Forces in Cells

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Forces in Cells
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Stimulus and Response

• Why animals and plants do what they do
• OR
• A fancy way of saying cause and effect in the
  animal world.

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What is a stimulus?

• Stimulus any change in an organisms environment
  that causes to the organism to react. It is a
  fancy way of saying cause.
• Stimulus singular
• Stimuli plural
• Example An animal is cold so it moves into the
  sun.
• (Add this word to your vocabulary book)

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What is a response?

• Response how the organism reacts to a stimulus
  and results in a change in behavior. It is a
  fancy way of saying effect.
• Example Getting a drink when you are thirsty.
• (Add this word to your vocabulary book)

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Examples of stimuli and their responses

• You are hungry so you eat some food
• A rabbit gets scared so it runs away
• You are cold so you put on a jacket
• A dog is hot so lies in the shade
• It starts raining so you take out an umbrella

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Plants also respond to their environment. When a
plant grows towards the light we call this
phototropism.
Add this word to your vocabulary. Dont forget
to draw a meaningful picture.
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There are two types of stimuli

• External Stimuli
• and
• Internal Stimuli

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External Stimulus

• External stimulus a stimulus that comes from
  outside an organism, or outside a cell.
• Examples
• 1. You feel cold so you put on a jacket.
• 2. A snake lunges at a rabbit so it runs away.
• 3. A venus fly trap senses a fly and snaps
  shut.
• Add this word to your vocabulary book.

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Internal Stimulus

• Internal stimulus a stimulus that comes from
  inside an organism, or inside a cell.
• Examples
• 1. You feel hungry so you eat some food.
• 2. A cat feels thirsty so it drinks water.
• Add this word to your vocabulary book.

10
Identify each action as an internal or external
stimulus.

• You have a stomach ache and decide to lay down.
• A bird is thirsty and drinks some water.
• A squirrel sees a cat and runs up a tree.
• A lion gets hungry and eats a gazelle.
• You see a spider and scream.

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Plant Responses -- Tropisms

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

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Guess These Tropisms
Chemotropism --
movement or growth in response to chemicals
Geotropism -- (gravitropism)
movement or growth in response to gravity
Hydrotropism --
movement or growth in response to water
Heliotropism --
movement or growth in response to sunlight
Thermotropism --
movement or growth in response to temperature
Thigmotropism --
movement or growth in response to touch or contact
Classify each as an internal or external stimulus.
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Internal Plant Stimulus

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

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On your paper, define the following terms that
apply to most cells. Dont forget to draw a
picture.

• Osmosis --
• Diffusion --
• Active Transport --
• Passive Transport --

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