Skip to 0 minutes and 0 secondsThe biological processes associated with life are achieved by sets of chemical reactions that take place inside cells. The sum of these reactions constitute cellular metabolism and they are organised into metabolic pathways, of

Skip to 0 minutes and 15 secondswhich there are two main types: catabolic pathways breakdown large biomolecules, such as fats, carbohydrates and proteins, providing key chemical building blocks for the anabolic pathways, which synthesise the biologically essential molecules that compose the cell. All biochemical reactions involve changes in energy as a result of the breaking and subsequent formation of chemical bonds. The biochemical field of bioenergetics studies the energy transformations that occur as a result of metabolic processes. An important concept to understand is, the energy released during metabolism is used to "do work" within cells, which may relate to movement of molecules, cells or organs, heating or setting up gradients of molecules.

Skip to 1 minute and 1 secondMuch energy within cells is tied up in the molecules it contains but when studying bioenergetics, biochemists focus on the proportion of energy that becomes "free" to do work. Over all catabolic pathways release energy and are described as exergonic, whereas anabolic pathways need to take in energy from the environment and thus are described as endergonic. Through millions of years of evolution, cells have developed efficient ways of temporarily storing energy in precise amounts in the form of high-energy covalent bonds contained in activated carrier molecules.

Skip to 1 minute and 37 secondsOften an energetically favorable reaction in the cell will be coupled to the energy requiring production of an activated carrier molecule, ensuring that the free energy released can be captured in a chemically useful form and not wasted as heat. Some activated carrier molecules such as nicotinamide adenine dinucleotide or NAD, for short, act as electron carriers, whereas others, such as, coenzyme A, carry an easily exchangeable chemical group. From your previous studies, you may already be familiar with the most abundant and versatile activated carrier molecule within all cells, which is called adenosine triphosphate, or ATP. This molecule is widely regarded as the near universal energy currency in cells, providing immediate access to energy for an enormous variety of cellular processes.

Skip to 2 minutes and 34 secondsEnergetically unfavorable reactions are often driven by coupling them to the favorable hydrolysis of the terminal phosphate of ATP, or the transfer of this phosphoryl group to another molecule to form a new high energy covalent bond. We will now turn our attention to some of the key biological processes that make ATP because ultimately, these generate the energy that sustains cellular life. One of the fundamental energy generating processes in cells is respiration - the oxidative of breakdown of complex organic molecules, such as glucose and fatty acids, to provide chemical energy in the form of ATP. As we review these processes, we will see some animations that illustrate them at a molecular level.

Skip to 3 minutes and 23 secondsThese animations were produced by researchers who work with Professor Sir John Walker, who as we saw in week one of this course, won the nobel prize in chemistry for his research in this area in 1997. Sugars are broken down by the process of glycolysis - a series of ten enzyme catalysed biochemical reactions which converts glucose into two molecules of pyruvate, coupled by the removal of electrons by NAD to give reduced Nad. The rest of the energy released by glycolysis is coupled to the synthesis of ATP molecules from ADP and inorganic phosphate, by a process known as substrate level phosphorylation. Alternatively, fatty acids are catabolised in the mitochondria by a repetitive four step sequence of enzyme catalyzed reactions called oxidation.

Skip to 4 minutes and 14 secondsEach passage through the pathway produces a single molecule each of reduced FAD, reduced NAD and Acetyl CoA. Acetyl CoA is also produced from pyruvate by the pyruvate dehydrogenase multi-enzyme complex in the mitochondrial matrix, coupled to the reduction of NAD, the two carbon fragment is then transferred from coenzyme A to the oxaloacetate to form citrate, which is then reconverted to oxaloacetate in a series of eight enzyme catalyzed steps known as the citric acid cycle. A single cycle of the pathway produces six molecules of reduced NAD, two molecules of reduced FAD and two molecules of ATP The final stage of respiration is known as oxidative phosphorylation.

