Cellular Energy: Definition, Overview, Cellular Respiration, Laws of Thermodynamics And An Excellent and Crucial Role of Enzymes.

Cellular Energy Definition

This component of the AP Biology curriculum is intended to provide an overview of what cellular energy is, the principles that govern it, and how cells must function in order to follow these rules and live, develop, and reproduce. We’ll take a general look at how cells collect and distribute energy in this section.

The first and second laws of thermodynamics will also be applied to how cells use energy. This involves how cells get energy via photosynthesis in chloroplasts and use energy through aerobic respiration in mitochondria. Though subsequent portions of the AP Biology curriculum go further into the complexity of those processes, we’ll focus on how cells develop highly organised systems of enzymes and combine exothermic events with endothermic reactions to save energy as efficiently as possible.

Cellular Energy Overview

So lift something substantial. Then repeat the process again and again. Are your muscles on fire? That’s the sensation of your muscles attempting to create as much energy as possible as quickly as possible in order to preserve homeostasis and enable you to do another repeat.

Energy is essential in muscle cells for one simple reason: it alters the structure of proteins in the muscle that reach out and grasp other proteins. The muscles lengthen as they retract, enabling you to lift a hefty item.

While the actual movement of your muscle cells takes energy since you are exerting physical effort, other reasons for a cell’s energy use are less visible. Cells need energy to create, destroy, store, and discharge a vast range of chemicals. Cells, on the other hand, control energy in ways that are compatible with thermodynamic rules — all the way from the sun to the chemicals that cells employ. You should absolutely know how this works since it will undoubtedly come up on the AP exam! So stick with us as we go through the fundamentals of cellular energy!

The vast bulk of energy flowing through the Earth began as energy tied to photons originating from the sun. These photons were stripped of their energy as they travelled through the chloroplasts of plants, and the energy was transferred via an electron transport chain and into the bonds of ATP through ATP synthase. The Calvin Cycle was then employed to convert this ATP into glucose. Photosynthesis is the intricate flow of energy that will be discussed in more depth in Section 3.5.

Cellular Respiration

The glucose molecule will then go on to fuel practically every other reaction in the universe. The breakdown of glucose by glycolysis begins the process of cellular respiration, which is explained further in section 3.6. The glycolysis products are subsequently transported to the mitochondria, where they are loaded with electrons through the citric acid cycle. These electron carriers then deposit their electrons into a different electron transport chain, which drives another ATP synthase enzyme. The ATP produced by this enzyme is dispersed throughout the cell. Cellular respiration is a complicated process.

A vast range of biological functions require the ATP molecules produced by cellular respiration. They are used to concentrate molecules into different compartments and establish ion gradients in primary active transport. These ATP molecules are also responsible for macromolecule synthesis, cell signalling, and cell motility. Thousands of functions in plants, animals, and fungi are fueled by the combination of photosynthesis and cellular respiration. However, if you’ve been paying attention, you’re undoubtedly asking why plants need cellular respiration in the first place. To put it another way, why do plants need cellular respiration when sunlight generates ATP?

The solution is straightforward. There is a limit to how much ATP a cell can store before it can no longer be generated. Because the enzymes that form ATP may also complete the reverse process, high ATP concentrations increase the likelihood of ATP being converted back to ADP. Furthermore, even in the dark, cells need energy. Plant cells would rapidly run out of energy if they didn’t have a storing molecule like glucose. Plants create lots of glucose and store energy in different ways that fuel the whole food chain, which is fortunate for humans!

Though AP Biology does not concentrate on the physics or chemistry of life on Earth, you will be assessed on a few ideas relevant to these disciplines. If you understand the principles of thermodynamics, for example, it may seem like living beings are breaking them. However, these rules would be useless if all of biology ignored them.

Cellular Energy

First Law of Thermodynamics

Take, for example, the first rule of thermodynamics. It argues that an isolated system has a limited quantity of energy that can only be changed into new forms, not generated or destroyed.

A living entity may seem to violate this rule as it develops, integrates energy into the bonds of freshly created molecules, and reproduces. While it is true that organisms continuously absorb energy and store it in new molecules, this does not mean that they are isolated systems.

