A molecule of glucose, which has the chemical formula C6H12O6, carries a packet of chemical energy just the right size for transport and uptake by cells. In your body, glucose is the “deliverable” form of energy, carried in your blood through capillaries to each of your 100 trillion cells. Glucose is also the carbohydrate produced by photosynthesis, and as such is the near-universal food for life.
ATP molecules store smaller quantities of energy, but each releases just the right amount to actually do work within a cell. Muscle cell proteins, for example, pull each other with the energy released when bonds in ATP break open (discussed below). The process of photosynthesis also makes and uses ATP - for energy to build glucose! ATP, then, is the useable form of energy for your cells.
Glucose is the energy-rich product of photosynthesis, a universal food for life. It is also the primary form in which your bloodstream delivers energy to every cell in your body. The six carbons are numbered.
Why do we need both glucose and ATP? Why don’t plants just make ATP and be done with it? If energy were money, ATP would be a quarter. Enough money to operate a parking meter or washing machine. Glucose would be a dollar bill (or $10) – much easier to carry around in your wallet, but too large to do the actual work of paying for parking or washing. Just as we find several denominations of money useful, organisms need several “denominations” of energy – a smaller quantity for work within cells, and a larger quantity for stable storage, transport, and delivery to cells.
Let’s take a closer look at a molecule of ATP. Although it carries less energy than glucose, its structure is more complex. “A” in ATP refers to the majority of the molecule – adenosine – a combination of a nitrogenous base and a five-carbon sugar. “T” and “P” indicate the three phosphates, linked by bonds which hold the energy actually used by cells. Usually, only the outermost bond breaks to release or spend energy for cellular work.
An ATP molecule, shown below, is like a rechargeable battery: its energy can be used by the cell when it breaks apart into ADP (adenosine diphosphate) and phosphate, and then the “worn-out battery” ADP can be recharged using new energy to attach a new phosphate and rebuild ATP. The materials are recyclable, but recall that energy is not!
How much energy does it cost to do your body’s work? A single cell uses about 10 million ATP molecules per second, and recycles all of its ATP molecules about every 20-30 seconds.
A red arrow shows the bond between two phosphate groups in an ATP molecule. When this bond breaks, its chemical energy can do cellular work. The resulting ADP molecule is recycled when new energy attaches another phosphate, rebuilding ATP.
Keep these energy-carrying molecules in mind as we look more carefully at the process which originally captures the energy to build them: photosynthesis. Recall that it provides nearly all of the food (energy and materials) for life. Actually, as you will see, we are indebted to photosynthesis for even more than just the energy and building blocks for life.
Photosynthesis: The Most Important Chemical Reaction for Life on Earth
What do pizza, campfires, dolphins, automobiles, and glaciers have in common? In the following section, you’ll learn that all five rely on photosynthesis, some in more ways than one. Photosynthesis is often considered the most important chemical reaction for life on earth. Let’s delve into how this process works and why we are so indebted to it.
Photosynthesis involves a complex series of chemical reactions, each of which convert one substance to another. These reactions taken as a whole can be summarized in a single symbolic representation – as shown in the chemical equation below.
We can substitute words for the chemical symbols. Then the equation appears as below.
Like all chemical equations, this equation for photosynthesis shows reactants connected by plus signs on the left and products, also connected by plus signs, on the right. An arrow indicating the process or chemical change leads from the reactants to the products, and conditions necessary for the chemical reaction are written above the arrow. Note that the same kinds of atoms, and number of atoms, are found on both sides of the equation, but the kinds of compounds they form change.
You use chemical reactions every time you cook or bake. You add together ingredients (the reactants), place them in specific conditions (often heat), and enjoy the results (the products). A recipe for chocolate chip cookies written in chemical equation form is shown below.
Compare this familiar recipe to photosynthesis below.
The equation shows that the “ingredients” for photosynthesis are carbon dioxide, water, and light energy. Plants, algae, and photosynthetic bacteria take in light from the sun, molecules of carbon dioxide from the air, and water molecules from their environment and combine these reactants to produce food (glucose).
Of course, light, carbon dioxide, and water mix in the air even without plants. But they do not chemically change to make food without very specific necessary conditions which are found only in the cells of photosynthetic organisms. Necessary conditions include:
enzymes - proteins which speed up chemical reactions without the heat required for cooking
chlorophyll - a pigment which absorbs light
chloroplasts - organelles whose membranes embed chlorophyll, accessory pigments, and enzymes in patterns which maximize photosynthesis
Within plant cells or algal cells, chloroplasts organize the enzymes, chlorophyll, and accessory pigment molecules necessary for photosynthesis.
When the reactants meet inside chloroplasts, or the very similar cells of blue-green bacteria, chemical reactions combine them to form two products: energy-rich glucose molecules and molecules of oxygen gas. Photosynthetic organisms store the glucose (usually as starch) and release the oxygen gas into the atmosphere as waste.
