Photosynthesis (the best thing ever)

Note: This program first aired on May 14, 2016.

Every day this time of year the world gets more and more green. Over night it seems like lawns spring up lushly, and the light green fuzz that covers the deciduous forest asserts itself more strongly every day. All of that green is a result of the growth of the photosynthetic structures of plants, structures that are replacing the ones that died last fall in preparation for the cold and dry winter season, or are growing anew from last year’s seeds. The primary photosynthetic structure for land plants is the leaf, a structure that has a lot of surface area relative to its volume. The surface area is important, as the leaf’s main job is to absorb solar radiation, so surface area equals absorption potential.

You all learned (I hope) in school that plants take carbon dioxide and water and sunlight and make sugar and oxygen gas*. The energy from the sunlight, electromagnetic radiation, gets transformed into chemical energy as stored in the bonds between the carbons and the hydrogens in the glucose molecule that is formed. Something we can’t store gets turned into something we can. It sounds quite simple, and if you look at them chemical equation for this reaction, it even looks quite simple. The reality though is far different, as my biology students this spring learned. Photosynthesis is amazing, and beautiful, and a process that virtually all life depends on, but it is not simple.

Nor should it be. Plants execute a very tricky procedure in fixing the carbon from carbon dioxide in to a glucose molecule. It isn’t easy to transform and store energy. Photosynthesis starts with light and that light energy gets transferred to electrons, electrons that are hanging out on chlorophyll molecules in the chloroplasts of plant leaves. Chlorophyll is a pigment that absorbs light, mostly red and blue light, because those wavelengths are especially good at activating chlorophyll’s electrons. Green light incidentally, is not good at all at exciting chlorophyll’s electrons, so it isn’t absorbed, it is reflected instead, which is why plants look green. Those electrons go through a series of steps in processes imaginatively called photosystems I and II. The light bumps them up to a high energy state, and over the course of photosystem I and II they bump back down, releasing that energy along the way. The end results of photosystems I and II are a whole bunch of temporary energy storage molecules called ATP, and a few electron acceptor molecules. ATP is adenosine triphosphate and is the go to molecule for temporary chemical energy in cells. The phosphate part is what makes it good for holding energy, but is also what makes it only able to do it temporarily, as the phosphate is very unstable. So ATP isn’t a viable long term storage solution, but cells make ATP to then use the energy stored in ATP to do other cellular business. Photosystems I and II are the light dependent parts of photosynthesis, and where the water (a reactant in the reaction) is used and the oxygen (a product of the reaction) gets formed.  The water gets split, and is where the electrons that get moved around ultimately come from, and the oxygen is a byproduct of splitting the water. Notice, no where in this part of photosynthesis have we talked about carbon or glucose. Carbon doesn’t have anything to do with this part.

The other nearly entirely separate part of photosynthesis is the light independent reaction—called the Calvin cycle. This is where all that ATP gets used, and those electron acceptor molecules from the photosystems give their electrons back up. Carbon dioxide gets incorporated into several different intermediate molecules with names like PGA and G3P and RuBP, and after 6 full turns of the Calvin cycle, you end up with enough carbons being fixed to make one glucose molecule. That requires a lot of electrons , and even more ATP. When you study the Calvin cycle you gain an appreciation for just how much energy really goes into making glucose, the energy molecule of life.  And note here, in the Calvin cycle, this is where the carbon comes into play. The carbon dioxide is a reactant in the chemical equation, and the glucose is the product linked to it. The water and oxygen gas that appear right next to the carbon and glucose on paper aren’t really connected to them at all in the plant cell, except for a few electrons.

And what that also means, if you think about it, is that the material of glucose, the carbon that makes up the main backbone of the glucose molecule comes only from the carbon dioxide the plant inhales from the atmosphere. That means that plants make food out of air. The material portion of that food, the actual atoms that make up the sugar we all eat, came out of thin air. And that is something I can’t get over, every time I think about how amazing plants are that is what I come back to, plants make food out of air (specifically the carbon dioxide gas) and oxygen out of water. I only wish I could make that much of a difference in the world.

