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PHOTOSYNTHESIS
9th grade – Biology
Miss Alexandra Martínez
GCI 2012-2013
Using the Energy in Sunlight
• Plants, algae and some bacteria capture
about 1% of the energy in sunlight that reaches
Earth and convert it to “chemical energy” through
the process of PHOTOSYNTHESIS
• Directly or indirectly, photosynthesis nourishes (feeds) almost the
entire world.
• Photosynthesis happen in 3 stages
Three Stages of Photosynthesis
• STAGE 1: Energy is captured from sunlight
• STAGE 2: Light energy is converted to chemical
energy which is temporarily stored in ATP and
NADPH
• STAGE 3: The chemical energy stored in ATP and NADPH powers
the formation of organic compounds, using carbon dioxide. It is
called the Carbon Fixation Stage
Stage 1: Absorption of Light
Energy
• Plants absorb energy from the sun by the use
of pigments (light absorbing substances)
• Pigments absorb only certain wavelengths of light
and reflect all the others.
• Plants use chlorophyll (a & b), which absorb red and blue, and
reflect green and yellow
• Plants use carotenoids, which absorb green and blue, and reflect
yellow, orange, red.
• Having both pigments enables plants to absorb more light energy
Stage 1: Absorption of Light
Energy
• Photosynthesis occurs in chloroplasts, where
clusters of pigments are embedded in the membrane of disk-shaped structures called Thylakoids.
• When light strikes a Thylakoid, energy is transferred to electrons in
chlorophyll causing electrons to be “energized” and be able to
capture the energy of sunlight
• These “energized” electrons jump to the thylakoid membrane where
they are used to power the second stage of photosynthesis
• When electrons hit the thylakoid membrane, an enzyme splits water
molecules, leaving H+, and the remaining oxygen atoms combine to
form oxygen gas, which leaves the membrane as a waste product.
Stage 2: Conversion of Light
Energy
• Excited electrons that leave chlorophyll are
used to produce new molecules, including ATP
and NADPH.
• The electrons are passed through a series of molecules along the
thylakoid membrane, which is the Electron Transport Chain
• Excited electrons loose some of their energy as they pass through
this chain, through the enzymes embedded there.
• The energy lost by the electrons is used to pump H+ inside the
thylakoid membrane. They help in the breaking down of water
molecules, keeping the H+ inside and generating a concentration
gradient, so they have a tendency to diffuse back out.
Stage 2: Conversion of Light
Energy
• These H+ are pumped out the membrane
through a specialized protein called ATP Synthase,
to produce ATP.
ADP + H+
ATP
• Other H+ are used to produce NADPH, which provides
high energy electrons needed to make carbon-hydrogen bonds in
the 3rd stage.
NADP+ + H+
NADPH
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SUMMARY
1. Pigments in the thylakoid of chloroplasts
absorb light energy
2. Electrons are excited and move through the electron transport
chain
3. Water molecules breakdown by an enzyme creating oxygen gas as
a waste product and H+
4. The H+ accumulate inside the thylakoid, setting a concentration
gradient that provide energy to make ATP and NADPH