How do cells make and use energy?

As a cellular biologist, I am well-versed in the intricate processes that cells undergo to generate and utilize energy. The primary source of energy for most cells is the chemical energy stored in food molecules. This energy is harnessed through a process known as cellular respiration, which is a series of metabolic pathways that convert biochemical energy from nutrients into adenosine triphosphate (ATP), the primary energy currency of the cell.

Cellular respiration can be divided into three main stages: glycolysis, the citric acid cycle (also known as the Krebs cycle or TCA cycle), and oxidative phosphorylation. Let's delve into each of these stages to understand how cells make and use energy.

1. Glycolysis: This is the first step in cellular respiration and occurs in the cytoplasm of the cell. It is an anaerobic process, meaning it does not require oxygen. During glycolysis, one molecule of glucose, a six-carbon sugar, is broken down into two molecules of pyruvate, a three-carbon compound. This process generates a net gain of two ATP molecules and two molecules of nicotinamide adenine dinucleotide (NADH), an electron carrier.

**2. Citric Acid Cycle (Krebs Cycle or TCA Cycle)**: Before entering the citric acid cycle, pyruvate is converted into a molecule called acetyl-CoA. This conversion occurs in the mitochondria and also produces one molecule of NADH and one molecule of carbon dioxide (CO2). The citric acid cycle is a series of chemical reactions that take place in the mitochondrial matrix. Each turn of the cycle uses acetyl-CoA and produces two molecules of CO2, one ATP, three NADH, and one molecule of flavin adenine dinucleotide (FADH2), another electron carrier. Since two molecules of pyruvate are produced from one glucose molecule, the cycle turns twice, yielding a total of two ATP, six NADH, and two FADH2.

3. Oxidative Phosphorylation: This is the final stage of cellular respiration and is the most efficient in terms of ATP production. It occurs in the inner mitochondrial membrane and consists of the electron transport chain (ETC) and chemiosmosis. The NADH and FADH2 produced in the previous stages donate their electrons to the ETC, which powers a series of protein complexes that pump protons (H+) across the inner mitochondrial membrane, creating an electrochemical gradient. Oxygen acts as the final electron acceptor and combines with electrons and protons to form water. The energy from this gradient is then used by ATP synthase to generate ATP from adenosine diphosphate (ADP) and inorganic phosphate (Pi). This process can produce up to 28 to 34 ATP molecules per glucose molecule, depending on the efficiency of the ETC and the cell's energy needs.

It's important to note that not all cells can perform oxidative phosphorylation. Some cells, such as those in muscle during intense exercise, may rely on glycolysis for a rapid but less efficient production of ATP, a process known as anaerobic respiration or fermentation.

In addition to the energy production through cellular respiration, cells also have other mechanisms to generate energy, such as glycogenesis, where glucose is stored as glycogen for later use, and lipolysis, where fats are broken down into fatty acids and glycerol to be used for energy.

The process of cellular respiration is a highly regulated one, with numerous enzymes and regulatory proteins ensuring that the reactions proceed efficiently and in response to the cell's energy needs. Disruptions in these processes can lead to various diseases and disorders, highlighting the importance of understanding and maintaining the balance of cellular energy production.

Cells need a source of energy, they get this energy by breaking down food molecules to release, the stored chemical energy.This process is called 'cellular respiration'. The process is happens in all the cells in our body. Oxygen is used to oxidize food, main oxidized food is sugar(glucose).