*Here’s that reaction equation: 6 CO₂ + 6 H₂O → C₆H₁₂O₆ + 6 O₂

References:

Look at any college level biology book and  you will find the basic mechanics of photosynthesis. I used Freeman’s Biological Science 5^th ed in my class this past year https://www.pearsonhighered.com/product/Freeman-Biological-Science-5th-Edition/9780321743671.html

Antlers

Note: This program first aired on May 7, 2016.

Walking in the woods in late winter or early spring, if you are very lucky, you may happen upon antlers, either singly or in a pair, dropped from one of two local species of Cervidae or deer family, the white tailed deer or the moose. The sexually mature males of each of these species drop their antlers in the winter after the mating season has ended for the year, to be found by gnawing rodents and the occasional and lucky human.

These species produce a new set of antlers every year, in a feat that pushes the boundaries of mammalian bone growth. Antlers are in fact bone, but have two distinct phases of development that are as different from each other as life and death. The first phase of antler development is the growth phase. This starts in early spring on a male cervid. The initiation of antler production in temperate and high latitude cervids is determined by photoperiod, or day length. The length the day is increasing in the spring, and this signal is picked up through the eyes and transmitted to the pineal gland, which regulates melatonin production. Melatonin has a regulatory effect for sex hormone production, and is involved in both the shedding of the old antlers and the production of the new ones.

Antler development begins on the pedicles, bony knobs that develop on the skulls of young male deer and moose. The surface of all bones in the body is covered by a vascular tissue called the periosteum, and the pedicles have a specialized periosteum that is antlerogenic, meaning it gives rise to the antlers. This specialized tissue is similar to some embryonic tissue or stem cells, and can self differentiate into antler tissue in a similar way that certain embryonic cells turn into bone. In fact, this tissue can be transplanted to other places on the deer’s body, and will result in the growth of an antler to the transplant site (so yes, you could make a unicorn deer). The sex hormones or androgens like testosterone seem to have the biggest impact, female deer can be induced to grow antlers if their hormones are manipulated correctly, and in the wild the female deer that are occasionally seen with antlers are actually hermaphrodites (possessing female genitalia but undescended testicles).

When the antlers first begin to grow they are soft and spongy, almost like cartilage. The tissue is mainly water, and the dry fraction of the growing bone is mostly protein with a small amount of minerals like calcium and phosphorous. The antlers are at this stage living tissue, full of blood vessels and nerves, and covered by a thin hair covered skin like membrane called velvet. Antlers grow like onion roots—from the tip, and at an incredibly fast rate, upwards of ¼ inch a day. The frame of the growing antler is constructed first, and is then slowly filled in as the summer goes on. In late summer as the days begin to shorten noticeably, testosterone levels go up and the antlers begin to calcify. The blood vessels that have nourished the growing bone structure die back and the bone dies but stays attached to the pedicles. Once the bone dies, the velvet dies as well, and bucks and bull moose rub the thin skin off, revealing the smooth mineralized mature bony antler underneath. The antlers are used during the mating season as males fight other males for access to females, and females make themselves available to the males of their choosing. After the fall mating season is complete, a hormonally mediated abscission zone forms at the boundary between the pedicle and the antler, and the bone there erodes away to the point that the antler falls off.

The fast growth rate of antlers, combined with the requirement to regrow the antlers on a yearly basis create a huge nutritional and metabolic burden on these animals. It takes a large amount of mineral nutrients, and an even larger amount of calories. Growing a large set of antlers certainly signals age, as the antlers grow larger each year, and also access to the natural resources required to produce them, poor nutrition can show up as poorly developed antlers.

