Download BIO 10 Lecture 2

BIO 10
Lecture 7
THE VITAL FORCE:
RESPIRATION
Respiration = the process by which living
organisms harvest the energy in highly
ordered, high energy molecules such as
carbohydrates and fats by oxidizing them
into CO2 and H2O to produce energy
usable for life processes
oxidation
(C6H10O5)n
CO2 + H2O + USABLE ENERGY
• Molecules like carbohydrates and fats are
energy-rich because they are highly
ordered
• That order can be converted to energy
when the molecule loses an electron
• When a molecule loses an electron during a
chemical reaction, it is said to be oxidized
• Stores less energy than it did before
• The loss of the electron releases energy
– When a molecule gains an electron during a
chemical reaction, it is said to be reduced
• Stores more energy than it did before
• The gain of the electron requires the input of energy
• During photosynthesis, plants take in CO2 +
H2O from the environment and, using the
energy from the sun, reduce these
molecules (add electrons to them)
• The result is starch, a highly reduced form
of carbon, hydrogen, and oxygen
• Long chains of simple sugars
• Highly ordered so very energy-rich
• Used to temporarily store the energy from
sunlight until it can be harvested by the cell
into a usable form via respiration
• May also be eaten by animals, who then harvest
the stored energy in an identical manner
Types of Respiration:
– Anaerobic
• Does not require oxygen
• Evolved first
• Very inefficient
• Takes place in the cytoplasm
• Can only support small life forms
– Prokaryotes
– Some small eukaryotes
– Can occur in large eukaryotes for brief periods
– Aerobic
• Requires oxygen
• Evolved later
• Highly efficient
• Takes place in the mitochondria
– Eukaryotes only
Usable Energy: ATP
• Cells cannot use the energy released during
the oxidation of carbohydrates and lipids
directly
– Must convert the energy stored in food into a
useable form or “currency”
– Much like we store the potential energy we earn by
working at our jobs as dollars in the bank, the
potential energy from food breakdown is stored as
ATP
This bond carries
small units of usable
energy around the cell
• Like dollars, ATP molecules carry energy in a form that
can be released at any time
• ATP also happens to carry just about the right amount
of energy to drive most cellular reactions
• During respiration, the energy stored in carbohydrates
and lipids is converted to the usable storage molecule
ATP
Photosynthesis: Plants use the photons from sunlight to
reduce carbon dioxide and water into carbohydrates
Carbohydrates
are stored as
potential
energy in the
plant cell
Respiration:
Carbohydrates are
oxidized to drive the
production of ATP
How is Respiration
Accomplished?
• General reaction:
– C6H12O6 +6O2 + ADP  6CO2 + 6H2O + ATP
(high energy)
(low energy products)
• Two stages:
Usable energy
• Glycolysis
– All respiring organisms
– 2 ATP molecules per glucose broken down
• Krebs Cycle and the Electron transport chain
– Eukaryotes only
– 34 additional ATP molecules per glucose
Glycolysis:
• Glucose enters the cytoplasm of the cell
• An enzyme immediately breaks apart one ATP
molecule into ADP and P (produces energy)
– The phosphate (P) group in then attached to the glucose,
creating glucose-6-phosphate
• At this point, the glucose-6-phosphate is rearranged
into fructose-6-phosphate and another phosphate is
added
• This requires one more ATP molecule
• The result is fructose-1,6 phosphate
• Note that, so far, no ATP have been produced – in
fact 2 ATP have been consumed!
– Ledger: -2 ATP
• Fructose-1,6 diphosphate is now cleaved into
two molecules of glyceraldehyde-3-phosphate
– Each of these molecules will now continue on
through glycolysis in an identical manner
• A molecule called NAD+ now strips an electron
from (oxidizes) each of the glyceraldehyde-3phosphate molecules
– The energy from this reaction is used to add
phosphate groups to each of the glyceraldehyde-3phosphates
• They are now 1,3-diphosphoglyceric acids
• Electron carrier molecules like NAD+ serve to help
carry the electrons down their energy gradients during
respiration
• NAD+ strips an electron off one molecule and (as
NADH) carries it to another molecule which accepts
the electron in a lower energy state than it was when
NAD+ got it
• At each step, therefore, energy can be harvested
NAD+ has been reduced
to NADH;
NADH now carries an
extra electron
NADH has been oxidized
to NAD+;
NAD+ can now accept
another electron
Note that as NAD+ accepts the electron, it also accepts
a hydrogen atom
NAD+ thus carries both electrons and H atoms in the cell
• In the final steps of glycolysis
– Both phosphates are stripped off the two 1,3diphosphoglyceric acid molecules
• Are added to ADP to create ATP
• In all, 4 ATP molecules are produced
LEDGER:
+ 2 ATP
In Some Organisms, Respiration
Ends with Glycolysis …
Plus side—
Reactions are very fast
Occurs in the cytoplasm
No oxygen required
Minus side—
Inefficient!
