Topic 9
How Does Life Use Energy?
Dr. George Lapennas
Dept. of Biology
Nature of science:
Search for mechanistic explanations –
ones that predict events based on
underlying rules and structures
Quick review of special characteristics
of living things (organisms)
1. Growth
2. Development (changes other than
growth during individual lifetime)
3. Reproduction (involves inheritance)
4. Ordered, complex structure;
adaptation
5. Movement (esp. animals)
6. Sensitivity/Responsiveness
7. Evolution (change over generations)
8. Consciousness/Rationality
9. Use of energy
Can life also be understood
mechanistically (vs vitalistically)?
An early mechanistic success:
Harvey’s partial explanation of the
pumping and circulation of the blood
Some 19th century steps from a
vitalistic toward a mechanistic
understanding of life
1. The cell theory
Living things are made up of large numbers of tiny
units called “cells” that come from previous cells
Prerequisite technical advance: Invention of the
microscope (early 1600’s)
Structure of an animal cell
Structure of a plant cell
2. In vitro (in glass) synthesis of
organic molecules
Wohler’s 1828 in vitro synthesis of urea, etc.
Eventually, chemists learned to synthesize
everything in vitro that organisms synthesize
in vivo.
General conclusion: There are no unique laws
of chemistry operating within living
organisms.
3. The fermentation controversy
“Fermentations” – chemical transformations that had
only been observed in association with living things
Buchner (1897) observed fermentation of fruit juice
by cell-free extract of yeast, yielding alcohol and
carbon dioxide (CO2)
Conclusion: Living cells are not required for
fermentation – only need some materials that were
present within the cells (now known to be enzymes proteins that act as catalysts to speed up reactions).
4. Cryptobiosis
Does life irreversibly end when life
processes cease?
or …
Can life processes be stopped and later restarted, so long as necessary structure
have been preserved?
Can the “clock” of life be stopped and then
re-started?
4. Cryptobiosis
Conclusion: Life processes only depend upon
the presence of certain matter in a certain
structural arrangement.
That matter and structure can persist during
drying or freezing when all processes cease.
Life processes can resume upon restoration of
water or thawing.
Why did mechanistic explanations
take so long to develop in biology?
- Because living things are much
more complex than anything else that
scientists study
- Many other discoveries had to be
made before the mechanisms of
biological structures and processes
could be effectively investigated.
Machinery of life: 4 classes of organic
“macro-molecules” assembled from smaller
“building blocks”
Machinery of life: 4 classes of organic
“macro-molecules” assembled from
building blocks
1. Proteins (structural; catalytic “enzymes”)
2. Nucleic acids (DNA, RNA; instructions for
inheritance as the structure of proteins)
3. Polysaccharides (energy storage; structure)
4. Complex lipids (energy storage; cell
membranes)
Amino acids are the building
blocks of Proteins
Protein structure – primary
structure
Protein structure – secondary
structure (local folding patterns)
Protein structure – tertiary
structure (overall folding pattern)
Protein function depends on form
DNA (molecule of inheritance)
Living matter seems to obey the
same laws of physics and
chemistry as non-living matter
Conservation of mass
Conservation of momentum
Gravitation
Chemical properties of elements
Laws of thermodynamics
Laws of thermodynamics
In any isolated system (no matter or energy can enter
or leave the system), including the entire universe:
First Law – the total amount of energy is constant,
though it can change form.
Laws of thermodynamics
In any isolated system (no matter or energy can enter or
leave), including the entire universe:
First Law –total amount of energy is constant, though it
can change form.
Second Law – Whenever anything actually happens,
the entropy (disorder) of the system increases.
Laws of thermodynamics
In any isolated system (no matter or energy can enter or
leave), including the entire universe:
First Law –total amount of energy is constant, though it
can change form.
Second Law – Whenever anything actually happens, the
entropy (disorder) of the isolated system increases.
- “Time’s Arrow” points in the direction of
increasing entropy (disorder) of the universe.
