One of the major topics on the AP® Bio exam is homeostasis. Most of the topics that you study in AP® Biology will force you to think: how does this relate to homeostasis? It is essential that you understand homeostasis completely and are able to relate it to a variety of topics. In this AP® Biology Crash Course Review we will summarize what homeostasis is, the many different levels where it is important that homeostasis is maintained, and finally a free response question that you could see on your AP® Biology exam.

Homeostasis is the state in which a system is functioning at an optimal level. It is essential to individual and group survival that homeostasis is maintained and kept relatively constant.

When outside influences disrupt homeostasis, feedback loops return the variables back to normal levels. Try and imagine an organism as a building with a thermostat. If you set the thermostat to 70 degrees and someone opens a window to the warm outdoors, the building will warm up, causing the thermostat to turn on and cool the building down. If someone opens up the freezer at that point, cooling down the building, the thermostat will turn off the air conditioning. Maintaining homeostasis in organisms is not all that different from maintaining temperatures in a building.

For living systems, homeostasis can occur at different levels. In this AP® Biology Crash Course Review of homeostasis, we will examine case studies of homeostasis at each level.

Homeostasis at the Organismal Level

Thermoregulation

Image Source: Wikimedia Commons

The analogy that we just set up in the introduction of this AP® Biology Review article is especially useful when thinking about thermoregulation. Body temperature works similarly to a thermostat by keeping the temperature constant in warm blooded species.

If the body is exposed to high heat, it will begin to sweat. Sweating is a mechanism that organisms use to thermoregulate; when an organism sweats, liquid is released from the body and onto the skin. The liquid perspiration is then able to evaporate from the skin. The process of evaporation releases energy into the environment, cooling the skin down.

When the opposite happens, and an organism is too cold, the body will react by shivering. Shivering causes the energy levels to rise, raising the internal body temperature.

It is important for an organism to remain at homeostasis. Straying too far from this ideal temperature can result in sickness or death.

Glucose Regulation

Blood sugar is another homeostatic parameter. The outside influence in this case is food; digestion increases glucose levels.

The hormone, insulin, lowers blood sugar, while glucagon prevents it from dropping too much. After eating, the pancreas reacts accordingly and produces the appropriate hormone to regulate blood sugar levels.

Diabetes is a disease that occurs when there is no natural production of insulin. Patients must inject themselves with the hormone in order to stay within the healthy blood sugar range.

Osmotic Regulation

Osmotic regulation relates to how the body keeps water and salts in a homeostasis. When blood has a high sodium concentration, it stimulates the hormone system to release vasopressin, a hormone that tells your kidneys to retain water. This, in turn, concentrates the urine and does not allow the organism to excrete as much water. When water is retained, it will decrease the concentration of sodium by solubilizing it.

Carbon Dioxide Regulation

Image Source: Wikimedia Commons

Muscles excrete carbon dioxide during exercise, thus increasing its levels in your blood. The carotid arteries’ receptors sense the changed environment and signal the brain to stimulate the lungs to increase respiration.

These are the processes that cause you to breathe more quickly and heavily as you exercise. Because you start increasing the rate of oxygen in your body, the concentration of carbon dioxide decreases.

Homeostasis at the Cellular Level

Cell reproduction is a popular topic on AP® Biology exam. The reason that cells reproduce so efficiently and often is due to their need to restore and keep homeostasis.

Cells in different parts of your body vary in lifespans. Regardless, dead cells need to be replaced for the organ and individual to function properly. Neighboring cells reproduce themselves in order to fill the void left by dead cells. Cells usually stop replicating when the intercellular space runs out.

Individually, cells contain their own homeostatic variables. Within the membrane, there are various pumping mechanisms that supply nutrients to the cell. When enough material is present, the pumping ceases; proton pumping mechanisms of membranes during respiration and photosynthesis are good examples of homeostasis at the cellular level.

Homeostasis at the Molecular Level

At a molecular level, organic molecule production must be limited for efficiency and prevention of harm. This homeostasis can be achieved by an enzyme used to stop the chemical processes. In glycolysis, ATP formed from glucose reacts with the enzyme that helped make it in order to prevent its own overproduction.

In the same fashion, essential amino acids regulate their own production. Isoleucine production is also inhibited by an enzyme that helped create it, theroninedeaminase.

Homeostasis at the Population Level

Homeostasis of an ecosystem depends greatly on a balanced population of different species.

The most obvious way populations are regulated is seen in the food chain. Predator and prey populations are affected by the supply of each other. There is an ideal range for both so that neither goes extinct.

Conversely, animals with symbiotic relationships sustain each other’s populations in a positive feedback loop. This means that increases in each factor perpetuates each other’s increase.

These animals develop adaptations that support each other’s survival. Co-evolution allows these populations counter death factors, like predators, and maintain homeostatic levels.

Free Response Questions

Now that we have a better understanding of homeostasis as part of this AP® Biology Review, let’s review a question that was on the 2004 AP® Biology exam.

• Blood-glucose levels
• Body temperature
• pH of the blood
• Osmotic concentration of the blood

Homeostasis, maintaining a steady-state internal environment, is characteristic of all living organisms. Choose three of the following physiological parameters and, for each, describe how homeostasis is maintained.

We have reviewed blood-glucose levels, body temperature, and osmotic homeostasis in the sections above. This question wants you to be specific and choose one organism; in this case, you could use humans. Here is an example of a free response for this question:

Homeostasis is maintained through a variety of mechanisms in all organisms. Humans maintain homeostasis of body temperature through two means: sweating and shivering. When the human body is too warm, the organism will sweat. The evaporation of the perspiration will cause the individual to lose energy on the skin, cooling it down. Humans also shiver when their bodies are too cold. Shivering causes energy to be expelled in the body, raising the temperature.

Humans also regulate blood-glucose levels through the pancreas organ. When humans eat food and blood glucose levels increase, insulin prevents it from increasing too drastically. When a human being is starving, a lack of blood glucose will cause glucagon to start to break down fat storage. Lastly, humans also have homeostasis in osmotic regulation. If human blood has too much salt in it, the kidneys will cause water retention to occur. The water retention will cause water to be added to the blood and the salt, lowering the salt concentration in the blood.

This is one answer that would be accepted by the AP® Biology exam on the topic of homeostasis! In this AP® Biology Crash Course Review we have defined homeostasis, reviewed examples of homeostasis, and finished up with a free response question. How else have you been studying homeostasis as part of your AP® Biology Review? What other examples have you found? Let us know in the comments section!

If you’re looking for more information about regulation, be sure to check out our article Negative Feedback: AP® Biology Crash Course Review

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