Author(s)

Enrique Hueyopa

Question

What is the effect of temperature on the rate diffusion of Iodine through a semipermeable membrane?

Standards

Investigation and Experimentation
    1.Scientific progress is made by asking meaningful questions and conducting careful
    investigations. As a basis for understanding this concept and addressing the content
    in the other four strands, students should develop their own questions and perform
    investigations. Students will:
        a.Select and use appropriate tools and technology (such as computer-linked
        probes, spreadsheets, and graphing calculators) to perform tests, collect data,
        analyze relationships, and display data.
        b. Identify and communicate sources of unavoidable experimental error.
        c. Identify possible reasons for inconsistent results, such as sources of error or
        uncontrolled conditions.
        d. Formulate explanations by using logic and evidence.

Experimental Design

Independent variable

The temperature variation of the iodine solution.

Dependent variables

The rate of diffusion of Iodine.

Series

Room temperature

Heated solution

Controls

semipermeable membrane

amount of iodine, water, starch used

containers, apparatus, probes

method

Materials

large plastic containers (able to hold more than 1 liter)

fruits and vegetable grocery plastic bags

eye dropper

graduated cylinder

distilled water

string

light sensors, temperature sensors, usb link adapters

Datastudio software

Procedures

Semipermeable membrane preparation

•  cut a grocery plastic bag to 25 cm
• cut two strings (one at 15cm and the other at 50 cm)
• fold the bottom (the side of the bag that is sealed) portion of the bag into thirds and tie with the short string (15cm)
• prepare a mixture of 500 ml distilled water and 20 g of starch
• pour the solution into the plastic bag and tie the open end with the 50 cm string 

iodine solution preparation

• pour 1000 ml of distilled water into the container
• pour 2 ml of iodine into the container

Experiment run

• place container in a microwave for two minutes
  
• place bag in the container with the long string hanging out of the container
• place probe into the solution and start the Data studio program and run

References

http://hyperphysics.phy-astr.gsu.edu/hbase/Kinetic/diffus.html
http://www.biologycorner.com/bio1/diffusion.html

Diffusion - the process by which molecules spread from areas of high concentratiion, to areas of low concentration. When the molecules are even throughout a space - it is called EQUILIBRIUM

Concentration gradient - a difference between concentrations in a space.

OSMOSIS Watch this animation of water molecules moving across a selectively permeable membrane. Water molecules are the small blue shapes, and the solute is the green. The solute is more concentrated on the right side to start with, which causes molecules to move across the membrane toward the left until equilibrium is reached. Start Animation

Selectively Permeable - membranes that allow some things through, the cell membrane is selectively permeable, water and oxygen move freely across the cell's membrane, by diffusion

Osmosis - the diffusion of water (across a membrane)

Water will move in the direction where there is a high concentration of solute (and hence a lower concentration of water.

A simple rule to remember is:

Salt is a solute, when it is concentrated inside or outside the cell, it will draw the water in its direction. This is also why you get thirsty after eating something salty.

Type of Solutions

	If the concentration of solute (salt) is equal on both sides, the water will move back in forth but it won't have any result on the overall amount of water on either side. "ISO" means the same
	The word "HYPO" means less, in this case there are less solute (salt) molecules outside the cell, since salt sucks, water will move into the cell. The cell will gain water and grow larger. In plant cells, the central vacuoles will fill and the plant becomes stiff and rigid, the cell wall keeps the plant from bursting In animal cells, the cell may be in danger of bursting, organelles called CONTRACTILE VACUOLES will pump water out of the cell to prevent this.
	The word "HYPER" means more, in this case there are more solute (salt) molecules outside the cell, which causes the water to be sucked in that direction. In plant cells, the central vacuole loses water and the cells shrink, causing wilting. In animal cells, the cells also shrink. In both cases, the cell may die. This is why it is dangerous to drink sea water - its a myth that drinking sea water will cause you to go insane, but people marooned at sea will speed up dehydration (and death) by drinking sea water. This is also why "salting fields" was a common tactic during war, it would kill the crops in the field, thus causing food shortages.

Diffusion and Osmosis are both types of PASSIVE TRANSPORT - that is, no energy is required for the molecules to move into or out of the cell.

Sometimes, large molecules cannot cross the plasma membrane, and are "helped" across by carrier proteins - this process is called facilitated diffusion.

Go to notes on ACTIVE TRANSPORT