Have you ever felt so hungry that you couldn't do anything? Our bodies need a constant amount of energy to function. Every action that we take from walking, running, sitting down, breathing, and even sleeping all require energy (Fig. 1).

In fact, it is estimated an average human of 70 kilograms consumes around 70 kilograms of energy each day. That means we consume our own weight in energy every single day. In this section, we will cover cellular energetics which is how energy is used by living systems.

Figure 1: All activities from sleeping to running require energy, Fabio Comparelli

Cellular Energetics Definition

Let's start by looking at the definition of cellular energetics.

In this overview, we will discuss the processes by which a cell uses energy and how these processes can change depending on the environment.

Cellular Energetics Function

Now, let's talk about the function of cellular energetics.

Cellular Energy

All living systems require constant energy to function. As we mentioned before, cellular energetics is the study of how energy is captured and then used by living systems. The function of energy in a living system is broad. For example, cells can use energy for repair, to divide mitotically, to bring in or expel molecules from the cell, and to power metabolism. Essentially, energy is necessary to power all the biological reactions within the cell. Cellularly energetics helps to provide that energy by breaking apart macromolecules. Importantly, cellular energetics expands beyond humans to plants and even bacteria as well.

To further place the importance of cellular energetics, imagine that a living cell is like a refrigerator. A refrigerator needs to be constantly powered to keep its contents colder than the outside environment. Following the second law of thermodynamics, we know that heat will flow from a hot object to a cold object. In our analogy, heat will naturally flow from the outside environment to inside the refrigerator. To counter this natural flow, a refrigerator needs to be constantly powered using electrical energy. Similarly, a cell needs to be constantly powered by energy to prevent natural energy leakage. If a cell does not have enough energy to overcome the natural energy leakage, the cell will die. Therefore, cellular energetics is critical for keeping cells alive.

To first understand how energy is acquired and used in biological reactions, a foundational understanding of enzymes is needed. Enzymes are proteins that can be found in nearly every biological reaction and without enzymes, survival is impossible.

Enzyme Structure and Function

Our bodies perform thousands of biological and chemical reactions each second. Each one of these reactions is critical for keeping us alive, however, without any assistance, they may take too long. Enzymes are biological catalysts that accelerate chemical reactions at a much faster rate. While enzymes speed up the reaction, enzymes themselves are not consumed during the reaction meaning that they remain unchanged throughout the reaction.

Each enzyme has a unique structure that allows it to specifically interact with its binding partners called substrates. The part of the enzyme that directly interacts with the substrate is called the active site. For an enzyme to catalyze a chemical reaction, the shape and charge of the substrate must be compatible with the active site of the enzyme. Similar to how a key cannot open every lock, an enzyme cannot catalyze every reaction. It can only catalyze reactions where the active site and substrates are compatible (Fig. 2).

Activation energy is the energy that is required for a chemical reaction to occur. Imagine trying to light a match on fire. Simply holding the match against the striking strip does not light the match because there is not enough energy. Some amount of force is necessary to generate friction and heat to reach the activation energy threshold of lighting the match. Enzymes catalyze reactions by lowering the activation energy threshold. Therefore, an enzyme-catalyzed reaction would need a smaller amount of force to light the match compared to an uncatalyzed reaction. Activation energy can be shown using the graph below (Fig. 3) with reactants on the left moving to products on the right. The "hill" is the activation energy that must be overcome for the reaction to occur.

Cellular Energetics Examples

One key component of cellular energetics is obtaining energy. Complex pathways involving multiple reactions are called cellular processes. Two examples of cellular processes that can be used to obtain energy are photosynthesis and cellular respiration. These two processes utilize a process called energy transfer to efficiently power multiple chemical reactions sequentially.

Energy Transfer

A cellular process involves a chain of multiple different chemical reactions each with its own unique enzymes. While some chemical reactions release energy, other reactions require energy. Cellular processes are considered sequential meaning that a product of one reaction becomes the reactant in the next reaction. This means that a reaction that releases energy can be coupled with a reaction that requires energy so that energy released from the first reaction can fuel the energy required for the second reaction. In this way, energy is efficiently transferred in a controlled manner.

