Download 12 photosynthesis

Photosynthesis
What is photosynthesis?
What is it for?
Where does it occur?
(Always?)
What are the two major processes?
How much detail will you need?
How can we know if photosynthesis
is taking place?
Variations on a theme….
Refer to chapters 6 and 7 in text.
What is photosynthesis?
“Photosynthesis involves
the conversion of light energy
into chemical energy.”
The primary storage of this chemical energy
will be in 6 carbon sugars.
ATP will be used as an intermediate
in this solar-to-carb energy transfer.
What is it for?
ATP will ultimately be re-charged
when energy is released from the carbs
in cellular respiration
when the cell has business to do.
far red↑
- Visible light is one part of the electromagnetic spectrum.
- The strongest absorption is on blue-violet (400-500nm)
and near red (650-700).
- The middle of the spectrum – green – is NOT absorbed:
It is reflected. Hence, plants appear green.
http://www.uic.edu/classes/bios/bios100/lecturesf04am/absorption-spectrum.jpg
The absorption spectrum of selected plant pigments:
http://www.mrothery.co.uk/images/Imag53.gif
The action spectrum of an unspecified plant:
- This shows the rate of photosynthesis
relative to light wavelength.
- Does the pattern look familiar? If so (it better!), why?
Why is it not just like… that other spectrum?
Where does
photosynthesis occur?
- The chloroplast is the
site of photosynthesis
in eukaryotes.
- Two chief parts:
“photo-”
light-dependent reactions
in thylakoids,
“-synthesis”
light-independent reactions
(aka Calvin cycle)
in stroma.
- note ATP/ADP and
NADPH/NADP+ bridging the two
- note two photosystems
(II comes before I) for light energy capture.
- note electron transport chain…
(See! Aren’t you glad you made the effort in respiration!)
http://www.55a.net/firas/ar_photo/11/photosynthesis-overview.gif
What are those two major processes, again? 1. Light-dependent reactions
-An e- is energized,
and handed off to the
electron transport chain.
- Water split, providing
e- to replace them, with
O2 off as waste. This is
photolysis.
- plastoquinone in
ETC pumps protons into
the thylakoid lumen…
continued next slide
http://cache.eb.com/eb/image?id=72207&rendTypeId=35
-Light absorbed by
chlorophyll a in
photosystem II:
-Light absorbed by
chlorophyll a in
photosystem I: ← NB
-2 e- are energized,
and handed off to NADP+,
which is reduced to
NADPH.
- The proton motive force
powers ATP synthase, in
photophosphorylation,
using chemiosmosis.
-ATP and NADPH head off
to the Calvin cycle.
http://cache.eb.com/eb/image?id=72207&rendTypeId=35
1. Light-dependent reactions (cont.)
1. Another view of the
light-dependent reactions
- This adds the free
energy of the reactants.
- Different wavelengths
stimulate PSII (680)
and PSI (700).
- e- -accepting
intermediates after
PSI
- ALSO, the
“cyclic pathway”:
http://www.agen.ufl.edu/~chyn/age2062/lect/lect_04/7_11.gif
Cyclic photophosphorylation: Instead of passing energized e- to NADP+,
some are sent back to the ETC:
This pumps out more H+ (therefore more ATP) without making NADPH,
because more ATP than NADPH is needed in the Calvin cycle!
(That default path is called non-cyclic photophosphorylation.)
Deep breath. Recap.
What’s the other process, again?
2. Light-independent reactions
- Carbon fixation:
- CO2 is attached
to RuBP by
Rubisco* (enzyme).
(H comes from
light-dependent reactions.)
- The intermediate
falls into two 3C units (3PG),
which are reduced
by ATP and NADPH.
- every third time around
a G3P, aka TP (a triose phosphate) is spun off,
which then may become glucose
or other organic molecules.
- Another ATP is invested to replace
the original RuBP.
* “probably the most abundant protein on Earth”
Not to panic: Here is the “Magical, Magical, POOF”
The regeneration stage can be broken down into steps.
1. Triose phosphate isomerase converts all of the G3P reversibly into dihydroxyacetone
phosphate (DHAP), also a 3-carbon molecule.
2. Aldolase and fructose-1,6-bisphosphatase convert a G3P and a DHAP into fructose 6phosphate (6C). A phosphate ion is lost into solution.
