B5: Communication, homeostasis and energy
5.1: Communication and homeostasis
5.1.1: Communication and homeostasis
- Often get qs that expect you to understand that urea conc has increased not because more has added, but
relatively, because water has been taken out
The need for communication systems in multicellular organisms
● Cell signalling is necessary for a multicellular organism to function in a coordinated way
● Evolution → increase in size and complexity of multicellular organisms → development of specialised
structures; ‘organs’ → each organ performs functions related to the whole body → communication between
cells, tissues and organs is essential (internal)
● Change in environment -- the cells that bring about the response to this change are almost always different
from those that detect the change → communication between receptor and effector is essential (external)
● Communication systems allow animals and plants to respond to changes in their internal and external
environments, and coordinate the activities of different organs. They enable maintenance of a relatively
constant internal state around a narrow range of conditions - it’s impossible to maintain a completely stable
state because everything causes minute change
● This process is known as ‘homeostasis’: the regulation of the internal conditions of a cell / organism in
response to internal and external changes → results in a dynamic equilibrium, with small fluctuations over
a narrow range of conditions
● Homeostasis is critically important for organisms as it ensures the maintenance of optimal conditions for
enzyme action → optimum enzyme activity → optimum rate of metabolic reactions → optimum cell
function
● Animals show a wider range of responses and generally have a greater variety of specialised cells
● Receptors + effectors are vital in a homeostatic system →
All communication systems include:
1) Receptors
2) Coordination system eg. brain, pancreas, spinal cord - receives and processes information from receptors
around the body
3) Effectors
Homeostasis in mammals relies on 2 different communication systems (which transfer information between
different parts of the body)
1) Nervous system
2) Endocrine system
Examples of physiological factors regulated by homeostasis in mammals:
- Core body temp - important to regulate because need to maintain optimum temp for enzymes in body.
Temp too high → enzymes denature and cannot function. Temp too low → rate of enzyme-controlled
reaction slows and the enzymes become ineffective
- Metabolic waste (eg. CO2 and urea)
- Blood pH
- Concentration of glucose in the blood - important to regulate because need to allow cells access to
respiratory substrates while also preventing high concs → dehydration → cell damage
- Water potential of the blood - important to regulate because prevents cells bursting/shrinking
, - Concentration of the respiratory gases (CO2 + O2) in the blood
The communication between cells by cell signalling
Autocrine
- Signals produced by signalling cells that can also bind to the ligand that is released
- TMT the signalling cell and the target cell can be the same, or a similar, cell
- Often occurs during the early development of an organism to ensure that cells develop into the correct
tissues and take on the proper function eg. in development of an embryo, autocrine signalling may help to
direct the differentiation of identical cells into the same cell type
- Regulates pain sensation
Paracrine
- Signals that act locally between cells that are close together
- Move by diffusion through the extracellular matrix
- In order to keep the response localised, paracrine ligand molecules are normally quickly degraded by
enzymes / removed by neighboring cells → reestablishes the concentration gradient for the signal → can
quickly diffuse through the intracellular space if released again
- Usually elicit rapid responses that are short-lived
- Eg. a synaptic signal
Endocrine
- Signals from distant cells
- Originate from ‘endocrine cells’, any of which are located in the endocrine glands
- Usually produce a slower response but an effect that lasts longer
- Ligands released are called ‘hormones’ - produced in one part of the body but affect target cells in other
distant regions in the body, travel the large distances in the bloodstream (relatively slow way of moving)
- Because of their form of transport, hormones get diluted and are present in low concentrations when they
act on their target cells - different from paracrine signaling, where local concentrations of ligands can be
very high
CELL SIGNALLING
● Signalling molecules prod by cells can be eg. proteins, amino acids, lipids, glycoproteins, phospholipids,
hormones (endocrine)
● Receptor molecules are proteins/glycoproteins. Usually in/on cell surface membrane, although some are in
the cytoplasm (eg. oestrogen receptors) because steroid hormones can diffuse through the CSM
● The signalling molecule binds to the receptor and causes specific changes in the receiving cell
● Production of histamine is an example of cell signalling
● Cell signalling is NOT exclusive to animals eg. plant hormone ethylene, which promotes fruit ripening.
