Where It Starts:Photosynthesis

Chapter 6

Biology Concepts and Applications, Eight Edition, by Starr, Evers, Starr. Brooks/Cole, Cengage Learning 2011.

6.1 Green Energy

Before photosynthesis evolved, Earth’s atmosphere had little free oxygen

Oxygen released during photosynthesis changed the atmosphere• Favored evolution of new metabolic pathways,

including aerobic respiration

Introduction

Autotroph organism that makes its own food using carbon from inorganic molecules (CO2) and energy

Heterotroph organism that obtains energy and carbon form organic compounds

Photosynthesis metabolic pathway by which most autotrophs capture light energy and use it to make sugars from CO2 and water

Electromagnetic energy• Travels in waves

• Is organized as photons Wavelength

• Distance between the crests of two successive waves of light

• Measured in nanometers (nm): 25 million nm = 1 inch Visible light A small part of a spectrum of

electromagnetic energy radiating from the sun • Between 380 and 750 nm

6.2 Sunlight as an Energy Source

Electromagnetic Spectrum

Photosynthetic Pigments

Photosynthesis begins when photons are absorbed by photosynthetic pigment molecules

Pigment is an organic molecule that absorbs only light of particular wavelengths• Photons not captured are reflected as color

Pigments Reflect Color

Major Photosynthetic Pigments Chlorophyll a

• Main photosynthetic pigment • Absorbs violet and red light (appears green)

Chlorophyll b, carotenoids, phycobilins• Absorb additional wavelengths

Collectively, photosynthetic pigments absorb almost all of wavelengths of visible light

Figure 6.3 in Text

Chlorophyll a

6.3 Exploring the Rainbow

Engelmann’s Experiment

Fig. 6.4a, p.96

alga

Outcome of T. Engelmann’s experiment.a

Prism

Bacteria (oxygen requiring)

Conclusion: Violet and red light are the best for driving photosynthesis

Absorption Spectra

Fig. 6.4b, p.96

Wavelength (nanometers)

b Absorption spectra for chlorophyll a (solid graphline) and chlorophyll b (dashed line). Compare thesegraphs with the clustering of bacteria shown in (a).

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Fig. 6.4c, p.96

Wavelength (nanometers)

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c Absorption spectra for beta-carotene (solid line)and one of the phycobilins (dashed line).

Key Concepts: THE RAINBOW CATCHERS

A great one-way flow of energy through the world of life starts after chlorophylls and other pigments absorb the energy of visible light from the sun’s rays

In plants, some bacteria, and many protists, that energy ultimately drives the synthesis of glucose and other carbohydrates

6.4 Overview of Photosynthesis

In the Chloroplast!! Photosynthesis proceeds in two stages

• Light-dependent reactions

• Light-independent reactions

Summary equation:

6H2O + 6CO2 6O2 + C6H12O6

Visual Summary of Photosynthesis

Fig. 6.13, p.104

sunlight

Calvin-Benson cycle

Light-DependentReactions

end products (e.g., sucrose, starch, cellulose)

ATP

Light-IndependentReactions

phosphorylated glucose

H2O

H2O O2

NADPHNADP+

CO2

ADP + Pi

Sites of Photosynthesis: Chloroplasts

Light-dependent reactions occur at a much-folded thylakoid membrane • Forms a single, continuous compartment inside the

stroma

• First stage of photosynthesis • light energy + H2O chemical energy (ATP & NADPH)

Light-independent reactions occur in the stroma (chloroplast’s semifluid interior) • Second stage of photosynthesis

•Use ATP & NADPH to assemble sugars from H2O and CO2

Sites of Photosynthesis

Sites of Photosynthesis

Products of Light-Dependent Reactions

Typically, sunlight energy drives the formation of ATP and NADPH

Oxygen is released from the chloroplast (and the cell)

Key Concepts:OVERVIEW OF PHOTOSYNTHESIS

Photosynthesis proceeds through two stages in chloroplasts of plants and many types of protists

First, pigments in a membrane inside the chloroplast capture light energy, which is converted to chemical energy

Second, chemical energy drives synthesis of carbohydrates

6.5 Light-Dependent Reactions

In the thylakoid membrane

Light-harvesting complexes • Absorb light energy and pass it to photosystems

which then release electrons•Photosystem a cluster of pigments and proteins

that converts light energy to chemical energy in photosynthesis

Electrons enter light-dependent reactions

1. Noncyclic Photophosphorylation

Electrons released from photosystem II flow through an electron transfer chain (ETC)• Electron transfer phosphorylation occurs

•Electrons that flow through the ETC set up a hydrogen ion gradient that drives ATP formation

• At end of chain, they enter photosystem I Photon energy causes photosystem I to release

electrons, which end up in NADPH Photosystem II replaces lost electrons by pulling them

from water (photolysis)• Photolysis process by which light energy breaks

down a molecule

Noncyclic Photophosphorylation

electron transfer chain

THYLAKOIDMEMBRANE

Fig. 6.8b, p.99

NADPH

THYLAKOIDCOMPARTMENT

STROMA

Photosystem IPhotosystem II

electron transfer chainlight energy light energy

oxygen(diffuses away)

2. Cyclic Photophosphorylation

Electrons released from photosystem I enter an electron transfer chain, then cycle back to photosystem I

