How this reading relates to what we are doing in class
• Science is not just about facts (verifiable observations and measurements).
• The POWER lies in how those facts come together in a story to explain how and why a natural phenomenon works the way it does.
• We are constructing those stories using facts
(or evidence) to construct evidence-based explanatory models of how cells work.

How this reading relates to what we are doing in class
• Uncertainty is a BIG part of science!
• Scientists are out to discover the unknown.
– Make hypotheses (educated guesses)
– Gather information to test those hypotheses
– Make revisions of hypotheses that include the new information
• We are doing this in our online discussions, in our reading guides, in our explanatory models, and even in our quizzes!!!

Unit #2: Flow of Energy through a Cell!
– http://www.youtube.com/watch?v=At7EltHApyE
– http://www.americanheart.org/presenter.jhtml?identifier=3053
– http://www.cdc.gov/heartdisease/facts.htm
– Whiteboards (10 min)
– Whole-class discussion (5 min)
– Any questions that we have about how energy flows through a cell

– Working cells control the transport of materials to and from the environment with membranes.
Transport of materials

– A closer look at our membranes
Hydrophobic region
Hydrophilic region

• Many small non-polar molecules can pass through
(or permeate) by diffusion
– Oxygen (O
2
), Carbon Dioxide (CO
2
)
– Water (H
2
O), although polar, is small enough to pass through
• Other charged and larger polar molecules cannot pass through (or permeate) and need membrane transport processes to do so.
– Ions: K+, Na+, H+
– Small hydrophilic molecules like glucose, amino acids, nucleotides
– Macromolecules like proteins and RNA
• Therefore, our cellular membranes are selectively permeable!!!

– Molecules contain heat energy.
• They vibrate and wander randomly - Brownian
Motion. http://sv.berkeley.edu/chemicalinteractions/menu.html
– Diffusion is one result of the movement of molecules.
• Molecules tend to spread into the available space.
• Diffusion is passive transport; no energy is needed.
• Water and small non-polar molecules like O2, CO2 travel across cell membranes through passive diffusion.
http://highered.mcgraw-hill.com/sites/0072495855/student_view0/chapter2/animation__how_diffusion_works.html

[solute]
– Osmosis is the passive transport of water across a selectively permeable membrane.
[solute]
= [solute]
[water]
= [water]
[water]
– http://highered.mcgraw-hill.com/sites/0072495855/student_view0/chapter2/animation__how_osmosis_works.html

– Osmoregulation is the control of water balance.
• Sodium-potassium pump essential to regulate cell volume through control of osmosis in many animal cells.
– Water balance in plant cells is different.
• They have rigid cell walls.
• They are at the mercy of the environment.
Turgid Flaccid

Lab 3 - Osmosis and Diffusion
• Here we will be going through an exercise of hypothesis-driven science.
• Brownian motion
– http://sv.berkeley.edu/chemicalinteractions/menu.html
• Selectively Permeable Membrane
– Hypothesis - educated guess, an idea you can test
– Making Predictions:
• If …[organize what you think you know and how you are testing your idea]
• Then …[predict what experimental result you would get]
• Because …[general principles; Ex: diffusion,membranes]
– Diffusion
• http://highered.mcgraw-hill.com/sites/0072495855/student_view0/chapter2/animation__how_diffusion_works.html

Lab 3 - Osmosis and Diffusion
• We will go through this on Tuesday!
• Constructing a scientific argument
– Re-state your claim/hypothesis
– Support or refute it with evidence or counterevidence from
• Verifiable observations,
• Verifiable measurements, and/or
• Reliable resources, other people’s data
– If claim/hypothesis refuted, state an alternative hypothesis

Lab 3 - Osmosis and Diffusion
• Osmosis evidence - the Egg!
– http://highered.mcgraw-hill.com/sites/0072495855/student_view0/chapter2/animation__how_osmosis_works.html
• Hypothesis:
– The solution that is hypertonic relative to the egg will…
– The solution that is hypotonic relative to the egg will…
• Prediction:
– If …[organize what you know and how you are testing your idea]
– Then …[predict your experimental result - what data will you actually have?]
– Because …[general principles about osmosis]

Lab 3 - Osmosis and Diffusion
• We will go through this on Tuesday!
• Constructing a scientific argument
– Re-state your claim/hypothesis
– Support or refute it with evidence or counterevidence from
• Verifiable observations,
• Verifiable measurements, and/or
• Reliable resources, other people’s data
– If claim/hypothesis refuted, state an alternative hypothesis

Lab 3 - Osmosis and Diffusion
– View of Elodea cells