Organisms must detect environmental change to survive. The coordination pathway runs stimulus to receptor to coordinator to effector to response, with each receptor type tuned to one class of stimulus. Plant tropisms, animal reflexes, and the eye's photoreceptors are different solutions to the same problem.
The coordination pathway runs stimulus to receptor to coordinator to effector to response.
All organisms must detect and respond to environmental change to survive. The coordination pathway is the universal architecture: a stimulus is detected by a receptor; the receptor sends information to a coordinator (the nervous or hormonal system); the coordinator directs an effector; the effector produces the response. Internal and external stimuli both run through the same five-element pathway.
Each receptor type is tuned to one class of stimulus. Photoreceptors detect light; mechanoreceptors detect pressure or vibration; chemoreceptors detect dissolved chemicals; thermoreceptors detect temperature; osmoreceptors detect osmolarity. Specificity is what allows an organism to process many input channels simultaneously without cross-interference.
Taxes are directional movements; kineses are undirected speed-and-turn responses.
Both taxes and kineses are innate behavioural responses found in motile organisms. They differ in whether the resulting movement is oriented relative to the stimulus.
Taxis and kinesis compared.
| Response type | Movement | Mechanism | Standard example |
|---|---|---|---|
| Taxis | Directional — positive (toward) or negative (away) | Organism steers continuously using sensory information | Bacteria swim toward higher glucose (positive chemotaxis) |
| Kinesis | Undirected — speed and turning rate change with stimulus intensity | Higher unfavourable intensity → faster, more turns; bias accumulates by chance | Woodlouse moves faster and turns more in dry air, reaching damp zones |
Direction is the load-bearing feature of any taxis answer. Movement toward the stimulus is positive; movement away is negative. When the data give speeds under different conditions, verify which speed corresponds to which direction before plugging any value into a calculation. The statistical tests that accompany taxis investigations have their own discipline — a null hypothesis must name both the stimulus type and the specific behaviour.
For a taxis, write the direction explicitly: positive (toward) or negative (away). On a distance-time calculation, use the speed value for the direction the question asks about. Selecting the wrong-direction speed is the dominant error in this topic's calculations.
Write a null hypothesis that names both the stimulus type and the specific behaviour.
No significant difference between observed and expectedis rejected — too general. State the p-value and compare it to 0.05 explicitly.
Plant tropisms are growth responses controlled by uneven IAA distribution.
Plants cannot locomote, so they respond to directional environmental stimuli through tropisms — growth responses in which the direction of growth is determined by the direction of the stimulus. AQA examines two tropisms: phototropism (response to light) and gravitropism (response to gravity).
IAA is the auxin, a plant growth hormone. It is produced in the meristematic tissue of the shoot tip — not contained or stored there. IAA moves through the zone of elongation and promotes cell elongation in shoots (not growth alone). The elongation is permanent (plasticity), not reversible (elasticity).
Light from one side causes IAA to diffuse to the shaded side of the shoot tip — the statement of movement is mandatory. Cells on the shaded side receive higher IAA, elongate more, and the shoot curves toward the light. Even illumination produces symmetric IAA distribution, equal elongation, and straight growth.
In a horizontally-placed root, IAA accumulates on the lower side. In roots, IAA inhibits elongation — the opposite of its shoot effect. The lower side elongates less; the upper side elongates more; the root curves downward (positive gravitropism). Credited phrasing: inhibits elongation or decrease in mean change in length.
Write
produces IAAfor the role of the shoot tip.Containsandstoresare ignored. For redistribution, writediffuses toormoves to—accumulates onalone does not name the movement.
In roots, write
inhibits elongationordecrease in mean change in length. Don't writedecreases lengthalone — it does not name the mechanism, and the same hormone promotes elongation in shoots, so the inhibition statement is what distinguishes the root case.
Investigation discipline matters. Serial dilutions of IAA require explicit volumes — dilute with water without volumes scores zero. Petri dish lids prevent evaporation of water from the IAA solutions, which would alter their concentration; prevent contamination is rejected because it does not explain the mechanism. Controlled variables alone cap a five-mark experimental design answer at three marks; replication-and-mean methodology and result-reading method carry the remaining two.
