# 13: Energy - Biology

13: Energy

## Contents

The total energy of a system can be subdivided and classified into potential energy, kinetic energy, or combinations of the two in various ways. Kinetic energy is determined by the movement of an object – or the composite motion of the components of an object – and potential energy reflects the potential of an object to have motion, and generally is a function of the position of an object within a field or may be stored in the field itself.

While these two categories are sufficient to describe all forms of energy, it is often convenient to refer to particular combinations of potential and kinetic energy as its own form. For example, macroscopic mechanical energy is the sum of translational and rotational kinetic and potential energy in a system neglects the kinetic energy due to temperature, and nuclear energy which combines potentials from the nuclear force and the weak force), among others. [ citation needed ]

Some forms of energy (that an object or system can have as a measurable property)
Type of energy Description
Mechanical the sum of macroscopic translational and rotational kinetic and potential energies
Electric potential energy due to or stored in electric fields
Magnetic potential energy due to or stored in magnetic fields
Gravitational potential energy due to or stored in gravitational fields
Chemical potential energy due to chemical bonds
Ionization potential energy that binds an electron to its atom or molecule
Nuclear potential energy that binds nucleons to form the atomic nucleus (and nuclear reactions)
Chromodynamic potential energy that binds quarks to form hadrons
Elastic potential energy due to the deformation of a material (or its container) exhibiting a restorative force as it returns to its original shape
Mechanical wave kinetic and potential energy in an elastic material due to a propagated deformational wave
Sound wave kinetic and potential energy in a fluid due to a sound propagated wave (a particular form of mechanical wave)
Radiant potential energy stored in the fields of propagated by electromagnetic radiation, including light
Rest potential energy due to an object's rest mass
Thermal kinetic energy of the microscopic motion of particles, a form of disordered equivalent of mechanical energy

The word energy derives from the Ancient Greek: ἐνέργεια , romanized: energeia, lit. 'activity, operation', [1] which possibly appears for the first time in the work of Aristotle in the 4th century BC. In contrast to the modern definition, energeia was a qualitative philosophical concept, broad enough to include ideas such as happiness and pleasure.

In the late 17th century, Gottfried Leibniz proposed the idea of the Latin: vis viva, or living force, which defined as the product of the mass of an object and its velocity squared he believed that total vis viva was conserved. To account for slowing due to friction, Leibniz theorized that thermal energy consisted of the motions of the constituent parts of matter, although it would be more than a century until this was generally accepted. The modern analog of this property, kinetic energy, differs from vis viva only by a factor of two. Writing in the early 18th century, Émilie du Châtelet proposed the concept of conservation of energy in the marginalia of her French language translation of Newton's Principia Mathematica, which represented the first formulation of a conserved measurable quantity that was distinct from momentum, and which would later be called "energy".

In 1807, Thomas Young was possibly the first to use the term "energy" instead of vis viva, in its modern sense. [2] Gustave-Gaspard Coriolis described "kinetic energy" in 1829 in its modern sense, and in 1853, William Rankine coined the term "potential energy". The law of conservation of energy was also first postulated in the early 19th century, and applies to any isolated system. It was argued for some years whether heat was a physical substance, dubbed the caloric, or merely a physical quantity, such as momentum. In 1845 James Prescott Joule discovered the link between mechanical work and the generation of heat.

These developments led to the theory of conservation of energy, formalized largely by William Thomson (Lord Kelvin) as the field of thermodynamics. Thermodynamics aided the rapid development of explanations of chemical processes by Rudolf Clausius, Josiah Willard Gibbs, and Walther Nernst. It also led to a mathematical formulation of the concept of entropy by Clausius and to the introduction of laws of radiant energy by Jožef Stefan. According to Noether's theorem, the conservation of energy is a consequence of the fact that the laws of physics do not change over time. [3] Thus, since 1918, theorists have understood that the law of conservation of energy is the direct mathematical consequence of the translational symmetry of the quantity conjugate to energy, namely time.

In 1843, James Prescott Joule independently discovered the mechanical equivalent in a series of experiments. The most famous of them used the "Joule apparatus": a descending weight, attached to a string, caused rotation of a paddle immersed in water, practically insulated from heat transfer. It showed that the gravitational potential energy lost by the weight in descending was equal to the internal energy gained by the water through friction with the paddle.

In the International System of Units (SI), the unit of energy is the joule, named after Joule. It is a derived unit. It is equal to the energy expended (or work done) in applying a force of one newton through a distance of one metre. However energy is also expressed in many other units not part of the SI, such as ergs, calories, British Thermal Units, kilowatt-hours and kilocalories, which require a conversion factor when expressed in SI units.

The SI unit of energy rate (energy per unit time) is the watt, which is a joule per second. Thus, one joule is one watt-second, and 3600 joules equal one watt-hour. The CGS energy unit is the erg and the imperial and US customary unit is the foot pound. Other energy units such as the electronvolt, food calorie or thermodynamic kcal (based on the temperature change of water in a heating process), and BTU are used in specific areas of science and commerce.

### Classical mechanics

In classical mechanics, energy is a conceptually and mathematically useful property, as it is a conserved quantity. Several formulations of mechanics have been developed using energy as a core concept.

Work, a function of energy, is force times distance.

This says that the work ( W ) is equal to the line integral of the force F along a path C for details see the mechanical work article. Work and thus energy is frame dependent. For example, consider a ball being hit by a bat. In the center-of-mass reference frame, the bat does no work on the ball. But, in the reference frame of the person swinging the bat, considerable work is done on the ball.

The total energy of a system is sometimes called the Hamiltonian, after William Rowan Hamilton. The classical equations of motion can be written in terms of the Hamiltonian, even for highly complex or abstract systems. These classical equations have remarkably direct analogs in nonrelativistic quantum mechanics. [4]

Another energy-related concept is called the Lagrangian, after Joseph-Louis Lagrange. This formalism is as fundamental as the Hamiltonian, and both can be used to derive the equations of motion or be derived from them. It was invented in the context of classical mechanics, but is generally useful in modern physics. The Lagrangian is defined as the kinetic energy minus the potential energy. Usually, the Lagrange formalism is mathematically more convenient than the Hamiltonian for non-conservative systems (such as systems with friction).

Noether's theorem (1918) states that any differentiable symmetry of the action of a physical system has a corresponding conservation law. Noether's theorem has become a fundamental tool of modern theoretical physics and the calculus of variations. A generalisation of the seminal formulations on constants of motion in Lagrangian and Hamiltonian mechanics (1788 and 1833, respectively), it does not apply to systems that cannot be modeled with a Lagrangian for example, dissipative systems with continuous symmetries need not have a corresponding conservation law.

