PhotosynthesisLight-Dependent Reactions

Calvin Cycle

Observe the animations highlighted above

Fig. 10-2

(a) Plants

(c) Unicellular protist10 µm

1.5 µm

40 µm(d) Cyanobacteria

(e) Purple sulfur bacteria

(b) Multicellular alga

• Photosynthesis occurs in plants, algae, certain other protists, and some prokaryotes– These organisms feed

not only themselves but also most of the living world

BioFlix: PhotosynthesisBioFlix: Photosynthesis

Structures of Photosynthesis

• Chloroplasts are structurally similar to and likely evolved from photosynthetic bacteria

• Leaves are the major locations of photosynthesis

• Their green color is from chlorophyll, the green pigment within chloroplasts

• CO2 enters and O2 exits the leaf through microscopic pores called stomata

Fig. 10-3a

5 µm

Mesophyll cell

StomataCO2 O2

Chloroplast

Mesophyll

VeinLeaf cross section

• Chloroplasts are found mainly in cells of the mesophyll, the interior tissue of the leaf– A typical

mesophyll cell has 30–40 chloroplasts

• Thylakoid• Grana• Stroma

The Photosynthesis Equation6 CO2 + 12 H2O + Light energy C6H12O6 + 6 O2 + 6 H2O

Reactants:

Fig. 10-4

6 CO2

Products:

12 H2O

6 O26 H2OC6H12O6

• Light-dependent reactions

– Occurs in Thylakoid

– Used H2O and light to produce ATP, NADPH, and O2

– NADPH is an electron carrier

• Calvin cycle– Occurs in stroma– uses carbon dioxide, ATP, and NADPH to

produce sugars

The Goal of Photosynthesis is to form high energy sugars.

This requires transforming light energy into usable chemical energy (ATP)

• ATP is form by the process of:

• Photophosphorylation – – The production of ATP using energy derived

from the redox reactions of an electron transport chain.

Redox Reactions• A chemical reaction involving the transfer of

one or more elections from one reactant to another; also called oxidation/reduction reactions

• In In oxidationoxidation, a , a substance loses substance loses electrons, or is electrons, or is oxidizedoxidized

• In In reductionreduction, a , a substance gains substance gains electrons, or is electrons, or is reduced (the reduced (the amount of amount of positive charge positive charge is reduced)is reduced)

Fig. 9-UN1

becomes oxidized(loses electron)

becomes reduced(gains electron)

Fig. 9-UN2

becomes oxidized

becomes reduced

The Two Stages of Photosynthesis: The Two Stages of Photosynthesis: A PreviewA Preview

• The light reactions (in the thylakoids):

– Split H2O

– Release O2

– Reduce NADP+ to NADPH

– Generate ATP from ADP by photophosphorylation

• The Calvin cycle (in the stroma) forms sugar from CO2, using ATP and NADPH

• The Calvin cycle begins with carbon fixation, incorporating CO2 into organic molecules

• Photosynthesis consists of the light reactions (the photo part) and Calvin cycle (the synthesis part)

Light

Fig. 10-5-1

H2O

Chloroplast

LightReactions

NADP+

P

ADP

i+

Light

Fig. 10-5-2

H2O

Chloroplast

LightReactions

NADP+

P

ADP

i+

ATP

NADPH

O2

Light

Fig. 10-5-3

H2O

Chloroplast

LightReactions

NADP+

P

ADP

i+

ATP

NADPH

O2

CalvinCycle

CO2

Light

Fig. 10-5-4

H2O

Chloroplast

LightReactions

NADP+

P

ADP

i+

ATP

NADPH

O2

CalvinCycle

CO2

[CH2O]

(sugar)

The light reactions convert solar energy to the chemical energy of ATP and NADPH

Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Fig. 10-7

Reflectedlight

Absorbedlight

Light

Chloroplast

Transmittedlight

Granum

• Chloroplasts are solar-powered chemical factories– Their

thylakoids transform light energy into the chemical energy of ATP and NADPH

The Nature of Sunlight

• Light is a form of electromagnetic energy• The electromagnetic spectrum is the entire range of

electromagnetic energy, or radiation • Visible light consists of wavelengths (including those

that drive photosynthesis) that produce colors we can see

WavelengthWavelength is the is the distance distance between between crests of wavescrests of wavesWavelength Wavelength determines the type determines the type of electromagnetic of electromagnetic energyenergy

UV

Fig. 10-6

Visible light

InfraredMicro-waves

RadiowavesX-raysGamma

rays

103 m1 m

(109 nm)106 nm103 nm1 nm10–3 nm10–5 nm

380 450 500 550 600 650 700 750 nm

Longer wavelength

Lower energyHigher energy

Shorter wavelength

Light and Pigments

• Pigments – light absorbing chemicals

• Chlorophyll – principle pigment in plants– Chlorophyll a– Chlorophyll b– Carotenoids– Xanthophyll

Why do leaves change colors?

