F214 Communication and Homeostasis (OCR)

Detailed notes on the first module, Communication and Homeostasis, of the F214 exam (OCR). Notes are taken from a combination of my textbook, past mark schemes and my own knowledge. None of the images belong to me - they were taken from various sources on the internet.

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  • Created on: 29-05-16 12:23
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F214 Communication and Homeostasis
(a) outline the need for communication systems within multicellular organisms, with reference to
the need to respond to changes in the internal and external environment and to coordinate the
activities of different organs
All living things need to maintain a certain limited set of conditions inside their cells. This is
because the cellular activities rely on the action of enzymes, which are sensitive to changes in
things such as temperature and pH. Cells produce waste products which may alter these factors.
Changes in the environment ­ both internal and external ­ must be monitored and the organism
must change its behaviour or physiology to reduce the stress. The environmental change is a
stimulus and the way in which the organism changes is its response.
Multicellular organisms have differently specialised cells which make up tissues, which make up
organs, and perform different functions. A communication system that covers the whole body is
therefore needed that allows rapid and specific communication between cells for both shortterm
and longterm responses.
(b) state that cells need to communicate with each other by a process called cell signalling
(c) state that neuronal and hormonal systems are examples of cell signalling
(d) define the terms negative feedback, positive feedback and homeostasis
Negative feedback is a process in which any change in a parameter brings about the reversal of
that change so that the parameter is kept fairly constant.
Positive feedback is a process in which any change in a parameter brings about an increase in that
Homeostasis is the maintenance of the internal environment in a constant state, within narrow
limits, despite external changes.
(e) explain the principles of homeostasis in terms of receptors, effectors and negative feedback
Stimulus receptor communication pathway (cell signalling) effector response
Sensory receptors monitor conditions. If they detect a change they will be stimulated to send a
message. A communication system such as the nervous system or the hormonal system acts by
signalling between cells. It is used to transmit a message from the receptor cells to the effector
cells. The message may or may not pass through a coordination centre such as the brain.
Effector cells bring about a response that reverses the change detected by the receptor cells.
(f) describe the physiological and behavioural responses that maintain a constant core body
temperature in ectotherms and endotherms, with reference to peripheral temperature receptors,
the hypothalamus and effectors in skin and muscles

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An ectotherm is an organism that relies on external sources of heat to regulate its body
temperature.…read more

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Motor neurones carry an action potential from the CNS to an effector such as a muscle or gland.
Relay neurones connect sensory and motor neurones.
Both neurones have many mitochondria, ribosomes, dendrites and ion pumps/channels. Many are
very long with a myelin sheath.…read more

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At these points only, the Na+ ions cause voltagegated sodium ion
channels to open to set up a new action potential. This is called saltatory conduction.
In myelinated neurones the local currents are longer, which speeds up the transmission of the
action potential.
(f) interpret graphs of the voltage changes taking place during the generation and transmission of
an action potential
1. The membrane starts in resting state ­ polarised with a potential difference of 60mV.
2.…read more

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Myelinated neurones are electrically insulated by a myelin sheath, which consists of Schwann cells
which produce the fatty substance myelin. They are wrapped around the neurone, so the sheath
consists of layers of membrane and cytoplasm. Every 13mm there are gaps called the nodes of
Ranvier, about 23mm long. Nonmyelinated neurones are enshrouded in a loosely wrapped
Schwann cell.…read more

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There are also voltagegated calcium ion channels (Ca2+ ) in the presynaptic membrane.
(j) outline the role of neurotransmitters in the transmission of action potentials
An action potential arrives at the synaptic knob, which causes voltagegated calcium ion channels
to open. Calcium ions diffuse down their concentration gradient into the synaptic knob, and cause
synaptic vesicles containing the neurotransmitter substance (acetylcholine) to move to and fuse
with the presynaptic membrane. Acetylcholine is released by exocytosis and diffuses across the
synaptic cleft.…read more

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One presynaptic neurone might diverge to several postsynaptic neurones. This would allow one
signal to be transmitted to several parts of the nervous system. It is useful in a reflex arc ­ a signal
goes to an effector and another informs the brain.
It ensures that signals are transmitted in the correct direction ­ only the presynaptic knob
contains the neurotransmitter and only the postsynaptic membrane has the specific receptors to
set up a new action potential.
It filters out lowlevel signals.…read more

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The receptor is associated with
an enzyme on the inner surface of the cell surface membrane, called adenyl cyclase.
Adrenaline (the first messenger) travels in the blood and binds to its specific, complementary
shaped receptor on the cell surface membrane. This activates the enzyme adenyl cyclase. The
adenyl cyclase converts ATP to cyclic AMP (cAMP). cAMP is the second messenger. cAMP activates
a cascade of enzymecontrolled reactions, first activating protein kinase, finally activating glycogen
phosphorylase, to hydrolyse glycogen.…read more

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The pancreas is a small organ lying below the stomach. It has both exocrine and endocrine
functions. The majority of the cells secrete digestive enzymes, which is the exocrine function. The
cells are in small groups surrounding a tubule which joins to other tubules to make up the
pancreatic duct. The release of the enzymes is triggered by nervous/hormonal stimulation and
they flow into the small intestine/duodenum.…read more


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