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← Origins of Life: Astrobiology & General Theories for Life - Energy

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Showing Revision 4 created 08/25/2019 by Kristina Helle.

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

  2. "In biology" I mean all of biology,
  3. from evolution to ecology,
    to physiology to cellular biology.
  4. And, the reason I can talk to you about it
    from such a wide swathe of biology
  5. is because we use energy
    for everything we do
  6. and because we spend a lot
    of our time and abilities
  7. just trying to get energy
    and make energy.
  8. For those reasons,
    there's infinitely many ways
  9. in which I could talk about energy
  10. and infinitely many ways
    I could describe it to you,
  11. but one of the most fun things to me
    and one of the arts of science to me
  12. is deciding how to draw the boxes
    that we're gonna use.
  13. And, to do that,
    you often have to frame a question
  14. or think about some objective
    you're trying to meet.
  15. And, the two I'm going to talk about
    in this talk:
  16. one is with relation to evolution -
    how do we think about energy
  17. in terms of evolution
    and what's needed for evolution?
  18. And, the other is more in relation to
    physiology and ecology,
  19. which really has to do with
    how do we get energy
  20. and how do we make energy.
  21. So, from an evolutionary perspective,
  22. the main thing we are concerned about
    is fitness,
  23. which is how many offspring
    or individuals
  24. we give to the next generation.
  25. To do that, we first have to grow
    and maintain ourselves to reproduce.
  26. So, one of the main boxes for evolution
    actually is development and growth.
  27. So, in this slide,
    I show a plant growing
  28. from early stages to later,
    and, in doing that,
  29. it has to create many more cells,
  30. it has to create new types of cells
    and new types of structures -
  31. so this takes a lot of energy
    and is a very energy intensive process.
  32. And, that's also true
    for animals and their growth.
  33. Here, there's a picture of these two birds
  34. crying out to their parents for food -
    to bring them food -
  35. because they need lots more food
    to grow and have energy
  36. to get to be big enough to reproduce.
  37. The next box I'll talk to you about
    for energy is maintenance,
  38. and that's sort of a less obvious
    visible one
  39. because you're not seeing cells change
    or structures change or reproducing -
  40. you sort of see things
    staying the same way they are visibly,
  41. but actually that takes a lot of energy
    just to replace cells that die
  42. or to feed cells energy
    just to keep living.
  43. As extreme examples,
    if you look at these redwood trees,
  44. it takes an enormous amount of energy
    just to keep water or sap
  45. pumping up to the leaves at the top -
    it's a huge distance they have to travel.
  46. And, you have to build structures
    to maintain them to go up to these leaves.
  47. So, you use a lot of energy
    just to keep pushing
  48. up to the tops of the trees.
  49. And, similarly, we use energy
    for all the structures in our body.
  50. But, there are extreme examples here too
  51. where -
    if we're thinking about a peacock -
  52. if it builds a whole array of feathers,
  53. that takes a lot of energy to build
    and to maintain.
  54. And, it's going to use that
    to attract a mate,
  55. which it again
    it needs for reproduction,
  56. which is important for evolution.
  57. As a brief aside,
    before I get to reproduction,
  58. from maintenance,
    one of the interesting things to me
  59. about us as humans is that,
    individually biologically,
  60. we use about the same amount of power
    or energy per time
  61. that you would see in a light bulb,
    but once you add in things -
  62. how much energy we use for cars
    or computers or light bulbs
  63. or heating our houses -
    we actually, each individual in the US,
  64. uses about the same amount of energy
    as a blue whale,
  65. so we're really enormous energy users
    in terms of our biological footprint.
  66. The last evolutionary box I'll talk about
    for energy is reproduction,
  67. which is where evolution sort of
    ultimately aimed most of the time.
  68. And, that can be things
    like oranges on a tree
  69. that tempt us to eat them
  70. because they're so pretty
    and flavorful and taste good,
  71. and then, we walk around
    and distribute those seeds
  72. to help our orange trees grow elsewhere
    and increase their numbers.
  73. Or also, an embryo
    growing inside a mother
  74. that takes a huge amount of energy
    and time to produce
  75. that's necessary for reproduction.
  76. So, growth, maintenance
    and reproduction
  77. are the main boxes that I think
    you can think about for evolution
  78. where energy is needed.
