Movement of Substances through a Cell Membrane BIOLOGY 2 GCSE Additional Science Chapter 3

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. GCSE Additional Science Chapter 3

GCSE Additional Science 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. Flow GCSE Additional Science Chapter 3 Flow

The Following Factors Affect the Rate of Diffusion: Concentration – the larger the difference between the concentration of molecules, the faster they will diffuse. Temperature – the higher the temperature, the faster diffusion of the molecules. Pressure – the higher the pressure on the molecules, the faster the movement (from the region of higher pressure to the region of lower pressure) GCSE Additional Science Chapter 3

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) GCSE Additional Science Chapter 3

Selectively permeable Membrane Osmosis Higher 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. Selectively permeable Membrane water GCSE Additional Science Chapter 3 sugar High water concentration Low sugar concentration Low water concentration High sugar concentration Weak solution Strong solution

Selectively permeable Membrane Osmosis Higher 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. Selectively permeable Membrane water GCSE Additional Science Chapter 3 sugar High water concentration Low sugar concentration Low water concentration High sugar concentration Weak solution Strong solution

Selectively permeable Membrane Osmosis Higher 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. Selectively permeable Membrane water GCSE Additional Science Chapter 3 sugar 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. capillary tube sucrose solution level tight knot GCSE Additional Science Chapter 3 sucrose solution Visking tube water

Practical Work GCSE Additional Science Chapter 3 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 5mm GCSE Additional Science Chapter 3 50mm Potato cylinder

The Importance of Osmosis to Cells Higher 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. GCSE Additional Science Chapter 3

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. GCSE Additional Science Chapter 3

The importance of Osmosis to Cells 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. GCSE Additional Science Chapter 3

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. GCSE Additional Science Chapter 3

Examples of 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. GCSE Additional Science Chapter 3

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. GCSE Additional Science Chapter 3