PHOTOSYNTHESIS

Photosynthesis

-converts sunlight into chemical energy

-very complex

-general reaction:

6CO2 + 6H20 → C6H12O6 + 6O2

• Light from the sun is composed of wavelengths (colors)

• The shorter the wavelength the higher the frequency, thus the higher the energy

• The longer the wavelength the lower the energy, thus the lower the energy

Sunlight (a.k.a. white light)

-sunlight is actually white light made of all wavelength colors

-sunlight is visible light

-different colors=different wavelengths of light

The Visible Spectrum

violet-blue-green-yellow-orange-red

380 nm 750 nm

The Electromagnetic Spectrum-This is what scientists call radiation waves-Radiation=energy that travels and spreads as it

goesExamples: X-rays, gamma rays, visible

light, microwaves, etc.

-The electromagnetic spectrum is organized according to wavelengths

-Wavelengths are measured in nanometers (nm)

-Gamma rays have the shortest wavelengths = 10-5 nm

(highest frequency and energy)

-Radio waves have the longest wavelengths =103 nm

(lowest frequency and energy)

PHOTONS-Photon=quantum=discreet amounts of light

energy-Photons are not objects, but each one has

a distinct amount of energyEx: violet photons contain almost twice as much energy as red photons

*violet wavelengths=380 nm=high frequency=high energy

*red wavelenghts=750 nm=low frequency=low energy

Chlorophyll (the photosynthetic pigment)

-Chlorophyll is a green photosynthetic pigment found in chloroplasts of plants

-There are two main types of chlorophyll(chlorophyll a and chlorophyll b)

-Green is the least effective color for photosynthesis because it is reflected

-What you see is reflected. -Everything else is absorbed

**Thus, red and blue are most effective for photosynthesis.

Absorption SpectrumsAbsorption spectrums are graphs that plot a pigment’s light absorption vs. wavelength

Absorption spectrum of chlorophyll

**Remember:Green wavelengths are between ~475 and 600 nm

Action spectrumsA. Action spectrums tell you how much photosynthesis is occurring at each wavelength

B. Made by illuminating chloroplast with different wavelengths of light and then plotting wavelength against some measure of photosynthetic rate

C. The photosynthetic rates could be measured by finding oxygen production, carbon dioxide absorption or light absorption Action spectrum for chlorophyll

Comparison of absorption and action spectra

Absorption spectrum for chlorophyll

Action spectrum for chlorophyll

*Almost no absorption at green wavelengths

*The photosynthetic rate is very low at green wavelengths

Light energy and water

A. In photosynthesis, light energy is used to split water molecules

B. This process is called photolysis = when a chemical is broken down by photons

C. Water is split into hydrogen ions, oxygen and electrons by photons

D. ATP is also produced

E. ATP and H ions will be used to fix CO2 to make organic molecules

F. Photosynthesis relies on water and sunlight for its initial reaction

General photosynthesis information

A. There are light dependent and light independent reactions

B. Light dependent reactions require lightC. Light independent reactions do not

require light or darkness.-they are independent of light or dark-DO NOT REFER TO LIGHT

INDEPENDENT REACTIONS AS DARK REACTIONS (darkness is not required)

ENRICHMENT ASSIGNMENT

1. READ: The light reactions and the Calvin cycle cooperate in converting light energy to the chemical energy of food: an overview (pp. 117&118 )

2. Briefly outline the section

Light Dependent ReactionsA. Light absorption

1. As chlorophyll absorbs light its electrons are raised to a higher energy level by photons at certain wavelengths

2. The electrons at higher energy levels are said to be excited electrons

3. The excited electrons cause the chlorophyll to become photoactivated

4. Photoactivation is the activation of a particular pigment’s electrons (It is caused by absorbing energy from photons.)

5. After photoactivation the electrons quickly return to their ground state

6. When electrons return to their ground state they give off a photon (discreet amount of energy)

7. The photon (energy) is released in the form of heat

8. This process explains the conversion of light energy into heat energy

B. Chlorophyll organization and light absorption

1. Chlorophyll is found in the thylakoids which are found in chloroplasts

2. Within the thylakoids chlorophyll is arranged into groups called photosystems

3. There are two photosystems:

-Photosystem I – best at 700nm (aka P700)

-Photosystem II – best at 680 nm (aka P680)

**Both photosystems are identical chlorophyll a molecules, except that they interact with different proteins of the thylakoids

6. Excited electrons that have absorbed photons of light pass from molecule to molecule until they reach the chlorophyll at the center of the photosystem

