Transcript CellSizeLimitations_Diffusion

Investigation 5.3
WHEN YOU FINISH YOUR QUIZ:
1) TURN IT IN TO YOUR HW BIN
2) GET OUT YOUR TEXTBOOK
3) OPEN TO PAGES 120-121
4) READ PROCEDURE FOR INV 5.3
Lab Write-Up
 Purpose: To discover the relationship between cell
size and diffusion
 Data Table:
Cube
Dimensions
3x3x3 cm
2x2x2 cm
1x1x1 cm
0.1x0.1x0.1 cm
Surface
Area (cm2)
Volume Surface Area to
(cm3)
Volume Ratio
Diffused
Distance (cm)
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)
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
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
Cell Size Limitations
WHY ARE CELLS SO SMALL?
HINT FOR NOTE-TAKING:
WRITE ONLY WHAT IS
UNDERLINED
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?
 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?
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
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
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.
Think of a mouse and an elephant
 Mice are always cold
 Elephants are too hot
Osmosis and Diffusion
WHAT IS OSMOSIS AND HOW DOES IT
AFFECT CELL DIFFUSION?
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
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