Cellular Respiration
I. Catabolism: Breaking down molecules for energy
II. Heterotrophic Nutrition
I. Catabolism involves
the break-down of molecules to release energy
The conversion of food into useful energy requires the
breakdown and gradual oxidation of food molecules. Catabolic reactions constantly occur in our
bodies and depend on the sugars produced by photosynthesis. In humans, the first stage of catabolism is
the digestive process that occurs in your stomach and intestines after every
meal (breakdown of large, complex food molecules). These compounds are then absorbed by the
intestine and passed on via the bloodstream to various cells in the body. Each cell then converts the sugars supplied
by digestion into fuel for its own use.
This final stage of catabolism is known as aerobic respiration (requires oxygen) and includes glycolysis, the citric acid cycle, and phosphorylation. When oxygen is unavailable to your cells,
organisms go into anaerobic respiration,
which includes glycolysis and fermentation.
v Cellular
Respiration: the process by which cells make ATP by breaking down organic
molecules.
o
Cellular Respiration Equation: an
oxidation-reduction reaction.
§ C6H12O6
+ 6 O2 → 6 CO2 + 6H20 + ATP
§ Oxidation
means loss of hydrogen electrons, reduction is gain of electrons (OIL RIG)
o
Cellular respiration has two stages:
§ Glycolysis
§ Oxidative
Respiration or Fermentation
o
Two types of cellular respiration:
§ Aerobic
(includes glycolysis + oxidative resp.)
§ Anaerobic
(glycolysis + fermentation)
v Glycolysis
o
Both anaerobic and aerobic pathways begin with glycolysis.
This is the process by which glucose is converted to pyruvic
acid (pyruvate), and some of its energy is released.
o
Glycolysis
occurs in the _____________________
of the cell.
o
One 6-carbon molecule of
___________________is oxidized to produce two 3-carbon molecules of
____________________.
o
What does “glycolysis”
mean?
o
In a series of 10 reactions, 1 molecule
of glucose splits to two identical molecules, each called pyruvate.
o
Glucose is a stable molecule that
doesn’t break down easily. The cell must use gain activation energy to begin glycolysis. This energy comes from ATP.
§ So,
ATP is necessary to begin the breakdown of glucose, but the entire process of glycolysis results in a net gain of two ATP
molecules.
v The
series of chemical reactions in glycolysis
1. Two
Phosphates attach to glucose, forming a new 6-carbon compound. The phosphate
groups come from 2 ATP.
2. The
6-carbon compound is then split into two 3-carbon molecules of PGAL.
3. The
two PGAL molecules are oxidized, and each receives another phosphate group
(provided by 2 NAD+ forming NADH), forming two new three-carbon
compounds.
4. Both
phosphate groups are removed from the three-carbon compounds. This reaction
produces two molecules of pyruvate. The phosphate
groups that were removed combine with a ADP to make
ATP (substrate-level phosphorylation). Four
phosphate groups were added = four molecules of ATP produced.
v Fermentation/Anaerobic
respiration
o
When there is no available oxygen, some
cells can convert pyruvate into other compounds
through additional biochemical pathways in the cytoplasm.
o
The term “fermentation” includes both glycolysis and these additional pathways.
o
Fermentation is the release of energy
from food in the absence of oxygen, but no additional ATP is made.
v Two
types of fermentation:
o
Lactic Acid Fermentation
o
Alcoholic Fermentation
v Beer
is produced by fermentation
o
Beers are obtained by the yeast fermentation
of malted cereal grains combined with hops and water.
§ Germans
consume the most beer at 40 gallons per person per year.
§ Beer
drinkers in US rank 14th in the world, with 156,900 millions of
barrels of beer a year (each barrel equals 31 gallons).
v How
Beer is Made: Early History
o
Early brewing attempts occurred around
7000 BC in Mesopotamia.
o
Egyptians and Greeks brewed alcoholic
beverages, but did not have the term “beer.”
o
English word “beer” stems from the
Celtic word “beor,” which referred to a malt brew
made by monks at a North Gaul monastery.
o
In the Middle
Ages, monasteries were the leading producers of beer.
o
Monks credited with many early brewing
techniques, such as the addition of hops to improve the aroma and help preserve
the beer.
v How
Beer is Made: Later History
o
European immigrants brought brewing
skills to America and founded a thriving beer industry
o
Bottled beer was introduced in 1875 by
Joseph Schlitz Brewing Co. in Milwaukee, WI
o
Canned beer was introduced in the 1930s
v How
Beer is Made
o
Beer requires properly prepared cereal
grain (usually barley and corn or rice), hops, pure water, and brewer’s yeast.
