Download Biology Chap 9 9-1 Chemical Pathways I. Chemical Energy and

Biology Chap 9
9-1 Chemical Pathways
I.
II.
III.
Chemical Energy and Food
A. There is a LOT of energy in food.
B. One gram of glucose (when burned in the presence of oxygen) releases 3811 calories of
heat energy!
C. A calorie is the amount of energy needed to raise the temperature of 1 gram of water by
1 degree Celsius.
D. **NOTE – A “Calorie” is different from a “calorie” [Calorie (like on food labels) is actually
a kilocalorie or 1000 calories – so if you look at a food packaged in Europe, it will say
“kcal” instead of Calories]
E. The process of “burning” glucose begins with the pathway called glycolysis.
1. Glycolysis only releases a small amount of energy
2. If oxygen is present, glycolysis leads to two other pathways
3. If no oxygen is present, glycolysis follows a different pathway
Overview of Cellular Respiration
A. This is the process of converting ADP molecules back into ATP (most of this conversion is
done through cellular respiration)
1. A muscle cell (working, i.e. not at rest) will convert ADP into ATP at a rate of
about 10 million molecules per second!
2. It requires the energy in food to do this
B. When there is oxygen, glycolysis is followed by the Krebs Cycle and the electron
transport chain. – these three steps make up the process of cellular respiration!
C. Cellular respiration releases energy by breaking down glucose (and other foods )
1. Oxygen + glucose  carbon dioxide + water + energy
2. But this process doesn’t take place all at once otherwise too much energy would
be released (all at once) – it would be lost as light and heat!
3. So it releases the energy a little bit at a time in many steps, converting ADP to
ATP
Glycolysis
A. This is the process of breaking one molecule of glucose into two molecule of pyruvic
acid (each of which contains 3 carbons).
1. This takes place in the cytoplasm of a cell.
B. ATP production
1. In glycolysis, a cell uses 2 ATP molecules to get the process of glycolysis started
2. The process then creates a total of four ATP molecules
a. Two when the 6 carbon sugar is broken into two, 3-carbon molecules
b. Two more when electrons are removed from the 3-carbon molecules
C. NADH production
IV.
1. Another reaction of glycolysis removes two high energy electrons from the
glucose molecule and passes them to an electron carrier called NAD+
a. NAD+ in cellular respiration works like NADP+ in photosynthesis
b. It carries two high energy electrons to other pathways in the cell
2. Two molecules of NADH are created in glycolysis, each when NAD+ accepts two
electrons and a H+
D. The remaining two 3-carbon molecules are pyruvic acid (or pyruvate) molecules
Fermentation
A. When oxygen is not present, glycolysis is followed by a different pathway
1. This two-step process is called fermentation
2. Fermentation does not require oxygen and is therefore anaerobic
3. It puts two high energy electrons back on to pyruvic acid by taking them from
NADH, creating NAD+
4. The two main types of fermentation are alcoholic fermentation and lactic acid
fermentation
B. Alcoholic Fermentation
1. Yeasts and some other microorganisms use this process
2. Pyruvic acid + NADH  alcohol + carbon dioxide + NAD+
3. This reaction is what causes bread dough to rise
C. Lactic Acid Fermentation
1. Pyruvic acid + NADH  lactic acid + NAD+
2. The body will produce lactic acid during times of high demand for energy when
it runs out of oxygen so that glycolysis can continue
a. Strenuous exercise requires lots of ATP
b. If the body cannot supply oxygen to the tissues fast enough, cells begin
to produce lactic acid so that the production of ATP can continue
c. The buildup of lactic acid in muscle tissue causes a painful, burning
sensation
3. Some prokaryotes also produce lactic acid as a waste product during
fermentation
a. This is used in the production of a large variety of foods to provide a
“tangy” flavor
a. Cheese, yogurt, sour cream, pickles, sauerkraut, etc
9-2 The Krebs Cycle and Electron Transport
I.
