slide 1 Flashcards
Photosynthesis
is the process
that converts
solar energy into
chemical energy
Autotrophs
sustain themselves without eating
anything derived from other organisms.
Autotrophs are the producers of
the biosphere,
producing organic molecules from CO2 and other
inorganic molecules
Almost all producers are photoautotrophs
using
the energy of sunlight to make organic molecules
through photosynthesis
Photosynthesis occurs in plants, algae, certain other protists (single-celled eukaryotes), and some prokaryotes
These organisms feed not
only themselves but also
most of the living world
Heterotrophs
obtain their organic material from
consuming all or parts of other organisms
Almost all heterotrophs, including humans
depend
on photoautotrophs for both food and oxygen.
The Earth’s supply of fossil fuels was formed from
the remains of organisms that died hundreds of
millions of years ago
Fossil fuels therefore represent stores of solar
energy captured by producers in the distant past.
The green color of plants is
from
chlorophyll, the green
pigment within chloroplasts
Although all green parts of a
plant contain chlorophyll,
leaves are the major site of
photosynthesis.
Chloroplasts are found mainly
in cells of the mesophyll
the
interior tissue of the leaf.
Each mesophyll cell contains
30–40 chloroplasts
stomata.
CO2 enters and O2 exits the
leaf through microscopic
pores called stomata
The chlorophyll is in the membranes of thylakoids (connected
sacs in the chloroplast)
thylakoids may be stacked in columns
called grana
The thylakoids are surrounded by stroma
a dense interior fluid
Photosynthesis is a complex series of reactions
that can be summarized as the following equation:
6 CO2 + 12 H2O + Light energy C6H12O6 + 6 O2 + 6 H2O
Chloroplasts split H2O into hydrogen and oxygen.
The electrons of hydrogen are incorporated into
sugar molecules and oxygen is released.
Photosynthesis is a redox process in which H2O
is
oxidized and CO2
is reduced.
Photosynthesis is an endergonic process
the
energy boost is provided by light
Photosynthesis reverses the direction of
electron
flow seen in cellular respiration
Photosynthesis consists of
The light reactions (the photo part) and the Calvin cycle (the synthesis part).
The light reactions (in the thylakoids)
– Split H2O – Release O2 – Reduce NADP+ to NADPH – Generate ATP from ADP by photophosphorylation
The Calvin cycle (in the stroma)
– forms sugar from CO2
, using ATP and
NADPH
Chloroplasts are solar-powered chemical factories
Their thylakoids transform light energy into the
chemical energy of ATP and NADPH
Light is a type of
electromagnetic
radiation wave
Light can also be considered discrete particles of energy called photons
Pigments are substances that absorb visible light
Leaves appear green
because chlorophyll
reflects and transmits
green light
Chlorophyll a is
the main
photosynthetic pigment.
Accessory pigments, such as
chlorophyll b
broaden the spectrum
used for photosynthesis.
Accessory pigments called
carotenoids
absorb excessive light
that would damage chlorophyll.
A spectrophotometer
measures a pigment’s
ability to absorb various wavelengths
This machine sends light through pigments and
measures the fraction of light transmitted at each
wavelength.
An absorption spectrum
is a graph plotting a
pigment’s light absorption versus wavelength
An action spectrum
profiles the relative
effectiveness of different wavelengths of radiation
in driving a process
Absorption spectra
show that violet-blue
and red light work best
for photosynthesis
When a pigment absorbs light,
it goes from a
ground state to an excited state, which is unstable
When excited electrons fall back to the ground
state, photons are given off
an afterglow called
fluorescence
If illuminated, an isolated solution of chlorophyll
will
fluoresce, giving off light and heat.
In chloroplasts a photosystem consists of
1) a reaction-center complex (a type of protein
complex) surrounded by,
2) light-harvesting complexes
The light-harvesting complexes (pigment
molecules bound to proteins)
transfer the energy
of photons to the reaction center.
A special pair of
chlorophyll a
molecules in the
reaction center is
excited by photons from the light harvesting complex and donates electrons to the primary electron acceptor
There are two interconnected types of photosystems:
- Photosystem II (PS II)
- Photosystem I (PS I)
Photosystem II (PS II)
functions first (the numbers
reflect order of discovery) and is best at absorbing a
wavelength of 680 nm (its chlorophyll a is called
P680)
Photosystem I (PS I)
is best at absorbing a
wavelength of 700 nm (its chlorophyll a is called
P700)
Photosystem II A photon hits a pigment and its energy is passed among pigment molecules until it excites P680.