  79. But now, I'm gonna shift gears
    and talk a little bit more
  80. about how it's needed
    for physiology and ecology,
  81. which has a lot to do with
  82. how do you get energy
    and how do you make energy,
  83. which evolution still plays a big role in
    because you need those things to survive,
  84. but it's not as explicit as
    if you do it this way.
  85. So, in terms of obtaining energy,
  86. this is a dramatic picture of an owl
    chasing down a mouse to eat for food,
  87. and that's one type of example
    of getting resources or food
  88. through what we call "active capture."
  89. But, other ways include things
    like grazing - like a cow in a pasture,
  90. or "sit-and-wait,"
  91. which would be like snakes or spiders
    waiting for prey items to come to them.
  92. And also, plants are a little bit
    like that
  93. in terms of getting energy.
  94. They're more like sit-and-waits,
    where they build structures
  95. and wait for things to come to them.
  96. So, as we see here, there's an extensive
    branching system for this tree,
  97. and when the leaves are all present
    on the limbs,
  98. it's using that to get light...
    from the environment,
  99. and get as much light in this little area
    as it can - within that canopy.
  100. And, that sort of branching system
    is reflected below ground
  101. in terms of root systems
    that it uses to get water nutrients
  102. from the ground as well,
    where it has to branch out
  103. and get as much resources as it can.
  104. Once resources are obtained,
  105. we have to process that to make energy,
  106. and, the first step in that for animals
    is the digestive system,
  107. which, you know, involves...
  108. going through our stomachs
    and things like that.
  109. But, the part I want to highlight here
  110. is that, in our guts,
    there are these microbial systems,
  111. often now called the "microbiome"
    or the "gut microbiome,"
  112. which we have to have
    to process energy.
  113. It's this own little world -
    an ecosystem - inside our bodies.
  114. And, basically, based on how it processes
    energy and the energy it needs,
  115. it really affects which bacteria you see
    and the diversity of bacteria you see,
  116. and when that's off
    it can really affect our digestive system.
  117. After we process energy
  118. and get it in a more usable form
    from what we took into our bodies,
  119. we still have to get it
    to the rest of our bodies -
  120. to our fingertip, our toe tip,
    or our head to use,
  121. and that's done by a branching system
    inside our bodies.
  122. It looks a lot like the branching system
    in trees outside
  123. or in their roots in the ground -
    and that's the cardiovascular system,
  124. where we use a heart to pump blood
    out to our limbs and to our head.
  125. And then, at the finer scale,
    we have capillaries or capillary beds,
  126. which is where the transfer of oxygen
    or other nutrients can take place.
  127. Once we distribute energy
  128. and get it to each cell that needs it
    to keep producing energy and living,
  129. the main way we make energy -
    at least in animals -
  130. is through mitochondria,
    and each mitochondria
  131. is like a little engine
    that takes oxygen and makes energy.
  132. And, it's actually a really old bacteria
    that we've brought in to -
  133. not "we" humans -
    but a long time ago
  134. cells brought in to make energy for them.
  135. So, it's a really ancient way
    of making energy.
  136. And, that begs the question of:
    if it's really ancient,
  137. is it really good at it -
    is it very efficient?
  138. And, I would think it would be
    because, if it's used that broadly,
  139. you would think it must be pretty good
    or you would reinvent the wheel somehow.
  140. But, what's interesting is
    if you compare...
  141. to something like solar panels,
  142. and compare like the grass
    and the trees in the background here
  143. to the solar panels in the foreground,
  144. the grass and the trees
    use photosynthesis to make energy,
  145. which is about three percent effective,
    but the solar panels can get up to
  146. about 30 percent efficiency,
    so about 10 times better,
  147. which was kind of shocking to me
    when I first learned about it -
  148. that they can do so much better.
  149. And, maybe this does suggest
  150. that biology can still evolve
    and do better.
  151. But, the catch here really is that
    solar panels use a lot of elements
  152. that aren't easily accessible
    to biological organisms -
  153. they take money to to either mine
    or to construct them the right way.
  154. When we think about
    sort of being efficient or evolving,
  155. it's always within constraints.
  156. So, I'd argue that biology is applied
    really well in the constraints it has,
  157. but we're able to get at things
    biology has not been able to get to.
  158. And, looking at this efficiency question
    from a different perspective,
  159. if you think back again to the networks,
  160. either for trees or the cardiovascular
    system within our bodies,
  161. there's a million ways
    you could build such a network.
