Transcript Third lecture, PPT

PHYS 1211 - Energy and
Environmental Physics
Lecture 9
Energy in Chemistry
and Biology
Michael Burton
This Lecture
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Chemical Energy
Biological Energy
Photosynthesis
Respiration
Energy in the Human Body
Chemical Energy
• Chemical Energy is energy contained in
matter as a result of its chemical structure
— i.e. the arrangement of its atoms and
molecules.
• Chemical Energy is released or absorbed in
chemical reactions.
– A reaction that releases energy is called an
exothermic reaction.
– A reaction that requires energy is called an
endothermic reaction.
Chemical Energy
• Chemical Energy is commonly measured in
kilojoules/mole (kJ/mol).
• A mole is a unit that measures the number
of molecules (or sometimes atoms or ions)
of a substance.
– 1 mole = 6.02  1023 molecules = NA
– NA is Avogadro’s number
– 1 mole of 12C has a mass of 12 grams
Example
In the reaction:
2H2 + O2 2H2O
DE = –483.6 kJ/mol of O2
Here DE is the change in Energy. The –ve sign
means an exothermic reaction. An
endothermic reaction would have a +ve
sign.
The energy per mole is measured at a
standard T and P of 25oC and 105 Pa (1 bar).
Bond Energies
• Chemical Energy is contained in chemical
bonds
– It can be thought of as the potential energy
associated with the electrical forces that bind
atoms together into molecules.
Bond energies
H-H bond
energy = 432 kJ/mol
O=O bond
energy = 494 kJ/mol
O-H bond
energy = 459 kJ/mol
Bond energies
H-H bond
energy = 432 kJ/mol
O=O bond
energy = 494 kJ/mol
O-H bond
energy = 459 kJ/mol
In the reaction 2H2 + O2  2H2O
We have to break 2 H-H bonds and one O=O bond and we then form 4 OH bonds (in the two H2O molecules).
So the total energy is 2  432 + 494 – 4  459 = –470 kJ/mol
(which is close to the actual value of –483) — such estimates are only
approximate as bond energies vary depending on precise structure of a
molecule.
Energy and Oxidation
• The concept of Chemical Energy applies to a
particular chemical reaction.
– We can’t in general talk about the chemical energy of a
substance.
• However, when we talk about the chemical
energy contained in fuel or food we mean the
energy released by combining with oxygen.
– i.e. Burning in oxygen (or oxidation).
• Because there is plentiful oxygen in the
atmosphere (for the Earth) this is an efficient way
to get energy.
Energy in Biology
• All living organisms require energy.
• As human beings we need energy to generate
the heat to maintain our body temperature,
and to provide mechanical energy in our
muscles.
• However, even a microbe needs energy just to
allow its fundamental chemistry to operate.
– Many chemical reactions involved in metabolism
are endothermic and require an energy source to
make them go.
Autotrophs and Heteroptrophs
• Organisms can be classified according to
the way they obtain energy.
• Autotrophs are organisms that can obtain
energy from light or inorganic chemical
reactions.
– Plants and some microbes are autotrophs.
• Heterotrophs are organisms that can only
obtain energy from other organisms.
– Animals, Fungi and many bacteria are
heterotrophs.
Autotrophs
• Autotrophs can be further divided according to
their source of energy.
• Chemoautotrophs obtain their energy from
inorganic chemical reactions.
– Mostly microbes that live in extreme environments.
• Photoautotrophs obtain their energy from
sunlight.
– By far the dominant primary source of energy. These
obtain energy from sunlight through the process of
photosynthesis. Includes plants and some bacteria.
Photosynthesis
Photosynthesis is carried out in green plants (such as
trees), but also in microorganisms called cyanobacteria
(often incorrectly called blue-green algae).
The pigment chlorophyll used in photosynthesis is
responsible for the green colour of plants.
Photosynthesis
• Photosynthesis involves the following overall
chemical reaction:
(sunlight)
(C6H12O6)
CO2 + H2O + energy  Glucose + O2
• Photosynthesis provides energy
– The chemical energy stored in the glucose and oxygen
can be reused.
• It also provides a source of organic chemicals
needed for life. The glucose can be further
processed into a host of other chemicals needed
for biological processes (e.g. proteins, DNA etc.)
Photosynthesis
A photosynthetic organism can
build all its complex biological
chemicals (proteins, nucleic
acids, lipids etc.) from water
and air (CO2) and a few other
elements (N, P etc.).
Heterotrophs (e.g. animals)
cannot do this and have to
obtain many of their organic
chemicals (as well as energy)
from food.
N, P etc.
Photosynthesis and Oxygen
• Photosynthesis was first evolved by
cyanobacteria at least about 2.4 billion
years ago.
• It is photosynthesis that created the oxygen
in the Earth’s atmosphere.
• We know from geological evidence that the
oxygen in the Earth’s atmosphere began to
build up over about 2.4–2.2 billion years
ago.
