EVOLUTION OF METABOLISM:
CELLULAR RESPIRATION
IMSS BIOLOGY ~ SUMMER 2012
LEARNING TARGETS
•
To understand the evidence for the evolutionary origin of cellular
respiration.
•
To understand the impact of oxygen on the evolution of
multicellular life on Earth.
•
To understand the processes by which cellular respiration harvests
chemical energy from food and converts it into ATP that fuels
cellular work.
•
To understand the distinctions between aerobic & anaerobic
respiration
•
To understand the role respiration plays on a cellular, organismal,
& ecological scales.
FOLLOW UP: PHOTOSYNTHESIS Q’S
• All plants and most other photosynthetic organisms
contain chlorophyll (primary pigment is chl a), but
• At least one group of bacteria (halobacteria) rely solely on
bacteriorhodopsin (derived from carotene) for
photosynthesis
THEORY OF
ENDOSYMBIOSIS
•
First proposed by Lynn
Margulis in 1960s
•
Much evidence to support
eukaryotic cellular
respiration originated via
endosymbiosis of aerobic
purple bacteria (alphaproteobacteria) which
ultimately became
mitochondria.
•
Note: this drawing shows
mitochondria
endosymbiosis in BOTH
animal and plant cells.
Both figures depict endosymbiotic origin of mitochondria. Which important details are
portrayed in the one above that are not below (and vice versa)?
SUPPORTING EVIDENCE FOR ENDOSYMBIOSIS
• Mitochondria have own genome (mtDNA) and are selfreplicating (divide independently of cell they live in)
• Genomes are much reduced from alpha-proteobacteria
(purple aerobic bacteria) ancestors
• mtDNA, mtRNA, ribosomes, and mechanisms of protein
synthesis and oxidative metabolism all similar to that of
proteobacteria
• Mitochondria have double phospholipid bilayers
• Evidence supports mitochondria arose ca. 2 bya in a
common ancestor of all extant eukaryotes
• mtDNA inherited by one’s mother (maternal lineage)
Human mtDNA contains 37 genes which
code for proteins needed for normal
mitochondrial function, e.g. 13 genes that
code for enzymes involved in oxidative
phosphorylation.
MITOCHONDRIA FUNCTIONS – BEYOND
OXIDATIVE METABOLISM
• Oxidative phosphorylation to make ATP
• Greatly increases capacities to make ATP!
• Many other cellular activities
• Assist in regulation of apoptosis (timed cell death)
• Genes code for proteins involved in synthesis of heme
and cholesterol
• Help keep intracellular levels of Ca+2 low inside neurons –
important in cell signaling pathways
Nuclear DNA is inherited from all ancestors. Mitochondrial
DNA is inherited from a single maternal lineage.
The Hunt for mtDNA
15 min.
ATMOSPHERIC O2 THROUGH EARTH’S HISTORY
• By 1.5 bya, photosynthesis-derived O2 accumulated to
significant levels in atmosphere; this in conjunction with
mitochondria  Cambrian Explosion during which
metazoan radiation occurred
LIFE IN AN OXYGENATED WORLD
• Oxygen is highly reactive and potentially toxic –
production of reactive oxygen species (radicals), or ROS
causes damage to cell membranes, DNA, RNA, proteins
(including enzymes and their cofactors)
•
Aging = accumulated cellular damage caused by ROS
EVOLUTION OF MULTICELLULARITY – FROM A METABOLIC PERSPECTIVE
•
Chloroplasts & mitochondria meet nrg demands of cell via their
electron transport systems which generate ROS  oxidative stress
and DNA damage in organelles
•
In unicellular organisms, DNA repair is only way to maintain pristine
chromosomal DNA in organelles, but DNA repair is limited.
•
Better strategy: avoid DNA damage by having separate germ and
somatic cell lines
• Germ (reproductive) cells: metabolically quiet early in development
 germ line DNA protected from oxidative damage
• Somatic cells: must be metabolically active as responsive to rapid
change
• Bendich, A.J. 2010.http://www.biology-direct.com/content/5/1/42
ENERGETICS OF GENOME COMPLEXITY
• All complex life composed of eukaryotic cells.
• Eukaryotic cells arose from prokaryotes just once in 4
billion years. Why haven’t prokaryotes evolved greater
complexity?
• Lane and Martin (2010) propose that prokaryotic genome
constrained by energetics
• Endoysymbiosis of mitochondria opened door for 200fold expansion in number of genes that can be expressed.
• 75% of cell’s total nrg budget allocated to protein
synthesis.
•
http://blogs.discovermagazine.com/notrocketscience/2010/10/20/the-origin-of-complexlife-%E2%80%93-it-was-all-about-energy
RESOURCES
•
Big themes in evolution of photosynthesis and respiration
http://www.shmoop.com/cell-respiration/evolution.html
•
Origins of development of eukaryotic organisms – mtDNA and
cpDNA http://www.biology-direct.com/content/5/1/42
•
Evolution
101http://evolution.berkeley.edu/evolibrary/news/071101_geneal
ogy
INTRODUCTION TO RESPIRATION
• Via the circulatory system, O2 is delivered to & CO2 is taken
away from tissues
• Recall that most metabolic processes of the body depend
on ATP
• What is the function of O2 in cellular respiration?
