Dynamic Equilibrium: Definition & Examples

Dynamic Equilibrium Definition

A system in dynamic equilibrium will have small changes that sum together to produce no net change. Many biological systems are in dynamic equilibrium, from the water inside a cell, to the dynamic equilibrium experienced by populations of predators and prey. Dynamic equilibrium is different from a static equilibrium, in which the parts do not move once they’ve reached equilibrium.

Dynamic equilibrium has different meanings in each science sub-discipline, such as biochemistry or ecology. In chemistry, the equilibrium of a reaction is the point at which the products and reactants have the lowest free energy. The dynamic equilibrium, on the other hand, is the point at which products are being generated as fast as they are falling apart. This may not be the same as the chemical equilibrium, as enzymes force many reactions far past their natural equilibriums by making products faster than they break apart. Due to this, chemist often refer to dynamic equilibrium as a dynamic steady-state, to clearly differentiate between the two points in a reaction.

Ecologists and population biologists will often refer to dynamic equilibrium when talking about populations of organisms. When studying the number of organisms in a population over time, many factors affect the growth of a population. Often, populations will go through periods of boom and bust. Ample resources cause high reproduction rates in all animals, leading to a much higher population. When the resources are distributed among this higher number, there are not nearly enough resources to go around. Thus, the population dies off. Ecologist see these cycles as a dynamic equilibrium that the population is stuck in, never really gaining or losing large numbers of individuals.

Examples of Dynamic Equilibrium

Glucose in an Organism

Throughout your entire lifetime, the glucose levels in your body remain relatively the same. Over the course of a day however, your body uses enormous amounts of glucose and must replace it. Each cell in your body requires glucose to function. As the cells use this glucose, the liver and your digestive system work quickly to replace it. Glucose from the food you eat is moved from the stomach and intestines into the bloodstream. The liver stores glucose as glycogen, and must break this large molecule down to release glucose into the blood. In your body, glucose is in dynamic equilibrium. While glucose has periods of high and low concentration, it is relatively stable. If glucose levels in your body fall out of dynamic equilibrium, or you cannot replace the glucose you use, you would eventually die.

Predator-Prey Dynamics

Ecologists often study the relationships between multiple species and their effects on each other. One relationship in nature that often shows dynamic equilibrium is the predatory-prey dynamic. Imagine a nature reserve that only contains rabbits and wolves. As the rabbit population increases, it provides more food for the wolf population. This sets both populations into dynamic equilibrium. The wolves, reaping the benefits of the increased rabbit population, also start to reproduce more. After a period of time, the wolf population also starts to increase dramatically. As more wolves are born and eat the rabbits, their populations eventually level off. The wolves, still reproducing at high levels, eventually start decreasing the rabbit population, which cannot keep up. The rabbits decrease, and eventually the wolves are left without enough food to support a large population. This dynamic equilibrium of both populations is interesting because it shows a direct cause and effect relationship between different species in an ecosystem.

Static Equilibrium – When a system reaches a point of stability in which no parts are still moving.
Equilibrium – A point in a reaction in which the lowest free energy exists on both sides of a chemical equation.
Free Energy – The energy in a system capable of causing a reaction.

Quiz

1. Many cell membranes have special proteins known as aquaporins that allow water molecules to pass through the membrane. If the cell is placed in an isotonic environment, which of the following will be obtained?
A. Static Equilibrium
B. Dynamic Equilibrium
C. Equilibrium will not be achieved

Answer to Question #1

B is correct. Although the total system will not exhibit a noticeable change, water molecules are constantly exchanged through the aquaporins. The rate of intake and output are the same in an isotonic environment, which has the same solute concentration as the inside of the cell. The cell relies on this dynamic equilibrium to continue to cycle cell nutrients and oxygen inside and outside of the cell.

2. Scenario 1: A box of bananas is balanced on a scale. The weight of the bananas is equivalent to the counterweight, and the scale is perfectly level. Scenario 2: A monkey enters and eats a banana. The scale falls out of balance, and the counterweight touches the floor. The researcher adds a banana to the box, evening the scales. The monkey eats another banana, and the cycle starts over.
How would you describe these two scenarios?
A. Dynamic Equilibrium; Static Equilibrium
B. Static Equilibrium; Dynamic Equilibrium
C. Both are dynamic equilibrium

Answer to Question #2

B is correct. In the first scenario, the scale is at equilibrium and nothing is changing. This is static equilibrium. In the second scenario, the scale is never really at equilibrium but is reset every time by the researcher. In this way, the scenario stays in a dynamic equilibrium. Although a dynamic equilibrium may have changes in the short term, the average over time is constant.

3. Organisms always exist in dynamic equilibrium, never static. Why?
A. The energy that sustains life is held in the bonds of molecules, which must be used.
B. All cell have membranes, which allow water molecules to exist in dynamic equilibrium.
C. Some cells exist in static equilibrium.

Answer to Question #3

A is correct. Glucose is the main molecule that organisms use to store and transfer energy. To get the energy from glucose and use it in processes like DNA replication, the glucose must be broken down. Other parts of the organism must replace this glucose if the process is to continue. While all cells do have membranes, not all membranes allow water to pass freely. In static equilibrium, nothing could be broken down, and no energy could be released.

Dynamic Equilibrium