Thursday, March 18, 2010


The word transport refers to how things are taken from one place and carry to another. In the diffusion lab, you will observe how cells can take in materials from the environment and also release materials to the environment based on concentration differences without the use of energy or ATP.


is defined as the movement of molecules from an area of high concentration to an area of low concentration and it requires no energy.  In nature, molecules are always vibrating.  When vibrating molecules collide they move in opposite directions and spread apart.

Part 1: 
In class we tested the permeability of a plastic bag (ARTIFICIAL CELL) filled with water, glucose, and starch. In this context, permeability refers to the ability of the plastic bag to allow some substances to go through it while preventing others from doing that.

An indicator is a chemical that changes color when it comes in contact with certain substances. When starch comes in contact with iodine, it turns black.

we placed a bag filled with starch solution and glucose in a beaker containing water and iodine (Iodine is a STARCH INDICATOR). See below.

Initial State

Below is the appearance of the cell a few hours later....
If the iodine diffuses through the membrane (plastic bag) and mixes with the starch solution, then it will turn BLACK!

Final State
The cell membrane is selectively permeable. That is, it has holes in it. It is the size of the holes that determines what goes in and out of the cell.

When starch comes in contact with iodine it turns BLACK. Iodine is a starch indicator.

Ask yourself...
Why did the content of the artificial cell or the plastic bag turned black?
  • The iodine molecules diffused into the artificial cell and turned black when it came in contact with starch.

How come the iodine solution in the beaker did not turn black?
  • The starch molecules are too large and could not diffuse out of the artificial cell or the plastic bag.

Identify the process.
  • The process is diffusion. (NOTE: There was a high concentration of iodine in the beaker and low concentration of iodine inside the plastic bag so the iodine moved from high concentration (in the beaker) to low concentration. (in the plastic bag)
"How would the result be different if the iodine was placed in the bag and the starch and the glucose were placed in the beaker?"
(Drawing the scenario might make it easier to answer the question)

Some substances that diffuse freely across the membrane are oxygen, carbon dioxide and water.

Part 2: OSMOSIS in Red Onion Cells:

is the diffusion of water through a cell membrane. When water goes through the cell membrane, no matter what direction, in or out, it is called "osmosis".

In class we observed osmosis in plant cell by:
  • FIRST putting a drop of water on a slide.
  • Putting a piece of red onion tissue (the outer skin) on top of the water.
  • Placing a cover slip on top of the tissue and press firmly.

When viewed under high power in the microscope, this is what it looks like!

Then to demonstrate osmosis we:

  • Put a drop of salt water on one side of the cover slip
  • Place a piece of paper towel on the other side of the cover slip.

This is how the cells looked after the cells exposure to salt water. Water diffused out of the cell

In another example of Osmosis we reversed the process by:
  • Putting a drop of fresh water on one side of the cover slip.
  • Placing a piece of paper towel on the other side of the cover slip.

Below is a picture after the water diffused back into the cell.

Observing Osmosis in Elodea
With Fresh Water

Magnified 400 X

With Salt Water

Magnified 1000 X

Osmosis in White Onions

Fresh water

400 X

With Salt water

400 X

Elodea Osmosis Lab Power Point



is defined as a series of processes that help maintain an organisms stable internal environment.

is the movement of particles from an area of greater concentration to an area of lesser concentration and it requires no energy. During diffusion, particles continue to spread until they are evenly distributed.

Is the diffusion of water through a cell membrane.

QUICK REVIEW! NOTE: The cell membrane also controls what goes in and out of the cell by active transport, using special transport proteins.

Active Transport:

is the movement of particles from an area of low concentration to an area of high concentration and it requires energy (ATP).


Let us review to other cellular processes, photosynthesis and cell respiration.
"An easy way to remember the formula for photosynthesis and cell respiration"

-What do plants need? (Plants need water, carbon dioxide and light)

-What do plants make? (Glucose and oxygen)

The Formula for photosynthesis 

Water + CO2 + light, change to glucose + O2 (photosynthesis takes place inside the chloroplast)

To obtain the formula for cell respiration
-Reverse the formula for photosynthesis and replace light with ATP

The formula for cell respiration:
glucose + O2 change to H2O +CO2 +ATP (cell respiration takes place inside the mitochondria)

All of the chemical reactions that take place inside an organism require the assistance of enzymes.

-Enzymes are biological catalyst that speed up the rate of chemical reactions without becoming part of the final product. 

-Enzymes are proteins and all proteins have specific shape. They interact with molecules called substrates. They must fit the substrate in order to make reaction go forward


Each chemical reaction that takes place in the body requires a different enzyme.

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