An Online Introduction to the Biology of Animals and Plants


Key Concepts


Section 2

Chapter 1





Plants, as well as some Protists and Monerans, can take small molecules from the environment and bind them together using the energy of light.  The incoming light energy is transformed into the energy holding the new molecules together, and the organisms use those molecules as an energy "fuel."  The basic process can be represented this way:

CO2    +    H2O      light   >    C6H12O6    +    O2
Carbon     Water                     Glucose       Oxygen
Dioxide                                     (sugar)                    


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 (dont forget that photosynthesizers, like any living things, require energy and get it through respiration processes, 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, huge molecules made up of hundreds, commonly thousands, of sugar molecules bound together.  This is why plant fibers are great sources of nutrition if you can break them down.  Breaking down plant fibers is chemically difficult - we humans cant, being limited to the more digestible starches put into seeds and fruits and tubers.  Plants use those 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 them 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 the components of 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 chemical processes.




Light can be understood as a combination of energy waves traveling outward from a light source, or as small packets moving from that source at the speed of light (each wave peak would correspond to a single packet).  Light always travels at the speed of light, altering only for the material through which its 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.

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 cant, 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.




This will be a very "bare bones" summation of the 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 uses the water (actually, the hydrogen part of the water, which releases the oxygen).  This part of photosynthesis shifts the light energy into energy of several carriers, 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.

Its interesting that photosynthesis is a sort of "mirror image" to

              aerobic respiration:      C6H12O6  +  O2   energy to ATP >   CO+  H2O

It uses glucose and oxygen and accesses the energy stored in the glucose, releasing carbon dioxide and water.  Even in some of the more detailed steps, the two processes still "reflect" each other.  For instance, the oxygen binds to hydrogen to make the water.




Plants, like all living things, are descended from ancient organisms that evolved and developed in the oceans.  The ocean-living plants that represent land plants ancestors are simple, so much so that they are often classified as Protistans - little multicellular complexity was needed to float in the light.

Even in the ocean, though, there are a number of different niches available for plants - in the shallows, many algae varieties exist that can attach to submerged objects, probably a feature that helped the land plants direct ancestors stick out of the water into the air.  But niches also exist in the deeper water, where there is a functional connection between the photosynthetic pigments used and the nature of how light interacts with water.

The surface of the water reflects some light, but the frequencies that get through get absorbed - you already know that the deeper you go, the darker it gets.  What you may not know is that not all frequencies get absorbed equally - the frequencies that green plants use dont get very far beneath the surface, but frequencies in the bluish and greenish ranges penetrate much further.  This has spurred the evolution of several classes of various-colored algae with different chlorophylls, capable of using these deeper-reaching frequencies to grow where the green algae cant.  Red algaes especially can occupy greater depths than the others, although Golden algaes can occupy a bit of a middle zone between green and red.  The color of Brown algaes seems more to do with other pigments and is apparently not really depth-related.


Informational Links


 A history of knowledge about photosynthesis.

A very weird thing:  Photosynthesis the Movie.

And The Photosynthesis Song.


Click on term to go to it in the text.
Terms are in the order they appear.


Basic Reaction of Photosynthesis
Uses for Glucose
Cell Walls
Plants & Aerobic Respiration 
Other Nutritional Requirements  
Nitrogen Needs 
Light:  Wavelength and Frequencies 
Light Absorption and Reflection  
Plant Pigments 
Light-Dependent Reaction 
Light-Independent Reaction 
Photosynthesis vs Aerobic Respiration
Types of Algae





Online Introduction to the Biology of Animals and Plants.

Copyright 2001-2014, Michael McDarby.   e-mail Contact.

Reproduction and/or dissemination without permission is prohibited.



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