Ch. 6 Where it all begins – PHOTOSYNTHESIS
Yesterday we discussed how all energy conversions in
biological systems begin with the sun!!
Harnessing the energy from sunlight to do work is
complicated, we would love to learn how to do it in an
economically sustainable way. But so far are temps have only
been moderately successful.
 Plants convert light energy into chemical energy.
 Most of the energy that reaches Earth’s surface is in
the form of visible light.
 Visible light is a small part of the electromagnetic
energy spectrum.
 Visible light travels in wavelengths between 380 and 750
nm, we see this as white light.
 Light is also organized in packets of energy called
photons.
 The longer the wavelength. The lower the amount of
energy. The shorter the wavelength, the higher the
amount of energy.
 Wavelengths shorter than 380 fall into the UV spectrum
and have enough energy to break chemical bonds of DNA
and other important molecules.
PIGMENTS
 Pigments are organic molecules which capture light
energy.
 Chlorophyll A is the most common pigment, found in
plants, protists, and bacteria. It absorbs violet and red
light, so it appears green to us.
 Accessory pigments – work together with chlorophyll a
to capture a wide range of wavelengths. They have
antioxidant properties to help protect plants from UV
light.
 They attract pollinators
 example –a tomato’s color changes from green to red
because it’s chlorophyll containing chloroplasts develop
into lycopene containing - chromoplasts
 In plants Chlorophylls are so abundantly mask the colors
of the other pigments, they break down faster in the
fall, allowing accessory pigments to be seen.
How do they trap the energy!?!
 The light capturing capability comes from a section of
atoms in which single bonds alternate with double bonds,
allowing for easy absorption of photons.
 sEach pigment is specialized for receiving light energy of
only certain wavelengths, REMEMBER-absorbing photon
excites electrons boosting them to a higher energy level,
they return quickly to a lower energy level by emitting
their extra energy.
 When the energy reaches a special pair of chlorophyll
molecules photosynthesis begins.
?? Where does photosynthesis occur??
?? What is an electron acceptor??
?? What replaces “lost” electrons??
?? What is some evidence that photosynthesis is taking
place??
?? What is the overall formula for photosynthesis??
Photosynthesis is divided into two main parts the light
dependent reaction [light reaction] and the light independent
reaction [dark reaction]
Light Dependent Reaction-occurs in the thylakoid membrane
within the chloroplasts, it generates ATP and NDPH to
provide energy for the light independent reaction, composed
of photosystem I and photosystem II
photosystem II
 Light energy in the form of photons excites a special
chlorophyll A molecule
 the electrons are passed through the thylakoid
membrane proteins and pigments [called the electron
transfer chain] which creates an electric-like current of
energy.
 This energy powers hydrogen ion membrane pumps which
move hydrogen ions from the stroma into the thylakoid
space, building up a concentration gradient. [Potential
energy]
 as the electrons reach the end of photosystem II they
have reached a lower energy level
 water molecules are split to yield 2 electrons, 2 hydrogen
ions and an oxygen. [This process is called photolysis and
photosystem II is the only biological systems strong
enough to oxidize water]
photosystem I
 the electrons are re-excited by the special chlorophyll a
molecule that begins photosystem I
 the electrons continue down the electron transfer chain
[ETC] and are sent to the stroma to meet up with the
electron acceptor NADP+
Chemiosmosis
 at the same time hydrogen ions will move through the ATP
synthase molecule causing it to spin, releasing hydrogen
ions into the stroma which will bond to NADP+ and form
NADPH
 the movement of the ATP synthase molecule increases
energy and allows ADP to bond with a phosphate generating
ATP
****now we have the energy to build glucose molecules****
There are two forms of the light reaction the non-cyclic
pathway just described and a cyclic pathway in which electrons
lost from photosystem I are cycled back to it. No NADPH is
formed and no oxygen is released. In both reactions hydrogen
ions are still pumped across the equipment. So ATP synthesis
still occurs. Plants use both processes. Some photosynthetic
bacteria can use either one process or the other using their
plasma membrane instead of a thylakoid membrane
photosynthetic protists also use both types.
http://www.youtube.com/watch?v=joZ1EsA5_NY Light
Reaction
http://www.youtube.com/watch?v=slm6D2VEXYs Dark
Reaction
Calvin-Benson Cycle
Light Independent Reaction [dark reaction]-uses carbon
dioxide to create glucose and other carbohydrate
molecules. Also called carbon fixation and the CalvinBenson cycle.
During Carbonfixation (Calvin Cycle) CO2 combines with
RuBP (5 carbon chain) with help from enzyme RuBisco to
create 6C chain which splits in half producing 2-3C chains
(PGAL) some for the PGAL will combine from Glucose 6C
chains while the rest recombines to start the cycle againATP & NADPH are used up in the process of making and
breaking bonds.
Alternative pathways
Several adaptations have evolved to conserve water and
allow photosynthesis to occur, some evolved a cuticle which
is a waxy coating, but it prevented gas exchange. So
stomata evolved which are little pore-like openings on the
bottoms of leaves, which can open and close.
C3 plants are the plants we reconsider normal method of
photosynthesis.
They close their stomata at night, but then oxygen cannot
escape, and it interferes with the production of sugar.
Both oxygen and carbon dioxide compete for RuBisCO if
too much oxygen is present, carbon dioxide produce and
not fixed. ATP and NADPH are used to convert the
pathways intermediates into a molecule that can enter the
Calvin-Benson cycle. Extra energy is involved but this way
sugars can still be made on hot dry day just a lot less.
This process is known a PHOTORESPIRATION.
C4 plants also use alternative pathways but does not have
a decline in sugar productivity, corn, switchgrass and
bamboo are examples of C4 plants. They perform the light
dependent reaction in mesophyll cells and the light
independent reaction in bundle sheath cells. Weird!
CAM plants are found in deserts and conserve water with
their alternative pathway.