3/27/2015
ENERGY
Energy is the ability to do work.
 All living things depend on energy.
 Energy comes in the form of light, heat,
electricity, or sound.
 Energy can be stored in chemical
compounds.
CHAPTER 8 PHOTOSYNTHESIS
ATP

ATP
ATP (adenosine triphosphate) is the energy
currency of cells.





Used to store and release energy in cells.
Consists of adenine (nucleotide), ribose
(sugar/carbohydrate), and three phosphate
groups.
Release of energy – by breaking the chemical
bonds to convert ATP into ADP.
Storage of energy – by creating a chemical bond
between ADP and P to create ATP.


Autotrophs and Heterotrophs

Autotrophs are organisms that make their own
food.




Unable to directly use the sun’s energy.


Must consume other organisms either by ingesting
(eating) them or decomposing them.
Animals are examples of heterotrophs.
Not good for long-term energy storage (use glucose
instead).
The Photosynthesis Equation

Use light energy from the sun to produce food
Plants are examples of autotrophs.
Heterotrophs obtain energy from the food they
consume.
Energy used for active transport, protein
synthesis, homeostasis, response to chemical
signals, bioluminescence, etc.
Cells only have a small amount of ATP.


Photosynthesis is the process whereby plants
use the energy of sunlight to convert water and
carbon dioxide into oxygen and high-energy
carbohydrates.
Carbon dioxide +water  sugar and oxygen
In addition to water and carbon dioxide,
photosynthesis requires light and chlorophyll, a
molecule in chloroplasts.
1
3/27/2015
The Photosynthesis Equation
1. LIGHT

Light travels to the Earth in the form of
sunlight

3 Requirements for Photosynthesis:
1. Sunlight
2. Pigments
3. Energy storing compounds

We perceive sunlight as white light, but is really a
mixture of many different wavelengths of light
Wavelengths of light that are visible to us are
known as the visible light spectrum
2. PIGMENTS

Pigments are light absorbing molecules that
help plants gather the sun’s energy
The main pigment found in plants is chlorophyll



Chlorophyll absorbs red and blue wavelengths of light,
but it reflects green making the plant appear green.
When the pigments absorb light they are also
absorbing the energy in that light, producing more
energy for the cell.

Other pigments are present too (they reflect other colors)
– think fall leaves.
INSIDE A CHLOROPLAST


Photosynthesis takes place in chloroplasts.
Chloroplasts contain saclike photosynthetic
membranes called thylakoids.




Thylakoids contain clusters of chlorophyll and other
pigments and proteins known as photosystems that are
able to capture sunlight.
Light dependent reactions take place here.
Thylakoids are arranged in stacks known as grana.
The region outside the thylakoid membrane is
called the stroma.

Light independent (dark) reactions take place here.
2
3/27/2015
3. Energy Storing Compounds

aka Electron carriers – compounds that can accept
a pair of high energy electrons and transfer them
(and their energy) to other molecules.

Occurs in 2 ways:




Light and Dark Rxns

Used to trap high energy electrons into chemical bonds.
1. Electron carrier NADP+ accepts a pair of high energy
electrons and an H+ and gets converted to NADPH.
2. AMP is converted to ADP which is then converted to
ATP.
Light-dependent reactions produce oxygen and
make energy storing compounds (ATP and
NADPH).



Light-independent (dark) reactions use the energy
stored in NADPH and ATP to make glucose.

NOTE: The energy stored in these molecules is
released by breaking chemical bonds to generate
things the cell needs, like glucose!
Occurs by converting ADP and NADP+ into ATP and
NADPH.
These reactions REQUIRE light.

Glucose is more stable and can store up to 100 times
more energy than NADPH and ATP.
These reactions do not require light.
Light Dependent Rxns

Photosynthetic membranes of chloroplast
(thylakoids) contain chlorophyll.


1. Light Rxns - Light absorption

Green plants contain photosystems.

