are best for photosynthesis. B Engelmann directed light through a prism so that bands of colors crossed a water droplet on a microscope slide. The water held a strand of Chladophora and oxygen-requiring bacteria. The bacteria clustered around the algal cells that were releasing the most oxygen—the ones that were most actively engaged in photosynthesis. Those cells were under red and violet light. These results constituted one of the first absorption spectra. alga 400 nm 500 nm 600 nm 700 nm Wavelength

Absorption Spectra Most photosynthetic organisms use a combination of pigments to drive photosynthesis An absorption spectrum shows which wavelengths each pigment absorbs best Organisms in different environments use different pigments

Absorption Spectra chlorophyll b phycoerythrobilin phycocyanobilin β-carotene chlorophyll a Light absorption Figure 6.4 Discovery that photosynthesis is driven by particular wavelengths of light. C Absorption spectra of chlorophylls a and b , β-carotene, and two phycobilins reveal the efficiency with which these pigments absorb different wavelengths of visible light. Line color indicates the characteristic color of each pigment. 400 nm 500 nm 600 nm 700 nm Wavelength

6.4 Overview of Photosynthesis In photosynthetic eukaryotes, photosynthesis occurs in chloroplasts An organelle that specializes in photosynthesis in plants and many protists Photosynthesis occurs in two stages Light-dependent reactions Light-independent reactions

Summary: Photosynthesis 6CO2 + 6H2O → light energy → C6H12O6 + 6O2

First Stage of Photosynthesis – Light-Dependent Reactions Noncyclic pathway Light energy is transferred to ATP and NADPH Water molecules are split, releasing O2 Occurs in the thylakoid membrane Folded membrane that make up thylakoids Contains clusters of light-harvesting pigments that absorb photons of different energies and convert light energy into chemical energy

Second Stage of Photosynthesis – Light-Independent Reactions Energy in ATP and NADPH drives synthesis of glucose and other carbohydrates from CO2 and water Occurs in the stroma A semifluid matrix surrounded by the two outer membranes of the chloroplast Sugars are built in the stroma

The Chloroplast two outer membranes of chloroplast stroma part of thylakoid membrane system: Figure 6.6 Zooming in on a chloroplast, the site of photosynthesis in a plant cell. The micrograph shows chloroplasts in cells of a moss leaf. thylakoid compartment, cutaway view

6.5 Light-Dependent Reactions Light-dependent reactions convert light energy to the energy of chemical bonds Photons boost electrons in pigments to higher energy levels Light-harvesting complexes absorb the energy Electrons are released from special pairs of chlorophyll a molecules in photosystems Electrons may be used in noncyclic or cyclic pathways of ATP formation

The Thylakoid Membrane Figure 6.7 A view of some of the components of the thylakoid membrane as seen from the stroma. Molecules of electron transfer chains and ATP synthases are also present, but not shown for clarity. photosystem light-harvesting complex

The Noncyclic Pathway – Photosystems and ETCs Photosystems (type II and type I) contain “special pairs” of chlorophyll a molecules that eject electrons Electrons lost from photosystem II are replaced by photolysis of water molecules Process by which light energy breaks down a water molecule into hydrogen and oxygen Electrons lost from a photosystem enter an electron transfer chain (ETC) in the thylakoid membrane

The Noncyclic Pathway – H+ and ATP In the ETC, electron energy is used to build up a H+ gradient across the membrane H+ flows through ATP synthase, which attaches a phosphate group to ADP ATP is formed in the stroma by electron transfer phosphorylation

The Noncyclic Pathway – NADPH Electrons from the first electron transfer chain from photosystem II are accepted by photosystem I Electrons ejected from photosystem I enter a different electron transfer chain in which the coenzyme NADP+ accepts the electrons and H+, forming NADPH ATP and NADPH are the energy products of light-dependent reactions in the noncyclic pathway

Noncyclic Pathway of Photosynthesis light energy light energy ATP synthase photosystem II electron transfer chain photosystem I electron transfer chain ADP, Pi e e– e– e– e– NADPH e e– e– thylakoid compartment H stroma Light energy ejects elec- trons from a photosystem II. The photosystem pulls replacement electrons from water molecules, which then break apart into oxygen and hydrogen ions. The oxygen leaves the cell as O2. The electrons enter an electron transfer chain in the thylakoid membrane. 4 Energy lost by the electrons as they move through the chain is used to actively transport hydrogen ions from the stroma into the thylakoid compart- ment. A hydrogen ion gradient forms across the thylakoid membrane. Light energy ejects elec- trons from a photosystem I. Replacement electrons come from an electron transfer chain. The ejected electrons move through a second electron transfer chain, then combine with NADP+ and H+, so NADPH forms. Hydrogen ions in the thylakoid compartment follow their gradient across the thylakoid membrane by flowing through the interior of ATP synthases. Hydrogen ion flow causes ATP synthases to phosphorylate ADP, so ATP forms in the stroma. Figure 6.9 Light-dependent reactions, noncyclic pathway. ATP and oxygen gas are produced in this pathway. Electrons that travel through two different electron transfer chains end up in NADPH.

The Cyclic Pathway When NADPH accumulates in the stroma, the noncyclic pathway stalls A cyclic pathway runs in type I photosystems to make ATP Electrons are cycled back to photosystem I and NADPH does not form Allows light-dependent reactions to continue when the noncyclic path stops

Pathways of Light-Dependent Reactions Figure 6.8 Summary of the inputs and outputs of the two pathways of light-dependent reactions.

