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
and produces oxygen gas. There is
another type of photosynthesis found in some bacteria called anoxygenic
photosynthesis, which uses hydrogen sulfide (H2S) as an
donor, producing sulfide as a result. We
will be concerned only with oxygenic photosynthesis which is the more
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
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).
Where does photosynthesis take place?
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 eukaryotes. 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.