§ Variation
in the quality of each of these items influences the overall quality of the
beer that is produced.
v Malting
o
Barley grains are soaked in cold water
until saturated, placed in shallow tanks, and allowed to germinate.
o
Enzymes are released that will later
convert starches in the grain to sugar for fermentation.
o
Once germination is complete, grain is roasted
to stop the germination process.
o
The grain is now referred to as a malt.
v Preparing
the Mash
o
The malt is crushed using iron rollers
and then transferred to the mash tank
o
Tank is copper or stainless steel
o
Malt is mixed with warm water until it
is of a porridge-like consistency (the mash).
o
Temperature is raised from 100-170
degrees (F) so that the enzymes react.
o
Enzymes break down starch and convert it
to simple sugars.
o
Mash then sits undisturbed so that
solids can descend to bottom of the tank.
v Brewing
the Wort
o
Liquid contained in the mash is
transferred to another tank called a lauter tun.
o
Most of the liquid is drawn out through
the bottom layer of mash solids.
o
Hot water is added to top of mash tank
to rinse the remaining liquid (now called wort) from
the mash.
o
Solid remains of grain are dried and
sold as animal feed.
o
Wort
travels to brew kettles where it is boiled to sterilize it and where hops are
added.
§ Hops
contribute to bitterness of beer.
o
After brewing is complete, finished wort is filtered again and pumped to fermentation tanks.
v Fermenting
o
In the fermentation tanks, the
atmosphere is controlled to prevent bad bacteria from interfering with yeast.
o
Yeast is added to the wort, and the temperature is slowly reduced over a period
of days to between 50 and 60 degrees.
o
Yeast grows, consuming sugar in the wort, and bubbles of CO2 form. Alcohol is also produced as a product of
fermentation. The wort
has now become beer.
o
Beer is transferred to aging casks:
longer storage=increased alcohol content
v Pasteurizing
o
Beer is sometimes heated to high
temperatures to stop yeast from making alcohol.
o
Pasteurization is not used in production
of genuine draft beers: these must be kept refrigerated to preserve their
flavor and slow the remaining yeast activity.
v Packaging
o
Beer is moved gently through piping to
bottling area to preserve natural carbonation.
o
Additional CO2 is added during bottling
o
When beer is dispensed from the keg, a
pressure apparatus called a “tapper” is used to apply a light pressure of CO2 to
the tapper head for dispensing: this improves the aroma of the beer
v Back
to Fermentation!
o
Anaerobic pathways are not efficient in
transferring energy. (only use 2.1% of available energy in glucose)
o
These pathways can provide enough energy
for unicellular or very small multicellular organisms
with limited energy requirements.
o
Most larger
organisms meet their energy requirements with aerobic respiration. However,
fermentation is an essential source of rapid bursts of energy.
v Aerobic
Respiration
o
When oxygen is available, most cells
send pyruvate down the pathway of aerobic
respiration. This pathway produces 20 times as much ATP as produced by glycolysis alone.
o
Aerobic respiration is the break down of ________________ in the presence of
_______________________.
o
Aerobic respiration takes place in the
______________________ of the cell.
o
Aerobic respiration has two major
stages: The Citric Acid Cycle and the Electron
Transport Chain.
§ The
______________ from glycolysis enters the space
inside the inner membrane of a mitochondrion (mitochondrial matrix).
§ It
then reacts with coenzyme A to form ____________________________. CO2,
and 2 NADH and H+ are produced in this reaction. (*transition reaction)
§ From
glycolysis to the Citric Acid Cycle, one glucose
molecules yields 4 ATP, 10 NADH, and 2 FADH2.
§ The
energetic electrons in the molecules of these NADH and FADH2 are used to make
more ATP in a series of reactions known as the ___________________________.
v Citric Acid Cycle
o
Reactions of this cycle were identified
by Hans Krebs (1900-1980).
o
Citric Acid Cycle takes place in the cytoplasm
of prokaryotes, but in the mitochondria of eukaryotes.
o
This biochemical pathway breaks down
Acetyl CoA, producing CO2, NADH, FADH2,
and ATP.
o
5 Main Steps, that all occur in the
mitochondrial matrix
1. A
2-carbon molecule of __________ combines with a 4-carbon compound,
______________, to produce a 6-carbon compound called ____________.
2. Citric
Acid releases a _____________ molecule and a __________ to form a 5-carbon
compound. The H atom is transferred to NAD+, reducing it to ______________.
