Biol 61 – Digestion
, like plants, produce cellular energy from sunlight, and can build the organic molecules they need through
are organisms that get their essential nutrients and organic molecules by eating other
organisms, and that is why they have digestive systems.
Whether it’s a unicellular organism or a single cell that is just
one of billions found in a multicellular organism, all heterotrophic cells need a source of fuel and organic building
We need the food calories for fuel and the organic molecules for building blocks, but we also need other essential
- that we can’t make ourselves.
There are compounds like amino acids and unsaturated
fatty acids that we can’t make but must get in our diet.
are organic substances needed in trace amounts.
are two general classes of vitamins, water soluble vitamins and fat soluble vitamins.
Water soluble vitamins are mostly
protein cofactors (or the building blocks for protein cofactors).
Excess supplies of these chemicals are usually
eliminated in the urine.
Fat soluble vitamins generally are not related to enzyme cofactors but play different roles.
Ingestion of too much of any of these fat soluble vitamins is potentially quite dangerous, since we have no mechanism
for removing them from the body - they will build up in the fat and can potentially cause vitamin intoxication.
soluble vitamins are vitamins A, D, E, and K.
All of the others are water soluble.
are inorganic substances,
ions like Cl- and Na+, iron, magnesium, etc.
Some we need, like salts, to maintain the correct ionic balance in our cells
and bodily fluids, while other inorganic substances play an important role as cofactors for the correct function of many
proteins in our body (see Table 41.1 & 41.2 for some examples).
If an organism can’t get enough calories to maintain
its own body weight, it is considered to be
But an organism can have plenty of calories from its diet
and yet be
because it is lacking required essential nutrients (so don’t just live off of macdonalds, doritos
and mountain dew, folks).
When we eat another organism, we usually start by ingesting big pieces that have to be broken down into successively
smaller fragments that can ultimately be absorbed by our cells and put into our blood to be distributed to all the cells of
So, a body goes to tremendous lengths to break food apart, digest it as much as it can, and get every last
molecule that’s worth keeping.
Prokaryotes absorb food molecules directly from their environment.
food processing involves:
(get it into the digestive system),
(move monomers into cells), and
(remove what you can’t use).
Single-cell protists and sponges digest their food