Membranes

Here’s a fun experiment that shows you how much stuff can pass through a membrane. Scientist call it the  semi-permeability of membranes.

Before we start, take out your science journal and answer this question: What do you think will happen when we stick a piece of celery into a glass of regular water. Anything special?

What if we add a teaspoon of salt to the water? Now do you think anything will happen?
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Let’s find out. First, you’ll need:

• 2 pieces of celery stalk
• salt
• 2 glasses
• a sensitive scale to weigh the celery

Note: If you don’t have a scale, you can rig up a balance by suspending two cups from a either end of of a pencil. Balance the center on a point (like another pencil) and you’ll be able to tell which is heavier at the end of this experiment (see image below).

You’ll need to measure the celery both before and after this experiment since you won’t be able to “read” the weight.

Trim your celery to be the same weight before you start your experiment.

Download Student Worksheet & Exercises

1. First, weigh the celery (both pieces) and record this in your journal.
2. Next, make your hypotonic solution (plain water). Fill a glass with water and stick your celery in for ten minutes.
3. Remove the piece of celery and pat dry. Weight it again and record your results. If you don’t see a weight difference, dip it in again for ten more minutes. Pat dry and weigh again.
4. Now make your hypertonic solution (salt water). Add a small amount of salt to the water (keep adding until no more can be dissolved and a small amount remains on the bottom).
5. Weight the second celery stalk and record it in your journal. Add this new celery stalk to the water. Wait impatiently for ten minutes. Remove and record the weight. Did you notice a difference? (Note – if you left the first one in for 20 minutes, make sure to leave this one in for the same amount of time.)

What effect did the salt solution have on the celery?  Did it change in appearance?  Did it feel different? Record your results in your journal!

Osmosis is the diffusion of water molecules through a membrane, where the water molecules move from high water concentration to areas of low water concentration.

Salt starts osmosis by attracting water and causing the water to move toward and across the membrane. Remember that salt is a solute, and when water is added to a solute, it spreads out (diffuses) the concentration of salt, and that creates a chemical solution.

Imagine the salt concentration inside a cell being the exact same as the salt concentration outside the cell – what would happen? Right – the water level will stay the same and nothing would happen. Now imagine there’s more salt inside of a cell than outside it. What happens now? The water moves through the membrane into the cell causing it to swell with water.

If the cell is placed in a higher concentration of salt (like sticking a carrot in a salt bath), the water will leave the cell, and that’s why the plant cells shrink and wilt. This is also why salt kills plants, becaise it takes water from the cells. This doesn’t just happen in plants, though. Animals can also get dehydrated if they drink ocean water.

Exercises

1. In what direction does water move?
2. What is the process by which water crosses membranes by itself?
3. What are all living things made of?
4. Did the celery in the fresh water weigh more or less? Why?
5. Did the celery in the salt water weigh more or less after a few minutes?

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