Photosynthesis: Using Light to Make Food

Energy classification Autotrophs—self nourishing

Obtain carbon from CO2

Obtain energy from light (photosynthesis) or chemical reactions (chemosynthesis)

Heterotrophs—use others for energy source Obtain carbon from autotrophs Obtain energy from autotrophs Even if ingest other heterotrophs, at some point the

original carbon & energy came from an autotroph

Carbon & Energy Enter life through photosynthesis (autotrophs) Released through glycolysis & cellular respiration

(heterotrophs)

ChlorophyllPlantsAlgaeSome bacteria

Transfer sun’s energy into chemical bondsConverts energy of photons to energy

stored in ATP Oxygen production is a byproduct

Three stagesLight-capturingLight-dependent

Convert light energy into chemical energyLight-independent

Form organic compounds (glucose)

CO2 + H2O => C6H12O6 (glucose) + O2

Remember that this is the opposite direction but the same basic reaction as cellular respiration.

Wavelength

Spectrum

Photons Packets of particle-like light Fixed energy (each photon a specific energy

wavelength) Think of them as bundles of energy, like an

electrified rubber ball Energy level

Low energy = long wavelength Microwaves, radio waves

High energy = short wavelength Gamma rays, x-rays

Only a small part of spectrum (400-750 nm) is used for vision & photosynthesis

The light that you see is REFLECTED, not absorbed.

Therefore, a green plant is reflecting the green part of the spectrum (and photons of that energy), not absorbing them; it absorbs all parts of the spectrum except green.

Molecules that absorb photons of only a particular wavelength

Chlorophyll aAbsorbs red, blue, violet lightReflects green, yellow lightMajor pigment in almost all photoautotrophs

Chlorophyll bAbsorbs red-orange, some blueReflects green, some blue

CarotenoidsAbsorb blue-violet, blue-green lightReflect red, orange, yellow lightGive color to many flowers, fruits,

vegetablesColor leaves in Autumn

AnthocyaninsAbsorb green, yellow, some orange lightReflect red, purple lightCherries, many flowersColor leaves in Autumn

PhycobilinsAbsorb green, yellow, orange lightReflect red, blue-green lightSome algae & bacteria

Pigment absorbs light of specific wavelentghCorresponds to energy of photon

Electron absorbs energy from photon Energy boosts electron to higher level Electron then returns to original level When it returns, emits some energy

(heat or photon)

Stage 1 (Light-Dependent)Light energy converted to bond energy of

ATPWater molecules split, helping to form

NADPHOxygen atoms escape

Stage 2 (Light-Independent)ATP energy used to synthesize glucose &

other carbohydrates

Occurs in thylakoids Electrons transfer light energy in

electron transport chain in photosystems

Photosystems—Clusters of chlorophyll, pigments, proteinsLight-gathering “antennae”Photosystem I (P680)—absorbs red light at 680nmPhotosystem II (P700)—absorbs far-red light at

700nm

Electrons transfer from photosystems Electron transfers pump H+ into inner

thylakoid compartment Repeats, building up concentration and

electric gradients Chemiosmosis!

H+ can only pass through channels inside ATP Synthase

Ion flow through channel makes protein turn, forcing Phosphate onto ADP

Phosphorylation!

Electrons continue until bonding NADP+ to form NADPH

NADPH used in next part of cycle Process is very similar to cellular

respiration!!!!Oxidative phosphorylation

ATP provides energy for bond formation NADPH provides hydrogen & electrons CO2 provides carbon & oxygen

CO2 in air diffuses into stroma CO2 attaches to rubisco (RuBP) Enters Calvin cycle (also called Calvin-

Benson)RuBP splits to form PGAPGA gets phosphate from ATP, then H+ and

electrons from NADPHForms PGALTwo PGAL combine to form glucose plus

phosphate group

Some PGAL recycles to form more RuBP Takes 6 “turns” of cycle to form one

glucose molecule 6 CO2 must be fixed and 12 PGAL must

form to produce one glucose molecule and keep the cycle running

*(G3P = PGAL)

Microscopic openings in leavesClose when hot & dryKeeps water insidePrevents CO2 & O2 exchange

Basswood, beans, peas, evergreens 3-Carbon PGA is first stable intermediate

in Calvincycle Stomata close, O2 builds up Increased O2 levels compete w/ CO2 in

cycle Rubisco attaches oxygen, NOT carbon to

RuBP This yields 1 PGA rather than 2 Lowers sugar production & growth of plant

12 “turns” rather than 6 to make sugars Better adapted to cold & wet

Corn, sugar cane, tropical plants Adapted to hot, dry climates Close stomata to conserve water

This limits CO2 entry and allows O2 to accumulate

This allows CO2 to remain high for Calvin cycle

Carbon stored in special cells, can be donated to Calvin cycle later

Requires 1 more ATP than C3, but less water lost & more sugar produced

Desert plants (cactus) Crassulcean Acid Metabolism (CAM) Opens stomata at night, uses C4 cycle Cells store malate & organic acids During day when stomata close, malate

releases CO2 for Calvin cycle