PHOTOSYNTHESIS
means "putting together with light."
• Autotrophs: Plants and plant-like organisms make
their energy (glucose) from sunlight or chemical
bonds
• Phototroph - use light as an energy source
• Chemotroph – obtain energy from chemicals
(inorganic)
Video - chemotroph
Heterotrophs
• Animals and other
organisms that
must get energy
from food instead
of sunlight or
inorganic
chemicals
• Depend on
autotrophs to
obtain energy
Why is Photosynthesis important?
Makes organic molecules (glucose) out of
inorganic materials (CO2 and H2O)
It begins all food chains/webs. Thus all life
is supported by this process.
It also makes oxygen gas!!
Plants use glucose as food for energy and as
a building block for growing.
6CO2 + 6H2O + sunlight  C6H12O6 + 6O2
Photosynthesis-starts to ecological food webs!
How do we know that plants make
carbohydrates from just carbon dioxide
water and light energy?
Experiments!
• Ex: Jan Baptisa van
Helmont (1648) planted a
willow branch weighing 5
pounds into 200 pounds of
soil and then after 4 years
the tree weighed 169 lbs.
and the soil was still nearly
200 lbs.
Photosynthesis
sunlight
Carbon dioxide + water
glucose + oxygen
absorbed by chlorophyll
6CO2 + 6H2O + energy  C6H12O6 + 6O2
As can be seen from the equation for photosynthesis, the
wood, bark, and root came from water and carbon
dioxide.
PHOTOSYNTHESIS
• 2 Phases
• Light-dependent reaction (Light Reactions)
• Light-independent reaction (Calvin Cycle)
• Light-dependent: converts light energy into
chemical energy; produces ATP molecules and
NADPH to be used to fuel light-independent reaction
• Light-independent: uses ATP, NADPH, and CO2 to
make simple sugars.
PHOTOSYNTHESIS
• Light-dependent reaction (LIGHT Reaction)
• Requires light
• Occurs in chloroplast- which have 2 membranes
• Thylakoids –system of membranes inside the inner
membrane; arranged as flattened sacs
• Grana – stacks of layered thylakoids
• Stroma – solution surrounding the grana
• membrane
Chloroplasts make the sugars!
Plant Cells
Elodea leaf x400
Chloroplasts
make the
oxygen too!
Plants
Leaves are green
because they
contain
the pigment:
chlorophyll
Leaves have a
large surface area
to absorb as much
light as possible
• Pigments: compounds that absorb different colors of white
light (ROY G BIV) – visible spectrum
Visible light is only a small part of the
electromagnetic spectrum (all forms of light).
When white light strikes an objet, its component colors can
be reflected or absorbed by the pigments in the object
Main pigment:
Chlorophyll a –directly involved
in light reactions
- absorbs less blue light but
more red light than chlorophyll b
Accessory pigments:
Chlorophyll b and Carotenoids –
enable plants to capture more
energy by absorbing colors that
chlorophyll a cannot
-These pigments absorb all
wavelengths (light) BUT green!
Why are plants green? - video
• LIGHT behaves as if it were composed of
"units" or "packets" of energy that travel in
waves. These packets are photons.
• The wavelength of light determines its color.
• Ted Talks – animation
• Photosynthesis - song
PHOTOSYSTEMS
Photosynthesis song
• There are 2 photosystems, photosystem II and
photosystem I.
• They contain chlorophyll a, chlorophyll b, and
carotenoids.
• They are embedded in the thylakoid membrane.
• There is a pair of “chlorophyll a” molecules at
the base of each photosystem that harnesses all
the energy captured by the pigments.
• Their job (chlorophyll a molecules) is to absorb
light energy and pass it on to the electrons.
THE LIGHT REACTIONS
STEP 1:
• Begins when pigments (PSI and PSII) absorb light
• Light energy forces electrons (e-) to enter a higher
energy level in a pair of chlorophyll a molecules of PSII
• “excited” e- leave chlorophyll a (oxidation reaction)
• Each oxidation reaction must be accompanied by a
reduction reaction so something has to accept the e-
THE LIGHT REACTIONS
STEP 2:
• Primary Electron Acceptor – accepts the e- lost from
chlorophyll a
STEP 3:
• Primary Electron Acceptor donates the e- the first
molecules of the ELECTRON TRANSPORT CHAIN
• As e- are passed along the chain, they lose most of the
energy the acquired when they were excited
• This energy is used to move protons (H+) into thylakoid
NADPH
THE LIGHT REACTIONS
STEP 4:
• Light is absorbed into PSI – happens at same
time that light is absorbed into PSII
• e- move from a pair of chlorophyll a molecules
in PSI to another Primary Electron Acceptor
• The e- lost by these chlorophyll a molecules are
replaced by the e- that have passed through the
ETC from PSII
NADPH
THE LIGHT REACTIONS
STEP 5:
• At the end of the second ETC the electrons
will be used to bond NADP+ and H+ to
make NADPH an energy storing compound
used in the Calvin Cycle.
