Introduction to Biology

Molecules and Cells

Chapter 10 - Photosynthesis 

The Base of the Food Chain Produces a Fuel for All

Plants, as well as some Protists and Monerans, can take small molecules from the environment and bind them together into glucose molecules using the energy of absorbed light.  Some of the incoming light energy is transformed into the energy holding the new molecules together, and the organisms use those molecules as an energy "fuel," for structure, and contribute it to the food chain, although they probably would prefer not to get eaten.   The basic process of photosynthesis can be represented this way:

CO₂    +    H₂O      light   _>    C₆H₁₂O₆    +    O₂
Carbon       Water                    Glucose       Oxygen
  Dioxide                                                                     

Structure of glucose.

In the case of organisms that live in water, the carbon dioxide and water are from their immediate surroundings;  for most land plants, the water is absorbed from the soil and the carbon dioxide from the atmosphere.

The glucose is used for two major purposes:  1)  it serves as an energy reserve for periods of darkness (don't forget that photosynthesizers, like any living things, require energy to run cellular processes and get it through respiration, commonly aerobic respiration; and 2)  it is used as a major component of structure:  the cell walls that surround almost all photosynthetic cells are made of starches, made up of hundreds, commonly thousands, of sugar molecules bound together.  This is why plant fibers are great sources of nutrition if you have the enzymes (or digestive bacteria in your gut) that can break them down.  Breaking down plant fibers is chemically difficult - we humans can't, being limited to the more digestible starches put into seeds and fruits and tubers.  Plants use those types of starches as sources of sugar fuels, and so build them into a molecule that is much easier to break down than the starch that holds their cells together.

Keep in mind that photosynthetic organisms are still living things, with protein-based chemistry, which means that they have nutritional requirements beyond carbon dioxide and water.  Proteins, unlike sugars and starches, contain a significant amount of nitrogen, which usually needs to be absorbed as nitrates (a nitrogen-oxygen molecule) to be usable.  Plants convert the nitrates into amino acids, which are then assembled into protein molecules.  The production and use of glucose for energy also requires ATP as an energy carrier;  ATP contains phosphorus, usually absorbed as phosphates (a phosphorus-oxygen molecule).  Anyone who takes care of plants knows that nitrates and phosphates are important ingredients in fertilizers.  Most photosynthesizers have other nutrient needs:  they make a few critical molecules with materials such as iron, or need small ions, such as sodium, for some of their chemistry.

Image of cellulose, the main plant structure starch.
 

Abstract from an article about the varying digestibilities of plant starches.
 

All of the amino acids - note the nitrogen in each one.
 

One form that nitrate takes.
 

A phosphate ion.
 

The basics of plants' nutritional needs.

The Energy Available in Light 

Light can be understood as a combination of energy waves traveling outward from a source, or as small packets moving from that source at the speed of light (each peak in the traveling waves would correspond to a single packet of energy, a photon).  Light always travels at the speed of light, altering only for the material through which it's moving (it goes slower in water, for instance), so a segment of a light beam with wave peaks more separated (a longer wavelength) would have fewer peaks absorbed by a surface (a lower frequency) in any given amount of time, and would hit that surface with less energy.  This means short wavelength = high frequency = more energy,  long wavelength = low frequency = less energy.  The only reason that this is important is that sunlight contains a fairly wide range of energy frequencies, but only a few are absorbed and used by chlorophyll, the energy-capturing molecule of photosynthesis.

You can tell a few of the frequencies that are not absorbed by chlorophyll (and a few other light-absorbing molecules) by looking at a plant.  That green you see is part of the reflected frequencies of light.  For the most part, absorption of the other frequencies of light is used in an energy conversion process that "spits" electrons through a system from the "excited" chlorophyll molecules.  Energy conversion can also involve reradiation - electrons on certain atoms will absorb a particular frequency, "jump" to the next electron level (which, with just one electron in it, will be unstable), then "jump" back, releasing the energy at a different frequency (because some has been used).  Some radiation is called ionizing radiation because it can make electrons jump completely off their atoms.

Although land plants absorb a variety of light frequencies, all frequencies are not equally powerful or useful:  while plants can absorb both red frequencies and purple frequencies, the purple have shorter wavelengths and carry more energy.  This is one of the reasons why "plant lights" are distinctly purple.

It is not unusual for land plants to use molecular supplements to absorb some frequencies that chlorophyll can't, and feed more energy into the photosynthesis process;  these pigments are commonly types of carotenoids.  The colors of leaves in the autumn reveals the carotenoids that have always been there but have been covered by huge amount of chlorophyll.  

Carotenoids can serve multiple roles:  they can be photosynthetic aids, but they may also minimize light damage (animals use pigments, like the human tan-producing molecule melanin, for similar protection) or even function in fighting disease.  Land plants may concentrate pigments, including carotenoids, in structure that need to stand out, such as the colors of flowers or mature fruits.  These colors signal animals that a food bribe is available, and then the animals are used to carry pollen or seeds.

Ranges of wavelengths associated with different types of energy. 
 

Graph showing the many wavelengths in the sunlight that hits the atmosphere, and the fraction of those that reach the surface.
 

The wavelengths absorbed by plant pigment molecules.
 

A page on photosynthetic pigments.
 

A page specifically on carotenoids.
 

How autumn leaves get their color.
 

Pigment use in flowers.
 

The Process Takes Two Big Steps 

This will be a very "bare bones" summary of photosynthetic chemistry:

Photosynthesis breaks down into a Light-Dependent Reaction and a Light-Independent Reaction.  The light-dependent reaction uses, not too surprisingly, light, but it also uses the water (actually, the hydrogen part of the water, which releases the oxygen).  This part of photosynthesis shifts the light energy into a chain of electrons which are used, in an electron-transport chain system similar to aerobic respiration, to make a lot of energy-carrying molecules, including a lot of ATP.  These energy carriers drive the light-independent reaction, which uses the carbon dioxide and actually makes the glucose.  For most plants on a typically sunny day, when the sun goes down the light-dependent reaction stops, but the backlog of energy carriers it has made may keep the light-independent reaction going until the middle of the night.

It's interesting that on several levels, photosynthesis is a sort of "mirror image" to

aerobic respiration:       

                C₆H₁₂O₆  +  O₂   energy to ATP _>   CO₂  +  H₂O

In aerobic respiration, the glucose is broken "in half," fed through a cycle that breaks all of the carbons away from each other, and sets up a chain of electrons used to make energy carriers for the cells.  Here, light sets up the electron chain, which is used to make carriers which then take single-carbon molecules and make larger carbon chains, with the last step sticking "halves" together to make glucose.  The real details lose that forward-backward reflection, but you get the basic idea.

In eukaryote cells, photosynthesis happens entirely in chloroplasts, with some processes happening on internal membranes and some dependent on differences between the chemistry of the spaces. 

A slightly more detailed explanation, with images.
 

More on the Light-Dependent Reaction.
 

More on the Light-Independent Reaction.
 

A movie showing the structures and processes working in a leaf.
 

The Photosynthesis Song (video).

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Introduction to Biology - Molecules & Cells.
For SCI-135.

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