PHOTOSYNTHESIS
The Dark and Light Reactions
ENERGY IS ESSENTIAL TO ALL
LIVING THINGS.
All energy on earth originates in nuclear
reactions in the sun.
PRODUCERS
PLANTS AND OTHER ORGANISMS CAPABLE
OF CARRYING OUT PHOTOSYNTHESIS
6C02+ 6H20+ energy C6H1206 +
602
PHOTOSYNTHESIS:
6 CO2 + 6 H2O + light energy  C6H12O6 + 6O2
PHOTOSYNTHESIS TAKES PLACE IN
THE CHLOROPLAST
LIGHT
REACTIONS:
OCCUR ON THE THYLAKOID
MEMBRANES
•convert
light and water into chemical energy
•produce
O2 as a waste product
HOW LIGHT WORKS
 Visible
radiation drives the light reactions
 Light is a type of energy called
electromagnetic energy.
 Different pigments embedded in thylakoid
membranes
DIFFERENT PIGMENTS ABSORB LIGHT OF
DIFFERENT WAVELENGTHS.

The wavelengths absorbed by the light harvesting
pigments are mainly in the blue and red range


This is why most plants appear green
The light harvesting pigments of chloroplasts are
1. chlorophyll a: absorbs in blue and red
2. chlorophyll b: absorbs in blue and orange
3. carotenoids: absorbs mainly blue-green range
Having all these different pigments broadens the
range of visible light that is usable for photosynthesis.
 Only chlorophyll a passes electrons to the electron
transport chain and thus is directly involved in the
light reactions.
 The other pigments convey the light energy to
chlorophyll a.

THE LIGHT REACTIONS
Photosystems I and II
STEP 1:
Pigments in photsystems II absorb light
 Energy from the light is absorbed by electrons
(increasing energy level)
 High-energy electrons are passed on to the
electron transport chain
 H2O molecules provide new electrons to the
chlorophyll to replace the ones that were lost
 Enzymes on the inner surface of the thylakoid
membrane break up each H2O molecule into 2
high-energy electrons
 Oxygen is released into the air as oxygen gas
 2 H+ ions are released inside the thylakoid
membrane

STEP 2:
High-energy electrons move through the electron
transport chain from photosystem II to
photosystem I
 b.Energy is used from the electrons to transport
the H+ from the stroma into the inner thylakoid

STEP 3:
Pigments in photosystem I use energy from light
to reenergize the electrons
 b.NADP+ picks up high-energy electrons on the
outer surface of the thylakoid membrane plus a
H+ ion and becomes NADPH

STEP 4:
H+ ions released during water-splitting and
electron transport the inside of the thylakoid
membrane becomes positively charged and the
outside negatively charged
 b.The difference in charges provides the energy to
make ATP

STEP 5:
H+ ions CANNOT directly cross the membrane
 b.ATP synthase (protein) allows H+ ions to pass
through it
 c.As H+ ions pass through this protein, the
protein rotates and ATP synthase binds to ADP
and a phosphate group – producing ATP

Water splits into H+ ions electrons and Oxygen
PHOTOLYSIS: the splitting of
water
Electrons
WATER
H+ ions
Oxygen
PHOTOSYNTHESIS: THE
DARK REACTIONS
ATP + NADPH2 + CO2C6H12O6
DARK REACTIONS: OCCUR IN THE
STROMA
Carbon dioxide is split, providing carbon to make
sugars.
 The ultimate product is glucose.
 While this system depends on the products from the
light reactions, it does not directly require light
energy.

CARBON FIXATION
3
CO2 molecules attach to 3, 5 carbon sugars
(RuBP)
 A 6 carbon sugar is formed and O2 is released
 This reaction is catalyzed by the enzyme
Rubisco.
 This 6 carbon sugar rearranges and splits into
2, 3 carbon sugars (PGA)
REDUCTION
6 ATP contribute phosphate groups
 8 NADPH drop off electrons
 These are used to make the high energy
compound G3P

REGENERATION
1 G3P is used to form glucose
 The others are used to regenerate the original 5
carbon sugar
 9 ATP and 6 NADPH are used per turn of the
calvin cycle
 These are regenerated by Light Reactions
 2 G3P = Glucose
