A "Consumer" Report
Ladies and gentlemen, we are consumers. Newsflash! I know, I know... it's America. Capitalism, materialism... these are familiar concepts. But I'm a biologist. It's what I do (observe the world around me critically). And lately I've been thinking about consuming in a biological context. Surprise, surprise. Recall, I've been using this blog to refresh readers (and myself) about the basics of biological science (click here to see where this started). Consumer Reports is a popular magazine complex interested in reviewing and rating products in hopes of helping consumers make smart buying decisions in the American (and global) marketplace. Today, I'll be submitting my own Consumer Report. This report, however, focuses on the underlying biological mechanisms--those relating to photosynthesis--responsible for the energy in what we consume. As always, I will attempt to paint a picture of photosynthesis from an evolutionary perspective.
You and I consume other living things, from which we obtain energy in a variety of different forms, mostly as proteins, lipids, and long chains of sugars called carbohydrates. As I have just discussed (last couple of posts), our bodies are made of millions and millions of cells capable of using energy bound up in the foods we eat after converting it to energetic currency called ATP through the processes of glycolysis and cellular respiration (if oxygen is present in our cells). But we never serve as the origin of any energy we use.
While our bodies can transform basic chemical compounds into more complex and useful forms (e.g., 10 of the essential amino acids used to synthesize proteins), we are constantly converting energy from one form to another (remember that first law of thermodynamics?). The question eventually arises, where does all this energy come from in the first place? The short answer isn't found on mother earth. It's the sun. A more fruitful explanation lies in the study of plants and other organisms (cyanobacteria, green algae), which we call autotrophs or producers, which can convert solar energy into chemical energy.
Evolution and Photosynthesis
First, a little digression... Evolutionary biologists (like myself) are scientists interested in all aspects of the evolution of biological species. While most evolutionary biologists do not concern themselves with the origins of life, other scientists' work often focuses on evaluating and empirically testing the significance of other kinds of evidence (i.e., astronomy, geology, early earth biochemistry, et cetera) about the archaeology of early life and the ancient earth environment. Sometimes scientists intend to demonstrate the validity of different theories about these latter phenomena, while at other times scientists make discoveries with implications for how we think about the history of life, while this was not their original intent; this second class of discoverers simply realize the implications of their new knowledge after laying it bare. Because photosynthesis is thought to have played a pivotal role in the evolution of life on earth, I choose this perspective as my starting point.
Isotopic studies of meteorite tungsten demonstrate confirm the long-held and well-supported idea that the earth is very, very old--around 4.6 billion years, give or take 10 million, to be more precise (Jacobsen 2003). Evidence suggests simple prokaryotic cells evolved between 3 and 4 billion years ago (Line 2001). Later, the processes of life's evolution generated single celled eukaryotes just over 2 billion years ago, which were first very simple then engulfed proto-plastids and evolved into photosynthesizing eukaryotes (as described in an earlier post, which you can read here) (Margulis 1970; Campbell and Reece 2005). Because a by-product of photosynthesis is oxygen, these creatures, which increased exponentially in number, played a major role in setting the environment up for more complex lifeforms (i.e., animals and land-dwelling plants) by increasing the atmospheric and aquatic O2 concentrations of the early earth (Dismukes et al. 2001; Campbell and Reece 2005). Recently, molecular sequence data of cyanobacteria chloroplasts are consistent with these other findings in demonstrating an ancient origin of photosynthesizing cells (Xiong et al. 2000). Not surprisingly, photosynthesis is still generating crucial oxygen used by most species today. Now that you know a bit about the evolutionary significance of photosynthesizing organisms. Thus, we turn to the process of photosynthesis itself.
Photosynthesis
Everyone who knows anything about this important process knows photosynthesis occurs in plants. Those more erudite scholars among you might even know this happens, more specifically, in the chloroplasts evolved from early plastids I mentioned. Those with heads high in the heavens (even concocting a nosebleed) of scientific knowledge might even remember that plant chloroplasts are divided into different parts and that different sub-cellular pathways carry out what are call "light" and "dark" reactions, that there are different photosystems, and or that electrons are involved in chemiosmosis during the sequence of super duper photosynthetic events. Allow me to clarify for those of us who aren't as smart as the nosebleeders.
