Photosynthesis
Photosynthesis in Overview
• Process by which plants and other autotrophs
convert the energy of sunlight into sugars.
• Requires sunlight, water, and carbon dioxide.
• Overall equation:
6 CO2 + 6 H20  C6H12O6 +
6 O2
• Occurs in the leaves of plants in organelles called
chloroplasts.
Leaf Structure
• Most photosynthesis occurs in the palisade layer.
• Gas exchange of CO2 and O2 occurs at openings
called stomata surrounded by guard cells on the
lower leaf surface.
• H2O is brought up from roots via tubes called xylem.
Palisade
Spongy
Chloroplast Structure
• Thylakoid – membrane bound
sacs
– Chlorophyll is embedded in the
membrane (found as part of
Photosystem I and Photosystem II)
– Electron Transport Chain and ATP
synthase is here
– Light reactions occur here
• Thylakoid lumen – liquid inside
thylakoid membranes
• Granum – a stack of thylakoids
• LAMELLA – unstacked
thylakoids that connect grana
• Stroma – liquid region outside
of the thylakoid membranes
(Calvin Cycle or dark reactions
occur here
Pigments
• Chlorophyll A is the most important photosynthetic
pigment. It is blue-green in colour and is the only
pigment that can participate directly in photosynthesis.
• Other pigments called antenna or accessory pigments
are also present in the leaf. They broaden the spectrum
that can be absorbed and transfer the energy over to
chlorophyll a
– Chlorophyll B (yellow / green)
– Carotenoids (orange / red)
– Xanthophylls (yellow / brown)
• These pigments are embedded in the membranes of the
chloroplast in groups called photosystems.
Figure 10.9 Location and structure of chlorophyll molecules in plants
The pigment molecules have a
large head section (porphyrin
ring with Mg at center) that is
exposed to light in the surface of
the membrane; the hydrocarbon
tail anchors the pigment
molecules into the lipid bilayer.
Figure 10.11 How a photosystem harvests light
Chlorophyll a
Photosynthesis: The Chemical Process
• Occurs in two main phases.
– Light reactions
• Light reactions are the “photo” part of photosynthesis.
Light is absorbed by pigments AND used to make
ATP and coenzymes NADPH that will drive the dark
reactions.
– Dark reactions (aka – the Calvin Cycle)
• Dark reactions are the “synthesis” part of
photosynthesis. ATP and NADPH generated in light
reactions are used to make glucose from CO2.
Figure 10.4 An overview of photosynthesis: cooperation of the light reactions and the Calvin
cycle (Layer 3)
Light Reactions
• Light-dependent reactions occur on the
thylakoid membranes.
– Light and water are required for this
process.
– Energy storage molecules are formed. (ATP
and NADPH)
– Oxygen gas is made as a waste product.
Dark Reactions
• Dark reactions (light-independent) occur in the
stroma.
– Carbon dioxide is “fixed” into the sugar
glucose.
– ATP and NADPH molecules created during
the light reactions power the production of
this glucose in a cyclic process
Figure 10.x1 Melvin Calvin