Edexcel Biology B is very scarce when it comes to finding information on it online. I've taken it upon my self to publish my own notes to help fellow students that have a hard time understanding the subject. The information in the text book is very large and can be hard to narrow down to a particu...
Dynamic equilibrium-a state of balance between continuing processes.
Feedback systems-the mechanisms involved in the control of conditions in an organism
The level of stimulation determines the amount of hormones released
pituitary gland produces and releases secretions that affect most of the other endocrine glands in the body
Nerve fibres- axons or dendrites that carry impulses
Sensory neurons carry impulses from receptors to CNS
Motor neurons carry impulse out from CNS to effectors in the body
Topic 9.1 - Homeostasis
i Know that homeostasis is the maintenance of a state of dynamic equilibrium.
Mammalian cells are very sensitive to changes however the internal conditions of the body
must be controlled. The maintenance state of dynamic equilibrium through the response of
the body through internal and externalStimuli is known as homeostasis
ii Understand the importance of maintaining pH, temperature and water potential in the body.
● pH affects the molecular structure of proteins so it must be maintained so that
enzymes operate at their optimum level. Hydrogen bonds disrupted
● Core temperature of the body needs to be stable so enzymes are at optimum and
don’t denature. A stable temperature also maintains the integrity of membranes so
that they can control the movement in and out of substances
● Water potential of body fluids must remain at a narrow level as osmotic effects could
destroys cells
iii Understand what is meant by negative feedback and positive feedback control.
Negative feedback provides a way of maintaining a condition e.g concentration of a
substance. For example a change in condition is registered by a receptor in which effectors
are stimulated to restore the equilibrium
,Positive feedback are effectors that work to increase the effect that has triggered the
response.
9.2 Chemical control in mammals
i Understand the principles of mammalian hormone production by endocrine glands and their
mode of action involving receptors on target cells.
One of the main forms of chemical control is brought about through hormones.Mammalian
hormones are produced by endocrine glands and are released into the blood. Hormones are
usually proteins or peptides. The cells of the target organs have specific receptors on the
surface of their membranes that bind to the hormone. This brings about a change in
membrane and elicits a response
, Different types of endocrine glands:
ii Know that there are two main modes of action in hormones:
● hormones attach to receptor sites and trigger the release of a second messenger that
activates specific enzymes in the cell, including adrenaline
● hormones enter cells and bind directly to transcription factors, including oestrogen.
Some hormones are released from direct stimulation of the endocrine gland by nerves. If the
gland is stimulated hormone is released if not hormone is not released. E.g adrenal medulla
of adrenal glands releases adrenaline when it is stimulated by sympathetic nervous system
Many hormones are also released from endocrine glands in response to another hormone or
chemical in the blood. E.g pituitary gland secretes several hormones in response to changes
in chemicals in blood such as glucose and salt
One way hormones work is by binding to specific receptors on the membrane of their target
cell. This triggers a series of membrane bound reactions that results in a formation of a
second messenger. This then activates different types of enzymes within the cell.
E.g most common messenger is a substance called cyclic AMP (formed by ATP) which
triggers responses like increased cellular respiration and contraction of muscles ect. Which
is how adrenaline has its effects
Hormones such as oestrogen bind to receptors inside the cells and form hormone-receptor
complexes which pass into the nucleus and act as transcription factors to regulate gene
expression.
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