The Earth’s original atmosphere
was similar to that of Mars and
Venus. It was formed by
volcanic outgassing and impacts
of comets and asteroids.
Composition:
CO2
N2
CO
H2O
SO2
(NOT O2)
The Great Oxygenation Event
(2.3 Billion Years Ago)
• The invention of photosynthesis changed the world forever.
• The oxygen produced as a by product is an incredibly
reactive chemical that easily reacts with most organic
chemicals. It would have been toxic to most life at the time.
• Probably caused the extinction of many species.
• Some species survived by hiding in oxygen free
environments. (obligate anaerobes).
• But some organisms evolved mechanisms to survive and
thrive in this “toxic waste”.
– Antioxidants — to protect them from the oxidising environment.
– Aerobic respiration — to use oxygen as an efficient source of energy.
Toxic Oxygen!
• Oxygen can be dangerous at above
normal atmospheric partial pressures
– e.g. for scuba divers particularly if using
oxygen rich mixtures.
• Oxygen may be a significant factor in
aging and degenerative diseases.
• We have evolved protection to
oxygen (using antioxidants) but only
just enough to survive atmospheric
oxygen levels.
Antioxidants
• Antioxidants are chemicals that protect us from
our toxic oxygen environment.
– Without them many of the key biological chemicals
such as DNA and proteins would be subject to
oxidative damage.
• Some antioxidants are made in the body
— others must be obtained from food.
– A well known example is Vitamin C (ascorbic acid).
– Lack of Vitamin C causes the disease scurvy — a major
problem for sailors on long voyages before its cause
was understood.
Antioxidants
• Fresh fruit and
vegetables are a good
source of
antioxidants
Respiration
• Respiration (cellular respiration) is the
inverse process to photosynthesis.
– It enables the energy stored in glucose and
oxygen to be retrieved and used.
Glucose + O2  H2O + CO2 + energy
(chemical energy
in the form of ATP)
• Respiration takes place in every cell of the
body.
Photosynthesis – Respiration Cycle
Sunlight
Photosynthesis (in green
plants and cyanobacteria)
energy + CO2 + H2O  Glucose + O2
Glucose + O2  H2O + CO2 + energy
Respiration (in every cell
of complex organisms)
Chemical Energy (ATP)
Adenosine Triphosphate (ATP)
• The molecule Adenosine
Triphosphate (ATP) is the
energy currency of living
cells.
• Removing one of the
phosphate groups (to make
ADP) releases energy.
(30.5 kJ mol–1)
• Energy must be supplied to
replace the phosphate
group.
Three Phosphate
(PO3 groups)
Adenosine
Chemical processes involved in
metabolism are driven by energy stored in
the form of ATP.
Anaerobic Respiration
• The cellular respiration reaction just
described is “aerobic respiration” making use
of oxygen.
• Anaerobic respiration is used when oxygen is
not available.
– Before the evolution of photosynthesis.
– By organisms that live in environments without
oxygen (e.g. obligate anaerobes).
– An alternative source of energy (animals use
anaerobic processes when energy is needed
rapidly).
Anaerobic Respiration
• Anaerobic processes
C6H12O6  2C2H5OH + 2CO2 + energy (2 ATP)
Glucose
Ethanol Fermentation
C6H12O6  2C3H6O6 + energy (2 ATP)
Lactic Acid Fermentation
• Aerobic Process
C6H12O6 + 6O2  6CO2 + 6H2O + energy (36-38 ATP)
The aerobic process is much more efficient. This is the
big advantage of living in an oxygen rich
environment.
Efficient energy production
The availability of
abundant oxygen and
efficient energy
production by aerobic
respiration allowed the
development of large
complex organisms.
Anaerobes are
invariably microbes
Chloroplasts
Photosynthesis in plants takes place in
structures called chloroplasts.
Chloroplasts are descended from the
cyanobacteria that first evolved
photosynthesis billions of years ago.
In the distant past these bacteria entered
into a symbiotic relationship with the
ancestors of plant cells, and are now fully
incorporated into the cells. We know this
because chloroplasts still have their own
DNA which we can compare with that of
cyanobacteria.
Mitochondria
Respiration takes place in structures
called mitochondria. They are found
in the cells of most “eukaryotic”
organisms — organisms with
complex cells that include plants and
animals.
Like chloroplasts these are also
descended from bacteria that entered
into symbiosis.
Structure of a Cell
This is a plant cell so it includes both chloroplasts and
mitochondria. An animal cell would include
mitochondria but no chloroplasts.
Energy in the Human Body
• Humans take in energy in the form of food and oxygen.
• The food is processed through the digestive system. The
energy component in the food are extracted in the form
of glucose.
• Cardiovascular System for processing of the oxygen
– The oxygen is taken in through the lungs. A substance in the blood
called haemoglobin attaches to oxygen molecules and allows
them to be carried in the blood.
