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NATHAN: Why does your
fridge start to smell,

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and where does that icky
black liquid come from?

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Wouldn't life just be
easier if things didn't rot?

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If things lasted forever?

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Well, maybe, but probably not.

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According to the USDA, the
average American household

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wastes 25% of its food,
and a lot of restaurants

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are even worse.

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So if we didn't
have decomposition,

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what would happen to
the food we throw away?

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Food in landfills normally gets
dealt with by bacteria, fungi,

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and protists that allow the
nutrients in food to return

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to the soil, and eventually,
other living things.

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These decomposers also
break down other dead stuff,

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like trees.

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In fact, they're pretty much the
only thing that can eat wood.

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So in a world without rot,
while your home may not

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have to worry about
being eaten by termites

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who rely on protists
in their stomach,

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pretty soon, forests
and landfills

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would be flooded with
a lot of dead stuff.

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How do we avoid this problem?

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Well, basically, in your
fridge, on a forest floor,

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in a dumpster, almost
anywhere, there

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are fungi, bacteria, and
protists that live entirely

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by eating dead stuff.

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These dead things can
be more or less divided

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into three categories.

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Carbohydrates--
sugar and starches.

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Lipids-- think fats.

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And proteins, like meats.

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All of these are
chemically different,

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so they each get broken
down by different enzymes

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in different ways before
being absorbed by decomposers.

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For example, proteases
break down proteins

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into amino acids, the
cell's building blocks.

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Lipids rely on lipases,
and carbohydrates on things

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like amylase and cellulases.

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So how does a perfectly
nice broccoli floret

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start giving off this
foul black liquid?

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Fruits and vegetables are
almost entirely made of water.

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So, on the most basic
level, you could

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say that cells are like
extremely complex water

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balloons.

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The exterior, or
cell wall, is made

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of cellulose, a complex
carbohydrate that

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gets broken down by enzymes
into small sugars the cell can

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get energy from.

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When a bacteria or
fungi or uses cellulase

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to eat the exterior
of a cell, it's like

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if I were to pop the balloon.

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That's the muck you
see in your fridge.

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That's how you get
that icky black liquid.

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What about that stink?

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For fruits and vegetables,
a lot of time, the smell

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happens after the icky
black liquid forms.

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Other bacteria that
weren't involved

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in the initial
colonization move in

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and start to stink
everything up.

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Meat get smelly when lipases
break down fat in the meat

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into glycerol and fatty
acids to energy sources.

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And fatty acids
are kind of gross.

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And so while your food rotting
may smells absolutely terrible,

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because of it, the
environment is able to recycle

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crucial nutrients it needs.

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And, well, that's
why we have all this.