3.Then fixation of another CO2 generates two more G3P.
4. F6P has two carbons removed by transketolase, giving erythrose-4-phosphate. The two carbons
on transketolase are added to a G3P, giving the ketose xylulose-5-phosphate (Xu5P).
5. E4P and a DHAP (formed from one of the G3P from the second CO2 fixation) are converted
into sedoheptulose-1,7-bisphosphate(7C) by aldolase enzyme.
6. Sedoheptulose-1,7-bisphosphatase (one of only three enzymes of the Calvin cycle that are unique
to plants) cleavessedoheptulose-1,7-bisphosphate into sedoheptulose-7-phosphate, releasing an
inorganic phosphate ion into solution.
7. Fixation of a third CO2 generates two more G3P. The ketose S7P has two carbons removed
by transketolase, giving ribose-5-phosphate (R5P), and the two carbons remaining
on transketolase are transferred to one of the G3P, giving another Xu5P. This leaves one G3P as the
product of fixation of 3 CO2, with generation of three pentoses that can be converted to Ru5P.
8. R5P is converted into ribulose-5-phosphate (Ru5P, RuP) by phosphopentose isomerase. Xu5P is
converted into RuP byphosphopentose epimerase.
9. Finally, phosphoribulokinase (another plant-unique enzyme of the pathway) phosphorylates RuP
into RuBP, ribulose-1,5-bisphosphate, completing the Calvin cycle. This requires the input of one ATP.
Thus, of six G3P produced, five are used to make three RuBP (5C) molecules (totaling 15
carbons), with only one G3P available for subsequent conversion to hexose. This requires
nine ATP molecules and six NADPH molecules per three CO2 molecules.
(courtesy Wikipedia)
OR, LOOK AT THIS → http://www.youtube.com/watch?v=JWxCfWOqIDc&safety_mode=true&persist_safety_mode=1&safe=active
http://bancroft.berkeley.edu/Exhibits/Biotech/calvin.html
'There is no such thing as pure science. By this I mean that
physics impinges on astronomy on the one hand, and chemistry
and biology on the other. The synthesis of a really new concept
requires some sort of union in one mind of the pertinent aspects
of several disciplines....It's no trick to get the right answer when
you have all the data. The real creative trick is to get the right
answer when you have only half of the data in hand and half
of it is wrong and you don't know which half is wrong. When
you get the right answer under these circumstances, you are
doing something creative.
From Calvin’s autobiography
Following the Trail of Light:
A Scientific Odyssey (1992):
What is used in the light-dependent reactions?
What is produced?
What is used in the light-independent reactions?
What is produced?
Make sure you write down the answers!
intermembrane space
outer membrane
stroma
inner membrane
lumen
granum
thylakoid
lamella
“space” in Mader
http://upload.wikimedia.org/wikipedia/commons/1/11/Chloroplast-new.jpg
Draw and annotate a diagram showing the structure
of a chloroplast as seen in an electron micrograph.
Why are thylakoids
so thin?
Why are there so many
of them?
Why the ‘empty’ (fluidfilled) space
around the grana?
Where are
chloroplasts found?
Why are they green?
Where is the light
actually absorbed?
botit.botany.wisc.edu/.../Chloroplast_EN.html
How can we know if photosynthesis is taking place?
6 CO2 + 6 H2O → C6H12O6 + 6 O2
-------------------------------------------------------------------
Directly:
1. Measure the CO2 consumed.
2. Measure the O2 generated.
Indirectly
3. Determine the net increase in biomass.
4. Measure the reduction of NADP+ by providing a
colorimetric substitute for NADP+
(This is what you do in the AP lab.)
What do you think would influence the rate of photosynthesis?
What would be the effect…, and why?
light? (intensity? duration?) temperature? pH? CO2 levels? ....
www.climateaudit.org/?p=884
http://www.marietta.edu/~spilatrs/biol103/photolab/rspCrve.gif
generalhorticulture.tamu.edu/.../light.html
Variations on a theme….
Oxygen actually binds to Rubisco more readily than does CO2,
so in low CO2 (like stomata closed to conserve water)
O2 is taken into the Calvin cycle, and CO2 is released:
Called photorespiration, but generates no ATP. Ouch.
