Forces in CellsForces in Cells

Stimulus and ResponseStimulus and Response

Why animals and plants do what they Why animals and plants do what they do do

OROR

A fancy way of saying A fancy way of saying causecause and and effecteffect in the animal world. in the animal world.

What is a stimulus?What is a stimulus?

Stimulus:Stimulus: any change in an any change in an organism’s environment that organism’s environment that causes to the organism to react. causes to the organism to react. It is a fancy way of saying It is a fancy way of saying “cause”.“cause”.

StimulusStimulus – singular – singular StimuliStimuli – plural – plural

Example: An animal is cold so it moves into the sun.Example: An animal is cold so it moves into the sun.

(Add this word to your vocabulary book)(Add this word to your vocabulary book)

What is a response?What is a response?

Response:Response: how the organism how the organism reacts to a stimulus and results reacts to a stimulus and results in a change in behavior.in a change in behavior. It is a It is a fancy way of saying “effect”.fancy way of saying “effect”.

Example: Getting a drink when you are Example: Getting a drink when you are thirsty.thirsty.

(Add this word to your vocabulary book)(Add this word to your vocabulary book)

Examples of stimuli and their Examples of stimuli and their responses:responses:

• You are hungry so you eat some foodYou are hungry so you eat some food

• A rabbit gets scared so it runs awayA rabbit gets scared so it runs away

• You are cold so you put on a jacketYou are cold so you put on a jacket

• A dog is hot so lies in the shadeA dog is hot so lies in the shade

• It starts raining so you take out an It starts raining so you take out an umbrellaumbrella

Plants also respond to their Plants also respond to their environment. When a plant environment. When a plant grows towards the light we call grows towards the light we call this this phototropismphototropism..

Add this word to your vocabulary. Don’t forget to draw a meaningful picture.

There are two types of There are two types of stimuli:stimuli:

External StimuliExternal Stimuli

andand

Internal StimuliInternal Stimuli

External StimulusExternal Stimulus

External stimulus: a stimulus that External stimulus: a stimulus that comes from outside an organism, comes from outside an organism, or outside a cell.or outside a cell.

Examples: Examples: 1. You feel cold so you put on a jacket.1. You feel cold so you put on a jacket.2. A snake lunges at a rabbit so it runs away.2. A snake lunges at a rabbit so it runs away.3. A venus fly trap senses a fly and snaps shut.3. A venus fly trap senses a fly and snaps shut.

Add this word to your vocabulary book.Add this word to your vocabulary book.

Internal StimulusInternal Stimulus

Internal stimulus:Internal stimulus: a stimulus that a stimulus that comes from inside an organism, comes from inside an organism, or inside a cell.or inside a cell.

Examples: Examples: 1. You feel hungry so you eat some food.1. You feel hungry so you eat some food.2. A cat feels thirsty so it drinks water.2. A cat feels thirsty so it drinks water.

Add this word to your vocabulary book.Add this word to your vocabulary book.

Identify each action as an internal or Identify each action as an internal or external stimulus. external stimulus.

A.A. You have a stomach ache and decide to You have a stomach ache and decide to lay down.lay down.

B.B. A bird is thirsty and drinks some water.A bird is thirsty and drinks some water.

C.C. A squirrel sees a cat and runs up a tree.A squirrel sees a cat and runs up a tree.

D.D. A lion gets hungry and eats a gazelle.A lion gets hungry and eats a gazelle.

E.E. You see a spider and scream.You see a spider and scream.

Plant Responses -- TropismsPlant Responses -- Tropisms

• Although plants cannot uproot themselves Although plants cannot uproot themselves in search of food and water, they can make in search of food and water, they can make tiny, slow movements in response to stimuli tiny, slow movements in response to stimuli of their environment (tropisms). They make of their environment (tropisms). They make these movements from signals in individual these movements from signals in individual cells. We have already discussed cells. We have already discussed phototropism. “Photo” means “light” and phototropism. “Photo” means “light” and “trop” means "turn, turning" or "tendency, “trop” means "turn, turning" or "tendency, affinity". Let’s look at some other tropisms.affinity". Let’s look at some other tropisms.

Guess These TropismsGuess These Tropisms

Chemotropism --

movement or growth in response to movement or growth in response to chemicals

Geotropism -- -- (gravitropism)(gravitropism)

movement or growth in response to movement or growth in response to gravity

Heliotropism --

movement or growth in response to movement or growth in response to sunlight

Hydrotropism -- movement or growth in response to movement or growth in response to waterwater

Thermotropism -- -- movement or growth in response to movement or growth in response to temperature

Thigmotropism -- -- movement or growth in response to touch or movement or growth in response to touch or contactcontact

Classify each as an internal or external stimulus.

Internal Plant StimulusInternal Plant Stimulus

• Turgor PressureTurgor Pressure – is a force exerted – is a force exerted outward on a plant cell wall by the outward on a plant cell wall by the water contained in the cell vacuole. water contained in the cell vacuole. This force gives the plant rigidity, and This force gives the plant rigidity, and keeps it standing up straight. Low keeps it standing up straight. Low turgor pressure results in a wilted turgor pressure results in a wilted plant. Turgor pressure of a cell is plant. Turgor pressure of a cell is largely maintained by largely maintained by osmosisosmosis, across , across the cell membrane the cell membrane

On your paper, define the On your paper, define the following terms that apply to following terms that apply to most cells. Don’t forget to most cells. Don’t forget to draw a picture.draw a picture.• Osmosis -- Osmosis --

• Diffusion -- Diffusion --

• Active Transport -- Active Transport --

• Passive Transport -- Passive Transport --

Homework: Chapter 5 workbook pages, section 1 (6th grade book).