1. What is photosynthesis?

            Photosynthesis is the driving force behind most of the life on our planet.  Photosynthetic organisms remove approximately 200 billion tons of carbon from the atmosphere each year.  Photosynthesis is responsible for producing all of the oxygen required by aerobic organisms, and was one of the driving factors of evolution early in the earth’s history.  Essentially, photosynthesis is the process of converting solar energy to chemical energy.  Unfortunately, it is not as simple as add sunlight and energy magically appears.  Photosynthesis is a complex process, by which atoms from water and carbon dioxide are cleaved and re-formed into organic molecules with high-energy bonds that can be used to fuel cellular processes.  The essential formula for photosynthesis which you learned in general biology is

 

6CO2 + 12 H2O + hv   --->   C6H12O6 + 6O2 + 6 H2O

 

where hv stands for energy from the sun. This is called oxygenic (oxygen evolving) photosynthesis, because it uses water as an electron donor and produces oxygen gas.  There is another type of photosynthesis found in some bacteria called anoxygenic photosynthesis, which uses hydrogen sulfide (H2S) as an electron donor, producing sulfide as a result.  We will be concerned only with oxygenic photosynthesis which is the more common and the only type found in eukaryotes.  Oxygenic photosynthesis is carried out in two steps, 1) the light reactions which use pigments to capture solar energy and 2) the dark reactions which use the energy from the light reactions to fix atmospherically derived carbon (CO2) into organic carbon (sugars).  It should be noted that the final product is not actually glucose as depicted in the equation above,we just don't usually discuss the last step when we talk about photosynthesis.  Glucose does result from photosynthesis, but it usually present in polymeric form either as sucrose (a dimer) or starch ( a polymer).

 overview of light/dark reactions

2. Where does photosynthesis take place?

    Photosynthesis occurs in many unrelated organisms from cyanobacteria that measure only a few microns (see figure at left) to giant sequoyah trees, big enough to drive a truck through.  The cyanobacteria are the most well-known photosynthetic prokaryotes, and it is their form of oxygenic photosynthesis that has been co-opted by the synechococcuseukaryotes.  In the cynobacteria, photosynthesis occurs on special infoldings of the plasma membrane called thylakoids.  Around 2.0 - 2.2 billion years ago (BYA) a single-celled eukaryote ingested a cyanobacterium, and instead of digesting it, retained it as an active intracellular symbiont.  Over thousands of generations, this "captured" cyanobacterium became reduced to a specialized organelle, the chloroplast.  All photosynthesis in green plants and algae occurs in chloroplasts.  Although there are many different kinds of chloroplasts, we will be concerned mostly with those found in the land plants.  The chloroplast of plants is surrounded by 2 membranes, the outer membrane is permable to small molecules whereas the inner membrane forms the permeability barrier of the organelle.  The inner membrane also containes transporters for phosphate and sugar precursors.  A series of membrane-bound pockets called thylakoids are found inside the chloroplast.  The thylakoid membranes are where the light reactions of photosynthesis take place.  In the land plants, the thlyakoids are usually stacked like pancakes into structures called grana (singular granum). The space formed by the pockets of the thylakoid membranes is called the lumen while the space surrounding the thylakoids (but inside the inner membrane) is called the stroma (see images below, also you may look here).  As you willl learn later, the way that these membranes are pocketed, and the distribution of photosynthetic molecules in and around those membranes, is vital for the proper functioning of the photosynthetic process.


A drawing of a chloroplast illustrating the different components.
A transmission electron micrograph of a chloroplast.  It is from images like this one that the diagram on the left was made.  See if you can identify the parts of the chloroplast in this image.

 
3. How  can light provide energy for plants?
    
    Light is composed of particles called photons that act like waves.  Light is only a small part of the broad electromagentic spectrum which is composed of everything from gamma rays to radio waves.  EM radiation  is characterized by its wavelength and its frequency, but for our discussions, we will be using wavelength.  It is important to note that the lower the wavelength of EM radiation, the higher its energy.  For example, an X-ray with a wavelength of 0.01 nm has much more energy than an infrared beam with a wavelength of 1000 nm.  Keep in mind that shorter wavelength equals more energy.  Visible light is also called photosynthetically available radiation (PAR) because it is only the narrow range of wavelengths that make up visible light that can fuel photosynthesis.  Visible light can be broken up into smaller increments based on wavelenghts.  Changes in the wavelength of visible light (PAR) result in a change of color.  For example light with a wavelength of 450 nm is blue while light with a wavelength of 650 nm is red.  Higher energy wavelengths are destructive to living tissue (think about what UV rays do to your skin) and lower energy wavelengths simply do not possess enough energy.

electromagnetic spectrum

We know that plants get the energy for photosynthesis from visible light (the PAR of sunlight), but how does electromagnetic energy become chemical energy to fuel this process?  Photosynthetic organisms produce specialized molecules called pigments that capture the energy from light and use it to fuel a series of redox reactions that store the captured energy in chemical bonds. 