### Chemistry

In the context of chemistry, energy is an attribute of a substance as a consequence of its atomic, molecular, or aggregate structure. Since a chemical transformation is accompanied by a change in one or more of these kinds of structure, it is invariably accompanied by an increase or decrease of energy of the substances involved. Some energy is transferred between the surroundings and the reactants of the reaction in the form of heat or light thus the products of a reaction may have more or less energy than the reactants. A reaction is said to be exothermic or exergonic if the final state is lower on the energy scale than the initial state in the case of endothermic reactions the situation is the reverse. Chemical reactions are usually not possible unless the reactants surmount an energy barrier known as the activation energy. The speed of a chemical reaction (at given temperature T) is related to the activation energy E by the Boltzmann's population factor e −E/kT – that is the probability of molecule to have energy greater than or equal to E at the given temperature T. This exponential dependence of a reaction rate on temperature is known as the Arrhenius equation. The activation energy necessary for a chemical reaction can be provided in the form of thermal energy.

### Biology

In biology, energy is an attribute of all biological systems from the biosphere to the smallest living organism. Within an organism it is responsible for growth and development of a biological cell or an organelle of a biological organism. Energy used in respiration is mostly stored in molecular oxygen [5] and can be unlocked by reactions with molecules of substances such as carbohydrates (including sugars), lipids, and proteins stored by cells. In human terms, the human equivalent (H-e) (Human energy conversion) indicates, for a given amount of energy expenditure, the relative quantity of energy needed for human metabolism, assuming an average human energy expenditure of 12,500 kJ per day and a basal metabolic rate of 80 watts. For example, if our bodies run (on average) at 80 watts, then a light bulb running at 100 watts is running at 1.25 human equivalents (100 ÷ 80) i.e. 1.25 H-e. For a difficult task of only a few seconds' duration, a person can put out thousands of watts, many times the 746 watts in one official horsepower. For tasks lasting a few minutes, a fit human can generate perhaps 1,000 watts. For an activity that must be sustained for an hour, output drops to around 300 for an activity kept up all day, 150 watts is about the maximum. [6] The human equivalent assists understanding of energy flows in physical and biological systems by expressing energy units in human terms: it provides a "feel" for the use of a given amount of energy. [7]

Sunlight's radiant energy is also captured by plants as chemical potential energy in photosynthesis, when carbon dioxide and water (two low-energy compounds) are converted into carbohydrates, lipids, and proteins and high-energy compounds like oxygen [5] and ATP. Carbohydrates, lipids, and proteins can release the energy of oxygen, which is utilized by living organisms as an electron acceptor. Release of the energy stored during photosynthesis as heat or light may be triggered suddenly by a spark, in a forest fire, or it may be made available more slowly for animal or human metabolism, when organic molecules are ingested, and catabolism is triggered by enzyme action.

Any living organism relies on an external source of energy – radiant energy from the Sun in the case of green plants, chemical energy in some form in the case of animals – to be able to grow and reproduce. The daily 1500–2000 Calories (6–8 MJ) recommended for a human adult are taken as a combination of oxygen and food molecules, the latter mostly carbohydrates and fats, of which glucose (C6H12O6) and stearin (C57H110O6) are convenient examples. The food molecules are oxidised to carbon dioxide and water in the mitochondria

and some of the energy is used to convert ADP into ATP.

The rest of the chemical energy in O2 [8] and the carbohydrate or fat is converted into heat: the ATP is used as a sort of "energy currency", and some of the chemical energy it contains is used for other metabolism when ATP reacts with OH groups and eventually splits into ADP and phosphate (at each stage of a metabolic pathway, some chemical energy is converted into heat). Only a tiny fraction of the original chemical energy is used for work: [note 1]

gain in kinetic energy of a sprinter during a 100 m race: 4 kJ gain in gravitational potential energy of a 150 kg weight lifted through 2 metres: 3 kJ Daily food intake of a normal adult: 6–8 MJ

It would appear that living organisms are remarkably inefficient (in the physical sense) in their use of the energy they receive (chemical or radiant energy), and it is true that most real machines manage higher efficiencies. In growing organisms the energy that is converted to heat serves a vital purpose, as it allows the organism tissue to be highly ordered with regard to the molecules it is built from. The second law of thermodynamics states that energy (and matter) tends to become more evenly spread out across the universe: to concentrate energy (or matter) in one specific place, it is necessary to spread out a greater amount of energy (as heat) across the remainder of the universe ("the surroundings"). [note 2] Simpler organisms can achieve higher energy efficiencies than more complex ones, but the complex organisms can occupy ecological niches that are not available to their simpler brethren. The conversion of a portion of the chemical energy to heat at each step in a metabolic pathway is the physical reason behind the pyramid of biomass observed in ecology: to take just the first step in the food chain, of the estimated 124.7 Pg/a of carbon that is fixed by photosynthesis, 64.3 Pg/a (52%) are used for the metabolism of green plants, [9] i.e. reconverted into carbon dioxide and heat.

### Earth sciences

In geology, continental drift, mountain ranges, volcanoes, and earthquakes are phenomena that can be explained in terms of energy transformations in the Earth's interior, [10] while meteorological phenomena like wind, rain, hail, snow, lightning, tornadoes and hurricanes are all a result of energy transformations brought about by solar energy on the atmosphere of the planet Earth.

Sunlight may be stored as gravitational potential energy after it strikes the Earth, as (for example) water evaporates from oceans and is deposited upon mountains (where, after being released at a hydroelectric dam, it can be used to drive turbines or generators to produce electricity). Sunlight also drives many weather phenomena, save those generated by volcanic events. An example of a solar-mediated weather event is a hurricane, which occurs when large unstable areas of warm ocean, heated over months, give up some of their thermal energy suddenly to power a few days of violent air movement.

In a slower process, radioactive decay of atoms in the core of the Earth releases heat. This thermal energy drives plate tectonics and may lift mountains, via orogenesis. This slow lifting represents a kind of gravitational potential energy storage of the thermal energy, which may be later released to active kinetic energy in landslides, after a triggering event. Earthquakes also release stored elastic potential energy in rocks, a store that has been produced ultimately from the same radioactive heat sources. Thus, according to present understanding, familiar events such as landslides and earthquakes release energy that has been stored as potential energy in the Earth's gravitational field or elastic strain (mechanical potential energy) in rocks. Prior to this, they represent release of energy that has been stored in heavy atoms since the collapse of long-destroyed supernova stars created these atoms.