6.6 Light-Independent Reactions Energy flow in the light-dependent reactions is an example of how organisms harvest energy from their environment Produces NADPH to pass to the light-independent reactions The cyclic, light-independent reactions of the Calvin-Benson cycle are the “synthesis” part of photosynthesis Enzyme-mediated reactions that build sugars in the stroma of chloroplasts

Carbon Fixation Extraction of carbon atoms from inorganic sources (atmosphere) and incorporating them into an organic molecule Builds glucose from CO2 Uses bond energy of molecules formed in light-dependent reactions (ATP, NADPH)

The Calvin-Benson Cycle The enzyme rubisco attaches CO2 to RuBP Forms two 3-carbon PGA molecules PGAL is formed PGAs receive a phosphate group from ATP, and hydrogen and electrons from NADPH Two PGAL combine to form a 6-carbon sugar Rubisco is regenerated

Calvin– Benson Cycle other molecules glucose 1 2 4 Figure 6.11 The Calvin–Benson cycle. This sketch shows a cross-section of a chloroplast with the reactions cycling in the stroma. The steps shown are a summary of six cycles of the Calvin–Benson reactions. Black balls signify carbon atoms. Appendix V details the reaction steps. 1 Six CO2 diffuse into a photosynthetic cell, and then into a chloroplast. Rubisco attaches each to a RuBP molecule. The resulting intermediates split, so twelve molecules of PGA form. 2 Each PGA molecule gets a phosphate group from ATP, plus hydrogen and electrons from NADPH. Twelve PGAL form. 3 Two PGAL may combine to form one six-carbon sugar (such as glucose). 4 The remaining ten PGAL receive phosphate groups from ATP. The transfer primes them for endergonic reactions that regenerate the 6 RuBP. glucose 3 other molecules Stepped Art

The Calvin-Benson Cycle – Illustrated 6 CO2 12 ATP 12 PGA 6 RuBP 6 ADP 12 ADP + 12 Pi Calvin– Benson Cycle 6 ATP 12 NADPH Figure 6.11 The Calvin–Benson cycle. This sketch shows a cross-section of a chloroplast with the reactions cycling in the stroma. The steps shown are a summary of six cycles of the Calvin–Benson reactions. Black balls signify carbon atoms. Appendix V details the reaction steps. 1 Six CO2 diffuse into a photosynthetic cell, and then into a chloroplast. Rubisco attaches each to a RuBP molecule. The resulting intermediates split, so twelve molecules of PGA form. 2 Each PGA molecule gets a phosphate group from ATP, plus hydrogen and electrons from NADPH. Twelve PGAL form. 3 Two PGAL may combine to form one six-carbon sugar (such as glucose). 4 The remaining ten PGAL receive phosphate groups from ATP. The transfer primes them for endergonic reactions that regenerate the 6 RuBP. 4 Pi 12 NADP+ 12 PGAL 10 PGAL sugar

6.7 Adaptations: Alternative Carbon-Fixing Pathways Environments differ, and so do the details of photosynthesis C3 plants C4 plants CAM plants

Stomata Small openings through the waxy cuticle covering epidermal surfaces of leaves and green stems Allow CO2 in and O2 out Close on dry days to minimize water loss Closing, however, prevents gas exchange as well

Photorespiration When stomata close, O2 levels rise and CO2 levels decline Reduces efficiency of sugar production in Calvin-Benson cycle Photorespiration At high O2 levels, rubisco attaches to oxygen instead of carbon Inefficient way to produce sugars as it requires more ATP and NADPH CO2 is produced rather than fixed

C3 Plants Plants that use only the Calvin–Benson cycle to fix carbon Forms 3-carbon PGA in mesophyll cells Used by most plants, but inefficient in dry weather when stomata are closed Example: barley

Adaptations to Photorespiration mesophyll cell CO2 O2 2PG RuBP Calvin– Benson Cycle PGA Figure 6.13 Adaptations of C4 and CAM plants minimize photorespiration. A Photorespiration. When stomata close during photosynthesis, O2 accumulates in tissues and CO2 declines. Rubisco initiates more photorespiration, shunting resources away from the sugar producing reactions of the Calvin–Benson cycle. ATP NADPH sugars

C4 Plants Plants that have an additional set of reactions for sugar production on dry days when stomata are closed; compensates for inefficiency of rubisco Forms 4-carbon oxaloacetate in mesophyll cells, then bundle-sheath cells make sugar Examples: corn, switchgrass, bamboo

Adaptations to Photorespiration mesophyll cell CO2 from inside plant oxaloacetate C4 Cycle bundle-sheath cell CO2 Figure 6.13 Adaptations of C4 and CAM plants minimize photorespiration. B In C4 plants, oxygen also builds up in leaves when stomata close during photosynthesis. An additional pathway in these plants keeps the CO2 concentration high enough in bundle sheath cells to prevent photorespiration. RuBP Calvin– Benson Cycle PGGA sugars

CAM Plants Crassulacean acid metabolism Plants with an alternative carbon-fixing pathway that allows them to conserve water in climates where days are hot Forms 4-carbon oxaloacetate at night, which is later broken down to CO2 for sugar production Example: succulents, cactuses

Adaptations to Photorespiration mesophyll cell CO2 froom outside plant oxaloacetaate C4 Cycle night day CO2 Figure 6.13 Adaptations of C4 and CAM plants minimize photorespiration. C CAM plants open stomata and fix carbon using a C4 pathway at night. When the plant’s stomata are closed during the day, the organic compound made during the night is converted to CO2 that enters the Calvin–Benson cycle. RuBP Calvin– Benson Cycle PGGA sugars

Points to Ponder Are “greening” efforts such as using biofuels and low-carbon imprint practices too little, too late? Why or why not? Why is radiation with greater or lesser wavelengths than visible light not generally used in biological processes? What conflicting “needs” confront a plant living in a hot, dry environment? What is the frequent result in C3 plants? How is the problem avoided in C4 plants?