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Key Concepts
• How do most small molecules cross the cell membrane?
• Why is osmosis important to cells?
• What is the difference between passive transport and active transport?
Key Terms• Selectively
permeable
• Diffusion
• Osmosis
• Passive transport
• Active transport
• Some substances can pass through the membrane while others cannot.
• Molecules move from an area of higher concentration to an area of lower concentration.
• The diffusion of water molecules through a selectively permeable membrane.
• The movement of dissolved materials through a cell membrane without using cellular energy.
• The movement of materials through a cell membrane using cellular energy.
Introduction
• Cells have structures that protect their contents from the world outside.
• All cells are surrounded by a cell membrane that separates the cell from the outside environment.
• The cell membrane is selectively permeable, which lets some things enter and leave the cell.– Oxygen– Carbon Dioxide– Water– Food molecules– Waste products
What is Diffusion?
• The process by which molecules move from an area of higher concentration to an area of lower concentration.
• The main method by which small molecules move across the cell membrane.
Animation of Diffusion• http://highered.mcgraw-hill.com/sites/0072495855/student_
view0/chapter2/animation__how_diffusion_works.html
• How does diffusion play a role in the following situations?– smelling cookies baking in your oven– making a pitcher of lemonade– adding chemicals to a pool
• Can you think of any other examples of diffusion?
What is Osmosis?
• The diffusion of water molecules through a selectively permeable membrane.
• Many cellular processes depend on osmosis because cells cannot function properly without adequate water.
How is Osmosis Related to Diffusion?
• Molecules tend to move from an area of higher concentration to an area of lower concentration.
• Water molecules move by diffusion from an area where they are highly concentrated through the cell membrane to an area where they are less concentrated.
Animation of Osmosis• http://highered.mcgraw-hill.com/sites/0072495855/student_
view0/chapter2/animation__how_osmosis_works.html
• Osmosis has a number of life-preserving functions.– assists plants in receiving water– helps in the preservation of fruit and meat– used in kidney dialysis
• Osmosis can be reversed to remove salt and other impurities from water.
What is Passive Transport?
• The movement of materials through a cell membrane without using cellular energy.
• Diffusion and osmosis are examples of passive transport.
Animation of Facilitated Diffusion
• http://highered.mcgraw-hill.com/sites/0072495855/student_view0/chapter2/animation__how_facilitated_diffusion_works.html
What is Active Transport?
• The movement of materials through a cell membrane using the cell’s energy.– The movement of a
substance in the opposite direction than they naturally move by diffusion.
• Minerals• Some sugars• Most amino acids
Methods of Active Transport
• Transport Proteins
• “pick up” molecules
• Examples of substances that are carried . . . – Calcium– Potassium– Sodium
• Engulfing
• Cell membrane surrounds and engulfs, or encloses, a particle.
• A vacuole is formed around the engulfed particle.
• Energy is required by the cell to perform this function.
Animation of Active Transport
• http://highered.mcgraw-hill.com/sites/0072495855/student_view0/chapter2/animation__how_the_sodium_potassium_pump_works.html
“Transport” is like Riding a Bike?
• Riding a bike down a hill . . .
• Pedaling a bike up a hill . . .
• Passive Transport • Active Transport
Why Are Cells Small?
• Cells become less efficient as they grow.
• The smaller they are, the easier it is for them to do their jobs.
• The smaller they are, the easier it is for substances to be moved in and out.