  162. We want those networks to span space
  163. to be able to get blood or water
    everywhere it needs to go,
  164. and we want them to do so
    in an efficient way
  165. so we don't spend
    a huge amount of energy
  166. just pumping blood around
    and losing energy pushing fluid around.
  167. And, if you think about all the ways
    in which networks could be built,
  168. you can look at... drive theory
    and look at data
  169. to see what's the most optimal.
  170. And, it turns out that biology
    has done a really good job
  171. of optimizing networks to be efficient
  172. And, one consequence of that,
  173. actually, as you look at metabolic rates
    on the y-axis here
  174. versus mass on the x-axis,
    you see a very clean systematic pattern
  175. where, the bigger something is,
    the more energy it uses -
  176. which isn't surprising.
  177. But, the surprising piece here
    is that it's nonlinear.
  178. So, you think about an elephant
    that's 10,000 times bigger than a mouse,
  179. it only uses about
    a thousand times more energy,
  180. which means - per cell -
    a cell from an elephant
  181. uses about ten times less energy
    than a cell from a mouse.
  182. So, you're gaining efficiency
    by getting bigger
  183. in this way of looking at it.
  184. And, just to make sure for people
    paying close attention to the axes here,
  185. they're logarithmic axes -
    so a curved line becomes a straight line,
  186. and, what would be the exponent
    of a mathematical equation
  187. becomes the slope.
  188. And, this pattern is true,
    not just for across these large...
  189. huge range of sizes
    in mammals or animals in general,
  190. but also for plants -
  191. xylem flux is a similar sort of measure
    of metabolic rate in plants.
  192. We plot that versus body size again.
  193. And again, you see
    a very clear straight line
  194. across a huge range in size for plants,
    and, again,
  195. an exponent or a slope
    that's close to 3/4.
  196. So, the same sort of pattern
    shows up again.
  197. And, another big effector,
    besides body size - after body size,
  198. the biggest effector of energy use
    across individuals is temperature.
  199. So, if you look in this figure,
    basically the warmer something is -
  200. if we think about a frog
    or a turtle or a plant -
  201. the warmer it is,
    the faster it uses energy,
  202. and that increases at exponential rates
    that are faster and faster and faster
  203. up to the point where
    you get extreme temperatures
  204. and things start to fall apart
    and things just start to die.
  205. But, up until that point or close to it,
  206. the warmer you are
    the faster you use that energy.
  207. And because, as we started seeing
    at the start of this talk,
  208. that we use energy for everything we do,
  209. understanding how mass
    and temperature affect metabolic rate
  210. or the power we produce
  211. tells us a lot about all kinds of
    other things in biology.
  212. So, for example, if we look at
    heart rates across mammals,
  213. another way to think about this
    in terms of mouse versus elephant
  214. is that an elephant's heart beats
    about ten times slower than a mouse.
  215. So, every time an elephant's heart
    beats once,
  216. the mouse will have beat
    ten times really quickly.
  217. Or, if you look at ecology
  218. when we correct for temperature
    and like versus size
  219. and we think about how much
    each individual produces in a system,
  220. that actually follows a very tight,
    clean pattern here as well
  221. and it's true across a huge,
    diverse variety of taxa
  222. that includes plants, mammals,
    insects, fish -
  223. almost everything you can think of.
  224. And finally,
    as another ecological example -
  225. this affects how many individuals we see
    or density, sorry -
  226. so how many individuals per area
    that we see
  227. where, the bigger you are
    or the warmer you are,
  228. the more energy you need
    the fewer individuals you get around
  229. in a very systematic.
  230. And, you see this systematic pattern
    for animals,
  231. which are the red dots
  232. and plants,
    which are the green dots.
  233. And, one of the interesting things here
  234. is that animals are much lower
    than the plants
  235. and that's because it has
    conversion efficiency,
  236. where plants have to convert
    sunlight into energy,
  237. and, basically, all animals
    either directly or indirectly
  238. get their energy from plants.
  239. So, they get about ten percent
    of the energy from plants
  240. that they can use
    to produce their numbers.
  241. So they're lower down
  242. because they lose a lot of efficiency
    in going from plants to animals.
  243. Those are the main messages
    I wanted to get across today,
  244. and I want to end by giving references.
  245. And, there's a lot of different ways
    these topics could go:
  246. there's so much to read in all of these.
  247. So, I try to give really big,
    all-encompassing references that,
  248. if you're interested in a topic,
    you can get in and go from there
  249. and search that lots more
    and find as much as you want.