The first module of HSC biology is ‘Maintaining A Balance’, and it is based almost entirely on the concept of homeostasis. In this article, I will discuss what homeostasis is, how it works, and why it is so important.
What is homeostasis?
The most obvious way to begin is with a definition:
“Homeostasis is the maintenance of a relatively stable internal environment, despite a changing external environment.”
This is the syllabus prescribed definition and therefore is what you should write every time a question asks about homeostasis. Starting with a strong definition very important when working towards a great answer.
Let’s look at what we can learn from that definition. Firstly, homeostasis is a broad concept; internal environment can refer to temperature, acidity (pH), water concentration or concentration of a range of other ions or molecules. All of these features must be kept within a narrow range for the body to work effectively, even if the external environment changes. For example, your core body temperature must be maintained at very close to 37oC (usually between 36oC and 38oC). On a cold day, your body must work to keep this from falling (using methods such as shivering). On hot days, your body must keep this down (using methods such as sweating).
Why do we need homeostasis?
HSC questions often ask why a particular aspect of homeostasis is important, or what the implications are if it fails. So before we look at how it is achieved, it is important to understand why homeostasis is necessary.
It’s all about enzymes
Enzymes are molecules in our body used to catalyse (speed up) almost all of the reactions needed to keep us alive. These reactions are collectively referred to as ‘metabolism’, and to give an idea of how many there are, this diagram is a simplified depiction of some of the reaction pathways in the body.
In understanding how enzymes work (and why sometimes they don’t), we must consider their structure and chemical composition. Enzymes are proteins, which are made up of long chains of amino acids in a specific sequence. This chain folds up into a particular shape depending on that sequence and it is the shape of the protein that allows it to perform its particular function.
Unfortunately, proteins are very sensitive molecules. If their conditions (such as temperature, pH etc.) are changed, they change their shape and fold up differently. Of course, this alters the function of the protein and almost always stops it from doing its job effectively. If the conditions are changed only slightly, the protein can often refold and go back to performing its function afterwards. However, if the conditions are changed significantly, the protein may be irreparably damaged. In this case, we say that the protein has been ‘denatured’. This case is particularly bad as completely new enzymes must be built before the cell can use them again.
Remembering that these enzymes are necessary to catalyse the massive number of reactions involved in our metabolism, if these enzymes cease to function effectively, metabolism will slow down, which quickly leads to damage to our cells and even death.
How do I fit that in an answer for the HSC?
In questions for the HSC (or other exams), we can condense all of that into the following paragraph:
“Enzymes are necessary for metabolism to proceed at a sufficient rate. However, they only work effectively in a narrow range of conditions. Outside this range, their function is reduced or entirely prevented, sometimes irreparably (denaturing). If this occurs, metabolism is slowed down and damage to cells can occur. This can lead to death if not resolved quickly.”
You can obviously adjust this answer slightly if the question is specifically asking about pH or temperature etc.
How is homeostasis achieved?
The most important mechanism in achieving homeostasis is ‘negative feedback’. Many students don’t quite understand what this means so I’ll give a definition:
“Negative feedback refers to processes where the response to a particular effect causes a reduction in that effect.”
That means that when a particular change occurs, the body acts to counteract that change. It is quickly clear why this is important to homeostasis; if the body experiences a change away from its stable state, the response must be to move back towards the stable state, that is, to resist or counteract the original change.
As a result, negative feedback mechanisms are incredibly common in the body in order to maintain stable conditions. For the most part, these mechanisms can be represented by a feedback loop. It is necessary in the HSC to understand what a feedback loop is and to give examples. To start with, we’ll look at a generic feedback loop:
Perhaps the most important thing you can take away from this is that feedback loops always start with a stable state. Far too many students start with the change from stable state, but that leaves nothing to loop back to after counteraction. The rest of the loop is relatively self-explanatory: A change occurs and is detected by receptors in the body. The message is then sent to a control centre (usually a specific part of the brain), which decides on an appropriate response and sends a message to the effector which will enact this response and counteract the initial change.
Now let’s look at drawing specific example of a feedback loop which is particularly important for HSC biology: temperature homeostasis.
In this case, we can show the effect of both increases and decreases in temperature. In both cases, the change is detected by thermoreceptors, (cells which detect temperature) and the message is sent to the hypothalamus (a particular part of the brain). However, the effector organ and response is obviously different in order to counteract the change.
Unless the question asks for a specific type of negative feedback, I suggest drawing this example in questions about feedback loops. It gives slightly more detail than a loop with only one direction of change which hopefully differentiates you as a better student.
However, using the generic feedback loop provided above, you can apply it to any specific example, provided you know how the change is detected and how it is counteracted.
If made it all the way through this article, thanks for bearing with me and I hope you’ve found it very useful in your understanding and your ability to answer questions on this topic. If you have any questions about this or a related topic, feel free to comment below. Similarly, if you have a different topic you’d like me to cover in an article, leave a comment here or on our facebook page.
Finally, if you’d like more specific help, maybe you’d like to try our tutoring at Top Marks Education. If you’re in year 11 or 12, you can claim a free one-on-one lesson to find out if we’re right for you!
Head Biology Teacher