- Changes that would reduce the entropy of the
universe cannot occur
“Spontaneous” changes
= changes that can happen
= “downhill” changes
“Spontaneous” processes can
happen
Two old hypotheses about
animals’ use of food
1. Assimilation - food is added to the body for
growth or to replace material lost through
“wear and tear”
Two old hypotheses about
animals’ use of food
1. Assimilation - food is added to body for growth
or to replace material lost through “wear and
tear”
2. Combustion - food is somehow “burned”
within the body, like fuel in a fire, generating
heat, and being consumed in the process
Reinterpretation of combustion and
animal respiration by Lavoisier
Lavoisier (late 1700’s)…
- Overthrew phlogiston theory and applied
new knowledge of gases to combustion
- Flames and animals do not produce
phlogiston,
- Both consume oxygen (O2) and
release carbon dioxide (CO2) and heat
Lavoisier and
Laplace
observed
quantitative
similarities
between
burning
charcoal and the
respiration of a
living animal.
Quantitative similarities between combustion
and respiration
Same/Similar ratios of …
heat produced / vol. of CO2 produced
and
vol. of O2 consumed / vol. of CO2 produced
“Slow combustion”
Lavoisier and Laplace hypothesized that
animals carry out a “slow combustion” of
fuel (process now called cellular
respiration).
They believed that the function of cellular
respiration was to make heat.
What do we know now about
the use of food by animals?
- Cells both ASSIMILATE food and use it as FUEL
FOR CELLULAR RESPIRATION
What do we know now about
the use of food?
- Cells both ASSIMILATE food and use it as FUEL
FOR CELLULAR RESPIRATION
- For most organisms, heat is just a useless
by-product of cellular respiration, not the
function of the process.
What is the primary function of
cellular respiration?
Cellular respiration provides energy to do
“cell work”.
What is the function of cellular
respiration?
Cellular respiration provides energy to do “cell
work”.
“Cell work” means “uphill” cellular processes
that would not be spontaneous (could not
occur) on their own,
… but can occur if “coupled” to some other,
highly spontaneous process, such that the
two processes occurring together increase
the entropy of the universe.
3 Types of
Cell Work:
-Active transport
-Movement
-Bio-synthesis
Digestion of macro-molecules
When we digest food macro-molecules, we
break them down into their building blocks
Examples:
proteins  amino acids
polysaccharides  simple sugars
Nucleic acids  nucleotides
Digestion of macro-molecules
When we digest food macro-molecules, we
break them down into their building blocks.
Blood carries building block to the cells,
where they are taken up, and some are
reassembled into new macro-molecules.
Digestion is “downhill”
Dismantling macro-molecules is a disordering
process that increases the entropy of the
universe.
Macro-molecule  building blocks + heat
(ordered, non-random) (disordered)
(random energy)
Assembly of macromolecules
simply by reversing digestion?
NO! Digestion is downhill (increases the
entropy of universe).
Assembly by simply reversing
digestion?
NO - Digestion is downhill (increases entropy of
universe)
So the reverse process (assembly simply by
reversing digestion) would be uphill (reduce
entropy of the universe), and can’t happen
A “spontaneous” processes can
happen; the reverse process cannot
Question: Then how can macromolecule assembly (and other
types of cell work) occur?
Answer: By “coupling” cell work
to some very downhill process
A spontaneous process can be reversed by
coupling it to a MORE spontaneous process
(represented by a larger weight).
Mechanical coupling
A spontaneous process can be reversed by
coupling it to a MORE spontaneous process (such
as a larger weight).
The COMBINED process is then downhill, and
increases the entropy of the universe.
We say: “The second, highly spontaneous,
process supplies energy to drive the uphill
process (which could not have occurred
alone).”
What highly spontaneous
process drives cell work?
The highly spontaneous process that
drives cell work is “splitting ATP”
ATP = Adenosine Tri-Phosphate
Splitting ATP
ATP

ADP + Phosphate + heat
(one larger
(two smaller
(random
molecule)
molecules)
energy)
Splitting ATP
ATP

ADP
+ Phosphate
+ heat
Splitting ATP is very downhill, and so can
drive uphill cell work.
