Photosynthesis
in
Higher
Plants
OINTRODUCTION
Green
plants
synthesise
the
food
they
need,
by
photosynthesis
and
all
other
organisms
depend
on
them
for
their
needs.
o
Photosynthesis
is
a
physico-chemical
process
by
which
plants
use
light
energy
to
drive the
synthesis
of
organic
compounds.
The
use
of
energy
from
sunlight
by
plants
doing
photosynthesis
is
the
basis
of
life
on
earth.
o
Photosynthesis
is
important
due
to
two
reasons
:
(a)
It
is
the
primary
sOurce
of
all
food
on
earth
and
(b)
It
is
also
responsible
for
the
release
of
Oxygen
into
the
atmosphere.
(2)
WHAT
DO
WE
KNOW?
Experiment
for
starch
formation
on
variegated
leaf
or
a
leaf
that
was
partially
covered
with
black
paper
&
exposed
to
light
showed
that
photosynthesis
occurred
only
in
green
part
of
leaves
in
the
presence
of
light.
o
Experiment
where
a
part
of
leaf
is
enclosed
in
a
test-tube
with
some
KOH
soaked
cotton
(which
absorbs
CO,),
while
other
half
is
exposed
to
air
and
set-up
kept
in
light
proved
that
CO,
is
needed
for
photosynthesis.
3)
EARLY
EXPERIMENTS
(1)
Joseph
Priestley
Using
a
burning
candle,
a
mouse,
mint
plant
and
a
bell jar
for
closed space,
hypothesised
that
plants
restore
to
the
air
whatever
burning
candles
or
breathing
animals
remove.
(2)
Jan
Ingenhousz
In
an
elegant
experiment
with
an
aquatic
plant,
showed
that
in
bright
sunlight
plants
produce
oxygen.
(3)
Julius
von
Sachs
Found
that
glucose
is
made
in
green
plant parts
and
stored
as
starch.
(4)
T.W.
Engelmann
Using
a
prism,
green
alga
Cladophora
and
aerobic
bacteria,
described
the
action
spectrum
of
photosynthesis,
which
roughly
resembles
the
absorption
spectrum
of
chlorophyll-a
and
b.
(5)
Cornelius
van
Niel
o
Demonstrated
that
photosynthesis
is
essentially
a
light
dependent
reaction
in
which
hydrogen
from
suitable
oxidisable
compound
reduces
C0,
to
carbohydrates.
o H,S
is
hydrogen
donor
for
purple
&
green
sulphur
bacteria.
H,0,
the
hydrogen
donor
in
green
plants
is
Oxidised
to
O,.
o
The
oxidation
product
is
sulphur
or
sulphate
in
purple
&
green
sulphur
bacteria
and
not
O,.
Hence
it
was
inferred
that
O,
evolved
by
green
plants
comes
from
H,0
and
not
from
CO,.
This
was
later
proved
by
using
radioisotopic
techniques.
The
correct
equation,
for
the
overall
process:
6CO,+124,0CH,0,
+
6H,O
+ 60,
(4)
WHERE
DOES
PHOTOSYNTHESIS
TAKE
PLACE
In
green
parts
of
the
plants,
mainly
in
the
mesophyll
cells
in
the
leaves,
which
have
large
number
of
chloroplasts.
o
Usualy
the
chloroplasts
align
themselves
along
the
walls
of
mesophyll
cells
to
get
optimum
quantity
of
the
incident
light.
CHLOROPLAST
ALIGNMENT
PARALLEL
o
In
low
or
optimum
light
intensity
to
get
maximum
incident
light
There
is
a
clear
DIVISION
OF
LABOUR
within
the
chloroplast.
MEMBRANOUS
SYSTEM
o
(Grana
+ Stroma
lamellae)
CHLOROPLAST
o
Responsible
for
trapping
light
&
synthesis
of
ATP
and
NADPH.
Directly
light
driven,
called
LIGHT
REACTION
(photochemical
reactions)
PERPENDICULAR
o
o
In
extremely
high
light
intensity
to
avoid
photo-oxidation.
o
STROMA
Enzymatic
reactions
to
synthesise
sugar,
which
in
turn
forms
starch,
takes place
Dependent
on
products
of
light
reactions
(ATP &
NADPH)
o
By
convention
called
DARK
REACTIONS
(Carbon
reactions)
o
However,
this
should
not
be
construed
to
mean
that
the
dark
reaction
occur
in
darkness
or
that
they
are
not
light-dependent.
10
Light
absorbed
Chapter
HOW
MANY
TYPES
OF
PIGMENTS ARE
INVOLVED
IN
PHOTOSYNTHESIS
Leaf-pigments
of
any
green
plant
can
be
separated
through
paper
chromatography
The
colour
in
leaves
is
due
to
four
pigments,
that
have
the
ability
to
absorb
light,
at
specific
wavelengths.
COLOUR
OF
THE
PIGMENTS
IN
THE
CHROMATOGRAM
i.
Chlorophyll-a
=
Bright
or blue
green
i.
Chlorophyll-b
=
Yellow-green
ii.
Xanthophyll
=
Yellow
iv.
Carotenoids
400
Yellow
to
yellow
orange
Rate
of
photosynthesis
Absorption
500
600
700
Wavelength
of
light
in
nanometies
(aim
The
wavelength
of
light
at
which
there
is
maximum
absorption
by
chlorophyll-a
i.e.,
in
blue
and
red
regions,
also
shows
higher
rate
of
photosynthesis.
Hence,
we
can
conclude
that
Chl-a
is
the
chief
pigment
associated
with
photosynthesis.
