Movement of Substances through a Cell Membrane - PowerPoint Presentation

Slide1

Movement of Substances through a Cell Membrane

BIOLOGY

Unit

1.1

Double Award

Unit 1.1

How do s

ubstances enter and leave

cells?

Molecules move quickly and at random from a region of high concentration to low concentration down a

concentration gradient

- this is called DIFFUSION.

Diffusion

Here the pink molecules will move from the left to the right until there are the same number of pink molecules on both sides.

Flow

Flow

Diffusion does not require an input of energy… it is a

passive

process.

The orange molecules move in the opposite direction. This is how substances such as oxygen and carbon dioxide enter and leave cells.

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, the cell membrane has to be selectively permeable (allows only some substances to enter and leave).

Osmosis

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

water

sugar

Selectively permeable Membrane

High water concentration

Low sugar concentration

Low water concentration

High sugar concentration

Weak solution

Strong solution

Osmosis

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

water

sugar

Selectively permeable Membrane

High water concentration

Low sugar concentration

Low water concentration

High sugar concentration

Weak solution

Strong solution

Osmosis

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

water

sugar

Selectively permeable Membrane

High water concentration

Low sugar concentration

Low water concentration

High sugar concentration

Weak solution

Strong solution

Practical Work

We 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

Practical Work

Potatoes 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

50mm

5mm

Potato cylinder

The Importance of Osmosis to Cells

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.

Higher

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

.Higher

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.

Higher

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. Higher

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.

Higher

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.