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Photosynthesis
Unit 3
1
Objectives
• Define Photosynthesis
• Explain how the light reaction
captures solar energy
• Describe how the Calvin cycle
makes sugar
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• Important as Carbon atoms are required for carbon
skeleton of all organic molecules
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• All radiation in this spectrum travels as waves
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5
• Short wavelength waves have more energy
than long wavelength waves
...
• This includes all the colours of the rainbow,
violet = the shortest wavelength and red the
longest
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dnr
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gov/ael/personals/pjpb/lecture/spectrum
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• When an atom absorbs a photon of
light energy, electrons are energized,
8
• This gain in energy moves the electron
from a lower energy atomic orbital to a
higher energy atomic orbital, which is
more distant from the nucleus
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The atom can return to its ground state,
which is the normal lowest energy level
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2
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Chloroplasts
• Lie mainly inside the leaf, in the cells of the
mesophyll, a layer with many air spaces and a
very high concentration of water vapour
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13
• 20-100 chloroplasts in each mesophyll
cell
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• Chlorophyll traps light energy which is
used to convert CO2 into Carbohydrates
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bio
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edu/biology/conn
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http://www
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co
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gif
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Structure of Chloroplast
• Chloroplasts are bounded
by two membranes, outer
and inner membrane
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17
http://micro
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fsu
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html
• Each grannum comprises several disk
shaped structures called thylakoids
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Photosynthetic Pigments
• Thylakoid membranes have several kinds of
pigments
• Pigments
– molecules which absorb visible light
• Chlorophyll a
• Chlorophyll b
• Caotenoids
• Phycobilins in cyanobacteria
20
• Pigments absorbs photons 9light NRG)
– And become excited
• Chlorophyll a
– absorb light in the blue region of the spectrum
• Chlorophyll b
– Absorb light in the red regions of the spectrum
• Thus reflect green light
...
• Each chlorophyll molecule has two main
parts, a complex ring and a long side
chain
...
This ring absorbs light energy
...
• All chlorophyll molecules are associated
with specific chlorophyll binding proteins in
the thylakoid membrane, this facilitates the
transfer of energy from one molecule to
the next
...
• Chlorophyll b
– is an accessory pigment
– Prevents damage to pigment chlorophyll a
– By accepting electrons
...
25
• When pigments enter excited state they can:
– Release NRG as heat/ light
• (Florescence)
– Transfer NRG to neighboring pigment
• Occurs in photosynthesis
– NRG can drive chemical rxns
• When pigment gives up e- it returns to grd
state
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Photosynthetic Pigments
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http://www
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com/glossary/images/chlorophyll
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– They absorb different wavelengths of light,
– thus expanding the spectrum of light that
provides energy for photosynthesis
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• Carotenoids
– protect chlorophyll a
– and other parts of the thylakoid membrane
– from excess light
– which can damage the photosynthetic
components
...
– pigments are packaged together
– and attached to thylakoid membranes
– by their long hydrocarbon chains
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• Excitation energy moves from pigments
• that absorb short wavelengths
• to pigments that absorb longer wave
lengths
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– i
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Chl A
• absorbs the longest wavelength
• Rxn centre
– light energy is transformed to chemical NRG
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Chl+ + acceptor
• ChlA becomes positively charged
(chemical oxidation)
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NADPH
34
Photosynthetic Pigments
Absorbtion
35
Photosynthesis
• 6CO2 + 12 H2O→ C6H12O6 + 6O2 + 6H2O
• Divided into two reactions:
– The light dependent reactions
• Thylakoids
• ATP and NADPH formation
– Carbon fixation reactions (Light Independent
reactions)
• stroma
36
Light Dependent Reaction
1
...
2
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3
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4
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1
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2
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•
12 H2O + 12 NADP+ + 18 ADP + 18 P→
+ 6 O2 + 12 NADPH + 18 ATP
38
Photosystems
• Two types of photosynthetic units:
– Arrangements of the Antenna Systems
– Photosystem I
• Chlorophyll a molecules has an absorption peak at
700 nm and is referred to as P700
...
39
Photosystems
• When a pigment molecule absorbs light
energy, it is passed from one pigment
molecule to another until it reaches to the
reaction centre
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Photosystem I, P700
• This electron is then transferred to a primary
electron acceptor from P700, which is the first of
several electron acceptors in a series
...
