PHOTOSYNTHESISPHOTOSYNTHESIS

AP BiologyAP Biology

CHAPTER 10CHAPTER 10

- The early atmosphere lacked oxygen. From the 1st plants, it took _____ years to produce oxygen. We now enjoy approximately _____ % oxygen in the atmosphere.

This made aerobic respiration possible.

This formed the ozone layer (O3), which protects us from harmful solar radiation

Photosynthesis is a Photosynthesis is a REDOXREDOX reaction reaction

Respiration is an Respiration is an exergonicexergonic RXN RXN (NRG released (NRG released from oxidation of sugar)from oxidation of sugar)

Photosynthesis is an Photosynthesis is an endergonicendergonic RXN RXN (NRG (NRG needed to reduce COneeded to reduce CO22))

Light = NRG Light = NRG (boost potential energy of electrons)(boost potential energy of electrons)

Water is split & electrons are transferred to COWater is split & electrons are transferred to CO22 reducing it to sugarreducing it to sugar

Photosynthesis has 2 stagesPhotosynthesis has 2 stages LIGHT REACTIONLIGHT REACTION (light dependent reaction): (light dependent reaction):

convert light energy to chemical bond energy in convert light energy to chemical bond energy in ATP and NADPHATP and NADPH Occurs on the thylakoidsOccurs on the thylakoids NADP+ NADP+ NADPH NADPH Oxygen is a byproductOxygen is a byproduct Generates ATPGenerates ATP

CALVIN CYCLECALVIN CYCLE (light independent reaction) – (light independent reaction) – take carbon dioxide and REDUCE it to take carbon dioxide and REDUCE it to carbs/organic compoundscarbs/organic compounds Occurs in the stromaOccurs in the stroma Carbon fixationCarbon fixation Does not require light directlyDoes not require light directly NADPH provides the reducing powerNADPH provides the reducing power APT provides the chemical energyAPT provides the chemical energy

AUTOTROPHSAUTOTROPHS = self-feeders = self-feeders Photoautotrophs – use light Photoautotrophs – use light Chemoautotrophs- use inorganic substances such as sulfur or Chemoautotrophs- use inorganic substances such as sulfur or

ammonia as an energy sourceammonia as an energy source ““producers”producers”

HeterotrophsHeterotrophs = other feeders = other feeders ConsumersConsumers

ChemoautotrophChemoautotroph = autotrophs that get their energy from chemicals = autotrophs that get their energy from chemicals

Chloroplasts are Chloroplasts are Mostly in the Mostly in the mesophyllmesophyll

CHLOROPLASTCHLOROPLAST

Cross section of a leafCross section of a leaf

Light can be Light can be Reflected (how you see things)Reflected (how you see things) Transmitted (passed through)Transmitted (passed through) Absorbed (changed from light Absorbed (changed from light

energy to another form)energy to another form)

Pigments – substances that Pigments – substances that absorb visible light. They absorb visible light. They absorb different wavelengths.absorb different wavelengths.

* Each pigment has a * Each pigment has a characteristic absorption characteristic absorption spectrum which can be spectrum which can be determined by a determined by a spectrophotometer. spectrophotometer.

PIGMENTS IN PLANTSPIGMENTS IN PLANTS

Chlorophyll aChlorophyll a molecules can participate directly in molecules can participate directly in the light RXN; accessory pigments help by the light RXN; accessory pigments help by transferring energy to chlorophyll atransferring energy to chlorophyll a

Chlorophyll bChlorophyll b – green-yellow pigment – green-yellow pigment CarotenoidsCarotenoids – yellow, orange, and/or pink – yellow, orange, and/or pink AnthocynaninAnthocynanin – Reds, purples and blues – Reds, purples and blues XanthophyllsXanthophylls - yellows - yellows

Visible SpectrumVisible Spectrum

Wavelength is the distance between the crests of Wavelength is the distance between the crests of electromagnetic waves.electromagnetic waves.

Visible light is detectable by the human eye (380-750 nm)Visible light is detectable by the human eye (380-750 nm) Light behaves as if it consists of particles called photons.Light behaves as if it consists of particles called photons. Sun radiates the full specturm of electromagnetic energySun radiates the full specturm of electromagnetic energy

Absorption SpectrumAbsorption Spectrumchlorophyll a -

"team captain"

chlorophyll b - accessory (antenna) pigments

Carotenoids etc - accessory (antenna) pigments

When chlorophyll When chlorophyll absorbs a photon, absorbs a photon, one of its electrons one of its electrons is boosted to a is boosted to a higher energy statehigher energy state..

Energy is captured in Energy is captured in a chemical bond.a chemical bond.

