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