Answer 1:
During photosynthesis, plants use the energy
from the sun in order to split water molecules
into their hydrogen and oxygen components. The
oxygen is then emitted into the atmosphere for us
animals to use to breathe. The hydrogen is used to
create cellular energy in the form of adenosine
5'-triphosphate (ATP) and through a more
complicated process into glucose. These reactions
are called photochemical or light reactions
because they require light to occur.
Check out this neat video describing the
process of photosynthetic electron transport and
ATP synthesis:
video
click here
If plants do not have access to light they can
break down glucose for energy. If they already
have enough glucose for the time being they store
it in the form of a complex carbohydrate molecule
called starch, which can be broken down for energy
later.
For more detailed information, check out this
article describing in detail the process of how
cell's (both eukaryotic and prokaryotic) get their
energy: cell
energy and cell functions
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