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).
- Kinetic Energy - the energy of movement (example: )
- Potential Energy - stored energy (example: )
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.
- Light, like other forms of electromagnetic radiation, is composed of subunits. A
subunit of electromagnetic radiation is called a QUANTUM (plural - quanta)
- The special name for a quantum that we can see (i.e., a unit of visible light) is a PHOTON.
- Light is odd among physical phenomena! It has properties of both a particle (it
bounces off solid matter, and its path can be bent if it travels through liquid) and a wave (it travels through space in a wave form).
- Not all photons are the same. Although all photons travel at the speed of light
(299,792,458 meters per second!), individual photons making up the light around us may be traveling at different wavelengths or at different frequencies, as shown in the illustration above.
- Photons of different wavelengths interact with matter in different ways. For
example, photons ranging in wavelength from about 380 nm to about 580 nm are received by the human eye and processed by the human brain as COLORS.
- The "color" of the photon depends on its wavelength.
- The shorter the wavelength, the higher the frequency, and the higher the energy.
- The highest energy photons are in the violet region; the lowest energy ones are in
the red region, with the visible spectrum running like so:
- Photons interact with matter in one of three ways. A photon striking matter (liquid, gas or solid) can be
- transmitted (meaning it passes through the matter)
- reflected (meaning it bounces off the matter and changes direction)
- absorbed (its energy is converted into the energy of the molecule it hits)
- Only absorbed photons have biological activity, because only those are giving up their energy to the matter which has absorbed them.
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
- RODS - very sensitive to low light levels; confer a "grainy" image;
cannot convey "color" information.
- CONES - not as sensitive to low levels of light; confer a sharp
image; can convey "color" information because they come in three
different types (in primates, anyway): red, green and blue.
- Some animals see all the colors we see (trichromatic, or "three
- Some animals have "limited" color vision (fewer types of cone
receptors than we have)
- Some animals cannot perceive color at all
- And some animals apparently can discriminate MORE colors than we
You usually can guess the animal's type of vision if you know whether it's...
- diurnal - active in daytime (usually has some degree of color vision)
- nocturnal - active at nighttime (usually cannot see color)
- crepuscular - active at dawn and dusk (may see some degree of color)
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
- six molecules of carbon dioxide plus
- 12 molecules of water
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.
- one molecule of glucose
- 6 molecules of oxygen
- 6 molecules of water
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
- one molecule of glucose in the presence of
- six molecules of oxygen and
- six molecules of water
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.)
- 6 molecules of carbon dioxide and
- 12 molecules of water
Plants REFLECT and TRANSMIT light in the yellow, green, and orange
region of the spectrum. This is why they look green to our eyes.