An excited electron from P680 is transferred to the primary electron acceptor. P680’s electrons are replaced by electrons from a water molecule.
Photosystem II Each electron “falls” down an electron transport chain from the primary electron acceptor of Photosystem II
The electron transport chain establishes a proton
gradient across the thylakoid membrane that
drives ATP synthesis
In Photosystem I
light energy excites P700 to pass
an electron to its primary acceptor, this electron is
replaced by the electron transport chain
photosystem 1
A second electron transport chain transfers
electrons to NADP+ reducing it to NADPH
The result of the light reaction is that ATP and NADPH
are supplied to the Calvin Cycle where they are used
to produce sugars
The Calvin Cycle:
Making Sugar
The Calvin cycle was described in
the 1950’s by Andrew Bensen,
James Bassham and Melvin Calvin
The Calvin cycle uses ATP and
NADPH (from light reactions) to
convert CO2
to sugar
The Calvin Cycle
The cycle is similar to that in the
citric acid cycle – the final product
of the reaction is an initial reactant.
The Calvin cycle has three phases
- CO2 acceptor fixes carbon
- Reduction of the sugar
- Regeneration of the CO2 acceptor
Phase 1: Carbon fixation:
In the first phase, the carbon in CO2 is attached to the 5-carbon sugar ribulose bisphosphate (RuBP) by the enzyme rubisco
(Rubisco)
Ribulose bisphosphate carboxylase
oxygenase
(Rubisco
Most abundant protein on Earth
Rubisco is relatively inefficient
fixing only 3 molecules/second and It is not very specific (occasionally binds O2 instead of CO2
calvin cycle
Phase 2: Reduction
In the second phase, ATP phosphorylates the sugar and NADPH donates electrons to raise the potential energy of the sugar. One molecule of the sugar glyceraldehyde 3-phosphate (G3P) leaves the cycle
Phase 3:
Regeneration
In the third phase,
additional G3P is
rearranged into
molecules of RuBP.
For the net production of
one molecule of G3P
the cycle must fix three
molecules of CO2
Alternative Mechanisms to Fix
Carbon in Hot, Dry Climates
Dehydration is a problem sometimes requiring
trade-offs with other metabolic processes,
especially photosynthesis.
On hot, dry days, plants close stomata, which
conserves H2O but also limits photosynthesis
Closing stomata reduces access to CO2 and
causes O2 to build up within cells.
Photorespiration
An apparently detrimental process called
photorespiration occurs while the stomata are
closed and O2 builds up in the mesophyll cells.
Photorespiration
• Rubisco binds O2 instead of CO2 • Additional CO2 is produced but lost from the leaf • ATP is consumed • Photosynthetic rate is reduced
The version of the Calvin cycle
shown previously fixes carbon
first in
a 3-carbon sugar –plants
that use this method are called
C3 plants
C4 plants
minimize photorespiration by using an enzyme, PEP carboxylase, with a higher affinity for CO2 than that of rubisco
In C4 plants PEP carboxylase fixes
CO2
in 4-carbon compounds
These are exported from mesophyll
cells to bundle sheath cells, where
they release CO2
for the Calvin cycle
Comparison: C3 – C4 Plants
C3 - carbon fixed first in a 3-carbon sugar C4 - a 4-carbon sugar is the first product - CO2 fixed first by PEP carboxylase (instead of rubisco) - Calvin cycle occurs in bundle sheath cells
CAM Plants
Some plants fix carbon
by crassulacean acid
metabolism (CAM)
1. CAM plants open their stomata at night, incorporating CO2 into organic acids.
2. During the day, the stomata close and the CO2 is released from the organic acids for use in the Calvin cycle.
Each day the Earth’s atmosphere receives enough energy to supply all of humanity’s energy consumption for 25 years!
Only ~1% of the solar
energy that reaches Earth
is converted to chemical
energy by plants
Significance of Photosynthesis
About 50% of sugars made in photosynthesis are
consumed in the cellular respiration of the plant.
The remainder are synthesized into proteins, lipids
and other molecules for the plant.
Some products are stored in roots, seeds and fruits
