Transcript Light Reactions

PHOTOSYNTHESIS
Chapters 10
Bell Ringer
• Answer the following question on a sheet of paper and
turn it into the tray. You may NOT use your notes, a
classmate, or your textbook.
A glucose-fed yeast cell is moved from an aerobic
environment to an anaerobic one. How would its rate of
glucose consumption change if ATP were to be generated
at the same rate?
What is Photosynthesis?
• Conversion of light energy from the sun to chemical
energy stored in sugar and other organic molecules
What is the ecologial context for
Photosynthesis?
• Organism acquire the organic
compounds that it uses for energy in
two major modes:
• Autotrophic
• Heterotrophic
What are autotrophs?
• Self-feeder
• Sustain themselves without eating
anything derived from other living beings
• Produce their organic molecules from
CO2 and other inorganic raw materials
• Are ultimate source of organic
compounds for all nonautotrophic
organism
• PRODUCERS
What are autotrophs?
• Plants are photoautotrophs
• Organisms that use light as a source of
energy to synthesize organic substances
What are heterotrophs?
• Unable to make their own food
• Live on compounds produced by other
organisms
• Completely dependent on other
photoautotrophs for food and oxygen
• “other-feeding”
• CONSUMERS
What is the site for Photosynthesis in
plants?
• Chloroplasts
• Found in green parts of plants
• About ½ million chloroplasts per square millimeter of
leaf surface
• Leaves are major site of photosynthesis in most
plants
Where does the green color come from?
• Chlorophyll
• Green pigment located within chloroplasts
• Light energy absorbed by chlorophyll drives the
synthesis of organic molecules in the chloroplast
• Mainly found in the mesophyll
What is the mesophyll?
• Mesophyll- tissue in
the interior of the
leaf. Where
chloroplasts are
found.
• Typical mesophyll
has 30 – 40
chloroplasts
What other parts of the plant are
important?
• CO2 enters the leaf and oxygen exits through the
stomatat
What is the stomata?
• Stomata-
microscopic pores in
the leaf that allow
CO2 and O2 enter
and exit.
What other parts of the plant are
important?
• Stroma –
• Dense fluid within the chloroplast
• Enclosed by envelope of two membranes
• Thylakoids –
• System of interconnected membranous sacs
• Segregate the stroma from interior of thylakoid
(thylakoid space)
• Stacked in columns, they are called grana
What is photosynthesis?
6CO2 + 12 H2O + Light energy -> C6H12O6 + 6 O2+ 6H2O
Photosynthesis
• The O2 given off in photosynthesis comes from H2O, not
CO2.
Two stages of Photosynthesis
• Light Reactions
• Calvin cycle (AKA Dark Reaction)
Photosynthesis
• Light Reactions- solar energy is captured (by
chlorophyll in the thylakoids) and converted into
chemical energy (ATP and NADPH).
• Photophosphorylation- creates ATP through the use of the
ETC in the light reactions.
Photosynthesis
• Dark Reactions/Calvin Cycle- chemical energy is
used to make organic compounds of food. (ie:
glucose) Occurs in stroma.
• Carbon Fixation- CO2 (from air) is combined with molecules
present in chloroplast to form organic molecules that are
reduced to carbohydrates. (w/NADPH)
Light Energy
• What is the nature of sunlight?
• Light is a form of energy known as
electromagnetic energy
• AKA – electromagnetic radiation
• Travels in rhythmic waves
• Wavelengths – distance between crests of
electromagnetic waves
• Electromagnetic spectrum – entire range of
radiation
Light Energy
• What is the nature of sunlight?
• Visible light – can be detected as various colors
by human eye
Light Energy
• Photons- packets of light
energy.
• Pigments- substances that
absorb visible light.
• Chlorophyll a, chlorophyll b,
carotenoids.
• Spectrophotometer-
instrument that measures the
ability of a pigment to absorb
various wavelengths of light.
