Photosynthesis Reactions - IWS.COLLIN.EDU -...

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Photosynthesis occurs in two stages linked by ATP and NADPH

NADPH is similar to NADH seen in mitochondria; it is an electron/hydrogen carrier

The complete process of photosynthesis consists of two linked sets of reactions

The light reactions and the Calvin cycle.

Photosynthesis Reactions

The Light Reactions

•  Occur in the grana •  Light energy is absorbed by the chlorophyll molecules to

split water and drive the transfer of electrons and H+ from water to the electron acceptor NADP+ reducing it to NADPH.

•  In this process, oxygen becomes released and ATP is produced ( as well as NADPH)


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The Calvin cycle

•  Occurs in the stroma and uses the energy generated during the light reactions

•  Forms sugar from carbon dioxide. This is called carbon fixation !

•  Since this is an endergonic (energy requiring, the making of a larger molecule from a smaller one) reaction, it uses ATP for energy and NADPH for reducing power

•  Sometimes called the “dark reaction” since these steps do not require light energy


Figure 7.5_s3

Light Reactions

(in thylakoids)

Calvin Cycle

(in stroma)

Sugar O2

NADPH

ATP

NADP+

ADP

P

H2O CO2

Light

Chloroplast

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Visible light is only a small part of the electromagnetic spectrum, the full range of electromagnetic wavelengths.

Electromagnetic energy travels in waves, and the wavelength is the distance between the crests of two adjacent waves.

The smaller the wavelength, the more energy is packed in that wave ( and the larger the wavelength, the less energy in that wave).

Light behaves as discrete packets of energy called photons. A photon is a fixed quantity of light energy.

Aspects of Light Energy

Increasing energy

10-5 nm 10-3 nm 1 nm 103 nm 106 nm 1 m 103 m

650 nm

380 400 500 600 700 750 Wavelength (nm)

Visible light

Gamma rays

Micro- waves

Radio waves

X-rays UV Infrared

Which of these elements of the electromagnetic spectrum are dangerous for us ?

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Many forms of Life on planet earth has evolved to use only a certain part of the electromagnetic spectrum .

This is called the visible light spectrum and includes the region of the electromagnetic spectrum between 380 and 750 nanometers.

•  This spectrum includes the colors of light we can see •  It also includes the wavelengths that drive photosynthesis


The way life reacts to the visible light spectrum is due to molecules that absorb the energy in the parts of the visible spectrum.

Pigments : Are substances that absorb visible light

The color of the pigment indicates what color is being absorbed and what color is being reflected.

For example, a red apple look red because molecules (pigments) in the apple skin absorb all colors except red, being reflected back into our eyes.


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Light

Reflected light

Absorbed light

Transmitted light

Chloroplast

Thylakoid


Why do leafs look green ?

Thus leaves look green because the color green is reflected. This part of the electromagnetic spectrum is NOT absorbed and NOT used for photosynthesis.

Those molecules in the leaves (chloroplasts) that absorb and use light energy are called the Plant pigments

•  They absorb some wavelengths of light and

•  Reflect or transmit other wavelengths ( the colors reflected are the colors we thus see).

Photosynthesis Pigments

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Photosynthesis Pigments The spectrophotometer can measure what is being absorbed and what

is being transmitted (not absorbed) at each wavelength.

White light

Refracting prism

Chlorophyll solution

Photoelectric tube

Galvanometer

Slit moves to pass light of selected wavelength

Green light

The high transmittance (low absorption) reading indicates that chlorophyll absorbs very little green light.

The low transmittance (high absorption) reading chlorophyll absorbs most blue light.

Blue light

1

2 3

4 0 100

0 100

Absorption Spectrum With the use of simple spectrophotometers one can

generate an absorption spectrum.

An absorption spectrum of a solution of pigments

•  Is a graph plotting light absorption (on Y-axis) versus wavelength (on X-axis).

This will tell us what part of visible light is absorbed by the pigments in solution and thus what part of visible light may/is important for action of those molecules involved.

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Chloroplasts contain several different pigments in the thylakoid membrane, which absorb light of different wavelengths.

•  Chlorophyll a

•  Chlorophyll b

•  Carotenoids


The 3 different pigments can be isolated from greeny leaves and analyzed for what spectrum of visible light they absorb.

Absorption Spectrum The combined action spectrum experiments helped reveal which wavelengths of light are photo-synthetically important. The results are shown below.

RESULTS

Abs

orpt

ion

of li

ght b

y ch

loro

plas

t pig

men

ts

Chlorophyll a

(a) Absorption spectra. The three curves show the wavelengths of light best absorbed by three types of chloroplast pigments. Which pigment absorbs what colors ? What colors are NOT absorbed ?

