Chapter 4 On the Move Diffusion Note: Particles (molecules) in liquids and gases about, at high speeds, in all directions. Particles (molecules) will escape from the concentrated area to places where there are fewer or no particles. Diffusion in Liquids A drop of potassium permaganate is placed in a test tube of water. Observe what happens. Question: What happens to the water after a few minutes? Where do you think the drop originates from? The top or bottom of the liquid? Diffusion in Liquids Diffusion in Gases A tube of bromine gas is connected via a valve to another empty container. Question: What is the colour of bromine gas? What happens in the left hand tube? What happens in the valve? What happens in the right hand container after a while? What can you say about the pathway of the bromine gas? Diffusion in Gases Diffusion The movement of molecules from a region of higher concentration to a region of lower concentration is called diffusion. Diffusion takes place in liquids and gases. Osmosis Osmosis is a special form of diffusion. The MOVEMENT OF WATER from a dilute solution to a more concentrated one through a PARTIALLY PERMEABLE MEMBRANE. This type of membrane, (also called semi-permeable) allows only water but not other (dissolved) substances to pass through. Osmosis Water concentration is lower in the beaker than in the bag. This causes water to move from the bag (left) into the beaker (right). Osmosis in an Animal Cell All cells (plants and animal) are surrounded by a partially permeable membrane and the cytoplasm contains dissolved substances eg. Salt and proteins. Whether or not water enters the cell depends on the relative concentration of the cytoplasm and the fluid surrounding it. An Animal Cell in Distilled Water What happens when there are more water molecules in the surrounding fluid than the cell? What do you think happens to the cell if it is placed in concentrated salt solution? A Plant Cell in Distilled Water Osmosis in Plants Plants gain water by osmosis through their roots, and it is osmosis that moves water into plant cells, making them turgid or stiff, and thus able to hold the plant upright. Test Byte 1 What happens if the water concentration is the same on both sides of the membrane? Diffusion vs Osmosis Diffusion Osmosis Random movement Random movement of water From higher to From higher to lower concentration lower concentration Remember: Molecules move in all directions in diffusion and osmosis. Osmosis is the diffusion of water. Test Byte 2 In what direction will the water flow in the following situations? What happens to the cell (if any)? Situation 1: Situation 2: Situation 3: Test Byte 3 In each of the following pictures, describe the relative concentration of water in the cell and the surrounding fluid.
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