Understanding Movement of Substances Through Cell Membrane
Substances enter and leave cells through diffusion and osmosis processes, driven by concentration gradients. The cell membrane plays a crucial role in selectively allowing substances to pass, ensuring cell survival. Osmosis can be demonstrated using Visking tubing, showing water molecules moving while larger sucrose molecules are blocked.
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Movement of Substances through a Cell Membrane BIOLOGY Unit 1.1 Double Award Unit 1.1
How do substances 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 The orange molecules move in the opposite direction. This is how substances such as oxygen and carbon dioxide enter and leave cells. Flow Diffusion does not require an input of energy it is a passive passive process.
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. Selectively permeable Membrane water sugar 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. Selectively permeable Membrane water sugar 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. Selectively permeable Membrane water 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. capillary tube 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. sucrose solution level tight knot sucrose solution Visking tube After about half an hour, the solution s level in the tube will have risen, and the water level will have dropped. water
Practical Work Potatoes can be used to demonstrate osmosis in living tissue. Distilled Water Potato cylinders of the same length and diameter are each placed in different concentrations of sucrose solutions for 20 minutes. 0.1M 0.2M 1M 0.5M 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. Labelled Petri dishes 5mm 50mm Cylinders showing no change will have cells with cytoplasm of equal strength to the sucrose solution that they were placed in. Potato cylinder
Higher 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.
Higher 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 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 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 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.
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.
