chap 3 photosynthesis

BIO 150 Metabolism and cell division

Photosynthesis

By the end of this lecture you will be able to:

1. Understand that ENERGY can be transformed from one form to another.2. Know that energy exist in two forms; free energy - available for doing work

or as heat - a form unavailable for doing work.3. Appreciate that the Sun provides most of the energy needed for life on

Earth.4. Explain why photosynthesis is so important to energy and material flow for

life on earth.5. Know why plants tend to be green in appearance.6. Equate the organelle of photosynthesis in eukaryotes with the chloroplast. 7. Describe the organization of the chloroplast.8. Understand that photosynthesis is a two fold process composed of the

light-dependent reactions (i.e., light reactions) and the light independent reactions (i.e. Calvin Cycle or Dark Reactions).

9. Tell where the light reactions and the CO2 fixation reactions occur in the chloroplast.

10. Define chlorophylls giving their basic composition and structure.11. Draw the absorption spectrum of chlorophyll and compare it to the action

spectrum of photosynthesis.12. Define the Reaction Centers and Antennae and describe how it operates.13. Describe cyclic photophosphorylation of photosynthesis.14. Describe noncyclic photophosphorylation of photosynthesis.

LEARNING OBJECTIVES

Energy can be transformed from one form to another

FREE ENERGY (available for work)

vs.HEAT

(not available for work)

THE SUN: MAIN SOURCE OF ENERGY FOR LIFE ON EARTH

• Almost all plants are photosynthetic autotrophs, as are some bacteria and protists– Autotrophs generate their own organic matter through

photosynthesis– Sunlight energy is transformed to energy stored in the

form of chemical bonds

(a) Mosses, ferns, andflowering plants

(b) Kelp

(c) Euglena (d) Cyanobacteria

THE BASICS OF PHOTOSYNTHESIS

Light Energy Harvested by Plants & Other Photosynthetic Autotrophs

6 CO2 + 6 H2O + light energy → C6H12O6 + 6 O2

Light use in photosynthesis

• Sun release energy in a broad spectrum of electromagnetic radiation.

• The energy of photon relates to its wavelength.

• Longer wavelength photons have low energy.• Short wavelength photons contain high

energy.• Photosynthesis use 2 wavelength – 450nm &

750nm

Electromagnetic Spectrum and Visible Light

Gammarays X-rays UV

Infrared & Microwaves Radio waves

Visible light

Wavelength (nm)

• When light strikes a leaf, 3 processes occur:• Light is absorbed - use to drive

photosynthesis.,• Light is reflected & light is transmitted – give

color to the object.

Why are plants green?

Reflected light

Transmitted light

Different wavelengths of visible light are seen by the human eye as different colors.

WHY ARE PLANTS GREEN?

Gammarays X-rays UV Infrared Micro-

wavesRadiowaves

Visible light

Wavelength (nm)

Pigments in chlorophyll

• Chlorophyll is the main photosynthetic pigment in tylakoid membranes.

• Chlorophyll a – a primary pigment absorbs blue violet and red wavelength.

• It initiates the light dependent reaction of photosynthesis.

• Chlorophyll b – an accessory pigment absorbs blue and red orange wavelength.

• Carotenoids – absorbs mainly blue green light and reflect mostly yellow, orange or red.

• Phycocyanin – absorbs green and yellow and reflect blue or purple wavelength.

