Photosynthesis
Converting Light Energy into Chemical Energy
How far down the rabbit hole must we go to look at photosynthesis?
Photosynthesis
A. Purpose of photosynthesisTo use sunlight to convert water
and carbon dioxide into high-energy
carbohydrates and oxygen.
B. Solar energy is converted into
chemical energy through
photosynthesis.
CO2 + H2O
Light Energy
Sugar
+ O2
1.
2.
3.
Plants use water +
carbon dioxide to
make sugar
Plants use sugars as
material for building
cells and energy to run
cells
Humans use sugars
for:
• Food
• Shelter
• Energy
• Clothing
• Medicines
Location of Photosynthesis
1.
2.
3.
Occurs in the leaves of green plants
Occurs in specific cells in the
middle layer of the leaf
Occurs in organelles – called
chloroplasts – within the cells of the
middle layer of the leaf
Photosynthesis Formula:
(6) CO2 + (6) H2O + light energy
(1) C6H12O6 + (6) O2
In the chloroplasts of photosynthetic organisms, the energy of
sunlight is transformed into the energy stored in chemical
bonds of carbohydrates (glucose).
CO2
H 2O
Light
Chloroplast
Chlorophyll
absorbs
LightDependent
Reactions
Other
materials
Calvin
Cycle:
H+ ions
ATP
LightIndependent
Reactions
NADPH
O2
Sugar (Glucose)
C6H12O6
F. Two Reactions in Photosynthesis
1. Light-dependent reactions convert light energy and the
splitting of water to chemical energy (ATP) and release H
and oxygen.
2. Light-independent reactions use the energy from the lightdependent reaction to convert CO2 and H to sugar
G. Light Dependent Reactions
2. Occurs in the thylakoid
membrane
1. Also called light reactions or photochemical reactions
Chloroplast
structure
3. Sunlight is absorbed by chlorophyll pigments in
photosystems located in the thylakoid membranes inside
the chloroplast.
Water is split into 2e-, 2 H+ and 1 O2 at Photosystem II.
4. Electrons move through the ETC from PS II to PS I.
Energy from these e- transport H+ from stroma to
thylakoid space.
5. Pigments in Photosystem I use energy from light to
convert NADP to NADPH.
6. ATP is produced from ADP using the energy in the
difference between the sides of the membrane.
7. ATP Synthase is the enzyme that converts ADP to ATP.
light dependent reaction
(click on the image to be redirected to the video)
Light Reaction (recap)
1. Light Absorption
Light energy absorbed by
pigments generates electrons that
are transferred to chlorophyll a,
the primary pigment in
photosystems.
•
Photosystems are bundles of
pigments responsible for
absorbing sun’s light energy.
Light Reaction (recap)
2. Electron Transport
Electrons are transported along the
ETC within the photosystems.
Electrons (-) and hydrogen ions
combine with NADP+ to produce
NADPH, stored energy!
A few words on ATP:
1. ATP, adenosine triphosphate,
is energy usable by cells.
2. Energy is stored and released
when phosphate bonds are formed
and broken.
Light-Independent Reaction
1. Also called dark reactions, Calvin cycle or photochemical
reactions
2. Three phase cycle that starts and finishes with a 5-carbon
molecule
3. The net reaction of the Calvin Cycle:
ATP + NADPH + Carbon Dioxide
Glucose
Calvin Cycle (Light-Independent Reaction)
C. Calvin Cycle basics:
1. Takes place in the stroma of the chloroplast
2. The cycle gets its energy to conduct the reaction via highenergy election from NADPH and ATP
3. The NADPH and ATP are two of the products of the lightdependent reactions
Calvin Cycle (Light-Independent Reaction)
D. Calvin Cycle- a step-by-step look:
1. 6 CO2 molecules enter the cycle from the atmosphere. The CO2
molecules combine with 6 5-carbon molecules. Result: 12 3carbon molecules.
2. The 12 3-C molecules are then converted into higher-energy
forms.
Calvin Cycle (Light-Independent Reaction)
D. Calvin Cycle- a step-by-step look:
3. Two of the 12 3-carbon molecules are removed from the cycle to
be used to make glucose and other compounds.
4. The remaining 10 3-C molecules are then converted back into
6 5-C molecules. These molecules then continue through the
cycle and combine
with six new CO2 to
begin the next cycle
LIGHT INDEPENDENT
REACTIONS
(Click the picture for the link)
Dark Reactions – These take place in the
STROMA
A. 6 CO2 molecules enter the Calvin Cycle
6 CO2 combine with 6 5-RuBP to form 6 6C
molecules that split into 12 3 C (PGAL)
molecules.
B. 12 PGA are converted into higher energy forms.
The energy for this conversion is supplied by
ATP and NADPH from the light reactions.
Dark Reactions.
C. 2 of the 12 PGAL molecules are removed and used to
produce one 6C glucose molecule.
D. The remaining 10 3 C molecules are converted back
into six 5 C RuBP molecules to begin the cycle again.
Dark Reactions.
The term “dark reaction” is confusing because these
reactions can occur in the light or the dark.
A better term is “light independent” reactions.
Dark Reactions.
Role of CO2 - is used to combine with RuBP to make
glucose. It comes from the atmosphere.
Role of RUBISCO – it is the enzyme that is used in the
reaction that makes glucose. It is the most abundant
protein in the world!
Role of ATP and NADPH – these molecules provide the
energy for these reactions.