### Cosmology

In cosmology and astronomy the phenomena of stars, nova, supernova, quasars and gamma-ray bursts are the universe's highest-output energy transformations of matter. All stellar phenomena (including solar activity) are driven by various kinds of energy transformations. Energy in such transformations is either from gravitational collapse of matter (usually molecular hydrogen) into various classes of astronomical objects (stars, black holes, etc.), or from nuclear fusion (of lighter elements, primarily hydrogen). The nuclear fusion of hydrogen in the Sun also releases another store of potential energy which was created at the time of the Big Bang. At that time, according to theory, space expanded and the universe cooled too rapidly for hydrogen to completely fuse into heavier elements. This meant that hydrogen represents a store of potential energy that can be released by fusion. Such a fusion process is triggered by heat and pressure generated from gravitational collapse of hydrogen clouds when they produce stars, and some of the fusion energy is then transformed into sunlight.

### Quantum mechanics

In quantum mechanics, energy is defined in terms of the energy operator as a time derivative of the wave function. The Schrödinger equation equates the energy operator to the full energy of a particle or a system. Its results can be considered as a definition of measurement of energy in quantum mechanics. The Schrödinger equation describes the space- and time-dependence of a slowly changing (non-relativistic) wave function of quantum systems. The solution of this equation for a bound system is discrete (a set of permitted states, each characterized by an energy level) which results in the concept of quanta. In the solution of the Schrödinger equation for any oscillator (vibrator) and for electromagnetic waves in a vacuum, the resulting energy states are related to the frequency by Planck's relation: E = h ν (where h is Planck's constant and ν the frequency). In the case of an electromagnetic wave these energy states are called quanta of light or photons.

### Relativity

When calculating kinetic energy (work to accelerate a massive body from zero speed to some finite speed) relativistically – using Lorentz transformations instead of Newtonian mechanics – Einstein discovered an unexpected by-product of these calculations to be an energy term which does not vanish at zero speed. He called it rest energy: energy which every massive body must possess even when being at rest. The amount of energy is directly proportional to the mass of the body:

m is the mass of the body, c is the speed of light in vacuum, E 0 > is the rest energy.

For example, consider electron–positron annihilation, in which the rest energy of these two individual particles (equivalent to their rest mass) is converted to the radiant energy of the photons produced in the process. In this system the matter and antimatter (electrons and positrons) are destroyed and changed to non-matter (the photons). However, the total mass and total energy do not change during this interaction. The photons each have no rest mass but nonetheless have radiant energy which exhibits the same inertia as did the two original particles. This is a reversible process – the inverse process is called pair creation – in which the rest mass of particles is created from the radiant energy of two (or more) annihilating photons.

In general relativity, the stress–energy tensor serves as the source term for the gravitational field, in rough analogy to the way mass serves as the source term in the non-relativistic Newtonian approximation. [11]

Energy and mass are manifestations of one and the same underlying physical property of a system. This property is responsible for the inertia and strength of gravitational interaction of the system ("mass manifestations"), and is also responsible for the potential ability of the system to perform work or heating ("energy manifestations"), subject to the limitations of other physical laws.

In classical physics, energy is a scalar quantity, the canonical conjugate to time. In special relativity energy is also a scalar (although not a Lorentz scalar but a time component of the energy–momentum 4-vector). [11] In other words, energy is invariant with respect to rotations of space, but not invariant with respect to rotations of spacetime (= boosts).

Some forms of transfer of energy ("energy in transit") from one object or system to another
Type of transfer process Description
Heat that amount of thermal energy in transit spontaneously towards a lower-temperature object
Work that amount of energy in transit due to a displacement in the direction of an applied force
Transfer of material that amount of energy carried by matter that is moving from one system to another

Energy may be transformed between different forms at various efficiencies. Items that transform between these forms are called transducers. Examples of transducers include a battery, from chemical energy to electric energy a dam: gravitational potential energy to kinetic energy of moving water (and the blades of a turbine) and ultimately to electric energy through an electric generator or a heat engine, from heat to work.

Examples of energy transformation include generating electric energy from heat energy via a steam turbine, or lifting an object against gravity using electrical energy driving a crane motor. Lifting against gravity performs mechanical work on the object and stores gravitational potential energy in the object. If the object falls to the ground, gravity does mechanical work on the object which transforms the potential energy in the gravitational field to the kinetic energy released as heat on impact with the ground. Our Sun transforms nuclear potential energy to other forms of energy its total mass does not decrease due to that in itself (since it still contains the same total energy even if in different forms), but its mass does decrease when the energy escapes out to its surroundings, largely as radiant energy.

There are strict limits to how efficiently heat can be converted into work in a cyclic process, e.g. in a heat engine, as described by Carnot's theorem and the second law of thermodynamics. However, some energy transformations can be quite efficient. The direction of transformations in energy (what kind of energy is transformed to what other kind) is often determined by entropy (equal energy spread among all available degrees of freedom) considerations. In practice all energy transformations are permitted on a small scale, but certain larger transformations are not permitted because it is statistically unlikely that energy or matter will randomly move into more concentrated forms or smaller spaces.

Energy transformations in the universe over time are characterized by various kinds of potential energy that has been available since the Big Bang later being "released" (transformed to more active types of energy such as kinetic or radiant energy) when a triggering mechanism is available. Familiar examples of such processes include nuclear decay, in which energy is released that was originally "stored" in heavy isotopes (such as uranium and thorium), by nucleosynthesis, a process ultimately using the gravitational potential energy released from the gravitational collapse of supernovae, to store energy in the creation of these heavy elements before they were incorporated into the solar system and the Earth. This energy is triggered and released in nuclear fission bombs or in civil nuclear power generation. Similarly, in the case of a chemical explosion, chemical potential energy is transformed to kinetic energy and thermal energy in a very short time. Yet another example is that of a pendulum. At its highest points the kinetic energy is zero and the gravitational potential energy is at maximum. At its lowest point the kinetic energy is at maximum and is equal to the decrease of potential energy. If one (unrealistically) assumes that there is no friction or other losses, the conversion of energy between these processes would be perfect, and the pendulum would continue swinging forever.

