The Need for energy
Cell Energy
Energy is essential to life. All living organisms
must be able to obtain energy from the
environment in which they live.
Plants and other green organisms are able to
trap the light energy in sunlight and store it in
the bonds of certain molecules for later use.

Work and the need for energy
• Several processes that require energy.
Active transport, cell division, movement of
flagella or cilia, and the production, transport,
and storage of proteins.
Energy is stored in the chemical bonds of that
molecule and can be used quickly and easily by
cells.

• The name of this energy molecule is
adenosine triphosphate, or ATP for short.
• ATP is composed of an adenosine molecule
with 3 phosphate groups attached.

Forming and breaking down ATP
• Adenosine with 1 phosphate = AMP
• Adenosine with 2 phosphate= ADP
• Adenosine with 3 phosphate = ATP
Adding phosphate require energy needing the
least and 3 the most.
Energy is stored in the bonds.
When this bond is broken energy is released.

• When the chemical bond between the second
and third phosphate groups in ATP is broken,
energy is released and the resulting molecule
is ADP.
• ADP can form ATP again by bonding with
another phosphate group.
• ADP can be used as energy also but will not
give off as much energy as ATP.

Photosynthesis: Trapping the sun’s
energy
• The cells of green organisms must trap light
energy and store it in a manner that is readily
usable by cell organelles- in the chemical
bonds of ATP.
• The process that uses the sunlight to make
simple sugar is called photosynthesis. These
simple sugars are then converted into complex
carbohydrates, such as starches, which store
energy.

Photosynthesis happens in two phases
1. light-dependent reactions-convert light
energy into chemical energy (ATP)
2. Light-independent reactions- use the ATP to
produce simple sugars.
Equation for photosynthesis
6CO2 + 6H2O  C6H12O6 + 6O2
productsreactants

Chloroplast and pigments
• Photosynthesis takes place in the membranes
of the thylakoid discs in chloroplasts.
• To trap the energy in the sun’s light, the
thylakoid membranes contain pigments,
molecules that absorb specific wavelengths of
the sunlight.
• Most common pigment is chlorophyll which
absorbs most wavelengths of light except
green.

• Since it chlorophyll can not absorb the
wavelength for green it is reflected, giving
leaves a green appearance.
• In the fall the, trees stop producing
chlorophyll in their leaves. Other pigments
become visible, giving leaves a wide variety of
colors.

Light-dependent reactions
• The first phase of photosynthesis requires
sunlight.
1. Sunlight strikes chlorophyll molecules in the
thylakoid membrane, the energy in the light
is transferred to electrons. (electrons become
excited or highly energized)
2. Electrons are passed from chlorophyll to an
electron transport chain, a series of proteins
embedded in the thylakoid membrane.

Light-dependent reactions
• capture energy in sunlight and transfer it
• - take place in thylakoid membrane of a
chloroplast
• - chlorophyll absorbs energy from sunlight
• - water is broken down (into H+ ions,
electrons, and oxygen)

Light-dependent reactions
• - oxygen is released as a waste product
• - NADPH is formed (functions like ATP =
energy) when electrons are added to NADP+
• - energy is transferred to make ATP (when H+
ions diffuse)
• - overall, oxygen is given off as a waste
product, NADPH and ATP are formed

Light-Independent reactions
The second phase of photosynthesis does not
require light. It is called the Calvin Cycle, which
is a series of reactions that use carbon dioxide to
form sugars.
Calvin cycle is named after Melvin Calvin.

Light-Independent reaction
• energy (NADPH and ATP) from light reactions
make sugars
• - occurs in stroma of chloroplast
• - does not need sunlight
• - carbon dioxide is needed
• - a simple sugar, glucose, is formed from
carbon dioxide and energy from ATP and
NADPH

Light independent reactions
• - overall, glucose, NADP+, and
ADP are created.
• NADP+ and ADP go back to the
light dependent reactions

Light-Independent