Today I'm going to talk to you
about energy in biology.

"In biology" I mean all of biology,

from evolution to ecology,
to physiology to cellular biology.

And, the reason I can talk to you about it
from such a wide swathe of biology

is because we use energy
for everything we do

and because we spend a lot
of our time and abilities

just trying to get energy
and make energy.

For those reasons,
there's infinitely many ways

in which I could talk about energy

and infinitely many ways
I could describe it to you,

but one of the most fun things to me
and one of the arts of science to me

is deciding how to draw the boxes
that we're gonna use.

And, to do that,
you often have to frame a question

or think about some objective
you're trying to meet.

And, the two I'm going to talk about
in this talk:

one is with relation to evolution -
how do we think about energy

in terms of evolution
and what's needed for evolution?

And, the other is more in relation to
physiology and ecology,

which really has to do with
how do we get energy

and how do we make energy.

So, from an evolutionary perspective,

the main thing we are concerned about
is fitness,

which is how many offspring
or individuals

we give to the next generation.

To do that, we first have to grow
and maintain ourselves to reproduce.

So, one of the main boxes for evolution
actually is development and growth.

So, in this slide,
I show a plant growing

from early stages to later,
and, in doing that,

it has to create many more cells,

it has to create new types of cells
and new types of structures -

so this takes a lot of energy
and is a very energy intensive process.

And, that's also true
for animals and their growth.

Here, there's a picture of these two birds

crying out to their parents for food -
to bring them food -

because they need lots more food
to grow and have energy

to get to be big enough to reproduce.

The next box I'll talk to you about
for energy is maintenance,

and that's sort of a less obvious
visible one

because you're not seeing cells change
or structures change or reproducing -

you sort of see things
staying the same way they are visibly,

but actually that takes a lot of energy
just to replace cells that die

or to feed cells energy
just to keep living.

As extreme examples,
if you look at these redwood trees,

it takes an enormous amount of energy
just to keep water or sap

pumping up to the leaves at the top -
it's a huge distance they have to travel.

And, you have to build structures
to maintain them to go up to these leaves.

So, you use a lot of energy
just to keep pushing

up to the tops of the trees.

And, similarly, we use energy
for all the structures in our body.

But, there are extreme examples here too

where -
if we're thinking about a peacock -

if it builds a whole array of feathers,

that takes a lot of energy to build
and to maintain.

And, it's going to use that
to attract a mate,

which it again
it needs for reproduction,

which is important for evolution.

As a brief aside,
before I get to reproduction,

from maintenance,
one of the interesting things to me

about us as humans is that,
individually biologically,

we use about the same amount of power
or energy per time

that you would see in a light bulb,
but once you add in things -

how much energy we use for cars
or computers or light bulbs

or heating our houses -
we actually, each individual in the US,

uses about the same amount of energy
as a blue whale,

so we're really enormous energy users
in terms of our biological footprint.

The last evolutionary box I'll talk about
for energy is reproduction,

which is where evolution sort of
ultimately aimed most of the time.

And, that can be things
like oranges on a tree

that tempt us to eat them

because they're so pretty
and flavorful and taste good,

and then, we walk around
and distribute those seeds

to help our orange trees grow elsewhere
and increase their numbers.

Or also, an embryo
growing inside a mother

that takes a huge amount of energy
and time to produce

that's necessary for reproduction.

So, growth, maintenance
and reproduction

are the main boxes that I think
you can think about for evolution

where energy is needed.

But now, I'm gonna shift gears
and talk a little bit more

about how it's needed
for physiology and ecology,

which has a lot to do with

how do you get energy
and how do you make energy,

which evolution still plays a big role in
because you need those things to survive,

but it's not as explicit as
if you do it this way.

So, in terms of obtaining energy,

this is a dramatic picture of an owl
chasing down a mouse to eat for food,

and that's one type of example
of getting resources or food

through what we call "active capture."

But, other ways include things
like grazing - like a cow in a pasture,

or "sit-and-wait,"

which would be like snakes or spiders
waiting for prey items to come to them.

And also, plants are a little bit
like that

in terms of getting energy.

They're more like sit-and-waits,
where they build structures

and wait for things to come to them.

So, as we see here, there's an extensive
branching system for this tree,

and when the leaves are all present
on the limbs,

it's using that to get light...
from the environment,

and get as much light in this little area
as it can - within that canopy.

And, that sort of branching system
is reflected below ground

in terms of root systems
that it uses to get water nutrients

from the ground as well,
where it has to branch out

and get as much resources as it can.

Once resources are obtained,

we have to process that to make energy,

and, the first step in that for animals
is the digestive system,

which, you know, involves...

going through our stomachs
and things like that.

But, the part I want to highlight here

is that, in our guts,
there are these microbial systems,

often now called the "microbiome"
or the "gut microbiome,"

which we have to have
to process energy.

It's this own little world -
an ecosystem - inside our bodies.

And, basically, based on how it processes
energy and the energy it needs,

it really affects which bacteria you see
and the diversity of bacteria you see,

and when that's off
it can really affect our digestive system.

After we process energy

and get it in a more usable form
from what we took into our bodies,

we still have to get it
to the rest of our bodies -

to our fingertip, our toe tip,
or our head to use,

and that's done by a branching system
inside our bodies.

It looks a lot like the branching system
in trees outside

or in their roots in the ground -
and that's the cardiovascular system,

where we use a heart to pump blood
out to our limbs and to our head.