RESPIRATION
• Involves ALL processes
that deliver O2 from
environment to the
tissues (cells), including
• breathing
• gas exhange between air &
blood and between blood &
tissues
• transport of respiratory gases
by blood
• use of O2 in cellular
respiration (aka “internal
respiration”)
CELLULAR RESPIRATION - OVERVIEW
• occurs primarily in mitochondria
• harvests nrg stored in organic “fuel” molecules, e.g.
glucose (C6H12O6) by enzymatically breaking these
molecules down to release their potential nrg
• traps and stores this potential nrg in a form that is usable
by the cell – ATP!
• uses O2
• produces waste products, CO2 & H2O (used in
photosynthesis)
• Alternative mode: at ocean’s bottom and other anoxic
environments, anaerobic organisms synthesize ATP using
nitrate or sulfur rather than oxygen
CELLULAR RESPIRATION - OVERVIEW
• The cells of aerobic organisms (e.g. plant & animals)
perform cellular respiration
• Cellular respiration is the primary way chemical nrg is
harvested from food and converted into ATP
ATP = adenosine triphosphate
“Currency” of cellular work
To get nrg to do cellular work, cells
must break down energycontaining substances to release
their potential nrg  stored as
ATP that can be later used by cell
to fuel an endergonic reaction
ATPase is enzyme that hydrolyzes
this terminal phosphate bond
http://faculty.ccbcmd.edu/biotutorials/energy/
atp.html#atp
GETTING THE PERSPECTIVE
• A HUGE amount of ATP is needed to fuel all the
cellular activities of an organism!
• Example
•Average human at rest uses ~45 kg (99 lbs.) of ATP
per day (but has surplus of < 1 g of ATP at any given
moment)
•Estimated that each cell generates & consumes
~10 x 106 molecules of ATP per sec!
• So, ATP production must be an on-going process in
order to remain alive
• Evidence of cellular respiration indicates life!!!
Is it Alive?
30 min.
Oxidation
Glucose loses electrons
(and hydrogen)
C6H12O6
Glucose
 6
O2
Oxygen
6
CO2
Carbon
dioxide
 6
H2O
Water
Reduction
Oxygen gains electrons (and hydrogen)
• Most common fuel molecule for cell. resp. is glucose
(C6H12O6)
• Overall redox reaction: glu oxidized (loses electrons) while
O2 reduced (gains electrons)
ROLE OF OXYGEN IN CELLULAR RESPIRATION
• Cellular respiration can produce up to 38 ATP
molecules for each glucose molecule consumed
• During cell. resp., hydrogen (H) & its bonding
electrons change “partners”
•H & electrons go from glu  O2, forming H2O (&
ATP)
•This H transfer is why O2 is so vital to cellular
respiration
CRITICAL REDOX REACTION
•
When hydrogen and water bind to form water, a “burst” of nrg is
released
•
This nrg released as electrons of hydrogen “fall” into their new
bonds with oxygen
•
But, the process needs to be stepped down for a cell to capture this
nrg and use it for cellular work.
ROLE OF ELECTRON
TRANSFER
• Cellular
respiration can
be considered a
controlled fall of
electrons that
releases nrg in a
stepwise way,
like walking
down a staircase
• 1st step: transfer of
electrons from
glucose to NAD+
(electron acceptor)
which reduces it to
NADH
• Rest of path:
electron transport
chain
• Involves series of
redox reactions
• Leads to
production of lots
of ATP
UNPACKING CELLULAR RESPIRATION
• The chemical reactions involved in cellular respiration are
grouped into three main stages
• Glycolysis
• Citric acid cycle
• Electron transport chain
THE BIG
PICTURE
• Electron transport chain functions like a chemical machine
• Uses nrg released by “fall” of electrons (“pulled” by O2) to pump
hydrogen ions (H+) across inner mitochondrial membrane  H+
more concentrated on one side of membrane
• This H+ can then rush “downhill” thru membrane protein that
“spins” the turbine to activate the enzyme, ATP synthase
Fig. 6.11
• Cyanide is a deadly poison, because it
• Binds to a protein complex in the chain and prevents
passage of electrons to O2
• Stops ATP synthesis
SUMMARY OF ATP YIELD DURING CELLULAR RESPIRATION
MONOMERS FROM FOOD MACROMOLECULES
SERVE AS FUEL FOR CELLULAR RESPIRATION
ANAEROBIC RESPIRATION
•
Some cells can actually work for (short) periods without O2, i.e.,
anaerobically
•
Sometimes referred to as fermentation
•
Involves glycolysis  lactic acid
• Without O2, electrons “dumped” from NADH onto pyruvic acid in
order to regenerate NAD+, so lactic acid is formed
CELLULAR RESPIRATION IN YEAST
• Yeast (the living organisms in soil sample C of the “It’s
Alive” activity) are facultative aerobes.
• Undergo aerobic respiration when O2 is present – as was
the case during the activity – sugar molecules, CO2 (what
caused the bubbling in the sample), water, and a high
yield of ATP
C6H12O6 + 6O2  6CO2 + 6H2O + ATP (hi yield)
• Undergo alcoholic fermentation when O2 is absent –
sugar molecules broken down into ethanol, CO2 and a low
yield of ATP
C6H12O6  6C2H5OH + 2CO2 + ATP (low yield)
Nrg flow & chemical cycling in ecosystems
FIG. 6.2: NRG FLOW & CHEMICAL CYCLING IN ECOSYSTEMS
• Misconception:
• Plants only perform photosynthesis
• Plants perform BOTH photosynthesis and cellular
respiration
• Plants must be able to harvest nrg (synthesize ATP) from
fuel molecules (e.g. glu) in order to grow & reproduce
just as animals do
• Animals perform cellular respiration only