Chapter 6 Where It Starts Photosynthesis

Chapter 6 Where It Starts Photosynthesis

6.1 Biofuels Coal, petroleum, and natural gas are fossil fuels the remains of ancient forests, a limited resource Biofuels such as oils, gases, or alcohols are made from organic matter that is not fossilized a renewable resource In the United States, biofuels are produced mainly from food crops such as corn, soybeans, and sugarcane Researchers are looking for ways to use non-food-plant matter such as switchgrass and agricultural wastes

Biofuels Research

Autotrophs and Heterotrophs Autotrophs harvest energy directly from the environment, and obtain carbon from inorganic molecules Plants and most other autotrophs make their own food by photosynthesis, a process which uses the energy of sunlight to assemble carbohydrates from carbon dioxide and water Animals and other heterotrophs get energy and carbon by breaking down organic molecules assembled by other organisms

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 shortest wavelengths (highest energy) range of most radiation reaching Earth s surface range of heat escaping from Earth s surface longest wavelengths (lowest energy) visible light gamma rays x-rays ultraviolet radiation near-infrared radiation radiation infrared microwaves radio waves 400 nm 500 nm 600 nm 700 nm

Wavelength and Energy

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

Some Photosynthetic Pigments

Take-Home Message: How do photosynthesizers absorb light? Energy radiating from the sun travels through space in waves and is organized as packets called photons The spectrum of radiant energy from the sun includes visible light; humans perceive different wavelengths of visible light as different colors; the shorter the wavelength, the greater the energy Pigments absorb light at specific wavelengths; photosynthetic species use pigments such as chlorophyll a to harvest the energy of light for photosynthesis

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

400 nm 500 nm 600 nm 700 nm Wavelength Photosynthesis and Wavelengths of Light bacteria alga

ANIMATED FIGURE: T. Englemann's experiment

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

Light absorption Absorption Spectra chlorophyll b phycoerythrobilin phycocyanobilin β-carotene chlorophyll a 400 nm 500 nm 600 nm 700 nm Wavelength

Take-Home Message: Why do cells use more than one photosynthetic pigment? A combination of pigments allows a photosynthetic organism to most efficiently capture the particular range of light wavelengths that reaches the habitat in which it evolved

6.4 Overview of Photosynthesis In plants and other photosynthetic eukaryotes, photosynthesis occurs in chloroplasts Photosynthesis occurs in two stages

Two Stages of Photosynthesis Light-dependent reactions (noncyclic pathway) First stage of photosynthesis Light energy is transferred to ATP and NADPH Water molecules are split, releasing O 2 Light-independent reactions Second stage of photosynthesis Energy in ATP and NADPH drives synthesis of glucose and other carbohydrates from CO 2 and water

Summary: Photosynthesis 6CO 2 + 6H 2 O light energy C 6 H 12 O 6 + 6O 2

The Chloroplast Chloroplast An organelle that specializes in photosynthesis in plants and many protists 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 (first stage of photosynthesis)

The Chloroplast Stroma A semifluid matrix surrounded by the two outer membranes of the chloroplast Sugars are built in the stroma (second stage of photosynthesis)

two outer membranes of chloroplast stroma part of thylakoid membrane system: thylakoid compartment, cutaway view Figure 6-5b p105

INTERACTION: Structure of a chloroplast To play movie you must be in Slide Show Mode PC Users: Please wait for content to load, then click to play Mac Users: CLICK HERE

Take-Home Message: Where do the reactions of photosynthesis take place? In the first stage of photosynthesis, light energy drives the formation of ATP and NADPH, and oxygen is released; in eukaryotic cells, these light-dependent reactions occur at the thylakoid membrane of chloroplasts The second stage of photosynthesis, the light-independent reactions, occur in the stroma of chloroplasts; ATP and NADPH drive the synthesis of carbohydrates

3D ANIMATION: Photosynthesis Bio Experience 3D

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

Figure 6-7 p106

The Thylakoid Membrane photosystem light-harvesting complex

The Noncyclic Pathway 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 the 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 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 chemiosmosis, or electron transfer phosphorylation

The Noncyclic Pathway 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 electron transfer chain light energy to second stage of reactions H + ADP + P i ATP synthase photosystem II photosystem I thylakoid compartment stroma

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

Take-Home Message: What happens during the light-dependent reactions of photosynthesis? In light-dependent reactions, chlorophylls and other pigments in thylakoid membrane transfer light energy to photosystems Photosystems eject electrons that enter electron transfer chains in the membrane; electron flow through ETCs sets up hydrogen ion gradients that drive ATP formation In the noncyclic pathway, oxygen is released and electrons end up in NADPH A cyclic pathway involving only photosystem I allows the cell to continue making ATP when the noncyclic pathway is not running; NADPH does not form; O 2 is not released

3D ANIMATION: Photophosphorylation

ANIMATED FIGURE: Noncyclic pathway of electron flow To play movie you must be in Slide Show Mode PC Users: Please wait for content to load, then click to play Mac Users: CLICK HERE

6.6 Energy Flow in Photosynthesis Energy flow in the light-dependent reactions is an example of how organisms harvest energy from their environment

Photophosphorylation Photophosphorylation is a light-driven reaction that attaches a phosphate group to a molecule In noncyclic photophosphorylation, electrons move from water to photosystem II, to photosystem I, to NADPH In cyclic photophosphorylation, electrons cycle within photosystem I

energy Excited P680 Excited P700 P700 (photosystem I) light energy P680 (photosystem II) light energy Energy flow in the noncyclic reactions of photosynthesis Stepped Art Figure 6-9a p108

energy Excited P700 P700 (photosystem I) light energy Energy flow in the cyclic reactions of photosynthesis Stepped Art Figure 6-9b p108