Absorption Spectrum
and Action Spectrum
• Absorption Spectrum –
• Graph plotting a pigment’s light absorption versus wavelength
• Action Spectrum –
• Profiles the relative effectiveness of different wavelengths of
radiation in driving the process of photosynthesis
• Prepared by illuminating chloroplasts and then plotting wavelengths
against some measure of photosynthetic rate
Absorption Spectrum
and Action Spectrum
Absorption Spectrum and Action Spectrum
Pigments
• Chlorphyll a –
• Considered Blue – green
• Absorb violet – blue and red
• Chlorophyll b –
• Considered Olive green
• Absorbs at slightly different wavelengths of red and blue
• Carotenoids –
• Hydrocarbons that are various shades of yellow and orange
• Absorb violet and blue-green
• Important function in photoprotection
• Photoprotection –
• - they absorb and dissipate excessive light that would otherwise damage
the chlorophyll or interact with oxygen, forming oxidative molecules that
might be dangerous to the cell
What happens when chlorophyll and other
pigments absorb light?
• When a photon of light is absorbed, one of the molecule’s
electrons is elevated to an orbital where it has more
potential energy
• Ground state – non-elevated electron
• Excited state – elevated electron
• Unstable
• Can’t remain long
Photosystems
• Photosystems-
composed of a reaction
center complex
surrounded by light
harvesting complexes
(pigment molecules +
proteins).
• PS II (P680) and PS I
(P700)
Photosystems
• What is a reaction –
center complex?
• Includes a special pair of
chlorophyll a molecules
• What is a light –
harvesting complex?
• Includes various pigment
molecules bound to
proteins
Photosystems
• Energy is transferred from
pigment molecule to a
pigment molecule within the
light-harvesting complex,
until it is passed into the
reaction-center complex.
• The reaction center complex
contains a molecule capable
of accepting electrons and
becoming reduced, called
the primary electron
acceptor.
Photosystems
• Thylakoid membrane is
populated by two types of
photosystems that
cooperate in the light
reaction
• PS II (P680) and PS I
(P700)
• Name in order of discover
Photosystems
• PS II (P680)
• Absorbs best at
wavelength of 680
nm
• PS I (P700)
• Absorbs best at
wavelength of 700
nm
Two stages of Photosynthesis
• Light Reactions
• Calvin cycle (AKA Dark Reaction)
Photosynthesis
• Light Reactions- solar energy is captured (by
chlorophyll in the thylakoids) and converted into
chemical energy (ATP and NADPH).
• Photophosphorylation- creates ATP through the use of the
ETC in the light reactions.
The Light Reactions
• As electrons fall back to its ground state an electron in
a nearby pigment is excited
• Photon of light is absorbed by chlorophyll a pigment
molecule in PS II exciting electrons.
• Electrons are passed along pigment molecules in the
light-harvesting complex, to the reaction center
complex, and ultimately to the primary electron
acceptor.
• Water molecule is split into 2 e-, 2 H+ and O. These
e- are transferred back to P680 and H+ is released to
lumen of thylakoid. O combines with O from previous
water splitting to release O2.
The Light Reactions
• Electrons are passed from primary electron acceptor in
PS II down the ETC to PS I. As electrons pass through
the ETC, ATP is generated.
• Meanwhile, PS I has absorbed light, excited electrons,
that are assed on to P700 and to primary electron
acceptor, leaving p700 without electrons.
• P700 accepts electrons from ETC (that came from PS II).
The Light Reactions
• Excited electrons are passed from primary electron
acceptor of PS I through a second ETC.
• Electrons move through a protein called ferredoxin and to
NADP+ reductase, where they are accepted by NADPH.
This stores the energy of the electrons into a form that
can be transferred to the Calvin Cycle. (No
chemiosmosis, thus no ATP in this ETC)
The Light Reactions
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Cyclic Electron Flow
• Cyclic Electron Flow- electrons take an alternative
pathway that uses PS I but not PS II.
Similarity in ETC
• Potential energy
stored in H+ gradient
• ATP synthase
• Similar electron
carriers
(cytochromes)
Differences in ETC
• Type of phosphorylation
• Oxidative in mitochondria
• Photophosphorylation in
chloroplasts
• Where electrons come from
• Mitochondria – organic
molecules
• Chloroplasts - water
• Where energy comes from
• Direction/location of H+
pumping
• Mito – protons pumped from
matrix out to intermembrane
space
• Chlor – pumps from stroma
into thylakoid space
Light Reactions
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Photosynthesis
• Dark Reactions/Calvin Cycle- chemical energy is
used to make organic compounds of food. (ie:
glucose) Occurs in stroma.
• Carbon Fixation- CO2 (from air) is combined with molecules
present in chloroplast to form organic molecules that are
reduced to carbohydrates. (w/NADPH)
Calvin Cycle
• Similar to citric acid cycle, although citric acid cycle
is catabolic, calvin is anabolic-
• Why is it anabolic?