The ancestors of this antler bearing group of animals didn’t have antlers, in fact they had tusks—enlarged canine teeth that they apparently used for the same purposes as antlers are used today. And whether it is a saber toothed deer or an Irish elk with its 12 foot wide antlers, these show that sexual selection, the selective force that deals only with getting a mate, is what drives some of the most remarkable feats of evolution known.

References:

From the Journal of Anatomy regarding ossification of  antler tissue in deer: http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1571559/

More on deer antlers from the Journal of Anatomy: http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1571559/

About photoperiod, pineal gland and melatonin http://millar.bio.ed.ac.uk/andrewM/Jo%20Selwood%20site/photoperiodism.htm

On the Irish Elk http://www.iwla-rh.org/html/DGIF_articles/deer_antlers.html

http://extension.missouri.edu/p/G9486

http://www.iwla-rh.org/html/DGIF_articles/deer_antlers.html

http://extension.missouri.edu/p/G9486

Viruses

Note: This program first aired on April 9, 2016. 

I thought this week we’d spend a little time talking about viruses, because I’ve been spending some quality time this winter with them. I imagine many of you out there listening have too.

Viruses are the strange fruit of the biosphere, they don’t quite meet all of the typical criteria for being alive, yet are clearly alive none the less, in a way that things like rocks and other inanimate objects are not. In the technical jargon, viruses are obligate intracellular parasites, which decoded means that viruses require access to the cells of a host organism to complete their reproduction; the amount of time they can survive outside the host cell varies from virus to virus, from just minutes to years. Regardless of how long they can lie dormant in the outside world, they can’t do anything active until they are inside a host cell.

The reason for this is largely due to their anatomy (or lack there of). Viruses are not cells, they do not have a phospholipid cell membrane. All other living cells do have these membranes. Additionally viruses lack the molecular machinery needed to reproduce their genetic information. The only things they can call their own are their viral genetic molecules (a piece of DNA or RNA) surrounded by a protein coating called a capsid. What cells lack in anatomy they make up for in ingenuity, they are the MacGyvers of the natural world. They can take the machinery and structure of a living cell and use it for their own nefarious purposes. It’s perfect really—why bother to evolve the enzymes required to replicate DNA or the organelles needed to generate a cell membrane, when you can just appropriate the use of some one else’s? Viruses do just that, many of them construct an envelope out of material from a host cell’s membrane, which functions as an invisibility cloak and grants passage past a host’s immune cells. Viruses lack the enzymes with which to replicate their genetic material, and have solved this problem by evolving the mechanism by which they hijack the replication molecules of a host cell. That is why they have to get inside a cell to do their business.

Viruses are incredibly small and incredibly diverse. Part of this diversity stems from the fact that viruses also are incredibly specific. Each virus is adapted to hijack a specific kind of cell, and only that kind. Generalists are the exception not the rule the virosphere. Human evolution has been shaped by viruses throughout our history; influenza, hepatitis, small pox, dengue, rabies, measles, polio, now we have zika and ebola to add to the mix. Each of these viruses has a favorite cell type, influenza goes for respiratory epithelial cells, polio prefers motor neurons.  Hepatitis reproduces primarily in the liver, causing the inflammation we recognize as the disease, and more than 200 different types of viruses, including the rhinoviruses, infect the cells lining the inside of your nose and cause the common cold. Developing vaccines to combat rapidly evolving or newly discovered viruses informs much of our biomedical research.  

All that moving in and out of our cells has left a lasting imprint not just on human social history, but on our very genome as well. The more we learn about the human genome, the more we realize that viruses have been a part of our world for a very long time. Scientists estimate that as much as 8 percent of our DNA is viral in origin. When the virus moves into a cell to reproduce, sometimes things don’t quite go according to plan, and instead of hijacking the cell machinery to make viral DNA, the viral DNA gets incorporated into the cell nuclear material (the same kinds of mechanisms are used in genetic engineering today).  If that viral DNA ends up in the genome of a germline cell (one that is destined to become a gamete), it can be passed on to offspring, which apparently happened a lot. There is evidence emerging that this viral DNA is actually influencing the expression of some of our human genes. The deeper we look the more connected we are.

