Photosynthesis

More energy needs = more air pollution

Researchers are turning to plants as a clean energy source

• Willows: grow fast, reproduce when cut, require low fertilizers and low pesticide

• Make 5-8 tons of wood/acre/year versus natural forests which make only 0.5-1 ton of wood/acre/year

2 Types of Feeders

Autotroph (Producer)• Plants, algae, some Protists

and some bacteria

• Capable of making organic from inorganic

• Phototrophs use light • Chemotrophs use chemicals

Heterotroph• Consumers

• Cannot make their own food from inorganic material

• Must consume, plant animal or both

Figure 7.1A Forest plants.

Figure 7.1B Wheat field.

Figure 7.1C Kelp, a large algae.

Figure 7.1D Micrograph of cyanobacteria (photosynthetic bacteria).

Chloroplast• Organelle where photosynthesis occurs

• Contains the pigment chlorophyll which traps energy in the form of light

• CO2 enters through the pours (stomata) on the leaves of plants

• H20 enters through the roots of the plant and is carried to chloroplast by veins

Other parts of the leave…

• Chloroplasts are concentrated in the cells of the mesophyll, the green tissue in the interior of the leaf

• An envelope of two membranes encloses the stroma, the dense fluid within the chloroplast

• A system of interconnected membranous sacs called thylakoids segregates the stroma from another compartment, the thylakoid space– Thylakoids are concentrated in stacks called grana

CO2 O2Stoma

Mesophyll Cell

Vein

Chloroplast

Mesophyll

Leaf Cross Section

Leaf

Outer and innermembranes

IntermembranespaceGranumStroma Thylakoid

space

Thylakoid

TEM 9750x

CO2 O2Stoma

Mesophyll Cell

Vein

Chloroplast

Mesophyll

Leaf Cross Section

Leaf

Chloroplast

Outer and innermembranes

IntermembranespaceGranumStroma Thylakoid

space

Thylakoid

Carbon dioxide

C6H12O6

Photosynthesis

H2OCO2 O2

Water

+ 66

Lightenergy

Oxygen gasGlucose

+ 6

Is the oxygen gas from the carbon dioxide or the water?

How do they know which product is from which reactant?

6 CO2 + 12 H2O

Experiment 1

C6H12O6 + 6 H2O + 6 O2

Notlabeled

6 CO2 + 12 H2O

Experiment 2

C6H12O6 + 6 H2O + 6 O2

Labeled

How do they know which product is from which reactant?

Radioactive ISOTOPES!

Reactants: 6 CO2

Products:

12 H2O

C6H12O6 6 H2O 6 O2

PHOTOSYNTHESIS IS A REDOX

What get’s oxidized?

Water….to become oxygen

What get’s reduced?

CO2….to become glucose

6 CO2 + 6 H2O C6H12O6 + 6 O2

Reduction

Oxidation

In other words………

Remember: CELLULAR RESPIRATION WAS ALSO A REDOX REACTION

What get’s oxidized?

Glucose….to become CO2

What get’s reduced?

Oxygen…….to become H20

6 CO2 + 6 H2OC6H12O6 + 6 O2

Reduction

Oxidation

In other words….

7.5 Overview: The two stages of photosynthesis are linked by ATP and NADPH

• Actually, photosynthesis occurs in two metabolic stages

– One stage involves the light reactions

– In the light reactions, light energy is converted in the thylakoid membranes to chemical energy and O2

– Water is split to provide the O2 as well as electrons

7.5 Overview: The two stages of photosynthesis are linked by ATP and NADPH

• H+ ions reduce NADP+ to NADPH, which is an electron carrier similar to NADH

– NADPH is temporarily stored and then shuttled into the Calvin cycle where it is used to make sugar

– Finally, the light reactions generate ATP

7.5 Overview: The two stages of photosynthesis are linked by ATP and NADPH

• The second stage is the Calvin cycle, which occurs in the stroma of the chloroplast

– It is a cyclic series of reactions that builds sugar molecules from CO2 and the products of the light reactions

– During the Calvin cycle, CO2 is incorporated into organic compounds, a process called carbon fixation

7.5 Overview: The two stages of photosynthesis are linked by ATP and NADPH

• NADPH produced by the light reactions provides the electrons for reducing carbon in the Calvin cycle

– ATP from the light reactions provides chemical energy for the Calvin cycle

– The Calvin cycle is often called the dark (or light-independent) reactions

H2O

NADP+

ADPP

LIGHTREACTIONS

(in thylakoids)

Light

Chloroplast

H2O

ADPP

LIGHTREACTIONS

(in thylakoids)

Light

Chloroplast

NADPH

ATP

O2

NADP+

H2O

ADPP

LIGHTREACTIONS

(in thylakoids)

Light

Chloroplast

NADPH

ATP

O2

CALVINCYCLE

(in stroma)

Sugar

CO2

NADP+

Light Reactions!

