Passive Transport by Diffusion
• Diffusion is the movement of molecules from an area of high concentration to an area of low concentration.(down the concentration gradient)
• Diffusion does not refer only to movement across a membrane.
How Diffusion Works
• http://highered.mcgraw-hill.com/sites/0072495855/student_view0/chapter2/animation__how_diffusion_works.html
Passive Transport by Osmosis
• Osmosis is the movement of water across a biological membrane.
• Water molecules move from an area of low concentration solute (hypotonic) to an area of high concentration (hypertonic)
• Water moves because the membrane is impermeable to the solute
How Osmosis works
• http://highered.mcgraw-hill.com/sites/0072495855/student_view0/chapter2/animation__how_osmosis_works.html
Passive transport by Facilitated Diffusion
• Molecules can enter a cell by diffusion through a specific channel proteins or carrier proteins embedded in the membrane
• This process mediated by a membrane protein is called facilitated diffusion
• Molecules move down the concentration gradient
Channel Proteins
• Form highly specific channel through the cell membrane
• Permit the passage of ions or polar molecules
• Some channel proteins remain open all the time, others have gates that can be opened or closed to allow or prevent the passage of particles
Carrier Proteins
• Bind to specific molecules, transport them across the membrane and release them
• Carrier molecules change shape while transporting molecules
• Usually transport ions or small polar molecules but can also transport amino acids and glucose
• Have a lower rate of diffusion than channel proteins
• http://highered.mcgraw-hill.com/sites/0072495855/student_view0/chapter2/animation__how_facilitated_diffusion_works.html
Active Transport
• Active transport is the movement of ions, molecules or minerals from an area of low concentration to an area of high concentration(against the concentration gradient)
• This requires energy in the form of ATP
Primary Active Transport
• Uses ATP directly to move molecules or ions from one side of the membrane to the other
• Ion pumps are carrier proteins that pump ions against the concentration gradient
• The sodium-potassium pump pumps Na+ out of the cell and K+ ions into the cell
How the Sodium-Potassium pump works
• http://highered.mcgraw-hill.com/sites/0072495855/student_view0/chapter2/animation__how_the_sodium_potassium_pump_works.html
Secondary Active Transport
• As an ion pump functions, a difference in charge, or electrical potential, builds up across the membrane
• One side of the membrane gains a more positive or negative charge compared to the other side due to the accumulation of positive or negative ions
Secondary Active Transport
• The combination of concentration gradient and electrical potential is called an electrochemical gradient
• This gradient stores potential energy that can be used by the cell
• This energy is used by another protein to transport other molecules across a membrane
Membrane-Assisted Transport
• Macromolecules are too large to cross the cell membrane through a channel or by means of a carrier protein
• Cells form vesicles to surround incoming or outgoing material and move it across the cell membrane
• Membrane-assisted transport requires energy
Endocytosis
• The process where a cell engulfs material by folding the cell membrane around it and then pinching it off to form a vesicle inside
• There are three methods:– Pinocytosis– Phagocytosis– Receptor-mediated endocytosis
Exocytosis
• Macromolecules and other large particles leave a cell by exocytosis
• Vesicles that contain cell products to be released or waste products to be excreted fuse with the cell membrane and empty their contents into the extracellular environment
Phagocytosis
• http://highered.mcgraw-hill.com/sites/0072495855/student_view0/chapter2/animation__phagocytosis.html