Stay well out there, wash your hands, cover your cough, stay home in bed if you are sick. Virus season will be over soon.

References:

University of California Berkley to the rescue again: http://www.ucmp.berkeley.edu/alllife/virus.html

The Journal Nature has a wonderful and well stocked educational website: http://www.nature.com/scitable/topicpage/cell-membranes-14052567

Viral DNA is in our DNA, and it can influence the sex of your kid: http://www.livescience.com/54247-ancient-viral-genes-may-determine-babys-sex.html

A list of human viral pathogens http://viralzone.expasy.org/all_by_species/678.html

It wouldn’t be a show about viruses without something from Carl Zimmer: http://phenomena.nationalgeographic.com/2015/02/01/our-inner-viruses-forty-million-years-in-the-making/

Spring Phenology

Note: This program first aired on March 19, 2016.

Years ago when I was in college a friend commented on the phenomenon of March break. He said that you leave for break and it is winter, and when you come back to school, it is spring. That was certainly the case this year, I went away for a week and when I returned I found the seeds germinating in the green house, wood cocks calling in the evening and elderberries in my yard breaking bud. The next day out walking in the spring like sunshine I came upon alders shedding pollen from their male catkins, another of my favorite early signs that spring. It may get cold again, and snow, but the mechanisms of spring are now underway, there is no turning back. I’ve been paying attention for enough time now that I know soon I’ll hear wood frogs on sunny afternoons with no wind, and after that will come the spring peepers. After the alders will come the blooms of beaked hazelnut, and after that red maple. The birds will return, one day I’ll look up to see turkey vultures and broad winged hawks, hear phoebes in the morning, and hermit thrushes in the evening.

Keeping track of the dates of all of these spring occurrences is a hobby I came to on my own. Do it for a few years and you have a living record of the seasons in your home ground, it becomes addictive. I can look back and see that last year I didn’t hear a wood cock at my house until April 1, this year I heard them on March 12, about the same time I heard them in 2013. Last year the alders started shedding pollen on April 14, this year they started a full month earlier, on March 13. I can make notes on the winter weather, but there is nothing as telling as seeing how early or late the first signs of spring arrive. Little did I know when I started that I was participating the time honored naturalist tradition of observing and documenting the timing of seasonal events in personal journals. It even has a name—phenology: the study of the timing of recurring natural events.

The neat thing is, phenology has made the big time. No longer is it just the pet of introverted nature nerds (like myself) everywhere, quietly rejoicing in the first bloom of shadbush, noting the emergence of the coltsfoot or watching for amphibian egg masses in local vernal pools. No, phenology is now big science. Climate scientists have discovered that naturalists’ records are a treasure trove of data related to how ecosystems are adapting and changing as climate warms. Just as arctic researchers are pouring over the journals of arctic explorers and sailors to document the changes in sea ice over the past two centuries, climate focused biologists and ecologists are reading the journals of 18^th and 19^th century naturalists to establish how the timing of natural events has changed in the industrial era. Looking at a large number of records, over a long period of time is best. If we just had to go on my ten years of data, what would we make of hearing the wood cocks in early March this year, not till April last year, and in mid March the year before that? We really couldn’t make much of these data, other than to broadly state that woodcocks call in the early spring, which we already know. Reading and compiling naturalists’ seasonal observations is one way scientists are finding data, another is the growing citizen science movement. Now you don’t have to simply keep a meticulous nature journal that you hope someone will find useful after you die, you can participate in a state or national phenological programs and start sending useful data to scientists immediately. The Signs of the Seasons program is based at the University of Maine and has New England as its focus area. The USA National Phenology Network collects similar information at a national level. Both have well developed online presences and I encourage you to check them out and consider volunteering.