### Conservation of energy and mass in transformation

Energy gives rise to weight when it is trapped in a system with zero momentum, where it can be weighed. It is also equivalent to mass, and this mass is always associated with it. Mass is also equivalent to a certain amount of energy, and likewise always appears associated with it, as described in mass-energy equivalence. The formula E = mc², derived by Albert Einstein (1905) quantifies the relationship between rest-mass and rest-energy within the concept of special relativity. In different theoretical frameworks, similar formulas were derived by J.J. Thomson (1881), Henri Poincaré (1900), Friedrich Hasenöhrl (1904) and others (see Mass-energy equivalence#History for further information).

### Reversible and non-reversible transformations

Thermodynamics divides energy transformation into two kinds: reversible processes and irreversible processes. An irreversible process is one in which energy is dissipated (spread) into empty energy states available in a volume, from which it cannot be recovered into more concentrated forms (fewer quantum states), without degradation of even more energy. A reversible process is one in which this sort of dissipation does not happen. For example, conversion of energy from one type of potential field to another, is reversible, as in the pendulum system described above. In processes where heat is generated, quantum states of lower energy, present as possible excitations in fields between atoms, act as a reservoir for part of the energy, from which it cannot be recovered, in order to be converted with 100% efficiency into other forms of energy. In this case, the energy must partly stay as heat, and cannot be completely recovered as usable energy, except at the price of an increase in some other kind of heat-like increase in disorder in quantum states, in the universe (such as an expansion of matter, or a randomisation in a crystal).

As the universe evolves in time, more and more of its energy becomes trapped in irreversible states (i.e., as heat or other kinds of increases in disorder). This has been referred to as the inevitable thermodynamic heat death of the universe. In this heat death the energy of the universe does not change, but the fraction of energy which is available to do work through a heat engine, or be transformed to other usable forms of energy (through the use of generators attached to heat engines), grows less and less.

The fact that energy can be neither created nor be destroyed is called the law of conservation of energy. In the form of the first law of thermodynamics, this states that a closed system's energy is constant unless energy is transferred in or out by work or heat, and that no energy is lost in transfer. The total inflow of energy into a system must equal the total outflow of energy from the system, plus the change in the energy contained within the system. Whenever one measures (or calculates) the total energy of a system of particles whose interactions do not depend explicitly on time, it is found that the total energy of the system always remains constant. [12]

While heat can always be fully converted into work in a reversible isothermal expansion of an ideal gas, for cyclic processes of practical interest in heat engines the second law of thermodynamics states that the system doing work always loses some energy as waste heat. This creates a limit to the amount of heat energy that can do work in a cyclic process, a limit called the available energy. Mechanical and other forms of energy can be transformed in the other direction into thermal energy without such limitations. [13] The total energy of a system can be calculated by adding up all forms of energy in the system.

Richard Feynman said during a 1961 lecture: [14]

There is a fact, or if you wish, a law, governing all natural phenomena that are known to date. There is no known exception to this law – it is exact so far as we know. The law is called the conservation of energy. It states that there is a certain quantity, which we call energy, that does not change in manifold changes which nature undergoes. That is a most abstract idea, because it is a mathematical principle it says that there is a numerical quantity which does not change when something happens. It is not a description of a mechanism, or anything concrete it is just a strange fact that we can calculate some number and when we finish watching nature go through her tricks and calculate the number again, it is the same.

Most kinds of energy (with gravitational energy being a notable exception) [15] are subject to strict local conservation laws as well. In this case, energy can only be exchanged between adjacent regions of space, and all observers agree as to the volumetric density of energy in any given space. There is also a global law of conservation of energy, stating that the total energy of the universe cannot change this is a corollary of the local law, but not vice versa. [13] [14]

This law is a fundamental principle of physics. As shown rigorously by Noether's theorem, the conservation of energy is a mathematical consequence of translational symmetry of time, [16] a property of most phenomena below the cosmic scale that makes them independent of their locations on the time coordinate. Put differently, yesterday, today, and tomorrow are physically indistinguishable. This is because energy is the quantity which is canonical conjugate to time. This mathematical entanglement of energy and time also results in the uncertainty principle – it is impossible to define the exact amount of energy during any definite time interval. The uncertainty principle should not be confused with energy conservation – rather it provides mathematical limits to which energy can in principle be defined and measured.

Each of the basic forces of nature is associated with a different type of potential energy, and all types of potential energy (like all other types of energy) appears as system mass, whenever present. For example, a compressed spring will be slightly more massive than before it was compressed. Likewise, whenever energy is transferred between systems by any mechanism, an associated mass is transferred with it.

In quantum mechanics energy is expressed using the Hamiltonian operator. On any time scales, the uncertainty in the energy is by

which is similar in form to the Heisenberg Uncertainty Principle (but not really mathematically equivalent thereto, since H and t are not dynamically conjugate variables, neither in classical nor in quantum mechanics).

In particle physics, this inequality permits a qualitative understanding of virtual particles which carry momentum, exchange by which and with real particles, is responsible for the creation of all known fundamental forces (more accurately known as fundamental interactions). Virtual photons are also responsible for electrostatic interaction between electric charges (which results in Coulomb law), for spontaneous radiative decay of exited atomic and nuclear states, for the Casimir force, for van der Waals bond forces and some other observable phenomena.

### Closed systems

Energy transfer can be considered for the special case of systems which are closed to transfers of matter. The portion of the energy which is transferred by conservative forces over a distance is measured as the work the source system does on the receiving system. The portion of the energy which does not do work during the transfer is called heat. [note 3] Energy can be transferred between systems in a variety of ways. Examples include the transmission of electromagnetic energy via photons, physical collisions which transfer kinetic energy, [note 4] and the conductive transfer of thermal energy.

Energy is strictly conserved and is also locally conserved wherever it can be defined. In thermodynamics, for closed systems, the process of energy transfer is described by the first law: [note 5]

## The 13 Types of Energy and Their Varied Applications and Functions

The enigmatic quantity called energy can be roughly defined as the ability of any physical entity to do work against exerted forces in the surroundings. Learn about various manifestations of energy, along with working mechanisms and related examples.

The enigmatic quantity called energy can be roughly defined as the ability of any physical entity to do work against exerted forces in the surroundings. Learn about various manifestations of energy, along with working mechanisms and related examples.

### Did You Know?

The word energy is derived from the ancient Greek word ἐνέργεια (pronounced energeia), meaning activity/operation. This term was probably coined first by Aristotle around 4th BCE., according to the available and discovered past records.

To fully grasp the working of the universe, one must be acquainted with the various kinds of energy. Every single event that occurs in this universe is an energy transformation of a particular type. The law of conservation of energy establishes two things―the sum total of energy in the universe is constant, and energy manifests itself in various forms, which can undergo transformation within these forms.

