Transcript Photosynthesis: Chapt. 10 - University of New England

Respiration vs. Photosynthesis
Photosynthesis and respiration as complementary processes in the living world.
Photosynthesis uses the energy of sunlight to produce sugars and other organic
molecules. These molecules in turn serve as food. Respiration is a process that uses
O2 and forms CO2 from the same carbon atoms that had been taken up as CO2 and
converted into sugars by photosynthesis. In respiration, organisms obtain the energy
that they need to survive. Photosynthesis preceded respiration on the earth for probably
billions of years before enough O2 was released to create an atmosphere rich in oxygen.
(The earth's atmosphere presently contains 20% O2.)
Photosynthesis:
Chapt. 8
• The Early Years:
– von Helmont (1600’s)
– Priestley (1700’s)
• From these studies it
was concluded:
– PS converts H20 &
CO2 to organic matter
& 02
Joseph Priestley
born 1733
Priestley became the first person ever to observe
photosynthesis in plants - the fact that they take in
carbon dioxide and release oxygen.
In 1772, Priestley placed a shoot of a green plant
into a container of water. He then covered the
container and lit a candle...The candle burned
longer than without the plant. Priestley also was
able to keep mice alive under the jar.
Priestley had just discovered what would later be
known as oxygen. He called the gas
dephlogisticated air, based on the phlogiston
theory (the idea that combustion was essentially
the process of losing a hypothetical substance
known as phlogiston) of the day.
Also: Invented soda pop
and the rubber eraser
“The injury which is continually done
to the atmosphere by the respiration
of such a large number of animals...
is, in part at least, repaired by the
vegetable creation.”
Jan Baptista van Helmont
(1580–1644)
Van Helmont describes his own experiment:
“I took an earthen vessel, in which I put 200
pounds of earth that had dried in a furnace,
which I moistened with rainwater, and I
implanted therein the trunk or stem of a
willow tree, weighing five pounds. And at
length, five years being finished, the tree
spring from thence did weigh 169 pounds
and about three ounces. … Lest the dust
that flew about should be mingled with the
earth, I covered the lip or mouth of the
vessel with an iron plate covered with tin
and easily passable with many holes. … I
again dried the earth up in the vessel, and
there was found the same 200 pounds,
wanting about two ounces. Therefore, 164
pounds of wood, bark, and roots, arose out
of water only.”
Photosynthesis
Life is powered by sunlight. The energy used by most living cells comes
ultimately from the sun. Plants, algae, and some bacteria use
energy from sunlight, particularly blue and red wavelengths, to build
molecules which later can be split through cellular respiration to
retrieve some of that energy.
Storing energy in molecules and then oxidizing those molecules to
retrieve the stored energy maintains all life on Earth. Plants are
often called ‘producers’ because they produce energy-storing
molecules used by almost all other organisms on Earth. By eating
plants, herbivores and carnivores ‘steal’ these energy-storing
molecules to maintain their own life processes.
Ultimately, the process of photosynthesis is the most important
chemical reaction on Earth.
Photosynthetic Equation
6CO2 + 6H2O
C6H12O6 + 6O2
six molecules of carbon dioxide plus six molecules of water
produce one molecule of sugar plus six molecules of oxygen
Where does PS occur?
Green leaves
More Specifically:
The Chloroplasts
Chloroplast Structure
• surrounded by double
membrane
• Contains additional
internal membranes
termed thylakoids
which may be
unstacked or stacked
(grana)
• internal region termed
stroma
• Text pg 69
Photosynthesis:
in two distinct reactions
Light Reactions
– absorb light
– produce oxygen
Dark Reactions
– fix CO2
Photosynthesis:
involves 2 separate pathways
• Light reactions.. light absorption,
oxygen production, uses sunlight E to
produce ATP & NADPH
• Dark reactions..CO2 uptake and
conversion to glucose (CO2 fixation).
Contains cyclic pathway to fix CO2
The Light Reactions
• requires light input
(light absorption)
• produces oxygen,
NADPH & ATP
• occur on thylakoid
membranes
A Closer look…
Light Absorption:
by Chlorophyll
Evidence:
Light absorption
spectrum of
chlorophyll matches
the effective light
wavelengths for rates
of photosynthesis
Text pgs. 140-141
Chlorophyll
• lipid molecule with a
porphyrin ring
structure and a long
HC tail
• HC tail embeds
chlorophyll in lipid
bilayer of thylakoid
• has a central Mg+
atom
• Text pg. 141
Chlorophyll Types
a&b
a in all photosynthetic eukaryotes and
cyanobacteria
b in higher plants & green algae
c in brown algae, diatoms & dinoflagellates
a&b
a&c
a&c
a&c
Accessory Pigments
• Increase efficiency of
photosynthesis by
absorbing light of different
wavelengths and passing
e- on to Chlorophyll
• Most common... the
carotenoids: lipids which
absorb in blue/green light
and thus reflect in
red/yellow.
• Add to Fall colors
• Text pg 140
Photosystems
Chlorophylls & accessory
pigments group together
to form a cohesive unit: a
Photosystem of two
components:
1. Light-harvesting
component: gathers light
E. and passes it around
2. Reaction center: Specific
Chl a molecule which
passes electrons on.
Text pg. 142
Photon
Reaction Center
Light Harvesting Pigments
From the Photosystem, e- are
passed along an Electron
Transport Chain..
The Photosynthetic Electron
Transport Chain (PETC)
Photon
PETC
Photosynthetic Electron
Transfer Chain (PETC)
• series of electron
carriers which take
electrons from
photosystem, and..
