Forces in Cells

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

What is a stimulus? Stimulus: any change in an organism’s 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)

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)

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

Plants also respond to their environment. When a plant grows towards the light we call this phototropism. Add this word to your vocabulary. Don’t forget to draw a meaningful picture.

There are two types of stimuli: External Stimuli and Internal Stimuli

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.

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.

Identify each action as an internal or external stimulus. A. You have a stomach ache and decide to lay down. B. A bird is thirsty and drinks some water. C. A squirrel sees a cat and runs up a tree. D. A lion gets hungry and eats a gazelle. E. You see a spider and scream.

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". Let’s look at some other 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". Let’s look at some other tropisms.

Guess These Tropisms ChemotropismChemotropism - - Chemotropism movement or growth in response to chemicals chemicals GeotropismGeotropism -- (gravitropism) Geotropism movement or growth in response to gravity gravity HeliotropismHeliotropism -- Heliotropism movement or growth in response to sunlight sunlight HydrotropismHydrotropism -- Hydrotropism movement or growth in response to water ThermotropismThermotropism -- Thermotropism movement or growth in response to temperature temperature ThigmotropismThigmotropism -- Thigmotropism movement or growth in response to touch or contact touch contacttouch contact Classify each as an internal or external stimulus.

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 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

On your paper, define the following terms that apply to most cells. Don’t forget to draw a picture. Osmosis -- Osmosis -- Diffusion -- Diffusion -- Active Transport -- Active Transport -- Passive Transport -- Passive Transport -- Homework: Chapter 5 workbook pages, section 1 (6 th grade book).