Download Chapter 9: Cellular Respiration

Chapter 9: Cellular Respiration
Chemical Energy and Food
- calorie is the amount of energy needed to raise the temperature of 1 gram of water 1 degree Celsius
- One gram of glucose (C6H12O6), when burned releases 3811 calories of heat
- The Calorie (capital “C”) that is used on food labels is a kilocalorie, or 1000 calories
Cellular Respiration
Cellular Respiration is basically the reverse of photosynthesis
enzymes
C6H12O6
Two stages:
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+ 6O2
__
-------------- 6CO2 +
6H20
+ energy
1. Glucose is converted to pyruvate and produce small amounts of
ATP and NADH
2. When oxygen is present pyruvate and NADH are used to produce large
amounts of ATP. When oxygen is absent pyruvate is converted to lactic
acid or ethyl alcohol.
Organisms obtain energy from organic molecules produced during photosynthesis
Stripping electrons from these organic molecules is used to make ATP
If cellular respiration took place in just one step, all of the energy from glucose would be released at
once, and most of it would be lost in the form of light and heat.
Cells release the chemical energy in food molecules a little bit at a time and trap little bits of energy by
using them to make ATP.
Aerobic – metabolic processes that require oxygen
Anaerobic – metabolic processes that do not require oxygen
Because more ATP per food molecule is produced when oxygen is present, aerobic pathways of cellular
respiration are the primary source of energy for most cells
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Stage 1. Glycolysis – splitting of glucose
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glucose is the starting material for cellular respiration
Cellular respiration is the process that releases energy by breaking down food molecules in the
presence of oxygen.
Glycolysis takes place in the cytoplasm. The Krebs cycle and electron transport chain (Stage 2) take
place inside the mitochondria.
the 1st stage is stage is splitting glucose into two 3-carbon molecules called pyruvate (pyruvic acid)
splitting glucose to pyruvate in a biochemical pathway is called glycolysis
At the pathway's beginning, 2 molecules of ATP are used up
glycolysis removes 4 high-energy electrons and passes them to an electron carrier called NAD+,
NADH holds the electrons until they can be transferred to other molecules
NADH helps to pass energy from glucose to other pathways in the cell.
The net gain of 2 ATP and 2 NADH are produced for every glucose molecule
the energy yield from glycolysis is small but the process is so fast that cells can produce thousands of
ATP molecules in just a few milliseconds.
glycolysis itself does not require oxygen.
when a cell generates large amounts of ATP from glycolysis, all of the cell's available NAD+ molecules
are filled up with electrons. Without NAD+, the cell cannot keep glycolysis going, and ATP production
stops.
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Stage 2. The Krebs Cycle and Electron Transport
- after glycolysis, ≈ 90 % of the energy in glucose is locked in the high-energy electrons of pyruvate
- Oxygen is required for the final steps of cellular respiration.
- Because the pathways of cellular respiration require oxygen, they are said to be aerobic.
- pyruvic acid is broken down into carbon dioxide in a series of energy-extracting reactions
- 3-carbon pyruvate is converted to a 2-carbon fragment removing CO2 and hydride ion with high energy
electrons
- CO2 is a byproduct and leaves the mitochondria and the cell
- The hydride ions convert NAD  NADH
- The 2-carbon fragment is called an acetyl group
- The acetyl group attaches to conenzyme A forming acetyl-CoA
Krebs Cycle
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Acetyl-CoA then adds the 2-carbon acetyl group to a 4-carbon molecule, producing a 6-carbon molecule
called citric acid.
- next phase of oxidative respiration called Krebs Cycle or called citric acid cycle produces ATP, electron
carriers and CO2
- two carbon fragment of acetyl-CoA is attached to a 4-carbon molecule in the mitochondria
the 6-carbon molecule is oxidized producing NADH and CO2
the resulting 5-carbon is oxidized to produce NADH, ATP and CO2
- the resulting 4-carbon molecule is oxidized and high energy electrons are attached to NAD and FAD to
produce NADH and FADH2
- these molecules carry most of the energy that was previously stored in glucose
- one cycle produces the same 4-carbon molecule that began the cycle again
- Every time you exhale, you expel the carbon dioxide produced by the Krebs cycle.
