First, notice where carbon is fixed by the enzyme nicknamed Rubisco. In C-3, C-4, and CAM plants, CO2 enters the cycle by joining with 5-carbon ribulose bisphosphate to form a 6-carbon intermediate, which splits (so quickly that it isn’t even shown!) into two 3-carbon molecules.
Now look for the points at which ATP and NADPH (made in the light reactions) add chemical energy (“Reduction” in the diagram) to the 3-carbon molecules. The resulting “half-sugars” can enter several different metabolic pathways. One recreates the original 5-carbon precursor, completing the cycle. A second combines two of the 3-carbon molecules to form glucose, universal fuel for life.
The cycle begins and ends with the same molecule, but the process combines carbon and energy to build carbohydrates – food for life.
So – how does photosynthesis store energy in sugar? Six “turns” of the Calvin cycle use chemical energy from ATP to combine six carbon atoms from six CO2 molecules with 12 “hot hydrogens” from NADPH. The result is one molecule of glucose, C6H12O6.
Lesson Summary
The single chemical equation below represents the overall process of photosynthesis as well as summarizes many individual chemical reactions that were understood only after hundreds of years of scientific exploration.
Chloroplasts are the organelles where the process of photosynthesis takes place in plants and algae.
Chloroplasts resemble blue green bacteria, containing their own DNA and ribosomes.
The Endosymbiotic Theory holds that chloroplasts once were independent prokaryotic cells, but were engulfed by other larger prokaryotes, forming the first eukaryotic cells.
Chloroplasts are made of membranes, which enclose stacks of membrane sacs called thylakoids.
The membranes sequence pigments and electron carrier molecules for efficient photosynthesis.
Thylakoids create compartments, which allow concentration gradients to store energy.
Pigment molecules absorb specific wavelengths (colors) of light; chlorophyll is the primary pigment in photosynthesis.
Electron carrier molecules form electron transport chains, which transfer energy in small steps so that the energy can be stored or used for work.
Photosynthesis consists of two groups of chemical reactions: the Light Reactions and the Calvin Cycle.
Light Reactions transform energy from sunlight into chemical energy, and produce and release oxygen gas.
When light strikes pigment molecules, electrons absorb its energy and are excited.
Light also provides energy to split water molecules into electrons, hydrogen ions, and oxygen gas.
The oxygen gas is released as “waste”, but it is the source of the oxygen in Earth’s atmosphere.
Two pathways capture the energy from excited electrons as chemical energy stored in the bonds of molecules; both pathways involve electron transport chains. One produces NADPH molecules, which stores energy and “hot hydrogen”.
A second pumps hydrogen ions into the thylakoids, forming an electrochemical gradient whose energy builds ATP molecules. This is “chemiosmosis”.
The Calvin Cycle uses the NADPH and ATP from the Light Reactions to “fix” carbon and produce glucose.
Stomata underneath plant leaves allow gases (CO2, H2O, and O2) to enter and exit the leaf interior.
Carbon dioxide enters the Calvin Cycle when an enzyme nicknamed “Rubisco” attaches it to a 5-carbon sugar. The unstable 6-carbon compound immediately breaks into two 3-carbon compounds, which continue the cycle.
Most plants fix CO2 directly with this pathway, so they are called C-3 plants.
Some plants have evolved preliminary fixation pathways, which help them conserve water in hot, dry habitats, but eventually the carbon enters the cycle along the “Rubisco” pathway. C-4 plants such as corn use a 3-carbon carrier to compartmentalize initial carbon fixation in order to concentrate CO2 before sending it on to Rubisco.
CAM plants such as jade plants and some cacti open their stomata for preliminary CO2 fixation only at night.
In the Calvin Cycle, the fixed CO2 moves through a series of chemical reactions, gaining a small amount of energy (or “hot hydrogens”) from ATP or NADPH at each step.
Six turns of the cycle process 6 molecules of carbon dioxide and 12 “hot hydrogens” to produce a single molecule of glucose.
The cycle begins and ends with the same 5-carbon molecule, but the process stores chemical energy in food for nearly all life.
Summary Animations
These interactive web sites depicts each step of photosynthesis in great detail.
http://www.johnkyrk.com/photosynthesisdark.html
http://www.johnkyrk.com/photosynthesis.html
Review Questions
Summarize Jan Van Helmont’s willow tree experiment. State his conclusion and the inference he made after his experiment, and explain how his data supports each. Finally, relate his findings to what we know today about the overall process of photosynthesis.
