The Process of
Photosynthesis
Chapter 8.3
Photosynthesis Summary
Light and Energy
Because light is a form of energy any
compound that absorbs it also absorbs
energy.
When chlorophyll absorbs energy from light
much of that energy is transferred directly to
electrons, raising their energy levels.
These high energy electrons are used to
make ATP (and NADPH – a high energy
electron carrier).
ATP and NADPH are used to power the Calvin
cycle that produces sugar.
Light Dependant Reactions
Photosystem I
Light Dependant Reactions
The light dependant reactions involve electron
transport between an electron donor (NADH) and
an electron acceptor (oxygen).
This creates a proton gradient which is used to
activate an enzyme known as ATP synthase which
re-phosphorylates ADP + P into ATP.
The light dependant phase is split into three parts;
photosystem II, electron transport chain, and
photosystem I.
Photosystem II
Photosystem II
• Light absorption – light enters the reaction center
and excites chlorophyll
• Electron capture – the electron is captured by the
electron acceptor
• Splitting water and releasing oxygen – water is
split and its electrons replace those lost in the
chlorophyll and H+ ions are released into the
thylakoid space.
• Oxygen is released as a waste product.
Electron Transport Chain
Electron transport chain (ETC) - Electrons are
pass through the chain to Photosystem I.
Energy generated is used to pump more H+ ions
into the thylakoid space or lumen from the
stroma.
This creates a gradient that will force H+ ions
back across the thylakoid membrane back into
the stroma. The movement of ions will provide
the power for ATP synthase.
Photosystem I
Photosystem I
• Photosystem I – electrons transferred
from ETC and chlorophyll is excited by
sunlight.
• NADP+ >>> NADPH – the excited
electrons are transferred in a second
ETC to NADP+ to produce NADPH
Hydrogen Movement and ATP
Formation
In chloroplasts light energy drives the conversion
of H2O to O2 and NADP+ to NADPH. NADPH will
be used later in the Calvin cycle.
The Thylakoid space fills up with H+ ions and
becomes positively charged (relative to the
outside). H+ ions pass back across the thylakoid
membrane, through ATP synthase.
As the ions pass through ATP synthase, it rotates
and the energy produced converts ADP + P to ATP.
NADP+ and NADPH
Nicotinamide adenine dinucleotide
phosphate or NADP+ is an electron carrier
used in anabolic reactions.
NADP+ is reduced to NADPH during the
light dependant reactions. When NADPH
is sent to the Calvin Cycle it supplies
energy and a source of Hydrogen in the
production of carbohydrates.
Photosynthesis
The Calvin Cycle
The Calvin cycle, also known as the light
independent reactions or the CO2 cycle,
involves the combining of existing
compounds and CO2 to make sugars.
These reaction occur in the stroma of
the chloroplasts.
The Calvin Cycle
Photosynthesis and Cellular
Respiration
Photosynthesis and cellular respiration are
closely related chains of reactions.
We can see that both these reactions are
basically the reverse of each other.
Photosynthesis involves using ATP to build
sugar.
Cellular respiration involves breaking
down sugar to produce ATP.
Photosynthesis and Cellular
Respiration