Photosynthesis

Photosynthesis is the cellular process by which organisms capture and store energy from the sun to produce sugar. While we often think that only plants undergo photosynthesis, it is a misconception that photosynthesis is exclusive to plants. In fact, photosynthesis is thought to have evolved from prokaryotic organisms, like cyanobacteria, that used photosynthesis to capture energy from the sun.

Taking a look at the net reaction for photosynthesis below, we can see that photosynthetic organisms convert carbon dioxide, water, and light into simple sugars and oxygen.

$6C{O}_{\mathit{2}}+6H_{\mathit{2}}O+light\to C_{\mathit{6}}{H}_{\mathit{12}}{O}_{\mathit{6}}+6O_{\mathit{2}}$

Importantly, the above equation is the net reaction and several intermediate reactions also take place that involves producing energy-rich molecules such as adenosine 5'-triphosphate (ATP) and reduced nicotinamide adenine dinucleotide phosphate (NADPH). In general, ATP is the currency of energy. Many biological reactions break apart ATP to generate enough energy to fuel the chemical reaction forward.

For eukaryotic organisms, many of these intermediate reactions take place in an organelle called the chloroplast (Fig. 4). Chloroplast is a membrane organelle that houses a green pigment called chlorophyll. Chlorophyll itself contains electrons that become excited when hit by solar energy from the sun. Proteins in the chloroplast membrane called electron transport chain (ETC) proteins will then use these excited electrons as an energy source to drive the formation of a proton gradient. Eventually, this proton gradient will be used to generate ATP, an energy-dense molecule. ATP will then be transferred to a region in the chloroplast called the stroma where the energy will be used to create simple sugars in a process called the Calvin cycle.

Cellular Respiration

Cellular respiration and fermentation are cellular processes performed by all living organisms that convert energy from biological macromolecules into energy in the form of ATP that can be used by the cell. While in the past, cellular respiration may have been expressed using the net equation below, it is important to note that like photosynthesis, cellular respiration and fermentation both involve multiple enzyme-catalyzed reactions.

$C_6{H}_{12}{O}_6+6O_2\to 6C{O}_2+6H_{2}O+energy\left(ATP\right)$

Since energy is formed by breaking apart large macromolecules, like glucose, into smaller molecules, cellular respiration is called a catabolic process. The main steps in cellular respiration include glycolysis, pyruvate oxidation, the Kreb's Cycle, and oxidative phosphorylation.

Glycolysis is the breakdown of glucose into ATP, NADH, and pyruvate. Pyruvate is then shuttled from the cytosol into the mitochondria where pyruvate oxidation occurs. Within the mitochondria, Kreb's cycle involves a series of enzyme-catalyzed reactions involving organic intermediate to form carbon dioxide, ATP, NADH, and FADH₂. The last set of cellular respiration is oxidative phosphorylation which takes place in the inner mitochondrial membrane. During oxidative phosphorylation, an electron transport chain (ETC) reaction occurs where electrons stored from glycolysis and the Kreb's cycle in the form of NADH and FADH₂ are passed down ETC proteins in the inner mitochondrial membrane (Fig. 5). As electrons are being passed, energy is being released which is used to fuel the formation of a proton gradient across the inner mitochondrial membrane. The final stop for de-energized electrons is oxygen. Finally, protons are transported down its gradient through membrane-bound ATP synthase which can fuel the energy required to make ATP from ADP. ATP can then be used to provide energy to all the processes of the cell.

While cellular respiration is typically performed aerobically meaning that oxygen is present, in the case of extreme exercise where the cell does not have enough oxygen, glycolysis can take place anaerobically, or without oxygen, in a process called fermentation. Fermentation produces ATP as well as alcohol and lactic acid as waste products.