Photosynthesis Part 2
Chloroplasts
• In green plants,
photosynthesis occurs
within organelles called
chloroplasts.
• Chloroplasts contain
photosynthetic
membranes arranged in
flattened sacs called
thylakoids which contain
chlorophyll.
• Thylakoids are arranged in
stacks called grana.
• The regions between the
grana are called stroma.
2 Types of Photosynthetic Reactions
The Light Reactions
• Takes place in the
presence of light
(during the daylight
hours).
• Light supplies the
energy for these
reactions to occur.
The Dark Reactions
• These reactions occur
without light (during the
night).
• However, the dark
reactions depend upon
the high energy chemical
products produced during
the light reactions.
Light and Dark Reactions Overview
The Light Reactions
• Begin when chlorophyll in the thylakoids of
the chloroplast absorb light.
• The chlorophyll molecules in these thylakoid
membranes are packaged into two
photosytems.
-Photosystem I
-Photosytem II
• Each photosystem contains hundreds of
chlorophyll molecules.
• Each chloroplast in a plant cell can contain
millions of these photosystems!
The Light Reactions and Photosystems
• Photosystems I and II are linked together in
structure and function.
• When photosystem II absorbs light, electrons
are passed to an electron transport chain.
• This chain uses the electrons’ energy to make
ATP.
• At the end of the chain electrons are passed to
photosystem I.
• When photosystem I absorbs light, its electrons
are passed to an electron carrier molecule
NADP to make NADPH2
The Light Reactions and Photosytems
• NADP is similar to the electron carrier NAD+
that we saw in cell respiration.
• Photosystem II provides photosystem I with a
continuous supply of electrons for these
reactions.
• When photosystem II absorbs light and loses
electrons, it replaces the lost electrons by
removing electrons from water (oxidizing
water).
• As a result Oxygen is produced.
• Here you can see directly how water is
necessary and oxygen is produced!
The Light Reactions and Photosystems
• In light, both photosystems absorb light at
the same time.
• Photosystem I uses absorbed light to
generate NADPH2 by reducing NAD+ .
• Photosystem II uses absorbed light to
generate ATP via an electron transport chain.
• These two high energy products, ATP and
NADPH2 are used to power the remaining
photosynthetic reactions – the dark reactions
The Light Reactions and Photosystems
Dark Reactions
Dark Reactions and the Calvin Cycle
• These dark reactions occur in the stroma of the
chloroplast.
• CO2 which diffuses into the stroma from the air
outside is fixed, or used to make glucose.
• The CO2 diffuses through tiny in pores in the
leaf called stomates or stomata.
• The incorporation of CO2 into an organic
compound during photosynthesis is called
Carbon fixation.
• Carbon Fixation: Occurs through a series of
enzyme controlled reactions called the Calvin
Cycle.
Calvin Cycle and Carbon Fixation
• The starting and ending compound in the Calvin
cycle is a 5-Carbon sugar called Ribulose
Biphosphate or RuBP.
• The cycle begins when CO2 reacts with RuBP
producing 2 molecules of a 3 carbon compound
called Phosphoglycerate or PGA.
• ATP produced from photosystem II, and
NADPH2 produced from photosytem I in the
light reaction are used to convert each PGA
molecule to a molecule of
phosphoglyceraldehyde or PGAL (G3P)
Calvin Cycle and Carbon Fixation
• Most of the PGAL (G3P)is used to make
more RuBP so the cycle can continue.
• For every 6 molecules of CO2 that react in
the cycle, 12 molecules of PGAL (G3P)
are formed.
• 10 of these PGAL (G3P) molecules are
used to make RuBP while 2 PGAL (3
carbon) molecules react to form 1 glucose
molecule (6 carbon)
Calvin Cycle and Carbon Fixation