Light Reactions!

• Chlorophyll a absorbs blue violet and red• Chlorophyll b absorbs blue and orange• Both reflect green• Carotenoids absorb blue-green light and reflect

yellow and orange.

• Light is absorbed in a unit quantity called photon– The shorter the wavelength, the greater the energy

Light

Chloroplast

Thylakoid

Absorbedlight

Transmittedlight

Reflectedlight

Light Reactions!

• Reactants: NADP, ADP+P, Water and light • Products: NADPH, ATP and oxygen

Light Reactions!

• Pigments in chloroplasts (chlorophyll

and carotenoids) are responsible for absorbing photons (capturing solar

power), causing release of electrons.

– The electrons jump to a higher energy level—the excited state—where electrons are unstable

– The electrons drop back down to their “ground state,” and, as they do, release their excess energy

Light Reactions!

• The energy released could be lost as heat or light… but here it is conserved as it is passed from one molecule to another molecule

– All of the components to accomplish this are organized in thylakoid membranes in clusters called photosystems

– Photosystems are light-harvesting complexes surrounding a reaction center complex

Reactioncenter complex

e–

Primary electronacceptor

Light-harvestingcomplexesPhoton

Photosystem

Transferof energy

PigmentmoleculesPair of

Chlorophyll a molecules

Thyl

akoi

d m

embr

ane

Light Reactions!• Two types of photosystems have been

identified and are called photosystem I and photosystem II

– Photosystem II, which functions first, is called P680 because its pigment absorbs light with a wavelength of 680 nm

– Photosystem I, which functions next, is called P700 because it absorbs light with a wavelength of 700 nm

Light Reactions!– Finally, the electrons reach the reaction

center where a primary electron acceptor accepts them and becomes reduced.

– This solar-powered transfer of an electron from the pigment (in the reaction center) to the primary electron acceptor is the first step of the light reactions.

NADPH

Photosystem II

e–

Millmakes

ATP Phot

on

Photosystem I

ATP

e–e–

e–

e–

e–

e–

Phot

on

The easier way to explain

Stroma

O2

H2O 12 H+

NADP+ NADPHPhoton

Photosystem II

Electron transport chainProvides energy forsynthesis of by chemiosmosis

+ 2

Primaryacceptor

1

Thylakoidmem-brane

P680

2

4

3Thylakoidspace

e–e–

5

Primaryacceptor

P700

6

Photon

Photosystem IATP

H++

The reality…..

Calvin Cycle

Calvin Cycle• Occurs in stroma

• The ATP and NADPH made in light reactions are used to power the process of carbon fixation.

• CO2 combines with 5-C ribulose biphosphate (RuBP) to make a very unstable 6-C compound, which splits immediately into 2 3-C compounds: G3P.

• What does this remind you of?!?!

Chloroplast

Outer and innermembranes

IntermembranespaceGranumStroma Thylakoid

space

Thylakoid

NADP+

NADPH

ATP

RuBP

3

6 ADP + P

G3P

P

Input:CO2

1

Rubisco

3 P

Step Carbon fixation

3-PGA6 P

CALVINCYCLE

6

6

6

6

P

Step Reduction

2

2

1

Calvin Cycle• For every 1 CO2 which enters the Calvin cycle, 2 G3P molecules are

created.

• So after 3 CO2 enter into the Calvin cycle (and 3 “turns” of the Calvin Cycle), ____ G3P molecules will be made. – 5 of the G3P molecules stay in the Calvin cycle, and 1 leaves.

• The G3P that leaves will be reduced to glucose

• The 5 Carbons that stay in the Calvin cycle are rearranged using ATP to reconstruct RuBP.

• So to make 1 glucose, the Calvin cycle needs to be turned 6 times!