Wavelength of light (nm)

Chlorophyll b

Carotenoids

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•  Chlorophyll a absorbs blue-violet and red light and reflects green.

•  Chlorophyll b absorbs blue and orange and reflects yellow-green.

•  Carotenoids •  broaden the spectrum of colors that can drive photosynthesis

(such as blue to violet) but reflect in the orange/red region •  They also appear to provide photo-protection by absorbing

and dissipating excessive light energy that would otherwise damage chlorophyll or interact with oxygen to form reactive oxidative molecules.

3 Photosynthesis Pigments

How effective each wavelength of light is in stimulating photosynthesis can be deduced by measuring how much oxygen is made over a certain period of time

Graphing the rate of oxygen production against the wavelength used creates an action spectrum ; it profiles the relative effectiveness of different wavelengths of radiation in driving photosynthesis in a leaf.

Action Spectrum

Light energy

PHOTOSYNTHESIS

6 CO2 6 + H2O

Carbon dioxide Water

C6H12O6 6 + O2

Glucose Oxygen gas

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Rat

e of

pho

tosy

nthe

sis

(mea

sure

d by

O2

rele

ase)

Action Spectrum

Action spectrum. This graph plots the rate of photosynthesis versus wavelength. The resulting action spectrum resembles the absorption spectrum for chlorophyll a but does not match exactly. This is partly due to the absorption of light by accessory pigments such as chlorophyll b and carotenoids.

•  Comparison between Action spectrum and Absorption spectrum

Pho

tosy

nthe

sis

Rat

e

Action vs Absorption Spectra

Abs

orpt

ion

Rat

e

Wavelength of light (nm)

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Action Spectrum •  The action spectrum for photosynthesis was first demonstrated by

Theodor W. Engelmann

400 500 600 700

Aerobic bacteria

Filament of alga

In 1883, Theodor W. Engelmann exposed alga to different wavelengths. He used aerobic bacteria, which concentrate near an oxygen source, to determine which segments of the alga were releasing the most O2 and thus photosynthesizing most. Bacteria congregated in greatest numbers around the parts of the alga illuminated with violet-blue or red light. Notice the close match of the bacterial distribution to the action spectrum in part in previous slide.

So what happens when photosynthetic pigments absorb light energy ?


When Pigments in chloroplasts absorb photons (capturing solar power), it

•  increases the potential energy of the pigments’ electrons and

•  sends the electrons into an excited (higher level) but unstable state.

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Generally, when isolated pigment molecules absorb light, their excited electrons drop back down to the ground state and release their excess energy (emission energy) as heat and some form of photon energy with less energy as initially absorbed (the basis of most fluorescence principles).


Excited state

Heat Photon of light

Chlorophyll molecule

Ground state

Photon (fluorescence)

Solution of Chlorophyll illuminated with UV light.

In normal white light, it looks green. The UV light excited electrons which now fall back to ground state, emitting photons with less energy


More energy Less energy

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In the thylakoid membranes, chlorophyll molecules are organized along with other pigments and proteins into photosystems.

When light is absorbed, the excited electrons are passed on to other electron acceptor molecules.

The solar-powered transfer of an electron from the reaction-center chlorophyll a pair to the primary electron acceptor is the first step in the transformation of light energy to chemical energy in the light reactions.

The Light Reaction

There are two photosystems, called PS II and PS I, that work in concert.

When PS II becomes excited by light energy, the result is •  splitting of water into Oxygen, H+ and electrons •  the H+ are released in the inner space of the thylakoids •  the electrons are funneled into an electron transport

chain

The electron chain passes the electrons from photosystem II to photosystem I and the energy released funnels more protons into the lumen of the thylakoids


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Thylakoid membrane

Thylakoid inner space

Chloroplast stroma

When PS I becomes excited by light energy, the result is •  Reduction of NADP into NADPH by accepting electrons

and H+

The buildup of hydrogen high inside the thylakoid (and thus low outside) will now drive the synthesis of ATP via an ATPsynthase by the movement of hydrogen from inside to outside.

This NADPH and ATP is required for the Calvin cycle that happens in the stroma…. The process that results in the making of sugars.


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ATP

NADPH

Electron transport

chain ramp Phot

on

Phot

on

Photosystem II Photosystem I

A “construction” analogy of PS II and PSI

Mitochondria/ Chloroplasts

So, when we compare mitochondria and chloroplasts, similar mechanisms are at work.

The buildup of hydrogen drives the production of ATP.

The ATP made by mitochondria is used to fuel all kinds of cellular activities

The ATP made by chloroplasts is only used to fuel the Calvin Cycle in order to make sugars.

