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Passive Transport. Section 4.1. Difference between active and passive transport. Cells maintain homeostasis by controlling the movement of substances across their cell membrane. No energy needed = passive transport Energy needed = active transport. Passive Transport.
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Passive Transport Section 4.1
Difference between active and passive transport • Cells maintain homeostasis by controlling the movement of substances across their cell membrane. • No energy needed = passive transport • Energy needed = active transport
Passive Transport • Differences in concentration = concentration gradient • Concentration is the same all through= Equilibrium • Passive transport can happen when there is a concentration gradient • Molecules in equilibrium continue to move but the movement is equal in all directions
Diffusion – the most common form of passive transport • Molecules move from an area of high concentration to an area of lower concentration • Many substances enter or leave cells by diffusing across the cell membrane.
Osmosis – a special type of diffusion • The diffusion of water through a selectively permeable membrane = osmosis. • Water goes into or out of a cell by osmosis
There are three possibilities for the direction of water movement: Water moves out - the cell shrinks. A solution that causes a cell to shrink is a hypertonic solution Water moves in - the cell swells. A solution that causes a cell to swell is a hypotonic solution Equal water movement – the cell stays the same. A solution that produces no change in cell volume is an isotonic solution Osmosis, continued
Other forms of passive transport 1. Diffusion Through Ion Channels • A transport protein with a pore through which ions can pass. • An ion can cross the cell membrane without touching the interior of the lipid bilayer.
Other forms of passive transport, continued • 2. Facilitated Diffusion • Carrier proteins - can bind to a substance on one side of the cell membrane, carry the substance across the cell membrane, and release it on the other side.