New Jersey City University BIOL 204 Principles of
Anatomy & Physiology I (2026) | Energy, Chemical
Reactions & Cellular Respiration
Energy in Biological Systems
Energy is the capacity to do work, essential for maintaining life processes.
In biological systems, energy exists primarily in two forms: potential
energy, which is stored energy of position or configuration, and kinetic
energy, which is energy of motion. Both forms are interchangeable;
potential energy can be converted into kinetic energy and vice versa. For
example, the concentration gradient across the plasma membrane stores
potential energy that can be harnessed when ions move down their
gradient, producing kinetic energy that powers cellular functions like nerve
impulses. Similarly, electrons in high-energy shells possess potential
energy that can be released as kinetic energy during electron transfer
reactions. These energy forms drive vital cellular activities such as muscle
contraction, nutrient absorption, and cellular transport.
Chemical Energy and Types of Energy
Chemical energy is a form of potential energy stored within molecular
bonds, and it is fundamental in cellular metabolism. When bonds are
broken during chemical reactions, this stored energy is released and
utilized for various cellular functions, including movement, synthesis of
molecules, and establishing concentration gradients across membranes.
Molecules such as triglycerides, glucose, and ATP serve as key chemical
energy reservoirs. Besides chemical energy, other energy forms include:
Electrical energy: movement of charged particles, e.g., ion flow across
membranes.
, Mechanical energy: movement of objects under force, e.g., muscle
contraction.
Sound energy: vibration of molecules, e.g., auditory vibrations.
Radiant energy: electromagnetic waves, e.g., visible light striking the
retina.
Heat: kinetic energy of atoms and molecules, usually not available to do
work but important in thermoregulation.
Thermodynamics Laws in Biology
The principles of thermodynamics govern energy transformations in living
organisms:
First Law (Law of Conservation of Energy): Energy cannot be created
or destroyed, only transformed from one form to another. For instance,
chemical energy in glucose is converted into ATP, heat, and mechanical
work.
Second Law: During energy transformations, some energy is lost as
heat, reducing the amount of usable energy. This explains why biological
systems are inefficient and require continuous energy input to maintain
order and perform work, such as warming muscles during activity.
Chemical Reactions and Metabolism
Metabolism encompasses all biochemical reactions within the organism,
critical for growth, repair, and maintenance. These reactions involve:
Bond breaking: destabilizing molecules to release energy.
Bond forming: creating new, more complex molecules. Reactions are
expressed as chemical equations with reactants (initial substances) on
the left and products (final substances) on the right. They can be
classified based on the nature of bond changes:
, Decomposition reactions: large molecules break into smaller parts
(catabolism).
Synthesis reactions: smaller molecules combine into larger structures
(anabolism).
Exchange reactions: groups are exchanged between molecules,
involving both bond breaking and forming.
Redox Reactions and Energy Transfer
Oxidation-reduction (redox) reactions involve transfer of electrons:
Oxidation: loss of electrons; molecules become oxidized.
Reduction: gain of electrons; molecules become reduced. Coenzymes
like NAD+ and FAD are essential for capturing and transferring energy
during these reactions. For example, NAD+ accepts electrons during
glucose oxidation, becoming NADH, which carries energy to the electron
transport chain for ATP synthesis.
Exergonic and Endergonic Reactions
Exergonic reactions: release energy because reactants have higher
energy than products (e.g., breakdown of glucose). They are
spontaneous and drive other reactions.
Endergonic reactions: consume energy to produce complex molecules
from simpler ones (e.g., protein synthesis). They are non-spontaneous
and require energy input, often supplied by ATP.
These reactions underpin cellular metabolism, with energy flow tightly
regulated to meet cellular demands.
ATP Cycling as Cellular Energy Currency
Anatomy & Physiology I (2026) | Energy, Chemical
Reactions & Cellular Respiration
Energy in Biological Systems
Energy is the capacity to do work, essential for maintaining life processes.
In biological systems, energy exists primarily in two forms: potential
energy, which is stored energy of position or configuration, and kinetic
energy, which is energy of motion. Both forms are interchangeable;
potential energy can be converted into kinetic energy and vice versa. For
example, the concentration gradient across the plasma membrane stores
potential energy that can be harnessed when ions move down their
gradient, producing kinetic energy that powers cellular functions like nerve
impulses. Similarly, electrons in high-energy shells possess potential
energy that can be released as kinetic energy during electron transfer
reactions. These energy forms drive vital cellular activities such as muscle
contraction, nutrient absorption, and cellular transport.
Chemical Energy and Types of Energy
Chemical energy is a form of potential energy stored within molecular
bonds, and it is fundamental in cellular metabolism. When bonds are
broken during chemical reactions, this stored energy is released and
utilized for various cellular functions, including movement, synthesis of
molecules, and establishing concentration gradients across membranes.
Molecules such as triglycerides, glucose, and ATP serve as key chemical
energy reservoirs. Besides chemical energy, other energy forms include:
Electrical energy: movement of charged particles, e.g., ion flow across
membranes.
, Mechanical energy: movement of objects under force, e.g., muscle
contraction.
Sound energy: vibration of molecules, e.g., auditory vibrations.
Radiant energy: electromagnetic waves, e.g., visible light striking the
retina.
Heat: kinetic energy of atoms and molecules, usually not available to do
work but important in thermoregulation.
Thermodynamics Laws in Biology
The principles of thermodynamics govern energy transformations in living
organisms:
First Law (Law of Conservation of Energy): Energy cannot be created
or destroyed, only transformed from one form to another. For instance,
chemical energy in glucose is converted into ATP, heat, and mechanical
work.
Second Law: During energy transformations, some energy is lost as
heat, reducing the amount of usable energy. This explains why biological
systems are inefficient and require continuous energy input to maintain
order and perform work, such as warming muscles during activity.
Chemical Reactions and Metabolism
Metabolism encompasses all biochemical reactions within the organism,
critical for growth, repair, and maintenance. These reactions involve:
Bond breaking: destabilizing molecules to release energy.
Bond forming: creating new, more complex molecules. Reactions are
expressed as chemical equations with reactants (initial substances) on
the left and products (final substances) on the right. They can be
classified based on the nature of bond changes:
, Decomposition reactions: large molecules break into smaller parts
(catabolism).
Synthesis reactions: smaller molecules combine into larger structures
(anabolism).
Exchange reactions: groups are exchanged between molecules,
involving both bond breaking and forming.
Redox Reactions and Energy Transfer
Oxidation-reduction (redox) reactions involve transfer of electrons:
Oxidation: loss of electrons; molecules become oxidized.
Reduction: gain of electrons; molecules become reduced. Coenzymes
like NAD+ and FAD are essential for capturing and transferring energy
during these reactions. For example, NAD+ accepts electrons during
glucose oxidation, becoming NADH, which carries energy to the electron
transport chain for ATP synthesis.
Exergonic and Endergonic Reactions
Exergonic reactions: release energy because reactants have higher
energy than products (e.g., breakdown of glucose). They are
spontaneous and drive other reactions.
Endergonic reactions: consume energy to produce complex molecules
from simpler ones (e.g., protein synthesis). They are non-spontaneous
and require energy input, often supplied by ATP.
These reactions underpin cellular metabolism, with energy flow tightly
regulated to meet cellular demands.
ATP Cycling as Cellular Energy Currency
