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Photosynthesis (The Light-Independent Stage (The Calvin Cycle (Ribulose…
Photosynthesis
The Light-Independent Stage
The Calvin Cycle
Ribulose biphosphate (
RuBP
) accepts
CO2
, forming a 6C molecule before breaking down into 2 glycerate-3-phosphae (
GP
) (3C) molecules, catalysed by
RuBisCO
2
GP
is reduced by
2 reduced NADP
using the energy from
2ATP
into 2 triose phosphate (
TP
)
It takes
6 turns
of the cycle to create
12TP
molecules, of which 10 are recycled and
2
are the
product
ATP
is used to convert 2
TP
into
RuBP
It takes place in the
stroma
TP Uses
TP can synthesis
glucose
TP can synthesis
amino acids
, fatty acids and glycerol (
lipds
)
5
molecules of
TP
can create
3 RuBP
molecules
RuBisCO has a optimum pH of 8, and is activated by large amounts of ATP and the cofactor magnesium
Factors affecting Photosynthesis
Light Intensity
Carbon Dioxide
Temperature
Increases in temperature
increase
rate of photosynthesis
up to
30*C
Above 30*C
, rates may
reduce
due to
photorespiration
: oxygen competing with carbon dioxide for RuBisCO
Above 45*C
, enzymes may denature,
reducing
photosynthetic rates
Water Stress
the conditions a plant will experience when water supply is limited
Cells become
plasmolyed
Plant roots produce abscisic acid that, when transloacted to the leaves, cause
stomata
to
close
, reducing gas exchange
The rate of photosynthesis is reduced
Tissues become
flaccid
and leaves
wilt
Roots are unable to replace water lost by transpiration
Chloroplast and Photosynthetic Pigments
Chloroplasts
Grana
Stroma
Photosynthetic Pigments
Photosynthesis and Respiration
Photosynthesis
Autotrophic Nutrition
-
Organic molecules are synthesised from non-organic material (water and carbon dioxide)
Plants are
photoautotrophs
, light energy is the source for autotrophic nutrition
6CO2 + 6H20 + energy from photons --(
chlorophyll
)--> C6H12O6 + H2O
Carbon Fixation
-
the process by which CO2 is converted into sugars
, it is a reduction reaction and endothermic, requiring energy, it helps regulate CO2 levels
Respiration
Heterotrophs
- obtain energy by digesting organic molecules
All living organisms respire, oxidising organic material, releasing chemical energy (exothermic)
C6H12O6 + 6O2 --> 6H2O + 6CO2 + energy
How they Interrelate
Photosynthesis and Respiration are opposites, allowing a balance of substances
Compensation Point
When photosynthesis and Respiration proceed at the same rate, with no net gain or loss of carbon dioxide
The time of day at which the plant is at
compensation point
is called the
compensation period
, this is different for each species
The Light-Dependent Stage
Photosystems
Photosystem I
(PSI) has a peak absorption of 700nm (P700)
Photosystem II
(PSII) has a peak absorption of 680nm (P680)
Water
An enzyme in
PSII
splits water in the presence of light, this is
photolysis
2H20 --> 4H+ + 4e- + O2
Photolysis creates a source of
protons
for
photophosphorylation
and donates
electrons
to chlorophyll to replace the lost electrons
Photophosphorylation
Non-Cyclic
Electrons in the
photosystem II
are hit by light and become excited which therefore leave the
chlorophyll
(magnesium) which becomes
oxidised
The electrons move through a series of
electron carriers
(
redox
reactions of
iron
) in the
thykaloid membrane
, losing energy each stage. The energy goes to help pump protons into the lumen
The electrons are replaced by electrons from
photolysis
The electrons are captured by
photosystem I
, replacing those lost from PSI due to excitation by light energy
Ferredocxin
accepts electrons from PSI and passes them to NADP in the stroma
Protons build up in the thykaloid, creating a
proton gradient
for
chemiosmosis
Protons diffuse out of
ATP Synthase
, the flow of protons causing
ADP
and
Pi
to for
ATP
The protons are accepted by
NADP
(along with the electrons) becoming
reduced NADP
, This is catalysed by
NADP reductase
Cyclic
Light strikes
PSI
, releasing a pair of electrons into the
electron carriers
ATP
is generated and eventually the electrons are
cycled
back into
PSI
Practical Investigations