Download Assignment 6 Cell Respiration

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Working in groups of two to four and armed with a textbook or some other reliable source
answer the following questions concerning cellular respiration (Chapter 6). This assignment is worth
30 points with the possible points for each question in parenthesis. Helpful web sites:
http://www.biology-pages.info/C/CellularRespiration.html
http://www.phschool.com/science/biology_place/biocoach/cellresp/fermentation.html
http://www.youtube.com/watch?v=Gh2P5CmCC0M
1. (5) In your own words, describe the overall goal of aerobic cellular respiration and the specific
cellular sites of each of the three processes (Glycolysis, Transport step, Krebs cycle, and the ETC)
which occur during cellular respiration?
The overall goal of aerobic cellular respiration is to liberate energy from the degradation of
primarily glucose molecules, but also other nutrient molecules, like amino acids or lipids and to
transfer this energy into the molecule called Adenosine Tri-Phosphate (ATP). The processes
involved are Glycolysis, the Krebs cycle, and the Electron Transport Chain (ETC) (also called the
electron transport system or shuttle (ETS)). Glycolysis occurs as a series of enzymatically driven
steps (10) which occur in the cytoplasm of all cells. The Krebs cycle is a series of enzymatically
driven steps (8) that occur in the mitochondrial matrix of eukaryotic cells. The ETC is run by a series
of reduction/oxidation linked molecules (called cytochromes) found embedded into the inner
mitochondrial membrane (imm.). Where would the Glycolytic enzymes, Kreb’s cycle enzymes and
ETC cytochromes be found in a prokaryotic cell?
2. (5) Describe the three major events of Glycolysis: Investment, Splitting, and Harvest to include
how each of these adds to the efficiency of the breakdown of glucose to pyruvic acid and producing
ATP?
Glycolysis is designed to take nutrient molecules like glucose and prepare them for complete
degradation via the Kreb’s Cycle. To accomplish this goal glycolysis is divided into three phases:
Investment, Splitting, and Harvest. The Investment stage of glycolysis involves the attachment of 2
phosphate groups to the glucose by splitting 2 molecules of ATP by an enzyme with the last name
of Kinase. This happens twice. The first time is to change a glucose molecule structurally to
prevent it from leaking back across the cell membrane via a facilitated diffusion protein channel.
The second time is to rearrange the glucose into a fructose that is a balanced molecule with a
phosphate group on either end of the 6C fructose molecule to allow for the ease of splitting the
molecule into two equal halves by an enzyme called Aldolase. The Splitting stage happens when an
enzyme (called aldolase) simply cuts the fructose into two 3 carbon chains called G3P, each with a
phosphate group on one end (Remember G3P from the Calvin cycle?). This step (splitting a bond)
liberates energy in the form of hydrogen electrons from two hydrogens, which are then removed
from the area by a molecule called Nicotinamide Adenine Dinucleotide (NAD+) forming NADH +
H+(by an enzyme called Dehydrogenase). So we have removed some energy but it is not yet into a
usable ATP molecule. When the electrons are removed and picked up by NAD+ the hydrogen
protons chase the NAD+ with the hopes of retrieving their lost electrons (This will happen later
during the running of the ETC). A phosphate group (designated as Pi or inorganic phosphate from
your diet) in the cell cytosol nearby is attracted to the site which liberated the electrons and
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Class Assignment 6
attaches itself to the carbon chain to prevent the molecule from simply reforming. This happens
twice or once for each of the 3C chains and the resulting molecules have phosphates on either end
of their structure once again. The Harvest involves the direct phosphorylation (or Substrate Level
Phosphorylation or SLP) of ADP by enzymes that can remove the phosphates from each of the 3
carbon chains and attach them to an Adenosine Di-Phosphate (ADP) forming ATP (by different
Kinases). This happens four times so we get a net yield of 2ATP (Why?). We also produce 2
3Carbon pyruvic acid (PAs) molecules that still can be broken down to liberate the rest of the
energy in the form of liberated electrons.
The net gains from glycolysis are 2ATP’s (4 total but it costs two to start), 2 PAs, and 2NADH +
H+’s per glucose.
3. (5) Describe the steps of the transport of pyruvic acid to the Krebs cycle enzymes?