• Chloroplasts absorb light energy and convert it to chemical energy

LightReflected

light

Absorbedlight

Transmittedlight

Chloroplast

THE COLOR OF LIGHT SEEN IS THE COLOR NOT ABSORBED

• Photosynthesis is the process by which autotrophic organisms use light energy to make sugar and oxygen gas from carbon dioxide and water

AN OVERVIEW OF PHOTOSYNTHESIS

Carbondioxide

Water Glucose Oxygengas

PHOTOSYNTHESIS

• The Calvin cycle makes sugar from carbon dioxide– ATP generated by the light

reactions provides the energy for sugar synthesis

– The NADPH produced by the light reactions provides the electrons for the reduction of carbon dioxide to glucose

LightChloroplast

Lightreactions

Calvincycle

NADP

ADP+ P

• The light reactions convert solar energy to chemical energy– Produce ATP & NADPH

AN OVERVIEW OF PHOTOSYNTHESIS

Chloroplasts: Sites of Photosynthesis

• Photosynthesis– Occurs in chloroplasts, organelles in certain

plants– All green plant parts have chloroplasts and carry

out photosynthesis• The leaves have the most chloroplasts• The green color comes from chlorophyll in the

chloroplasts• The pigments absorb light energy

Photosynthesis occurs in chloroplasts

• In most plants, photosynthesis occurs primarily in the leaves, in the chloroplasts

• A chloroplast contains: – stroma, a fluid – grana, stacks of thylakoids

• The thylakoids contain chlorophyll– Chlorophyll is the green pigment that captures light

for photosynthesis

Mesophyll

Leaf cross sectionChloroplasts Vein

Stomata

Chloroplast Mesophyllcell

CO2 O2

20 m

Figure 10.4a

Outermembrane

Intermembranespace

Innermembrane

1 m

Thylakoidspace

ThylakoidGranumStroma

ChloroplastFigure 10.4b

The Two Stages of Photosynthesis: A Preview

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

• The light reactions (in the thylakoids)– Split H2O

– Release O2

– Reduce NADP+ to NADPH– Generate ATP from ADP by

photophosphorylation

© 2011 Pearson Education, Inc.

• 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


Light

LightReactions

Chloroplast

NADP

ADP+ P i

H2OFigure 10.6-1

Light

LightReactions

Chloroplast

ATP

NADPH

NADP

ADP+ P i

H2O

O2

Figure 10.6-2

Light

LightReactions

CalvinCycle

Chloroplast

ATP

NADPH

NADP

ADP+ P i

H2O CO2

O2

Figure 10.6-3

Light

LightReactions

CalvinCycle

Chloroplast

[CH2O](sugar)

ATP

NADPH

NADP

ADP+ P i

H2O CO2

O2

Figure 10.6-4

Stages in photosynthesis

• Light reaction and dark reaction.• Light reactions requires direct energy of light

to make energy carrier molecules used in dark reaction.

• Light reaction occur in the grana and dark reaction take place in the stroma.

A Photosystem: A Reaction-Center Complex Associated with Light-Harvesting Complexes

• A photosystem consists of a reaction-center complex (a type of protein complex) surrounded by light-harvesting complexes

• The light-harvesting complexes (pigment molecules bound to proteins) transfer the energy of photons to the reaction center


Figure 10.13

(b) Structure of photosystem II(a) How a photosystem harvests light

Thyl

akoi

d m

embr

ane

Thyl

akoi

d m

embr

ane

PhotonPhotosystem STROMA

Light-harvestingcomplexes

Reaction-centercomplex

Primaryelectronacceptor

Transferof energy

Special pair ofchlorophyll amolecules

Pigmentmolecules

THYLAKOID SPACE(INTERIOR OF THYLAKOID)

Chlorophyll STROMA

Proteinsubunits THYLAKOID

SPACE

e

Figure 10.13a

(a) How a photosystem harvests light

Thyl

akoi

d m

embr

ane

PhotonPhotosystem STROMA

Light-harvestingcomplexes

Reaction-centercomplex

Primaryelectronacceptor

Transferof energy

Special pair ofchlorophyll amolecules

Pigmentmolecules

THYLAKOID SPACE(INTERIOR OF THYLAKOID)

e

• A primary electron acceptor in the reaction center accepts excited electron and is reduced as a result

• Solar-powered transfer of an electron from a chlorophyll a molecule to the primary electron acceptor is the first step of the light reactions


