Stimulus & Responses

StimulusA change in organism’s

environmental conditions which can affect the organism in either a positive or negative way.

Types of Stimuli▪ A stimulus can either be external or internal.

▪ External stimuli- come from factors found in the environment.Examples- temperature, predators, presence

of water or food, etc▪ Internal stimuli- come from factors within an

organismExamples- germs, dehydration, lack of

energy, pain

Stimulus

Is this stimulus external or internal?

ResponseAn organisms reaction to change.

Response

HomeostasisHomeo- sameStasis- state of balance

The purpose of response to stimuli in organisms is to maintain homeostasis, a balance within it’s internal systems.

Examples

▪ Stimulus = Is this internal or external?▪ Response =

Lack of water

Wilting

Tropisms▪ Tropism = plant’s growth response

toward or away from a stimulus

Tropisms▪ Tropism = plant’s growth response

toward or away from a stimulus

▪ Positive tropism

▪ Negative tropism

Tropisms▪ Tropism = plant’s growth response

toward or away from a stimulus

▪ Positive tropism = plant grows toward a stimulus

▪Negative tropism

Tropisms▪ Tropism = plant’s growth response

toward or away from a stimulus

▪ Positive tropism = plant grows toward a stimulus

▪Negative tropism = plant grows away from a stimulus

Hydrotropism

Hydrotropism = a plant’s roots grow toward water

Is this positive or negative?

PhototropismPlants grow toward a source of light

Examples

ThigmotropismA plant grows toward or away from pressure

Positive turns toward pressure

Negative turns away from pressure

Geotropism▪ A plant grows with or against gravity▪ Also called Gravitropism

Negative- A plant’s stems or trunk grow up, against the pull of gravity.

Positive- A plant’s roots grow down, with the pull of gravity

Regardless of the slope of the land, the tree grows up against gravity.

Regardless of the position a seed is planted in, the roots grow down with gravity and the stem up against gravity.

Dormancy▪ When plants can go through a period of

inactivity. During this time, the seeds wait to germinate until conditions are favorable for growth and development of a new plant. ▪ For example: The seeds of a longleaf

pine tree must be exposed to fire (heat) before they will germinate and grow.

Working Together

▪ Plant tropisms work together to maintain homeostasis within the plant

- geotropism

+ phototropism

+ geotropism

+ hydrotropism