Unit III: Evolution & Energy
Module VII: Getting energy into food
Photosynthesis
I.
Introduction
Sooooo . . . . . .how exactly do plants trap that sunlight and store it in
sugar molecules???
Recall that, in cellular respiration, energy is extracted from glucose by
removing high-energy electrons & then sending then down the
electron transport chain to rebuild ATPs.
(ADP +P + energy  ATP)
Thus it would seem logical for plants to use sunlight to convert low
energy electrons to high-energy electrons. Then they could combine
the high-energy electrons (in H atoms – again!) with, hmmm, let’s say
carbon dioxide, to build a sugar such as glucose (C6H12O6)
II. Overview
Scientists like to divide photosynthesis into two parts:
 (The plant doesn’t really care!!) 
1. Light reactions Sunlight is trapped & put in ”temporary storage”
(Require light)
2. Dark reactions  “Trapped sunlight is combined with CO2 to produce – you
guessed it! GLUCOSE!!
(Don’t require light; but don’t need dark either!!)
Recall that in cellular respiration, high-energy electrons were
transported from the Krebs cycle to the ETC by the carrier molecule
NADH. In photosynthesis, high-energy electrons hitch a ride (from the
light reactions to the dark reactions) in a different taxi, NADPH.
But let’s get to know our host organisms a little better before we
plunge into the details of photosynthesis. .
III. Plant Anatomy
In unit IV, (Yea! The end is nearing!) we will study the plant kingdom in
detail. Here we will only examine the parts of a higher plant in order to
better understand photosynthesis:
Photosynthesis occurs in green plants, but where???
Most plants have three non-reproductive organs: roots, stem, and
leaves.
Click on the plant structure (above) which is the plant’s main organ of
photosynthesis.
* (Important! Do not skip this step!) *
IV.
Highlights of Photosynthesis
You may recall from your physics course that electrons can sometimes
absorb photons of light and “jump” to a higher energy level:
http://www.lightlink.com/sergey/java/java/atomphoton/index.html
As you just observed, the photon is usually re-emitted after a short time
and the electron returns to its ground state.
However, if an electron acceptor is nearby, the excited electron can be
transferred to this new molecule. Then the energy of the photon remains
with the excited electron.
I.
Light reactions:
Occur on membrane surface of Thylakoid disk in chloroplast
Trap radiant energy (light) and temporarily store in
molecules of NADPH & ATP
In the light reactions . . ..
1) chlorophyll and its accessory pigments act as solar
collectors and absorb photons of sunlight
2) The photons excite (energize) electrons of chlorophyll.
3) An electron acceptor NADP steals the excited electrons;
now it becomes NADPH:
NADP + excited electron  NADPH
4) NADPH is the “taxi cab” which carried the excited
electron (H atom) to the dark reaction in the stroma to
make some GLUCOSE!! (Remember, you already learned that
the energy of glucose was stored in the high energy electrons
associated with its H atoms!!)
Now let’s watch a cartoon of the light reactions!! (OH BOY!)
Illustration: http://www.biology.com/learning/photo/concepII.html
(Be sure to wait for the flash of light to start the process; it takes a
few seconds!)
5) Some of the trapped sunlight also stored in molecules of
ATP
6) Both the ATP and NADPH molecules will carry the
trapped sunlight to the Dark Reactions . . ..
BUT WAIT!!!!!
II.
Dark Reactions (Calvin Cycle)