• There are two types of photosystems in the thylakoid membrane

• Photosystem II (PS II) functions first (the numbers reflect order of discovery) and is best at absorbing a wavelength of 680 nm

• The reaction-center chlorophyll a of PS II is called P680


• Photosystem I (PS I) is best at absorbing a wavelength of 700 nm

• The reaction-center chlorophyll a of PS I is called P700


Linear Electron Flow

• During the light reactions, there are two possible routes for electron flow: cyclic and linear

• Linear electron flow, the primary pathway, involves both photosystems and produces ATP and NADPH using light energy


• A photon hits a pigment and its energy is passed among pigment molecules until it excites P680

• An excited electron from P680 is transferred to the primary electron acceptor (we now call it P680+)


Figure 10.14-1

Primaryacceptor

P680

Light

Pigmentmolecules

Photosystem II(PS II)

1

2e

• P680+ is a very strong oxidizing agent• H2O is split by enzymes, and the electrons are

transferred from the hydrogen atoms to P680+, thus reducing it to P680

• O2 is released as a by-product of this reaction


Figure 10.14-2

Primaryacceptor

H2O

O2

2 H

+1/2

P680

Light

Pigmentmolecules


1

2

3

e

e

e

• Each electron “falls” down an electron transport chain from the primary electron acceptor of PS II to PS I

• The electron transport chain consists of plastoquinone (Pq), cytochrome complex and plastocyanin (Pc).

• Energy released by the fall drives the creation of a proton gradient across the thylakoid membrane

• Diffusion of H+ (protons) across the membrane drives ATP synthesis


Photophosphorylation

• The released energy is harnessed by the thylakoids membrane to produce ATP.

• The ATP synthesis is called photophosporylation because it is driven by light energy.

Figure 10.14-3

Cytochromecomplex

Primaryacceptor

H2O

O2

2 H

+1/2

P680

Light

Pigmentmolecules


Pq

Pc

ATP

1

2

3

5

Electron transport chaine

e

e

4

• In PS I (like PS II), transferred light energy excites P700, which loses an electron to an electron acceptor

• P700+ (P700 that is missing an electron) accepts an electron passed down from PS II via the electron transport chain


Figure 10.14-4

Cytochromecomplex

Primaryacceptor

Primaryacceptor

H2O

O2

2 H

+1/2

P680

Light

Pigmentmolecules


Photosystem I(PS I)

Pq

Pc

ATP

1

2

3

5

6

Electron transport chain

P700Light

e

e

4

e

e

• Each electron “falls” down an electron transport chain from the primary electron acceptor of PS I to the protein ferredoxin (Fd)

• The electrons are then transferred to NADP+ and reduce it to NADPH by NADP+ reductase.

• The electrons of NADPH are available for the reactions of the Calvin cycle

• This process also removes an H+ from the stroma


Figure 10.14-5

Cytochromecomplex

Primaryacceptor

Primaryacceptor

H2O

O2

2 H

+1/2

P680

Light

Pigmentmolecules


Photosystem I(PS I)

Pq

Pc

ATP

1

2

3

5

6

7

8

Electron transport chain

Electron transport

chain

P700Light

+ HNADP

NADPH

NADP

reductase

Fd

e

e

e

e

4

e

e

Photosystem II Photosystem I

Millmakes

ATP

ATP

NADPH

e

e

e

ee

e

e

Phot

on

Phot

on

Figure 10.15

Cyclic Electron Flow

• Cyclic electron flow uses only photosystem I (P700) and produces only ATP, but not NADPH.

• Occurs when cells require additional ATP and thre is no NADP+ to reduce to NADPH.

• No oxygen is released during this reaction.• Cyclic electron flow generates surplus ATP,

satisfying the higher demand in the Calvin cycle


• Non-cyclic electron flow produces ATP and NADPH roughly in equal quantities but Calvin cycle consumes more ATP than NADPH.

• If ATP runs low, the Calvin cycle will slow down and NADPH will accumulate.

