Energy
• Cells use oxygen in cellular respiration, which
harvests chemical energy from food molecules.
• The waste products are CO2 and H2O
• Cells are able to convert about 40% of the
chemical energy stored in foods to energy for
cellular work.
• The other 60% generates heat.
• This explains why exercise makes you hot!
• Sweating helps release this excess heat.
Energy
• Energy - the capacity to do work.
• Kinetic energy - energy of motion.
• Potential energy – stored energy.
• Chemical energy – potential energy
available for release in a chemical reaction
Energy Transformations
• Thermodynamics – the study of energy
transformations.
• 1st law of thermodynamics – (law of energy
conservation) Energy can be transferred,
and transformed, but it cannot be created
or destroyed.
Energy Transformations
• 2nd law of thermodynamics – during every
energy transfer or transformation, some
energy becomes unstable & unusable
(most often it converts to heat).
– Heat is a disordered form of energy, an its
release makes the universe more random.
• Entropy – a measure of this disorder.
– Energy conversions increases the entropy
(disorder) of the universe.
Chemical Reactions
• Exergonic reaction – “energy outward” a
chemical reaction that releases energy.
Example: Wood burning – as the cellulose (a
polysaccharide) burns, each of the
glucose molecules rich in potential energy
release this energy in the form of heat and
light.
Chemical Reactions
• Cellular respiration – an exergonic
chemical process that uses oxygen to
convert the chemical energy stored in food
molecules to a form of chemical energy
(ATP) that the cell can use to perform
work.
Chemical Reactions
• Endergonic reactions – “energy inward”
yield products that are rich in potential
energy.
• Example: Photosynthesis, starts with
energy-poor reactants (CO2 and H2O) and,
using energy absorbed from sunlight,
produces energy-rich sugar molecules.
Chemical Reactions
• Metabolism – (metabole = change) the
total of an organism’s chemical reactions
(exergonic & endergonic).
• Metabolic pathway – a series of chemical
reactions that either builds a complex
molecule or breaks one down.
Chemical Reactions
• Energy coupling – the use of energy
released from exergonic reactions to drive
endergonic reactions.
– All cells have this ability.
– ATP molecules are key to this ability.
ATP drives cellular work
• ATP (adenosine triphosphate) Energy
storing molecule that powers nearly all
forms of cellular work.
– Adenosine = a nitrogenous base adenine +
ribose sugar.
– Triphosphate = a chain of 3 phosphate
groups.
ATP drives cellular work
• The 3 phosphates are negatively charged
causing a mutual repulsion.
• This allows the triphosphate chain to be
easily broken by hydrolysis.
• When a bond between the 3 phosphates is
broken, one phosphate group leaves,
energy is released, and ADP (adenosine
diphosphate) remains.
ATP drives cellular work
• The hydrolysis of ATP is an exergonic
reaction – it releases energy.
• The removed phosphate is transferred to
some other molecule, a process called
phosphorylation.
• Phosphorylation is an endergonic reaction.
• The two together form energy coupling.
3 Types of cellular work
• Chemical – the
phosphorylation of reactants
provides energy to drive
endergonic synthesis of
products.
3 Types of cellular work
• Mechanical – the transfer of
phosphate groups to special
motor proteins in muscle cells
causes the proteins to change
shape and pull on actin
filaments, in turn causing the
cells to contract.
3 Types of cellular work
• Transport – ATP drives the
active transport of solutes
across a membrane against
their concentration gradient by
phosphorylating membrane
proteins.
ATP drives cellular work
• ATP drives all 3 types.
• Work can be maintained because ATP is a
renewable resource that cells regenerate.
– Working muscle cells consume and
regenerate 10 million ATP molecules each
second!