Living Cells Diffusion and Osmosis Mr G Davidson

Diffusion Diffusion is the movement of molecules from a region of high concentration to a region of low concentration down the concentration gradient. Molecules of liquids and gases are constantly moving, and they will always try to spread out until there is an equal distribution. Saturday, December 08, 2018 Mr G Davidson

Diffusion A purple crystal of potassium permanganate is added to a beaker of water. It is left for a day. The particles of the crystal begin to diffuse into the water. Water Crystal Saturday, December 08, 2018 Mr G Davidson

Diffusion The molecules from the crystal have spread out from the crystal (high conc.) to the water (low conc.) This continues until the molecules are equally spread out. Saturday, December 08, 2018 Mr G Davidson

Importance of Diffusion Diffusion is important to all living organisms as it allows important substances to pass into and out cells. In unicellular organisms oxygen is constantly diffusing into the cell as it uses it up, and carbon dioxide is constantly diffusing out. Saturday, December 08, 2018 Mr G Davidson

Diffusion in Unicellular Organisms Respiration in the cell uses up oxygen and produces carbon dioxide High concentration of oxygen Low concentration of carbon dioxide Oxygen diffuses into the cell Carbon dioxide diffuses out of the cell Saturday, December 08, 2018 Mr G Davidson

Diffusion in Multicellular Organisms Diffusion is also important to multicellular organisms. Blood is used to carry gases around the body. Saturday, December 08, 2018 Mr G Davidson

Diffusion in Multicellular Organisms This blood picks up and loses the gases in the lungs of mammals. In the lungs oxygen diffuses into the blood. When this blood reaches the tissues, the oxygen diffuses into the tissue cells from the blood. Saturday, December 08, 2018 Mr G Davidson

Diffusion in Multicellular Organisms Air sac in lungs High oxygen concentration Low oxygen concentration High oxygen concentration Low oxygen concentration Saturday, December 08, 2018 Mr G Davidson

Diffusion in Multicellular Organisms In the tissues the oxygen is used up in respiration and carbon dioxide is produced. In the tissues carbon dioxide diffuses from the tissue cells into the blood. When this blood reaches the lungs, the carbon dioxide diffuses into the air sacs and this air is then breathed out. Saturday, December 08, 2018 Mr G Davidson

Diffusion in Multicellular Organisms Air sac in lungs Low CO2 concentration High CO2 concentration Low CO2 concentration High CO2 concentration Saturday, December 08, 2018 Mr G Davidson

Osmosis Osmosis is a special type of diffusion. Osmosis is the movement of water molecules from a region of high water concentration to a region of low water concentration across a selectively permeable membrane. A selectively permeable membrane only allows small molecules to pass through it. Saturday, December 08, 2018 Mr G Davidson

Osmosis Sugar molecule Selectively permeable membrane Water molecule Low water concentration High water concentration Saturday, December 08, 2018 Mr G Davidson

diffuse from high water through the pores in the Osmosis The sugar molecules try to diffuse through the membrane but can’t get through the pores The water molecules diffuse from high water conc.to low water conc. through the pores in the membrane. Saturday, December 08, 2018 Mr G Davidson

Osmosis The following shows a model cell made of visking tubing. Water 2% sugar solution H2O H2O 98% water H2O H2O In this case the bag will gain mass H2O H2O Saturday, December 08, 2018 Mr G Davidson

Osmosis If we always think of the water concentration, we can easily explain what happens in terms of osmosis. A hypotonic solution will always lose water. A hypertonic solution will always gain water. An isotonic solution will neither gain nor lose water. Saturday, December 08, 2018 Mr G Davidson

In this case, water enters the cell and the cell becomes TURGID. Water Concentrations Hypotonic solution (water) High water conc. Low water conc. Plant cell In this case, water enters the cell and the cell becomes TURGID. Saturday, December 08, 2018 Mr G Davidson

In this case, water leaves the cell and the cell becomes FLACCID. Water Concentrations Hypertonic solution (5% salt solution) Low water conc. High water conc. Plant cell In this case, water leaves the cell and the cell becomes FLACCID. Saturday, December 08, 2018 Mr G Davidson

Water Concentrations Isotonic solution Plant cell In this case, water neither leaves nor enters the cell and the cell remains unchanged. Saturday, December 08, 2018 Mr G Davidson

Water Concentrations If a cylinder of potato tissue is placed in a hypertonic solution the cells lose weight and the tissue feels soft. If a cylinder of potato tissue is placed in a hypotonic solution the cells gain weight and the tissue feels firm. HYPOTONIC SOLUTION HYPERTONIC SOLUTION Saturday, December 08, 2018 Mr G Davidson

Plant Cells When plant cells are in a hypotonic solution they Water When plant cells are in a hypotonic solution they gain water by osmosis. This fills the large vacuole which forces the cell membrane out against the cell wall. This is what makes the cells go turgid and allows plants to stand upright. Water Saturday, December 08, 2018 Mr G Davidson

Plant Cells When plant cells are in a hypertonic solution they Water When plant cells are in a hypertonic solution they lose water by osmosis. This empties the large vacuole and pulls the cell membrane away from the cell wall. This is called PLASMOLYSIS and the cell is soft and said to be PLASMOLYSED. Water Saturday, December 08, 2018 Mr G Davidson

Animal Cells Water When animal cells are placed in a hypotonic solution, they too gain water. However, there is no strong cell wall and the cell membrane eventually ruptures. Water Saturday, December 08, 2018 Mr G Davidson

Animal Cells Water When animal cells are placed in a hypertonic solution, they too lose water by osmosis and the cell shrinks. Water Saturday, December 08, 2018 Mr G Davidson

Animal Cells Some unicellular animals, such as Paramecium, have a problem because they live in fresh water which is hypotonic. This means they constantly gain water by osmosis. In order to prevent them from bursting they have special “contractile vacuoles” which they use to expel water. They do this by filling up and then going to the cell membrane where they burst, “spitting” out the excess water. Contractile vacuoles Paramecium Saturday, December 08, 2018 Mr G Davidson