Saturday, May 14, 2016
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
Look at any college level biology book and you will find the basic mechanics of photosynthesis. I used Freeman’s Biological Science 5th ed in my class this past year https://www.pearsonhighered.com/product/Freeman-Biological-Science-5th-Edition/9780321743671.html
Saturday, May 7, 2016
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.
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