Discussion Points: Photosynthesis (it’s wicked cool)
Converting Light Energy to Chemical Energy
Photosynthesis is a process used by plants and other organisms capture the sun's light energy and
produce glucose, water and oxygen. It is pretty
much the coolest process ever because
photosynthesis is essential for the survival of all
aerobic life on earth. That is any organism that
needs oxygen to survive such as you. Not only does
photosynthesis maintain normal levels of oxygen in
the atmosphere (around 21% of the air we
breathe) but it is ultimately, directly or indirectly,
the source of energy for nearly all life on earth. For
example, next time you eat a beef hamburger
think about the fact that the cow got its energy
from eating grass and other vegetation, and the
vegetation made its own energy through
photosynthesis.
Photosynthesis occurs in plants, algae, and many species of bacteria. Of course these organisms
don’t photosynthesise because of an overwhelming desire to do good for the world but because
they are creating their own food. Remember, cells make energy (ATP) to perform their functions by
breaking down glucose through the process of cellular respiration. Mobile organisms, such as us, are
able to forage for food as a source of glucose but plants have a much harder time shopping for
groceries because they aren’t as mobile. As such they produce their own glucose for cellular
respiration. Organisms that photosynthesize are called photoautotrophs meaning they can capture
light (photo-) to produce their own energy. The term autotroph refers to organisms that produce
complex organic compounds, in this case glucose, from simple substances. The simple substances
that photoautotrophs use are carbon dioxide and water. Therefore the summarized equation for
photosynthesis is:
Light energy
Carbon Dioxide + Water
6CO2
+ 6H2O
Glucose + Oxygen
C6H12O6 +
6O2
Photosynthesis occurs in a specialised organelle called the chloroplast (see Figure 1). Plant cells are
remarkable in that they have two organelles specialized for energy production: chloroplasts, which
create energy via photosynthesis, and mitochondria, which generate energy through respiration, a
particularly important process when light is unavailable.
Chloroplasts are one of several different types of plastids, plant cell organelles that are involved in
energy storage and the synthesis of metabolic materials. Chloroplasts are ellipsoid-shaped and are
enclosed in a double membrane. Most of the volume of the inside of a chloroplast is taken up by
stroma, a semi-fluid material that contains dissolved enzymes. The stroma also contains DNA and
RNA which means that chloroplast are able to replicate themselves.
Higher plants contain membrane bound closed hollow disks called thylakoids. The thylakoid disks
are stacked in piles called grana which are attached to one another by lamellae. Each thylakoids is
embedded with molecules called chlorophyll
which absorb light energy in the form of photons.
This light energy is then converted to chemical
energy (glucose) through photosynthesis.
Chlorophyll is a pigment which actually gives
leaves their green colour. There are actually
several types of pigments in plants which can be
separated and identified through
chromatography (like we did in our prac).
Photosynthesis occurs in two stages. These stages
are called the light-dependent reactions and the
light-independent reactions (also known as the
Calvin cycle). The light reactions take place in the
presence of light and occur in the grana. The dark
reactions which occur in the stroma do not require direct light, however dark reactions in most
plants occur during the day.
Reciprocal Reactions
If you compare the equations for cellular respiration and photosynthesis you’ll notice that they are
pretty much the exact opposite of each other.
Photosynthesis: 6CO2 + 6H2O + light energy --> 6O2 + C6H12O6
Cellular respiration: 6O2 + C6H12O6 --> 6H2O + 6CO2 + ATP energy
This is because they are reciprocal reactions. In
photosynthesis, plant cells use light energy to drive a
reactions in which carbon dioxide and water are
combined to produce energy-rich organic molecules.
These energy-rich organic molecules are, in turn, broken
down to make energy available to meet the lifesustaining activities of cells.