Energy Transduction: Photosynthesis and Respiration

Photosynthesis and Respiration are the two processes by which living things (1) collect and store energy and (2) release that stored energy to do work.

What is Energy?

ENERGY (E) is defined as the capacity to do work. Energy can be measured in units called Joules (J). One J = about 1/4 calorie; and one calorie is the amount of energy needed to raise the temperature of 1 gram of water by 1oC (Celsius).

FIRST LAW OF THERMODYNAMICS: Energy cannot be created or destroyed; it can only change in form.

SECOND LAW OF THERMODYNAMICS: All systems in the universe tend to go from a state of order to a state of chaos (ENTROPY). Entropy is unordered energy that is unavailable to do work.


Light Energy: The Origin of Photosynthesis

It all begins with The Sun, which provides all the earth's energy in the form of LIGHT.


A Side Note about Light and Vision in Animals

  • What we perceive as "white" light is actually all "colors" of light: Photons of every wavelength hitting our retina in about equal intensity. They average out to make "white.

  • What we perceive as "black" is the absence of light: few or no photons are hitting our retina at all, and there is no color stimulus.

  • Colors are all in the mind: Color is a psychophysical phenomenon. Our retinas contain two types of photoreceptors that are stimulated by photons, and send a message to the brain:
  • What you perceive in your brain is triggered by a physical thing (light), but how you perceive it depends on the wiring of your brain.

    You usually can guess the animal's type of vision if you know whether it's...


    Energy Transductions in Biological Systems

    Autotroph - (auto = "self"; troph - "feeding") an organism that captures energy and stores it in the chemical bonds of organic molecules that it manufactures from inorganic molecules via photosynthesis.

    (a.k.a. - "producer") Heterotroph - (hetero = "other"; troph - "feeding") an organism that eats other organisms to obtain energy. (a.k.a. - "consumer")

    Storing the Energy: Photosynthesis

    The most common means by which autotrophs make organic molecules (sugar) is PHOTOSYNTHESIS. Autotrophs that capture light energy are called PHOTOAUTOTROPHS. (There are other kinds of autotrophs, but we won't discuss them here.)

    Overall, the chemical reaction of photosynthesis is as follows:

    Light energy + plant enzymes
    6CO2 + 12H2O ------------------------------------------------> C6H12O6 + 6O2 + 6H2O

    ...which means that it takes in the presence of light and the proper enzymes in the cell, to make The sugar (glucose) is the storage form for energy in plants, and it's often converted into long chains for long-term storage as CARBOHYDRATE. The oxygen and water are side products that are not used by the plant in this reaction.

    WHY STORE SUGAR AND CARBOHYDRATES? What does the plant do with them, once it has them? It uses them to (1) manufacture its body and for (2) energy storage.


    Releasing the Energy: Cellular Respiration

    ALL living organisms burn organic molecules to release the stored energy and use it to drive their own chemical reactions. The process living organisms use to release the energy stored in sugar is called CELLULAR RESPIRATION, and its chemical equation is exactly the opposite of photosynthesis:

    about 16 enzymatic rxns
    C6H12O6 + 6O2 + 6H2O ------------------------------------> 6CO2 + 12H2O + ENERGY

    ...which means that

    can be "burned" to release stored energy as well as the "waste" products of
  • Plants ABSORB light primarily in the violet/blue and the orange/red region of the visible spectrum. This is the light energy packaged in sugar as chemical energy. (The sugar can be broken down to re-release the energy so the cell can do work.)

  • Plants REFLECT and TRANSMIT light in the yellow, green, and orange region of the spectrum. This is why they look green to our eyes.