Chloroplast Ultrastructure
Chloroplasts, like mitochondria, have two outer layers--outer and inner membranes--and are filled with interconnected networks of membranous sacs known as thylakoids. Chlorophyll is a special pigment in thylakoid membranes that give plants their green color, as you shall see.
As I mentioned, photosynthesis involves two different phases. These are the light reactions and the Calvin cycle. These are crucial to your life and mine and to life's evolution.
Ladies and gentlemen, we are consumers. Newsflash! I know, I know... it's America. Capitalism, materialism... these are familiar concepts. But I'm a biologist. It's what I do (observe the world around me critically). And lately I've been thinking about consuming in a biological context. Surprise, surprise. Recall, I've been using this blog to refresh readers (and myself) about the basics of biological science (click here to see where this started). Consumer Reports is a popular magazine complex interested in reviewing and rating products in hopes of helping consumers make smart buying decisions in the American (and global) marketplace. Today, I'll be submitting my own Consumer Report. This report, however, focuses on the underlying biological mechanisms--those relating to photosynthesis--responsible for the energy in what we consume. As always, I will attempt to paint a picture of photosynthesis from an evolutionary perspective.
You and I consume other living things, from which we obtain energy in a variety of different forms, mostly as proteins, lipids, and long chains of sugars called carbohydrates. As I have just discussed (last couple of posts), our bodies are made of millions and millions of cells capable of using energy bound up in the foods we eat after converting it to energetic currency called ATP through the processes of glycolysis and cellular respiration (if oxygen is present in our cells). But we never serve as the origin of any energy we use.
While our bodies can transform basic chemical compounds into more complex and useful forms (e.g., 10 of the essential amino acids used to synthesize proteins), we are constantly converting energy from one form to another (remember that first law of thermodynamics?). The question eventually arises, where does all this energy come from in the first place? The short answer isn't found on mother earth. It's the sun. A more fruitful explanation lies in the study of plants and other organisms (cyanobacteria, green algae), which we call autotrophs or producers, which can convert solar energy into chemical energy.
Evolution and Photosynthesis
First, a little digression... Evolutionary biologists (like myself) are scientists interested in all aspects of the evolution of biological species. While most evolutionary biologists do not concern themselves with the origins of life, other scientists' work often focuses on evaluating and empirically testing the significance of other kinds of evidence (i.e., astronomy, geology, early earth biochemistry, et cetera) about the archaeology of early life and the ancient earth environment. Sometimes scientists intend to demonstrate the validity of different theories about these latter phenomena, while at other times scientists make discoveries with implications for how we think about the history of life, while this was not their original intent; this second class of discoverers simply realize the implications of their new knowledge after laying it bare. Because photosynthesis is thought to have played a pivotal role in the evolution of life on earth, I choose this perspective as my starting point.
Isotopic studies of meteorite tungsten demonstrate confirm the long-held and well-supported idea that the earth is very, very old--around 4.6 billion years, give or take 10 million, to be more precise (Jacobsen 2003). Evidence suggests simple prokaryotic cells evolved between 3 and 4 billion years ago (Line 2001). Later, the processes of life's evolution generated single celled eukaryotes just over 2 billion years ago, which were first very simple then engulfed proto-plastids and evolved into photosynthesizing eukaryotes (as described in an earlier post, which you can read here) (Margulis 1970; Campbell and Reece 2005). Because a by-product of photosynthesis is oxygen, these creatures, which increased exponentially in number, played a major role in setting the environment up for more complex lifeforms (i.e., animals and land-dwelling plants) by increasing the atmospheric and aquatic O2 concentrations of the early earth (Dismukes et al. 2001; Campbell and Reece 2005). Recently, molecular sequence data of cyanobacteria chloroplasts are consistent with these other findings in demonstrating an ancient origin of photosynthesizing cells (Xiong et al. 2000). Not surprisingly, photosynthesis is still generating crucial oxygen used by most species today. Now that you know a bit about the evolutionary significance of photosynthesizing organisms. Thus, we turn to the process of photosynthesis itself.
