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Investigation 5.3

This lab investigates the relationship between cell size and diffusion rates using agar cubes of different sizes and Sodium Hydroxide as the diffusion substance. The lab focuses on measuring diffusion distances and discussing the limitations of cell size. The concepts of diffusion, volume vs. surface area, and osmosis are explored.

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Investigation 5.3

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  1. Investigation 5.3 When you finish your quiz: Turn it in to your HW bin Get out your Textbook Open to pages 120-121 Read procedure for inv 5.3

  2. Lab Write-Up • Purpose: To discover the relationship between cell size and diffusion • Data Table:

  3. Lab Procedure • We will use “cells” of different sizes to test diffusion rates • Cells are made of agar • Agar comes from seaweed: it is the main substance in Jello • How will we test diffusion? • Agar also contain phenolphthalein solution • What is phenolphthalein? • It is an indicator for the presence of a base • Changes color in the presence of a base • What will we use as the diffusion substance? • A base: Sodium Hydroxide (NaOH)

  4. Lab Write-Up • Hypothesis: Which cube size will be the best at diffusing chemicals? Why? • Cube size ____ will be best at diffusing chemicals because… • Discussion: • Questions 1,2,4,7,8 (pg. 121) • Conclusion: • Parts A-E • Side note: You only get ONE CUBE of agar per group! • Old Carpenter’s saying: Measure TWICE, cut ONCE

  5. When you are done: • Turn your quiz into the HW bins • Switch lab books with your partner • Grade your partner’s Investigation 5.3 Write-Up • Make sure both your names and the total points (out of 10) are on the top • Open textbooks to pg 119: Investigation 5.2 and pre-read the procedure

  6. Cell Size Limitations Why are cells so small? Hint for Note-taking: write only what is underlined

  7. Cell Size and Diffusion • ALL cells had a diffusion distance of around 0.5 cm • This took about 10 minutes • Would this work for an actual cell?

  8. Most cells are between 2µm and 200µm • A micrometer is 1 millionth of a meter! • Too small to be seen with naked eye • Why can’t organisms be one big giant cell?

  9. Diffusion (passive transport) • Particles (molecules and atoms) move from HIGH concentration to a LOWER concentration. • This is random movement. • The cell does not have to work or expend energy, so no ATP is used. • Diffusion in Action

  10. Limits to Cell Size • Diffusion limits cell size • Movement of food or wastes is from higher concentration to lower concentration • The larger the distance, the slower the diffusion rate • A cell 20 cm across would require months for nutrients to get to the center

  11. Volume vs. Surface Area • As an object like a cube gets bigger, volume increases more rapidly than surface area. • When the cell size doubles (for example from 1 cm3 to 2 cm3), there will be 8x as much volume as before, but only 4x as much surface area as before.

  12. Think of a mouse and an elephant • Mice are always cold • Elephants are too hot

  13. Osmosis and Diffusion What is osmosis and how does it affect cell diffusion?

  14. What is Osmosis? • Osmosis: Diffusion of water across a membrane • Different types of membranes will allow some, none, or all particles to move across them • When water moves across, this is osmosis

  15. Types of Membranes • Permeable: Allows particles to pass through in either direction • Semi-Permeable: Only allows some particles to pass through • Impermeable: Does not allow particles to pass through • Differentially Permeable: Allows particles to pass through in only ONE direction based on osmotic differential

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