4. So what is a pigment?

    Although they all absorb light not all pigments are created equal, some absorb light from certain wavelengths better than others.  The most important pigment in plants is chlorophyll.  There are actually two types of chlorophyll in plants, chlorophyll a (chl a) and chlorophyll b (chl b) (there is also a third, chlorophyll c found in some algae).  Chlorophyll is composed of two parts; the first is a porphyrin ring with magnesium at its center, the second is a hyrophobic phytol tail..  The ring has many delocalized electrons that are shared between several of the C, N, and H atoms; these delocalized electrons are very important for the function of chlorophyll.  The tail is a 20 carbon chain that is highly hydrophic and stabilizes the molecule in the hydrophobic core of the thylakoid membrane.
chlorophyll structure
 Structurally, the only difference between chlorophyll a and b is the functional group  indicated in green.  The CH3 group is present in chl a where chl b has a CHO  group.  Functionally, they are very different.  Chlorophyll a and b absorb differnt wavelengths better than others.  For instance chl a absorbs best at 450 and 680 nm, where chl b absorbs best at 500 and 640 nm.  While chlorophyll a is directly involved in the redox reactions of the light reactions, chl b  functions as an accessory pigment, meaning it is not directly involved in the light reactions.  Accessory pigments absorb light and pass the energy from the light to the chl a in the reaction center where the first stage of the light reactions take place.   Other accessory pigments can be present such as xanthophylls and the more well known carotenoids.  The most well known carotenoid is beta-carotene which absorbs different wavelengths than the chlorophylls.  Click on the link here  to see an example of the different wavelengths of light that Chlorophyll a, chlorophyll b, and beta-carotene absorb.  Think about how this will affect the wavelengths we can percieve with our eyes.  Also, compare the absorbtion spectra of the different pigments with the action spectrum of photosynthesis.

Did you notice that these main pigments do not absorb green light well?  Chlorophyll a and b both absorb blue light and red light best, resulting in an overall green appearance, whereas beta-carotene absorbs blue and some green light best resulting in an orange color (carotene was first derived from carrots, hence its name).  During the summer, chlorophylls dominate, resulting in leaves of plants being green.  In the autumn, when deciduous plants are getting ready for winter, they digest their chlorophyll resulting in the accessory pigments like carotenes becoming dominant, hence the bright red and orange colors of fall foliage.

5. How do pigments convert electromagnetic energy into chemical energy?

    Now we are getting into the nitty- gritty of photosynthesis.  Here we will learn about the moelcular mechanisms by which pigments and other molecules work together to capture energy and convert into a usable form.  The core components of the light reactions are found in two spacially and functionally distinct
click to watch the movieunits called photosystems.  These are easily enough, named Photosystem I (PSI) and  Photosystem II (PSII), after the order of their discovery.  People often have problems with this, because the light reactions actually start with Photosystem II, and the products of those reactions are used as substrates in Photosystem I.  So it is with Photosystem II that we will begin.  The fist component of Photosystem II is composed of the Light Harvesting Complex (LHC) and the reaction center.  The LHC is composed of hundreds of molecules of chlorophylls and accessory pigments.  Most of the Chl a in a cell is actually involved in light harvesting.  These are referred to as  antenna chlorophylls.  All the pigment molecules in the LHC are constantly absorbing light, and when light of a certain wavelength (<680 nm) is absorbed, the absorbed energy is transfered from one molecule to another until it reaches the reaction center.  Because it works optimally with light of <680 nm, the core reaction center of photosystem II is called the P680 complex.  The reaction center contains a special pair of chlorophylls bound to a number of reaction center proteins in the thylakoid membrane that finally absorb the energy from the light.  This absorbed energy moves one of the P680 chlorophylls into the excited state which is highly unstable.  The Chl a molecule gets rid of this energy by ejecting an electron.  This results in the Chl a molecule being oxidized; since it was oxidized by light this process is known as photo-oxidation.  The high-energy electron is immediately taken up by the Primary electron acceptor also known as the quinone Q. Now we have a reduced Q which is a strong reducing agent and an oxidized chlorophyll a which is a strong oxidizing agent, this process is called charge separation.  The high-energy electron is immediately dumped from Q into the electron transport chain to fuel ATP production, we will discuss that in more detail later.  In order to return to the ground state, the chlorophyll needs an electron from somewhere, so it uses an electron stripped from water.  Water is split in the oxygen evolving complex located on the inner surface of the thylakoid membrane near the reaction center.  The driving force of this reaction is the oxidizing power of chlorophyll  a, but the O2 evolving complex contains proteins that catalyze the oxidation of water by Chl a as well as a quartet of Mn+ ions that stabilize the O- ions until they can form a stable O2 molecule.  Each molecule of water that is split yields 2 e-, 2 H+, and 1/2 O2,  so for every 4 photons of light absorbed by the PSII reaction center, two molecules of water are split, one molecule of diatomic oxygen is created, and four protons are pumped into the lumen of the thylakoid.  To see a movie of this whole process click on the picture (the movie is from Molecular Cell Biology, Lodish et. al., (c) 2000 Freeeman publishers).