Photosynthesis
Everyone who knows anything about this important process knows photosynthesis occurs in plants. Those more erudite scholars among you might even know this happens, more specifically, in the chloroplasts evolved from early plastids I mentioned. Those with heads high in the heavens (even concocting a nosebleed) of scientific knowledge might even remember that plant chloroplasts are divided into different parts and that different sub-cellular pathways carry out what are call "light" and "dark" reactions, that there are different photosystems, and or that electrons are involved in chemiosmosis during the sequence of super duper photosynthetic events. Allow me to clarify for those of us who aren't as smart as the nosebleeders.
Chloroplast Ultrastructure
Chloroplasts, like mitochondria, have two outer layers--outer and inner membranes--and are filled with interconnected networks of membranous sacs known as thylakoids. Chlorophyll is a special pigment in thylakoid membranes that give plants their green color, as you shall see.
As I mentioned, photosynthesis involves two different phases. These are the light reactions and the Calvin cycle. These are crucial to your life and mine and to life's evolution.
Light Reactions use solar radiation to convert carbon dioxide and water into useable chemical forms
Generally, light reactions convert solar energy into chemical energy trapped in the form of ATP and an electron carrier, NADPH (similar to NADH and FADH, from discussion of cellular respiration). By solar energy, I mean light. Let me explain. Light is electromagnetic radiation or energy travelling in the form of waves with the properties of particle packages of energy, or photons. Different wavelengths of light are visible and invisible (see electromagnetic spectrum). Light with low wavelengths is intense (has more energy; e.g., violet light) while the converse is true of long wavelength light (e.g., red light). When light from the sun (which emits all wavelengths of light) hits objects on earth (mostly visible light reaches the surface), some visible light is reflected by those objects, which may absorb or transmit (allow to pass through them) other wavelengths. Chemicals called pigments absorb many frequencies of visible light and reflect a narrow range of wavelengths. So, we see that the chemical composition of matter is highly variable, thus objects take on different colors because their pigments or absorption characteristics (absorption spectra) allow only certain colors to be reflect and processed by visual sensory organs of living things, (e.g., our eyes). And that's a simple (albeit incomplete) explanation of why we perceive objects to be different, specific colors!! Isn't that cool? Nonetheless, there are deeper truths these facts hold in store for photosynthesis.
Thylakoid membranes are peppered with chemical complexes called photosystems. In broad terms, photosystems consist of chemical complexes (groups of large molecules) containing photosynthetically important plant pigments (which I expand on later) surrounding a central region where special pigment molecules are housed. The light reactions of photosynthesis depend upon the excitation of electrons in photosystems and the transport of these electrons down protein chains to generate energy bound in ATP and NADPH.
To elaborate, there are two photosystems at work in plant cells, photosystem I and photosystem II. Here's how they do the work of the light reactions. Pigments in photosystem II abosorb light from the sun at wavelengths of 410-510 nm and 610-700 nm (both are approximations). This light is in the form of an individual photon which energetically must exactly match the difference between excited electron states and the ground state of the pigment molecule (most commonly discussed are chlorophyll a, chlorophyll b, and carotenoids). The photon's energy is transferred directly between pigment molecules in photosystem I's outer complex until it reaches the base of the inner region, where two special chlorophyll molecules are located. These molecules, because they absorb light best at 680 nm, are termed P680's. Absorption of the photon powers a single electron from one of the P680's to a higher energy state, a higher orbital (if you know chemistry). The excited electron reduces a molecule called the primary electron acceptor. Nearby, an enzyme quickly breaks a proximate water molecule into its constituent parts: oxygen, two protons (H+), and two electrons. The electrons drop back to each fill the void left in each P680 molecule at the base of the inner photosystem region from giving electrons to the primary electron acceptor. (hang with me here!) Next, the primary acceptor drops its newly gained electron in one of two places, electron transport chains (ETC; we'll call them electron transport chains one and two, ETC1 and ETC2!). For now, we consider what happens down each individual ETC.