In truth, creatures are linked to the whole universe’s energy. Though none of the energy in the cosmos is wasted, it is continually combining and dissociating into other forms.

Cellular Energy

Second Law of Thermodynamics

Take a look at the second law of thermodynamics now. According to this rule, systems progress from more ordered to less ordered, resulting in the greatest level of entropy. Entropy is a measure of a system’s disorder, or lack of order. Organisms are reducing entropy in their local surroundings by continually merging smaller molecules into bigger ones.

However, this is one of the reasons why organisms need a continual supply of energy from the sun: they are always fighting the breakdown of bigger molecules into smaller molecules while attempting to become as organised as possible. As it drives activities and is lost as heat, energy moves from the sun, via photosynthesis and cell respiration, and back into the cosmos. Without this steady supply of energy, life on Earth would swiftly devolve into entropy and perish.

Cells have a few tricks up their sleeves to keep accumulating energy, creating ordered systems, growing, and reproducing despite the laws of thermodynamics being against them.


To begin with, enzymes are one of the most crucial instruments for establishing timely, organised reactions. Enzymes are used by cells to carry out hundreds of reactions at various times and in various regions of the cell. Hundreds of enzymes function in catabolism to break down molecules, while anabolism uses a totally separate set (though there is some overlap).

Furthermore, enzymes that are necessary for comparable operations are often physically adjacent to one another, and they may easily access the products of a previous enzyme. This maintains activities well ordered, ensuring that the cell receives the correct result at the conclusion of a lengthy process involving hundreds of enzymes.

Enzymes are also significant because they reduce the amount of activation energy required for a reaction to begin, but they have no impact on the amount of energy generated or absorbed by individual processes. Another method used by cells is linking exothermic and endothermic processes to use the energy generated by the former to fuel the latter. Cells may be as efficient as possible while battling the rules of thermodynamics by arranging enzymes and particular processes.

Let’s return to the original scenario we used to begin this session to better understand how cells maintain order and energy flow.

When you contract a muscle, bands of proteins in each muscle fibre shorten, shortening the whole muscle. The protein strands need ATP energy to travel in order to do this. When ATP is converted to ADP, an ATP molecule attaches to the protein myosin, which catalyses the release of energy.

The myosin head lurches forward as a result of the energy. A “power stroke” is completed when the myosin head binds to the thin actin filament and the myosin protein returns to its usual location. This shortens the muscle by pulling the actin and myosin strands together. This process will continue as long as you flex your muscles, which plainly consumes a lot of ATP.

Despite the fact that your cells store a small quantity of ATP, continuous flexing causes your muscles to run out of energy. As a result, the mitochondria in muscle cells work swiftly to replenish the ATP that has been depleted. The Krebs cycle rapidly pumps out electron carriers and modest quantities of ATP using a highly organised succession of enzymes.

These electron carriers swiftly reach the inner mitochondrial membrane and the electron transport chain, where the energy they convey is used to form a hydrogen ion gradient. The ATP synthase enzyme, which catalyses the production of ATP molecules that may be delivered to cells, is powered by this gradient. In essence, this well-ordered mechanism collects energy from several exothermic processes to produce ATP, which is a highly endothermic activity.

While this system is very complicated, there is one catastrophic error that might cause it to fail completely. There is a shortage of oxygen. The electron transport chain’s last electron acceptor is oxygen. Without oxygen, the whole sequence of processes comes to a halt, starting with the electron transport chain and working its way back to the Krebs cycle’s many reactions.

Cells, thankfully, have a backup generator. When there is no oxygen present, cells switch to a new set of enzymes to finish the lactic acid fermentation process from the leftover glucose molecules. This mechanism creates far less ATP, but it permits the cell to preserve order and function until the body can provide oxygen again.

Phew! That was a lot of information! However, it should demonstrate how energy flows throughout a normal cell and how cells resist the rules of thermodynamics by maintaining a highly organised system of enzymes that match energy-releasing activities with reactions that need energy input! Don’t worry about the technicalities or specifics; we’ll cover them in the AP Biology curriculum later.

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