Here in the 21rst century, I’ve inadvertently discovered another way of keeping records of spring phenological events. Looking back through Facebook posts I see that I have excitedly posted every year the day I first heard woodcocks peenting in the marsh beyond my house. Ditto for wood frogs. And nature oriented friends are doing the same. This past winter wasn’t long or particularly hard, but it was weird, and people are ready for the predictability of the signs of spring. It is refreshing that so many people find something as simple as hearing a woodcock so exciting. I think we’ve finally found what Facebook is good for.

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References:

Read all about UMaine’s Signs of the Seasons Citizen Science New England Phenology program, find out about training sessions and sign up to participate! https://umaine.edu/signs-of-the-seasons/

The USA National Phenology Network collects citizen science data at a national level https://www.usanpn.org/

Using Thoreau’s journals to track climate change: http://loe.org/shows/segments.html?programID=12-P13-00018&segmentID=4

A more science oriented article about Thoreau’s journals, including a researcher from Acadia National Park https://www.elsevier.com/connect/tracking-climate-change-with-the-help-of-henry-david-thoreau

Biology Midterm:What we learn from teaching

Note: This program first aired on March 5, 2016.

This semester has been a busy one, as I am teaching the freshman biology class for our Ocean Studies majors, and find myself relearning topics I haven’t thought about in a long time. This week we reached the midway point of the semester, and in the time honored tradition of academic institutions everywhere, we are having a midterm exam. I don’t give a lot of tests in my classes, but I do feel that it is prudent to stop every once in a while and take stock of where we’ve been and what we’ve learned. Just as the students review their notes and try to prioritize the content of the last seven weeks, so do I. I want to craft an exam that emphasizes the most important ideas and provides an opportunity for students to actually apply those ideas, rather than simply parrot them back at me.

In reviewing my notes from the first several weeks it became clear we had covered some serious ground. It took a few days to distill out the big ideas, but eventually they floated to the top. We started by thinking about the biggest idea you can in a biology class—what is life? How do we determine when something is alive or not? In many ways this is clear cut, things that are alive are made of cells, use energy, have genetic information, have a means of reproducing them selves and evolve over time. The longer you spend in biology though the more amazing and wonderous life gets, and I hope that my students will spend their careers parsing its beautiful nuances, rather than resting on these five broad criteria. A central tenant of modern biology is Cell Theory: all living things are made of cells and all cells arise from other cells. When you think of the implications of that in your own body, all of your cells, the trillions of cells in your body, came from one original cell, the fertilized egg in your mother’s womb. And all of us as leaves on the tree of life, came from one original cell, a mash up of phospholipids and primitive RNA in a hydrothermal vent or mud volcano nearly 4 billion years ago.

We are doing the cellular biology part of freshman biology this semester, trying to understand how those fundamental units of life work. To do that we cover the basic classes of biological molecules: proteins, nucleic acids, carbohydrates and lipids. Each of these classes plays a critical role in cell function, and therefore life. Proteins have the highest diversity, because they have over 20 different subunits (called amino acids) that can be arranged like beads on a string in a nearly infinite number of different combinations. Carbohydrates also have relatively high diversity, based not on having lots of different kinds of sub units—because carbohydrate subunits are very similar and there aren’t that many of them—instead carbohydrate subunits have many many different ways they can get hooked together. If proteins and carbohydrates were Lego sets, proteins would be a set with many different kinds of  small Lego blocks, carbohydrates would be a set with lots of blocks that were all the same, but each of which had several places it could be connected to another block.

We also looked at nucleic acids, which look a bit like proteins, but actually hold all of our genetic information, and at lipids, which make up the membranes of all of our cells and the specialized organelles within those cells. The cell is an amazing structure, functioning on a scale we can scarcely understand, even though we are made of cells. Gravity doesn’t really matter to a cell, the forces that dominate the cellular environment are electrochemical gradients, concentrations of various ions and molecules and electrons that push and pull substances to one side of a cell membrane or another. The pull is passive, it happens without any work involved. The push however, requires an energy source. To push a substance against a concentration gradient takes energy, just like pushing something heavy uphill. And what we find in cellular function is that, just like Sisyphus, our cells keep pushing things up hill, just to let them roll down again. In the Greek myth this was a punishment, but in cellular respiration it is a clever trick. Our cells use spontaneous reactions, reactions that result in a release of free energy, reactions that roll down hill on their own, to power non spontaneous, energy absorbing reactions, up hill reactions in a perfect system of energetic coupling.

