Transcript Slide 1

Chapter 10
Photosynthesis
Autotrophs
• Chemoautotroph - An
organism, that obtains its
nourishment through the
oxidation of inorganic
chemical compounds as
opposed to photosynthesis.
• Photoautotroph – An
organism that uses radiant
energy (light) as a source
of energy to synthesize
carbohydrates
Chloroplast structure
Thylakoid membrane
Inner and
Outer
membranes
Granum
Stroma (Fluid
interior)
Cross-section of leaf
Chloroplasts can be found in the mesophyll cells
All parenchyma layers = Mesophyll layer
Bundle sheath cell
Chlorophyll is a Photoreceptor
•
Chlorophyll is found in the chloroplasts of green plants, and is what makes green
plants, green.
•
The basic structure of a chlorophyll molecule is a porphyrin ring, with a central atom.
This is very similar in structure to the heme group found in hemoglobin, except that in
heme the central atom is iron, whereas in porphyrin it is magnesium.
•
There are actually 2 types of chlorophyll, named a and b. They differ only slightly, in
the composition of a sidechain (in a it is -CH3, in b it is CHO). Both of these two
chlorophylls are very effective photoreceptors.
What is Electromagnetic Radiation?
• Electromagnetic radiation can be described in terms of a
stream of photons, which are massless particles (they
actually have EXTREMELY LOW MASS) each traveling
in a wave-like pattern and moving at the speed of light
(only in a vacuum).
• So, all electromagnetic radiation travels at the speed of
light (c) which is 299,792,458 meters per second
(1,079,252,848.8 km/h).
• Each photon contains a certain amount (or bundle) of
energy, and all electromagnetic radiation consists of
these photons.
• The only difference between the various types of
electromagnetic radiation is the amount of energy found
in the photons.
• Radio waves have photons with low energies,
microwaves have a little more energy than radio waves,
infrared has still more, then visible, ultraviolet, X-rays,
and ... the most energetic of all ... gamma-rays.
How can Electromagnetic
Radiation affect us?
• EMR comes from all kinds of different sources – the primary
source being outer space. EMR also comes from the sun
(all heavenly bodies), man-made objects such as radios,
etc.
• EM radiation carries energy and momentum, which may be
transferred to any matter when it interacts with the matter.
– For example, Ultraviolet radiation (from the sun) can cause damage
to our DNA – resulting in skin cancer.
– Microwave radiation agitates molecules of water, producing heat and
eventually “cooking” the matter (hopefully food)
The Electromagnetic Spectrum
Are all the photons different?
• All the photons are the same, they just
contain different amounts of energy and
therefore travel at different wavelengths
• Radio waves, visible light, X-rays, and all
the other parts of the electromagnetic
spectrum are fundamentally the same
thing. They are all electromagnetic
radiation
Rainbows
• In empty space (vacuum) all photons travel with
the same speed or velocity.
• Photons are slowed down when they pass
through different media such as water, glass or
even air. This slowing down accounts for the
refraction or bending of light by optical lenses.
• The energy of the photon is not changed, but the
wavelength is.
• Different energy optical photons are slowed by
different amounts in glass or water; this leads to
the dispersion of light and the appearance of
rainbows.
The Visible Spectrum
• Some photons have energy and
wavelengths that allow us to see them
• This is visible light – what is light?
• Zillions of photons moving in a wave-like
pattern with energy levels that make them
visible to the human eye (between 400
and 700 nanometers).
• Some animals like fish and snakes, can
see photons (wavelengths) that we cannot
The Visible Light Spectrum
Visible Light
• So visible light appears white, but it is a
collection of photons traveling at different
wavelengths and frequencies.
• A greater intensity of light means more
photons present.
What happens to photons once
they hit matter?
• Photons interact with electrons in all matter- one
photon interacts with one electron.
• The collision causes the electrons to be ejected
from their shells (they enter an excited state).
They eventually may return to ground state.
• If the energy of the photon is high enough, the
electron leaves the matter
• The photons are destroyed and others are
created during these collisions with electrons
• this is called the photoelectric effect.
Chlorophyll absorbs blue light
As the chlorophyll in leaves decays in the autumn, the green colour
fades and reveals the oranges and reds of carotenoids.
There are three major pigment types:
1. Chlorophylls
2. Caretonoids and
3. Phycobilins (phycoerythrin, phycocyanin)
These main pigment types each have a characteristic absorption spectrum.
Chlorophylls absorb blue/red and caretonoids absorb blue/green light.
Photosynthetic pigments in plants do not effectively absorb green and yellow
light, which is why plants appear green or yellow-green.
Phycobilins are found (in addition to the first two pigments) in cyanobacteria (also
termed ‘blue-green’ algae) that absorb the portions of the spectrum not effectively
absorbed by plants. Cyanobacteria (which are Prokaryotic - i.e. do not possess
sub cellular components) were probably the first photosynthetic organisms.
