Form follows Function

One of the overarching themes of biology is that form follows function, which means that the shape of an object directly relates to to its intended purpose or job. We see this at all levels in the hierarchy of biological organization from atoms up to the biosphere. This principle expands far beyond biology, the phrase was actually initially coined by an architect, Louis Sullivan. In 1896, Sullivan wrote: “Whether it be the sweeping eagle in his flight, or the open apple blossom, the toiling work-horse, the blithe swan, the branching oak, the winding stream at its base, the drifting clouds, over all the coursing sun, form ever follows function, and this is the law.”

In architecture, this occurs by humans intentionally designing a form to meet the desired function. In biology, this is the result of evolution by natural selection. Organisms with phenotypes that are best suited for the function are better equipped to survive, reproduce, and pass on these forms to the next generation. Form follows function can be observed at every biological level.

At the molecular level, the shape (structure) of a protein determines its function and the function of a protein often directs how it is shaped three-dimensionally (i.e., tertiary structure). For example, there are two basic shapes for proteins: fibrous (stringy) and globular (round). Collagen is an example of a fibrous protein (Figure 28.1A) that gives strength to our skin to prevent it from tearing and for this purpose, it is shaped like a rope. This is an example of form follows function at the molecular level of proteins. The function of globular proteins, such as hemoglobin (Figure 28.1B), is to transport oxygen in the blood so that all cells of the body can receive oxygen to perform metabolic processes. A sphere is an efficient shape for packing tightly together within a red blood cell without getting tangled.

At the cellular level, the structure of skeletal muscle cells allows them to have the function of contraction, which allows us to move. Skeletal muscle cells that make up your biceps are attached to both ends of the humerus bone by tendons and are packed full of contractile proteins (actin and myosin). When the contractile proteins contract, they shorten the muscle cell, which then pulls on the ends of the humerus and allows you to flex your forearm. But how a muscle is used also affects its structure and function – an overused muscle will often hypertrophy or in other words, have a larger cross-sectional area. A cell can change its shape and structure, and even its volume, if demands drastically change (e.g., more force production is needed).

At the level of an organism’s body, form follows function relates intimately to the body’s anatomy and physiology. Anatomy is the study of the structure of the body (e.g., where the quadriceps muscle is located) and physiology is the study of how the body functions (e.g., how the quadriceps muscle contracts). Consider the heart as an example. From an anatomical perspective, the heart consists of four hollow chambers (atria and ventricles) and is made of cardiac muscle cells. From a physiological perspective, this structure allows the heart to have the function of pumping blood around the body. However, if the structure of the heart changes (e.g., some of the heart chambers become stretched out or dilated), then the heart’s function decreases as the heart can no longer pump as much blood, which will eventually cause congestive heart failure.

At the ecosystem level, the structure of the environment affects the function of ecosystem processes. For example, in coral reefs, the corals, which are the foundation species, provide protection and habitat for other species (Figure 28.2). The physical structure of the reef built by the coral protects other species, such as fish, from ocean waves and currents and gives them a place to hide from predators. If the coral reef’s structural integrity is damaged, fish at lower trophic levels will die since they are more vulnerable to the ocean waves, currents, and predators. This will deprive fish at higher trophic levels of prey, and the entire ecosystem could collapse. This is an example of form follows function at the broader ecosystem level.

Homeostasis

In order to function properly, cells require appropriate conditions such as proper temperature, pH, and appropriate concentration of diverse chemicals. These conditions may, however, change from one moment to the next. Organisms are able to maintain consistent internal conditions within a narrow range almost constantly, despite environmental changes, by activation of regulatory mechanisms. Animal organs and organ systems constantly adjust to internal and external changes through a process called homeostasis (meaning “steady state”). These changes might be in the level of glucose or calcium in blood or in external temperatures. Homeostasis means to maintain dynamic equilibrium in the body. It is dynamic because it is constantly adjusting to the changes that the body’s systems encounter. It is equilibrium because body functions are kept within specific ranges. Even an animal that is apparently inactive is maintaining this homeostatic equilibrium.