Using the overall equation for photosynthesis, explain which components relate to J.B. Priestley’s observation that “Plants restore the air that animals injure.”
Explain how the structure of a chloroplast – its membranes and thylakoids – makes its function – the chemical reactions of photosynthesis – more efficient.
Summarize the Endosymbiotic Theory. What evidence related to chloroplasts supports this theory?
Name the two stages (sets of reactions) which make up the process of photosynthesis.
Match the major events with the stage of photosynthesis in which they occur. Stages Light Reactions
Calvin Cycle
Major Events
Carbon dioxide is fixed.
Electrons in chlorophyll jump to higher energy levels.
Glucose is produced.
NADPH and ATP are produced.
NADPH and ATP are used.
Oxygen gas is released.
Water is split.
Use your understanding of pigments to explain why the living world appears green. Then think a little further and offer a hypothesis to explain why the world is not black!
Explain the value of cycles of chemical reactions, such as the Calvin Cycle.
Explain how their various methods of carbon fixation adapt C-3, C-4, and CAM plants to different habitats.
We humans depend on photosynthesis, and our actions in turn affect photosynthesis. Explain how humans depend on photosynthesis for: food
building materials for furniture and homes
fuel for vehicles, heat, and electricity
breathable air
Explain how the following actions would affect photosynthesis:
We may clear-cut a forest for timber and parking lot space
When we burn fossil fuels for transportation or heat, we release CO2 into the atmosphere
When we dam up and overuse water in a certain area, the area water table drops
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://micr
o.magnet.fsu.edu/primer/lightandcolor/lightandenergyintro.html.
Laurence A. Moran and Pearson Prentice Hall, “Fixing Carbon: The Rubisco reaction,” 10 July 2007. Sandwalk: Strolling with a Skeptical Biochemist,
http://sandwalk.blogspot.com/2007/07/fixing-carbon-rubisco-reaction.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
accessory pigment
A molecule which absorbs colors of light other than blue-violet and red, and then transfers the energy to chlorophyll.
ATP synthase
Ion channel and enzyme complex that chemically bonds a phosphate group to ADP, making ATP as H+ ions flow through the ion channel.
Calvin Cycle
The second stage of photosynthesis, which can proceed without light, so its steps are sometimes called “light-independent” or “dark” reactions; results in the formation of a sugar.
carbon fixation
The process which converts carbon dioxide in the air to organic molecules, as in photosynthesis.
chlorophyll
The primary pigment of photosynthesis.
chloroplast
The organelle in plant and algal cells where photosynthesis takes place.
electron carrier
A molecule which transfers energy-carrying electrons within an electron transport chain.
electron transport chain (ETC)
A series of electron-carrying molecules which accept and pass along energy-carrying electrons in small steps, allowing the energy lost at each transfer to be captured for storage or work.
endosymbiotic theory
The theory which states that chloroplasts and mitochondria originated as independent prokaryotic cells which were engulfed by larger prokaryotic cells to form the first eukaryotic cells.
glucose
The carbohydrate product of photosynthesis; serves as the universal fuel for life.
light-dependent reactions
The first set of reactions of photosynthesis; requires sunlight; also called the light reactions.
NADPH
An energy carrier molecule produced in the light reactions of photosynthesis; used to build sugar in the Calvin cycle.
photolysis
The light reaction process of splitting water molecules into electrons, hydrogen ions, and oxygen gas.
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.
photosystem
A cluster of proteins and pigments found in chloroplasts and active in photosynthesis.
pigment
A molecule which absorbs specific wavelengths of light energy and reflects others and therefore appears colored.
RuBisCo
The enzyme that combines one molecule of CO2 with a 5-carbon sugar called ribulose biphosphate (RuBP); the most abundant enzyme on earth.
stomata (singular stoma)
Openings on the underside of a leaf which allow gas exchange and transpiration.
thylakoid
Flattened sac-shaped compartment within a chloroplast, made of membranes embedded with molecules which carry out photosynthesis.
Points to Consider
Recall Priestley’s early observation that plants “restore the air.” Name some ways that plants and algae affect the atmosphere.
Which of your own activities affect photosynthesis? Think “globally” in addition to “locally” and add large-scale human activities to your list. Are there any changes you could make in your life which could promote photosynthesis and a healthy atmosphere?
You learned in this chapter that plants make "food" which life needs for energy. But is it usable energy? Or does it need to be converted into some other type of energy? What do you think and why?
