Why respond? Survival and irritability
A stimulus is any change that a living thing can detect, such as light, heat, touch or a chemical. Irritability (also called sensitivity) means being able to detect a stimulus and respond to it. Every living thing has it, at every level:
- Single cells (e.g. bacteria, Euglena) move towards food or light.
- Plants respond slowly, by growing in a direction.
- Animals respond fast, with nerves and muscles, and also with hormones.
Responding helps an organism find food, stay at a good temperature and escape danger. Organisms that respond well survive and have more young, so natural selection keeps these responses.
The response chain: stimulus → receptor (detects it) → coordinator (brain, spinal cord, or a plant's cells) → effector (muscle or gland) → response.
Taxis and kinesis: simple movements
Taxis
A taxis is a movement of the whole organism in a set direction related to the stimulus. Towards it is positive; away is negative.
- Euglena swims towards light (positive phototaxis) so it can photosynthesise.
- Earthworms move away from light (negative phototaxis), which keeps them in damp soil.
Kinesis
In a kinesis the organism does not move towards or away. Instead, its speed and rate of turning change with how strong the stimulus is.
- Woodlice in dry air move fast and turn a lot. This raises the chance of reaching a damp place.
- In damp air they slow down and turn less, so they stay there.
Kinesis is useful when the stimulus has no clear direction, like humidity.
Tropisms and IAA in plants
A tropism is a growth of part of a plant towards or away from a stimulus coming from one direction.
- Shoots: positive phototropism (towards light), negative gravitropism (up).
- Roots: positive gravitropism (down) and usually negative phototropism.
The control chemical is IAA (indoleacetic acid), a kind of auxin. It is made in the tips of shoots and roots and spreads by diffusion.
In a shoot
Light from one side → IAA moves to the shaded side → IAA makes those cells elongate (get longer) → the shaded side grows more → the shoot bends towards the light.
In a root
Gravity → IAA collects on the lower side → in roots IAA slows cell elongation → the upper side grows more → the root bends down.
Reflexes: fast, automatic protection
A reflex is a fast response that you do not think about. A simple reflex arc uses three neurones:
- Sensory neurone carries the impulse from the receptor to the spinal cord.
- Relay neurone (inside the spinal cord) passes it on.
- Motor neurone carries it to the effector muscle.
Reflexes are fast because the path is short and the brain does not have to decide. They are inborn, so they protect even a newborn. Examples: pulling away from heat, blinking, the knee jerk.
Receptors: the Pacinian corpuscle and the eye
Each receptor responds to one kind of stimulus only. It works as a transducer: it changes the energy of the stimulus into a nerve impulse.
Pacinian corpuscle (pressure)
It sits deep in the skin. A nerve ending is wrapped in many layers (lamellae) like an onion. Pressure squashes the layers and stretches the membrane. Stretch-mediated sodium channels open, Na⁺ ions rush in, and the inside becomes less negative. This change is the generator potential. If it reaches the threshold, an action potential (impulse) is sent. A bigger press gives a bigger generator potential, but the impulse itself is all-or-nothing.
Rods and cones (light)
| Rods | Cones | |
|---|---|---|
| Pigment | rhodopsin, breaks in dim light | iodopsin, needs bright light |
| Colour | no (black and white) | yes (three types: red, green, blue) |
| Wiring | many rods share one neurone | one cone, one neurone |
| Result | high sensitivity, low acuity | high visual acuity (detail), low sensitivity |
When many rods share one bipolar neurone, their small generator potentials add up (summation) to pass the threshold even in dim light — but the brain cannot tell which rod was hit, so the image is less sharp.
Control of heart rate
Heart muscle is myogenic: it starts its own beat. The beat begins at the SAN (sinoatrial node, the pacemaker) in the right atrium. The wave spreads over the atria, reaches the AVN, waits a moment, then goes down the bundle of His and the Purkyne fibres so the ventricles squeeze from the bottom up.
The medulla of the brain changes the rate:
- Chemoreceptors in the aorta and carotid arteries detect pH. Exercise → more CO₂ → lower blood pH → more impulses along the sympathetic nerve → SAN fires faster.
- Baroreceptors detect blood pressure. High pressure → more impulses along the parasympathetic (vagus) nerve → SAN slows down.
This is negative feedback: the change is sensed and the heart works to undo it.
Try it
Choice chamber at home: put a damp paper towel in one half of a box and a dry one in the other. Gently place 10 woodlice (or ants) in the middle, cover with a dark cloth, and count each side every 2 minutes for 10 minutes. Predict first: which side ends up with more? Then let them go outside. In the 3D: on step 5, slowly raise the pressure. At which number does the impulse first fire?
Key formulas and definitions
- Response chain: stimulus → receptor → coordinator → effector → response
- Taxis = directional movement of whole organism; kinesis = change in speed/turning, not direction
- Tropism = directional growth; IAA: shoots — more elongation; roots — less elongation
- Generator potential ≥ threshold → action potential (all-or-nothing)
- Rods: sensitivity (summation); cones: visual acuity
- Heart: SAN → atria → AVN → bundle of His → Purkyne fibres; medulla: sympathetic ↑, vagus ↓
Worked examples
1. Woodlice were placed in a choice chamber with dry and humid sides. After 10 minutes, 16 of 20 were on the humid side. Explain the result.
This is kinesis. In dry air the woodlice moved faster and turned more, so they left dry places quickly. In humid air they slowed down and turned less, so they stayed. Over time most collected on the humid side, which stops them drying out.
2. A shoot tip is lit from the left. Predict and explain what happens.
IAA moves to the shaded (right) side. There IAA makes cells elongate more. The right side grows longer than the left, so the shoot bends to the left, towards the light (positive phototropism).
3. Why can we see in dim light with rods but not read small print?
Several rods connect to one bipolar neurone. Their small generator potentials add up (summation) and pass the threshold, so rods are very sensitive. But because many rods share one neurone, the brain cannot tell exactly which rod was stimulated, so detail (acuity) is low.
4. During a run, blood CO₂ rises. Describe how heart rate changes.
CO₂ lowers blood pH. Chemoreceptors in the aorta and carotid arteries detect this and send more impulses to the medulla. The medulla sends more impulses along the sympathetic nerve to the SAN. The SAN fires faster, so heart rate rises. More blood flow removes CO₂ faster.
Common mistakes
- Mixing up taxis and kinesis. Taxis has a direction; kinesis only changes speed and turning.
- Saying IAA moves towards the light. It moves to the shaded side.
- Saying IAA always speeds growth. In roots, high IAA slows cell elongation.
- Thinking a stronger press makes a bigger nerve impulse. The generator potential grows, but the action potential is all-or-nothing; a stronger stimulus gives more impulses per second.