The reflex arc generates a fast, automatic response that bypasses conscious processing.
A reflex arc is a fixed neural pathway that produces a rapid, automatic, involuntary response. The pathway bypasses conscious decision-making in the brain, which is what makes reflexes fast enough to protect — the motor response is already underway while the sensory signal is still travelling toward the brain for conscious awareness. The sequence runs stimulus → receptor → sensory neurone → intermediate neurone (in the spinal cord) → motor neurone → effector → response.
Write
impulseoraction potentialfor what travels along a neurone. Don't writesignalormessage— both are rejected. The substitution is the most consistently enforced verbal-floor in this topic.
A worked example. Constriction (bright light): the circular muscles of the iris contract; the radial muscles relax. Dilation (dim light): the radial muscles contract; the circular muscles relax. Both muscle names with both actions are required — muscles constrict alone is rejected because it does not name which muscles do what.
The Pacinian corpuscle converts pressure into a generator potential through stretch-mediated sodium ion channels.
The Pacinian corpuscle is a mechanoreceptor in the dermis, concentrated in fingertips, soles, joints, and tendons. Each corpuscle contains a single myelinated sensory neurone surrounded by concentric rings of connective tissue — the lamellae — separated by a gel matrix. The lamellae act as a mechanical filter that transmits deformational force inward to the neurone membrane.
- Stretch-mediated sodium ion channels in the axon membrane are closed at rest. The complete term is mandatory; dropping
ionscaps the answer, andstretch-mediated membranescores zero. - Pressure deforms the lamellae and, through them, the axon membrane underneath.
- The deformation opens the channels mechanically, exposing the channel pore to the extracellular fluid.
- Sodium ions diffuse in down the concentration gradient into the axon. The credited phrasing is
diffuse inormove in down the concentration gradient;influxis rejected. - The inward flow of positive charge produces depolarisation — the generator potential. If threshold is reached, an action potential is initiated in the sensory neurone.
Write
stretch-mediated sodium ion channels— the complete term. Droppingionsfromsodium ionscaps the answer at 2 of 3 marks;stretch-mediated membranescores zero on that step.
Write
diffuse in down the concentration gradient. Don't writeinflux. The credited phrasing names both the direction of movement and the driving force;influxnames only the direction.
Any three of the five steps score full marks (3/3), but only when phrased with the exact required terminology. Dropping ions from sodium ions caps the answer at 2 of 3. Stretch-mediated membrane earns zero on that step — the full term stretch-mediated sodium ion channels is required.
Rod cells trade resolution for sensitivity; cone cells trade sensitivity for resolution and colour.
The retina contains two types of photoreceptor — rods and cones — which differ on four axes: pigment, light requirement, colour discrimination, and wiring to the optic nerve. The blind spot is the point where the optic nerve exits the retina; it contains no photoreceptors and produces no signal.
Pigment: rhodopsin. Light: dim. Colour: none (monochromatic). Wiring: retinal convergence — many rod cells synapse onto a single bipolar neurone. The pooled signals summate spatially; weak inputs combine to cross threshold even when single inputs cannot. The cost is spatial resolution: the brain cannot tell which individual rod fired.
Pigment: iodopsin, in three varieties — red-sensitive, green-sensitive, and blue-sensitive cones. The -sensitive qualifier is mandatory. Light: bright. Colour: full. Wiring: each cone has its own dedicated bipolar neurone, so spatial information is preserved. Cones cannot pool signals; they require bright light to reach threshold.
Write
red-sensitive cone,green-sensitive cone,blue-sensitive cone. Dropping the-sensitivequalifier and writingred cone,green cone, orblue coneis rejected — the cones do not contain coloured pigment that emits colour; they absorb different wavelength ranges differentially.
Colour perception arises from the relative stimulation of all three cone types simultaneously, not from any single cone type responding alone.
Key terms
- IAA
- action potential
- threshold
- generator potential
- cone
- convergence
- depolarisation
- refractory period
- all-or-nothing
- cell elongation