Movement of Substances through a Cell Membrane

Movement of Substances through a Cell Membrane

BIOLOGY 2

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How do Substances Enter and Leave Cells?

Molecules move quickly, at random, through a liquid – this is called DIFFUSION. Substances move from a region of high to low concentration.

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Here the pink molecules will move from the left to the right until there are the same number of pink molecules on both sides. The orange molecules move in the opposite direction. This is how substances such as oxygen and carbon dioxide enter and leave cells.

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The Following Factors Affect the Rate of Diffusion:

1. Concentration – the larger the difference between the concentration of molecules, the faster they will diffuse.

2. Temperature – the higher the temperature, the faster diffusion of the molecules.

3. Pressure – the higher the pressure on the molecules, the faster the movement (from the region of higher pressure to the region of lower pressure)

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The Effect of the Cell Membrane on Diffusion

If a cell membrane was totally permeable (allows everything in and out), the cell would die in no time. Even though useful substances would enter the cell easily, the cell’s content would diffuse out. In order to survive therefore, the cell membrane has to be selectively permeable (allows only some substances to enter and leave)

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OsmosisHere we can see the water diffusing through a selectively permeable membrane, from a region of high concentration to a region of lower water concentration.

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Selectively permeable Selectively permeable MembraneMembrane

High water concentrationHigh water concentrationLow sugar concentrationLow sugar concentration

Low water concentration Low water concentration High sugar concentrationHigh sugar concentration

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Weak solution Strong solution


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Practical WorkWe can use a special plastic called Visking tubing to demonstrate osmosis.

Here we can see that the water molecules can move through the tubing but the sucrose molecules are prevented. The reason for this is to do with the size of the molecules. The sucrose molecule is larger than the water and cannot fit through the tiny holes in the wall of the tubing. Water can move into the sucrose solution, but the sucrose cannot go into the water.

After about half an hour, the solution’s level in the tube will have risen, and the water level will have dropped.

water

Visking tube

sucrose solution

capillary tube

tight knot

sucrose solution level

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Practical WorkPotatoes can be used to demonstrate osmosis in living tissue.

Potato cylinders of the same length and diameter are each placed in different concentrations of sucrose solutions for 20 minutes.

The length and mass of each cylinder should then be re-measured. The potatoes that have been in strong sucrose solutions will have reduced in length and mass, whilst the ones placed in weak sucrose solutions will have gained length and mass.

Cylinders showing no change will have cells with cytoplasm of equal strength to the sucrose solution that they were placed in.

Distilled Water 0.1M 0.2M

0.5M 1M

Labelled Petri dishes

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5mm

Potato cylinder

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The Importance of Osmosis to Cells

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If a red blood cell was in a solution containing less water than itself (hypertonic), then the cell would lose water. It would shrivel and die.

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Homeostasis

It is very important therefore to keep the blood constant so that the cells in it do not gain or lose too much water.

The term for keeping these internal conditions constant is Homeostasis.

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The importance of Osmosis to Cells

If a red blood cell was in a solution that had more water in it than was in the cell (hypotonic), then it would gain water. It would expand, burst and die.

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Active Transport

In many cases substances are moved into or out of cells against a diffusion gradient i.e. opposite to the movement of the molecules.

Energy is needed for this process, and so it’s called active transport.

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Examples of Active Transport

In the body we see active transport in the small intestine, when digested food is absorbed into the blood in the opposite direction to diffusion.

In a plant, rare minerals from the soil are pulled into the roots opposite to the direction of diffusion.

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Summary

Diffusion is the movement of particles down a concentration gradient.

Osmosis is the diffusion of water molecules through a selectively permeable membrane.

Active transport is the movement of molecules against a concentration gradient – this requires energy.

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Documents

Movement of Substances through a Cell Membrane