Photosynthesis
Energy is the ability to do work
• Living things depend on energy.
• Organisms that make their own food =
autotrophs
• Plants and some other organisms are able to
use light energy from the sun to make food
• Organisms that cannot make their own food =
heterotrophs
Chemical Energy
• Adenosine triphosphate (ATP) – chemical
compound used to store and release energy
ATP
• ATP consists of adenine (A), ribose (a 5-carbon
sugar, remember RNA?), and three phosphate
groups.
• The 3 phosphate groups hold high energy
bonds
ADP
• ADP is adenosine diphosphate – meaning it
has two phosphate groups instead of three
• Cells can add a phosphate group to ADP to
make ATP to store energy.
– ATP is like a full battery. ADP is like a battery that
needs recharging
Releasing energy
• The energy stored in ATP is used by breaking
the chemical bonds between the second and
third phosphate groups.
Why ATP is important
• Powers cell membrane sodium-potassium
pump – vital for nervous system connections
• Powers movement of organelles
• Powers synthesis of proteins and nucleic acids
• Powers responses to chemical signals
– Firefly light (chemical = luciferin) is powered by
ATP in a process called bioluminescence
Firefly luciferin
Photosynthesis
Photosynthesis – plants use the energy of
sunlight to convert water and carbon dioxide
into high energy carbohydrates (sugar and
starch) and oxygen (as waste)
Investigating Photosynthesis
• Where does the mass of a tree come from?
• Trees start as seedlings and grow into trees
– Metasequoia (Dawn Redwood)
Jan van Helmont’s Experiment
• 1643 – Belgian physician – “Where does mass
of tree come from?”
• Watered plant for five years
• Seedling grew into tree – over 75 kg (~200 lb)
• Mass of soil didn’t change over five years
• Concluded mass was from water
• Water = “hydrate” part of carbohydrate, what
about “carbo-”?
Joseph Priestley’s Experiment
• 1771 – Joseph Priestley, English minister
• Lit candle, placed jar over candle, watched
candle go out.
• Concluded oxygen was needed for fire
• Placed sprig of mint plant in jar
• After a few days, the candle could be relighted
and would stay lit for a while
• Concluded plants produce oxygen
Jan Ingenhousz’ Experiment
• 1779 – Ingenhousz, Dutch scientist
• Found that plants only produce oxygen when
exposed to light
• Concludes plants need sunlight to produce
oxygen
The Photosynthesis Equation
Carbon dioxide + Water + Light  Sugar + Oxygen
6CO2
+ H2O + Light  C6H12O6 + O2
• Photosynthesis uses the energy of sunlight to convert water
and carbon dioxide into high-energy sugars and oxygen
Light and Pigments
• In addition to water
and carbon dioxide,
photosynthesis
requires light and
chlorophyll, which is
a molecule in
chloroplasts, an
organelle in plant
cells.
Light and Pigments
• Energy from the sun
travels to Earth in the
form of light.
• Light is of various
wavelengths.
• Your eyes see the
wavelengths as
different colors.
Light and Pigments
• Plants gather the sun’s
energy with light
absorbing molecules
called pigments.
• Main pigment is
chlorophyll – two
types chlorophyll a
and chlorophyll b.
Light and Pigments
• Chlorophyll readily absorbs
light in the red and blueviolet regions of the
spectrum.
• Chlorophyll does not
absorb light well in the
green region – thus the
leaves of plants appear
green.
• Because light is energy, the
plants are absorbing the
energy from that light.
Inside a Chloroplast
• Photosynthesis takes place inside chloroplasts
• Contain saclike photosynthetic membranes
called thylakoids.
• Thylakoids are arranged in stacks called grana.
Inside a Chloroplast
• Chlorophyll and other pigments in the
thylakoid membrane are organized into
photosystems.
• The region outside the thylakoid membrane is
called the stroma.
• Two reactions in photosynthesis:
– Light-dependent: inside thylakoid membrane
– Calvin cycle (Light-independent/dark): in stroma
Chloroplast
Electron Carriers
• When sunlight strikes a leaf, it ‘excites’
electrons, meaning that they gain energy.
• Plant cells use electron-carriers to move these
high energy electrons (think of hot coals) –
process called electron transport.
Electron Carriers
• NADP+ (Nicotinamide adenine dinucleotide
phosphate) is an electron carrier.
• NADP+ accepts and holds 2 high-energy
electrons along with a hydrogen ion (H+)
• When it is holding these, it is converted to
NADPH. Now it can carry these electrons to
chemical reactions elsewhere in cell
Light-Dependent Reactions
• Light-dependent reactions require light.
• This is why plants need light to grow.
• The light-dependent reactions of plant cells
produce oxygen as a gas and convert ADP and
NADP+ into the energy carriers ATP and NADPH
Light-Dependent Reactions
• Step 1: Pigments in photosystem II (discovered
second, hence name) absorb light
– High energy electrons passed on to electron transport
chain (ETC)
– Thylakoids break up water molecules to use 2 H+ ions and
oxygen is released – source of O2 we breathe
• Step 2: Electrons move through ETC from
photosystem II to photosystem I
– H+ ions moved from stroma to inner thylakoid
• Step 3: Pigments in photosystem I use energy from
light to energize electrons. NADP+ picks these up
and H+ ions to become NAPH
Light-Dependent Reactions (Cont.)
• Step 4: As electron pass from chlorophyll to
NAP+, H+ ions are pumped across membrane,
providing energy to make ATP.
• Step 5: An enzyme, ATP Synthase, helps H+
ions cross the membrane
– ATP Synthase binds ADP and a phosphate group to
produce ATP
Confused yet?
The Calvin Cycle (Light-Independent
Reactions)
• The ATP and NADPH formed by the lightdependent reactions have chemical energy
but not enough to sustain plant.
• During the Calvin Cycle, plants use that energy
to make energy that can be stored = highenergy sugars
Calvin Cycle (Light-Independent
Reactions)
• Step 1: 6 Carbon dioxide molecules enter cycle
from atmosphere. Combine with six 5-carbon
molecules = 12 3-carbon molecules
• Step 2: 12 3-carbon molecules converted into
high-energy forms. Energy to do this comes from
ATP and NAPH produced earlier
• Step 3: Two 3-carbon molecules removed from
cycles to produce sugars for metabolism and
growth in the form of sugars, lipids, and proteins.
• Step 4: Remaining carbon molecules are recycled
for later Calvin Cycles.
Factors affecting
Photosynthesis
• Water
– Plant adaptation –
plants in dry
conditions have a
waxy coating on
leaves to prevent
water loss
• Temperature
• Intensity of Light
– Maximum rate
– Conifers in Winter
may only occasionally
carry out
photosynthesis