A. Chemical Energy and ATP
*Energy is the ability to do work.
All life functions depend on energy
to fuel them.
-There are many forms of energy :
light, heat, electricity, etc.
-Energy can be stored in chemical
bonds and then released by living
things as chemical fuel for life
sustaining functions. One of the
most important compounds that
cells use to store and release
energy is adenosine triphosphate
(ATP) – the basic energy source
used by all types of cells.
• Adenine
• A 5-carbon sugar called ribose
• Three phosphate groups, which
are the key to ATP’s ability to
store and release energy.
*Adenosine diphosphate
(ADP) is used in this
process. When a cell has
energy available, it can
store small amounts of it
by adding phosphate
groups to ADP molecules
to produce ATP.
-ADP is like a
rechargeable battery that
powers the machinery of
the cell.
*Cells can release energy
stored in ATP by the controlled
breaking of the chemical bonds
between the second and third
phosphate groups.
-Cells can add or subtract these
phosphate groups as needed to
either store or release energy.
-ATP can easily release and
store energy by breaking and
re-forming the bonds between
its phosphate groups.
-Cells use energy provided by ATP to :
• Carry out active transport
(Endo/exocytosis)
• Power movement : muscle
contraction, cilia, flagella, etc.
• Synthesize (make) proteins,
respond to chemical signals at the
cell surface, or produce light.
A. Chlorophyll and Chloroplasts
*Energy from the sun travels to the Earth as
light. Plants gather the sun’s energy with
light-absorbing pigments, ex : chlorophyll
inside chloroplasts.
Chlorophyll – the principal pigment of plants
and other photosynthetic organisms.
-Chlorophyll in plants comes in different kinds :
chlorophyll a and chlorophyll b. Both absorb
light well in the blue-violet and red regions of
the visible spectrum but not in the green
region. Leaves reflect the green light until
temperatures drop late in the year and the
chlorophyll molecules break down.
A. Chlorophyll and Chloroplasts
-As chlorophyll absorbs light, a large fraction of that light
energy is transferred directly to electrons in the
chlorophyll molecule itself. By raising the energy levels of
these electrons, light energy can produce a steady supply
of high-energy electrons to make photosynthesis work.
-Photosynthesis occurs inside of chloroplasts.
Chloroplasts have two major parts :
Thylakoids – saclike photosynthetic membranes that are
interconnected and arranged in stacks known as grana.
-Pigments (ex : chlorophyll) are located in the thylakoid
membranes.
Stroma – the fluid portion of the chloroplast outside of the
thylakoids.
*High-energy electrons made by chlorophyll need
special electron “carriers” to transport them to
other molecules.
Electron carrier – A compound that can accept a
pair of high-energy electrons and transfer them,
along with most of their energy, to another
molecule, ex : NADP+.
NADP+ (nicotinamide adenine dinucleotide
phosphate) – carrier molecule that transfers highenergy electrons from chlorophyll to other
molecules. It accepts and holds two high-energy
electrons and a hydrogen to become NADPH.
*Conversion of NADP+ into NADPH traps energy
from the sun in chemical form.
(Light energy)
6CO2 + 6H2O
(Reactants)
C6H12O6 + 6O2
(Products)
(Light energy)
Carbon dioxide + water
sugar + oxygen
Recall : Photosynthesis uses the energy of sunlight to
convert water and carbon dioxide (reactants) into
high-energy sugars and oxygen (products).
-Plants use the sugars to make complex carbohydrates
such as starches, and to provide energy for the
synthesis of other compounds, including proteins and
lipids.
*Requires the direct involvement of light and
light-absorbing pigments. Light-dependent
reactions use energy from sunlight to
produce energy-rich compounds such as ATP.
-This happens inside the thylakoid
membranes of the chloroplast. Water is
required in these reactions as a source of
electrons and hydrogen ions and oxygen is
released as a byproduct.
Light Reactions :
-Takes in H2O and light
-Gives off O2
-ADP and NADP+ are converted into ATP and
NADPH to power the Calvin cycle
*No light is required to power the light-independent
reactions (Calvin cycle). These reactions take place
outside the thylakoids in the stroma.
-Plants absorb carbon dioxide from the atmosphere
and complete photosynthesis by producing carboncontaining sugars and other carbohydrates. Carbon
fixation occurs here.
-During the light-independent reactions, ATP and
NADPH molecules produced from the lightdependent reactions are used to produce highenergy sugars from carbon dioxide.
Calvin Cycle :
-Carbon fixation occurs (uses CO2 to make glucose)
-6 molecules of carbon dioxide are used to make a
single 6-carbon sugar molecule.
• Temperature – enzymes help
photosynthesis occur. Too high or too
low and they don’t work. Cold
temperatures can cause
photosynthesis to stop entirely.
• Light intensity – high light intensity
increases the rate of photosynthesis
but it will eventually reach a maximum
rate of photosynthesis.
• Availability of water – shortage of
water can slow or stop photosynthesis.
Water loss can also lead to damage of
plant tissues.