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.