Example of coupling:
ATP-driven assembly of a protein
spontaneous?
1) Amino acids + heat  protein
2)
ATP
 ADP + Phosphate + heat
no
YES
---------------------------------------------------------------------------(1+2) Amino acids + ATP  protein + ADP
+ Phosphate + heat
yes
ATP
splitting
also
drives
the
other
types of
cell
work
Regeneration of ATP
Human cells contain only enough ATP to last
about 30 seconds.
Regeneration of ATP
Human cells contain only enough ATP to last 30
seconds.
We must constantly regenerate ATP by
putting 3rd phosphate back on ADP to “make
ATP” again (= ATP synthesis)
What process is downhill
enough to drive uphill ATP
synthesis?
Since ATP splitting is downhill, putting the
third phosphate back on must be uphill.
How can we drive ATP synthesis?
What process is downhill
enough to drive uphill ATP
synthesis?
Since ATP hydrolysis (splitting) is downhill, putting
the third Phosphate back on must be uphill.
How can we drive ATP synthesis?
Couple it to something even more downhill –
something highly spontaneous - but what?
What process is downhill
enough to drive uphill ATP
synthesis?
Since ATP hydrolysis (splitting) is downhill, putting
the third Phosphate back on must be uphill.
How can we drive ATP synthesis?
Couple it to something even more downhill –
something highly spontaneous - but what?
The “slow combustion” of food = cellular
respiration.
Coupling cell respiration and
ATP synthesis
spontaneous?
ADP + Phosphate + heat  ATP
Food + O2
 CO2 + H20 + heat
no
YES
---------------------------------------------------------------------------Food + O2 + ADP
 ATP + CO2 + heat
yes
Coupling cell respiration and
ATP synthesis
spontaneous?
ADP + Phosphate + heat  ATP
Food + O2
 CO2 + H20 + heat
no
YES
---------------------------------------------------------------------------Food + O2 + ADP
 ATP + CO2
yes
The COMBINED process makes ATP and increases the
entropy of the universe.
The ATP Cycle
“Metabolic pathways”
Metabolism = all the chemical processes of
cells
Metabolic pathway = sequence of reactions by
which chemical changes such as cell
respiration are carried out in many small
steps, each catalyzed by an enzyme
Metabolic pathways
Metabolism = the chemical processes of cells
Metabolic Pathway = sequence of reactions by
which chemical changes such as cell
respiration are carried out in many small steps.
Cellular respiration using the sugar glucose
as fuel takes place in three phases,
involving 20 separate reactions, and 20
different enzymes.
3 Stages of glucose “burning”
The ten
steps of
Stage 1
(glycolysis)
Stage 1 (glycolysis) occurs in the cytosol
(“cell juice”); Stages 2 & 3 in mitochondria
Mitochondrion
Enzymes = protein catalysts
Enzymes are proteins that act as catalysts –
enzymes speed up chemical reactions.
Structure of an enzyme
Where does the food come from
that is used as fuel in cellular
respiration?
Animals eat plants
Where does the food come from
that is used as fuel in cellular
respiration?
Animals eat plants,
… or they eat animals that have eaten
plants.
Where does the food come
from that is used as fuel in
cellular respiration?
Animals eat plants, or they eat animals
that have eaten plants.
Plants make their own food by …
Where does the food come
from that is used as fuel in
cellular respiration?
Animals eat plants, or they eat animals
that have eaten plants.
Plants make their own food by …
… photosynthesis
Photosynthesis
spontaneous?
CO2 + H2O
Photons of light
(ordered energy)
 Glucose + O2
 heat
no
YES
(random energy)
-----------------------------------------------------------------------------CO2 + H2O + Photons  Glucose + O2 + heat
yes
Light supplies the energy to drive synthesis of glucose
in photosynthesis
Where photosynthesis occurs
NOTE WELL: Plant cells use the food they make in
the same way that animal cells do - by cellular
respiration