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PHOTOSYNTHESISJJA Obico, InsructorDepartment of Biology

ENERGY

REDOX REACTION

Energy is transferred from one molecule to another via redoxreactions.

The reduced form of a molecule thus has a higher level of energy than the oxidized form

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Redox in covalently bonded molecules

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Photosynthesis Trivia

On a global scale, photosynthesis makes about 160 billion metric tons of carbohydrate per year

1 metric ton= 1000 kg

Equivalent to: a stack of 60 trillion copies of a thick book; 17 stacks of books reaching from earth to sun!!!

PhotosynthesisTWO STAGES:

1. Light reaction- “photo”

- light energy chemical energy (ATP, NADPH)

Occurs in the thylakoids- Occurs in the thylakoids

2. Dark reaction – Calvin cycle- “synthesis”

- Light independent reactions

- Carbon fixation; Makes sugar

- Occurs in the stroma

Nature of LIGHT

A form of electromagnetic energy/ radiation

Visible light- segment of EM spectrum vital to life

W l h h d b fWavelength- the distance between crests of EM waves

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Pigments

Pigments- substances that absorb light

Pigments in the chloroplast

Chlorophyll a blue green; primary pigment Chlorophyll a- blue-green; primary pigment

Chlorophyll b- yellow green; acessory pigment

Carotenoids- shades of yellow and orange

Absorption spectrumGraph of pigment’s light absorption versus wavelength

The light reaction can perform work with those wavelengths of light that are absorbed.

In the thylakoid are several pigments that differ in their absorption spectrum.

Chlorophyll a, the dominant pigment, absorbs best in the red and blue wavelengths, and least in the green.Other pigments with different structures have different absorption spectra.

Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Fig. 10.8a

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Collectively, these photosynthetic pigments determine an overall action spectrum for photosynthesis.

An action spectrum measures changes in some measure of photosynthetic activity (for example, O2 release) as the wavelength is varied.

Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Fig. 10.8b

The action spectrum of photosynthesis was first demonstrated in 1883 through an elegant experiment by Thomas Engelmann.

In this experiment, different segments of a filamentous alga were exposed to different wavelengths of light.Areas receiving wavelengths favorable to photosynthesis should produce excess O2.Engelmann used the abundance of aerobicabundance of aerobicbacteria clustered along the alga as a measure of O2production.

Fig. 10.8c

The action spectrum of photosynthesis does not match exactly the absorption spectrum of any one photosynthetic pigment, including chlorophyll a.

Only chlorophyll a participates directly in the light reactions but accessory photosynthetic pigments absorb light and transfer energy to chlorophyll a.

Chlorophyll b, with a slightly different structure than chlorophyll a, has a slightly different absorption spectrum and funnels the energy from these wavelengths to chlorophyll a.gy g p yCarotenoids can funnel the energy from other wavelengths to chlorophyll a and also participate in photoprotection against excessive light.

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LIGHT REACTIONS

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Photosystems

Chlorophyll is organized along with other proteins and smaller organic molecules into photosystems

PHOTOSYSTEMSHas light gathering “antenna complex” consisting of a cluster of chloro a, b and carotenoidmoleculesß

Reaction centerOne of the chloro aWhere first light driven chemical reaction of photo occursIncludes PRIMARY ELECTRON ACCEPTOR

PHOTOSYSTEMPHOTOSYSTEM I

P700 (700 nm wavelength)

PHOTOSYSTEM IIP680

Two Routes for electron flow

1. Non cyclicpredominant route; PS I and PS IIATP and NADPH

2. Cyclicalternative route; makes use of PS IATP only

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Non cyclic electron flow Cyclic electron flow

What is the significance of cyclic electron flow?

Noncyclic- produces ATP and NADPH

Calvin cycle- consumes more ATP than NADPH

A h l l h hNADPH concentration- helps regulate which pathway to take (cyclic or noncyclic)

If ATP runs low for Calvin; NADPH accumulates

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Calvin CycleyDark Reactions/ Light Independent Reaction

Calvin cycle

Uses ATP and NADPH to convert CO2 to sugar

Direct product: Glyceraldehyde-3-phosphate (G3P) NOT glucose 3 Carbon sugarg g

Cycle must take place 3X for the net synthesis of glucose

Carbon fixation – incorporation of CO2 into organic compounds

Calvin cycle3 Phases

1. Carbon fixationCO2 joins ribulose biphosphate (RuBP) by RuBPcarboxylase (Rubisco)Product is a 6-carbon unstable molecule that breaks into 2 3 b l l d 3 h h l t3-carbon molecule named 3-phosphoglycerate

2. Reduction3- phosphoglycerate receives a phosphate group 1,3 biphosphoglycerateNADPH reduces 1,3 biphosphoglycerate

3. Regeneration of CO2 acceptor (RuBP)

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Fig. 10.17.1

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Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Fig. 10.17.2

Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Fig. 10.17.3

Alternative mechanisms of carbon fixation

PhotorespirationC3 plants- first organic product of carbon fixation is a 3-carbon molecule (3 phosphoglycerate)

Some plants close stomata during hot, dry days.

CO2 entry is prevented; Calvin cycle starves.

Inside the leaf, O2 production increases because of photosynthesisphotosynthesis

Rubisco can also accept O2

Product splits

One 2-carbon compound is exported from the chloroplast

Mitochondria and peroxisomes break this product

CO2 is released

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Photorespiration is a wasteful processp

It does not generate ATP and produces no food

C4 plantsPreface Calvin cycle with 4-carbon compound

Sugarcane and corn

Presence of BUNDLE SHEATH CELLS

Calvin cycle is confined in the chloroplast of bundle h h llsheath cells

Carbon fixation happens in the MESOPHYLL CELLSCO2 joins PEP (Phosphoenolpyruvate) by PEP carboxylase(Pepco) forms oxaloacetatePepco- higher affinity with CO2

Mesophyll cells- pump CO2 into bundle sheath so Rubisco can function well

CAM plants

Crassulacean acid metabolism

Succulent plants

Cacti, pineapple, CrassulaceaeCacti, pineapple, Crassulaceae

Open their stomata during night and close during the day

Store organic acid into VACUOLES

CO2 is release from the organic acid during the day when products of light reactions are available (ATP, NADPH)

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