7. The photosystem (the chlorophyll) will then pass the excited electrons to a chain of electron carriers

C. Oxygen production1. Photosystem II absorbs light

2. Its electrons become excited

3. Photosystem II donates its electrons to an electron transport chain and the flow of electrons will generate an ATP molecule

4. Photosystem II has been oxidized (LEO)

5. To get the electrons back (that were donated) an enzyme in the center of photosystem II breaks a water molecule (photolysis)

6. The water is split into hydrogen ions, oxygen and electrons

7. Electrons are donated to PS II (GER)

8. Oxygen and hydrogen ions are byproducts

9. Oxygen is released to the atmosphere

10. The production of oxygen in photosynthesis is done by photolysis and requires sunlight

D. ATP Production

1. an excited electron from the center of PS II is donated and passed along a chain of electron carriers

2. The energy for ATP is generated via a proton gradient that is created as electrons move through an ETC (chemiosmosis)

3. ATP is eventually formed when the H+ ions move through ATP synthase

**their energy is harnessed to bring a phosphate group and ADP

together

4. The electrons from PS II are eventually donated to PS I (after they go through the ETC)

5. When ATP is produced in this manner it is called non-cyclic photophosphorylation(the book calls it non-cyclic electron flow)

E. NADPH Production

1. NADPH = nicotinamide adenine dinucleotide phosphate-oxidase

2. After PS I accepts the electrons that were donated by PS II (the ones that went through the ETC), PS I becomes photoactivated

3. Next PS I donates its excited electrons to NADP+ reductase via another ETC

4. NADP+ reductase is an enzyme that assists in the reduction of NADPH

5. The reduction happens when NADP+ accepts two excited electrons from PS I and a H+ ion from the stroma

6. NADPH is then formed

NADP+ + H+ + 2E-NADP+ reductase

NADPH

***Non-cyclic electron flow is responsible for generation of NADPH and ATP***The purpose of NADPH and ATP production is to provide reducing power and chemical energy to drive the Calvin cycle (to make sugar)

F. Cyclic photophosphorylation(in the book it is cyclic electron flow)1. PS II is not involved2. Produces ATP but not NADPH3. ATP is made via chemiosmosis (the same way as non-cyclic photophosphorylation)4. How it works:

a. PS I absorbs lightb. the excited electrons are given to an

electron acceptorc. the electrons pass through an ETC to

produce ATP via chemiosmosisd. At the end of the ETC the electrons go

back to PS I and the process starts again

**Remember in the chloroplast. . .** 1. Chemiosmosis involves the pumping of H+

ions through the membrane.2. The protons go from the stroma to the

thylakoid space. 3. This creates a proton gradient. 4. The protons later flow through ATP synthase

(back to the stroma) and their energy is captured in order to join a phosphate with ADP

5. This produces ATP.

Go to web animation of light-dependent reactions

Assignment

#1. Draw and annotate a chloroplast.-Include: grana

thylakoids thylakoid membrane stroma ribosomes double membrane circular DNA fat/oil droplets

Light-independent reactions(light not required)

A. Calvin cycle-

1. takes place in the stroma

2. begins with a 5 carbon sugar called ribulose biphosphate

3. Ribulose biphosphate = RuBP

4. ATP and NADPH from the light dependent reactions drive the Calvin cycle

5. ATP provides the energy

6. NADPH provides reducing power

7. RuBP is a carbon dioxide acceptor

8. The reaction is catalyzed by the enzyme ribulose biphosphate carboxylase

9. RuBP carboxylase=rubisco

10. 3RuBP and 3CO2 form:

6 3-Phosphoglycerate

11. ATP is broken down to convert

6 3-Phosphoglycerate to

6 1,3-Biphosphoglycerate

12. NADPH reduces

6 1,3-Biphosphoglycerate to

6 Glyceraldehyde 3-phosphate

13. Only one of the G3P molecules will be converted to glucose, sucrose, starch, fatty acids or amino acids

14. Five G3P molecules will be converted back to RuBP to keep the Calvin cycle continuing

Calvin cycle (more info.)