Calvin Cycle
• Similar to citric acid cycle, although citric acid cycle
is catabolic, calvin is anabolic-
• building carbohydrates from smaller molecules and
consuming energy (endergonic)
Calvin Cycle
• CO2 enters the Calvin Cycle from the light reactions
and exits as sugar.
• The carbohydrate produced in the Calvin Cycle is
not actually glucose, but a 3-carbon sugar called
G3P (glyceraldehyde-3- phosphate).
• To synthesize 1 molecule of G3P, the process has to
happen 3x fixing 3 molecules of CO2.
• Expends 9 ATP and 6 NADH.
Calvin Cycle
• Three phases:
• 1: Carbon Fixation• 2: Reduction-
• 3: Regeneration of CO2 Acceptor (RuBP)-
Calvin Cycle
• 1: Carbon Fixation- CO2 is attached to 5-C
molecule (ribulose bisphosphate- RuBP) to form
a 6-C molecule. Enzyme: Rubisco.
Calvin Cycle
• 1: Carbon Fixation
CO2 enters one at a time and attaches to a 5C molecule called ribulose bisphosphate
(RuBP).
The enzyme involved in this first reaction is
called rubisco. (This is the most abundant
protein in chloroplasts and thought to be the
most abundant protein on earth).
Results in a 6-C intermediate, so unstable that
it splits in half immediately- forms a molecule
called 3-phosphoglycerate
Calvin Cycle
• 2: Reduction- molecule from phase 1 is reduced
(by NADPH) to become 6 molecules of
glyceraldehyde 3-phosphate (G3P). One G3P is
released.
• Each 3-phosphoglycerate receives a phosphate
molecule from ATP to form 1,3bisphosphogylcerate. This molecules gains
electrons (is reduced) by NADPH to become
glycaraldehyde 3-phosphate
Calvin Cycle
• 2: Reduction- molecule from phase 1 is reduced
(by NADPH) to become 6 molecules of
glyceraldehyde 3-phosphate (G3P). One G3P is
released.
• G3P is the same three carbon sugar that glycolysis
splits glucose into
• For every 3 molecules of CO2, 6 molecules of G3P
are formed, but only 1 can be considered a net gain
Calvin Cycle
• 2: Reduction- molecule from phase 1 is reduced
(by NADPH) to become 6 molecules of
glyceraldehyde 3-phosphate (G3P). One G3P is
released.
• For every 3 molecules of CO2, 6 molecules of G3P
are formed, but only 1 can be considered a net gain
• Cycle began with 15 C’s worth of carbohydrates in
the form of three 5-C sugar RuBP
• At the end, there are 18 C’s worth of carbohydrates
in the form of 6 G3P
Calvin Cycle
• 3: Regeneration of CO2 Acceptor (RuBP)-
other 5 molecules of G3P are rearranged to
create 3 more CO2 acceptors (RuBP)
• Spends 3 ATP
Net
• For net synthesis of G3, the Calvin Cycle consumes 9
ATP and 6 NADPH
• Light reaction regenerates the ATP and NADPH
• G3P becomes starting material for metabolic pathways
that synthesize other organic compounds, including
glucose and other carbohydrates
• Neither Light or Dark reaction can make carbon from CO2
Alternative Methods to Carbon fixation
• Evolved in hot, arid climates
• Evaporative loss of water from leaves
• On hot days, plants close stomata to limit that loss
(limiting CO2 intake, increasing O2 concentration in
leaves)
• Favors process called photorespiration
• Normal plants are C3 plants (1st organic product is a 3C
compound)
What is photorespiration?
• Rubisco can bind O2 and add it to the Calvin Cycle
instead.
• The product splits and 2C compound leaves the
chloroplasts
• Peroxisomes and mitochondria rearrange the compound
and split it – releasing CO2
• Consumes ATP
• Photo – light
• Respiration – consumes O2
Alternative Methods to Carbon fixation
• C4 Plants
• Use alternate mode of carbon fixation forming a 4C compound
as its product
• CAM plants
• Crassulacean acid metabolism takes place
• In many succulent (water-absorbing) plants
• Cacti, pinepples
• Open stomata at night, close them during day
• At night take up CO2 and incorporate it into organic acids
• Mesophyll cells store the organic acids they make during the
night in their vacuoles until morning
• During day, CO2 is released from organic acids and is used