Photosynthesis
What is Photosynthesis?
All of the energy used by
living cells comes ultimately
from the sun, capture by
plants and algae through
this process.
Cells of plant leaves
contains organelles called
chloroplasts that actually
carry our photosynthesis.
Stages:
1. Capturing energy from
sunlight.
2. Using the energy to
make ATP.
3. Using ATP to build
carbohydrates.
Autotrophs – organisms that
can make their food.
Heterotrophs – organisms that
cannot make their own food.
Light – dependent
reaction
Light –
independent
reaction
Stage 1: Capturing
Energy from Sunlight
Photons – tiny packets of
energy in light.
Pigments – molecules that
absorb light
Chlorophyll – main pigment
used in photosynthesis
Leaf cells of Hydrilla
plant as seen under
the microscope.
Thylakoids - membranebound compartment inside
chloroplasts and
cyanobacteria.
- It is the site that
contains the chlorophyll
used to absorb light and use
it for biochemical reactions.
Process of Photosynthesis
Step
Step 5.
1. Light
Light Dependent
Dependent
Step 2. Light Dependent
Step
Step 6.
3. Light
Light Dependent
Dependent
Step 4. Light Dependent
Step 7. Calvin Cycle
The
CO2ATP
and and
H2ONADPH
enters the
is
used
leaf by the calvin cycle as
aLight
power
hits
source
the pigment
for
in
converting
the membrane
carbon
of adioxide
from
thylakoid,
the atmosphere
splitting the
into
H2O
simple
into O2sugar glucose.
The
The calvin
electrons
cycle
move
converts
down
3CO2
to enzymes
molecules from the
atmosphere
Sunlight hitstothe
glucose
second
The
pigment
second
molecule
of two allowing
major
stages
the enzymes
in photosynthesis
to convert
(following
ADP to ATP
the
and
light
NADP+
reactions),
gets converted
involving
to NADPH
atmospheric CO2 fixation
and reduction of the fixed
carbon into carbohydrate.
The chemical equation for
photosynthesis
Stage 2: Using the Energy
to Make ATP
Stroma – semiliquid
substance found inside the
chloroplast surrounding the
thylakoid
Diagram showing the
steps that take place
during
photosynthesis.
Photosystem – a network
found in the membranes of
thylakoids in which
chlorophyll pigments are
grouped together
Thylakoids - membranebound compartment inside
chloroplasts and
cyanobacteria.
- It is the site that
contains the chlorophyll
used to absorb light and use
it for biochemical reactions.
Stage 2: Using the Energy
to Make ATP
Each chlorophyll molecule
within the photosystem
network is capable of
capturing protons.
Diagram showing the
steps that take place
during
photosynthesis.
A lattice of proteins holds
the pigments of the
photosystem in close
contact with one another.
When light strikes the
chlorophyll molecule in the
photosystem, the resulting
excitation passes from
chlorophyll molecule to
another.
The excited electron does
not transfer physically-it is
the energy that passed from
one chlorophyll to another.
Stage 2: Using the Energy
to Make ATP
Eventually, the energy
arrives at a key chlorophyll
molecule touching a
membrane-bounded protein.
It is transferred to the
protein, which, in turn
passes it to a series of
proteins that utilize the
energy to make ATP and
build organic molecules.
The excited electrons are
passed on to the electron
transport chain, a series of
membrane embedded
electron chain.
Light hits a second
chlorophyll molecule. Water
is split and oxygen is
released into the
atmosphere.
Electrons, Nicotinamide
adenine dinucleotide
phosphate (NADP+) and H
unite to form NADPH + H+.
The Hydrogen ions move
back across the thylakoid
membrane and generate the
energy for the formulation of
ATP.
Stage 2: Using the Energy
to Make ATP
The end products of the light reactions, then are
NADPH + H+, O2 and ATP.
O2 is released into the atmosphere; NADPH + H+ and ATP are
released into the stroma of chloroplasts and are used to drive the
dark reactions of photosynthesis.
Stage 3: Using ATP to
Build Carbohydrates
Goal of photosynthesis: To
capture carbon atoms from
carbon dioxide in air and use
them to make carbohydrates
that store energy.
Diagram of the Calvin Cycle
In a series of reactions,
plants produce a number of
carbon-containing
molecules.
From these molecules,
plants can then assemble
more complex
carbohydrates such as
glucose and other
compounds needed for
energy and growth.
This series of reactions is
called the Calvin Cycle.
(Named after its discoverer,
Melvin Calvin of University
of California.)
The Calvin Cycle
The Calvin cycle uses ATP
and NADPH to convert CO2
to sugar:
Carbon enters the Calvin
cycle as CO2 and leaves as
sugar.
ATP and NADPH produced
by the light reactions are
used in the Calvin cycle to
reduce carbon dioxide to
sugar.
ATP is the energy source,
while NADPH is the reducing
agent that adds high-energy
electrons to form sugar.
The Calvin cycle is similar to
the Krebs cycle in that the
starting material is
regenerated by the end of
the cycle.
The Calvin cycle actually
produces a three-carbon
sugar glyceraldehyde 3phosphate (G3P).
The Calvin Cycle
For the Calvin cycle to synthesize one molecule of sugar (G3P), three
molecules of CO2 Must enter the cycle. The cycle may be divided into
three phases:
1. Carbon Fixation.
A CO2 molecule combines with a five-carbon acceptor
molecule, ribulose-1,5-bisphosphate (RuBP). This step makes a sixcarbon compound that splits into two molecules of a three-carbon
compound, 3-phosphoglyceric acid (3-PGA). This reaction is catalyzed
by the enzyme RuBP carboxylase/oxygenase, or rubisco.
The Calvin Cycle
2. Reduction.
In the second stage, ATP and NADPH are used to convert the
3-PGA molecules into molecules of a three-carbon sugar,
glyceraldehyde-3-phosphate (G3P). This stage gets its name because
NADPH donates electrons to, or reduces, a three-carbon intermediate
to make G3P.
3. Regeneration
Some G3P molecules go to make glucose, while others must
be recycled to regenerate the RuBP acceptor. Regeneration requires
ATP and involves a complex network of reactions, which my college bio
professor liked to call the "carbohydrate scramble."