Skip to 5 minutes and 3 secondsHere, the numerous reduced NAD and FAD molecules that were produced as a result of the preceding pathways, donate high energy electrons to a chain of proteins embedded on the inner mitochondrial membrane with molecular oxygen acting as the final electron acceptor. The energy released as a result of the passage of electrons along the respiratory chain is then used to transport protons across the inner mitochondrial membrane - this creates an electrochemical proton gradient. Protons move back into the mitochondrial matrix along this gradient by facilitated diffusion, through intrinsic channel proteins associated with the enzyme ATP synthase. This process, known as chemiosmosis, releases energy which can be harnessed by the enzyme ATP synthase in order to synthesize ATP from ADP - nature's energy currency.

Skip to 6 minutes and 10 secondsDevelopment of chemiosmotic theory by Peter Mitchell was one of the key milestones in biochemistry and which received the chemistry nobel prize in 1978. Similarly, plants use chemiosmosis to generate ATP in the light-dependent

Skip to 6 minutes and 27 secondsreactions of photosynthesis via two alternative pathways: cyclic photophosphorylation and noncyclic photophosphorylation. Solar energy is captured by chlorophyll pigments of the photosystems in the chloroplasts of plants inducing electron transfer processes that create the electrochemical proton gradient across the thylakoid membrane. So far, we've only explored the metabolism of carbohydrates, but the metabolism of nitrogen-containing compounds is equally important, as nitrogen is a key component of the amino acids of proteins and the nucleotides that are so important for the synthesis of DNA and RNA. Some microbes rely on anaerobic nitrate respiration, or denitrification pathways, to generate metabolic energy using nitrate is a terminal electron acceptor of the electron transport chains.

Skip to 7 minutes and 20 secondsThis pathway is particularly significant, since the intermediates of denitrification, which are nitric oxide and nitrous oxide are significant greenhouse gases. An understanding of bioenergetics is important for the treatment of numerous human diseases whose mechanisms are associated with disruptions in mitochondrial processes. Moreover, as we will explore further this week, the process of photosynthesis is inspiring solutions to the growing demand for sustainable energy, in the form of novel biofuels and advanced biotechnology products that can convert solar energy into a useful form.

Introduction to metabolism and bioenergetics

Metabolism is central to all cellular life. The video describes the cellular and molecular changes that deliver energy for organisms through different pathways. It highlights two of the most fundamental processes, photosynthesis and cellular respiration.

Focusing on respiration, the video describes how electrons pass along the respiratory chain from one protein to the next and generate a proton gradient across the mitochondrial membrane. As protons move back into the mitochondrial matrix in a process known as chemiosmosis, the molecular machine known as ATP synthase generates ATP. Professor Sir John Walker was one of the researchers who discovered the structure of ATP synthase, winning the Nobel Prize in Chemistry for this research in 1997. Through further studies a detailed understanding of the structure and function of ATP synthase has been obtained - an example of this can be viewed in our Gallery of Molecules.

Professor Walker’s research group have also prepared some beautiful animations that illustrate how ATP synthase functions at a molecular level. Further details about this molecular machine and the animations that show how it works can be seen by clicking on the links below.

Technical terms in simplified form


Endergonic reactions are ones in which energy is absorbed from the surroundings. In simple terms, it takes more energy to start the reaction than what is obtained out of it, so the total energy change is a negative net result.


Exergonic reactions are ones in which a positive flow of energy is released to the surroundings. In simple terms, more energy is obtained out of the reaction that is required to start it, so the total energy change is a positive net result.


Cellular respiration refers to sets of metabolic reactions that take place in the cells of organisms to convert biochemical energy from nutrients into cellular energy, with the release of waste products. Nutrients that are commonly used in respiration include sugars, amino acids and fatty acids. The most common oxidizing agent used as an electron acceptor in respiration is molecular oxygen (O2), though some organisms can use other chemicals as their final electron acceptor.

Share this video:

This video is from the free online course:

Biochemistry: the Molecules of Life

UEA (University of East Anglia)