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TRANSPORT THROUGH CELL MEMBRANES
Diffusion and Osmosis
Learning Outcomes:
B9 - Analyse the structure and function of the cell membrane:
describe passive transport processes including diffusion, osmosis, and facilitated transport
explain factors that affect the rate of diffusion across a cell membrane
predict the effects of hypertonic, isotonic, and hypotonic environments on osmosis in animal cells
Recall that the cell membrane is selectively permeable, allowing only certain things to enter and exit the cell
Several mechanisms are involved in the movement of materials in and out of the cell
Some of these are passive (do not require energy), while others are active (require energy)
Diffusion & OsmosisDiffusion & Osmosis
Passive - don’t require energy - “goes with the flow”
Movement of substances with the concentration gradient (from high to low concentration)
Diffusion
Movement of molecules from an area of high concentration to an area of low concentration until equilibrium is reached
Substances that can pass freely through the cell membrane will do so by diffusion, if a concentration gradient is present (higher concentration on one side of the membrane)
Examples - gases (O2, CO2), lipid-soluble (non-polar) molecules
Rate of diffusion
Factors that affect the rate of diffusion:Temperature
Size of molecules
Concentration gradient(how would each of these affect the rate?)
Osmosis
Is the diffusion of water across a differentially permeable membrane
Water moves in and out of cells by osmosis through the cell membrane
Membrane requirements
Membrane must be permeable to water but impermeable to the solute
Must have different solute concentrations on either side of the membrane
Water moves toward higher solute concentration (ex. toward the salt)
Solute stays where it is(“salt sucks!”)
Osmosis in cells
Osmotic pressure is pressure due to movement of water across a membrane
This is important in the functioning of many living systems (ex. circulatory system)
Tonicity
Refers to the relative solute concentrations of solutions in two places
Determines the direction of water movement (ex. In or out of cells)
Hypertonic = higher concentration
Hypotonic = lower concentrationIsotonic = equal concentrations
These terms are used to compare two solutions
Cell in a hypertonic solution
Solute concentration is greater outside the cell
Net movement of water OUT of the cell (cell shrinks)
Cell in a hypotonic solution
Solute concentration is less outside than inside the cell
Net movement of water INTO the cell (cell expands and may burst)
Cell in an isotonic solution
Solute concentrations are equal inside and outside of the cell
No net movement of water (equal movement in both directions)
Animal and Plant Cells
Blood Cells