Chapter 8 Section 2 - Photosynthesis
Photosynthesis
 Light energy is trapped and converted into chemical
energy during photosynthesis.
Cellular Energy
Overview of Photosynthesis
Purpose of Photosynthesis
 To convert light energy into an energy that
organisms can use.
Cellular Energy
Overview of Photosynthesis
Purpose of Photosynthesis
 Plants make sugars through
photosynthesis, an energy that all life on
earth can use.
 Protists such as algae and certain
bacteria can use photosynthesis as well.
 Without photosynthetic organisms, there
would be no life on earth!
Cellular Energy
Photosynthesis
 The Reactants of Photosynthesis are:
 Light – from the sun
 Carbon Dioxide (CO2) – from the air
 Water (H2O) – from the roots
 The Products of Photosynthesis are:
 Glucose (C6H12O6) - sugar
 Oxygen (O2)
Cellular Energy
Overview of Photosynthesis
 Photosynthesis occurs in two phases:
 Light-dependent reactions (Light
reactions)
 Light-independent reactions (sometimes
called dark reactions or Calvin cycle)
Overview of Photosynthesis
 Occurs in the chloroplasts of plant cells.
Cellular Energy
Photosynthesis
Phase One: Light Reactions
 Takes place in the
thylakoids.
 Chloroplasts
 Chloroplasts are the
organelles that
capture light energy.
 Disk-shaped
Cellular Energy
Photosynthesis
Phase One: Light Reactions
 Chloroplasts
 2 compartments:
 Thylakoid: membranes that contain stacks called
grana; where light reactions take place.
 Photosystems I and photosystems II contain lightabsorbing pigments and protein complexes
important in the light reactions.
 Stroma: Fluid-filled space outside grana.
 Where light-independent reactions take place.
Cellular Energy
Photosynthesis
Phase One: Light Reactions
 Pigments
• Light absorbing molecules found in thylakoid
membranes.
•
Different pigments absorb different
wavelengths of light and reflect others.
•
Chlorophylls are the major light-absorbing
pigments in plants.
•
Chlorophylls absorb most in blue-violet
region of visible light spectrum and reflect
light in green spectrum.
Chloroplast Pigments
• Chloroplasts contain several pigments:
– Chlorophyll a
– Chlorophyll b
– Carotenoids
Cellular Energy
Photosynthesis
Basics of the Light Reactions
 The absorption of light is the first step of
photosynthesis.
 When light strikes a thylakoid in a chloroplast,
energy is transferred to and excites electrons in
chlorophyll.
https://www.youtube.com/watch?v=YeD9idmcX0
w
Section 2 Cellular Energy
Photosynthesis
Basics of the Light Reactions
 Energized electrons are used to break down
(split) water molecules.
o Oxygen is released through openings on the
underside of the leaf called stomata.
o Activated electrons are passed along a series
of molecules along the thylakoid membrane,
the electron-transport chain. This generates
energy in the form of NADPH.
o Energy produced from protons (H+) is used to
synthesize ATP.
Cellular Energy
Photosynthesis
Basics of the Light Reactions
 Energy stored is stored temporarily in energy
carrying molecules, ATP and NADPH that will be
used in the Calvin cycle (light-independent reaction).
https://www.youtube.com/watch?v=hj_WKgnL6MI
https://www.youtube.com/watch?v=joZ1EsA5_NY
Review: ATP CYCLE
• Occurs continuously in cells
• About 10 million new ATP molecules are made in every
cell every second!!!
2. Releasing Energy
Energy released by
breaking bonds – used
to power cells
1. Stored Energy
Energy stored in
chemical bonds.
4. Making ATP
Energy released by other
chemical reactions and
processes can be used to
bond a phosphate to ADP to
make ATP
ADP
+
P
3. Energy Depleted
ADP has less
chemical energy
than ATP
Phase Two: The Calvin Cycle (Light-Independent
Reactions)
 A series of enzymeassisted reactions in
which chemical energy
stored in ATP and
NADPH is used to
synthesize
carbohydrates such as
glucose.
Phase Two: The Calvin Cycle (Light-Independent
Reactions)





Does not require light.
Occurs in the stroma of chloroplast.
Requires CO2.
Uses ATP and NADPH as fuel to run.
Makes glucose sugar from CO2 and hydrogen.
 Plants use sugars formed during the Calvin cycle as a source
of energy and to form starch, which stores energy, and
cellulose for structure of cell wall.
https://www.youtube.com/watch?v=E_XQR800AgM
Chloroplast
Light
Stroma
NADP
Stack of
thylakoids
ADP
+P
Light
reactions
Calvin
cycle
Sugar used for
 Cellular respiration
 Cellulose
 Starch
 Other organic compounds
Cellular Energy
Photosynthesis
Factors That Affect The Rate Of Photosynthesis
 Light intensity
 If you increase the amount of light, then
photosynthesis will increase up to a point.
 Temperature
 CO2 concentrations
 Water
Cellular Energy
Alternative Pathways
 Many plants in extreme climates have
alternative pathways for photosynthesis.
 C4 plants
 In hot, dry environments where light
energy is plentiful but water is scarce.
 Sugarcane
 Corn
 Some grasses
Cellular Energy
Alternative Pathways
 CAM plants
 Water conserving plants.
 Open their stomata at night.
 Thick cuticle.
 Cacti
 Orchids
 Pineapple