This is where the light reactions occur.
The light reactions are divided into 4 processes:





1. Light absorption
2. Electron transport
3. Oxygen production
4. ATP production
1. Light Rxns - Light absorption
2. Light Rxns - electron transport


Photosystem II


Electrons in pigments absorb light – start off
photosynthesis.
High energy electrons then passed to electron
transport chain.
Clusters of pigment molecules that absorb
energy from sunlight.
High energy electrons move through the
photosystems and are then released to
electron carriers.
During electron transport, high energy
electrons are passed along electron carriers
in the photosynthetic membrane.




These carriers are called the electron transport
chain.
As the electrons pass, their energy is used by
proteins to pump H+ ions from the stroma into the
thylakoid space.
Energy and H+ used to convert NADP+ into
NADPH.
At the end of the chain, electrons get transferred
to photosystem I.
3
3/27/2015
4. Light Rxns - NADPH
production
3. Light Rxns - Oxygen Production

Electrons are getting used up by chlorophyll
and must be replaced!



This occurs by taking electrons from water to
replace those used by chlorophyll.
Enzymes break water into 2 electrons, 2 H+, and
1 oxygen atom.


Because hydrogen (H+) ions were released
inside the thylakoid membrane as a product of
the splitting of water molecules, the inside of
the membrane becomes positively charged,
while the outside is negatively charged.

This difference in charges creates a gradient that
provides the energy to make ATP from ADP.


At the send of a short electron transport chain,
NADP+ in the stroma uses high energy electrons
and H+ ions to become NADPH.
This is the source of nearly all oxygen on Earth – we
NEED it!
5. Light Rxns - ATP production

Photosystem I – pigments here use energy
from light to re-energize the electrons they
receive.
ATP synthase – enzyme in thylakoid membrane that
binds a P to ADP to create ATP.
Chemiosmosis – light dependent reactions produce
NADPH and ATP
Light Rxns - SUMMARY


Light reactions summary
The light reactions USE: water, light energy,
chlorophyll pigments
The light reactions PRODUCE: oxygen,
NAPDH, ATP
Dark Rxns -The Calvin Cycle




Dark reactions = light independent reactions =
Calvin cycle
ATP and NADPH can hold large amounts of
chemical energy, but only for a few minutes.
The Calvin Cycle uses CO2 as well as ATP
and NADPH from the light-dependent
reactions to produce glucose that can be
stored in the plant for long periods of time.
This process does not require light, but often
takes place while the sun is shining.
4
3/27/2015
Calvin Cycle


Carbon dioxide molecules enter the cycle
from the atmosphere.
The carbon dioxide molecules combine with
5-carbon molecules already in the chloroplast.


This reaction is catalyzed by the enzyme rubisco.
Calvin Cycle



The result is 3-carbon molecules.
The energy from breaking ATP into ADP and
NADPH into NADP+ is used to convert the 3carbon molecules into PGAL.
Most PGAL is recycled.
1 of 6 PGAL molecules formed is used to
make glucose.


3 factors affect photosynthesis
Calvin Cycle - SUMMARY




Calvin Cycle USES: NADPH, ATP ,CO2
Calvin cycle PRODUCES: glucose (C6H12O6)
Plants use glucose for energy.
Organisms that eat plants indirectly also use this
energy from glucose.



1. Temperature – enzymes that control the
reactions of photosynthesis work best at 0-35
degrees C.
2. Light intensity – higher intensity is better.
3. Light wavelength – red, blue work best
(green is the worst).
4. Water – a raw material; shortage slows or
stops photosynthesis and damages plant
tissues.
5. Enzymes – necessary for the process!
Photosynthesis in extreme
conditions



C4 and CAM plants have biochemical adaptations that minimize water
loss while still allowing photosynthesis to take place in intense sunlight.
C4 plants - have a specialized chemical pathway that allows them to
capture even very low levels of carbon dioxide and pass it to the Calvin
cycle; this enables photosynthesis to keep working under intense light
and high temperatures
 Requires extra energy in the form of ATP to function.
 Examples: corn, sugar cane, and sorghum.
CAM plants - admit air into their leaves only at night. In the cool darkness,
carbon dioxide is combined with existing molecules to produce organic
acids, “trapping” the carbon within the leaves. During the daytime, when
leaves are tightly sealed to prevent the loss of water, these compounds
release carbon dioxide, enabling carbohydrate production.
 Examples: pineapple trees, many desert cacti
5