6. What about the electron transport chain and Photosystem II ?

    Once the P680 complex converts a photon of light into chemical energy in the form of a reduced primary electron receptor (Q), the captured energy can be put toclick here for full size image work.  The high-energy electrons used to reduce the quinone Q fall through a series of  quinones and cytochromes that make up the PSII electron transport chain until they reach the soluble electron carrier plastocyanin.  As the electrons move through the PSII electron transport chain their energy is used to fuel proton pumps to create a charge difference across the thylakoid membrane.  The first stage of the PSII electron transport chain is the Q cycle which uses the energy from Q reduction to snatch protons from the stroma of the chloroplast and move them into the lumen of the thylakoid through intermediary Q molecules.  After the primary e- acceptor Q recieves 2 e- from P680 it absorbs two H+ from the stroma.  The 2 protons are pumped into the thylakoid lumen and the electrons are passed on to the cytochrome b/f complex.  Movement of the electrons through the cytochrome b/f complex is used to pump more protons into the thylakoid lumen until the electrons reach plastocyanin.  Plastocyanin is a soluble electron carrier the moves throughout the thylakoid lumen, but its functional destination is the PSI reaction center. Click on the image to see a diagram, notice the changes in energy as measured using redox potential.

7. If PSII can convert sunlight to energy, why is there a PSI?

    PSII does indeed convert sunlight into chemical energy, but the work of photosynthesis is not done.  Not all of the components for carbohydrate synthesis are yet click here for full sizepresent, it will be photosystem I that completes the light reactions and forms the remaining fuel  for carbohydrate synthesis.  Photosystem I has a light harvesting complex and a reaction center very much like PSII.  Because the proteins associated with the pair of special Chl a molecules in the reaction centerare different, PSI functions optimally with photons of light with an wavelength of 700 nm, so the reaction center of PSI is called the P700 complex.  When 700 nm light is captured by the PS I LHC it is moved to the P700 chlorophylls, where once again the result is an excited state P700.  P700 sheds a high-energy electron and this time instead of being reduced by water, the electrons from PSII that were carried by plastocyanin are used to return P700 to its ground state.  The high-energy electron generated by light capture is moved through intermediaries in the P700 reaction center to a soluble protein called ferrodoxin.  Ferrodoxin is an Fe and S containing protein located on the stromal surface of the thylakoid membrane.  Ferodoxin passes the electron on to FAD (flavin adenine dinucleotide: a coenzyme that functions as an electron carrier) which in turn passes it on to NADP+ (nicotinic adenine dinucleotide phosphate).  Two electrons along with a proton from  the stroma convert each NADP+ to NADPH, the final product of photosystem I.

8. But what about all those protons that got pumped into the thylakoid lumen?

    One of the byproducts of PSII was the pumping of several protons from the stroma into the thylakoid lumen.  To review, for every 2 photons of light absorbed by the P680 complex, the O2 evolving complex liberates 2 H+ from water, Q pumps 2 protons into the lumen from the stroma, the cytochrome b/f complex pumps 2 H+ in from the stroma, and finally NADP+ removes one H+ from the stroma when it is reduced by FAD.  That results in a gain of 6 H+ in the lumen and a loss of 5 H+ from the stroma.  Movement of the protons inside the thylakoid lumen is very important, because it creates a concentration gradient across the membrane.  With more protons inside the lumen than the stroma, the pH is much lower inside the lumen.  This pH difference across this membrane contains the potential to do work, this electrical potential energy is called proton-motive force, the greater the pH (concentration) difference, the greater energy available to so work.  The thylakoid membrane contains an ATP synthase you should remember from the mitochondria called the F0F1 complex. Because the thylakoid membrane forms a barrier against proton diffusion, they can only move through the pore of the F0F1 complex.  The F0F1 complex uses the energy released by the protons moving through the pore to add a high-energy phosphate ion (Pi+) to ADP resulting in ATP formation.  For every 4 protons transported through the  F0F1 complex, one molecule of ATP is created.  This process of using the proton gradient to fuel an energy requiring process is known as chemiosmosis.  For a full view of the light reactions see the figure below, to follw these reactions in relation to free energy click HERE. .
Overview of the light reactions