Diffusion Iodine and Starch Reaction

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watch?v=2Th0PuORsWY&f

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1. What caused the color change you observed?

• Starch solution inside the dialysis tubing

• Water outside the dialysis tubing

• Iodine added to the water• --- REACTION! Color

change! How?

2. What was able to diffuse across the membrane?

• Starch solution changed color• Water in the beaker didn’t have a reaction• What can you conclude:

– About the movement of starch molecules?– About the movement of iodine molecules?– About the movement of water molecules?

What molecules were able to diffuse in/out the membrane?

• Water• Iodine• Why couldn’t

starch move acrossthe membrane?

3. Was the beaker of water hypertonic or hypotonic compared to the starch solution in

the tubing?• Hypertonic = more solute (high concentration) • Hypotonic = less solute (low concentration)

• What’s a solute? – The substance you’re dissolving (starch)

• What’s a solvent? – The substance you’re dissolving the solute IN (water)

3. Where is the highest concentration of solute? The lowest concentration?

4. Why was the weight of your tubing different after adding iodine to the water?

• Water moves from high low concentration

• Water moved into the dialysis tubing due to … OSMOSIS!

• Would water still move across the membrane without using iodine?

How does the tubing represent a cell membrane?

• Selectively (semi) permeable – only certain molecules can cross the membrane– Water, iodine, NOT starch

• Normally, cells exist in isotonic solution– What’s isotonic? What’s the solution?

• Osmosis can cause cells to shrivel up, swell, or even burst!

How can water cause a cell to burst?• When cells are in a “freshwater” solution• Water rushes in, bursting the cell

What about plant cells?• What do plant cells have that animal cells

do not? • Notice the cell wall does not collapse (or

burst), but the cell membrane can collapse

What allows water molecules in/out of the cell?

• Aquaporins – water channel proteins– Type of facilitated diffusion, because it requires

“help” from a specific channel protein• Special protein channels are embedded in

the lipid bilayer – a double-layered sheet that forms a strong, flexible barrier called the …

• YOU GUESS IT! CELL MEMBRANE!

The Lipid Bilayer

2 sheets of lipids make the lipid bilayer

-- PHILIC = loving -- PHOBIC = hating

Why is the hydrophilic portion on the outside and the hydrophobic portion on the inside of

the two sheets?

Recap -• Osmosis is… movement

of WATER molecules from high to low concentration– This creates osmotic

pressure - a force produced by the net movement of water

• Diffusion is… movement of molecules from high to low concentration

Types of Transport• ACTIVE TRANSPORT• Moves materials

against concentration gradient

• Requires energy!• Endocytosis - import

– Phagocytosis• Exocytosis - export

PASSIVE TRANSPORT• Occurs due to

concentration gradient

• Requires no energy• Diffusion• Osmosis