PhotosynthesisPhotosynthesis Earliest life forms survived by metabolizing Earliest life forms survived by metabolizing

high-energy inorganic moleculeshigh-energy inorganic molecules About 3 billion years ago, some primitive About 3 billion years ago, some primitive

organisms evolved the ability to organisms evolved the ability to photosynthesizephotosynthesize

they combined carbon dioxide and water to they combined carbon dioxide and water to make glucosemake glucose

further evolution resulted in more efficient further evolution resulted in more efficient photosynthesis that produced molecular photosynthesis that produced molecular oxygen as a by-productoxygen as a by-product

as oxygen began to accumulate in the as oxygen began to accumulate in the atmosphere, cells evolved the ability to use the atmosphere, cells evolved the ability to use the oxygen for cellular respirationoxygen for cellular respiration

All living things are either producers or All living things are either producers or depend on producersdepend on producers

most producers are photoautotrophs – most producers are photoautotrophs – absorb and convert light energy into stored absorb and convert light energy into stored chemical energy in organic molecules chemical energy in organic molecules through the process of through the process of photosynthesisphotosynthesis (plants, algae and cyanobacteria)(plants, algae and cyanobacteria)

Heterotrophs – consumers and Heterotrophs – consumers and decomposers that obtain organic decomposers that obtain organic compounds from producers or other compounds from producers or other consumersconsumers

photosynthesis sustains almost all living photosynthesis sustains almost all living things in the biosphere things in the biosphere

The Visible Light Spectrum – light The Visible Light Spectrum – light travels in wavestravels in waves

Includes all the colors of the rainbow – Includes all the colors of the rainbow – violet has the shortest wavelength and red violet has the shortest wavelength and red has the longest has the longest

Light is composed of particles or packets of energy called Light is composed of particles or packets of energy called photonsphotons

When a pigment molecule absorbs a photon of light, one When a pigment molecule absorbs a photon of light, one of its electrons is energized and is then accepted by an of its electrons is energized and is then accepted by an electron acceptor molecule in photosynthesiselectron acceptor molecule in photosynthesis

Absorption spectrum – spectrophotometers are used to Absorption spectrum – spectrophotometers are used to measure the relative abilities of different pigments to measure the relative abilities of different pigments to absorb different wavelengths of light – absorption absorb different wavelengths of light – absorption spectrum is a plot of the absorption of light of different spectrum is a plot of the absorption of light of different wavelengths wavelengths

Action spectrum – shows how effective various Action spectrum – shows how effective various wavelengths of light are in causing wavelengths of light are in causing photosynthesis photosynthesis

Chloroplasts – organelles containing the Chloroplasts – organelles containing the green pigment, chlorophyll green pigment, chlorophyll Bounded by an outer and inner membraneBounded by an outer and inner membrane inner membrane encloses a fluid-filled stroma which contains the inner membrane encloses a fluid-filled stroma which contains the

enzymes for the dark reactionsenzymes for the dark reactions thylakoids are suspended in the stroma – fluid filled sacs (stack of thylakoids are suspended in the stroma – fluid filled sacs (stack of

thylakoids is a granum) – thylakoid membranes contain chlorophyll thylakoids is a granum) – thylakoid membranes contain chlorophyll and the enzymes that catalyze the light reactionsand the enzymes that catalyze the light reactions

Chloroplasts have their own DNA (circular chromosome) and Chloroplasts have their own DNA (circular chromosome) and ribosomes (70s) ribosomes (70s)

Pigments Pigments

chlorophyll a is the main photosynthetic chlorophyll a is the main photosynthetic pigmentpigment

absorbs light mostly in the red and blue regions absorbs light mostly in the red and blue regions of the spectrumof the spectrum

chlorophyll a – pigment that initiates the light chlorophyll a – pigment that initiates the light reactionsreactions

chlorophyll b – acts as an accessory pigmentchlorophyll b – acts as an accessory pigment carotenoids – other accessory pigments which carotenoids – other accessory pigments which

are yellow and orange – absorb different are yellow and orange – absorb different wavelengths of light to broaden the spectrum of wavelengths of light to broaden the spectrum of light available for photosynthesis – they pass light available for photosynthesis – they pass the energy to chlorophyll athe energy to chlorophyll a

Photosynthetic equation:Photosynthetic equation: 6CO6CO22 + 6H + 6H22O O C C66HH1212OO66 + 6O + 6O22

involves many steps – divided into two involves many steps – divided into two major sets of reactions: light-dependent major sets of reactions: light-dependent reactions, and the light-independent reactions, and the light-independent reactions (dark reactions or carbon fixation reactions (dark reactions or carbon fixation reactions)reactions)

Light-dependent reactions take place in the Light-dependent reactions take place in the thylakoid membranethylakoid membrane

Light-independent reactions take place in Light-independent reactions take place in the stroma the stroma

The Light-Dependent Reactions The Light-Dependent Reactions Energy from the sun is temporarily stored in the Energy from the sun is temporarily stored in the

molecules of ATP and NADPH – energy is then molecules of ATP and NADPH – energy is then used to run the light-independent reactions used to run the light-independent reactions