Absorption of light wavelengths
Spectrophotometry
• Using photons of visible light to measure
and analyze materials.
• You can use it to determine how much
light a particular solution absorbs
• You can also use it to determine how
much light a particular solution transmits
(allows to pass through)
• You can also determine the wavelength of
light the sample absorbed or transmitted.
Light Reactions
- Occur in the thylakoid membrane
- They capture the energy from light
- Photons of light excite electrons in chlorophyll which jump
to a primary electron acceptor in the photosystem
- The e- leaves am “electron hole” in its place, which is
replaced by the photolysis of water
- The electrons travel down an electron transport chain in
the thylakoid membrane
- The proteins of the e- transport chain get reduced and
oxidized. When reduced, they are able to pump H+ from the
stroma, into the thylakoid space, creating a proton motive
force
- The H+ then diffuse back into the stroma, via ATP
synthase
- ATP synthase is energized by the H+ force to
phosphorylate ADP into ATP
- The final electron recipient is NADP+ (nicotinamide
dinucleotide phosphate) a cousin of NAD+
Dark Reactions
- Do not really happen in the dark
- Do not require light
- Carbon is fixed in this reaction (From
atmospheric CO2)
- Energy from the ATP and electrons from
NADPH (generated in the light reactions),
is used to reduce the CO2 into G3P, a 3carbon sugar
What happens to when light “hits”
chlorophyll molecules
• Photons of light excite certain electrons in
the pigments
• These electrons “jump” to a higher state,
leaving an “electron hole” behind in the
ground state
• If the electron is not “captured”, it falls
back to ground state, releasing the energy
from the photon as heat and fluorescent
light
Chlorophyll excited by UV light
fluoresces red
LIGHT REACTIONS
The thylakoid membranes of plant chloroplasts have two different kinds of
photosystems each with its own set of light harvesting chlorophyll and carotenoid
molecules and the photochemical reaction centre.
Photosystem I - is maximally excited by light at longer wavelengths. (P700)
Photosystems II - is maximally excited by shorter wavelengths. (P680)
The purpose of light reactions
1) To build the chemiosmotic or proton gradient.
2) Generate ATP.
3) Reduce NADP+ to NADPH.
Non-cyclic Electron flow – starts at photosystem II and ends at
NADPH
Light Reaction Facts
• O2 is created as a by-product of the
Splitting of water (Photolysis)
• Proton Motive Force created for ATP
Synthase (Some H+ come from photolysis
of water and some pumped into lumen by
electron transport chain)
• ATP produced for Dark Reactions
• NADPH produced to reduce CO2 in Dark
Reactions (e- deliverer)
CALVIN CYCLE
(Dark Reactions)
CALVIN CYCLE FACTS
• The fixation of the CO2 is carried out by a giant enzyme
ribulose biphosphate carboxylase/oxidase
(RUBISCO) which is the most abundant enzyme on
earth. This enzyme is very sluggish it works much slower
than most other enzymes. (i.e. ~ 3 molecules of
substrate per sec. compared with ~1000/sec for others).
Therefore, there are many copies of this enzyme in the
stroma ~ 50% of chloroplast protein and most abundant
protein in the world!
• The Facts on G3P (Glyceraldehyde - 3 - Phosphate)
Plants also make other sorts of molecules from the
products of the Calvin cycle. For example G3P is used
by most seed plants to fashion a number of lipids and
amino acids as well as Nitrogen bases (DNA and RNA).
Calvin Cycle Facts, cont’d.
•
The cycle spends ATP as an energy source and
consumes NADPH as reducing power for adding high
energy electrons to make the sugar
•
There are three phases of the cycle
1. CO2 fixation 2. Reduction (NADPH2 with energy
from ATP) 3. Regeneration of CO2 acceptor (RuBP)
•
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For every 3 molecules of CO2 that enter the cycle
one G3P is made and released from the cycle
6 NADPH are used for reduction
9 ATP are used for energy
Cyclic Light Reactions
Cyclic Light Reactions
• In cyclic electron flow, the electron begins in a
pigment complex called photosystem I, passes
from the primary acceptor to the rest of the
electron transport chain, before returning to
chlorophyll in photosystem I.
• This pathway is known as cyclic
photophosphorylation, and it produces neither
O2 nor NADPH. ATP is produced. In bacterial
photosynthesis, a single photosystem is used,
and therefore is involved in cyclic
photophosphorylation.
Why do plants switch from noncyclic to cyclic?
• Non-cyclic (linear) produces equal
amounts of ATP and NADPH
• But the Dark reactions need more ATP
than NADPH
• So when there is plenty of NADPH but not
enough ATP, the plant switches-off half of
the electron transport chain.
Photorespiration?
Qu’est-ce que c’est?
• O2 has an inhibitory effect upon photosynthesis.
As shown in the following graph, in the presence of elevated O2 levels, photosynthesis rates are lower.