3. The
5-carbon compound releases another __________________, forming a 4-carbon
compound. NAD+ is reduced to NADH. A molecule of _________ is made from
ADP.
4. The
4-carbon compound releases a _________ to form another 4-carbon compound. The
H+ is used to reduce FAD to FADH2.
5. The
4-carbon compound releases another ______________ to regenerate
________________, which keeps the Citric Acid Cycle operating. The H atom reduces
NAD+ to NADH.
o
One glucose molecule causes two turns of
the Cycle (one glucose produces 2 pyruvate
molecules, which form 2 Acetyl CoA molecules, each of
which go through the Citric Acid Cycle).
§ The
two turns produce 6 NADH, 2 FADH, 2 ATP, and 4 CO2 molecules.
§ CO2
is a waste product that diffuses out of cells and is given off by the organism.
o
After the two turns of the Citric Acid
Cycle, the bulk of energy released by glucose still has not been transferred to
usable ATP. There are now 4 molecules of ATP—2 from glycolysis,
and 2 from the Citric Acid Cycle.
o
However, the 10 total molecules of NADH
(2 from glycolysis, 2 from the transition reaction,
and 6 from the Citric Acid Cycle) and the 2 molecules of FADH2 (both
from the Citric Acid Cycle) drive the next stage of aerobic respiration—the
electron transport chain.
v
Electron Transport Chain (PINBALL AGAIN!)
o
The ETC makes up the second stage of
_______________ respiration.
§ In
prokaryotes, this chain lines the cell membrane.
§ In
eukaryotes, the chain lines the inner membrane of the _________________________(christae).
o
ATP is produced by the electron
transport chain when NADH and FADH2 release H atoms.
§ The
process of producing ATP from energy released by the electron transport chain
is called oxidative phosphorylation.
o
The loss of H atoms results in the
return to NAD+ and FAD molecules, which go back to the Citric Acid Cycle to be
reused.
§ Electrons
in the H atoms are at a high energy level, but they lose some of their energy
as they are passed along the series of molecules in the electron transport
chain.
o
This energy is taken and used to send
the protons of the H atoms from inside the mitochondrial matrix to the outer
side of the inner mitochondrial membrane.
o
Now, there is a high concentration of H
protons in the space between the inner and outer mitochondrial membranes.
§ The
concentration gradient of protons drives the synthesis of ATP. As these protons
move down the concentration gradient into the mitochondrial matrix, ATP synthase within the inner mitochondrial membrane makes ATP
from ADP.
§ ATP
can only be made if electrons continue to move from molecule to molecule in the
electron transport chain.
o
Oxygen is the final acceptor of
electrons. By accepting an electron (which produces water), more electrons can
come down the chain.
·
http://www.science.smith.edu/departments/Biology/Bio231/etc.html
v Aerobic
respiration: energy yield
o
A maximum yield of 38 ATP molecules can be
produced through aerobic respiration.
How much ATP can each step yield?
o
The actual yield varies from cell to
cell, and most eukaryotic cells produce only 36 ATP molecules per glucose.
o
Approximately 39% of energy is harvested
from glucose in the form of ATP.
II. Heterotrophic
Nutrition
Humans (and other heterotrophs) are dependent on the consumption of other
organisms to undergo catabolism to fill our energy needs. In addition, we utilize the materials
produced during digestion to perform anabolic activities, such as the building
of complex carbohydrates and proteins.
The steps in the process of our nutrition are as follows:
1. Ingestion: we put
the food into our mouth
2. Digestion: enzymes
within our bodies break down polysaccharides into simple sugars (monosaccharides), proteins into amino acids, fats into
fatty acids and glycerol, and nucleic acids into nucleotides. These molecules are absorbed by our
intestines and passed to other parts of the body. They are further broken down by our cells
into simple, 2-4 carbon molecules (such as Acetyl CoA). These simple fragments can then be degraded
further and used for catabolism to
release energy (leaving the end products of CO2, H2O, and NH3) or for anabolism to store energy and build new
biological molecules.
v References
for this lecture
v http://www.sirinet.net/~jgjohnso/respiration.html
v http://www.sp.uconn.edu/~terry/images/anim/ETS.html
v http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/M/Metabolism.html