Chlorophyll molecules and enzymes that run the light-dependent reactions are embedded in the membranes of the thylakoids

pigments are organized in the thylakoid pigments are organized in the thylakoid membrane into units called antenna complexesmembrane into units called antenna complexes

each antenna complex traps light and transfers each antenna complex traps light and transfers the energy to a reaction center the energy to a reaction center

•Antenna complex and associated enzymes are organized into Photosystems

•Reaction center in Photosystem I absorbs at a peak of 700 nanometers (P700) – Photosystem II absorbs at a peak of 680 nanometers (P680)

Noncyclic Photophosphorylation – both Noncyclic Photophosphorylation – both photosystems are used and electrons are photosystems are used and electrons are ultimately passed to NADPH – electron ultimately passed to NADPH – electron “hole” left behind is filled by splitting water “hole” left behind is filled by splitting water and removing electrons – Oand removing electrons – O22 gas is gas is

formed and releasedformed and released continuous, one-way flow of electronscontinuous, one-way flow of electrons

ATP is generated by ATP is generated by chemiosmosischemiosmosis

called called photophosphorylationphotophosphorylation because light energy i because light energy is used to s used to phosphorylatephosphorylate ADP ADP

Cyclic Photophosphorylation – simplest Cyclic Photophosphorylation – simplest light-dependent reactionlight-dependent reaction

energy from the sun is used to cycle energy from the sun is used to cycle electrons through Photosystem I to produce electrons through Photosystem I to produce ATP (used by ancient bacteria) ATP (used by ancient bacteria)

Light-independent reactions (Carbon Light-independent reactions (Carbon Fixation Reactions or Dark reactions) Fixation Reactions or Dark reactions)

energy from ATP and NADPH is used to energy from ATP and NADPH is used to form glucose from COform glucose from CO22

Most plants go through the Calvin (CMost plants go through the Calvin (C33) )

Cycle Cycle

COCO22 enters the cycle by reacting with enters the cycle by reacting with

ribulose biphosphate (RuBP) – reaction is ribulose biphosphate (RuBP) – reaction is catalyzed by the enzyme catalyzed by the enzyme rubiscorubisco

six carbon dioxide molecules are “fixed” to six carbon dioxide molecules are “fixed” to make one glucosemake one glucose

at the end of each cycle, RuBP is at the end of each cycle, RuBP is reformed reformed

PhotorespirationPhotorespiration Plants need to have their stomata (pores in the Plants need to have their stomata (pores in the

leaves) open in order for gas exchange to occur leaves) open in order for gas exchange to occur (CO(CO22 in and Oin and O22 out) out)

When it gets too hot, the stomata close and When it gets too hot, the stomata close and COCO22 concentrations drop and Oconcentrations drop and O22 concentrations concentrations increaseincrease

RuBP also tends to combine with ORuBP also tends to combine with O22 in a in a process called photorespirationprocess called photorespiration

OO22 is used up and CO is used up and CO22 is generated (as in is generated (as in cellular respiration) however photorespiration cellular respiration) however photorespiration does not produce any energy and stops the Cdoes not produce any energy and stops the C33 pathwaypathway

if this continues for a long enough time (dry, hot if this continues for a long enough time (dry, hot weather) then the plant dies from lack of energy weather) then the plant dies from lack of energy (glucose) (glucose)

CC44 plants reduce photorespiration plants reduce photorespiration CC33 plants have almost all their chloroplasts in the mesophyll plants have almost all their chloroplasts in the mesophyll

cellscells plants that are adapted to dry, hot weather often use the Cplants that are adapted to dry, hot weather often use the C44

pathway - have chloroplasts in both the mesophyll AND pathway - have chloroplasts in both the mesophyll AND bundle-sheath cells (surround the vascular bundles)bundle-sheath cells (surround the vascular bundles)

•the mesophyll cells contain a 3-carbon the mesophyll cells contain a 3-carbon molecule called PEP instead of RuBPmolecule called PEP instead of RuBP•the COthe CO22 combines with PEP to form a 4- combines with PEP to form a 4-

carbon molecule of oxaloacetate carbon molecule of oxaloacetate •oxaloacetate acts as a shuttle to transport oxaloacetate acts as a shuttle to transport carbon dioxide to the bundle-sheath cells carbon dioxide to the bundle-sheath cells where it releases COwhere it releases CO22 and increases CO and increases CO22

concentrations to allow the regular Cconcentrations to allow the regular C33 cycle cycle

to proceedto proceed•disadvantage is that Cdisadvantage is that C44 pathway uses more pathway uses more

energy (ATP) than the Cenergy (ATP) than the C33 pathway pathway

Comparison of CComparison of C33 and C and C44 Plants Plants

Most plants are CMost plants are C33 plants plants Called CCalled C33 because first product of carbon because first product of carbon

fixation is a 3 carbon compound (PGA)fixation is a 3 carbon compound (PGA) CC44 plants – called C plants – called C44 because first because first

compound formed in carbon fixation is 4 compound formed in carbon fixation is 4 carbons (oxaloacetate)carbons (oxaloacetate)

CC44 plants thrive in deserts and mid-summer plants thrive in deserts and mid-summer when sun is plentiful but water scarcewhen sun is plentiful but water scarce

CC33 plants do well in cool, wet, cloudy plants do well in cool, wet, cloudy climates because Cclimates because C33 pathway is more pathway is more energy-efficientenergy-efficient