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cell signalling
the way in which cells communicate with each other
stimulus
the environmental change the body responds to
response
the behavioural or physiological change
survival and activity
all living things need to maintian a certain limited set of conditions inside their cells
these include:
a suitable temperature
a suitable pH
an aqueous environment to keep substrates and products….
the threat from changing environments 1
the external environment for any organism is not fixed but fluctuates and even directionally changes
these changes can place stress on the organisms
e.g. colder environments lead to more heat loss
an organism must monitor the surroundings and change its behaviour or physiology to reduce the stress
the threat from changing environments 2
the environment may change slowly as the seasons cycle through. these changes elicit a gradual response
coat thickness will change to provide varied levels of insulation
however, sometimes the environment may change rapidly. a predator appearing or the change from darkness in a burrow to sunlight. these stimuli produce a much more rapid response
changing internal environments 1
in multicellular organisms many of the cells and tissues are not exposed to the external environment. they are protected by epithelial tissues and organs such as skin or bark
in animals the internal cells and tissues are bathed in tissue fluid
as cells undergo metabolic processes they use some substrates and create new waste products. these can be toxic. these substances move out of the cells into the tissue fluid
the cell’s activities has altered the environment surrounding it
changing internal environments 2
carbon dioxide is a waste product from cells. of this is allowed to build up in the tissue fluid it will alter pH leading to altered protein and enzyme action
the accumulation of waste acts as a stimulus to cause the removal of waste
in the carbon dioxide example lower pH stimulates greater breathing activity. the build up of waste can also act directly on the cells to lead to reduced activity
coordinating the activities of different organs
differentiation means that multicellular organisms are more efficient than single-celled organisms
the cells that monitor the blood may be in a different part of the blood well away from the sources of the waste products
they may be of some distance away from the tissue or organ that specialises in the removal of waste from the body
multicellular organisms must have a good communication system to manage this.
a good system will:
cover the whole body, enable cells to communicate
enable specific, rapid communication
allow both short and long term responses
cell signalling
cell signalling is how cells communicate with each other. in this process one cell will release a chemical that is detected by another cell
the second cell will then respond to the signal released
the two systems that work by cell signalling are the;
neuronal system
hormonal system
cell signalling neuronal
an interconnected network of neurones that signal to each other across synapse junctions
the neurones can conduct a signal very quickly enable rapid responses to stimuli that may be changing quickly
cell signalling hormonal
a system that uses the blood to transport its signals
the signal is released by cells in an endocrine organ directly into the blood
the hormone is transported throughout the whole body but only recognised by specific target cells. this is due to specific shapes
the hormonal system enables longer-term responses
effector
a cell, tissue or organ that brings about a response
homeostasis
maintaining a constant internal environment despite changes in external and internal factors
negatives feedback
the mechanisms that reverses a change, taking the system back to the optimum
positive feedback
the mechanism that increases a change, taking the system further away from the optimum
sensory receptors
cells/nerves endings that respond to a stimulus in the internal or external environment and can create action potentials
control mechanisms
set point
receptor
controller
effector
feedback loop
standard response pathway
stimulus
receptor
communication pathway
effector
response
sensory receptors
these monitor changes in the environment
when one of the receptors detect a change it will be stimulated and sends a message to an effector
communication system
such as the neuronal system or hormonal system. they act by signalling between cells. it sends a message from the receptor cells to the effector cells via a coordination centre, usually in the brain. the message to the coordinator is the input and the message to the effectors is the output.
effector
these cells bring about a response
feedback system
when the effector responds to the output the condition changes
this change is detected by the receptor
this will alter the response pathway
the input will be changed
negative feedback
to maintain a constant internal environment any change from the optimum must be reversed
this mechanism that brings the conditions back towards the optimum is known as negative feedback
the receptor detects the change in condition (stimulus) and sends an input
the coordination centre produces an output and the effector responds
the effector brings about a change that reverses the initial change, moving the system closer to the original and the stimulus is reduced
the receptors detect the reduction in stimulus and reduce the input
the output is also reduced so the effector reduces their activity
negative feedback summary
for negative feedback to work, a number of processes must occur:
a change must be detected
the change must be signalled to other cells
there must be an effective response that reverses the change in condition
maintaining a constant internal environment
while negative feedback systems can maintain a relatively constant set of conditions they will never be perfectly constant. there will be some variation around the optimum conditions
when the stimulus occurs it takes time to respond and can often lead to slight overshooting
however, as long as the variation is not too great the conditions will remain within an acceptable range
taking the temperature example the body will remain warm enough for enzymes but cool enough to avoid damage.
positive feedback
this response is much less common than negative feedback
in this case the response increases the original change, destabilising the system which is often harmful
below a certain core body temperature enzyme becomes less active and the exergonic reactions that release heat are slower. this will further cool the body.
positive feedback
occasionally positive feedback can be beneficial. an example of this is seen at the end of pregnancy- dilation of the cervix
as the ceric begins to stretch this causes the posterior pituitary gland to secrete the hormone oxytocin
oxytocin increases the uterine contractions which stretch the cervix more, causing further release
once the cervix is fully dilated, the baby can be born. the child birth ends the production
ecotherm
an organism that relies on external sources of heat to maintain body temperature
endotherm
an organism that uses heat from metabolic reactions to maintain body temperature
advantages of ectothermy
do not use energy to keep warm
therefore:
-less of their food is used in respiration
-more of the energy and nutrients gained from food can be converted to growth
-they need to find less food
-they can survive for long periods without food
disadvantages of ectothermy
they are less active in cooler temperatures to the risk from predators is greater as they are unable to escape and cannot take advantage of food that is available while they are cold
advantages of endothermy
can maintain a constant body temperature regardless of the external temperature
remain active when the external temperatures are low, which means they can take advantage of prey that may be available or escape from predators
inhabit colder parts of the planet
disadvantages of endothermy
use a significant part of their energy intake to maintain body temperature in the cold
need more food
use for growth a lower proportion of the energy and nutrients gained from food
may overheat in hot weather
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