The Krebs Cycle
A. Overview
1. After glycolysis is finished, 90% of the energy in glucose is still there – “trapped”
in the pyruvic acid molecules
2. In the presence of oxygen, pyruvic acid moved into the second stage of cellular
respiration called the Krebs Cycle
II.
III.
3. Pyruvic acid is broken down into carbon dioxide in a series of reactions
4. Citric acid is the first compound formed in this cycle so the Krebs Cycle is also
known as the citric acid cycle
B. How it works
1. Begins when pyruvic acid enters the mitochondria
2. One carbon is removed from pyruvic acid and is released as carbon dioxide
3. The remaining 2-carbon molecule join to a compound called coenzyme A which
forms acetyl-CoA
4. Acetyl-CoA then adds the 2-carbon acetyl group to a 4-carbon compound (that
is already in the mitochondria) to form the 6-carbon compound called citric acid
1. CoA breaks away and repeats its part of the process
5. Citric acid is then broken down further, releasing more carbon dioxide (first into
a 5-carbon molecule then into a 4-carbon molecule  the same 4-carbon
molecule that combines with the acetyl group from acetyl-CoA!
6. In addition, as the citric acid is being broken down, electrons are being
transferred to carrier molecules converting NAD+ into NADH and FAD into FADH2
1. Those high energy electrons are then used to make large amounts of
ATP
Electron Transport
A. The high energy electrons from NADH and FADH2 then enter the electron transport
chain.
1. In eukaryotic cells this takes place in the inner membrane of the mitochondria
2. In prokaryotic cells this takes place in the cell membrane
B. These electrons travel from protein to protein in the chain until finally the electrons are
combined with hydrogen ions and oxygen to form water (4 hydrogen ions, 1 oxygen
molecule to make 2 water molecules)
C. At each step on the electron transport chain, the energy from the high energy electrons
is used to transport hydrogen ions across the inner membrane of the mitochondria and
into the intermembrane space (between the inner and outer membrane) –
1. **Note** this is against the concentration gradient!
2. This also makes the intermembrane space slightly positively charged and the
mitochondrial matrix slightly negatively charged
D. The differences in charges that build up helps power a protein “pump” at the site of ATP
synthase, a protein in the inner membrane
1. It isn’t really a pump but allows hydrogen ions to flow (or diffuse) across the
inner mitochondrial membrane along the concentration gradient
2. As hydrogen ions pass through ATP synthase, it spins, grabbing an ADP molecule
and attaches a phosphate to it, producing ATP
E. In general, each pair of high energy electrons that enters the electron transport chain
provides enough energy to make three molecules of ATP.
The Totals
A. Glycolysis is able to make 2 molecules of ATP
IV.
V.
B. In the presence of oxygen, the Krebs cycle and electron transport combine to make 34
more molecules of ATP for a total of 36 molecules of ATP from just one molecule of
glucose
C. The final products of the reactions also include water and carbon dioxide
D. This represents 38% of the total energy available from glucose – the rest is lost as heat
Energy and Exercise
A. Initial energy comes from ATP already stored in a cell and from cellular respiration
B. After about 5-6 seconds, this ATP is nearly gone and most ATP is being produced by
lactic acid fermentation which lasts for about 90 seconds
1. As lactic acid builds up, the only way for the body to get rid of it is through a
chemical reaction that requires lots more oxygen
2. This creates an “oxygen debt” in athletes needing quick bursts of energy (like
sprinters) that is “paid back” with heaving breathing (or panting) after the race
C. For longer energy needs (long races, games, etc), only cellular respiration provides
enough ATP (and it is a slower process) so these athletes have to pace themselves
1. The body can store energy in other carbohydrates such as glycogen which is
slower to break down than glucose (lasting 15-20 minutes)
2. After those sources of energy are gone, the body switches to breaking down
other stored molecules (such as fats) to obtain energy
3. This is why aerobic activity is so recommended for weight control
Comparing Photosynthesis and Cellular Respiration
A. Generally opposites of each other
1. Photosynthesis is energy absorbing; cellular respiration is energy releasing
2. Opposite reactants and products