Almost every physical quantity can be precisely defined, except energy, which can only be indirectly observed and measured as it manifests itself in different forms. Therefore, work and energy are very closely related, and have the same unit. Energy is a scalar physical quantity, i.e., it can be completely described by specifying the magnitude. Also, it should be noted that when the perspective of studies related to energy changes from macroscopic to microscopic and vice-versa, the form also might change. For example, mechanical energy like friction in the macroscopic view might be only thermal energy at the microscopic scale. The unit is joules according to the International System of Units (SI). When it manifests itself in the form of heat, it is measured using the unit of ‘calorie’ or ‘kilocalorie’.

### Different Forms of Energy

Energy can be further characterized through its observed properties. All the types can be broadly divided into two types―Potential and Kinetic Energy. The sum total of both these energies of a particle always remains constant, when there are no frictional forces operating on it.

It is basically defined as the summation of potential and kinetic energies of a body, which is affected by external forces. If the body is not affected by any external force, then the mechanical energy ‘Me’ remains constant, i.e., the body is isolated from any external forces. This is a hypothetical scenario, and in reality, forces like friction act on all bodies, though their values are very less. Thus, this energy can be simply represented as:

where, ‘Ep’ is the total potential energy, and ‘K’ is the kinetic energy.

Numerous modern devices convert other forms into mechanical energy and vice-versa, like thermal power plants (heat to Me), electric generators (Me to electricity), turbine (Kinetic energy to Me), etc.

The conservation of mechanical energy is also dependent on whether two bodies experience collision that is either elastic or non-elastic. In the former type, energy is conserved as the original shape and form is regained, whereas in the latter type, deformation of the bodies is permanent, and a different form of energy like heat may emerge from it. In this case, energy may not be conserved but might increase or decrease, depending on the nature of collision and the extent of deformation.

The inherent and dormant entity stored in any physical system, due to its position and structure in an environment, along with applied forces is called potential energy. The mass value of a body plays an important role in deciding it. For example, imagine an archer with a bow and an arrow is ready to launch it. When the arrow is made ready for launching and the taut bowstring is pulled back, at that position, the string has elastic potential energy stored in it. In this position, the string has the ‘potential’ to perform the work to launch the arrow.

There are various forms of potential energy, depending on the kind of forces involved, such as gravitational potential energy, chemical potential energy, electrical potential energy, magnetic potential energy, and nuclear potential energy. When a force is applied on a body, work is done in a specific direction. This work is represented by taking into account the potential energy of that body, which is denoted by a negative sign, as the energy may increase or decrease depending on whether the work is done against or in the force direction, respectively. This is represented as:

where, ‘W’ is work done, and ‘δEp’ is the potential energy present in the body.

It is mainly possessed by a particle or a body due to its motion. It is subdivided primarily into rotational kinetic energy and vibrational kinetic energy. In the above example, when the archer releases the bowstring, the arrow gets launched when the stored elastic potential energy gets converted into kinetic energy. The bowstring in motion possesses kinetic energy. Thus, any particle in motion has this kind of energy. Kinetic energy of a body that is not undergoing rotation is given by the following formula:

K = (M × V2) ÷ 2 —– equation 3

where, ‘K’ is the total kinetic energy, ‘M’ is mass of the body, and ‘V’ is the velocity at which it is traveling. For a rotating body, the kinetic energy is represented as:

where, ‘I’ and ‘W’ are the moment of inertia and angular velocity of the body.

Kinetic energy varies according to the frame of reference of an observer, along with inertia. For example, if a car passes an observer who is stationary, then the speeds of both objects are relative to each other, and hence the car possesses kinetic energy with a positive value. But, if both the observer and car are traveling at the same speed, then this energy is equivalent to zero.

It can be studied or estimated by measuring the temperature of the body or substance under consideration. It exists due to the vibrational, rotational, and translational motion of the body, along with the potential energy of its atoms and molecules. It is a part of the internal thermodynamics of an object, and mainly exists due to the loss of kinetic energy occurring during atomic collisions. This energy is a combination of both kinetic and potential energies of the object, and is characterized by the heat absorption aspect of the atoms, molecules, and other sub-atomic particles. In case of a gas that consists of atoms of the same element, then the thermal energy is equivalent to the entire kinetic energy of that gas. Thermal energy can be easily represented in the form of an equation that describes a mono-atomic gas in the following manner:

K = (M × V2) ÷ 2 —– from equation 3

Thus, if a gas has ‘N’ molecules, then its thermal energy can be represented as.

where, ‘k’ is the Boltzmann constant, and ‘T’ is the measured temperature or the heat of the body.

From the above formula, it is clear that this energy operates by the processes of absorption or emission of heat, during its transfer from one portion of the system to another.

It is derived from the electrical potential energy that exists between charges, which is delivered in the form of an electric current. When you connect the terminals of a battery with a bulb, electrical energy flows between the two terminals, in the form of an electric current. This process takes place due to the transfer of electrons through the wire, between the terminals. This type of energy can also exist in combination with other fundamental energies, which are stated below:

### Electromagnetic Energy

As the name suggests, it is present in the form of electromagnetic waves that vary in frequency and amplitude. Both the electric and magnetic components are perpendicular to each other, and also to the direction of energy propagation.

### Electrochemical Energy

The generation of electricity with the help of chemical reactions involves electrochemical energy. An amazing example is that of the fuel cell, wherein electricity can be generated due to the reactions triggered inside a device that contains a mixture of different components.

### Electrostatic Energy

It is the least harnessed one, and is present when two bodies undergo a frictional interaction or collision, which can create minor electrical charges. For example, rub a comb on a woolen material and hold it over small paper pieces they are lifted up because of the static electricity created by rubbing both the objects.

When an object or body is characterized by polar movement, i.e., the existence of two poles, which have exactly opposite characteristics, then the entity that controls all the related processes is called magnetic energy. The force that is exerted is in the form of a magnetic field, and the North and South poles of this field are situated exactly opposite to each other. A popular example is that of our planet, the Earth, which behaves like a giant magnet. The magnetic energy travels in the form of magnetic lines, which extend from the North to the South pole, creating the magnetic field.

Often, the terms ‘electromagnetic energy’ or ‘electromagnetism’ are used, as electricity and magnetism can exist in combined form in the form of waves. In case of this type, the strength of the field depends on several factors such as magnetic dipole moment, strength of the current produced, amount of magnetic material present, etc. A common example that incorporates the use of this energy is that of the electromagnet. This device is utilized in our everyday lives, and it consists mainly of a wire coiled around a metallic material. When an electric current is passed through the wire, a magnetic field is formed, which can be further used for different purposes depending on its strength and the associated magnetic forces.