ETC1 is similar to the ETC found in mitochondria, which functions in cellular respiration. As with mitochondrial ETCs, ETC1 breaks the explosive change in energetic state of the electrons from photosystem I into a small series of steps, each yielding ATP. Eventually (quickly), our little electron is passed to photosystem I. That's right! Electrons go from photosystem II to photosystem I. Kind of weird, huh? The explanation for this apparent misnomer is that the photosystems were named in order of discovery, with scientists later determining the order of electron passage contradicted the nomenclature.
The last compound in ETC1 drops our electron directly onto the special pigment molecules called P700's, chlorophylls that absorb light best at 700 nm (go figure), which are at the base of the central region of photosystem I. Again, this transfer of energy blasts electrons from P700's to an excited state, which reduces photosystem I's primary electron acceptor. This acceptor, in turn, drops the electron down the other ETC (i.e., ETC2), where, upon reaching the bottom of the chain, it is passed on to an enzyme called NADP+ reductase. This enzyme requires two electrons from the light reactions (photosystem I) to reduce NADP+ to NADPH, an electron carrier molecule. And that's it for the light reaction phase!
ATP generation
I told you that electrons dropping down ETC1 make ATP and those funneled down ETC2 cause NADP+ to be reduced to NADPH. What I said was true. However, I was holding something back; namely, the mechanism responsible for these changes. It is a familiar mechanism, chemiosmosis (if you don't know what this means, read this)! This is the same mechanism driving the generation of the majority of ATP in aerobically respiring living cells, oxidative phosphorylation, a process of cellular respiration.
As I have stressed in talking about cellular respiration, ATP is generated by an enzyme, ATP synthase, as a result of a H+ gradient in mitochondria. The same is true of photosynthesis. In mitochondria, ATP is made as H+ are pumped down their concentration gradient from the mitochondrial intermembrane space into the inner space of the organelle (mitochondrial matrix), where ADP is phosphorylated into ATP. In plants, things are not much different. Obviously, the electron transport chains are different. Less obviously, however, is the fact that another similarity exists. In mitochondria AND thylakoids, the ETC's and ATP synthases are embedded in the same membrane (the inner membrane of mitochondria, the only membrane in thylakoids) and ETCs supply force to pump H+ into a different space (against their passive transport directionality, therefore requiring active transport) which accumulate and run easily through ATP synthase literally turning the machinery to generate energy for ATP production. Whew! Clear as mud right? But where does that ATP go next? How is it actually used in cells of plants? What kind of work do these cells accomplish and why is it important to you and me as consumers? Simple (ha ha).
The Calvin Cycle
In brief, the Calvin cycle uses ATP and NADPH to turn carbon dioxide into polysaccharides that benefit consumers like us! In a sense, this process is partially analogous to a reversal of the citric acid cycle (CAC). On the one hand, the CAC uses oxidized sugars to generate chemicals storing energy. On the other hand, the Calvin cycle expends energy to make sugars, which are stored in plant tissues and consumed by animals as food. The Calvin cycle is comprised of three phases, (1) carbon fixation, (2) reduction, and (3) regeneration of the starting materials (carbon dioxide). While some think of the Calvin cycle as generating sugars directly, this is not the case. Instead, it yields three carbon sugars which are also intermediates in CAC's, glyceraldehyde-3-phosphate, or G3P. Enzymes separate from the Calvin cycle use G3P as a starting material for glucose genesis, which only involves a few steps.
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Dismukes, et al. 2001. The origin of atmospheric oxygen on earth: the innovation of oxygenic photosynthesis. PNAS 98(5):2170-2175.
Jacobsen, S.B. 2003. How Old Is Planet Earth? Science (Washington). 300(5625):1513-1514..
Line, M.A. 2001. The enigma of the origin of life and its timing. Microbiology 148:21-27.
Margulis, L. 1970. Origin of eukaryotic cells. New Haven, Yale University Press.
Xiong, et al. 2000. Molecular evidence for the early evolution of photosynthesis. Science 289(5485):1724 - 1730.
For info on other citations, see References post.
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