The sea slug. “Elysia pusilla” by Katharina Händeler, Yvonne P. Grzymbowski, Patrick J. Krug & Heike Wägele – Händeler K., Grzymbowski Y. P., Krug P. J. & Wägele H. (2009) “Functional chloroplasts in metazoan cells – a unique evolutionary strategy in animal life”. Frontiers in Zoology 6: 28. doi:10.1186/1742-9994-6-28 Figure 1F.. Licensed under CC BY 2.0 via Wikimedia Commons.
One of my students wanted to figure out how to make animals photosynthesize. Well, this article indicates that sea slugs have figured out how eat and digest the algae but keep the algal chloroplasts alive in their guts so the sea slug can use the fats and carbohydrates the chloroplasts produce (the stealing of the algal organelles is called kleptoplasty). To maintain the chloroplasts, the slugs have actually had to incorporate some of the algae DNA into their own chromosomes–this is called horizontal gene transfer and it’s what scientists try to do with gene therapies.
My biology students are doing an exercise in genetics and heredity that requires them to combine the genes of two parents to see what their offspring might look like. They do the procedure twice — to create two kids — so they can see how the same parents can produce children who look similar but have distinct differences. To actually see what the kids look like, the students have to draw the faces of their “children”.
“I’m not going to claim that child as my own!”
I was walking through the class when I heard that. Apparently one student, who’d had a bit of art training, was paired with another student who had not.
Fortunately, I was able to convince the more practiced artist to give the rest of the class a lesson on how to draw faces. She did an awesome job; first drawing a female face and then adapting it a bit to make it look more male.
If nothing else, I tried to make sure that the other students registered the idea that proportion is important in drawing biological specimens — like faces — from real life. Just getting the proportions right made a huge difference in the quality of their drawings. The forehead region should be the largest (from the top of the head to the eyebrows), then the area between the eyebrows and the bottom of the nose, then the nose to lips, and then, finally, the region from lips to chin should be shortest. You can see the proportion lines in the picture above.
The adaptation stage, where she made the facial features more masculine, was also quite useful. The students had to think about what were typical male features and if there were a genetic basis to things like square chins.
Although all of the other students’ drawings improved markedly, including her simulated spouse’s, I don’t think my art-teaching student was absolutely happy with the end results after the one lesson. She ended up handing in two drawings of her own even though everyone else (including her partner) did one each.
However, having all the students on the same page, working with the same basic drawing methods, helped improve the heredity exercise because it reduced a lot of the variability in the pictures that resulted from different drawing styles and skill levels.
I also think that taking these interludes for art lessons are quite useful in a science class, since it emphasizes the importance of accurate observation, shapes student’s abilities to represent what they see in diagrams, and demonstrates that they can — and should — be applying the skills they learn in other classes to their sciences.
Everyone knows about Charles Darwin, but hardly anyone remembers Alfred Russel Wallace, who came up with the idea of natural selection at the same time as Darwin. Darwin’s publication of the On the Origin of Species was spurred on by Wallace. Flora Lichtman and Sharon Shattuck shed a little light on Wallace with this video:
Indeed, from the introduction of On the Origin of Species:
I have more especially been induced to [publish], as Mr. Wallace, who is now studying the natural history of the Malay archipelago, has arrived at almost exactly the same general conclusions that I have on the origin of species. Last year he sent to me a memoir on this subject, with a request that I would forward it to Sir Charles Lyell, who sent it to the Linnean Society, and it is published in the third volume of the Journal of that Society. Sir C. Lyell and Dr. Hooker, who both knew of my work—the latter having read my sketch of 1844—honoured me by thinking it advisable to publish, with Mr. Wallace’s excellent memoir, some brief extracts from my manuscripts.
Ships tracks in black, plotted on a white background, show the outlines of the continents and the predominant tracks on the trade winds. Image and caption from Sapping Attention.
For an interesting historical contrast — that highlights the change from wind to engine powered ships and the opening of the Panama and Suez canals — above is Ben Schmidt’s image created from the log books of U.S. ships in the 19th century, while below is a figure by Ben Halpern showing modern shipping patterns.
This map shows the frequency of shipping traffic along shipping routes around the world, ranging from low (blue) to high (red). Image and caption from SeaWeb.
The first image also clearly shows the triangular trade routes between the Americas, Europe and Africa.
Schmidt also has some wonderful videos showing, among other things, the routes of whaling ships that are pushed farther and farther out as they drive whale populations toward extinction.
As an exercise to transition from ecology to biochemistry in Biology class, I had students follow the energy from the Sun to humans via potatoes. After all, we’ve been putting together food webs, following energy through the food chain, and now I want to start talking about the short and long chained biochemical molecules like glucose and starch, at least at a general level.
Leaves in the forest canopy capturing sunlight. Photosynthesis in action. Because of all the captured (and reflected) sunlight the floor of the forest beneath the canopy is dark with very little undergrowth. Note: these are not potato trees, potatoes tend to grow under the ground, and potato plants are short, bushy herbs.
So, we start with photosynthesis. The leaves of the potato plant capture sunlight and combine water and carbon dioxide to produce glucose with oxygen as a by-product.
This reaction takes radiative energy from the Sun, and stores it as chemical energy in the bonds of the glucose molecule.
A glucose molecule that stores the energy from photosynthesis. The carbon and oxygen atoms in the ring are highlighted.
Glucose is a simple sugar, one of the basic carbohydrate molecules (my bio class has not done the testing for carbohydrates yet, but we will soon). Simple carbohydrates are monomers that can be chained together to produce more complex molecules.
Sizable packets of solar energy stored in the chemical bonds of the carbs.
The potato plant chains together a series of glucose molecules it produces by photosynthesis into long chained polymers called starches. Starches are good for long-term storage of the energy because, for one thing, they don’t dissolve in water the way glucose does. (A good metaphor for this might be to have students carry a handful of beads to represent a bunch of glucose molecules versus carrying a string of beads to represent the starch).
Starch molecules form by chaining together glucose molecules. Water is a byproduct.
The large stash of energy consolidated into the starch is an inviting target for animals like humans. We eat things like potatoes to get the starches, only we usually refer to them by their other name, carbs. Carbs are short for complex carbohydrates: since glucose is a simple carbohydrate, a chain of glucoses is called a complex carbohydrate. This is why people on low-carb diets try to avoid foods like potatoes.
For those of us who do eat potatoes, however, we need to break the starches down into their constituent glucose molecules to get the energy. When we eat potatoes, we chew (masticate) them to break down the cell walls and expose the starches to the enzymes, like amylase in our saliva, that breaks apart the long carbohydrate chains into simple glucose molecules. Enzymes, like amylase, are catalysts. Catalysts are substances that accelerate a chemical reaction, but are not used up in the process.
The body extracts these glucose molecules from the digested food in the small intestines. The glucose is absorbed through the small, finger-like, capillary-filled villi that line the small intestines, and gets into the blood plasma. The circulatory system transports the glucose in the plasma to cells throughout the body.
Cells use the glucose for energy by reversing the photosynthesis reaction, in a reaction called respiration:
So the cells use respiration to liberate the energy the potato plant captured from the Sun.
Teachers’ Note
I very much liked how this exercise worked. Trying to follow the energy through the plant and human, while using as much biological vocabulary as possible, really worked to integrate our discussions of anatomy and ecology, and helped introduce biochemistry. I think I’ll try other exercises like this, where students try to follow a specific atom through the human body or through the environment (as we study global biogeochemical cycles). It might also be useful to use this as an example of how isotopic tracers work.
