Photosynthesis has two phases: the light-dependent reaction and the light-independent reaction. In the light-dependent reaction, chlorophyll uses energy from sunlight to split water molecules and produce oxygen, ATP, and NADPH. These products are then used in the light-independent Calvin-Benson cycle to convert carbon dioxide into glucose using ATP and NADPH as an energy source. The oxygen, glucose, and regeneration of ATP/NADPH are essential for sustaining life.

1. Photosynthesis

2. ATP
• Remember…
– ATP is cellular energy
– Energy from the chemical
bonds of ATP is released
when the bond between
the 2nd and 3rd phosphate
groups is broken.
– It then becomes adenosine
DIphosphate – ADP
– ATP is then recharged! It
uses energy to gain an
extra P and become ATP
again

3. Metabolism
• Life is sustained by inputs of energy,
however not all forms of energy can
sustain life.
• The sun is abundant, but cannot directly
power protein synthesis or energy-requiring
reactions, it must first be converted to
chemical bond energy

4. Photosynthesis
• Energy flow through
ecosystems begins when
photosynthesizers [plants]
capture sunlight and convert
it to chemical energy
• This chemical energy can
power the reactions of life,
and can be stored for use at
a later time

5. Photosynthesis
• Autotrophs are producers
– Make food using energy
from environment and
carbon from inorganic
molecules
• Heterotrophs are
consumers
– Obtain carbon from organic
compounds assembled by
other organisms

6. Photosynthesis
• Photosynthesis: Metabolic pathway that uses light energy
to turn carbon dioxide (CO2) and water (H2O) into
carbohydrates (sugar)
– Also creates OXYGEN!
• The process is cyclical, the products from one reaction
are the reactants for the next reaction
chlorophyll

7. Photosynthesis
• Photosynthesis has 2 parts
– Light-dependent: converts
light energy into chemical
energy; produces oxygen
and ATP to be used in
light-independent reaction
– Light-independent: does
not require light energy,
uses CO2 and H2O to
build sugars; powered by
ATP, also called the
CALVIN CYCLE

8. Chloroplasts
• Photosynthesis occurs in the chloroplast
• The chloroplast is a plastid that carries
out photosynthesis
– Plastids are double membrane organelles
that contain pigment
• Chloroplasts resemble the photosynthetic
bacteria they evolved from, so
photosynthesis in eukaryotic cells is
similar to bacterial photosynthesis

9. Chloroplasts
• Thylakoid membrane: Highly folded into stacks
of interconnected thylakoids [Light-dependent
reactions]
– Folds in this membrane form disks called thylakoids
– The membrane encloses a single, continuous
internal space
• Stroma: Cytoplasm-like fluid inside the
chloroplast [Light-independent reactions]
– thylakoid membrane, ribosomes, and chloroplast’s
DNA are suspended in the stroma

11. Photosynthesis
• Pigment: organic molecule that
selectively absorbs certain
wavelengths of light
• If a wavelength is not absorbed, it is
reflected, and that gives the pigment
its color
• Chlorophyll a is the most common
pigment in plants
– It absorbs violet, red, and orange
light, but reflects green!

12. Photosynthesis
• Most photosynthetic
cells have accessory
pigments that capture
other wavelengths of
light
– Other functions (attract
pollinators; antioxidants)
– Disassembly of
chlorophylls during
autumn reveals

13. 3. The released electron goes through electron transport chain. The
electron is bounced from between proteins in the thylakoid
membrane. Note: Energy lost along electron transport chain
4. The electron is eventually passed to another chlorophyll molecule
and then on to the final electron acceptor (ferredoxin NADP+
reductase) where the electron transport chain ends.
5. Ferredoxin NADP+ reductase then gives the electron to an
electron carrier NADP which turns it into NADPH. NADPH carriers
the electron to the next phase.
H+
O2
H+
H+
Ferredoxin
NADP NADPH

14. Phase 1: Light Reaction
1. Light hits a leaf, causing an electron in chlorophyll to enter an
excited state (higher energy level) within the chloroplast’s thylakoid
membrane.
2. The excited electrons leaves the chlorophyll and an enzyme
splits a water molecule to replace the electron lost by the
chlorophyll.
– Oxygen gas is formed and exits through the membrane and into the
atmosphere. H+ ions accumulate inside the thylakoid space.
H2O H+
O2
Ferredoxin

15. 6. As more copies of steps 1 through 5 occur, more
and more H+ ions from step 2 begin to build up in
the thylakoid space. H+ ions begin to pass from
an area of higher concertation (thylakoid space)
to an area of lower concentration (stroma)
through an enzyme called ATP synthase. This
process called, chemiosmosis, creates ATP.
H+
H+
H+
Ferredoxin
ADP ATP
H+
H+
H+
H+
H+
H+

16. Phase 1: Light Reaction
Summary
• Lost energy used to recharge ATP from ADP
• NADPH produced from e- transport chain
– Stores energy until transfer to stroma
– Plays important role in light-independent
reaction (LIR)
• Total byproducts: ATP (Used in LIR), NADPH
(Used in LIR), O2 (Exits plant)

17. Phase 2: Dark Reaction (Light-Independent)
• In the second phase,
also called the Calvin-
Benson Cycle, carbon
dioxide, ATP and NADPH
are used to make sugar
(glucose) in the stroma.
• The “synthesis” part
• Plants must make
glucose to store energy
long term. ATP is only
short term energy!

18. Phase 2: Dark Reaction
1. In the stroma, an enzyme called RUBISCO takes a few
molecules of carbon dioxide and connects it to a
molecule called RuBP and turns it into a 3 carbon
molecule called PGA
2. Then more enzymes use ATP and NADPH from the
light reactions to rearrange the molecules in PGA to
store energy and create a few molecules called G3P.
3. NOTE: When ATP and NADPH are used as energy it
they become ADP and NADP respectively. The NADP
and ADP are sent back to the light reactions to get
recharged.
4. Some G3P’s are sent into the cytoplasm to make
glucose. Some G3P molecules are used to make
RuBP that are needed to restart the dark reactions.

19. ATP
Light
Reactions
Calvin
Cycle
Light
H2O
O2
CO2
Sugar
(Glucose)
Chloroplast

20. Alternative Pathways
• Plants in some climates have alternative ways of
doing photosynthesis to help them survive
better.

21. Alternative Pathways
Alternatives

22. Rate of Photosynthesis
• Light intensity, carbon dioxide levels,
and temperature effect the rate of
photosynthesis
– As light increases, rate of
photosynthesis increases
– As CO2 increases, rate of
photosynthesis increases
– Temperature Low = Rate of
photosynthesis low
– Temperature Increases = Rate of
photosynthesis increases
– If temperature too hot, rate drops

23. Chemosynthesis
• The synthesis of organic
compounds by bacteria or other
living organisms using energy
from reactions involving inorganic
chemicals, typically in the
absence of sunlight.
• They use chemicals instead of
water as an electron donor before
the electron transport chain.
• They generally don’t produce

24. Light-Independent Reactions
• The Calvin–Benson cycle
– Builds sugars in the stroma of chloroplasts
– Not powered by light energy
• Driving force is ATP and NADPH formed by the
light-dependent reactions
– Uses carbon atoms from CO2 to make
sugars