Cambridge A Levels A2 Biology Chapter 14 Homeostasis
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Cambridge A Levels A2 Biology
Institution
Chapter 14 Homeostasis: 58 pages
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Chapter 14 Homeostasis
14.1 Homeostasis in Mammals
14.2 Homeostasis in Plants
14.1 Homeostasis in Mammals
Homeostasis
In order to function properly and efficiently, organisms have different control systems that
ensure their internal conditions are kept relatively constant.
Homeostasis: The process of maintaining constant internal body conditions.
Examples of physiological factors that are controlled by homeostasis in mammals:
• Core body temperature
• Metabolic waste (eg. carbon dioxide and urea)
• Blood pH
• Concentration of glucose in the blood
• Water potential of the blood
• Concentration of the respiratory gases (carbon dioxide and oxygen) in the blood
Internal Environment
The internal environment of an organism refers to all the conditions inside the body in which
cells function.
For a cell, its immediate environment is the tissue fluid that surrounds it.
Many features of the tissue fluid influence how well the cell functions.
Four features of tissue fluid that influence cell activities are:
• Temperature
Low temperatures slow down metabolic reactions:
At high temperatures proteins, including enzymes, are denatured and cannot function.
• Water potential
If the water potential decreases, water may move out of cells by osmosis, causing
metabolic reactions in the cell to slow or stop; if the water potential increases, water
may enter the cell causing it to swell and maybe burst
,• Concentration of glucose in the blood
Glucose is the fuel for respiration, so lack of it causes respiration to slow or stop,
depriving the cell of an energy source.
Too much glucose may cause water to move out of the cell by osmosis, again disturbing
the metabolism of the cell.
• pH
Enzyme activity is influenced by pH; the pH of cytoplasm is between 6.5 and 7.0 and if it
fluctuates outside this range, enzymes will function less efficiently and will be denatured
at extreme values of pH.
,Homeostatic Control
The majority of homeostatic control mechanisms in organisms use negative feedback to
maintain homeostatic balance.
Negative feedback minimises the difference between the actual value of the factor and the
ideal value or set point.
Negative feedback control loops involve:
A stimulus
▪ Any change in a factor.
▪ Internal stimuli and external stimuli.
A receptor
▪ Detects a stimulus that is involved with a condition / physiological factor.
▪ Sends information (input) about the changes they detect through the nervous
system to a central control in the brain or spinal cord.
A coordination system (nervous system and endocrine system)
▪ Transfers information between different parts of the body.
▪ The central control instructs an effector to carry out an action.
▪ Corrective action: a response or series of responses that return a physiological
factor to the set point so maintaining a constant environment for the cells within
the body.
An effector (muscles and glands)
▪ Carries out a response. (output)
Outcome of a negative feedback loop:
The factor / stimulus is continuously monitored.
The factor fluctuates around a particular ‘ideal’ value or set point.
If there is an increase in the factor, the body responds to make the factor decrease.
If there is a decrease in the factor, the body responds to make the factor increase.
, The two coordination systems in mammals:
• Nervous system
Information is transferred in the form of electrical impulses.
Electrical impulses are transmitted along neurones.
• Endocrine system
Uses chemical messengers called hormones that travel in the blood, in a form of
long-distance cell signalling.
Control mechanisms that involve positive feedback:
If a person breathes air that has very high carbon dioxide content, a high concentration of
carbon dioxide in the blood results.
This is sensed by carbon dioxide receptors, which cause the breathing rate to increase.
So, the person breathes faster, taking in even more carbon dioxide, which stimulates the
receptors even more, so the person breathes faster and faster.
Positive feedback cannot play any role in keeping conditions in the body constant.
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