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Where It Starts – Photosynthesis
Chapter 7
Hsueh-Fen Juan
Oct. 02 & 09, 2012
ABC video: Solar Power
Impacts, Issues:Biofuels
Coal, petroleum, and natural gas were once ancient forests, a limited resource; biofuels from wastes are a renewable resource
7.1 Sunlight as an Energy Source
Photosynthetic organisms use pigments to capture the energy of sunlight
Photosynthesis• The synthesis of organic molecules from
inorganic molecules using the energy of light• 光合作用定義:利用光能將無機分子轉變為有機分
子的代謝途徑
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)
Electromagnetic Spectrum of Radiant Energy
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)• 因此平常葉子呈綠色即因葉綠素含量豐富之故,但
一到秋天,葉綠素大量破壞流失,其他色素反而較穩定,因此葉子呈現黃、紅等色
Photosynthetic Pigments
Collectively, chlorophyll and accessory pigments absorb most wavelengths of visible light
The light-trapping part of a pigment is an array of atoms, and electrons of these atoms occupy one large orbital that spans all of the atoms
Electrons in such arrays in pigment molecules absorb photons of light energy, boosting electrons to a higher energy level
Energy is captured and used for photosynthesis
Some Pigments in Photosynthesizers
Two Photosynthetic Pigments
7.2 Exploring the Rainbow
Engelmann identified colors of light that drive photosynthesis (violet & red, 光合色素所吸收的 ) by using a prism to divide light into colors• Algae ( 藻類 ) using these wavelengths gave off
the most oxygen ( 使用好氧菌來測試 ) An absorption spectrum ( 吸收光譜 ) shows
which wavelengths a pigment absorbs best• Organisms in different environments use different
pigments ( 地球不同地方能收到的陽光波長也不同 )• 為啥藻類喜歡吸收 500-600nm 的光?因為海水對此
波長的光吸收率最差,因此,水生藻類大多富含能吸收此段波長光的光合色素
Photosynthesis and Wavelengths of Light
7.1-7.2 Key Concepts:The Rainbow Catchers
The flow of energy through the biosphere starts when chlorophylls and other photosynthetic pigments absorb the energy of visible light
7.3 Overview of Photosynthesis
Chloroplast• An organelle that specializes in photosynthesis in
plants and many protists
Stroma• A semifluid matrix surrounded by the two outer
membranes of the chloroplast• Sugars are built in the stroma• Stroma contains DNA and some ribosomes
Overview of Photosynthesis
Thylakoid membrane ( 囊狀膜 )• Folded membrane that make up thylakoids ( 葉
綠囊 )• The space inside all of the disks is a single,
continuous compartment• Contains clusters of light-harvesting pigments
that absorb photons of different energies Photosystems (type I and type II)• Groups of molecules that work as a unit to
begin the reactions of photosynthesis• Convert light energy into chemical energy
Overview of Photosynthesis
Light-dependent reactions• Light energy is transferred to ATP and NADPH
• Water molecules are split, releasing O2
Light-independent reactions• Energy in ATP and NADPH drives synthesis of
glucose and other carbohydrates from CO2 and water
Animation: Photosynthesis overview
Summary: Photosynthesis
Sites of Photosynthesis in Plants
Fig. 7-5a, p. 111
Fig. 7-5b, p. 111
Fig. 7-5c, p. 111
Animation: Sites of photosynthesis
7.4 Light-Dependent Reactions
In the first stage of photosynthesis, light energy drives electrons out of photosystems
The electrons may be used in a noncyclic or cyclic pathway of ATP formation
Capturing Energy for Photosynthesis
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
The Thylakoid Membrane
Cyclic and Noncyclic Pathways
Electrons from photosystems take noncyclic or cyclic pathways, forming ATP
Replacing Lost Electrons
Electrons lost from photosystem II are replaced by photolysis of water molecules, which dissociate into hydrogen ions and oxygen
Photolysis ( 光解 )• Process by which light energy breaks down a
molecule such as water
Electron Flow in a Noncyclic Pathway
Electrons lost from a photosystem enter an electron transfer chain in the thylakoid membrane
Electron transfer chains• Organized arrays of enzymes, coenzymes, and
other proteins that accept and donate electrons in a series
• 分清楚: light-harvesting complex 和 electron transfer chains
Harvesting Electron Energy
Light energy is converted to chemical energy• Entry of electrons from a photosystem into the
electron transfer chain is the first step in light-dependent reactions
ATP forms in the stroma• Electron energy is used to build up a H+
gradient across the membrane• H+ flows through ATP synthase (ATP 合成酶 ),
which attaches a phosphate group to ADP• ATP synthase is a membrane transport protein
(H+ cannot simply diffuse through a lipid bilayer )
Fig. 7-8, p. 113
to second stage of reactionsThe Light-Dependent Reactions of Photosynthesis
ATP synthase
light energylight energy NADPH ATP
ADP + Pi
photosystem IIelectron transfer chain photosystem I
thylakoid compartment
stroma
A Light energy drives electrons out of photosystem II.
C Electrons from photosystem II enter an electron transfer chain.
E Light energy drives electrons out of photosystem I, which accepts replacement electrons from electron transfer chains.
G Hydrogen ions in the thylakoid compartment are propelled through the interior of ATP synthases by their gradient across the thylakoid membrane.B Photosystem II pulls
replacement electrons from water molecules, which dissociate into oxygen and hydrogen ions (photolysis). The oxygen leaves the cell as O2.
