Plants and Photosynthesis

Plants and Photosynthesis

Photosynthesis• Organisms

–Autotrophs: “Self Feeders”

•Photo-: Light

•Chemo-: Oxidize inorganics (Ex: Sulfur, Ammonia), unique to bacteria

–Heterotrophs: “Other Feeders”

History• Jean-Baptiste van Helmont (1600’s)

–grew willow tree

•Weighed soil before and after

•Added only water

–Tree gained 75 kg

–No change in mass of soil

–Concluded: mass in plants comes from water

Site of Photosynthesis

MesophyllMesophyllCellsCells

MesophyllMesophyllCellsCells

Upper EpidermisUpper EpidermisUpper EpidermisUpper Epidermis

LowerLowerEpidermisEpidermis

LowerLowerEpidermisEpidermisVeinVeinVeinVein StomaStomaStomaStoma

Site of Photosynthesis

Inner & OuterInner & OuterMembranesMembranes

ThylakoidsThylakoids

GranumGranum

StromaStroma

PhotosynthesisConversion of Light E into Chem E

• Light E

–Travels in waves (photons)

–Wavelength (): crest to crest (measured in nm) inversely related to frequency

•Higher frequency = more E

–Different = different properties

Nature of Light

Gamma RaysGamma Rays X-RaysX-Rays UVUV InfraredInfrared Micro-Micro-waveswaves

RadioRadioWavesWaves

Visible LightVisible Light

400400 450450 500500 550550 600600 650650 700700 750750Wavelength (nanometers)Wavelength (nanometers)

•Visible spectrum is ~380–750 nm

Nature of Light• Pigments absorb certain and reflect or

transmit others

Nature of Light• Spectrophotometers measure amount

of Light pigments absorb or reflect

Nature of Light• Pigments

–Absorb and reflect light

–Specific pigment = specific light

–Chlorophylls

•a and b – both absorb blues and reds

•a is 1 pigment for photosynthesis – focuses solar E onto a pair of e-s

Nature of Light• Accessory pigments – funnel the E they

collect to a central Chlorophyll A–Carotenoids

•Carotenes – reflect oranges•Xanthophylls – reflect yellows

–Phycocyanins – reflect blues• Some accessory pigments provide

photoprotection against excess light–Carotenoids in human eyes serve

same function

Absorption/Action Spectra

00

2020

4040

6060

8080

100100

% L

ight

Abs

orpt

ion

% L

ight

Abs

orpt

ion

400400 450450 500500 550550 600600 650650 700700 750750Wavelength (nanometers)Wavelength (nanometers)

400400 450450 500500 550550 600600 650650 700700 750750

ChlorophyllChlorophyll CollectivelyCollectively

Visible LightVisible Light

PhycocyaninPhycocyanin

CarotenoidsCarotenoids

Engelmann’s Experiment• Simple experiment in 1883

• Compare to action spectrum

Photosynthesis• Can be divided into

–Light-dependent rxn

•Makes E storing compounds NADPH and ATP to fuel L-i rxn

•Occurs in thylakoids

–Light-independent rxn

•Uses NADPH and ATP to produce glucose, a more stable form of E

•Occurs in stroma

Photosynthesis

Light-dependent rxn

Light-dependent rxn• Light is absorbed in photosystem II, an

“antenna complex” of hundreds of pigments that funnel E to a reaction center

• Rxn Center: central chlorophyll a

molecule next to a protein, the 1° e- acceptor

Light-dependent rxn• Chemi- osmosis

Photosynthesis

Light-dependent rxn

Light-dependent rxn• The e-s from the broken bonds slide

down the ETC, slowly losing E

• The e-s are recharged by sunlight in photosystem I and are passed along more carrier proteins to NADP+, reducing it to NADPH

H+

H+

H+

H+

H+

H+

H+H+

H+

H+

H20

O2sun Light-dependent

H+

H+

H+

H+

H+

H+H+

H+

H+

H+

sunLight-dependent

ADP ATPH+

H20

O2sun

Light-dependent rxn summary• H2O is broken up by sunlight

• O2 is released as waste

• e-s flow down ETC, pump H+ ions, and finally make NADPH

• H+ ions diffuse across thylakoid membrane and help form ATP

• ATP and NADPH move on to the light-independent rxn

Photosynthesis

L-i rxn – C fixation

L-i rxn – Reduction• 12 ATPs phosphorylate the 12 3PGs to

form 12 1,3 bisphosphoglycerates

• A pair of e-s from NADPH reduces each 1,3 bisphosphoglycerate to glyceraldehyde-3-phosphate (G3P)

–The electrons reduce a carboxyl group to a carbonyl group

L-i rxn – Reduction

L-i rxn – Reduction• Two G3Ps can now be removed from

the cycle to make glucose or be used for as any other carb the plant cell needs

Light-independent rxn summary• Carbon Fixation

–CO2 needed to begin the process• Synthesis of G3P (Glyceraldehyde 3 phosphate)

–ATP and NADPH are used

• Regeneration of 5C compound

–Need more ATP to reset the cylce

Photorespiration• Stomata not only allow gas exchange,

but transpiration also

• Hot, dry day – stomata close

–Problem: CO2 , O2

• Rubisco can bind either CO2 OR O2 to RuBP

• When O2 binds, no useful cellular E is produced

Photorespiration• When rubisco adds O2 to RuBP, RuBP

splits into a 3-C piece and a 2-C piece

–The 2-C fragment is exported from the chloroplast and degraded to CO2 by mitochondria and peroxisomes

• Photorespiration decreases photosynthetic output by siphoning organic material from the Calvin cycle

• Up to 50% of the C fixed by Calvin cycle can be drained away on a hot, dry day

C4 Plants• Mesophyll cells use PEP carboxylase to

fix CO2 to phosphoenolpyruvate, forming oxaloacetate (4C)

–PEP carboxylase has a very high affinity for CO2 and can fix CO2 efficiently when rubisco cannot - on hot, dry days with the stomata closed

C4 Plants• Oxaloacetate then dumps the extra

CO2 into the Calvin cycle in bundle-sheath cells

• Rubisco can then work with a high concentration of CO2, thus minimizing photorespiration

• C4 plants thrive in hot regions with intense sunlight

–Examples: sugar, corn

C4 Plants

CAM Plants• Crassulacean Acid Metabolism

• CO2 is fixed at night, but NO photosynthesis takes place at night

• During the day, the light reactions supply ATP and NADPH to the Calvin cycle and CO2 is released from the organic acids

CAM Plants• Allows plants to keep their stomata

closed during the hot, dry hours of day and open in the cooler hours of night

–Less water is lost in the process

–Less photorespiration occurs

–Ex: succulent plants, cacti, pineapples, and several other plant families

CAM Plants

• Both C4 and CAM plants add CO2 into organic intermediates before it enters the Calvin cycle

–In C4 plants, carbon fixation and the Calvin cycle are spatially separated

–In CAM plants, carbon fixation and the Calvin cycle are temporally separated

• Both eventually use the Calvin cycle to incorporate light energy into the production of sugar