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The light and dark reactions

Jean Battinieri

on 13 September 2018

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Transcript of Photosynthesis

Understanding Photosynthesis
Thank you for your attention!
The chloroplast
The Light Reaction
The Calvin Cycle / Dark reaction / Light independent reaction
6CO + 12 H O + sunlight 6O + 6H O + C H O
the chloroplast is thought to once have been a free living organism - it has its own DNA, ribosomes, cytoplasm and a membrane
Now it lives within a plant cell providing energy to the cell and getting a home - ENDOSYMBIOTIC relationship
plants provide energy to you and most other living things.
there are 2 reactions that occur in the chloroplasts the light reaction and the Calvin cycle or the dark reaction
the ruben / kamen experiment
20 years after van Neil made his hypothesis it was proven by ruben and kamen
used O as a tracer to follow Oxygen during photosynthesis
during one experiment they "tagged" the oxygen from the CO - no evidence the of isotope appeared in the released O during another experiment they "tagged" the oxygen from the water it did turn up on the released oxygen
heterotrophs - use energy from plants
leaves are the major sites of photosynthesis
there are about a half a million chloroplasts per millimeter of leaf surface
chlorophyll is the green pigment found in the chloroplasts
Van Helmont
wanted to know if plants grew by taking water out of the soil
potted a plant massed everything
gave it only water
massed everything after a few years
soil remained at the same mass
said mass came from water
took a candle and place it under jar watched as flame went out
said something was needed to keep fire burning
did the experiment again this time with a plant in it
saw that the flame burned longer
studied aquatic plants
saw they produced bubbles during the day but not at night
said they must need sunlight in order to go through photosynthesis
Van Neil 1930
studied bacteria that go through photosynthesis but make glucose from CO but don't release Oxygen
said Oxygen released did not come from CO
hypothesized that plants split water as a source of hydrogen releasing oxygen as a by-product
Two photosystems make up the light reaction

Photosystem I
discovered first
reaction center absorbs light having a wavelength of 700 nm (in the red part of the spectrum) P700

Photosystem II
discovered second
reaction center absorbs light having a wavelength of 680 nm - (in the red part of the spectrum) P680