We learned that at this level, biology is really chemistry. Life is just an organized system that fights entropy, the tendency of a system towards disorder in the absence of input of energy. Life organizes and inputs that energy, pushing the burden back up the hill, over and over and over again, forming the chemical bonds that would otherwise NOT form. I hope my students have been touched by this wonder. I know I certainly have.

Watching the Planets

Note: This program first aired February 6, 2016.

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My reminder note

Stargazing has been one of those things I’ve been putting off for later in life, when I can’t do other things. I figure when I can’t get out and run and hike as well as I once did, gazing at the night sky and devoting time to learning the stars would be a good pastime. Even though I am supposedly not actively pursuing celestial knowledge, I love learning about the stars and their mysterious and classical names. I try to pay attention to space weather and set my alarm for the middle of the night when a strong aurora or meteor shower is forecast. I watch the moon habitually.

So when I heard the buzz about the line up of the planets taking place right now in the morning sky, I knew I had to check it out. You can see the planets at various places in the night sky, they are the stars that move, counfounding early sky watchers. The stars are fixed in their spots in the sky, and rotate around Polaris (the current north star) in their positions as a whole. The planets don’t do this, and come and go from the sky as they move along their own orbits around the sun. When we do see them, they travel along the plane of the ecliptic, the flat plane on which all of the planets orbit the sun.  What the plane of the ecliptic looks like in the sky is a broad arc from south east to south west (its also where you will more or less find the constellations of the zodiac). Right now there are 5 planets visible along this arc in the hour or two before dawn, from east to west: Mercury, Venus, Saturn, Mars, Jupiter. Seeing them all laid out like that across the sky really helps you visualize the ecliptic and figure out where you should be looking if you are looking for planets (they won’t be found anywhere else in the sky).

My local library has a lendable telescope, many local libraries in Maine now have them thanks to a Cornerstones of Science grant. I’ve been looking at it since they got it, and with all these planets in the sky, I finally had a good and time sensitive reason to check it out. After a test drive early evening moon viewing, I set up the telescope in the only spot at my house with a view of the southern part of the sky, and set my alarm for 5 am. The sky was dark and clear when I ventured out the next morning and fixed the scope on the brightest thing in the sky. A round white disc came into view, with three bright dots next to it and I realized I was looking not only at Jupiter but three of its moons. I located Saturn and finessed the focus knob to the outer reaches of its capabilities, wondering why Saturn wasn’t round like Jupiter. As I feathered the focus I realized that the oval smudge I was seeing was Saturn with its rings. The gap between the rings and the planet came in and out of focus, and I stopped breathing for a moment in astonishment. I was standing in my driveway, looking at my own little patch of sky seeing something at best 746 million miles away with my own two eyes. I gave Venus and Mars a shot, but nothing compared to seeing Saturn, and I spent the rest of the morning darkness alternating between Saturn’s rings and enjoying my new found understanding, gleaned from comparing planets in the telescope, that Jupiter is REALLY BIG.

I encourage you to get to your local library and check out a telescope, or if you are lucky make connections with a local astronomy club. If nothing else, on a clear pre dawn morning get yourself out of bed and find a southern view. Arced across the sky you will see 5 of our sister planets on full display. The show only lasts until February 20^th, so don’t wait.

References:

http://bangordailynews.com/2015/01/29/outdoors/a-look-at-the-earths-tilt-in-2015/

http://www.nytimes.com/2016/01/27/science/how-to-get-five-planets-into-a-single-photograph.html?login=email&hpw&rref=science&action=click&pgtype=Homepage&module=well-region&region=bottom-well&WT.nav=bottom-well&_r=0

http://www.cornerstonesofscience.org/

What does it mean to say something is alive?