D Energy lost by the electrons as they move through the chain causes H+ to be pumped from the stroma into the thylakoid compartment. An H+ gradient forms across the membrane.
F Electrons from photosystem I move through a second electron transfer chain, then combine with NADP+ and H+. NADPH forms.
H H+ flow causes the ATP synthases to attach phosphate to ADP, so ATP forms in the stroma.
NADP+
Noncyclic Pathway of Photosynthesis
Electron Flow in a Cyclic Pathway
7.5 Energy Flow in Photosynthesis
Energy flow in the light-dependent reactions is an example of how organisms harvest energy from their environment
Photophosphorylation
Energy Flow in Light-Dependent Reactions
Fig. 7-9a, p. 114
Fig. 7-9b, p. 114
7.3-7.5 Key Concepts:Making ATP and NADPH
Photosynthesis proceeds through two stages in the chloroplasts of plants and many types of protists
In the first stage, sunlight energy is converted to the chemical bond energy of ATP
The coenzyme NADPH forms in a pathway that also releases oxygen ( 有 NADPH 有氧,反之則無 )
7.6 Light-Independent Reactions: The Sugar Factory
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 ( 固碳 )• Extraction of carbon atoms from inorganic
sources (atmosphere) and incorporating them into an organic molecule
• Builds glucose from CO2
• Uses bond energy of ATP and the reducing power of NADPH formed in light-dependent reactions
The Calvin-Benson Cycle
Enzyme rubisco attaches CO2 to RuBP
• Forms two 3-carbon PGA molecules ( 因六碳不穩定,馬上變成兩個三碳 )
PGAL is formed• PGAs receive a phosphate group from ATP,
and hydrogen and electrons from NADPH (PGA吃了這三樣玩意形成 PGAL)
• Two PGAL combine to form a 6-carbon sugar Rubisco is regenerated (rubisco 這酶要記一下 )
Inputs and Outputs of the Calvin-Benson Cycle
The Calvin-Benson Cycle
Animation: Calvin-Benson cycle
7.7 Adaptations: Different Carbon-Fixing Pathways
Environments differ, and so do details of photosynthesis• C3 plants• C4 plants• CAM plants
Stomata
Stomata ( 單數: stoma ,氣孔 )• 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
C3 Plants
C3 plants• Plants that use only the Calvin–Benson cycle
to fix carbon • Forms 3-carbon PGA in mesophyll cells ( 葉肉
細胞 )• Used by most plants, but inefficient in dry
weather when stomata are closed
Photorespiration
When stomata are closed, CO2 needed for light-independent reactions can’t enter, O2 produced by light-dependent reactions can’t leave
Photorespiration ( 光呼吸 )• At high O2 levels, rubisco attaches to oxygen
instead of carbon
• CO2 is produced rather than fixed
• 光呼吸成因是 O2 累積過多,而非 CO2 無法進來
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
C3 and C4 Plant Leaves
Animation: C3-C4 comparison
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 CO2 for sugar production
• Example: succulents, cactuses
A CAM Plant
Jade plant (Crassula argentea)
C3, C4, and CAM Reactions
Fig. 7-13a, p. 117
C3
Fig. 7-13b, p. 117
C4
Fig. 7-13c, p. 117
CAM
7.6-7.7 Key Concepts:Making Sugars
The second stage is the “synthesis” part of photosynthesis, in which sugars are assembled from CO2
The reactions use ATP and NADPH that form in the first stage of photosynthesis
Details of the reactions vary among organisms
7.8 Photosynthesis and the Atmosphere
The evolution of photosynthesis dramatically and permanently changed Earth’s atmosphere
Different Food Sources
Autotrophs (自營 )• Organisms that make their own food using
energy from the environment and inorganic carbon
Heterotrophs (異營 )• Organisms that get energy and carbon from
organic molecules assembled by other organisms
Two Kinds of Autotrophs
Chemoautotrophs (化學自營 )• Extract energy and carbon from simple molecules
in the environment (hydrogen sulfide, methane)• Used before the atmosphere contained
oxygen Photoautotrophs (光合自營 )• Use photosynthesis to make food from CO2 and
water, releasing O2
• Allowed oxygen to accumulate in the atmosphere
Earth With and Without Oxygen Atmosphere
Effects of Atmospheric Oxygen
Selection pressure on evolution of life• Oxygen radicals
Development of ATP-forming reactions• Aerobic respiration
Formation of ozone (O3) layer
• Protection from UV radiation
7.8 Key Concepts:Evolution and Photosynthesis
The evolution of photosynthesis changed the composition of Earth’s atmosphere
New pathways that detoxified the oxygen by-product of photosynthesis evolved
7.9 A Burning Concern
Earth’s natural atmospheric cycle of carbon dioxide is out of balance, mainly as a result of human activity
The Carbon Cycle
Photosynthesis locks CO2 from the atmosphere in organic molecules; aerobic respiration returns CO2 to the atmosphere
• A balanced cycle of the biosphere
Humans burn wood and fossil fuels for energy, releasing locked carbon into the atmosphere• Contributes to global warming, disrupting
biological systems
Fossil Fuel Emissions
The sky over the Ne York City on a sunny day.
7.9 Key Concepts: Photosynthesis, CO2 & Global Warming
Photosynthesis by autotrophs removes CO2 from the atmosphere; metabolism by all organisms puts it back in
Human activities have disrupted this balance, and contribute to global warming