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Diffusion and osmosis Describing Solutions Sucrose Solution – Solution: One or more substances distributed uniformly in another substance. – Solute: The substance dissolved in a solution. – Solvent: The substance in which the solute is dissolved. • Concentration: the measurement of the solute in a fixed amount of solvent. – Ex: 2% sugar solution = 2 grams sugar + 100 ml water. Passive Transport • Molecules moving across the plasma membrane without use of cell’s energy. Think: Why do you think this is called “passive” transport? concentration gradient – Molecules are always moving. – From area of high concentration to area of low concentration. – Equilibrium is reached when concentrations are equal. (No concentration gradient) Hint: Nature loves balance. To remember which way molecules think of a rock rolling down a hill – it does not take as much energy as pushing it up a hill. Diffusion • The movement of any molecule in solution from high concentration to low concentration – Membranes allow some molecules to pass others not to pass. • Ability to pass depends on: – Size – Lipid soluble. – Appropriate pore Hint: molecules that pass the lipid bilayer of the cell (plasma) membrane are hydrophobic, i.e. nonpolar. Everything else must go through a protein channel. Passive Transport:Osmosis • Osmosis – Movement of water molecules through a membrane from high concentration area to low concentration. Think: what is the difference between osmosis & diffusion? Terms relating to osmosis. • These terms always describe the surrounding solution COMPARED to the cell. – Hypertonic: high solutes - low water. – Hypotonic: low solutes high water. – Isotonic: equal solutes equal water. Most cells do not explode when filled with water, they would from the excess water first. How Cells Deal With Osmosis • Freshwater single cell organisms: • Water moves in due to osmosis. • Excess water collected by contractile vacuole. • Water is expelled from organism. Contractile vacuole collecting water in a paramecium and squirt it out if too much enters the cell. How Plant Cells Deal With Osmosis • Plant cells have turgor pressure due to water wanting to move into the cell. (keep their shape) • Central water vacuole fills. • Pressure against the cell wall is turgor pressure. Look: what happened to the shape of the cell wall? Filled water vacuole due to hypotonic environment. How Cells Deal With hypertonic solution • Water leaves cell due to hypertonic solution. • Cytoplasm and organelles move to center of cell. Elodea cell in hypertonic solution. How Animal Cells Deal With Osmosis • Animal cells must be in isotonic solution. • Animal cells in a hypotonic solution gain water. – Cytolysis results. • Animal cells in hypertonic solution lose water. RBC’s cell in Isotonic solution. RBC’s cell in hypotonic solution. RBC’s cell in hypertonic solution. Summarize the lecture in a paragraph with a diagram that shows the following: • How water moves when a cell has more water inside than out and visa versa. • What the following terms mean: solute, solvent, solution. • The diffusion of an drop of ink in a glass of water over time. • Explain this cartoon in terms of how osmosis works.