Let’s review the chemical equation for photosynthesis once more, this time at the level of atoms as in the equation below.
Look closely at its primary purpose: storing energy in the chemical bonds of food molecules. The source of energy for food is sunlight energy. The source of carbon atoms for the food molecules is carbon dioxide from the air, and the source of hydrogen atoms is water. Inside the cells of plants, algae, and photosynthetic bacteria, chlorophyll, and enzymes use the light energy to rearrange the atoms of the reactants to form the products, molecules of glucose and oxygen gas. Light energy is thus transformed into chemical energy, stored in the bonds which bind six atoms each of carbon and oxygen to twelve atoms of hydrogen – forming a molecule of glucose. This energy rich carbohydrate molecule becomes food for the plants, algae, and bacteria themselves as well as for the heterotrophs which feed on them.
One last detail: why do “6”s precede the CO2, H2O, and O2? Look carefully, and you will see that this “balances” the equation: the numbers of each kind of atom on each side of the arrow are equal. Six molecules each of CO2 and H2O make 1 molecule of glucose and 6 molecules of oxygen gas.
Lesson Summary
All organisms require a constant input of energy to do the work of life.
Energy cannot be recycled, so the story of life is a story of energy flow – its capture, transformation, use for work, and loss as heat.
Life runs on chemical energy.
Food is chemical energy stored in organic molecules.
Food provides both the energy to do life’s work and the carbon to build life’s bodies.
The carbon cycles between organisms and the environment, but the energy is “spent” and must be replaced.
Organisms obtain chemical energy in one of two ways.
Autotrophs make their own carbohydrate foods, transforming sunlight in photosynthesis or transferring chemical energy from inorganic molecules in chemosynthesis.
Heterotrophs consume organic molecules originally made by autotrophs.
All life depends absolutely upon autotrophs to make food molecules.
The process of photosynthesis produces more than 99% of all food for life, forming the foundation of most food
chains.
Only three groups of organisms – plants, algae, and some bacteria – carry out the process of photosynthesis.
All organisms use similar energy-carrying molecules for food and to carry out life processes.
Glucose (C6H12O6,) is a nearly universal fuel delivered to cells, and the primary product of photosynthesis.
ATP molecules store smaller amounts of energy and are used within cells to do work.
Chlorophyll and NADPH molecules hold energy temporarily during the process of photosynthesis.
The chemical equation below summarizes the many chemical reactions of photosynthesis.
The equation states that the reactants (carbon dioxide, water and light), in the presence of chloroplasts, chlorophyll and enzymes, yield two products, glucose and oxygen gas.
Chlorophyll is a pigment that absorbs sunlight energy.
Chloroplasts are the organelles within plant and algal cells that organize enzymes and pigments so that the chemical reactions proceed efficiently.
In the process of photosynthesis, plants, algae, and blue green bacteria absorb sunlight energy and use it to change carbon dioxide and water into glucose and oxygen gas.
Glucose contains stored chemical energy and provides food for the organisms that produce it and for many heterotrophs.
Photosynthesized carbohydrates (represented here by glucose) make up the wood we burn and (over hundreds of millions of years) the coal, oil, and gas we now use as fossil fuels.
Most of the oxygen gas is waste for the organisms which produce it.
Both CO2 consumed and O2 produced affect the composition of earth’s atmosphere; before photosynthesis evolved, oxygen was not part of the atmosphere.
Review Questions
Compare the behavior of energy to the behavior of matter in living systems.
Water and carbon dioxide molecules are reactants in the process of photosynthesis. Does this mean they are “food” for plants, algae, and blue-green bacteria? Use the definition of “food” to answer this question.
Compare autotrophs to heterotrophs, and describe the relationship between these two groups of organisms.
Name and describe the two types of food making found among autotrophs, and give an example of each. Which is quantitatively more important to life on earth?
Trace the flow of energy through a typical food chain (describing "what eats what"), including the original source of that energy and its ultimate form after use. Underline each form of energy or energy-storing molecule, and boldface each process which transfers or transforms energy.
Trace the pathway that carbon atoms take through a typical food chain, beginning with their inorganic source.
The fact that all organisms use similar energy-carrying molecules shows one aspect of the grand "Unity of Life." Name two universal energy-carrying molecules, and explain why most organisms need both carriers rather than just one.
A single cell uses about 10 million ATP molecules per second. Explain how cells use the energy and recycle the materials in ATP.
Discuss the importance of photosynthesis to humans in terms of food, fuel, and atmosphere. In what ways could you affect the process of photosynthesis to conserve these benefits?
Using symbols, write the overall chemical equation for photosynthesis, labeling the reactants, necessary conditions, and products. Then write two complete sentences which trace the flow of (1) energy and (2) atoms from reactants to products.
