Chapter 11: Photosynthesis
Leaving Certificate Biology
Higher Level
Photosynthesis
Chlorophyll
Carbon
+ water
dioxide
Sunlight
glucose + oxygen
Chlorophyll
6CO2 + 6H2O
Sunlight
C6H12O6 + 6O2
• Photosynthesis is the process of using
sunlight energy and chlorophyll to produce
glucose from carbon dioxide and water
Photosynthesis
• Light: from the sun
• Carbon dioxide comes from either
– Atmosphere (animals breathe out carbon
dioxide and burning of fuels creates carbon
dioxide)
– Respiration within the leaf
• Water: from the ground through the roots
Photosynthesis
• General process:
– Sunlight is absorbed by chlorophyll (found in
chloroplasts)
– This absorbed energy is used to split water into
three different components:
• Hydrogen ions (also called protons) go into the pool of
protons of the chloroplast
• Oxygen passes out of plant cell and leaf into the
atmosphere where animals breathe it in
• Electrons are given to chlorophyll
– Chlorophyll that has energy because of sunlight
transfers that energy to its electrons
• These electrons get combined with protons and CO2 to
make glucose
Structure of chloroplasts
• Chlorophyll absorbs all colours in white light
except green – that is why plants appear green
Photosynthesis
•
Photosynthesis occurs in two stages:
1. Light stage:
•
•
Light-dependent – requires light
Occurs in thylakoid membranes of the chloroplast
Chlorophyll
molecules
Granum
(stacks of
thylakoids)
Thylakoid
membrane
Stroma
Photosynthesis
•
Photosynthesis occurs in two stages:
2. Dark stage:
•
•
Light-independent – does not require light to occur
Occurs in the stroma of the chloroplast
Chlorophyll
molecules
Granum
(stacks of
thylakoids)
Thylakoid
membrane
Stroma
Light Stage
• Sunlight photon
strikes a cluster of
chlorophyll
molecules called a
photosystem
• The chlorophyll
molecules transfer
the energy to a
reaction center
chlorophyll (RCC)
Light Stage
• The energy is absorbed by an electron which travels
to a higher orbit as a result
• The RCC is now unstable and ordinarily the electron
would give out its excess energy in the form of
fluorescence
• However, nature has evolved a way of slowing this
process down so that the electron’s energy can be
released slowly to make carbohydrate
• The energised electron is picked up by an electron
acceptor and can take one of two paths:
– Cyclic pathway (pathway 1)
– Non-cyclic pathway (pathway 2)
Cyclic Pathway
Cyclic Pathway
• The energised electron from RCC is
transferred along the electron transport chain
• It gives off it excess energy slowly to power
the production of ATP
• When the electron has given off all of its
energy it returns to the RCC
Non-Cyclic Pathway
Occurs in
the thylakoid
membrane
Non-Cyclic Pathway
• Two energised electrons are captured by
NADP+ to become NADP–
• NADP+ + 2e– → NADP–
• NADP– attracts protons because of its
negative charge
• NADP– + H+ → NADPH
• NADPH is the electron and proton carrier
and transfers them to the dark stage
(Calvin Cycle)
• The electrons are those from the RCC
Non-Cyclic Pathway
• Due to the NADPH carrying the electrons
lost from RCC to the dark stage, the RCC
is therefore electron-deficient (positivelycharged: RCC+)
• The RCC+ splits two water molecules
producing 4 electrons (4e–), 4 protons (4H+
ions) and oxygen (O2)
• RCC+ + 2H2O → RCC+ + 4e– + 4H+ + O2
Non-Cyclic Pathway
Fates of the Products of the Splitting of Water
• The electron from water has too much energy to go
back to the RCC and so is passed along the
electron transport chain producing ATP in the
process
• When the electron has lost all its energy in the
electron transport chain it then goes back to the
RCC
• The protons go into the proton pool within the
chloroplast
• The oxygen does one of two things:
– Leaves the leaf and diffuses out into the atmosphere
– Is used by the cells of the leaf or other regions of the
plant for respiration
Light Stage Products
• The products of the light stage which go
onto the dark stage are:
– ATP
– NADPH
• The by-product of the light stage which are
either released into atmosphere or used in
respiration is:
– O2
Dark Stage (Calvin Cycle)
CO2
STROMA
NADPH
Back to
light
stage
NADP+
ATP
ADP
& Pi
Carbohydrate
(Glucose)
Back to
light
stage
Dark Stage (Calvin Cycle)
• Carbon dioxide enters leaf through the stomata
and enters leaf cells (ground tissue) and then
enters the chloroplasts (specifically the stroma)
• NADPH and ATP leave the thylakoid membrane
and enter the stroma
• Using energy supplied by ATP, carbon dioxide is
reduced by the H+ ions and electrons from the
NADPH
• The product of the dark stage is carbohydrate
(glucose) which is then changed into starch and
stored within the cell (vacuole) or transported
through the phloem to another area of the plant
where it may be needed (e.g. at the meristems)
or stored (e.g. root tubers)
SUMMARY