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Bio 140


on 1 July 2013

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Transcript of Photosynthesis

The sun bathes the earth with sunlight. This light travels to Earth in waves.
Sunlight consists of discrete particles called
As a result of the electrons alling back to the ground state, energy is released in a burst of light and heat.
This energy eventually reaches the
reaction-center complex
Here, a pair of chlorophyll a molecules transfers an excited electron to the
primary electron acceptor
The oxygen is released as a byproduct and electrons from the hydrogen atoms are transferred to the chlorophyll a molecules.

These electrons fill the empty space left by transfer of the excited electrons to the electron acceptor.
Meanwhile, water travels through the roots and up the tree.
Enzymes work to split the water molecules into hydrogen and oxygen atoms.

From the primary electron acceptor of PSII, the electron travels through the

The energy released as the electron falls down the ETC is harnessed by the
a proton pump that pumps the H+ ions (leftover from water) into the thylakoid space from the stroma, thus creating a proton gradient across the thylakoid membrane.

This concentration gradient is used to drive ATP synthesis. The ATP generated will be used in the Calvin Cycle.
Cytochrome Complex
Electron Transport Chain
The electron completes its journey down the ETC and is transferred to PSI.
Another photon of light excites the the electrons of a PSI pigment. The energy travels to the reaction-center complex and the electron is passed on to the primary electron acceptor.
From the primary electron acceptor of PSI, the electrons fall down another electron transport chain to the
protein .
ferredoxin (Fd)
The enzyme catalyzes the transfer of the electrons on ferredoxin to NADP+.
NADP reductase
H+ ions within the stroma are attached, creating
the final product of .
Carbon dioxide enters the leaves through the stomata.
With the help of an enzyme called
, the carbon dioxide molecules
bind to a 5-carbon sugar, .
The unstable 6-carbon product is unstable and quickly breaks down into two 3-carbon sugar molecules of
3-phosphoglycerate (PGA)
The NADPH and ATP generated during the light reactions will be used in The Calvin Cycle.
The Calvin Cycle
The Light Reactions
Using the ATP and NADPH generated through the light reactions, the
PGA molecules are first phosphorylated and then reduced. These reactions yield two molecules of

glyceraldehyde-3-phosphate (G3P)
When several G3Ps have been produced, some combine to make , while others are reused within the cycle.
In order to generate a new glucose molecule, the cycle must continue several times. Each turn of the cycle adds only one carbon atom from each molecule of carbon dioxide.
With the help of ATP, the remaining G3P is phosphorylated and converted back to RuBP, in order act as the carbon dioxide acceptor and the cycle begins again.
The Light Reactions
The Calvin Cycle
The byproducts ADP and NADP+ from the phosphorylation and reduction process are used by the light reactions.
Gist of


The entire process of photosynthesis occurs within chloroplasts.
The , connected sack-like membranes, can
be stacked into columns called .

thylakoid membrane
are .
Photosynthetic pigments
such as ,
absorb light and are housed within the

of the photosystem.

The light reactions occur within the thylakoids.
convert light energy into chemical energy: ATP and NADPH.
ATP provides the energy and NADPH supplies the electrons for
which converts carbon dioxide to sugar. Its byproducts, ADP and NADP+, are recycled through the light reactions.
solar photons
when struck by a photon, pigments within the light-harvesting complex become temporarily excited.
Embedded within the
light-harvesting complexes
Kopidlansky, K. (2007, March 21). Biology made easy: A guide to
Photosynthesis. Retrieved from http://voices.yahoo.com/biology-made-easy-guide-photosynthesis-246120.html?cat=58
Farabee, M. J. (2010, May 18). Photosynthesis. Retrieved from http://
Groleau, R. (01, November 2001). Illuminating photosynthesis.
Retrieved from http://www.pbs.org/wgbh/nova/nature/photosynthesis.html
Kyrk, J. (2012, January 18). Photosynthesis. Retrieved from http://
How the calvin cycle works. (2006). Retrieved from http://
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