Dry environments cause problems for photosynthesis:
Two specialized alternative photosynthetic adaptations
that you will want to know…
C4 pathway:
In arid climates is helps to be able to just crack the windows:
In order to increase the carbon locally available to the Rubisco
carbon is first fixed, more aggressively, to PEP by PEP carboxylase,
resulting in a 4-C molecule.
This is carried to bundle sheath cells around the leaf vein,
(which is where the Calvin cycle occurs in these plants)
and handed off into the Calvin cycle.
About 7600 species of plants use C4 carbon fixation, which
represents about 3% of all terrestrial species of plants. All these
7600 species are angiosperms. C4 carbon fixation is less
common in dicots than in monocots, with only 4.5% of dicots
using the C4 pathway, compared to 40% of monocots. ... Fortysix percent of grasses are C4 and together account for 61% of
C4 species. These include the food crops maize, sugar
cane, millet, and sorghum. (Wikipedia)
CAM pathway:
www.steve.gb.com/science/photorespiration.html
Another route is to simply close the windows during the day:
By attaching the CO2 to some other organic molecule at night,
and then releasing it to Rubisco in the presence of light,
while the stomata are closed:
In both C4 and CAM carbon is first fixed by some other molecule:
- is C4 this is separated from Calvin by space (structural).
- in CAM they are separated by time.
The majority of plants possessing CAM are either epiphytes (e.g., orchids,
bromeliads) or succulent xerophytes (e.g., cacti…), but CAM is also found
in …[a scattering of others].
Plants which are able to switch between different methods of carbon fixation
include … Dwarf Jade Plant, which normally uses C3 fixation but can use
CAM if it is drought-stressed,[and … Purslane, which normally uses C4 fixation
but is also able to switch to CAM when drought-stressed.
CAM has evolved convergently many times. It occurs in 16,000 species
(about 7% of plants), belonging to over 300 genera and around 40 families,
but this is thought to be a considerable underestimate. It is found
in quillworts (relatives of club mosses), in ferns, and in gymnosperms, but the
great majority of plants using CAM are angiosperms (flowering plants).
(Wikipedia)
Welwitschia ↑:
Google this one!
Rafflesia arnoldii,
producing the world’s largest flower,
doesn’t photosynthesize.
↑http://scienceray.com/biology/earths-largest-flower/
Wikipedia →
The flower of Rafflesia arnoldii grows to a diameter of around
one meter (3 ft) and weighing up to 11 kilograms (24 lb).[citation
needed] It lives as a parasite on the Tetrastigma vine, which grows
only in primary (undisturbed) rainforests.Rafflesia lacks any
observable leaves, stems or even roots, yet is still considered a
vascular plant. Similar to fungi, individuals grow as thread-like
strands of tissue completely embedded within and in intimate
contact with surrounding host cells from
which nutrients andwater are obtained. This plant produces no
leaves, stems or roots and does not have chlorophyll. It can only
be seen when it is ready to reproduce. Perhaps the only part
of Rafflesiathat is identifiable as distinctly plant-like are the
flowers; although, even these are unusual since they attain
massive proportions, have a reddish-brown coloration and stink
of rotting flesh, which is why it was nicknamed the "carrion
flower". This scent attracts insects such asflies which then
pollinate the rare plant.
How do reactant molecules reach the chloroplasts in leaves?
To make one molecule of glucose takes
how much ATP and NADPH?
Compare C4 and CAM pathways
to each other,
and to the C3 (conventional) pathway.
In the light reactions, what is the electron donor?
Where do the electrons end up?
How might you measure photosynthesis?
Describe how the two stages of photosynthesis
are dependent on each other
Relate chloroplast form to function.
photosynthesis
chemiosmosis
absorption spectrum
cyclic photophosphorylation
pigment
non-cyclic photophosphorylation
action spectrum
RuBP
chloroplast
rubisco
light-dependent reactions
3PG
thylakoid
G3P
light-independent reactions
TP
Calvin cycle
outer membrane
stroma
inner membrane
photosystems
lamella
electron transport chain
intermembrane space
chlorophyll a
granum
photosystem II:
6 CO2 + 6 H2O → C6H12O6 + 6 O2
photolysis
photorespiration
thylakoid lumen
PEP
photosystem I
PEP carboxylase
ATP synthase
bundle sheath cells
photophosphorylation,
CAM pathway
C4 pathway