• transfers the electron to NADP+ in the presence
of enzyme ferredoxin-NADP+ reductase
...
• P700 becomes positively charged when it
gives up an electron to the primary
electron acceptor, the missing electron is
replaced by one donated by photosystem
II
...
44
• Energy passes to
1
...
2
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Electron transport chain
4
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• Molecule of P680 that gives up energized
electron is positively charged
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• Reaction is catalyzed by a unique
manganese containing enzyme, water is
broken into 2 electrons, 2 protons and
oxygen
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Electron Transport Chain
• Non cyclic electron transport
– As e-s are transported along the chain
connection P680 and P700, they lose energy
...
– Energy from this gradient is harnessed to
produce ATP from ADP by chemiosmosis
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49
REDOX Chain
50
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Electron Transport Chain
• Cyclic electron transport
– Only involves P700
– Electron passed to electron acceptors, as
they are passed on energy lost, lost energy
used to pump protons across thylakoid
membrane
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REDOX Chain
53
Chemiosmosis
• Photophosphorylation
– Electrons lose energy at each transfer within
the electron transport chain
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• Chemiosmosis
– The mechanism by which the phosphorylation
of ADP is coupled to diffusion down a proton
gradient
...
• This pumping of protons results in a proton gradient
across the thylakoid membrane
...
• This accumulation of protons causes the pH of the
thylakoid interior to fall to a pH of about 5 in the
thylakoid lumen, compared to of about 8 in the
stroma
...
• However the channel membrane protein ATP
synthase will facilitate the diffusion of protons
...
• Motion of protons through the enzyme induces
conformational change in the enzyme necessary
for ATP synthesis
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56
PQ -
Plastoquinone
PC -
Plastocyanin
Fd -
Ferredoxin
57
Carbon Fixation Reaction
• ATP and NADPH produced in light
dependent phase are used to transfer
energy to reduce CO2 to carbohydrates in
the stroma
...
58
• 12 NADPH + 18 ATP + 6 CO2 → C6H12O6
+ 12 NADP+ + 18 ADP + 18 P + 6 H2O
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Calvin Cycle
• Carbon fixation occurs in the stroma through the
Calvin cycle
...
• 10 of these enzymes are present in glycolysis
...
– Reduction of 3- Phosphoglycerate (3PG) to
Glyceraldehyde 3-phosphate (G3P)
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61
Photosynthesis
• RubisCO is a key catalyst of the Calvin
cycle,
• RubisCO makes up over 50% of the total
chloroplast proteins
• catalyzes formation of phosphoglyceric
acid,
62
• key compound in biosynthesis ofglucose
...
CO2 Fixation or Uptake
• 6 molecule of CO2 reacts with 6 phosphorylated
five carbon compound, ribulose bisphosphate
(RuBP)
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66
• Unstable 6 carbon molecules produced (6)
yields 2 molecules of phosphoglycerate,
(12 PGA) with 3C each
...
• Rubisco is the most abundant protein in
chloroplast
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Glyceraldehyde-3-phosphate (Carbon)
reduction
• 2 steps in which energy and reducing power of
ATP and NADPH are used to convert PGA into
glyceraldehyde-3-phosphate (G3P)
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• Combination of 2 G3P leads to formation
of fructose or glucose
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3
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• Through a series of 10 reactions,
• these 30 C become
• rearranges into 6 molecules of ribulose
phosphate
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C3 plants
• Crops such as soy bean, wheat and potatoes
might not yield as much carbohydrate from
photosynthesis as expected, especially during
hot times of summer
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74
• Photosynthesis occurs in mesophyll cells
inside the leaf, entry and exit of gases
controlled by stomata, tiny pores
concentrated on the underside of the leaf
...
Photosynthesis then uses
up remaining CO2 and produces O2 which
accumulates in the chloroplasts
...
• Acts as a carboxylase and oxygenase as
at high levels of O2 competition for enzyme
active sites occur with CO2
...
• This process is called photorespiration
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succulent-plant
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png
• http://micro
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fsu
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html
• Brock Biology of Microorganisms (11th Edition) Published in
February 1, 2005, Prentice Hall
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• Biology 4th edition :
Peter Raven & George Johnson
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The McGraw-Hill
Companies, Inc
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2008
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79