It is the fundamental physical entity that controls the reactions occurring or involving both organic and inorganic compounds and substances, and also controls life-related processes. Chemical energy can be manifested in other forms such as heat, light, electricity, etc., from different sources. When the energy decreases after a reaction, it is then transferred to the surrounding environment or media, and hence the process is called exothermic. Similarly, if a body absorbs energy, its energy value increases, thus making it an endothermic process.

The motive force that powers the human body is provided by the chemical energy that is derived through the process of respiration, which involves the formation and breaking of inter-atomic molecular bonds. Through molecular rearrangements, along with compound formation and breakdown, the biological world derives energy. For example, the formation of glucose from the process of photosynthesis is useful for energy generation in a plant cell.

This type of energy is often represented in the form of the Rydberg constant, which is given as:

‘Me’ is the mass at zero motion, ‘E’ is the charge, ‘eo’ is the space permittivity, ‘H’ is the Planck constant, and ‘C’ is the light speed.

Sound is heard as the result of compressions and rarefactions produced in air as a medium. Thus, the sound energy is derived from the oscillatory motion of air molecules. The vibrations produced when the waves travel through this medium are absorbed and interpreted accordingly. These vibrations are parallel to each other and are in the same direction as that of the wave propagation. Humans and other living beings have the extraordinary character of hearing sound waves with the help of special ear components. When the ear catches sound energy, the waves are amplified and are passed onwards with the help of auditory nerves. The brain then interprets the signals, thus providing us with the feeling of hearing. Sound does not travel in vacuum, i.e., outer space, as compression and rarefaction is not possible in such a medium.

When sound energy is released from an object, the waves spread in all directions, and are a combination of both potential and kinetic energy densities of the body. For example, if a car passes an observer, the first kind of energy that is experienced by the person consists of the sound waves, and their strength depends on several parameters like wave frequency and amplitude, distance between the observer and the vehicle, the total area of the surroundings, etc.

It is propagated by electromagnetic waves through space for example, the light received from the Sun is an example of radiant energy. The spectrum of electromagnetic radiation is vast―from radio waves to the high-frequency gamma rays. The energy derived from this source is directly proportional to the frequency of waves. Humans can only detect the visible light spectrum of electromagnetic radiation, and all other wavelengths are invisible. Majority of light energy that is received by our planet is in the form of the Sun’s rays.

Light energy or power is measured mainly by a unit called radiant flux. There have been several theories that attempt to explain the propagation of light waves through any medium including space. The most famous one is the Quantum theory, which states that light travels in the form of small packets of particles called quanta, and each quantum shows dual personality, i.e., it can behave as a wave as well as a particle. Light energy is often accompanied with other kinds like heat, sound, chemical, and magnetic. It can be said that this energy is a secondary form and exists only when another type undergoes transformation due to several processes. These might include chemical reactions, nuclear fission and fusion processes, absorption, reflection, refraction, etc.

The force of attraction that exists between two bodies having substantial mass values is called gravitational force, and this phenomenon is controlled by the entity called gravitational energy. According to Newton’s law of gravitation, any two bodies having masses will exert a force on each other that will tend to attract both of them. This force is directly proportional to the product of their masses and inversely proportional to the square of distance between them. This force is represented as:

where, ‘G’ is the gravitational attraction, ‘g’ is the gravitational constant, R is the distance between the two objects, and ‘M1’ and ‘M2’ are the masses of both the bodies, respectively.

Gravitational energy is the weakest one of all in our Universe, but the force caused by it could be very strong in some celestial objects like black holes, wherein it is theorized that the gravitational forces would be so strong that not even light can escape from its attraction. On our planet, this energy helps to keep us stable and balanced. The heavier the body in terms of mass, the higher would be its gravitational attraction. Hence, as the Sun contributes the maximum mass of our solar system, its high gravity makes it possible the revolution of every planet around it.

It is a type of potential energy, and it is mainly derived from processes involving nuclear fission and nuclear fusion. In the former one, a radioactive elemental atom is divided or separated, further giving rise to daughter elements, and releasing a tremendous amount of energy. This principle is used in case of nuclear reactor and other associated technological applications. In the latter type, two atoms of an element combine with each other and fuse. This process also leads to the release of high amount of energy, and the prime example where this process is said to occur is that of the Sun it is theorized that in this star, nuclear fusion is taking place at its core portions.

Nuclear power has several applications in the modern world, and since several decades, this energy is utilized to produce electricity and heat supplies. Entire ships and submarines can be operated on the basis of a nuclear source. Some nations also use this energy form to make nuclear weapons. The electricity production is done with the help of a nuclear reactor and radioactive material. The atomic nuclei are bombarded with electrons, which cause them to split and form daughter elements. The energy released is used to power generators, which further produce electric power.

When you stretch a rubber band and then release it, the inter-atomic forces makes it snap back to its original condition. The stored elastic potential energy is converted into kinetic energy to create the reversible motion, which brings the elastic band to its original position. Thus, elastic energy more or less makes it possible exert tensional and compressional forces on an object. The work done depends on the magnitude of these forces.

For example, when a spring is extended, the stored potential energy makes it possible for the stretching of the material, and when the extensional forces are removed, it reverts back to its original position. Another example is the one, which was described earlier in this article―the bow and arrow description. In this instance, the bow string is stretched till a particular point, and after the arrow is released, it reverts back to its original position due to the elastic energy that is present during its stretching.

After a certain point, elasticity might get converted to plasticity, wherein the object gets permanently deformed. This happens because each material has its own limit of elasticity, and beyond this limit, the elastic forces stop operating. This can be easily observed with the Young’s modulus experiment.

It is defined as the energy, which is present by virtue of existence of tensional forces on an object’s surface. Such forces are typically present on still water, viscous liquids, stretched rubber material, etc. When two materials come in contact with each other (mostly liquids) and do not form any sort of mixture, surface tensions are created, which are governed by this type of energy. For example, the capillary motion in plant tissues, the formation bubbles and soap films on water, immiscibility between oil and water, etc are all instances of surface energy. This type exists under a particular limit of external forces, and when these forces increase beyond a certain limit, then the energy is released. Surface tensional forces are represented as:

where ‘dW’ is the work done and represents total surface energy of the body, ‘γ’ is surface tension, and ‘Sa’ is the surface area of the body.