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Mitochondria/ Chloroplasts

In mitochondria, the production of ATP is referred to as oxidative phosphorylation via the chemiosmosis mechanisms ( the making of ATP via the use of an ETC, hydrogen gradient and the use of oxygen).

An electron transport chain also produces a gradient of H+ across the thylakoid membrane, which drives H+ through ATP synthase, producing ATP. Because the initial energy input is light (“photo”), this chemiosmotic production of ATP is called photophosphorylation.

The Importance of Photosynthesis: A Review

•  A review of photosynthesis

Light reactions: • Are carried out by molecules in the thylakoid membranes • Convert light energy to the chemical energy of ATP and NADPH • Split H2O and release O2 to the atmosphere

Calvin cycle reactions: • Take place in the stroma • Use ATP and NADPH to convert CO2 to the sugar G3P • Return ADP, inorganic phosphate, and NADP+ to the light reactions

O2

CO2 H2O

Light

Light reaction Calvin cycle

NADP+ ADP

ATP

NADPH

+ P 1

RuBP 3-Phosphoglycerate

Amino acids Fatty acids

Starch (storage)

Sucrose (export)

G3P

Photosystem II Electron transport chain

Photosystem I

Chloroplast

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Light

Thylakoids

H2O CO2

O2

NADP+

ADP

P

ATP

NADPH G3P

3-PGA

RuBP

Chloroplast

Sugars

Photosystem II

Photosystem I

Light Reactions

Electron transport chain

Calvin Cycle

(in stroma)

Stroma

Cellular respiration

Other organic compounds

Cellulose

Starch

The Importance of Photosynthesis

7.13 CONNECTION: Photosynthesis may moderate global climate change

!  The greenhouse effect operates on a global scale.

–  Solar radiation includes visible light that penetrates the Earth’s atmosphere and warms the planet’s surface.

–  Heat radiating from the warmed planet is absorbed by gases in the atmosphere, which then reflects some of the heat back to Earth.

–  Without the warming of the greenhouse effect, the Earth would be much colder and most life as we know it could not exist.

© 2012 Pearson Education, Inc.

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Sunlight

ATMOSPHERE

Some heat energy escapes into space

Radiant heat trapped by CO2 and other gases



!  The gases in the atmosphere that absorb heat radiation are called greenhouse gases. These include

–  water vapor,

–  carbon dioxide, and

–  methane.


Methane source

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! Remember that photosynthesis captures CO2 into carbohydrates

!  The carbohydrates of plants become cellulose, wood,….

! When plants, trees, animals died million of years ago, they became submerged into sediments of swamps, rivers, oceans and eventually became covered with sand, mud,…

! Over time the carbohydrates, woods became converted to what we know as fossil fuels.



!  The history of the earth includes the movement of CO2 from above the air into the deeper regions of the earth


!  The industrial revolution has been the beginning of increased exploration and use of fossil fuels such as coal, oils, petroleums,…

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! Since 1850, there has been an increased and progressive acceleration in the usage and combustion of fossil fuels

! And this implies, a faster release of the hidden CO2 captured by those fossilized organisms resulting in a 40% increase in atmospheric CO2 concentrations since 1850



These Increasing concentrations of greenhouse gases have been linked to global climate change (global warming).


Sunlight

ATMOSPHERE

Some heat energy escapes into space

Radiant heat trapped by CO2 and other gases

There has been a slow but steady rise in Earth’s surface temperature and an overall change in climatic conditions in different regions of the world.

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!  The presence of large amount of oceans and algae that perform photosynthesis has slowed down the effect.

!  CO2 also dissolves easier in water providing a buffer for CO2 changes

!  But… too much CO2 can make oceans acidic…


GreenHouse Effect Data

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!  The predicted consequences of continued CO2 increase in the atmosphere and global climate change include –  melting of polar ice,

–  rising sea levels, acidification of oceans

–  extreme weather patterns,

–  droughts,

–  increased extinction rates, and

–  the spread of tropical diseases.




Effects of increasing carbon dioxide and temperature on coral reefs.

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! Widespread deforestation has aggravated the global warming problem by reducing an effective CO2 sink.

! Global warming caused by increasing CO2 levels may be reduced by

–  limiting deforestation,

–  reducing fossil fuel consumption, and

–  growing biofuel crops that remove CO2 from the atmosphere.




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Global Warming

Bad Quotes "Trees cause more pollution than automobiles.”

Quote by Ronald Reagan, 1981

“The dangers of carbon dioxide? Tell that to a plant, how dangerous carbon dioxide is”

Quote by Santorum, presidential candidate (2012)

Why are these examples of politician who obviously do not understand the importance of plants and ecology ?

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