Homeostatic regulation

The goal of homeostasis is the maintenance of equilibrium around a point or value called a set point. A set point is the physiological value around which the normal range fluctuates. A normal range is the restricted set of values that is optimally healthful and stable. For example, the set point for normal human body temperature is approximately 37°C (98.6°F). Physiological parameters, such as body temperature and blood pressure, tend to fluctuate within a normal range a few degrees above and below that point. Maintaining homeostasis requires that the body continuously monitor its internal conditions. A change in the internal or external environment is called a stimulus and is detected by a sensor.  the response of the system is to adjust the deviation parameter toward the set point. For instance, if the body becomes too warm, adjustments are made to cool the animal. If the blood’s glucose rises after a meal, adjustments are made to lower the blood glucose level by getting the nutrient into tissues that need it or to store it for later use.

When a change occurs in an animal’s environment, an adjustment must be made. The sensor senses the change in the environment, then sends a signal to the control center (in most cases, the brain) which in turn generates a response that is signaled to an effector. The effector is often a muscle (that contracts or relaxes) or a gland that secretes hormones.

Feedback Mechanisms

Homeostasis is maintained by negative feedback loops. Any homeostatic process that changes the direction of the stimulus is a negative feedback loop. It may either increase or decrease the stimulus, but the stimulus is not allowed to continue as it did before the receptor sensed it. In other words, if a level is too high, the body does something to bring it down, and conversely, if a level is too low, the body does something to make it go up. An example is animal maintenance of blood glucose levels. When an animal has eaten, blood glucose levels rise. This is sensed by the nervous system. Specialized cells in the pancreas sense this, and the hormone insulin is released by the endocrine system. Insulin causes blood glucose levels to decrease, as would be expected in a negative feedback system, as illustrated in Figure 28.3. However, if an animal has not eaten and blood glucose levels decrease, this is sensed in another group of cells in the pancreas, and the hormone glucagon is released causing glucose levels to increase. This is still a negative feedback loop, but not in the direction expected by the use of the term “negative.” Another example of an increase as a result of the feedback loop is the control of blood calcium. If calcium levels decrease, specialized cells in the parathyroid gland sense this and release parathyroid hormone (PTH), causing an increased absorption of calcium through the intestines and kidneys and, possibly, the breakdown of bone in order to liberate calcium. The effects of PTH are to raise blood levels of the element. Negative feedback loops are the predominant mechanism used in homeostasis.

A positive feedback loop maintains the direction of the stimulus, possibly accelerating it. Few examples of positive feedback loops exist in animal bodies, but one is found in the cascade of chemical reactions that result in blood clotting, or coagulation. As one clotting factor is activated, it activates the next factor in sequence until a fibrin clot is achieved. The direction is maintained, not changed, so this is positive feedback. Another example of positive feedback is uterine contractions during childbirth, as illustrated in Figure 28.4. The hormone oxytocin stimulates the contraction of the uterus. This produces pain sensed by the nervous system. Instead of lowering the oxytocin and causing the pain to subside, more oxytocin is produced until the contractions are powerful enough to produce childbirth.

Adjusting the Set Point

It is possible to adjust a system’s set point. When this happens, the feedback loop works to maintain the new setting. An example of this is blood pressure: over time, the normal or set point for blood pressure can increase as a result of continued increases in blood pressure. The body no longer recognizes the elevation as abnormal and no attempt is made to return to the lower set point. The result is the maintenance of an elevated blood pressure that can have harmful effects on the body. Medication can lower blood pressure and lower the set point in the system to a more healthy level. This is called a process of alteration of the set point in a feedback loop.

Changes can be made in a group of body organ systems in order to maintain a set point in another system. This is called acclimatization. This occurs, for instance, when an animal migrates to a higher altitude than that to which it is accustomed. In order to adjust to the lower oxygen levels at the new altitude, the body increases the number of red blood cells circulating in the blood to ensure adequate oxygen delivery to the tissues. Another example of acclimatization is animals that have seasonal changes in their coats: a heavier coat in the winter ensures adequate heat retention, and a light coat in summer assists in keeping body temperature from rising to harmful levels.

Adapted from

Clark, M.A., Douglas, M., and Choi, J. (2018). Biology 2e. OpenStax. Retrieved from https://openstax.org/books/biology-2e/

Barrickman, N., Bell, K., and Cowan, C. (n.d.) Human Biology. Pressbooks. Retrieved from https://slcc.pressbooks.pub/humanbiology/chapter/chapter-12-organ-systems-of-the-human-body/

Sullivan, L. H. (1986). The tall office building artistically considered. Lippincott’s Magazine, 57, 403-407