In solid objects, surface energy is usually present in combination with elastic energy. When a solid is stretched this energy is mostly measured in the form of heat. The volume of the deformed body remains more or less same, as compared to the original object. Contact angles are also measured in order to determine this type of energy.

As seen in the above-described sections, the physical entity called energy can work in myriad forms and kinds, and can also exist in combination with the various types. This entity is governed by a single doctrine, which is also known as Newton’s 3rd law of motion. It states that energy can neither be created nor destroyed, and only can be changed from one form to another. This law is applicable to the entire Universe, at least till the extent discovered by mankind.

## Contents

Quantum biology is an emerging field most of the current research is theoretical and subject to questions that require further experimentation. Though the field has only recently received an influx of attention, it has been conceptualized by physicists throughout the 20th century. It has been suggested that quantum biology might play a critical role in the future of the medical world. [5] Early pioneers of quantum physics saw applications of quantum mechanics in biological problems. Erwin Schrödinger's 1944 book What is Life? discussed applications of quantum mechanics in biology. [6] Schrödinger introduced the idea of an "aperiodic crystal" that contained genetic information in its configuration of covalent chemical bonds. He further suggested that mutations are introduced by "quantum leaps". Other pioneers Niels Bohr, Pascual Jordan, and Max Delbruck argued that the quantum idea of complementarity was fundamental to the life sciences. [7] In 1963, Per-Olov Löwdin published proton tunneling as another mechanism for DNA mutation. In his paper, he stated that there is a new field of study called "quantum biology". [8]

### Photosynthesis Edit

Organisms that undergo photosynthesis absorb light energy through the process of electron excitation in antennae. These antennae vary among organisms. For example, bacteria use ring-like antennae, while plants use chlorophyll pigments to absorb photons. Photosynthesis creates Frenkel excitons, which provide a separation of charge that cells convert into usable chemical energy. The energy collected in reaction sites must be transferred quickly before it is lost to fluorescence or thermal vibrational motion.

Various structures, such as the FMO complex in green sulfur bacteria, are responsible for transferring energy from antennae to a reaction site. FT electron spectroscopy studies of electron absorption and transfer show an efficiency of above 99%, [9] which cannot be explained by classical mechanical models like the diffusion model. Instead, as early as 1938, scientists theorized that quantum coherence was the mechanism for excitation energy transfer.

Scientists have recently looked for experimental evidence of this proposed energy transfer mechanism. A study published in 2007 claimed the identification of electronic quantum coherence [10] at −196 °C (77 K). Another theoretical study from 2010 provided evidence that quantum coherence lives as long as 300 femtoseconds at biologically relevant temperatures (4 °C or 277 K) . In that same year, experiments conducted on photosynthetic cryptophyte algae using two-dimensional photon echo spectroscopy yielded further confirmation for long-term quantum coherence. [11] These studies suggest that, through evolution, nature has developed a way of protecting quantum coherence to enhance the efficiency of photosynthesis. However, critical follow-up studies question the interpretation of these results. Single molecule spectroscopy now shows the quantum characteristics of photosynthesis without the interference of static disorder, and some studies use this method to assign reported signatures of electronic quantum coherence to nuclear dynamics occurring in chromophores. [12] [13] [14] [15] [16] [17] [18] A number of proposals emerged trying to explain unexpectedly long coherence. According to one proposal, if each site within the complex feels its own environmental noise, the electron will not remain in any local minimum due to both quantum coherence and thermal environment, but proceed to the reaction site via quantum walks. [19] [20] [21] Another proposal is that the rate of quantum coherence and electron tunneling create an energy sink that moves the electron to the reaction site quickly. [22] Other work suggested that geometric symmetries in the complex may favor efficient energy transfer to the reaction center, mirroring perfect state transfer in quantum networks. [23] Furthermore, experiments with artificial dye molecules cast doubts on the interpretation that quantum effects last any longer than one hundred femtoseconds. [24]

In 2017, the first control experiment with the original FMO protein under ambient conditions confirmed that electronic quantum effects are washed out within 60 femtoseconds, while the overall exciton transfer takes a time on the order of a few picoseconds. [25] In 2020 a review based on a wide collection of control experiments and theory concluded that the proposed quantum effects as long lived electronic coherences in the FMO system does not hold. [26] Instead, research investigating transport dynamics suggests that interactions between electronic and vibrational modes of excitation in FMO complexes require a semi-classical, semi-quantum explanation for the transfer of exciton energy. In other words, while quantum coherence dominates in the short-term, a classical description is most accurate to describe long-term behavior of the excitons. [27]

Another process in photosynthesis that has almost 100% efficiency is charge transfer, again suggesting that quantum mechanical phenomena are at play. [18] In 1966, a study on the photosynthetic bacteria Chromatium found that at temperatures below 100 K, cytochrome oxidation is temperature-independent, slow (on the order of milliseconds), and very low in activation energy. The authors, Don DeVault and Britton Chase, postulated that these characteristics of electron transfer are indicative of quantum tunneling, whereby electrons penetrate a potential barrier despite possessing less energy than is classically necessary. [28]

Seth Lloyd is also notable for his contributions to this area of research.

### DNA mutation Edit

Deoxyribonucleic acid, DNA, acts as the instructions for making proteins throughout the body. It consists of 4 nucleotides guanine, thymine, cytosine, and adenine. [29] The order of these nucleotides gives the “recipe” for the different proteins.

Whenever a cell reproduces, it must copy these strands of DNA. However, sometimes throughout the process of copying the strand of DNA a mutation, or an error in the DNA code, can occur. A theory for the reasoning behind DNA mutation is explained in the Lowdin DNA mutation model. [30] In this model, a nucleotide may change its form through a process of quantum tunneling. [31] Because of this, the changed nucleotide will lose its ability to pair with its original base pair and consequently changing the structure and order of the DNA strand.

Exposure to ultraviolet lights and other types of radiation can cause DNA mutation and damage. The radiations also can modify the bonds along the DNA strand in the pyrimidines and cause them to bond with themselves creating a dimer. [32]

In many prokaryotes and plants, these bonds are repaired to their original form by a DNA repair enzyme photolyase. As its prefix implies, photolyase is reliant on light in order to repair the strand. Photolyase works with its cofactor FADH, flavin adenine dinucleotide, while repairing the DNA. Photolyase is excited by visible light and transfers an electron to the cofactor FADH-. FADH- now in the possession of an extra electron gives the electron to the dimer to break the bond and repair the DNA. This transfer of the electron is done through the tunneling of the electron from the FADH to the dimer. Although the range of the tunneling is much larger than feasible in a vacuum, the tunneling in this scenario is said to be “superexchange-mediated tunneling,” and is possible due to the protein's ability to boost the tunneling rates of the electron. [30]

### Vibration theory of olfaction Edit

Olfaction, the sense of smell, can be broken down into two parts the reception and detection of a chemical, and how that detection is sent to and processed by the brain. This process of detecting an odorant is still under question. One theory named the “shape theory of olfaction” suggests that certain olfactory receptors are triggered by certain shapes of chemicals and those receptors send a specific message to the brain. [33] Another theory (based on quantum phenomena) suggests that the olfactory receptors detect the vibration of the molecules that reach them and the “smell” is due to different vibrational frequencies, this theory is aptly called the “vibration theory of olfaction.”