Light is needed to make NADPH and ATP by energizing the two photosystems found in the thylakoid membranes of the chloroplasts.
The flow of electrons through the photosystems make this possible; there are two routes for the electrons the cyclic and the more frequently used noncyclic process.
The nature of sunlight
light is a form of energy known as electromagnetic energy
it travels in waves
electromagnetic spectrum shows the visible light
the visible light spectrum goes from 380nm to 750 nm
light is emitted in photons - particles of light
the shorter the wavelength the greater the energy of each photon
the visible light is what drives photosynthesis
light can either be reflected, transmitted, or absorbed
substances that absorb light are called pigments
the wavelengths that are absorbed disappear
the color we see is the color most reflected or transmitted by the pigment
chlorophyll absorbs red and blue light and reflects green light
there are two types of chlorophyll; chlorophyll a and chlorophyll b
chlorophyll a is blue green
chlorophyll b is yellow green
chlorophyll a directly participates in the light reaction
chlorophyll b is an accessory pigment - they absorb light and transfer the energy to chlorophyll a
other accessory pigments are carotenoids - these help to absorb light but also can protect the chlorophyll from excess light
Within the thylakoid membrane the chlorophyll is organized along with proteins and other kinds of smaller organic molecules into PHOTOSYSTEMS
Chlorophyll a is the ONLY molecule close to the reaction center
the primary electron acceptor also shares a place near the reaction center
The noncyclic pathway
photosystem II absorbs light and excites the electron which moves into P680 reaction center
this electron needs to be replaced
water splits into 2(H ) + O
Oxygen immediately bonds with another oxygen to form O which is released into the atmosphere
the electron moves through the Electron Transport Chain (ETC) via electron carriers
as electrons move through ETC electron releases energy and helps to convert ADP to ATP - called noncyclic photophosphorylation
the mechanism for photophosphorylation is chemiosmosis
Photosystem I
Light energy excites an electron to primary acceptor of photosystem I
The electron from photosystem II reaches the bottom of the ETC it fills an electron hole in the P700
the excited electron goes to a second ETC and through a series of proteins
the electrons get transfered to an electon carrier called NADP+
NADP+ converts to NADPH which will provide power to the Calvin cycle
The Cyclic Pathway
under certain conditions photoexcited electrons take an alternative path and uses photosystem I but not photosystem II
electrons cycle back to P700
there is no production of NADPH and no release of Oxygen
ATP is generated - cyclic photophosphorylation
the calvin cycle consumes more ATP than NADPH so this is a way to make up the difference
the carbohydrate produced directly from the Calvin cycle is actually not glucose but a 3 carbon sugar called glyceraldehyde - 3 phosphate (G3P).
to make one molecule of this sugar it needs to cycle 3 times and use 3 CO
Carbon fixation
each CO molecule is attached to a five-carbon sugar named ribulose biphosphate (RuBP)
this produces a six-carbon intermediate so unstable that it immediately splits in half to form two molecules of 3-phosphoglycerate (for each CO )
each molecule of 3-phosphoglyerate picks up a phosphate group from ATP
then NADPH donates electrons (reduction) to creating six G3P (PGAL)
one of these six is a net gain and exits the cycle to be used by the plant cell
the other five continue to regenerate RuBP
Regeneration of CO acceptor
the five molecules of G3P (PGAL) are rearranged into three molecules of RuBP using 3 molecules of ATP
The PGAL produced becomes the starting material for metabolic pathways that synthesize other organic compounds, including glucose and other carbohydrates.
Alternative mechanisms of carbon fixation
Factors that affect photosynthesis
amount of light
amount of CO2
amount of water
metabolic adaptions
compromise between photosynthesis and the prevention of excessive water loss from a plant
CO enters the leaf through the stomata also where transpiration takes place
hot dry days most plants close their stomata which then conserves water
but also reduces the amount of photosynthesis that can take place - limiting the amount of CO intake
CO amounts decrease and O amounts increase
these conditions favor photorespiration
most plants are considered C3 plants because they use the enzyme Rubisco to add CO2 to ribulose biphosphate creating a 3 carbon compound 3-phosphoglycerate
important agriculture plants that use this process include rice, wheat, and soybeans
they produce less food when there stomata close on hot dry days
O2 increase in the air spaces of the leaf and rubisco adds O2 to the calvin cycle instead of CO2
this creates a two-carbon compound that leaves the chloroplasts and goes to the mitochondria and peroxisomes where it is broken down into CO2
this process is photorespiration
unlike normal respiration NO ATP is generated and NO food is generated
it is not known if this process is benefical to plants
this process can drain 50% of the carbon fixed by the Calvin cycle
some plants have developed other pathways to fix carbon that is not as wasteful - C4 and CAM
C4 plants
they preface the Calvin cycle with an alternate mode of carbon fixation that forms a four-carbon compound as its first product
agrigulture plants that use this method include sugarcane, corn, and members of the grass family
these plants have a unique leaf anatomy
there are 2 distinct types of photosynthetic cells: bundle sheath cells and mesophyll cells
bundle sheath cells are arranged around the vein
mesophyll cells are between the bundle sheath cell and the leaf surface
the carbon cycle only occurs in the chloroplasts in the bundle sheath cells
prior to the cycle CO2 is incorporated into organic compounds in the mesophyll
1st step CO2 is added to phosphoenolpyruvate (PEP) to form the 4 carbon product oxaloacetate
the enzyme PEP carboxylase adds CO2 to PEP
PEP carboxylase can fix CO2 more efficiently than Rubisco when it is hot and dry and the stomata are partially closed
Mesophyll cells export their four-carbon products (malate) to the bundle sheath cells through the plasmodesmata
In the bundle sheath cells the four carbon compound releases CO2 which is used by the Rubisco in the Calvin cycle
this is an adaptation that reduces photorespiration and enhances sugar production
CAM pathway
second photosynthetic adapation in arid conditions evolved in succulent (water storing) plants such as cacti and pinapples
plants open their stomata during the night and close them during the day - reverse of other plants
this helps conserve water but also prevents CO from enterning
at night plants take in CO and incorporate it into a variety of organic acids
this mode of carbon fixation is called crassulacean acid metabolism or CAM
the mesophyll cells of CAM plants store the organic acids made at night in vacuoles until morning when stomata close
During the day light reactions make ATP and NADPH for the Calvin cycle and the CO is released from the organic acids made at night
A review:
light reactions capture energy and use it to make ATP and NADPH which are transferred to the Calvin cycle
Calvin cycle uses ATP and NADPH and sugar from CO
the sugar made in the chloroplasts supplies the entire plant with chemical enegy and carbon skeletons to synthesize all the major organic molecules of a cell
about 50% of organic material is used for cellular respiration in the mitochondria
the sugar glucose is linked together to make a poly saccharide - cellulose - the main ingredient in the cell wall
Extra sugar is stockpiled by making starch, storing some in the chloroplasts, and some in the roots, tubers, seeds, and fruits.


copy and paste this website to view an animation on the Calvin Cycle

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