• Final Products: Glucose, ADP, and NADP+

CO2

ATPNADPH

Input

CALVINCYCLE

G3POutput:

NADP+

NADPH

ATP

RuBP

3

6 ADP + P

G3P

P

Input:CO2

1

Rubisco

3 P

Step Carbon fixation

3-PGA6 P

CALVINCYCLE

6

6

6

6

P

Step Reduction

2

2

1

NADPH

ATP

RuBP

3

P

G3P

P

Input:CO2

1

Rubisco

3 P

Step Carbon fixation

3-PGA6 P

CALVINCYCLE

6

6

6

6

P

Step Reduction

2

2

G3P5 P

3

3

G3P1 P

Glucoseand othercompounds

Output:

Step Release of onemolecule of G3P

1

NADP+

6 ADP +

NADPH

ATP

RuBP

3

P

G3P

P

Input:CO2

1

Rubisco

3 P

Step Carbon fixation

3-PGA6 P

CALVINCYCLE

6

6

6

6

P

Step Reduction

2

2

G3P5 P

3

3

G3P1 P

Glucoseand othercompounds

Output:

Step Release of onemolecule of G3P

1

Step Regeneration of RuBP4

4ATP3

3 ADP

NADP+

6 ADP +

Other info about plants…

• In hot climates, many plants close up their stomata to prevent water loss.

• This prevents CO2 from getting in and O2 from getting out

• They start to use the O2 during carbon fixation in the Calvin cycle. O2 combines with RuBP and makes a 2C molecules

• Can’t make sugar….but they are reducing the carbon fixation by 50%

Evolutionary link!

• C4 plants keep their stomata mostly closed when climate becomes hot and dry….. They open stomata only at night

• An enzyme in mesophyll has a high affinity for CO2 and will continue to fix carbon and allow it to enter Calvin cycle even when levels of CO2 are low.

• What is carbon fixation?– When inorganic carbon molecules (CO2) are reduced to

organic molecules (G3P)

Global Warming and Greenhouse Gas

• Greenhouse gases reflect the heat back to the Earth, thus keeping our temperatures fairly constant in the atmosphere

• Greenhouse gases are a natural and necessary thing for our survival

• GHG: CO2, water vapor, methane, ozone, nitrous oxide

The problem comes when the levels of greenhouse gases

continue to rise

Atmosphere

Sunlight

Some heatenergy escapesinto space

Radiant heattrapped by CO2and other gases

Increase in greenhouse gases may be causing increase in Earth’s average

temps

• The increase in burning fossil fuels is leading to the increase in greenhouse gases

Are plants responsible for increase in methane?

Frank Keppler and Thomas Rockmann found a new source of methane in living plants

• CH4 Methane (natural gas) is an important greenhouse gas

• Pound for pound it is 23 times more effective than carbon dioxide at trapping heat in the atmosphere

• Roughly 600 million metric tons of methane are produced annually

Methane, Plants and Climate Change

By 2001, the Intergovernmental Panel on Climate Change (IPCC) believed all major sources of methane had been identified

Prior to Keppler and Rockmann’s discovery, the major known sources of methane were:

• Anaerobic bacteria (including those that live in the digestive system of cows and other ruminants)

• Forest and savannah fires• Burning of fossil fuels

So how do you explain the fluctuations in methane levels BEFORE humans burning fossil

fuels (pre Industrial Revolution)

Methane production in plants

• They found that living plants produced 10 to 100 times more methane than dried plant material

• The amount of methane produced by each plant is small but the cumulative amount is

HUGE!!

60-240 million tons of methane/year

DON’T BLAME ME!!!

Additional Data

• Strengthening their case, two independent lines of evidence were discovered that supported links between plants, methane, and climate changes

Huge amts of methaneare shown over tropical Forests and savannah’s

Methane, Plants and Climate Change

• Methane (CH4) is best known as “natural gas.”

• Methane is an important greenhouse gas. • Pound-for-pound, methane is 23 times more effective than carbon dioxide in trapping heat. • Human activities have tripled the levels of atmospheric methane over the past 150 years.• Roughly 600 million metric tons of methane are produced annually. •Knowing all sources of methane is essential if its levels are to be controlled.

• Concept Review

Understanding plants produce methane helps explain puzzling

observations

The plant-methane link also means that global warming will drive

further increases in temperature

Warmer Earth More plants

More methane

“Global Warming Blame the Forests”

Sometimes, the media misrepresents the findings of the scientists

So here’s the facts…

• Plants have been emitting methane for hundreds of millions of years

• They have NOT caused the sharp increase in methane seen since the Industrial Revolution

FACTS FACTS FACTS

• Plants are an essential source of the oxygen on which life as we know it depends

• The major cause of global warming is NOT the plants…..it’s the fossil fuels

• The benefit of the plants outweighs the harm caused by the methane those forests give off