• Chlorophyll a

• Chlorophyll b

Component of a Chloroplast• Thylakoid – Saclike

photosynthetic membranes – Light-dependent

reactions occur here

• GranumGranum – Stack of – Stack of thylakoids thylakoids

• StromaStroma – Region – Region outside the thylakoid outside the thylakoid membranemembrane– Reactions of the Calvin Reactions of the Calvin

Cycle occur hereCycle occur here

The Light-Dependent Reactions

• Photophosphorylation is the process of creating ATP using a Proton gradient created by the Energy gathered from sunlight.

• ChemiosmosisChemiosmosis is the process of using is the process of using Proton movement to join ADP and Pi. This Proton movement to join ADP and Pi. This is accomplished by enzymes called is accomplished by enzymes called ATP ATP synthasessynthases or or ATPasesATPases. .

NADP+ + e- + Energy NADPH• NADP+ (Nicotinamide

adenine dinucleotide phosphate)– Electron, hydrogen, and energy

carrier

Light-Dependant

Reactions

1. Photosystem II

• Chlorophyll absorbs lightChlorophyll absorbs light• Electrons on a chlorophyll molecule (p680) Electrons on a chlorophyll molecule (p680)

absorb energy from light and become absorb energy from light and become “energized” “energized”

• High-energy (“energized”) electrons are passed High-energy (“energized”) electrons are passed on to the electron transport chainon to the electron transport chain– Electrons are passed to pheophytin molecule then to Electrons are passed to pheophytin molecule then to

plastoquinone Qa then to plastoquinone Qb then to ETC.plastoquinone Qa then to plastoquinone Qb then to ETC.

• Chlorophyll’s electrons are replenished by the Chlorophyll’s electrons are replenished by the breakdown of Hbreakdown of H22OO

• HH22O is broken down into 2H+ ions, OO is broken down into 2H+ ions, O22, and 2 e-. , and 2 e-. Electrons are used to replenish chlorophyll’s lost Electrons are used to replenish chlorophyll’s lost electrons.electrons.

2. Electron Transport Chain• The molecules of the electron transports

chain use high-energy electrons to push H+ ions from the stroma into the inner thylakoid space.

3. Photosystem I

• Chlorophyll absorbs light-energy and re-Chlorophyll absorbs light-energy and re-energized the electrons from photosystem II.energized the electrons from photosystem II.

• NADP+ picks up these high-energy NADP+ picks up these high-energy electrons and H+ to become NADPH.electrons and H+ to become NADPH.

4. Hydrogen Ions

• ChemiosmosisChemiosmosis• Electrochemical GradientElectrochemical Gradient• Hydrogen ions build up inside the thylakiod Hydrogen ions build up inside the thylakiod

membrane.membrane.– High concentration of H+ inside the membrane (Strong High concentration of H+ inside the membrane (Strong

Positive Charge)Positive Charge)– Low concentration of H+ outside the membrane Low concentration of H+ outside the membrane

(Negative Charge)(Negative Charge)

– Provides the energy to form ATPProvides the energy to form ATP

5. ATP formation

• H+ work to reach equilibrium.H+ work to reach equilibrium.

• Pass through the ATPsynthasePass through the ATPsynthase

• Movement of H+ ions through the Movement of H+ ions through the ATPsynthase powers ATP productionATPsynthase powers ATP production

Do Now…Do Now…

• What is the function of NADPH?What is the function of NADPH?• How is light energy converted into chemical energy How is light energy converted into chemical energy

during photosynthesis?during photosynthesis?• Can the complete process of photosynthesis take place Can the complete process of photosynthesis take place

in the dark? Explain your answer. in the dark? Explain your answer. • Explain what happens to a molecule of water in the light Explain what happens to a molecule of water in the light

dependant phase of photosynthesis.dependant phase of photosynthesis.• If OIf O22 is a waste/byproduct of photosynthesis, track where is a waste/byproduct of photosynthesis, track where

it came from to where it exits the plant.it came from to where it exits the plant.

