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Photosynthesis
What is it?
What is an electron transport chain?
What is the Calvin Cycle?
What is Glycolysis?
Photosynthesis
• Plant photosynthesis occurs in
leaves and green stems within
specialized cell structures called
chloroplasts.
• One plant leaf is composed of tens
of thousands of cells, and each cell
contains 40 to 50 chloroplasts.
Light Dependent Reactions
• Photosynthesis is a very complex
process, and for the sake of
convenience and ease of understanding,
plant biologists divide it into two stages.
• In the first stage, the light-dependent
reaction, the chloroplast traps light
energy and converts it into chemical
energy contained in nicotinamide
adenine dinucleotide phosphate
(NADPH) and adenosine
triphosphate(ATP), two molecules used
in the second stage of photosynthesis.
Light-independent Reactions
• In the second stage, called the
light-independent reaction
(formerly called the dark reaction),
NADPH provides the hydrogen
atoms that help form glucose, and
ATP provides the energy for this
and other reactions used to
synthesize glucose.
Electron Transport
• Photosynthesis relies on flows of energy
and electrons initiated by light energy.
• Light energy causes the electrons in
chlorophyll and other light-trapping
pigments to boost up and out of their
orbit; the electrons instantly fall back
into place, releasing resonance energy,
or vibrating energy, as they go, all in
millionths of a second.
• Chlorophyll and the other pigments
are clustered next to one another
in the photosystems, and the
vibrating energy passes rapidly
from one chlorophyll or pigment
molecule to the next, like the
transfer of energy in billiard balls.
• Photosynthesis begins when light
strikes Photosystem I pigments
and excites their electrons. The
energy passes rapidly from
molecule to molecule until it
reaches a special chlorophyll
molecule
In Summary
• The transfer of electrons in a step-bystep fashion in Photosystems I and II
releases energy and heat slowly, thus
protecting the chloroplast and cell from
a harmful temperature increase.
• It also provides time for the plant to
form NADPH and ATP. In the words of
American biochemist and Nobel laureate
Albert Szent-Gyorgyi, “What drives life
is thus a little electric current, set up by
the sunshine.”
Light-independent Reactions
• The chemical energy required for
the light-independent reaction is
supplied by the ATP and NADPH
molecules produced in the lightdependent reaction.
• The light-independent reaction is
cyclic, that is, it begins with a
molecule that must be regenerated
at the end of the reaction in order
Calvin Cycle
• The light-independent reactions
use the electrons and hydrogen
ions associated with NADPH and
the phosphorus associated with
ATP to produce glucose.
• These reactions occur in the
stroma, the fluid in the chloroplast
surrounding the thylakoids, and
each step is controlled by a
Cellular Respiration
Glycolysis
• process in which cells produce the
energy they need to survive. In cellular
respiration, cells use oxygen to break
down the sugar glucose and store its
energy in molecules of adenosine
triphosphate (ATP).
• Cellular respiration is critical for the
survival of most organisms because the
energy in glucose cannot be used by
cells until it is stored in ATP. Cells use
ATP to power virtually all of their
activities—to grow, divide, replace worn
out cell parts, and execute many other
tasks.