A. Carbon is:

-absorbed as carbon dioxide

-released as sugar

B. ATP=energy for reactions

NADPH=reducing agent

C. Net sugar production per turn (3 carbon dioxide and 3 RuBP) is 1 G3P.

Phases of the Calvin CycleA. Carbon fixation:

1. Every RuBP is attached to a CO2

3RuBPRubisco

3CO2(a 5 carbon

sugar)

converts to a very unstable6 carbon molecule that isimmediately converted to 6 3-carbon molecules

6 glycerate-3-phosphate

**For every RuBP and CO2 pair, two three carbon molecules are formed

3RuBP + 3CO2 → 6 glycerate-3-phosphate

B. Reduction

1. molecules of glycerate-3-phosphate are phosphorylated to glycerate-1,3-biphosphate

*when ATP is hydrolyzed to ADP

2. glycerate-1,3-biphosphate is reduced when NADPH donates its electrons

*NADPH→NADP+

3. 6 molecules of triose-phosphate are produced

*one is removed from the Calvin cycle and used by the plant to produce sugar

*the other 5 are recycled back into the Calvin cycle and converted back to 3RuBP

C. Regeneration (of RuBP)

1. 5 triose-phosphate (G3P) molecules go through a complex series of reactions to form 3 RuBP

2. The Calvin cycle starts over

3. CO2 will be received by RuBP again

More on the Calvin Cycle

A. Start with 15 total carbons in 3 RuBP

*Remember RuBP is a 5 C molecule

B. 3 CO2 is added for a total of 18 carbons

C. 1 triose-phosphate (G3P) is released (a 3 C molecule is released)

D. The other 5 triose-phosphate molecules are recycled back into 3RuBP(15 C are recycled)

E. Net gain of carbons=3 (the single triose phosphate that was released)

F. Energy consumed during the Calvin cycle =9 ATP and 6 NADPH

G. ATP and NADPH will regenerate in the light-dependent reaction

H. There must always be light dependent reactions for light independent reactions to occur

I. The products of the light reactions are used as fuel for the Calvin cycle

1. Outline photosynthesis (light-dependent and light-independent)

2. Explain how the light-independent reaction depends on the light –dependent reaction

Measuring Photosynthesis*can be done three ways

A. Production of oxygen

1. Aquatic plants release oxygen in bubbles during photosynthesis

2. If the bubbles are collected, their volume can be measured

B. Carbon dioxide absorption

EX: 1. Leaves take in CO2 from the air

2. You could pot a plant in an enclosed

environment and measure the CO2 before and after

EX: 1. Aquatic plants absorb CO2 from the water

2. If plants take up CO2, the pH of the water will rise

3. You could use pH indicators to measure pH before and after

C. Increase in Biomass (measure the increase in sugar molecules)

1. Measure how much mass a plant gains over time2. to do this the plant must be completely dehydrated (dead)3. It is best to measure batches of plants

-select a few from each bunch to be dehydrated at different times)

4. Biomass is an indirect measurement of the photosynthetic rate

Limiting Factors in Photosynthesis

A. For photosynthesis to occur the following criteria must be met:

-suitable temperature

-presence of: chlorophyll

light

carbon dioxide

water

B. Changes to one limiting factor will change the rate of photosynthesis

C. Limiting factors are those that are near their minimum or maximum level

D. Limiting factors determine the rate-limiting stepFor example: If light intensity is the limiting factor, the light dependent reaction will limit the rate of photosynthesis.The limiting-step will be the reduction reaction in the Calvin cycle (when the products of the light dependent stage are needed)

E. Light as a limiting factor1. At low light NADPH and ATP are not produced (b/c they are light-dependent products)

2. If NADPH and ATP are not produced the Calvin cycle will stop at the reduction and phosphorylation reactionsR

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Light intensity

The effect of light intensity on photosynthesis

*At high intensity photosynthesis plateaus

*Light intensity is directly proportional to the rate of photosynthesis

*Light is not usually the limiting factor

F. Carbon dioxide as a limiting factor

1. If there is little or no carbon dioxide the Calvin cycle is limited at carbon fixation

2. RuBP and NADPH will acculmulate

**Carbon dioxide is often a limiting factor because it is never at a high concentration in the atmosphere

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Carbon dioxide concentration

The effect of carbon dioxide on photosynthesis

*There is no photosynthesis when carbon dioxide is low*Carbon dioxide and photosynthesis are directly proportional*At high carbon dioxide concentrations photosynthesis plateaus

G. Temperature as a limiting factor

1. At low temperature the enzymes that catalyze the reactions work slowly

2. At high temperature rubisco is ineffective (it is denatured)

3. Carbon fixation becomes the rate-limiting step

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Temperature

The effect of temperature on photosynthesis

*As temperature increases so does the rate of photosynthesis*After the optimum temperature is surpassed the rate quickly falls

Review Questions 1. Compare action spectra and absorption spectra.

2. What are the functions of ATP and NADPH produced in non-cyclic photophosphorylation

3. What is the purpose of cyclic photophosphorylation?

4. What is the advantage of non-cyclic photophosphorylation over cyclic photophosphorylation?

5. What is the purpose of the Calvin cycle?

6. Explain the relationship between the structure of the chloroplast and its function.