The principles of homeostasis
Homeostasis: the regulation of the internal conditions of a cell / organism in response to internal and external
changes, in response to internal and external changes
- dynamic equilibrium, with small fluctuations over a narrow range of conditions. It’s impossible to maintain
a completely stable state because everything causes minute change
- Receptors + effectors are vital in a homeostatic system
,Sensory receptors: structure in the body that can detect changes in organism’s internal + external environment
(stimuli)
[__ information from sensory receptors is transmitted to the brain → impulses transmitted along the motor neurones
to the effectors
[__Effectors: muscles / glands that react to the motor stimulus to bring about a change in response to the stimulus
→ restore equilibrium
- Responses include muscle contraction / hormone release
Differences between receptors and effectors
➔ Key difference is that receptors detect stimuli, while effectors produce a response to the stimuli
➔ Receptors are specialised cells of sensory organs, while effectors are mainly muscles and glands (so present
all over the body)
➔ Receptors are connected to sensory neurones, while effectors are connected to motor neurones
➔ Receptors transmit information to the CNS, while effectors receive information/commands from the CNS
Negative feedback systems
- Most of the feedback systems/homeostatic mechanisms in the body involve -tve feedback
- Change initiates a series of events which then reverse that change
- Change from normal value of a physiological factor detected by sensory receptors → effectors work to
reverse the change + restore conditions to their base level → reverses the initial stimulus
- Eg. control of blood sugar levels, temp control, water balance of the body
Positive feedback systems
+ Relatively few +tve feedback systems in the body
+ Change in internal environment of body detected by sensory receptors → effectors stimulated to reinforce
the change → increase initial stimulus (response)
+ Causes proliferation rather than control
, + Unrestricted +tve feedback mechanisms would result in level of products getting out of control, so there is
always an external factor which can bring the +tve feedback loop to an end
+ Eg. blood clotting cascade
- Damaged blood vessel
- Platelets stick to damaged region
- Platelets release factors that initiate clotting + attract more platelets
- These newly attracted platelets also add to the +tve feedback system, which cont until a clot is
formed
+ Eg. during childbirth
- Baby’s head presses against the cervix
- Stimulates production of oxytocin (hormone)
- Oxytocin stimulates the uterus to contract, intensifying uterine contractions→ pushes baby’s head
even harder against the cervix
- Stimulates secretion of even more oxytocin → cont until baby is born -- birth stops secretion of
oxytocin, ending the +tve feedback loop
Differences between negative feedback and positive feedback
➔ Key difference is their response to change: positive feedback amplifies the stimulus→ results in more of a
product/output signal, while negative feedback reduces the stimulus → results in less of a product/output
signal
[__ positive feedback moves away from a target point, while positive feedback moves towards a target point
➔ Negative feedback is more associated with the stability of the system - resists change, instead of enhancing
it (as with positive feedback)
➔ Positive feedback is a less frequent process than negative feedback
➔ Positive feedback may require external interruption, while negative feedback does not
➔ Positive feedback may be associated with vicious cycles, and even death
➔ Positive feedback has a wider range than negative feedback
The physiological and behavioural responses involved in temperature control in ectotherms
and endotherms.
Thermoregulation: maintenance of a relatively constant core body temp, in order to maintain optimum enzyme
activity
Changes in temperature of an organism’s surroundings depend on a number of physical processes, including:
- Exothermic chemical reactions eg. waste heat from respiration causes incr temp
- Latent heat of evaporation - objects cool as water evaps from surface because heat necessary to change
water → water vapour is extracted from the skin
- Radiation - loss of heat (in form of EM radiation) from hot objects into cooler surroundings. Main way the
human body loses heat, also main way we gain heat from external sources eg. the Sun
- Convection - heating / cooling by currents of air / water
- Conduction - heating as a result of the collision of molecules -- transfer of heat energy from a warmer
material to a cooler one. Air is not a good conductor of heat, but water + the ground are. The body can lose
heat to the air by condition but because air is a better insulator, the trapping of a layer of air around the
body reduces further heat loss by radiation (why wearing thick clothes made of materials that trap air eg.
wool keeps you warm in cold conditions)