Only 2 ATP molecules produced from
each glucose molecule
There is much more energy available in
the pyruvic acids – how to get it out?
A Better Way: The Krebs Cycle
and the Electron Transport Chain
Pyruvic acid (the final breakdown product of
glycolysis) is shuttled into the mitochondria for
further breakdown and the production of more ATP
Evolved later, but generates MUCH larger quantities
of energy (36 total ATP per glucose molecule when
combined with glycolysis)
Occurs only in mitochondria (only in eukaryotes) and
requires oxygen.
The mitochondrion
has a doublemembrane
4 parts:
-Outer membrane
-Intramembrane space
-Inner membrane
-Inner compartment
• The Kreb’s Cycle takes place in the inner
compartment:
– Each of the two pyruvic acids travels into the
mitochondrion, where it is converted into acetyl
coenzyme-A
• This reaction strips an electron off (oxidizes) the pyruvic
acid and NAD+ picks up the electron
• One molecule of CO2 is also produced
– Diffuses through the cell membrane into the bloodstream
– Picked up by hemoglobin and transported to the lungs for
exhalation
– The acetyl coenzyme-A then undergoes a series
of reactions that oxidizes it completely to CO2
• At each step, NAD+ (or a similar molecule called FAD)
pick up the stripped off electrons
• For each molecule of pyruvic acid that
goes through this process the outcome
is:
– 3 NADH (6 total per glucose)
– 1 FADH2 (2 total per glucose)
– 1 ATP (2 total per glucose)
• NADH and FADH2 then continue on to
the next stage, taking their precious
electrons and hydrogens to the electron
transport chain …
• The Electron Transport Chain (ETC) is a
series of electron carrier molecules
embedded in the mitochondrial inner
membrane
– When a molecule of NADH arrives, it dumps its
electron to the first carrier, which accepts the
electron in a lower energy state than it was when
NAD+ picked it up
– This carrier then passes the electron along to the
next carrier in the chain, which accepts it in a yet
lower energy state
– The movement of electrons at each transfer releases
energy
– The energy is used to power the movement of H+ ions (from
the oxidation of NADH) across the inner mitochondrial
membrane from the inner compartment into the outer
compartment
Note that at each step, the downhill run of the electrons
provides the electron carrier molecules with the energy to
pump H+ ions against their concentration gradient across the
inner mitochondrial membrane
• Hydrogen ions are then
allowed to flow downhill
(with their concentration
gradient) through an
enzyme in the membrane
called ATP synthase
– Facilitated transport
•
Like a water wheel
spinning, as the ions pass,
energy is used to transfer
phosphate onto ADP to
make ATP.
• The Greatest amount of ATP is made in this
stage (32 ATP per glucose).
• At the end of the ETC, which carrier accepts
the electron? OXYGEN!
1/2 O2 + 2 electrons + 2 H+ = H2O
• Which is why organisms that use the Kreb
Cycle and the ETC to produce ATP have to
breathe in oxygen
• It’s also why we can sustain a multi-cellular
body with an enormous requirement for ATP
• The miracle of the mitochondrion is that it
evolved a way to harvest the energy of pyruvic
acid in small chunks to produce ATP
• Fats, proteins, carbohydrates, and sugars
other than glucose can also enter the
pathway to be converted to energy
• Food eaten in excess of caloric demands can
also be converted from amino acids, fatty
acids, and sugars into proteins, fats, and
carbohydrates for structure or storage
– 98 percent of energy reserves of animals are fats
Short Review of Lecture 7
• What is the difference between anaerobic and
aerobic respiration? What do the two processes
have in common?
• What is the difference between oxidation and
reduction reactions? Which yield energy? Which
require energy?
• Where does glycolysis take place?
• Where does the Kreb Cycle take place?
• Where does the electron transport chain take
place?
• Why are all multicellular eukaryotes obligate
anerobes?