The resulting PAs (pyruvic acids) from glycolysis are then transported one at a time to the
mitochondrial matrix for further degradation. A problem arises since two mitochondrial
membranes have to be crossed in eukaryotic cells. To accomplish this transport a special taxi cab
molecule called Co-enzymeA (CoA) was designed to cart the PA, but CoA can only transport a 2C
chain molecule (a two seater taxi). To accommodate CoA a CO2 is removed from each PA to form a
2C acetyl group. The liberation of CO2 by Dehydrogenase enzymes also liberates electrons and
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Class Assignment 6
NAD+ is waiting in the wings to steal electrons from whom? You guessed it hydrogen. This
happens twice or once for each PA.
The net gains from the transport step are 2CO2’s and 2NADH + H+’s per glucose.
4. (5) Describe the two major divisions of the Krebs cycle: Destroying and Rearranging. In your
answer includes how each of these adds to the efficiency of the breakdown of acetic acid and
producing ATP?
The Krebs cycle can be grossly divided into two halves the splitting or destroying side which
destroys each acetyl group by cutting out 2CO2’s and liberating electrons (from?) and the
rearranging side to get back to the starting molecule oxaloacetic acid . To start the destruction
each acetyl group being transported by CoA is combined to a starting molecule called oxaloacetic
acid (OA) to form citric acid (CA) (the other name for the cycle) by the enzyme Citrate Synthase.
2CO2’s are split from the CA by Dehydrogenase enzymes and each time a CO2 is released the
electrons from hydrogen are given to NAD+ (think energy) to form NADH + H+. Just like in
glycolysis, when the electrons are removed a phosphate (from your diet) can fill the void on the
molecule to prevent it from collapsing back on itself and then the phosphate can be enzymatically
removed to ADP to form 1ATP (via SLP) (The GTP in the figure below is an intermediate step.).
The rearrangement side of the cycle is designed to reform the starting molecule of OA
(oxaloacetic acid). To do these steps electrons are stripped from the remaining acetyl group (from
whom?) and given not only to an NAD+ to form NADH + H+, but also a different electron carrier
molecule called Flavin Adenine Dinucleotide (FAD+) to form FADH2 (This molecule can bind both of
the hydrogen atoms hence it is referred to as FADH2). Once we have reformed OA we can begin the
cycle with the second acetyl group from the other pyruvic acid from the original glucose.
The net gains are 4CO2’s, 6 NADH + H+’s, 2FADH2’s, and 2ATP’s per glucose.
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Class Assignment 6
5. (5) Describe the electron transport chain, system, or shuttle (ETC or S) in five steps or less.
The ETS is a series of oxidation/reduction molecules (called cytochromes) tied together and
embedded into the inner mitochondrial membrane. The NADH + H+’s arrive at the beginning of the
chain and give the electrons (from whom?) to the first molecule of the chain (reducing it), who
passes the electrons to its neighbor reducing the neighbor while oxidizing itself. This electron
passing continues through the rest of the transport molecules. The FADH2’s have to drop off their
electrons farther down the chain, but the transport mechanism is the same. As the electrons are
passed from cytochrome to cytochrome they lose energy and the energy loss is used to pump the
hydrogen ion (H+) across the imm. and into the space between the inner and outer mitochondrial
membranes. Eventually the electrons are passed to an oxygen atom by the last cytochrome on the
inside of the inner membrane (Why do you need to breathe?). As the hydrogen concentration
gradient builds up across the imm. the hydrogen’s seek a means to return to their electrons. The
hydrogen’s can only return across the imm. via a specific portal called ATP synthase
(IcanmakeATPdoggoneitase). As the hydrogen’s pass through the portal, ATP synthase unites ADP
with a Pi forming ATP and the hydrogen’s bind with the oxygen to form a water molecule (called
metabolic water). Each pair of electron passed through the chain from the NADH + H+’s will yield
the production of 3ATP, whereas those from FADH2’s will yield 2ATP. This is called oxidative
phosphorylation (OP) via chemiosmosis. Please see the figure on the next page.
What is the total ATP production for Aerobic Respiration, Glycolysis only, Kreb’s cycle only, and
ETS only?
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Class Assignment 6
6. (5) Describe Figure 7.11 in the textbook or why is having a complete balanced diet as it applies to
your eating habits, attempts to lose or gain weight, or maintaining a healthy life style?
I’ll leave this answer for you to ponder after learning the previous material and studying the
figure.
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Class Assignment 6