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Osmosis

Osmosis

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This lesson will explain the process of osmosis and will describe the role of osmosis in our bodies.

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Tutorial

What's Covered

Welcome to today’s lesson on osmosis. In this lesson today, you are going to learn about the process and roles of osmosis. Specifically, today’s lesson will cover:

  1. Osmosis Overview
  2. Concentration
  3. Tonicity
  4. Passive Transport

1. Osmosis Overview

Osmosis is the movement of water across a selectively permeable membrane. When you’re talking about osmosis you’re talking about water. If you think back to diffusion, diffusion is the movement of any substance from an area of high concentration to low concentration. However, when you talk about osmosis, you’re focusing specifically on water moving across a selectively permeable membrane.

Terms to Know

Osmosis

The movement of water across a selectively permeable membrane

Selectively Permeable

A membrane that can regulate which molecules are able to pass through the membrane and which can't

This means the membrane can determine what passes through it. Some membranes are not going to allow solutes to pass. Instead of those solutes being able to pass back and forth to even out concentrations, water is going to have to pass back and forth in order to even out concentrations.

2. Concentration

So the purpose of osmosis is to even solute concentrations across the membrane by moving water. To get a better understanding, take a look at the image below.

There are two beakers, and there is an equal amount of water in each of the beakers. Now, let's say you drop two tablespoons of salt into the beaker on the left (the dots represent molecules of salt). In the one on the left, you have quite a few molecules of salt filling up that water.

Now, let's say you only drop in half a tablespoon into the beaker on the right. You'll notice that you have the same amount of water in each of our beakers, but the one on the left has a significant amount more salt than the one on the right. Therefore, the one on the left would have a higher concentration of salt than the beaker on the right would. Think of concentration as the ratio of solutes to solvent.


3. Tonicity

Tonicity closely relates to concentration. Tonicity describes the concentration of solutes across a membrane.

Term to Know

Tonicity

The concentration gradient of a solute in a solution

When you have a concentration of solutes equal across the membrane, the concentration of solutes on one side of the membrane is equal to the concentration of solutes on the other side of the membrane, you refer to that as being isotonic.

Term to Know

Isotonic

A type of tonicity where there is an equal amount of solute on both sides of a membrane

Most of the time, there is an equal amount of water moving into and out of cells; you're not going to have a huge net movement of water. Most cells generally have equal amounts of water moving into and out of it.

The the side of the membrane that has a higher concentration of solutes is going to be referred to as hypertonic, while the side of the membrane with a lower concentration of solute is going to be hypotonic.

Terms to Know

Hypotonic

A type of tonicity where there is a lower concentration of solutes outside of a cell than inside a cell

Hypertonic

A type of tonicity where there is a higher concentration of solutes outside of a cell than inside a cell


4. Passive Transport

Osmosis is the form of passive transport. What this means is that it does not require the use of ATP, or cellular energy, in order for it to happen. It occurs naturally, because water will be moving from an area of high concentration to an area of low concentration to even out the concentration across a membrane.

Term to Know

Passive Transport

Movement of molecules across a membrane without the use of energy

Try It

QUESTION: You have a cell that has a high concentration of solutes inside of the cell and a lower concentration of solutes outside of the cell. If the solutes cannot pass through the membrane, water needs to move. So which direction would water move in this case to even out the concentration?

ANSWER: Water would move into the cell.

QUESTION: You have a cell that has a high concentration of solutes inside of the cell and a lower concentration of solutes outside of the cell. Which direction is water going to flow if the solutes can't flow back and forth?

ANSWER: Water is going to have to flow out of the cell to even out those concentrations.

Take a look at the examples of cells below to better understand.

For the first cell you'll notice the inside of the cell would be referred to as hypotonic because it has a lower concentration, while the inside of the cell would be hypertonic because it has a higher concentration. Water is going to have to flow into the cell to even out that concentration.

For the second cell, you can see that the outside of the cell is hypertonic, having more solutes than the inside of the cell. Whereas the inside of the cell is hypotonic, having less solutes.

And in the last cell, you'll notice we have the same number of solutes inside and outside the cell. So water is going to move equally into and out of the cell in this case. You’re not going to have a net movement of water-- the same amount of water flowing in as flowing out naturally.

The type of conditions that a cell is in can have an affect on the cell.

IN CONTEXT

Red blood cells are very sensitive to tonicity and solute concentration. When red blood cells are in certain conditions, it's going to affect what the cell looks like. Take a look at the three images of red blood cells below.



If a red blood cell is in a hypotonic solution, water is going to move into the blood cell, and it's going to cause the blood cell to expand.

However, when blood cells are in hypotonic conditions, they will generally explode. If a red blood cell is in an isotonic condition, you have equal amounts of water moving into and out of the cell. This will not have an effect on what that cell looks like.

Finally, if a blood cell is in a hypertonic solution, then there is a higher concentration of solutes outside than inside. So water is going to move out of the cell. This will cause the red blood cell to shrivel up.

Summary

This lesson has been an overview on the process of osmosis, as well as the role of osmosis in our bodies. Specifically you looked at concentration, tonicity, and passive transport. Finally, you got some examples of how the conditions that a cell is in can have an effect on what the cells looks like-- whether it's hypotonic, hypertonic, or isotonic conditions.

Keep up the learning and have a great day!

Source: THIS WORK IS ADAPTED FROM SOPHIA AUTHOR AMANDA SODERLIND

TERMS TO KNOW
  • Osmosis

    The movement of water across a selectively permeable membrane

  • Tonicity

    The concentration gradient of a solute in a solution

  • Isotonic

    A type of tonicity where there is an equal amount of solute on both sides of a membrane

  • Hypotonic

    A type of tonicity where there is a lower concentration of solutes outside of a cell than inside a cell

  • Hypertonic

    A type of tonicity where there is a higher concentration of solutes outside of a cell than inside a cell

  • Selectively Permeable

    A membrane that can regulate which molecules are able to pass through the membrane and which can't

  • Passive Transport

    Movement of molecules across a membrane without the use of energy