• The rise of NADPH will stimulate a temporary shift from noncyclic to cyclic electron flow until ATP supply is enough.

Figure 10.16

Photosystem I

Primaryacceptor

Cytochromecomplex

Fd

Pc

ATP

Primaryacceptor

Pq

Fd

NADPH

NADP

reductase

NADP

+ H

Photosystem II

A Comparison of Chemiosmosis in Chloroplasts and Mitochondria

• Chloroplasts and mitochondria generate ATP by chemiosmosis, but use different sources of energy

• Mitochondria transfer chemical energy from food to ATP; chloroplasts transform light energy into the chemical energy of ATP

• Spatial organization of chemiosmosis differs between chloroplasts and mitochondria but also shows similarities


• In mitochondria, protons are pumped to the intermembrane space and drive ATP synthesis as they diffuse back into the mitochondrial matrix

• In chloroplasts, protons are pumped into the thylakoid space and drive ATP synthesis as they diffuse back into the stroma


Mitochondrion Chloroplast

MITOCHONDRIONSTRUCTURE

CHLOROPLASTSTRUCTURE

Intermembranespace

Innermembrane

Matrix

Thylakoidspace

Thylakoidmembrane

Stroma

Electrontransport

chain

H Diffusion

ATPsynthase

H

ADP P iKey Higher [H ]

Lower [H ]

ATP

Figure 10.17

• ATP and NADPH are produced on the side facing the stroma, where the Calvin cycle takes place

• In summary, light reactions generate ATP and increase the potential energy of electrons by moving them from H2O to NADPH


Figure 10.18

STROMA(low H concentration)

STROMA(low H concentration)

THYLAKOID SPACE(high H concentration)

LightPhotosystem II

Cytochromecomplex Photosystem ILight

NADP

reductase

NADP + H

ToCalvinCycle

ATPsynthase

Thylakoidmembrane

21

3

NADPH

Fd

Pc

Pq

4 H+

4 H++2 H+

H+

ADP+P i

ATP

1/2

H2O O2

Concept 10.3: The Calvin cycle uses the chemical energy of ATP and NADPH to reduce CO2 to sugar

• The Calvin cycle, like the citric acid cycle, regenerates its starting material after molecules enter and leave the cycle

• The cycle builds sugar from smaller molecules by using ATP and the reducing power of electrons carried by NADPH


• Dark reaction occurs in stroma of the chloroplast and involves many reaction.

• Each reaction are catalyzed by different enzymes.

• This reaction do not require light.• Both light reaction and dark reaction occur at

the same time in chloroplasts during daylight.• Each dependent upon each other.

• Carbon enters the cycle as CO2 and leaves as a sugar named glyceraldehyde 3-phospate (G3P)

• For net synthesis of 1 G3P, the cycle must take place three times, fixing 3 molecules of CO2

• The Calvin cycle has three phases– Carbon fixation (catalyzed by rubisco)– Reduction– Regeneration of the CO2 acceptor (RuBP)


Input3 (Entering one

at a time)CO2

Phase 1: Carbon fixation

Rubisco

3 P P

P6

Short-livedintermediate

3-Phosphoglycerate3 P P

Ribulose bisphosphate(RuBP)

Figure 10.19-1


at a time)CO2


Rubisco

3 P P

P6


3-Phosphoglycerate6

6 ADP

ATP

6 P P1,3-Bisphosphoglycerate

CalvinCycle

6 NADPH

6 NADP

6 P i

6 PPhase 2: Reduction

Glyceraldehyde 3-phosphate(G3P)

3 P PRibulose bisphosphate

(RuBP)

1 PG3P

(a sugar)Output

Glucose andother organiccompounds

Figure 10.19-2


at a time)CO2


Rubisco

3 P P

P6


3-Phosphoglycerate6

6 ADP

ATP

6 P P1,3-Bisphosphoglycerate

CalvinCycle

6 NADPH

6 NADP

6 P i

6 PPhase 2: Reduction

Glyceraldehyde 3-phosphate(G3P)