NADPH does not form, oxygen is not released

ATP Formation

In both pathways, electron flow through electron transfer chains causes H+ to accumulate in the thylakoid compartment• A hydrogen ion gradient builds up across the

thylakoid membrane

H+ flows back across the membrane through ATP synthases • Results in formation of ATP in the stroma

6.6 Energy Flow in Photosynthesis

6.6 Energy Flow in Photosynthesis

Key Concepts: MAKING ATP AND NADPH

In the first stage of photosynthesis, sunlight energy is converted to the chemical bond energy of ATP

The coenzyme NADPH forms in a pathway that also releases oxygen

6.7 Light Independent Reactions:The Sugar Factory

Light-independent reactions proceed in the stroma

Carbon fixation: Enzyme rubisco attaches carbon from CO2 to RuBP to start the Calvin–Benson cycle • Calvin Benson cycle light-independent

reactions of photosynthesis

• Carbon fixation process by which carbon from an inorganic source gets incorporated into an organic molecule.

• Rubisco carbon fixing enzyme

Calvin–Benson Cycle

Cyclic pathway makes phosphorylated glucose• Uses energy from ATP, carbon and oxygen from

CO2, and hydrogen and electrons from NADPH

Reactions use glucose to form photosynthetic products (sucrose, starch, cellulose)

Six turns of Calvin–Benson cycle fix six carbons required to build a glucose molecule from CO2

Light-Independent Reactions

Fig. 6.10, p.101

6CO2

12 PGA 12 ATP

12 ADP + 12 Pi

12 NADPH

12 NADP+

12 PGAL

phosphorylated glucose

1 Pi

10 PGAL

4 Pi

ATP

6 ADP

6 RuBP

Calvin-Bensoncycle6

f It takes six turns of theCalvin–Benson cycle (sixcarbon atoms) to produceone glucose molecule andregenerate six RuBP.

e Ten of the PGAL get phosphate groups from ATP. In terms of energy, this primes them for an uphillrun—for the endergonic synthesis reactions thatregenerate RuBP.

d The phosphorylatedglucose enters reactions that form carbohydrateproducts—mainly sucrose, starch, and cellulose.

a CO2 in air spaces inside aleaf diffuses into a photosyntheticcell. Six times, rubisco attaches acarbon atom from CO2 to the RuBPthat is the starting compound forthe Calvin–Benson cycle.

b Each PGA molecule getsa phosphate group from ATP, plus hydrogen and electronsfrom NADPH. The resultingintermediate is called PGAL.

c Two of the twelve PGALmolecules combine to forma molecule of glucose withan attached phosphate group.

6.8 Adaptations: Different Carbon-Fixing Pathways

Environments differ• Plants have different details of sugar production

in light-independent reactions

On dry days, plants conserve water by closing their stomata gaps that open on plant surfaces that allow water vapor and gases to diffuse across the epidermis• O2 from photosynthesis cannot escape

Plant Adaptations to Environment

C3 plants• High O2 level; Rubisco

attaches to O2 instead of CO2 to RuBP; Photorespiration reduces efficiency of sugar production

Plant Adaptations to Environment

C3 plants• Photorespiration

•Reaction in which rubisco attaches oxygen instead of CO2 to ribulose bisphosphate

•Plant loses carbon instead of fixing it.

• Extra energy is need to make sugars on dry days

Plant Adaptations to Environment

C4 plants• Carbon fixation occurs twice, in two different cells to

minimize photorespiration

• First reactions release CO2 near rubisco, limit photorespiration when stomata are closed

Examples: Corn, bamboo

Fig. 6.11b2, p.102

CO2 from inside plant

Calvin-Benson

cycle

RuBP

sugar

PGA

C4cycle

CO2

oxaloacetate

b C4 plants. Oxygen also buildsup in the air spaces inside the leaveswhen stomata close. An additionalpathway in these plants keeps theCO2 concentration high enough toprevent rubisco from using oxygen.

Plant Adaptations to Environment

CAM plants • Type of C4 plant that conserves water by fixing carbon

twice, at different times of the day in the same cell•Day time C4 reactions•Night time Calvin-Benson cycle

• Open stomata and fix carbon at night

Fig. 6.11c2, p.102

Calvin-Benson

cycle

C4cycle

sugar

nightday

CO2 from outside plant

PGA

CO2

oxaloacetate

RuBP

c CAM plants open stomata andfix carbon with a C4 pathway at night.When stomata are closed during theday, organic compounds made duringthe night are converted to CO2 thatenters the Calvin–Benson cycle.

Key Concepts: MAKING SUGARS

Second stage is the “synthesis” part of photosynthesis

Enzymes speed assembly of sugars from carbon and oxygen atoms, both from carbon dioxide

Reactions use ATP and NADPH that form in the first stage of photosynthesis

ATP delivers energy, and NADPH delivers electrons and hydrogens to the reaction sites

Details of the reactions vary among organisms

Animation: C3-C4 comparison

Animation: Calvin-Benson cycle

Animation: Energy changes in photosynthesis

Animation: Noncyclic pathway of electron flow

Animation: Photosynthesis overview

Animation: Sites of photosynthesis

Animation: Wavelengths of light