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Bioenergetics: Energy Transfer Within Cells

Bioenergetics is a branch of biochemistry that investigates the transformation of energy in biological systems. It involves studying the production, storage, and consumption of energy within cells – most commonly through reactions involving adenosine triphosphate (ATP). This molecule can be thought of as a currency that all cell spends to drive various cellular processes.

ATP is produced in the mitochondria, this explains why the mitochondrion is often considered the powerhouse of the cell, and in chloroplasts, an organelle found only in plants. Although both the mitochondria and the chloroplast produce ATP, the raw material used by these organelles to produce ATP is different. The mitochondria uses sugar to produce ATP while the chloroplast uses light energy to produce ATP.

To understand bioenergetics, we rely heavily on the understanding of thermodynamics – a branch of physics concerned with the relationship between heat and other forms of energy. This is because chemical reactions obey the laws of thermodynamics. Therefore, bioenergetics is a multidisciplinary subject where we utilise our knowledge of biology, chemistry, and physics to understand what is going on within organisms at the cellular level!

Every reaction that occurs within a cell can be studied using bioenergetics, with the most notable reactions being cellular respiration through the oxidative phosphorylation reaction and photosynthesis, which occurs in the mitochondria and chloroplasts, respectively, within eukaryotic cells. Prokaryotic organisms can also carry out cellular respiration and photosynthesis, although the reactions occur directly on the plasma membrane instead as they do not contain membrane-bound organelles.

There are three laws of thermodynamics, namely:

Open systems are systems where energy can be transferred between a system and its environment; while closed, or isolated, systems are systems where energy transfer between the system and its environment does not occur.

In bioenergetics, we only use the first law of thermodynamics and the second law of thermodynamics to understand how processes happen.

Below, I will focus on these two.

The first law of thermodynamics

The first law of thermodynamics, also known as the Law of Conservation of Energy, states that the change in internal energy of a system equals the net heat transfer into the system minus the net work done by the system.

Heat transfer is a process driven by temperature differences, such as ice melting in your drinks on a hot summer day. The ice absorbs heat energy, causing the water and air surrounding the ice to cool down. Meanwhile, work is done when a force changes the location of an object in a particular direction. The difference between how much heat is transferred into or out of the system and the work done by the system is stored as internal energy.

Now, what is this internal energy?

The internal energy of a system is the sum of the kinetic and potential energies, and thus mechanical energy, of its atoms and molecules. When dealing with the internal energy of a system, we do not look at individual atoms and molecules but the averages or distribution of the atoms and molecules.

An example of the first law of thermodynamics in biological systems is in the form of metabolism. Digested food is converted into chemical energy in the body and at the same time, the internal energy of the body is increased. At the same time, the maintenance of our body temperature and movement releases the internal energy stored within our body as a form of work. Therefore, when the amount of energy of the food that is consumed is equal to the work done in maintaining life, the net change in internal energy is zero. When the amount of energy in the food consumed is more than work done, the internal energy is stored as fat and vice versa, which is how dieting or overeating can affect your weight!

The second law of thermodynamics

The second law of thermodynamics states that the total entropy of an isolated system increases over time, with the state of maximum entropy being thermodynamic equilibrium. It also states that as energy is transferred or transformed, useful energy is wasted, and hence the total available energy in the system to do useful work also decreases.

To understand what this means, it is important to familiarise ourselves with the following properties that make up the equation that allows us to apply the second law of thermodynamics:

1) Enthalpy is the heat content of the reacting system. Exothermic reactions are reactions in which heat is released when products are formed (denoted by negative sign) whereas endothermic reactions are reactions where heat is absorbed to form the products (denoted by a positive sign).

2) Entropy is a quantitative expression for the disorder within a system. When the products from a chemical reaction is less complex and more disordered than the reactants, the reaction has a gain in entropy, denoted by a positive sign. One way to think about entropy is using the process of diffusion, where molecules at a region of higher concentration move to a region of lower concentration.

Note that while this helps us to understand what entropy is in a rough sense, it is not accurate to think of entropy as the disorder of a system. A more accurate explanation is given in footnote 1 (scroll to the end of this article!).

3) Gibbs free energy is the amount of energy that can be used to do work. In a reaction where ΔG is negative, free energy is released to do work and the reaction is said to be exergonic, an example being cellular respiration. In a reaction where ΔG is positive, free energy has to be used for the reaction to proceed and the reaction is said to be endergonic, an example being photosynthesis.

Do check out this video from Bozeman Science explaining the terms in greater detail.

If everything tends to disorder, how is it possible that living organisms, composed of molecules much more highly organised than their surroundings, exist? In practice, the term ‘isolated system’ stated in the second law of thermodynamics refers to the universe, and so living organisms on themselves are not isolated or closed systems, but rather open systems constantly exchanging materials and energy with their surroundings. Order within a living system is maintained and disorder leaves the organism mainly in the form of heat.

The equation for Gibbs free energy is useful in helping us determine the direction in which a chemical reaction will take. A reaction in which the formation of products are favourable is known as a spontaneous reaction. That is, the reaction releases free energy, and the products formed contain lower energy than the reactants. It must be noted that spontaneous reaction does not mean a reaction that occurs rapidly or without energy input, as spontaneous reactions can be slow and still need to overcome the activation energy of the reaction.

Since temperature in the equation is in the Kelvin temperature, it can only have a positive value. Therefore, when ΔH is negative and ΔS is positive, ΔG will always be negative and the reaction is spontaneous. When ΔH is positive and ΔS is negative, ΔG will always be positive and non-spontaneous.

When both ΔH and ΔS are positive, a situation where one force is favourable but the other is not, it is the temperature that determines the value of ΔG and hence whether the reaction is spontaneous or not. In a situation where ΔH and ΔS are negative, the enthalpy change of the reaction has to be much larger than TΔS so that ΔG is negative.

This is summarised in the table below:

If this sounds a little confusing, check out these videos which explain these well!

What is the relevance of the table to bioenergetics? Well, many biological processes are actually non-spontaneous reactions, being entropically unfavourable. Therefore, these biological reactions are usually coupled with another reaction that is entropically favourable such that the net ΔG is a negative value, with the most common reaction being the hydrolysis of inorganic phosphate on an ATP molecule.


  1. The view of entropy as "disorder" is easy to understand but factually inaccurate. Entropy is intricately linked to the idea that the extent to which energy is dispersed depends on how much energy is transferred as heat. This is why it is usually explained as 'disorder', where the "disorder" is actually heat. There is also a second interpretation of entropy which uses the Boltzmann distribution, but the explanation is more complex. I would recommend this video by Khan Academy to learn more about the Boltzmann distribution to those who are interested!


Low Jia Yi

BSc Biochemistry with a Year in Industry

Imperial College London


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