P5G3P

ATP

3 ADP

Phase 3:Regeneration ofthe CO2 acceptor(RuBP)

3 P PRibulose bisphosphate

(RuBP)

1 PG3P

(a sugar)Output

Glucose andother organiccompounds

3

Figure 10.19-3

Concept 10.4: Alternative mechanisms of carbon fixation have evolved in hot, arid climates

• Dehydration is a problem for plants, sometimes requiring trade-offs with other metabolic processes, especially photosynthesis

• On hot, dry days, plants close stomata, which conserves H2O but also limits photosynthesis

• The closing of stomata reduces access to CO2

and causes O2 to build up• These conditions favor an apparently wasteful

process called photorespiration© 2011 Pearson Education, Inc.

Photorespiration: An Evolutionary Relic?

• In most plants (C3 plants), initial fixation of CO2, via rubisco, forms a three-carbon compound (3-phosphoglycerate)

• In photorespiration, rubisco adds O2 instead of CO2 in the Calvin cycle, producing a two-carbon compound

• Photorespiration consumes O2 and organic fuel and releases CO2 without producing ATP or sugar


• Rubisco adds O2 to the Calvin cycle instead of CO2

• The product split and two carbon compound formed called glycolic acid and exported from the chloroplast

• Mitochondria and perosixomes then break the glycolic acid to CO2.

• Photorespiration may be an evolutionary relic because rubisco first evolved at a time when the atmosphere had far less O2 and more CO2

• Photorespiration limits damaging products of light reactions that build up in the absence of the Calvin cycle

• In many plants, photorespiration is a problem because on a hot, dry day it can drain as much as 50% of the carbon fixed by the Calvin cycle


C4 Plants

• C4 plants minimize the cost of photorespiration by incorporating CO2 into four-carbon compounds in mesophyll cells

• This step requires the enzyme PEP carboxylase• PEP carboxylase has a higher affinity for CO2 than

rubisco does; it can fix CO2 even when CO2 concentrations are low

• These four-carbon compounds are exported to bundle-sheath cells, where they release CO2 that is then used in the Calvin cycle


• CO2 bound to the 3 carbon molecule known as phospoenol pyruvate (PEP) to form 4 carbon compounds called oxaloacetate

• This reaction catalyzed by enzyme PEP carboxylase

• Oxaloacetate then converted to the intermediate malate which was then sent to adjacent bundle sheath through plasmodesmata.

• Malate then is decarboxylated and dehydrogenated to form pvruvate and NADPH and CO2 is released.

• Bundle sheath cells are impermeable to CO2, result with a high concentration of CO2 within the bundle sheath cells

• The increase of CO2 concentration minimizes photorespiration and increase sugar production.

• The pyruvate goes back to the mesophyll cell and reacts with ATP to regenerate PEP and ready to accept another CO2

Figure 10.20

C4 leaf anatomy The C4 pathway

Photosyntheticcells of C4 plant leaf

Mesophyll cell

Bundle-sheathcell

Vein(vascular tissue)

Stoma

Mesophyll cell PEP carboxylase CO2

Oxaloacetate (4C) PEP (3C)

Malate (4C)

Pyruvate (3C)

CO2

Bundle-sheathcell

CalvinCycle

Sugar

Vasculartissue

ADP

ATP

Figure 10.20a

C4 leaf anatomy

Photosyntheticcells of C4 plant leaf

Mesophyll cell

Bundle-sheathcell

Vein(vascular tissue)

Stoma

Figure 10.20bThe C4 pathway

Mesophyll cell PEP carboxylase CO2

Oxaloacetate (4C) PEP (3C)

Malate (4C)

Pyruvate (3C)

CO2

Bundle-sheathcell

CalvinCycle

Sugar

Vasculartissue

ADPATP

• In the last 150 years since the Industrial Revolution, CO2 levels have risen gratly

• Increasing levels of CO2 may affect C3 and C4 plants differently, perhaps changing relative abundance of these species

• The effects of such changes are unpredictable and a cause for concern


Advantages of C4 pathway functions in addition to C3 pathway

• Photosynthesis can progress even at a very low CO2 concentration.