LIGHT REACTIONLIGHT REACTION

Occur in the thylakoid membraneOccur in the thylakoid membrane Reduce NADP+ Reduce NADP+ NADPH NADPH Give off O2 as a by-productGive off O2 as a by-product Generate ATP (photophosphorylation)Generate ATP (photophosphorylation)

LIGHT DEPENDENT LIGHT DEPENDENT REACTIONREACTION

ON BOARDON BOARD

PHOTOSYSTEMSPHOTOSYSTEMS

Pigments are assembled into Pigments are assembled into photosystemsphotosystems in the thylakoid in the thylakoid membrane (light collecting units). Each are composed of membrane (light collecting units). Each are composed of

1. 1. antenna complexantenna complex:: several hundred pigments each with several hundred pigments each with different absorption spectra – they absorb photons from a wide different absorption spectra – they absorb photons from a wide rage of lightrage of light

2. 2. reaction-center chlorophyllreaction-center chlorophyll:: one of many chlorophyll a one of many chlorophyll a molecules in each complex can actually transfer an excited molecules in each complex can actually transfer an excited electron to start the light reaction. These pigments are located electron to start the light reaction. These pigments are located in the reaction center.in the reaction center.

3. 3. primary electron acceptorprimary electron acceptor : traps high energy e- released : traps high energy e- released from the reaction center. This energy is stored as ATP and from the reaction center. This energy is stored as ATP and NADPHNADPH

2 Photosystems (PS) in 2 Photosystems (PS) in

photosynthesisphotosynthesis PS IIPS II comes first – absorbs best at 680 (aka p680) comes first – absorbs best at 680 (aka p680) PS IPS I comes second – absorbs best at 700 (aka p700) comes second – absorbs best at 700 (aka p700) p680 and p700 are identical chlorophyll a molecules p680 and p700 are identical chlorophyll a molecules

but each is associated with a different protein.but each is associated with a different protein.

Light reactionLight reaction – 2 routes – 2 routes

Noncyclic flow Noncyclic flow (NOT (NOT A CYCLE)A CYCLE)

Occurs in the Occurs in the thylakoid membranethylakoid membrane

Passes e- from water Passes e- from water to NADP+ (photolysis)to NADP+ (photolysis)

Produces ATP by Produces ATP by noncyclic noncyclic photophosphorylationphotophosphorylation

Produces NADPHProduces NADPH

Produces OProduces O22

NONCYCLIC e- flowNONCYCLIC e- flow

1.1. At PSII =Water is split with the help of sunlight (which At PSII =Water is split with the help of sunlight (which excites the e-); oxygen is given off as waste, e- are excites the e-); oxygen is given off as waste, e- are carried by a primary electron acceptor to the electron carried by a primary electron acceptor to the electron transport chaintransport chain

2.2. The e- are passed down protein carriers (in doing do The e- are passed down protein carriers (in doing do provides energy for chemiosmotic synthesis of ATP).provides energy for chemiosmotic synthesis of ATP).

3.3. At PS I = the sunlight excites the e- again! The e- get At PS I = the sunlight excites the e- again! The e- get shot up to another primary acceptor.shot up to another primary acceptor.

4.4. The e- are passed down another ETC and with the The e- are passed down another ETC and with the help of NADP+ reduchelp of NADP+ reducasease NADP+ picks up 2 H+ and NADP+ picks up 2 H+ and becomes NADPH+H+ becomes NADPH+H+

CYCLICCYCLIC ELECTRON FLOW ELECTRON FLOW

e- leave chlorophyll a at the reaction center return to the reaction center.e- leave chlorophyll a at the reaction center return to the reaction center. Photons are absorbed by PSI (p700) releases high energy e- to the Photons are absorbed by PSI (p700) releases high energy e- to the

primary e- acceptor which passes them to the cycle.primary e- acceptor which passes them to the cycle. Absorption of two photons of light sends a second pair of e- through the Absorption of two photons of light sends a second pair of e- through the

cyclecycle FUNCTION: to produce additional ATP without the generation of NADPH FUNCTION: to produce additional ATP without the generation of NADPH

or evolving oxygen. or evolving oxygen.

ChemiosmosisChemiosmosis Coupeling of exergonic e- flow down ETC Coupeling of exergonic e- flow down ETC

to endergonic ATP production by creation to endergonic ATP production by creation of an electrochemical proton gradient of an electrochemical proton gradient across the membrane. across the membrane.

ETC in Mitochondria vs ChloroplastETC in Mitochondria vs Chloroplast Mitochondria: Mitochondria:

Transfer chem. Energy from food to ATP. High energy e- Transfer chem. Energy from food to ATP. High energy e- pass down chain are extracted by the oxidation of food pass down chain are extracted by the oxidation of food moleculesmolecules

Inner mitochondrial membrane pumps protons from matrix Inner mitochondrial membrane pumps protons from matrix out of the intermembrane spaceout of the intermembrane space

Chloroplast:Chloroplast: change light energy into chemical energy. Photosystems change light energy into chemical energy. Photosystems

capture light energy to drive electrons to the top of the capture light energy to drive electrons to the top of the chain. chain.

