Introduction
Welcome to topic 3.3 of AP Biology! In this topic, we’ll cover cellular energy and how all organisms need it in order to operate properly.
The cellular processes that take place in the next two topics are to provide the cell with cellular energy and use it in order for cells to function properly.
We’ll also touch upon how core metabolic processes are shared, conserved, and fundamental processes and features support the concept of a common ancestry between all organisms.
Let’s put our energy into learning this! (sorry bad joke we know)
What is Cellular Energetics?
Cellular energetics is the study of how organisms capture, store, and use energy to power life processes. Energy is the capacity to do work. There are multiple types of energy, however we’ll mainly focus on potential energy and chemical energy.
The First Law of Thermodynamics states that energy cannot be created or destroyed. However, energy can be converted from one form to another. For example, potential energy turns into kinetic energy when something moves.
According to the Second Law of Thermodynamics, energy transfer/transformations are never 100% efficient. Some energy is always lost in less usable forms, such as heat. This also leads to an increase in the disorder (entropy) of the universe. This law is why living organisms require a constant input of energy to maintain their stability as not all energy used will be put towards the needed process.
In living systems, chemical energy is converted into other forms through exergonic and endergonic reactions. Endergonic reactions require energy to take place, whereas exergonic reactions release energy. Endergonic and exergonic reactions may be “coupled,” meaning the heat needed for an endergonic reaction is supplied by an exergonic reaction right before it.
Exergonic (spontaneous) reactions provide energy to allow endergonic (nonspontaneous) processes to occur. This linkage of reactions is crucial for cellular metabolism. Metabolism is the sum of all chemical reactions in an organism, including both catabolism (the breakdown of substances) and anabolism (the creation of new substances).
Although exergonic reactions release energy, the reaction might not occur naturally without a bit of energy to get it going. This is because the reactants must first turn into a high-energy molecule, called the transition state,before creating the products. The transition state is sort of a reactants-products hybrid state that is difficult to achieve. In order to reach this state, a certain amount of energy (referred to as the activation energy) is required. The enzymes we talked about in the topic 3.1 (here’s a link) are catalysts that lower activation energy.
But How is Energy Important?
Energy is essential to life and all living systems require an input of energy to operate. Life requires different systems of producing and using energy in order to maintain a balance.
Energy input must exceed energy loss in order to maintain a balance of energy within the cells to adequately power cellular processes. An excess in energy loss or energy production may result in cellular death. In order to maintain this equilibrium, cellular processes that release energy (exergonic reactions) may be paired with cellular processes that require energy (endergonic reactions). Energy-related pathways in biological systems may be sequential, meaning that the product of a reaction is typically the reactant or a subsequent step in the energy pathway.
ATP: The Energy Currency
ATP stands for adenosine triphosphate, the universal energy carrier molecule in cells. It consists of an adenine, a ribose sugar, and three phosphate molecules (hence the tri- in triphosphate). Most of the energy from ATP comes from its high-energy phosphate bonds.
Using hydrolysis, ATP is broken into ADP and Pi (inorganic phosphate), which is eventually recharged into ATP so the process can begin again.
One ATP hydrolysis reaction releases about 7.3 kcal/mol of free energy. ATP hydrolysis is paired with endergonic reactions as ATP provides the energy necessary to enable cells to perform non-spontaneous reactions. Some examples of this would include active transport and muscle contraction.
Common Ancestry Among Organisms
Core metabolic pathways are used across all currently recognized domains, indicating a common ancestry among all organisms. Domains are used to classify all living organisms based on fundamental similarities in their cellular structures and genetic makeup.
One of these core metabolic pathways is glycolysis: the breaking down of glucose. Glucose must be broken down in all organisms eventually to create energy.
Our next two topics will focus on photosynthesis and cellular respiration, respectively. These have the same core metabolic pathways in order to secure a balance of energy production and usage in their respective organisms.