• This happens in the stroma of the
chloroplast.
Replacing Electrons
• A special water splitting enzyme exists at the
base of PSII.
• The water splitting enzyme splits water into
its parts.
• Water (H2O)
• H+: The hydrogen proton will accumulate inside the
thylakoids in the lumen.
• O2: The oxygen will be released by the thylakoids
and it will end up in our atmosphere.
• The electrons from the hydrogen atom will enter at
the base of PSII.
CHEMIOSMOSIS
• the process of the movement of protons down their
concentration gradient for the synthesis of ATP
• concentration of (H+) is great inside the thylakoid (from the
splitting of water and the H+ that were pumped on by the ETC)
• concentration gradient causes the H+ to diffuse through ATP
synthase (an enzyme that helps make ATP by using the energy
from the moving H+)
• That energy bonds ADP with a phosphate to make ATP in the
stroma.
• Some of the H+ flowing into the stroma can be used to make
NADPH.
• Both ATP and NADPH will be used in the Calvin Cycle (The Dark
Reactions)
• In plants and simple animals, waste products are removed
by diffusion. Plants, for example, excrete O2, a product of
photosynthesis.
PHOTOSYNTHESIS
• What affects photosynthesis?
• Light intensity: as light increases, rate of
photosynthesis increases
PHOTOSYNTHESIS
• What affects photosynthesis?
• Carbon Dioxide: As CO2 increases, rate of
photosynthesis increases
PHOTOSYNTHESIS
• What affects photosynthesis?
• Temperature:
• Temperature Low = Rate of photosynthesis low
• Temperature Increases = Rate of photosynthesis increases
• If temperature too hot, rate drops
Stoma
This opening how plants exchange gases!
Can you name the two important gases that go
in and out of the leaves?
Why are the
stomata
located on the
underside of
leaves?
"Thanks for the Glucose!“
LET’S PRACTICE! - game
Calvin Cycle
The Calvin Cycle –carbon fixing
1. 6 CO2 molecules enter the cycle.
2. Enzyme “RuBisCo” combines six 5-carbon (RuBp)
molecules with the carbon from CO2 and forms
them into twelve 3-carbon molecules
3. 12 ATP and 12 NADPH form the twelve 3-carbon
molecules into twelve High-energy 3-carbon
molecules (G3P)
4. 2 (G3P)of the twelve 3-carbon molecules are
combined to form a 6-carbon sugar
5. 6 ATP molecules are used to convert the 10
remaining 3-carbon molecules back into the six 5carbon molecules the cycle began with (RuBp)
Calvin Cycle
Types of Photosynthesis
• C3 Photosynthesis
• C4 Photosynthesis
• CAM Photosynthesis
C3 Photosynthesis : C3 plants
• Called C3 because the CO2 is first incorporated into a 3carbon compound.
• Stomata are open during the day.
• RUBISCO, the enzyme involved in photosynthesis, is
also the enzyme involved in the uptake of CO2.
• Adaptive Value: more efficient than C4 and CAM plants
under cool and moist conditions and under normal light
because requires less machinery (fewer enzymes and no
specialized anatomy).
Most plants are C3.
C4 Photosynthesis : C4 plants
• Called C4 because the CO2 is first incorporated
into a 4-carbon compound.
• Stomata are partially closed during the hottest
parts of the day.
• Have an enzyme that allows CO2 to be taken into
the plant very quickly when CO2 levels are low
and O2 levels are high.
• Photosynthesizes faster than C3 plants.
• Has better water use efficiency because plants do
not need to keep stomata open as much
• C4 plants include corn, sugar cane, and many of
our summer annual plants.
CAM Photosynthesis : Crassulacean Acid Metabolism
• Named after plant family in which it was first found
(Crassulaceae) & because CO2 is stored as an acid before use in
photosynthesis.
• Stomata open at night (evaporation rates are lower) and are closed
during day.
• CO2 is converted to an acid and stored during the night.
• During the day, the acid is broken down and the CO2 is released to
RUBISCO for photosynthesis
• Better water use efficiency than C3 plants under arid conditions
due to opening stomata at night (no sunlight, lower temperatures,
lower wind speeds, etc.)
• include many succulents such as cactuses and agaves and also
some orchids and pineapples
Videos and other resources:
• Carbon Cycle Ted Talks - video
• Bozeman Science – video
• Crash Course –video
• Calvin Cycle game