The study of immunology is essential within Biology Topics to understand the immune system.
How do we Respond to Stimuli?
All the living things respond to changes around them. The changes in surroundings to which living things respond are called stimuli (‘stimuli’ is the plural of ‘stimulus’). The living things show response to external stimuli such as heat, light, touch, sound, smell, taste, water and chemicals. The response of living things is usually in the form of some movement of their body part.
For example, if a man touches a very hot object accidentally, he quickly pulls his hand away from the hot object. In this case, the stimulus is heat and the man responds by moving his hand away from the hot object. If a barefooted man steps on a sharp object like a thorn (or a nail), he quickly pulls away his foot from the sharp object. In this case, the stimulus is pain (produced by sharp object) and the man responds by moving his foot away from the sharp object.
If we move from a dark place into bright sunshine suddenly (or switch on a bright light), then our eyes shut themselves automatically for a moment (till they adjust to the changed bright surroundings). In this case, bright light is the stimulus and we respond by shutting the eyes momentarily. If we see our favourite food, our mouth starts watering. In this case, the sight of favourite food is the stimulus and watering of mouth is our response to it.
Other animals also respond to stimuli (or changes in their surroundings). For example, when we move towards a bird, the bird flies away from us. In this case, fear is the stimulus and the bird responds by moving away. Similarly, birds fly away when they hear a loud sound. In this case loud sound is the stimulus and the birds respond by moving away. Some animals are attracted by light and hence go towards a source of light. On the other hand, there are certain animals which do not like light and hence go away from it. For example, moths and certain beetles are attracted by light. We can see these moths and beetles flying around a street light at night, especially during rainy season. In this case, the stimulus is light and the moths and beetles respond by moving towards the source of light.
On the other hand, the animals like cockroach and earthworm do not like light. The cockroach and earthworm always try to go away from light to darkness. For example, when the light in the kitchen is switched on at night, then the cockroaches begin to run to their places of hiding in the kitchen (where light cannot reach). In this case also, the stimulus is light but the cockroaches respond by moving away from it. Similarly, wild animals run away when bright light is flashed towards them. Here also the stimulus is bright light and the wild animals respond by running away from it.
Plants also respond to changes around them. In other words, plants also respond to stimuli. Plants respond to stimuli such as touch, sunlight, gravity, water and chemicals, etc. This will become clear from the following examples. We will first discuss the case of Mimosa plant which is commonly known as “sensitive.
plant” or “touch-me-not” plant. A Mimosa plant is shown in Figure (a). If we touch the leaves of a Mimosa plant with our fingers, then its leaves fold up and droop [see Figure (b)]. In this case, the “touch” of finger is the stimulus and the Mimosa plant responds by folding its leaves.
We will now describe the response of plants to light (or rather sunlight). If a potted plant is kept in the open space, the stem of plant grows straight up towards the source of light “sun” [see Figure (a)].
Let us keep the potted plant having straight stem near the window in a dark room so that sunlight falls on it from the right side only (through the window). After some days we will see that the stem of plant bends towards the right side from where the light is coming [see Figure (b)], From this we conclude that the stem of a growing plant bends towards light. In this case, the stimulus is sunlight and the plant responds by bending its stem towards it. The plant stems bend towards sunlight because the leaves need sunlight to make food.
The flowers of some plants open up in the morning in bright light but close after sunset when it gets dark. For example, the flowers of dandelion plant open up in the morning in response to light but close after sunset when it becomes dark (see Figure).
In some plants, however, the flowers close during the day time and bloom (open up) only at night. For example, the petals of moonflower close during the day when there is bright light but open up at night when it is dark and there is no light (see Figure). Non-living things do not respond to changes in their surroundings. For example, a rock is a non-living thing which does not respond to stimuli like heat, light, touch, sound and smell, etc.