Further Reading / Supplemental Links
Graham Kent, “Light Reactions in Photosynthesis” Animation. Bio 231 Cell Biology Lab, October 2004. Available on the Web at:
http://www.science.smith.edu/departments/Biology/Bio231/ltrxn.html.
Illustrator: Thomas Porostocky; Writer: Lee Billings; Map data adapted from MODIS observations by NASA's Terra and Aqua satellites; Graph data and reference: Biology, 4th ed., Neil A. Campbell, Benjamin/Cummings Publishing Company, 1996. “Crib Sheet #10, Photosynthesis.” Seed Magazine, August 2007. Available on the Web at:
http://www.seedmagazine.com/news/uploads/cribsheet10.gif.
John Mynett, “Photosynthesis Animations.” Biology4All, 01 January 2002. Available on the Web at:
http://www.biology4all.com/resources_library/details.asp?ResourceID=43
Kenneth R. Spring, Thomas J. Fellers, and Michael W. Davidson, “Introduction to Light and Energy.” Molecular Expressions Optical Microscopy Primer. The Physics of Light and Energy, Last modified Aug 23, 2005. Available on the Web at
http://micro.magnet.fsu.edu/primer/lightandcolor/lightandenergyintro.html.
“Photosynthesis,” “Electron Transport Chain” and “ATP Synthase” Animations. Virtual Cell Animation Collection, Molecular and Cellular Biology Learning Center, no date given. Available on the Web at:
http://vcell.ndsu.nodak.edu/animations/photosynthesis/index.htm.
Vocabulary
ATP
Adenosine triphosphate, the energy-carrying molecule used by cells to do work.
autotroph
An organism capable of transforming one form of energy – usually light – into the food, or stored chemical energy, they need to do work.
chemosynthesis
Process by which a type of autotroph makes food using chemical energy in inorganic molecules.
chlorophyll
The primary pigment of photosynthesis.
chloroplast
The organelle in plant and algal cells where photosynthesis takes place.
consumers
Heterotrophs, which must eat or absorb organic food molecules because they are incapable of producing them.
energy
The ability to do work.
food
Organic (carbon-containing) molecules which store energy in the chemical bonds between their atoms.
food chain
A pathway which traces energy flow from producers through consumers.
glucose
The carbohydrate product of photosynthesis; serves as the universal fuel for life.
heat
Thermal energy, the energy of vibrations in molecules – the “lowest” form of energy, which cannot easily be used for useful work.
heterotrophs
Organisms which must consume organic molecules because they are incapable of synthesizing the food, or stored chemical energy, they need to work.
inorganic molecules
Molecules which do not contain carbon (with a few exceptions such as carbon dioxide) and are not necessarily made by living organisms.
NADPH
An energy carrier molecule produced in the light reactions of photosynthesis and used to build sugar in the Calvin cycle.
organic molecule
A molecule which contains carbon, made by living organisms; examples include carbohydrates, lipids, and proteins.
photosynthesis
The process by which plants, algae, and some bacteria transform sunlight into chemical energy and use it to produce carbohydrate food and oxygen for almost all life.
producer
An autotroph, capable of synthesizing food molecules; forms basis of food chains.
Points to Consider
Why do some people describe photosynthesis by plants as “making food from thin air"?
Before we conclude this analysis of “the most important chemical reaction for life on Earth,” solidify your understanding of its importance by returning to the pizza, campfires, dolphins, automobiles, and glaciers. Can you connect them all to the chemical equation for photosynthesis (Figure below)?
You’ll be able to make more connections after studying the next chapter on cellular respiration. Can you already connect carbon dioxide and oxygen to automobiles?
Figure 4.4
Lesson 4.2: Into the Chloroplast: How Photosynthesis Works
Lesson Objectives
Understand that hundreds of years of scientific exploration have contributed to our understanding of photosynthesis.
Explain the
contributions of Van Helmont, Priestley, and Melvin Calvin to our understanding of photosynthesis.
Describe the structure and function of chloroplasts, thylakoids, and pigments.
Explain how electron carrier molecules form electron transport chains.
Trace the flow of energy and materials through the Light Reactions, including chemiosmosis.
Trace the flow of energy and materials through The Calvin Cycle.
Compare and contrast C-3, C-4, and CAM pathways for carbon fixation.
Introduction
Life requires photosynthesis for fuel and for the oxygen to burn that fuel. Since the Industrial Revolution (late 18th and early 19th centuries), we humans have relied on products of ancient photosynthesis for enormous quantities of fossil fuel energy. And, knowingly or not, we have also benefited from photosynthesis to remove the carbon dioxide produced when we burn those fuels. So it may not surprise you that biologists have studied this critical process in great detail. The goals of this lesson are:
to discuss how scientists have explored this most important chemical reaction for life on earth
to encourage you to appreciate just a little of its intricate beauty, and
to understand how your own decisions and actions can influence the process of photosynthesis.
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