How Do Cells Capture Energy Released By Cellular Respiration?

Living organisms form an energy chain in which plants produce food that animals and other organisms use for energy. The main process that produces food is **photosynthesis** in plants and the principal method of converting the food to energy is cellular respiration.

During photosynthesis, plants capture light energy and use it to power chemical reactions in the plant cells. The light energy lets plants combine carbon from carbon dioxide in the air with hydrogen and oxygen from water to form glucose.

In cellular respiration, organisms such as animals eat food containing glucose and break down the glucose into energy, carbon dioxide and water. The carbon dioxide and water are expelled from the organism and the energy is stored in a molecule called adenosine triphosphate or ATP. The energy transferring molecule used by cells is ATP, and it provides the energy for all other cell and organism activities.

Living organisms are either single-cell **prokaryotes or eukaryotes**, which can be single-celled or multicellular. The main difference between the two is that prokaryotes have a simple cell structure with no nucleus or cell organelles. Eukaryotes always have a nucleus and more complicated cell processes.

Single cell organisms of both types can use several methods to produce energy and many use cellular respiration as well. Advanced plants and animals are all eukaryotes and they use cellular respiration almost exclusively. Plants use photosynthesis to capture energy from the sun but then store most of that energy in the form of glucose.

Both plants and animals use the glucose produced from photosynthesis as an energy source.

Photosynthesis produces glucose, but the glucose is just a way of storing chemical energy and can't be used by cells directly. The overall photosynthesis process can be summarized in the following formula:

**6CO₂ + 12H₂O + light energy → C₆H₁₂O₆ + 6O₂ + 6H₂O**

The plants use photosynthesis to convert light energy into chemical energy and they store the chemical energy in glucose. A second process is needed to make use of the stored energy.

Cellular respiration converts the chemical energy stored in glucose into chemical energy stored in the ATP molecule. ATP is used by all cells to power their metabolism and their activities. Muscle cells are among the kinds of cells that use glucose for energy but convert it to ATP first.

The overall chemical reaction for cellular respiration is as follows:

**C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + ATP molecules**

The cells break glucose down into carbon dioxide and water while producing energy that they store in ATP molecules. They then use the ATP energy for activities such as muscle contracting. The complete cellular respiration process has three stages.

Glucose is a carbohydrate with six carbon atoms. During the first stage of the cellular respiration process called **glycolysis**, the cell breaks the glucose molecules into two molecules of pyruvate, or three-carbon molecules. To get the process started takes energy so two ATP molecules from the cell's reserves are used.

At the end of the process, when the two pyruvate molecules are created, energy is released and stored in four ATP molecules. Glycolysis uses two ATP molecules and produces four for each glucose molecule processed. The net gain is two ATP molecules.

Glycolysis starts in the cell cytoplasm but the cell respiration process mainly takes place in the **mitochondria**. The kinds of cells that use glucose for energy include almost every cell in the human body with the exception of highly specialized cells such as blood cells.

The mitochondria are small membrane-bound organelles and are the cell factories that produce ATP. They have a smooth outer membrane and a highly folded inner membrane where the cellular respiration reactions take place.

The reactions first take place inside the mitochondria to produce an energy gradient across the inner membrane. Subsequent reactions involving the membrane produce the energy used to create ATP molecules.

The pyruvate produced by glycolysis is not the final product of cellular respiration. A second stage processes the two pyruvate molecules into another intermediate substance called acetyl CoA. The acetyl CoA enters the citric acid cycle and the carbon atoms from the original glucose molecule are completely converted to CO₂. The citric acid root is recycled and links to a new acetyl CoA molecule to repeat the process.

The oxidation of the carbon atoms produces two more ATP molecules and converts the enzymes NAD⁺ and FAD to NADH and FADH₂. The converted enzymes are used in the third and last stage of cellular respiration where they act as electron donors for the electron transport chain.

The ATP molecules capture some of the energy produced but most of the chemical energy remains in the NADH molecules. The citric acid cycle reactions take place inside the mitochondria.

The electron transport chain (ETC) is made up of a series of compounds located on the inner membrane of the mitochondria. It uses electrons from the NADH and FADH₂ enzymes produced by the citric acid cycle to pump protons across the membrane.

In a chain of reactions, the high energy electrons from NADH and FADH₂ are passed down the series of ETC compounds with each step leading to a lower electron energy state and protons being pumped across the membrane.

At the end of the ETC reactions, oxygen molecules accept the electrons and form water molecules. The electron energy originally coming from the splitting and oxidation of the glucose molecule has been converted into a proton energy gradient across the inner membrane of the mitochondria.

Because there is an imbalance of protons across the inner membrane, the protons experience a force to diffuse back into the interior of the mitochondria. An enzyme called ATP synthase is embedded in the membrane and creates an opening, allowing the protons to move back across the membrane.

When the protons pass through the ATP synthase opening, the enzyme uses the energy from the protons to create ATP molecules. The bulk of the energy from cellular respiration is captured at this stage and is stored in 32 ATP molecules.

ATP is a complex organic chemical with an adenine base and three phosphate groups. Energy is stored in the bonds holding the phosphate groups. When a cell needs energy, it breaks one of the bond of the phosphate groups and uses the chemical energy to create new bonds in other cell substances. The ATP molecule becomes adenosine diphosphate or ADP.

In cellular respiration, the energy liberated is used to add a phosphate group to ADP. The addition of the phosphate group captures the energy from glycolysis, the citric acid cycle and the large amount of energy from the ETC. The resulting ATP molecules can be used by the organism for activities such as movement, looking for food and reproduction.