A revision summary of OCR A level Biology Communication & Homeostasis 5.1.1
Made by a student who achieved A* in A level Biology.
Covers all the points within the OCR Biology Specification.
Condenses 3 OCR specification books and class notes.
Need for communication systems
Communication - Cell
Cells must control:
signalling
- glucose conc (respiration, osmosis, water conc)
Neuronal system –
- Temperature & pH (enzymes)
interconnected network
- CO2 (dissolves & becomes acidic affecting pH) & O2 conc
of neurones that signal to
- Toxins/ inhibitors
each other across
External environment – air, water, soil. Organisms responds to reduce
synapses.
stress by behavioural or physiological changes.
- Quick rapid short-
Internal environment – tissue fluid surrounding cells. Activity of cells
term
alters the tissue fluid as products diffuse in e.g. CO 2 can change pH
Hormonal system –
- Blood flow maintains composition of tissue fluid by delivering &
blood transports signals
removing substances.
which are recognised by
Organ systems – multi-cellular organisms have specialised cells for
target cells to elicit
each function = more efficient. Communication is needed so tissues/
response
organs can work together.
- Slower, longer-
Homeostasis – maintenance of the internal environment within a normal range despite
changes in external environment.
1. Stimulus (change in environment)
2. Receptor (detect change in external environment - thermoreceptor, osmoreceptor,
chemoreceptor)
3. Communication system (info transmitted to brain and impulses sent along motor neurones to)
4. Effector (muscles/ glands that react to the motor stimulus to bring about change)
5. Response (movement or secretion of hormone).
Negative Feedback – effectors work to reverse of any change in internal conditions back to base
level.
- (Temp, blood glucose, water control)
Positive Feedback – effectors are stimulated to increase change detected
- Childbirth – head of baby presses against cervix & stimulates oxytocin production. Oxytocin
stimulates contraction of uterus, pushing head of baby even harder against uterus. (Clotting
Definition Advantages Disadvantages
Ectotherm - Obtain heat Less food is used in respiration Less active in cooler temp & at
from external sources. Body More energy gained from food risk from predators, cannot
temp fluctuates with converted into growth take advantage of food
environment Have to find less food available while they are cold.
Use behavioural Survive for long periods without
mechanisms to control body food
temp range
Endotherm - Generate heat
Maintain constant body temp Significant part of energy intake
within their bodies despite external conditions used to maintain body temp
(exergonic reactions) toRemain active when external temp Need more food
maintain body temp. is low (escape predators & hunt Less energy available for
prey) growth
Inhabit cooler parts of planet Overheat in hot temp
Endotherms temp control – exergonic chemical reactions release energy in form of heat
(respiration). ↑respiration in muscles/ liver to release heat.
Core body temp – thermoreceptors in the thermoregulatory centre of the hypothalamus
detects changes in core body temp & receives nerve impulses from peripheral thermoreceptors in
the skin of extremities.
Peripheral body temp – receptors in the skin detect change in surface temp
Organ Too Hot Too Cold
Skin Sweat glands secrete sweat which uses heat ↓sweat secreted = ↓evaporation
from blood to evaporate off skin surface Hairs lie flat to create layer of still
Hairs lie flat to reduce trapped air insulating air
Liver ↓respiration = ↓heat released ↑respiration = ↑heat
Skeletal ↓contractions = ↓heat released Spontaneous muscle contractions
muscles release heat
Blood Vasodilation allows heat to be lost from blood Vasoconstriction diverts blood away
vessels through radiation from skin surface
Ectotherms behavioural strategies
Heating Cooling
lies in sun to gain heat by radiation. lies in mud/water to lose heat by evaporation
Pressing against hot rocks/ the earth to gain heat stands high to lose heat by convection
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