Calvin

Cycle

Calvin

Cycle

The Calvin CycleThe Calvin Cycle

1.1. 6 CO6 CO22 molecules enter the cycle. molecules enter the cycle.2.2. Enzyme “rubisco” combines 6 5-carbon (RuBp) Enzyme “rubisco” combines 6 5-carbon (RuBp)

molecules with the carbon from COmolecules with the carbon from CO22 and forms them and forms them into 12 3-carbon moleculesinto 12 3-carbon molecules

3.3. 12 ATP and 12 NADPH form the 12 ATP and 12 NADPH form the 12 3-carbon molecules into 12 3-carbon molecules into 12 High-energy 3-carbon molecules (G3P)12 High-energy 3-carbon molecules (G3P)

4.4. 2 (G3P)of the 12 3-carbon molecules are combined to 2 (G3P)of the 12 3-carbon molecules are combined to form a 6-carbon sugarform a 6-carbon sugar

5.5. 6 ATP molecules are used to convert the 10 remaining 6 ATP molecules are used to convert the 10 remaining 3-carbon molecules back into the 6 5-carbon 3-carbon molecules back into the 6 5-carbon molecules the cycle began with (RuBp) molecules the cycle began with (RuBp)

Calvin

Cycle

Calvin

Cycle

Factors Affecting Photosynthesis

• Water supply

• Amount of sunlight

• Temperature

Types of Photosynthesis

• C3 Photosynthesis

• C4 Photosynthesis

• CAM Photosynthesis

C3 Photosynthesis : C3 plants.

• Called C3 because the CO2 is first incorporated into a 3-carbon compound.

• Stomata are open during the day. • RUBISCO, the enzyme involved in photosynthesis, is

also the enzyme involved in the uptake of CO2. • Photosynthesis takes place throughout the leaf. • Adaptive Value: more efficient than C4 and CAM plants

under cool and moist conditions and under normal light because requires less machinery (fewer enzymes and no specialized anatomy)..

• Most plants are C3.

C4 Photosynthesis : C4 plants. • Called C4 because the CO2 is first incorporated into a 4-carbon

compound. • Stomata are open during the day. • Uses PEP Carboxylase for the enzyme involved in the uptake of CO2.

This enzyme allows CO2 to be taken into the plant very quickly, and then it "delivers" the CO2 directly to RUBISCO for photsynthesis.

• Photosynthesis takes place in inner cells (requires special anatomy called Kranz Anatomy)

• Adaptive Value: • Photosynthesizes faster than C3 plants under high light intensity and

high temperatures because the CO2 is delivered directly to RUBISCO, not allowing it to grab oxygen and undergo photorespiration.

• Has better Water Use Efficiency because PEP Carboxylase brings in CO2 faster and so does not need to keep stomata open as much (less water lost by transpiration) for the same amount of CO2 gain for photosynthesis.

• C4 plants include several thousand species in at least 19 plant families. Example: fourwing saltbush pictured here, corn, and many of our summer annual plants.

CAM Photosynthesis : CAM plants. CAM stands for

Crassulacean Acid Metabolism • Called CAM after the plant family in which it was first found

(Crassulaceae) and because the CO2 is stored in the form of an acid before use in photosynthesis.

• Stomata open at night (when evaporation rates are usually lower) and are usually closed during the day. The CO2 is converted to an acid and stored during the night. During the day, the acid is broken down and the CO2 is released to RUBISCO for photosynthesis

• Adaptive Value: – Better Water Use Efficiency than C3 plants under arid conditions due to

opening stomata at night when transpiration rates are lower (no sunlight, lower temperatures, lower wind speeds, etc.).

– May CAM-idle. When conditions are extremely arid, CAM plants can just leave their stomata closed night and day. Oxygen given off in photosynthesis is used for respiration and CO2 given off in respiration is used for photosynthesis. This is a little like a perpetual energy machine, but there are costs associated with running the machinery for respiration and photosynthesis so the plant cannot CAM-idle forever. But CAM-idling does allow the plant to survive dry spells, and it allows the plant to recover very quickly when water is available again (unlike plants that drop their leaves and twigs and go dormant during dry spells).

• CAM plants include many succulents such as cactuses and agaves and also some orchids and bromeliads