Chapter 5: Cellular Respiration
Lesson 5.1: Powering the Cell: Cellular Respiration and Glycolysis
Lesson Objectives
Clarify the relationship between breathing and cellular respiration.
Trace the flow of energy from food molecules through ATP to its use in cellular work.
Compare cellular respiration to burning.
Analyze the chemical equation for cellular respiration.
Briefly describe the role of mitochondria in producing ATP.
Compare cellular respiration to photosynthesis.
Show how carbon and oxygen atoms cycle through producers, consumers, and the environment.
Recognize that glycolysis is the first and most universal of three stages in cellular respiration.
Explain why biologists consider glycolysis to be one of the oldest energy production pathways.
Describe how some of the energy in glucose is transferred to ATP in the cytoplasm, without oxygen.
Introduction
You know that humans deprived of oxygen for more than a few minutes will quickly become unconscious and die. Breathing, also known as respiration, is essential for human life, because the body cannot store oxygen for later use as it does food. The mammalian respiratory system, shown in Figure below features a diaphragm, trachea, and a thin membrane whose surface area is equivalent to the size of a handball court - all for efficient oxygen intake. Other forms of life employ different types of respiratory organs: fish and aquatic amphibians and insects flaunt gills, spiders and scorpions develop "book lungs," and terrestrial insects use an elaborate network of tubes called tracheae, which open via spiracles, as shown in Figure below and Figure below. A constant supply of oxygen gas is clearly important to life. However, do you know why you need oxygen?
Figure 5.1
The human respiratory system is only part of the story of respiration. Diaphragm, lungs, and trachea take air deep into the body and provide oxygen gas to the bloodstream. The fate of that oxygen is the story of cellular respiration.
Figure 5.2
Spiracles in this Indian Luna Moth () caterpillar connect to a system of internal tubes (tracheae) which carry oxygen throughout the animal's body.
Figure 5.3
Gills in this alpine newt larva, , bring blood close to an extensive surface area so that the newt can absorb dissolved oxygen gas from its watery habitat.
Many people would answer that oxygen is needed to make carbon dioxide, the gas exhaled or released by each of the respiratory systems listed above. However, CO2 is waste product. Surely, there is more to the story than just gas exchange with the environment! To begin to appreciate the role of oxygen inside your body, think about when your breathing rate increases: climbing a steep slope, running a race, or skating a shift in a hockey game. Respiration rate correlates with energy use, and that correlation reflects the link between oxygen and energy metabolism. For this reason, the chemical reactions inside your cells that consume oxygen to produce usable energy are known as cellular respiration. This chapter will introduce you to the overall process of cellular respiration, and then focus on the first stage, which by itself does not require oxygen.
An Overview of Cellular Respiration
Another way to think about the role of oxygen in your body - and a good starting point for understanding the whole process of cellular respiration - is to recall the last time you sat by a campfire (see below figure) and noticed that it was "dying." Often people will blow on a campfire to keep it from "dying out." How does blowing help? What happens in a campfire?
Figure 5.4
Analyzing what happens when wood burns in a campfire is a good way to begin to understand cellular respiration.
You know that a fire produces light and heat energy. However, it cannot "create" energy (remember that energy cannot be created or destroyed). Fire merely transforms the energy stored in its fuel – c
hemical energy – into light and heat. Another way to describe this energy transformation is to say that burning releases the energy stored in fuel. As energy is transformed, so are the compounds that make up the fuel. In other words, burning is a chemical reaction. We could write our understanding of this energy-releasing chemical reaction up to this point as:
Now return to what happens when you blow on a fire. The fire was "dying out," so you blew on it to get it going again. Was it movement or something in the air that promoted the chemical reaction? If you have ever "smothered" a fire, you know that a fire needs something in the air to keep burning. That something turns out to be oxygen. Oxygen gas is a reactant in the burning process. At this point, our equation is:
To complete this equation, we need to know what happens to matter, to the atoms of oxygen, and to the atoms of the fuel during the burning. If you collect the gas rising above a piece of burning wood in an inverted test tube, you will notice condensation - droplets appearing on the sides of the tube. Cobalt chloride paper will change from blue to pink, confirming that these droplets are water. If you add bromothymol blue (BTB) to a second tube of collected gases, the blue solution will change to green or yellow (Figure below), indicating the presence of carbon dioxide. Thus, carbon dioxide and water are products of burning wood.
Figure 5.5
Bromothymol blue (BTB) changes from blue to green to yellow as carbon dioxide is added. Thus, it is a good indicator for this product of burning or cellular respiration.
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