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Cellular Respiration
How do Organisms Get Energy?
Photoautotrophs (e.g. plants)
Chemoautotrophs (e.g. archaebacteria)
Through the sun
Make organic compounds without sun’s energy
Heterotrophs (e.g. animal, fungi, most
Through plants
Glucose is the ultimate source of energy
glucose stores energy in its bonds and cellular
respiration is a way to acquire this energy
Exergonic Redox Reaction
C6H12O6 + 6O2  6H2O + 6CO2 + ENERGY
as the bonds WITHIN the glucose molecule are
broken down, energy is released
 To summarize even more, glucose stores energy in
its bonds and cellular respiration is a way to acquire
this energy!
 These bonds are broken through EXERGONIC
 C6H12O6 oxidized to 6CO2 , and 6O2 reduced to
 each carbon in C6H12O6 is converted to CO2
 each hydrogen in C6H12O6 is converted to ½H2O
Combustion of Glucose
In test tube
In cellular respiration
In cellular respiration energy is released from combustion of glucose.
34% of this energy goes to ATP; 66% goes to heat
Aerobic vs Anaerobic Respiration
C6H12O6 + 6O2  6H2O + 6CO2 + ENERGY
In the above equation oxygen is the electron
acceptor in the oxidation of glucose
Most organisms are obligate aerobes – they
require oxygen and cannot survive without it
Obligate anaerobes (some species of bacteria)
use other molecules as the final electron
acceptor and must live in environments that has
no oxygen
Facultative anaerobes can tolerate aerobic and
anaerobic conditions
Highly folded
Folds of the inner
Protein-rich liquid
Energy Transfer Terminology
Substrate-level Phosphorylation:
ATP forms directly in an enzyme-catalyzed reaction.
Oxidative Phosphorylation:
ATP forms indirectly through a series of enzymecatalyzed redox reactions involving oxygen as the final
electron acceptor.
Done mainly through compounds called energy carriers
These reactions harness energy, which is eventually
transferred to ATP
Energy Carriers
NAD+ and FAD+ are low energy, oxidized
coenzymes that act as electron acceptors.
 When an electron(s) are added to these
molecules, they become reduced to NADH
and FADH2.
 In this case, reducing a molecule gives it
more energy.
Aerobic Respiration: Overview
Occurs in Four Distinct Stages:
Glycolysis: 10-step process in the cytoplasm.
Pyruvate Oxidation: 1-step process in the
mitochondrial matrix.
Krebs Cycle: 8-step cyclical process in the
mitochondrial matrix.
Electron Transport Chain : Multi-step
process in the inner mitochondrial membrane.
Glycolysis - Overview
Takes Place in the Cytoplasm
Enzymes break down glucose (6 carbons) into two
smaller molecules of pyruvate (3 carbons), releasing
Does not require oxygen
Glycolysis – Key parts of process
2 ATPs are used in steps 1 & 3 to prime
glucose for splitting.
 F 1,6-BP splits into DHAP and G3P.
 DHAP converts to G3P.
 2 NADH are formed in step 6.
 2 ATP are formed by substrate-level
phosphorylation in both steps 7 and 10.
 2 pyruvates are produced in step 10.
Energy Yield & Products:
4 ATP produced – 2 ATP used = 2 net ATP
2 pyruvates
Further processing in aerobic
cellular respiration
(if oxygen is available)
Glycolysis – Energy Created
Creates 2 molecules of ATP (2 x 31 kJ/mol)
Yields 62 kJ of energy, from a possible 2870 kJ/glucose (only
a 2.2% energy conversion)
Most energy is still trapped in pyruvate and the 2 NADH
molecules, but some lost as heat
Earliest cells in Earth’s history thought to have used this
method of energy metabolism since oxygen is not required
and enough energy is produced to sustain life in unicellular
simple organisms today use glycolysis for energy
In multicellular organisms glycolysis takes place first in the
cytoplasm, and then more processes in the mitochondria to
yield more energy
Pyruvate Oxidation
(if oxygen is present…)
The following occurs for each pyruvate:
CO2 removed.
NAD+ reduced to NADH and the
2-carbon compound becomes acetic
Coenzyme A (CoA) attaches to acetic
acid to form acetyl-CoA.
Pyruvate Oxidation
Pyruvate Oxidation
Energy Yield & Products:
2 acetyl-CoA
2 CO2 (released as waste)
CoA comes from vitamin B5
Proteins, lipids, and carbohydrates are catabolized to
It can be used to make fat or ATP
[ATP] determines what pathway this molecule takes
If O2 is present, ‘acetyl CoA’ moves to the Kreb’s Cycle
(aerobic respiration)
If O2 is NOT present, ‘acetyl CoA’ becomes ‘lactate’
(anaerobic respiration / fermentation)