• It be able to store CO2 temporarily so that it can be used later.

• Be able to avoid photorespiration. Photorespiration is a wasteful process and reduces their photosynthetic efficiency.

CAM Plants

• Some plants, including succulents, use crassulacean acid metabolism (CAM) to fix carbon

• CAM plants open their stomata at night, incorporating CO2 into organic acids

• Stomata close during the day, and CO2 is released from organic acids and used in the Calvin cycle


• Develop a special carbon fixation pathway known as CAM.

• At night when the stomata open, CAM plants use the enzyme PEP carboxylase to fix CO2 and turn to oxaloacetate.

• The oxaloacetate then converted to malate and stored in cell vacuole.

• During day time, exchange of gas cannot occur because of the closing stomata, CO2 then taken from malate by decarboxylation process.

Sugarcane

Mesophyllcell

Bundle-sheathcell

C4 CO2

Organic acid

CO2

CalvinCycle

Sugar(a) Spatial separation of steps (b) Temporal separation of steps

CO2

Organic acid

CO2

CalvinCycle

Sugar

Day

Night

CAM

Pineapple

CO2 incorporated(carbon fixation)

CO2 releasedto the Calvincycle

2

1

Figure 10.21

Figure 10.21a

Sugarcane

Figure 10.21b

Pineapple

Features C3 plant C4 plant CAM plant

Representative spp (e.g)

Rice, wheat, soybean Maize, sugarcane Cactus, pineapple

Efficiency Less than C4 and CAM More than C3 More than C3

Carbon fixation Once Twice Twice

Site for carbon (C) fixation and C.cycle

Mesophyll cells Initial carbon fixation – mesophyll cells Calvin cycle – bundle sheath cells

Both occur in mesophyll cells but at different period of time

CO2 acceptor RuBP RuBP, PEP RuBP, PEP

CO2 fixing enzyme Rubisco Rubisco, PEP carboxylase

Rubisco, PEP carboxylase

Rate of photorespiration

High Low Low

Rate of photosynthesis High High High

Yields Low High High

Factors affecting photosynthesis process

• Light intensity• Concentration of carbon dioxide • Temperature • Concentration of chlorophyll• Concentration of oxygen• Water • Chemical compounds

Autotrophic prokaryotes

• Prokaryotes consist of bacteria and the cynobacteria

• Certain prokaryotes are autotrophic – able to make their own complex organic compounds as food.

• Cynobacteria – photosynthetic organisms. They use CO2, water and energy from sun to produce sugar and other substance without the presence of chloroplast but use the photosynthetic pigment located in membrane system in the cytoplasm.

• Autotrophic bacteria• Photosynthetic bacteria• Chemosynthetic bacteria

Photosynthetic bacteria• Use pigment known as bacteriochlorophyll to

capture light• This pigment is similar as chlorophyll found in plants• The source of hydrogen to reduce CO2 does not

come from water but from other molecule.• Purple sulphur bacteria use hydrogen sulphide as a

source of hydogen.• Brown or red bacteria use organic compound such

as ethanoic acid as the source of hydrogen

Chemosynthetic bacteria• This type of bacteria produce complex organic

compound from CO2 and water but the chemical energy is not coming from light but from specific chemical reaction which they able to catalyze.

• Nitrifying bacteria like Nitrosomonas and Nitrococcus get its energy from oxidation of ammonia to nitrite in the nitrification process in nitrogen cycle.

• Other chemical such as H2S, NH3 (ammonia), Fe2+.

It's not that It's not that easy bein' easy bein' green… but it green… but it is essential for is essential for life on earth!life on earth!

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chap 3 photosynthesis

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