Take-Home Message: How does energy flow during the reactions of photosynthesis? Light provides energy inputs that keep electrons flowing through electron transfer chains Energy lost by electrons as they flow through the chains sets up a hydrogen ion gradient that drives the synthesis of ATP alone, or ATP and NADPH

6.7 Light-Independent Reactions The cyclic, light-independent reactions of the Calvin-Benson cycle are the synthesis part of photosynthesis Calvin-Benson cycle Enzyme-mediated reactions that build sugars in the stroma of chloroplasts

Carbon Fixation Carbon fixation Extraction of carbon atoms from inorganic sources (atmosphere) and incorporating them into an organic molecule Builds glucose from CO 2 Uses bond energy of molecules formed in light-dependent reactions (ATP, NADPH)

The Calvin-Benson Cycle The enzyme rubisco attaches CO 2 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

1 4 Calvin Benson Cycle 2 3 other molecules glucose Stepped Art Figure 6-10 p109

ANIMATED FIGURE: Photosynthesis overview To play movie you must be in Slide Show Mode PC Users: Please wait for content to load, then click to play Mac Users: CLICK HERE

Take-Home Message: What happens in lightindependent reactions of photosynthesis? Light-independent reactions of photosynthesis run on the bond energy of ATP and energy of electrons donated by NADPH; both formed in the light-dependent reactions Collectively called the Calvin Benson cycle, these carbonfixing reaction use hydrogen (from NADPH), and carbon and oxygen (from CO 2 ) to build sugars

6.8 Adaptations: Different Carbon-Fixing Pathways Environments differ, and so do details of photosynthesis: C3 plants C4 plants CAM plants

Stomata Stomata Small openings through the waxy cuticle covering epidermal surfaces of leaves and green stems Allow CO 2 in and O 2 out Close on dry days to minimize water loss

C3 Plants 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

Photorespiration When stomata are closed, CO 2 needed for light-independent reactions can t enter, O 2 produced by light-dependent reactions can t leave Photorespiration At high O 2 levels, rubisco attaches to oxygen instead of carbon CO 2 is produced rather than fixed

A C3 Plant: Barley palisade mesophyll cell spongy mesophyll cell

mesophyll cell CO 2 O 2 ATP glycolate NADPH PGA RuBP Calvin Benson Cycle B On dry days, stomata close and oxygen accumulates inside leaves. The excess causes rubisco to attach oxygen instead of carbon to RuBP. This is photorespiration, and it makes sugar production inefficient in C3 plants. sugars Figure 6-11b p110

C4 Plants 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

A C4 Plant: Millet mesophyll cell bundle-sheath cell

B C4 plants. Oxygen also builds up inside leaves when stomata close during photosynthesis. An additional pathway in these plants keeps the CO 2 concentration high enough in bundle-sheath cells to prevent photorespiration. mesophyll cell bundle-sheath cell CO 2 from inside plant oxaloacetate PGA CO 2 C4 Cycle Calvin Benson Cycle RuBP sugars Figure 6-12b p110

CAM Plants 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 CO 2 for sugar production Example: succulents, cactuses

A CAM Plant: Jade Plant

mesophyll cell CO 2 from outside plant oxaloacetate C4 Cycle night day CO 2 PGA Calvin Benson Cycle RuBP sugars Figure 6-13a p111

ANIMATED FIGURE: Carbon-fixing adaptations To play movie you must be in Slide Show Mode PC Users: Please wait for content to load, then click to play Mac Users: CLICK HERE

Take-Home Message: How do carbon-fixing reactions vary? When stomata are closed, oxygen builds up inside leaves of C3 plants; rubisco then can attach oxygen (instead of carbon dioxide) to RuBP; photorespiration reduces the efficiency of sugar production, so it can limit the plant s growth Plants adapted to dry conditions limit photorespiration by fixing carbon twice: C4 plants separate the two sets of reactions in space; CAM plants separate them in time

Biofuels Revisited The first cells on Earth were chemoautotrophs that extracted energy and carbon from inorganic molecules in the environment, such as hydrogen sulfide and methane The evolution of photosynthesis dramatically and permanently changed Earth s atmosphere Photoautotrophs use photosynthesis to make food from CO 2 and water, releasing O 2 into the atmosphere

Earth s Early Atmosphere

Earth With an Oxygen Atmosphere

Effects of Atmospheric Oxygen Selection pressure on evolution of life Oxygen radicals Development of ATP-forming reactions Aerobic respiration Formation of ozone (O 3 ) layer Protection from UV radiation

The Atmospheric Carbon Cycle Photosynthesis removes carbon dioxide from the atmosphere, and locks carbon atoms in organic compounds Aerobic organisms break down organic compounds for energy, and release CO 2 into the atmosphere Since photosynthesis evolved, these two processes have constituted a more or less balanced cycle of the biosphere Today, Earth s atmosphere is out of balance the level of CO 2 is increasing, mainly as a result of human activity

Fossil Fuels When we burn fossil fuels, carbon that has been locked for hundreds of millions of years is released back into the atmosphere, mainly as carbon dioxide Today, we release about 28 billion tons of carbon dioxide into the atmosphere each year, more than ten times the amount we released in the year 1900 Increased atmospheric CO 2 contributes to global warming and disrupts natural biological systems

Fossil Fuel Emissions

Renewable Energy Sources Biofuels are a renewable source of energy The carbon in plant matter comes from atmospheric CO 2, fixed by photosynthesis Making biofuel production economically feasible is a high priority for today s energy researchers Research Homework: Research different forms of biofuels. Which form seems the most reasonable choice: Biomass? Biogas? Look into biofuel use in Brazil. Do you think the United States should use their program as a template?