What you'll learn

• How responding to internal and external stimuli increases survival.
• How indoleacetic acid (IAA) controls phototropism and gravitropism in flowering plants.
• How taxes and kineses help mobile organisms stay in favourable conditions.
• How a three-neurone reflex protects the body, and how to investigate animal movement using a choice chamber or maze.

Why response matters

Organisms survive better when they can detect changes and respond appropriately. A change might be external, such as light, humidity, temperature or gravity, or internal, such as a change in blood water potential.

This section is A-level-only, so the focus is on linking the stimulus to the survival value of the response.

Plant responses: growth rather than movement

Flowering plants cannot usually move from place to place, but they can change the direction of their growth. These growth responses are slower than animal movements, but they are still highly adaptive.

A growth factor is a chemical that affects growth. In plants, growth factors are made in growing regions and move to other tissues, where they regulate cell elongation. A meristem is a region of plant tissue where cells divide and growth occurs, such as the tip of a shoot or root.

Important examples are:

• Phototropism: growth response to light.
• Gravitropism: growth response to gravity.

IAA and differential cell elongation

The main growth factor you need here is indoleacetic acid, usually shortened to IAA. It is a type of auxin.

IAA does not have the same effect everywhere. Its effect depends on both its concentration and the organ involved:

• In shoots, higher IAA concentration generally stimulates cell elongation.
• In roots, higher IAA concentration can inhibit cell elongation because roots are more sensitive to IAA.

This difference explains why the same redistribution of IAA can make shoots and roots bend in opposite directions.

The diagram summarises how IAA distribution causes bending in phototropism and gravitropism.

Phototropism in shoots

When light comes from one side, IAA moves to the shaded side of the shoot. In shoots, IAA stimulates elongation, so cells on the shaded side elongate more than cells on the lit side. This makes the shoot bend towards the light.

This is positive phototropism, and it increases the chance of photosynthesis because the leaves and shoot are directed towards light.

Gravitropism in shoots and roots

When a seedling is placed horizontally, gravity causes IAA to accumulate on the lower side of both the shoot and root.

In a shoot, high IAA stimulates elongation on the lower side, so the shoot bends upwards. This is negative gravitropism because the shoot grows away from gravity.

In a root, high IAA inhibits elongation on the lower side. The upper side elongates more, so the root bends downwards. This is positive gravitropism because the root grows towards gravity.

Investigating IAA and root growth

You could investigate the effect of IAA concentration on seedling root growth by growing seedlings in different IAA concentrations, usually measured in mol dm⁻³, and measuring root length after a fixed time.

Good design means you should:

• use a suitable range of IAA concentrations, including a control with no added IAA
• use seedlings of the same species and similar starting size
• keep temperature, light, water availability and time constant
• repeat each concentration and calculate a mean
• measure root growth using the same method each time

Simple responses in mobile organisms

Animals that can move can maintain themselves in favourable environments using simple movement responses. These do not need complex learning or decision-making.

Taxes

In a taxis, the animal’s direction of movement depends on the direction of the stimulus.

Examples include:

• positive phototaxis: movement towards light
• negative phototaxis: movement away from light
• positive chemotaxis: movement towards a chemical

Kineses

In a kinesis, the animal does not move directly towards or away from the stimulus. Instead, the stimulus affects how fast it moves or how often it changes direction.

For example, woodlice may move more rapidly and change direction more often in dry conditions. This increases the chance that they leave the dry area. In damp conditions, they move less and remain there for longer. Over time, more woodlice collect in the damp region.

Required practical 10: animal movement

You need to be able to describe an investigation into how an environmental variable affects animal movement, using either a choice chamber or a maze.

A common setup uses woodlice and humidity. One side of the chamber is made dry using a desiccant such as silica gel, and the other side is made damp using water. The woodlice are placed in the central region, and the number in each side is counted after fixed time intervals.

Variables in a choice chamber

The independent variable is the environmental condition you change, such as humidity or light intensity.

The dependent variable is what you measure, such as the number of animals in each region after 5 minutes.

The control variables are factors kept the same, such as temperature, chamber size, light intensity, number of animals and duration of the investigation.

Improving validity and reliability

To improve the investigation:

• use several animals, not just one
• repeat the experiment and calculate a mean
• swap which side is damp or dry to check for side bias
• allow a short acclimatisation period before recording
• handle animals gently and return them to their habitat afterwards

Reflexes: fast protective responses

Some responses need to happen very quickly. A reflex is rapid, automatic and involuntary, meaning it does not require conscious thought before the response happens.

A simple protective reflex can involve three neurones:

• a sensory neurone, which carries impulses from a receptor to the central nervous system
• a relay neurone, which passes impulses within the central nervous system
• a motor neurone, which carries impulses to an effector

A receptor detects the stimulus. An effector is a muscle or gland that produces the response. A synapse is a junction between neurones where signals pass from one neurone to the next.

You do not need details of the dorsal and ventral roots of the spinal cord for this section.