Photosynthesis involves two stages: the light-dependent reactions and the Calvin cycle. In the light reactions, solar energy is absorbed by pigments like chlorophyll and used to produce ATP and NADPH. In the Calvin cycle, ATP and NADPH fuel the reduction of carbon dioxide into carbohydrates like glucose. Photosynthesis is essential as it produces oxygen and food for all organisms on Earth.

1. Chapter 6
Pathways of Photosynthesis

2. Photosynthesis produces food and
released oxygen
Photosynthesizers are autotrophs that produce
their own food
• Photosynthesis
– Converts solar energy to chemical energy (glucose)
• Autotrophs
– Produce their own food
• Heterotrophs
– Consumers
• Nearly all food chains start with plants or algae

3. In eukaryotes, chloroplasts carry
out photosynthesis
• Materials for photosynthesis
– CO2
• Comes through stomata
• Comes from organisms breathing out
– H2O
• Comes in through roots
• The materials diffuse into chloroplasts, where
photosynthesis occurs
• Thylakoids are within chloroplasts and
chlorophyll is found inside of them
• Chlorophyll is within thylakoids and absorbs
solar energy

4. Photosynthesis involves two sets of
reactions: the light reactions and the Calvin
cycle reactions
• Oxidation
– Loss of hydrogen atoms
– Loses electrons
• Reduction
– Gain of hydrogen atoms
– Gains electrons
• This type of reactions is called a redox reaction

5. • Light reactions
– Only occur in the presence of light
– Energized electrons from the breakdown of H2O are
taken up by a coenzyme (NADP+)
– NADP+ also combines with H+ (a reduction reaction)
to form NADPH
– ATP is also produced

6. • Calvin Cycle
– Light independent (can occur with or without light)
– CO2 reduces to CH2O, which can be used to form
glucose
– H+ comes from NADPH
– Energy comes from ATP

7. The light reactions capture solar
energy
Solar energy is absorbed by
pigments
• Solar energy
– Radiant energy from the sun
• Visible light
– Type of radiation used for
photosynthesis and vision
– Shorter wavelength means more
energy
• Violet-shortest wavelength, most
energy
• Red-longest wavelength, least
energy

8. • Chlorophyll a, chlorophyll b, and carotenoids are
found in photosynthesizing cells and are capable
of absorbing various portions of visible light
• Green is reflected instead of absorbed, this is
why plants appear green

9. During fall, enzymes that rebuild chorophyll work
at a slower speed due to lower temperatures.
Chlorophyll eventually disintegrates, leaving only
carotenoids to absorb light. Carotenoids reflect
yellow, orange, and red, which is why leaves
appear to change colors.

10. Solar energy boosts electrons to a
higher energy level
Antenna molecule
– Takes in light energy
– Pass energy to the
reaction center

11. Electrons release their energy as
ATP forms
• Chloroplasts use
electrons energized by
solar energy to generate
ATP by way of electron
transport chain
– High energy electrons
enter
– Low energy electrons leave
– Energy is released each
time the electron is
transferred
– Energy is ultimately used to
make ATP

12. The Calvin cycle reactions
synthesize carbohydrates
ATP and NADPH from the light dependent reactions are
needed to produce a carbohydrate
• CO2 comes in from the atmosphere and combines with
RuBP (an enzyme that speeds the Calvin cycle)
• CO2 is reduced to form CH2O
– ATP and NADPH are needed for this to occur
– ATP breaks down to ADP + P
– NADPH breaks down to NADP+
• G3P (glyceraldehyde-3-phosphate) (CH2O)is then
produced, which is the building block for many
carbohydrates (glucose, starch, and cellulose) and RuBP
• The ADP + P and NADP+ can go back to the light
reactions to form ATP and NADPH again

13. http://media.pearsoncmg.com/bc/bc_0media_bio/bioflix/bioflix.htm?cc5photosynthesis

14. Tropical rain forests and global
climates change
• Rain forest vegetation absorbs large amounts of
CO2
• CO2 traps radiant heat from the sun and warms
the world, similar to a greenhouse, which is why
it is called a greenhouse gas
• Without greenhouse gasses, Earth would be
about 33˚C cooler
• With excess greenhouse gasses, climate
patterns will be disrupted (more heatwaves,
droughts, and storms) and more hurricanes and
tornadoes will occur

15. • Burning fossil fuels releases CO2, clearing
rainforests usually involves burning them (slash
and burn), releasing the trapped CO2, and
reducing the amount of vegetation that can
absorb CO2
• At warmer temperatures, photosynthesis rates
have been seen to decrease
• Cut down forests => more CO2 in the air =>
warmer temperatures => lower photosynthesis
rates => less CO2 is absorbed
• Protecting remaining forests and reestablishing
deforested areas may help to slow climate
change