Note: This program first aired January 30, 2016.

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This semester I am teaching the first year biology course at the small college where I work. Our first class met today and I thought it right to address the elephant in the room first thing. Yes, we are studying biology, but what does it actually mean to say something is alive?

When I asked the question out loud, there was silence in the room. When it comes down to it, we can generally discern whether something is alive or not, but most of us have never really thought about what it is that allows us to make that judgment. What are the criteria that determine whether or not something is alive? There is no universally agreed upon set of characteristics that describe what life is, but most definitions contain some version of the following:

First, things that are alive use energy. They have to get energy from some where, energy is required to do what living things do, which is combat entropy. Entropy is the tendency of a closed system to proceed from highly ordered to disorder. Spontaneous chemical reactions move in this direction, everything moves in this direction without the input of additional energy. Life brings that energy into the system and maintains those higher order molecules. Life is higher order. The energy comes either from the sun (for photosynthesizers) or from a food source (for heterotrophs) or from chemical bonds (for the rare chemosynthesizers).

Everything that is alive is made of cells. We didn’t know this until we had microscopes. Cells are small enough that we were unaware of their existence, until the technology existed to be able to see them. Then, they were everywhere we looked. And by the 1800’s it was clear, everything that is alive is made of cells. And just what is a cell? A cell is best defined by it’s plasma membrane. The cell membrane is what separates that which is inside the cell, the part that is alive, from that which is outside the cell, that which is not alive (in the case of a single celled organism). It defines the cell in space. The membrane controls what gets into the cell and what gets out. The cell is the fundamental unit of life, and the membrane is what delineates the shape of that unit.

Things that are alive contain and use information. They contain genes, the genetic information which directs the cell to make the protiens and enzymes that keep it functioning. Thing that are alive can also sense information from the environment and can respond to that information with actions that enable the organism to maintain homeostasis or regulate its internal environment. This occurs at every level of life, from bacteria to large complex multicellular organisms.

Living things can and must reproduce themselves, some would say this is the imperative of life. Without reproduction, after the first organic molecule formed or the first cell was created, life would have ended. Without reproduction, we would have to rely on organic molecules having to chemically evolve again and again, starting from scratch each time. And that would not have gotten us very far.

Lastly things that are living evolve. Evolution is simply a change in gene frequency in a population over time, frequencies that respond to selective pressure. Versions of genes that increase an individual’s fitness tend to be passed on, as fitter individuals get to reproduce more. Versions of genes that don’t support fitness tend to go away, because less fit individuals don’t get to reproduce. The genes in a population at any given time reflect the reproductive capacity of the individuals in the population. And while it is populations that evolve, not individuals, individuals are part of the process, it is individuals that suffer the direct impact of selective pressure, and individuals that do or do not get to pass on their genes. In saying that things that are alive evolve really means then that individuals are part of a larger process inherent to life.

In class we reviewed these criteria, and then evaluated several different examples for whether or not we would consider them alive. The tree was easy, so was the granite rock. The stack of firewood sparked some debate. Yes, it was dead now, but it had been alive, and had some of the attributes we covered. Then I showed them a picture of a foamy yellow pile of material on the forest floor. It was a slime mold, but I didn’t tell them that. No one knew what it was, but they had the tools to assess its “aliveness”, they knew the questions to ask, the observations to make. Our criteria may not be universally accepted, but it provides a framework for thinking systematically and critically about the world we live in. My students are at the beginning of their careers as scientists, and these are exactly the tools they need.

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References:

Biological Science, 5/E

Scott Freeman, Kim Quillin, Lizabeth Allison, 

ISBN-10: 0321743679 • ISBN-13: 9780321743671

©2014 • Benjamin Cummings • Cloth, 1416 pp

Pearson Publishing