Year 2
Homeostasis
Name:
Teacher:
1
, Principles of homeostasis and negative feedback
What is homeostasis?
Homeostasis is the maintenance of the internal environment in a constant state within an organism,
despite external changes. Organisms can only function if internal conditions such as temperature, pH,
water potential and blood glucose concentration are kept within a narrow range.
Importance of homeostasis
It is particularly important to maintain the right core body temperature and blood pH.
Q: Why?
It is also important to maintain the right blood glucose concentration because cells need glucose for
respiration to produce ATP, and blood glucose concentration also affects water potential of the blood.
A. Temperature (also see Year 1 Biological Molecules booklet)
A recap on how temperature affects rate of an enzyme controlled reaction:
2
3
1
1–
2–
3–
If body temperature is too low, enzyme activity is reduced, slowing the rate of metabolic reactions. If
it is too high, enzymes will denature and metabolic reactions will stop. The highest rate of enzyme
activity happens at their optimum temperature – about 37 C in humans.
2
,B. pH (also see Year 1 Biological Molecules booklet)
pH is calculated based on the concentration of hydrogen ions (H+) in the environment. The greater
the concentration of H+, the lower the pH (more acidic). pH is expressed on a logarithmic scale.
Each value on a logarithmic scale using log10 is ten times larger than the value before, eg. A solution
of pH 3 contains ten times more H+ than solution at pH4. Converting values to a logarithmic scale
makes it easier to plot both very small and very large values on the same axis of a graph.
You can work out pH of a solution using the following equation:
pH = - log10 [H+]
NB. [H+] is the concentration of H+ in a solution in moldm-3.
Eg 1. A solution has a H+ concentration of 0.01 moldm-3. Calculate its pH.
Eg 2. A blood sample has a H+ concentration of 3.9 x 10-8 moldm-3. What is the pH of the blood?
A recap on how pH affects rate of an enzyme controlled reaction:
If blood pH is too acidic or too alkaline, enzymes become denatured.
Why?
Different enzymes can have different optimum pHs, eg. Enzymes in the stomach work best at low pH,
whereas enzymes in the gut work best at neutral pH.
3
, C. Blood glucose concentration
If blood glucose concentration is too high, the water potential of the blood is reduced to a point where
water molecules move out of the cells into the blood by osmosis. This can cause the cells to crenate
(shrivel up).
If blood glucose concentration is too low, cells are unable to carry out normal activities because there
isn’t enough glucose for respiration to provide ATP.
Negative feedback
A process that brings about a reversal of any change in conditions. It ensures that an optimum steady state
can be maintained, as the internal environment is returned to its original set of conditions after any change.
Any deviation from the norm or set point results in a response being taken which restores the norm. Once
the norm is restored, the response is inhibited.
Homeostasis involves multiple negative feedback mechanisms for each thing being controlled. This is
because having more than one mechanism gives more control over changes in your internal environment.
If you had only one negative feedback mechanism, the response would be slower and would have less
control.
4
Homeostasis
Name:
Teacher:
1
, Principles of homeostasis and negative feedback
What is homeostasis?
Homeostasis is the maintenance of the internal environment in a constant state within an organism,
despite external changes. Organisms can only function if internal conditions such as temperature, pH,
water potential and blood glucose concentration are kept within a narrow range.
Importance of homeostasis
It is particularly important to maintain the right core body temperature and blood pH.
Q: Why?
It is also important to maintain the right blood glucose concentration because cells need glucose for
respiration to produce ATP, and blood glucose concentration also affects water potential of the blood.
A. Temperature (also see Year 1 Biological Molecules booklet)
A recap on how temperature affects rate of an enzyme controlled reaction:
2
3
1
1–
2–
3–
If body temperature is too low, enzyme activity is reduced, slowing the rate of metabolic reactions. If
it is too high, enzymes will denature and metabolic reactions will stop. The highest rate of enzyme
activity happens at their optimum temperature – about 37 C in humans.
2
,B. pH (also see Year 1 Biological Molecules booklet)
pH is calculated based on the concentration of hydrogen ions (H+) in the environment. The greater
the concentration of H+, the lower the pH (more acidic). pH is expressed on a logarithmic scale.
Each value on a logarithmic scale using log10 is ten times larger than the value before, eg. A solution
of pH 3 contains ten times more H+ than solution at pH4. Converting values to a logarithmic scale
makes it easier to plot both very small and very large values on the same axis of a graph.
You can work out pH of a solution using the following equation:
pH = - log10 [H+]
NB. [H+] is the concentration of H+ in a solution in moldm-3.
Eg 1. A solution has a H+ concentration of 0.01 moldm-3. Calculate its pH.
Eg 2. A blood sample has a H+ concentration of 3.9 x 10-8 moldm-3. What is the pH of the blood?
A recap on how pH affects rate of an enzyme controlled reaction:
If blood pH is too acidic or too alkaline, enzymes become denatured.
Why?
Different enzymes can have different optimum pHs, eg. Enzymes in the stomach work best at low pH,
whereas enzymes in the gut work best at neutral pH.
3
, C. Blood glucose concentration
If blood glucose concentration is too high, the water potential of the blood is reduced to a point where
water molecules move out of the cells into the blood by osmosis. This can cause the cells to crenate
(shrivel up).
If blood glucose concentration is too low, cells are unable to carry out normal activities because there
isn’t enough glucose for respiration to provide ATP.
Negative feedback
A process that brings about a reversal of any change in conditions. It ensures that an optimum steady state
can be maintained, as the internal environment is returned to its original set of conditions after any change.
Any deviation from the norm or set point results in a response being taken which restores the norm. Once
the norm is restored, the response is inhibited.
Homeostasis involves multiple negative feedback mechanisms for each thing being controlled. This is
because having more than one mechanism gives more control over changes in your internal environment.
If you had only one negative feedback mechanism, the response would be slower and would have less
control.
4