And then, at the finer scale,
we have capillaries or capillary beds,

which is where the transfer of oxygen
or other nutrients can take place.

Once we distribute energy

and get it to each cell that needs it
to keep producing energy and living,

the main way we make energy -
at least in animals -

is through mitochondria,
and each mitochondria

is like a little engine
that takes oxygen and makes energy.

And, it's actually a really old bacteria
that we've brought in to -

not "we" humans -
but a long time ago

cells brought in to make energy for them.

So, it's a really ancient way
of making energy.

And, that begs the question of:
if it's really ancient,

is it really good at it -
is it very efficient?

And, I would think it would be
because, if it's used that broadly,

you would think it must be pretty good
or you would reinvent the wheel somehow.

But, what's interesting is
if you compare...

to something like solar panels,

and compare like the grass
and the trees in the background here

to the solar panels in the foreground,

the grass and the trees
use photosynthesis to make energy,

which is about three percent effective,
but the solar panels can get up to

about 30 percent efficiency,
so about 10 times better,

which was kind of shocking to me
when I first learned about it -

that they can do so much better.

And, maybe this does suggest

that biology can still evolve
and do better.

But, the catch here really is that
solar panels use a lot of elements

that aren't easily accessible
to biological organisms -

they take money to to either mine
or to construct them the right way.

When we think about
sort of being efficient or evolving,

it's always within constraints.

So, I'd argue that biology is applied
really well in the constraints it has,

but we're able to get at things
biology has not been able to get to.

And, looking at this efficiency question
from a different perspective,

if you think back again to the networks,

either for trees or the cardiovascular
system within our bodies,

there's a million ways
you could build such a network.

We want those networks to span space

to be able to get blood or water
everywhere it needs to go,

and we want them to do so
in an efficient way

so we don't spend
a huge amount of energy

just pumping blood around
and losing energy pushing fluid around.

And, if you think about all the ways
in which networks could be built,

you can look at... drive theory
and look at data

to see what's the most optimal.

And, it turns out that biology
has done a really good job

of optimizing networks to be efficient

And, one consequence of that,

actually, as you look at metabolic rates
on the y-axis here

versus mass on the x-axis,
you see a very clean systematic pattern

where, the bigger something is,
the more energy it uses -

which isn't surprising.

But, the surprising piece here
is that it's nonlinear.

So, you think about an elephant
that's 10,000 times bigger than a mouse,

it only uses about
a thousand times more energy,

which means - per cell -
a cell from an elephant

uses about ten times less energy
than a cell from a mouse.

So, you're gaining efficiency
by getting bigger

in this way of looking at it.

And, just to make sure for people
paying close attention to the axes here,

they're logarithmic axes -
so a curved line becomes a straight line,

and, what would be the exponent
of a mathematical equation

becomes the slope.

And, this pattern is true,
not just for across these large...

huge range of sizes
in mammals or animals in general,

but also for plants -

xylem flux is a similar sort of measure
of metabolic rate in plants.

We plot that versus body size again.

And again, you see
a very clear straight line

across a huge range in size for plants,
and, again,

an exponent or a slope
that's close to 3/4.

So, the same sort of pattern
shows up again.

And, another big effector,
besides body size - after body size,

the biggest effector of energy use
across individuals is temperature.

So, if you look in this figure,
basically the warmer something is -

if we think about a frog
or a turtle or a plant -

the warmer it is,
the faster it uses energy,

and that increases at exponential rates
that are faster and faster and faster

up to the point where
you get extreme temperatures

and things start to fall apart
and things just start to die.

But, up until that point or close to it,

the warmer you are
the faster you use that energy.

And because, as we started seeing
at the start of this talk,

that we use energy for everything we do,

understanding how mass
and temperature affect metabolic rate

or the power we produce

tells us a lot about all kinds of
other things in biology.

So, for example, if we look at
heart rates across mammals,

another way to think about this
in terms of mouse versus elephant

is that an elephant's heart beats
about ten times slower than a mouse.

So, every time an elephant's heart
beats once,

the mouse will have beat
ten times really quickly.

Or, if you look at ecology

when we correct for temperature
and like versus size

and we think about how much
each individual produces in a system,

that actually follows a very tight,
clean pattern here as well

and it's true across a huge,
diverse variety of taxa

that includes plants, mammals,
insects, fish -

almost everything you can think of.

And finally,
as another ecological example -

this affects how many individuals we see
or density, sorry -

so how many individuals per area
that we see

where, the bigger you are
or the warmer you are,

the more energy you need
the fewer individuals you get around

in a very systematic.

And, you see this systematic pattern
for animals,

which are the red dots

and plants,
which are the green dots.

And, one of the interesting things here

is that animals are much lower
than the plants

and that's because it has
conversion efficiency,

where plants have to convert
sunlight into energy,

and, basically, all animals
either directly or indirectly

get their energy from plants.

So, they get about ten percent
of the energy from plants

that they can use
to produce their numbers.

So they're lower down

because they lose a lot of efficiency
in going from plants to animals.

Those are the main messages
I wanted to get across today,

and I want to end by giving references.

And, there's a lot of different ways
these topics could go:

there's so much to read in all of these.

So, I try to give really big,
all-encompassing references that,

if you're interested in a topic,
you can get in and go from there

and search that lots more
and find as much as you want.