• ultimately carry
electrons to NADP+
Photosystems
• Expts. in the 1940’s suggested that light
photons are absorbed at 2 different points
along the same PETC.…
• In fact, there are two Photosystems in
operation
• Text pg. 143
Photon
Photon
PETC
PETC
Two Photosystems
operate in light absorption
• PS I max. light absorption at 700nm (P700)
• PS II max. abs. at 680nm (P680)
So how does it all work?
• 2 separate photosystems (I, II)
• PETC: groups of electron acceptors
• Final e- acceptor is NADP+
– which is reduced to NADPH
The Z Scheme
e- acceptors
Photon
e- acceptors
NADPH
Photon
PS I
H+ + O 2
PS II
H2O
The Z Scheme
The Z Scheme:
1. PS II : absorbs light at 680nm. Chl a at
reaction center becomes activated & passes on
e- . Lost e- from Chl a is replaced by water,
releasing O2
2. e- carried on to PS I
3. PSI : passes e- onto NADP+ . Requires
additional Energy to do this. Energy comes from
light of 700nm
4. NADP+ is reduced to NADPH
Text pg. 143
So what have we done?
• Chl a reaction center (P680) gets hit by
light
• Passes e- to PETC
• P680 replenishes lost e- by splitting H20
H 20
02
• P700 picks up e- , and gets hit by more
light
• Passes e- further along PETC
• Finally e- used to reduce NADP+
End Result of Light Reactions
•
•
•
•
Split water
Formed oxygen
Reduced NADP+ to NADPH
What about ATP production?
The Light Reactions
• requires light input (light absorption)
• occur on thylakoid membranes
• produces oxygen, NADPH & ATP
Light Reactions:
ATP Synthesis
•
Photophosphorylation:
as e- are passed from
one part of PETC to next,
H+ are pumped across
thylakoid membrane and
out an ATP synthase
particle (CF1)
• Text pg. 145
Photophosphorylation
Occurs in two ways:
• Non-cyclic
Photophosphorylation
• Cyclic
Photophosphorylation
• Text pg 144
Noncyclic Photophosphorylation
• produces: oxygen, 2 ATP’s, NADPH
H2O
PSII
PSI
NADP+
NADPH
H+
H+
Cyclic Photophosphorylation
• ATP only produced
• In evolution, probably first PS
PSI
PETC
ATP
Remember…Photosynthesis
occurs in two distinct reactions
Light Reactions
– absorb light
– produce oxygen
Dark Reactions
– fix CO2
The Dark Reactions
• Energy stored in ATP and NADPH used
to drive CO2 fixation to carbohydrates
• may go on in light or dark, provided
sufficient energy is available
• requires a molecule which will attach
CO2 to it
• occurs in a cyclic pattern (The Calvin
Cycle) Text pg 148
Dark Reactions: Calvin Cycle
Involves a 5C sugar...
RuBP combining w.
carbon dioxide..
CO2 + (5C) RuBP
…and ending up as:
two (3C)phosphoglycerate
(3PG) formed
Along Calvin Cycle: Phosphoglycerate (3PG)
transforms to:
Glyceraldehyde 3- phosphate (G3P)
• some G3P goes to remake RuBP
• excess G3P goes to make sugars (glucose)
• Text p 148
Calvin Cycle
• combines 1 CO2 with 5C sugar (RuBP) to
produce 2 PGA
• Enzyme named Rubisco does step 1 above
• Cycle’s main product is 3C molecule G3P
• Cycle requires 3 ATP and 2 NADPH
• 6 turns of cycle required to produce a 6C
compound such as glucose
• Text pg 148
G3P =
Glyceraldehyde 3-Phosphate
• 3 Carbon compound
• Main product of Calvin cycle
• Used for synthesis of starch in plastids
How does G3P make
glucose?
• 3 turns of Calvin cycle produces 6 G3P
molecules
• 6 G3P = 18 Carbon atoms fixed (6x 3C
= 18C fixed).
• Of these, 15C go to reform 3 new RuBP,
while 3C remaining (1 G3P) goes to
produce glucose
\ 6 turns of Calvin required to make 1
glucose
Calvin Cycle:
Summation
Every 6 turns:
• Produces 12 (3C) G3P = 36C
– of this 30 C Regenerate 6 (5C) RuBP
– and 6C produce 1 (6C) glucose
• And uses 18 ATP & 12 NADPH
6CO2 + 18 ATP+ 12 NADPH
C6H12O6
Calvin Cycle:
Summation
Rubisco
• Ribulose 1,5-bisphosphate carboxylase
• enzyme which catalyzes the joining of CO2 with
RuBP
• makes up ~20% plant protein
• most abundant protein on planet (by far!)
• in low CO2 conditions (high O2), oxygen
interferes with Rubisco.....Photorespiration
• Photorespiration may waste 20-50% of C fixed
Rubisco
Inside plant cells rubisco forms the bridge
between life and the lifeless, creating
organic carbon from the inorganic carbon
dioxide in the air.
Rubisco takes carbon dioxide and attaches it
to ribulose bisphosphate, a short sugar
with five carbon atoms. Rubisco then
clips the lengthened chain into two
identical phosphoglycerate pieces (PGA),
each with three carbon atoms.
Phosphoglycerates are familiar molecules in
the cell, and many pathways are available
to use it. Most of the phosphoglycerate
made by rubisco is recycled to build more
ribulose bisphosphate, which is needed to
feed the carbon-fixing cycle. But one out
of every six molecules is skimmed off and
used to make sucrose (table sugar) to
feed the rest of the plant, or stored away
in the form of starch for later use.
•Contains 2992 hydrogen bonds, which
force the 16 protein chains to assume
208 helices, 248 beta-strands and 456
turns.
•Enzyme is composed of two subunits:
The small subunit is made up of 123
amino acids. The protein contains a fourstranded antiparallel sheet , which is
flanked by two helices. Some turns
stabilize the loops.
The large subunit contains 475 amino
acids.