Pumps protons from stroma into the lumen as a reservoir. Pumps protons from stroma into the lumen as a reservoir. ATP forms in the stroma where it drives sugar synthesis ATP forms in the stroma where it drives sugar synthesis during the calvin cycle.during the calvin cycle.

Light Independent ReactionLight Independent Reaction“Calvin Cycle”“Calvin Cycle”

Occurs in the stromaOccurs in the stroma Similar to the Krebs Cycle – starting Similar to the Krebs Cycle – starting

material is regeneratedmaterial is regenerated C enters as COC enters as CO22 and leaves as and leaves as

carbohydratescarbohydrates ATP (chemical energy) and NADPH ATP (chemical energy) and NADPH

(reducing power) are energy sources(reducing power) are energy sources Calvin cycle produces 3-C sugar (G-3-P)Calvin cycle produces 3-C sugar (G-3-P)

1. Carbon dioxide enters thru stomates) and bonds to RuBP (enzyme action)

2. ATP and NADPH (from light reaction) are unstable and must be used quickly! They are used to form molecules of PGAL.

PGAL – regenerates RuBP for light ind. to continue. ATP input. (PGAL is also G3P or glyceraldehyde 3-phosphate)

PGAL is used to produce glucose, which is stable and can be stored!

C4 plantsC4 plants Incorporate carbon dioxide Incorporate carbon dioxide

into 4-C compounds.into 4-C compounds. Corn, sugarcane and Corn, sugarcane and

important agricultural important agricultural grassesgrasses

Leaf anatomy of C4 plants Leaf anatomy of C4 plants spacially segregates the spacially segregates the calvin cycle from the initial calvin cycle from the initial corporation of CO2 into corporation of CO2 into organic compounds.organic compounds.

ALTERNATIVE MECHANISIMS OF ALTERNATIVE MECHANISIMS OF CARBON FIXATIONCARBON FIXATION

C4 Plants: HOT ARID CLIMATESC4 Plants: HOT ARID CLIMATES Calvin cycle in most plants produces 3-PGA as the Calvin cycle in most plants produces 3-PGA as the

first intermediate  - these are called C3 plants first intermediate  - these are called C3 plants because first intermediate has 3 carbons (rice, wheat because first intermediate has 3 carbons (rice, wheat and soybeans)and soybeans)

C4 plants produce 4-C compounds initially. (ex. corn, C4 plants produce 4-C compounds initially. (ex. corn, sugarcane and grasses)sugarcane and grasses)

   STEP 1:STEP 1: CO CO22 added to (PEP) to form oxaloacetate – added to (PEP) to form oxaloacetate –

a four carbon product. In comparison to RuBP – PEP a four carbon product. In comparison to RuBP – PEP has a higher affinity to COhas a higher affinity to CO22 and none for O and none for O22. This can . This can fix COfix CO22 efficiently under hot, dry conditions that efficiently under hot, dry conditions that cause the stomata to close and Ocause the stomata to close and O22 concentration to concentration to rise. rise.

   STEP 2:STEP 2: After CO After CO22 fixed by the mesophyll cells they fixed by the mesophyll cells they

convert oxaloacetate to another 4-C compound convert oxaloacetate to another 4-C compound (usually malate)(usually malate)

   STEP 3STEP 3 Mesophyll cells export the 4-C products Mesophyll cells export the 4-C products

through plasmodesmata to bundle-sheath cells. through plasmodesmata to bundle-sheath cells.   

CAM PlantsCAM Plants

CAM PlantsCAM Plants : VERY ARID : VERY ARID CONDITIONS - CONDITIONS - NIGHTNIGHT

Plants open their stomata Plants open their stomata mostly at night and closes mostly at night and closes them during the day.them during the day.

++Conserves water, Conserves water, --but but doesn’t allow COdoesn’t allow CO22 in….. in…..

COCO22 taken in at night and taken in at night and incorporated into organic incorporated into organic acids. Carbon fixation is acids. Carbon fixation is called called crassulacean acid crassulacean acid metabolismmetabolism (CAM) (CAM)

Acids stored Acids stored Day – light reaction runs as Day – light reaction runs as

normal and acids release normal and acids release COCO22 and calvin cycle runs. and calvin cycle runs.

CAM PlantsCAM Plants

Crassulacean Acid Metabolism Crassulacean Acid Metabolism occurs mainly in Crassulacean species (and other occurs mainly in Crassulacean species (and other

succulentsucculent plants). plants). The chemical reaction of the carbon dioxide The chemical reaction of the carbon dioxide

accumulation is similar to that of C4 plants but here are accumulation is similar to that of C4 plants but here are carbon dioxide fixation and its assimilation not carbon dioxide fixation and its assimilation not separated spatially but in time separated spatially but in time

arid regions arid regions uptake of carbon dioxide during the night uptake of carbon dioxide during the night The prefixed carbon dioxide is stored in the vacuoles The prefixed carbon dioxide is stored in the vacuoles

as malate, and is used during the daytime for as malate, and is used during the daytime for photosynthesis. photosynthesis.