– The circulatory system allows the glucose and oxygen to be
carried around the body where it is supplied to the mitochondria
of all the cells.
Digestive System
The digestive system extracts
nutrients from food.
For example carbohydrates and
sugars are broken down to
make glucose.
The glucose is passed into the
blood (mainly in the small
intestine).
Cardiovascular System
In the lungs oxygen is extracted
from the inspired air. A protein
called Haemoglobin attaches to the
oxygen molecules and allows the
oxygen to be carried through the
blood.
The oxygen rich blood is pumped
by the heart through the arteries
which split into a network of
smaller blood vessels and
eventually into the capillaries.
Energy distribution
The circulatory system
carries the glucose and
oxygen around the body to
all its cells where the
mitochondria carry out
cellular respiration
converting the glucose and
oxygen to energy.
The resulting carbon
dioxide is then carried
back through the veins to
the lungs where it passed
into the expired air.
Energy is particularly needed by some
cells e.g. muscle cells that have many
mitochondria.
VO2max
• VO2max is a measure of the
maximum rate of inspiration of
oxygen while exercising.
– Measured in litre min–1 or ml kg–1min–1
• It is a measure of the maximum
rate of aerobic respiration.
• Typical value for an untrained
male is about 3.5 l min–1.
• Trained endurance athletes
achieve about 7 l min–1.
VO2max and Power
C6H12O6 + 6O2  6CO2 + 6H2O + energy (2817 kJ mol–1)
Glucose
A VO2max of 7 l min–1 is 0.3125 mol of O2 min–1
= 0.052 mol of glucose per minute
= 146.7 kJ min–1
= 2445 W
Remember that earlier we calculated the power output of an endurance
athlete as about 400 W. The difference between these two figures is due to
the efficiency of muscles in converting energy into mechanical form (which
is about 15%).
The rest of the energy ends up as heat — which is of course why we get hot
when exercising.
Anaerobic Respiration
• Human muscles can extract energy using
anaerobic respiration. This enables short
periods of exertion at rates well above that
limited by VO2max.
• This is used by sprinters for example. The
body goes into “oxygen debt” and has to
take in oxygen to break down the anaerobic
products such as lactic acid.
Bomb Calorimeter
• The energy content of food is
measured by burning it in a
device called a bomb
calorimeter.
• The sample is placed in a
container with high pressure
oxygen and ignited electrically.
• The heat produced is measured
by means of the temperature
increase in a surrounding water
bath.
Energy content of foods
Food Component
fat
ethanol (alcohol)
proteins
carbohydrates
organic acids
polyols
Energy Density
kcal/g
9
6.3
3.2
4
2.1
2.4
kJ/g
38
26
13
17
9
10
Remember: 1 Food calorie is a kilocalorie = 4184 Joules or 4.184 kJ.
Energy Expenditure (70kg human)
Activity
Power (W) kcal/hour
Lying still (awake)
89
Sitting at rest
116
Walking (4.2 kph)
232
77 =1848
per day
100
(basal rate)
200
Jogging (8.5 kph)
662
569
Maximal activity
(untrained)
1673
1438
These figures are measured by calorimetry — i.e. they measure the total
energy used, not the mechanical energy produced.
Energy Requirement
• The actual energy requirement (in the form of
food) will be the basal rate + the energy needed
for activity during the day.
• This varies depending on level of activity but
might typically be 2400–2900 calories for a 70kg
body weight.
• Can be much higher for extreme levels of activity.
– A tour de France cyclist “burns” 6000–9000 calories
per day.
Calories Consumed
Excess Energy
• Many countries have average energy consumption
above typical daily requirements.
• Excess energy is stored as fat.
• This energy storage is an important evolutionary
adaptation enabling animals to survive with an
unpredictable food supply.
– When food is abundant we eat more than we need and
store the excess energy as fat.
– When food is scarce we can use the stored fat as an
energy source.
Humpback Whales
• Some mammal species can
live off stored fat for
extended periods.
– Humpback Whales feed only in
summer in Antarctic waters.
– They fast through the winter
months when they migrate north
to their breeding grounds (up to
25,000 km round trip).
– It was the stored fat that was the
main target of the whaling
industry in the 19th and early 20th
century. It was used to produce
whale oil - an important
commodity before the the
petroleum industry developed.
Whale hunting (~1840)
The Obesity Problem
• In developed countries food is readily available.
– There is a tendency to overeat — This is just what we
are evolutionarily programmed to do in such
circumstances.
• Many of these countries have an epidemic of
obesity.
– Resultant health problems that include cancer,
cardiovascular disease and many others.
• Also supports a large diet and exercise industry.
World Food Crisis
• At the same time in many poor countries
there are food shortages and high food
prices (particularly in the last year or so).
• Average energy consumptions only just
above the basal level.
• Many people undernourished.
Next lecture
• The next lecture will be the first of two looking
at fossil fuels (coal, oil and natural gas).