A system is a set of interacting parts or components in which neither the components nor the whole can be understood without understanding the interactions.

Biological phenomena that occur at all of the hierarchical levels of organization (see Figure 1.5), from the synthesis of large molecules within a living cell, to the consumption of one individual organism by another, can be analyzed as systems (see Figure 1.7). Systems analysis allows us to explore many interesting questions, such as "What determines the average amount of Protein T in a cell?" or "How will introduction of foxes to a new area affect the abundance of voles?"

Systems analysis starts by identifying how a system is organized—what its components are, and how their amounts are determined by interactions among and between components and the environment outside the system.

Let's consider a simple system with three components: A, B, and C (see Figures 1.6 and 1.8). The amount of each component depends on processes that add to or subtract from it (see the word equation on page 8). For example, the amount of the second component, B, depends on a process A → B that converts A to B and on a process B → C that converts B to C. The rate of the first conversion is likely to depend on the amount of A, and the rate of the second conversion is likely to depend on the amount of B. But these rates also can be influenced by amounts of later or "downstream" system components. In this example, the downstream component C may influence the process A → B, a phenomenon known as feedback (see Figure 1.8).

The amounts of each component may depend not only on interactions among the components, but also on inputs from or losses to the external environment. For example, the amount of the third component, C is increased by the process B → C that converts B to C, but is reduced if there is loss of C from the system, for example by export of protein breakdown products from the cell, or death of fox and owl predators. Similarly, the amount of the first component, A, is decreased by the process A → B and increased if there is input from outside—for example by import of amino acids by the cell, or absorption of sodium into the gut from ingested food, or growth of grass fueled by solar energy input (see Figure 1.7).

This Animated Tutorial illustrates three important properties of biological systems—that they are dynamic, depending on continual inputs of energy and materials to exist; that they are organized; and that the interactions among components influence their organization. The tutorial uses the three-component system illustrated in Figure 1.8, with inputs into A from the outside and losses from C to the outside.

The Systems Model

The basic, "No feedback" model starts with the amount of A = 100, of B = 0, and of C = 0. For each time unit, the model adds a constant amount to A and subtracts an amount from A that increases with A. Each time unit, the amount subtracted from A is added to B, and an amount that increases with B is subtracted from B. The amount subtracted from B is added to C, and an amount that increases with C is also subtracted from C.

If "Positive feedback" is checked, the amount subtracted from A is augmented by an amount that increases with C. If "Negative feedback" is checked, the amount subtracted from A is decreased by an amount that increases with C.

If "No input" is checked, the input to A from the external environment is set to zero.

The simulation shows the amounts of the three components, A, B, and C, for 100 time steps.


Biological Systems are Dynamic and Depend on Inputs to Exist

To appreciate the dynamic nature of biological systems, begin by checking "No feedback." Then click on "Start/Stop" to begin the simulation. Notice that the amounts of A, B, and C change through time—they are dynamic. How do they change? Do they reach a steady value at some point—in other words, do they reach an equilibrium? Write down the final amounts for A, B, and C.

The fact that the abundances of the three components reach an equilibrium does not mean that the system has "stopped". The equilibrium is maintained by a steady movement of energy and materials through the system, as the next activity illustrates.

Click "Reset," "No input," "No feedback," and "Start/Stop." By clicking "No input" you have changed the organization of the system: you have removed the input to A. Notice that once again, the amounts of A, B, and C change through time. Do they change in the same way they did with input to A? What are the final amounts of A, B, and C? Do they differ from the final amounts you obtained when there was input into A? What can you conclude about how the presence or absence of an interaction influences the structure of a system? What can you conclude about the relevance of inputs of energy and materials to the functioning of biological systems?


Biological Systems Exhibit a Particular Organization

The organization of any biological system can be seen in the particular components that comprise it and in the interactions among the components. Furthermore, these aspects of organization or structure determine the relative amounts of the components at any time. These relative amounts can be thought of as the state of the system at that time, another aspect of its organization.

To see how the exact interactions determine the state of the system, de-select "No input" to allow input to A once again. Now click on "Positive feedback." This changes the organization of the system so that there is a positive effect of C on the rate of the A → B conversion. Reset the system and click "Start/Stop." Observe how A, B, and C change. Do they change in the same way they did with input but no feedback? Write down the equilibrium amounts of A, B, and C. How do these amounts compare to those with no feedback? Can you visualize how increasing the rate of the A → B conversion with no other change in rates of input or loss affects the system dynamics and equilibrium amounts of the components?

Now compare positive feedback with negative feedback by clicking "Negative feedback," "Reset," and "Start/Stop." This again changes the organization of the system to include a negative effect of C on the rate of the A → B conversion. Compare the dynamics and equilibrium amounts of A, B, and C with what you observed with positive feedback and with no feedback. Can you visualize how changing the rate of the A → B conversion, with no other changes affects the system dynamics and equilibrium amounts of the components?

No feedback
Positive feedback
Negative feedback
No input

In this tutorial, you have seen how a systems model can be used to understand the properties of biological systems. Regardless of the details of components, interactions, and inputs, all models of biological systems exhibit the properties illustrated by this simple, three-component system: they are dynamic, depending on continual inputs of energy and materials to exist; they are organized and have structure; and the interactions among components and with the external environment influence their structure.

Throughout the textbook, you will encounter examples of biological systems and system models, with different inputs, feedbacks, and the like. You may wish to return to this system simulation from time to time to refresh your perspective on the fascinating, dynamic, and complex behaviors of biological systems.

Textbook Reference: Concept 1.2 Life Depends on Organization and Energy