Photosynthesis - 1
The energy needed for life on our planet originates with the sun.
As we have
discussed, living organisms require a source of organic fuel molecules to provide
energy for cell functioning. Organisms that can use energy from the sun and
carbon from their physical surroundings to produce their own organic molecules,
including fuel molecules, are autotrophs. The majority of autotrophs produce
their organic molecules by the process of photosynthesis. Photosynthesis
transforms light energy into chemical energy, and uses that chemical energy to
produce organic molecules, typically glucose, from water and carbon dioxide.
Most photosynthetic organisms (or photoautotrophs) are plants or protists that
Many prokaryotes are also photosynthetic.
have chlorophyll pigments.
Some bacteria, such as the purple sulfur bacteria, have
different light-capturing pigments and photosynthetic mechanisms.
studied in microbiology. Recall that heterotrophs are organisms that obtain their
organic fuel molecules pre-formed from the environment.
Animals, fungi, many
protists and many bacteria are heterotrophs.
Not all autotrophs are photosynthetic; a tiny proportion of living organisms, the
chemoautotrophs, manufacture organic fuel molecules by chemosynthesis.
Chemosynthetic autotrophs (or chemoautotrophs) use energy from chemical
reactions involving inorganic atoms and molecules, such as S, Fe, H and N, to make
Chemosynthesis sustains some deep seabed ecosystems
that surround sulfur vents.
To be complete, some green and purple non-sulfur
bacteria are photoheterotrophs, using light to reduce organic compounds.
Bacterial energy and carbon source versatility are discussed in microbiology.
The products of photosynthesis, along with a number of inorganic atoms and
molecules, are the basis for the biological molecules used for structure and
metabolism for all living organisms as well as the fuel molecules we use in cell
The process of photosynthesis also produces oxygen gas as a "by-product", the
very same molecule that is used in aerobic cell respiration, without which most
organisms on earth would not survive.
Photosynthesis produces about 160 billion metric tons of carbohydrate annually,
an amount most of us can't even imagine.
Plants being efficient organisms use
less than they produce and we depend on the plants "leftovers" for survival. This
dependence, for humans, is greater than just obtaining food directly from an
autotroph or processed through the food chain by a series of heterotrophs, many
of which use tremendous amounts of photosynthetic output to make those
conversions. Our lifestyles depend on products of or past products of
As you read and learn the photosynthetic pathways,
consider that you would not be here without it!