C3 Plants
• Most plants fix carbon via Rubisco, to make a 3-carbon
compound called 3-phosphoglycerate
• These plants are called C3 plants because the first
compound made upon CO2 fixation is this 3-carbon
compound.
• Plants found in temperate biomes are all C3 plants. Also
agricultural plants such as rice, wheat, soy, are C3
plants.
• In C3 plants, all photosynthesis steps (light as well as
dark reactions) take place in the chloroplasts of the
mesophyll cells
Cross-section of leaf
Chloroplasts can be found in the mesophyll cells
All parenchyma layers = Mesophyll layer
Bundle sheath cell
Photorespiration – A problem with C3 plants
• On hot, dry days, plants partially close their
stomata, thus reducing CO2 concentrations in
the leaves.
• Since O2 is still being made from the light
reactions, O2 concentrations increase in the
leaves.
• Rubisco’s active site can bind both O2 and
CO2.
• So when CO2 levels drop and O2 levels
increase, O2 competes for the Rubisco active
site.
Photorespiration – A problem with C3 plants
• Rubisco now fixes O2 instead of CO2 in the Calvin
cycle
• The final product is no G3P (sugar), but an
intermediate product that peroxisomes and
mitochondria in the plant cells split to release CO2.
• Because O2 is used and CO2 is released in the
presence of light, this reaction is called
Photorespiration. Unlike cellular respiration however,
no ATP is generated.
• Photorespiration is wasteful! No sugar is made and the
ATP generated from light reactions is used.
Photorespiration, cont’d.
• This is because there is competition between O2 and CO2 for
the active site on the Rubisco enzyme of the Calvin cycle.
• In other words, Rubisco can bind to both, CO2 and O2 equally
well
In "normal" photosynthesis, CO2 is joined
with RUBP to form 2 molecules of 3PGA
In the process called photorespiration, O2
replaces CO2 in a non-productive (less sugar
and no ATP made), wasteful reaction.
O2 is taken in and CO2 is released, hence
Photorespiration
Does Photorespiration have ANY benefits?
• Researcher believe that photorespiration, as wasteful as
it is, may protect the plant from damage caused by too
much light.
• In other words, if light reactions are still taking place,
NADPH and O2 are building up, but the Calvin cycle is
not progressing due to low CO2 levels – this can harm a
plant.
• So photorespiration alleviates this buildup by using some
of the O2 to push forward the Calvin cycle.
Hence, the evolution of C4 plants…
• C4 plants are so named because the first compound
made upon CO2 fixation is a 4-carbon compound such
as oxaloacetate or malate
• In C4 plants, the photosynthesis steps are split between
the chloroplasts of two different types of cells: The
mesophyll cells and the bundle-sheath cells
• The mesophyll cells fix carbon dioxide via an enzyme
called PEPc (phosphenolpyruvate carboxylase) to make
the 4-carbon oxaloacetate
• Examples of C4 plants : various grasses, corn,
sugarcane, many hardy weeds, etc.
PEPc vs. Rubisco.
Hungry anyone?
• PEPc as a much higher affinity to CO2 than O2,
unlike rubisco which swings both ways
• Therefore, PEPc binds to CO2 even when CO2
levels are depleted.
• The mesophyll then exports its 4-carbon malate
to the bundle-sheath cells via plasmodesmata
• The 4-carbon malate releases CO2 in the
bundle-sheath cells, which rubisco fixes into 3phosphoglycerate.
• The Calvin cycle can proceed as usual now.
C4 Plants - Synopsis
•
In C4 plants, the
mesophyll cells can keep
the CO2 concentrations in
the bundle-sheath cells
high enough to keep
rubisco from binding O2.
• Therefore, C4
photosynthesis minimizes
photorespiration and
increases sugar production.
• Found in plants belonging
to hot, biomes with intense
sunlight.
What is a bundle-sheath cell?
Parenchyma or
• Thick-walled plant cell surrounding veins that
functions in a different version of photosynthesis
called C4 photosynthesis.
CAM Plants
• Another evolutionary adaptations to hot and arid
conditions is the Crassulacean metabolism.
• These plants perform C4 photosynthesis, but they keep
their stomata completely closed during the day and open
them at night.
• At night, the mesophyll cells use PEPc to fix CO2 and
store it as 4-carbon organic compounds such as malate
• In the daytime, when their stomata close, the organic
compounds release CO2 for Rubisco to fix into 3phosphoglycerate and the Calvin cycle proceeds as
usual.
Crassulacean Metabolism or CA plants
• Succulents such as
various cacti and
pineapple, perform
CAM photosynthesis
Aloe vera
C4 vs. CAM
Comparison of ALL three types of
photosynthesizing plants
Why does Rubisco bind O2?
• Possible evolutionary remnant – Earth’s
early atmosphere had low O2 if any at all.
• When Rubisco evolved in the first
Photoautotrophs, it did not matter that its
active site was not specific to CO2 only,
since no O2 was present