The vibration theory of olfaction, created in 1938 by Malcolm Dyson [34] but reinvigorated by Luca Turin in 1996, [35] proposes that the mechanism for the sense of smell is due to G-protein receptors that detect molecular vibrations due to inelastic electron tunneling, tunneling where the electron loses energy, across molecules. [35] In this process a molecule would fill a binding site with a G-protein receptor. After the binding of the chemical to the receptor, the chemical would then act as a bridge allowing for the electron to be transferred through the protein. As the electron transfers through and that usually would be a barrier for the electrons and would lose its energy due to the vibration of the molecule recently bound to the receptor, resulting in the ability to smell the molecule. [35] [36]

While the vibration theory has some experimental proof of concept, [37] [38] there have been multiple controversial results in experiments. In some experiments, animals are able to distinguish smells between molecules of different frequencies and same structure, [39] while other experiments show that people are unaware of distinguishing smells due to distinct molecular frequencies. [40] However, it has not been disproven, and has even been shown to be an effect in olfaction of animals other than humans such as flies, bees, and fish. [ citation needed ]

### Vision Edit

Vision relies on quantized energy in order to convert light signals to an action potential in a process called phototransduction. In phototransduction, a photon interacts with a chromophore in a light receptor. The chromophore absorbs the photon and undergoes photoisomerization. This change in structure induces a change in the structure of the photo receptor and resulting signal transduction pathways lead to a visual signal. However, the photoisomerization reaction occurs at a rapid rate, in under 200 femtoseconds, [41] with high yield. Models suggest the use of quantum effects in shaping the ground state and excited state potentials in order to achieve this efficiency. [42]

#### Quantum vision implications Edit

Experiments have shown that the sensors in the retina of human eye is sensitive enough to detect a single photon. [43] Single photon detection could lead to multiple different technologies. One area of development is in quantum communication and cryptography. The idea is to use a biometric system to measure the eye using only a small number of points across the retina with random flashes of photons that “read” the retina and identify the individual. [44] This biometric system would only allow a certain individual with a specific retinal map to decode the message. This message can not be decoded by anyone else unless the eavesdropper were to guess the proper map or could read the retina of the intended recipient of the message. [45]

### Enzymatic activity (quantum biochemistry) Edit

Enzymes may use quantum tunneling to transfer electrons long distances. It is possible that protein quaternary architecture may have evolved to enable sustained quantum entanglement and coherence. [46] More specifically, they can increase the percentage of the reaction that occurs through hydrogen tunneling. [47] Tunneling refers to the ability of a small mass particle to travel through energy barriers. This ability is due to the principle of complementarity, which hold that certain objects have pairs of properties that cannot be measured separately without changing the outcome of measurement. Electrons have both wave and particle properties, so they can pass through physical barriers as a wave without violating the laws of physics. Studies show that long distance electron transfers between redox centers through quantum tunneling plays important roles in enzymatic activity of photosynthesis and cellular respiration. [48] [49] For example, studies show that long range electron tunneling on the order of 15–30 Å plays a role in redox reactions in enzymes of cellular respiration. [50] Without quantum tunneling, organisms would not be able to convert energy quickly enough to sustain growth. Even though there are such large separations between redox sites within enzymes, electrons successfully transfer in a generally temperature independent (aside from extreme conditions) and distance dependent manner. [47] This suggests the ability of electrons to tunnel in physiological conditions. Further research is needed to determine whether this specific tunneling is also coherent.

### Magnetoreception Edit

Magnetoreception refers to the ability of animals to navigate using the inclination of the magnetic field of the earth. [51] A possible explanation for magnetoreception is the entangled radical pair mechanism. [52] [53] The radical-pair mechanism is well-established in spin chemistry, [54] [55] [56] and was speculated to apply to magnetoreception in 1978 by Schulten et al.. The ratio between singlet and triplet pairs is changed by the interaction of entangled electron pairs with the magnetic field of the earth. [57] In 2000, cryptochrome was proposed as the "magnetic molecule" that could harbor magnetically sensitive radical-pairs. Cryptochrome, a flavoprotein found in the eyes of European robins and other animal species, is the only protein known to form photoinduced radical-pairs in animals. [51] When it interacts with light particles, cryptochrome goes through a redox reaction, which yields radical pairs both during the photo-reduction and the oxidation. The function of cryptochrome is diverse across species, however, the photoinduction of radical-pairs occurs by exposure to blue light, which excites an electron in a chromophore. [57] Magnetoreception is also possible in the dark, so the mechanism must rely more on the radical pairs generated during light-independent oxidation.

### Other biological applications Edit

Other examples of quantum phenomena in biological systems include the conversion of chemical energy into motion [58] and brownian motors in many cellular processes. [59]

## Mechanics and energetics of human locomotion on sand.

Moving about in nature often involves walking or running on a soft yielding substratum such as sand, which has a profound effect on the mechanics and energetics of locomotion. Force platform and cinematographic analyses were used to determine the mechanical work performed by human subjects during walking and running on sand and on a hard surface. Oxygen consumption was used to determine the energetic cost of walking and running under the same conditions. Walking on sand requires 1.6-2.5 times more mechanical work than does walking on a hard surface at the same speed. In contrast, running on sand requires only 1.15 times more mechanical work than does running on a hard surface at the same speed. Walking on sand requires 2.1-2.7 times more energy expenditure than does walking on a hard surface at the same speed while running on sand requires 1.6 times more energy expenditure than does running on a hard surface. The increase in energy cost is due primarily to two effects: the mechanical work done on the sand, and a decrease in the efficiency of positive work done by the muscles and tendons.

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### Caiman red blood cells carry bicarbonate, not blood plasma

Bautista et al. find that rather than carry bicarbonate in their blood plasma, caiman carry the anion in their red blood cells, thanks to their specially modified haemoglobin.

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