- the ATP produced directly in the Krebs cycle can be used for cellular activities.
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Electron Transport Chain
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NADH and FADH2 made in the Krebs cycle carry electrons through a electron transport chain by a
series of oxidative-reduction reactions in the membrane of mitochondria to convert ADP into ATP
In eukaryotes, the electron transport chain is located in the inner membrane of the mitochondrion
In prokaryotes, the same chain is in the cell membrane.
Each electron transport chain passes high-energy electrons to proton-pumping membrane channels
Every time 2 high-energy electrons transport down the electron transport chain, their energy is used to
transport hydrogen ions (H+) across the membrane.
During ele ctron transport, H+ ions build up in the intermembrane space, making it positively charged.
The other side of the membrane, from which those H+ ions have been taken, is now negatively charged.
The inner membranes of the mitochondria contain protein spheres called ATP synthases
The protons build up outside and move back in via ATP synthases and cause it to rotate
Each time it rotates, the enzyme grabs a low-energy ADP and attaches a phosphate, forming high-energy
ATP
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ATP leaves the mitochondria through other protein channels where it can be used by the cell for other
purposes
On average, each pair of high-energy electrons that moves down the electron transport chain provides
enough energy to produce three molecules of ATP from ADP
The total number of ATP molecules produced by cellular respiration is 36
After the energy from the electrons has been expended, hydrogen atoms carrying the electrons are joined
with oxygen to produce H20
The final acceptor for the electron chain is oxygen
Energy from pyruvate can not be extracted unless oxygen is present to siphon off the electrons from the
electron transport chain
As long as oxygen is available, high energy electrons from food molecule can continue to produce ATP
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The 36 ATP molecules per glucose represent about 38 percent of the total energy of glucose
62 percent is released as heat, which is one of the reasons your body feels warmer after vigorous
exercise.
Fermentation
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Fermentation releases energy from food molecules by producing ATP in the absence of oxygen.
if no oxygen is present, pyruvate produced by glycolysis has a different fate
if no oxygen is present electrons can not flow down the electron transport chain but back up and remain
on their carriers NADH
therefore the cells NAD+ becomes saturated with electrons
with no more NAD+ to carry away electrons the pathway of electron transport chain back-up and the
Krebs cycle can not proceed
when oxygen is absent another acceptor for the electrons is formed
Because fermentation does not require oxygen, it is said to be anaerobic
The two main types of fermentation are:
1. lactic acid fermentation
2. alcoholic fermentation
lactic acid fermentation
under anaerobic conditions electrons from glycolysis are added to organic molecules
in animals lactic acid is produced by adding electrons back to pyruvate
when your muscles use all the available oxygen, the limited 2 ATP from glycolysis is all that is available
This process regenerates NAD+ so that glycolysis can continue
Without enough oxygen, the body is not able to produce all of the ATP that is required
this causes the tired feeling in your muscles
lactic acid build up makes your muscles feel sore
however, if oxygen becomes available, lactic acid can be converted back to pyruvate which can enter
oxidative respiration
alcoholic fermentation
fungi and plants have a different pathway
they convert pyruvate to a 2-carbon molecule by removing CO2
the electrons from glycolysis is added to these molecules to produce ethyl alcohol
wine and beer contain ethyl alcohol by yeast (fungi) performing fermentation
Alcoholic fermentation produces carbon dioxide as well as alcohol
Alcoholic fermentation causes bread dough to rise
When yeast in the dough runs out of oxygen, it begins to ferment, giving off bubbles of carbon dioxide
that form the air spaces you see in a slice of bread
The small amount of alcohol produced in the dough evaporates when the bread is baked.
Other Fuels used for Cellular Respiration
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glucose is obtained by eating carbohydrates such as starch and sugar
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fatty acids can also be used in cellular respiration to gain energy
Cells Control Rate of Cellular Respiration
- the rate of cellular respiration slows down when the cell has enough ATP by feedback inhibition
- enzymes in glycolysis and the Krebs cycle are regulated by an allosteric site when ATP levels are high
causing the enzymes to deactivate
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