Diffusion and Osmosis: How does stuff get into and out of a cell? The Cell in its Environment Ch 7, Section 2 L3 Biology
Dec 11, 2015
Diffusion and Osmosis: How does stuff get into
and out of a cell?
The Cell in its Environment
Ch 7, Section 2
L3 Biology
Diffusion• Movement of molecules from an area of
high concentration to an area of lower concentration
• Continues until equilibrium is reached.
Diffusion
• Occurs without control or energy use – it is called Passive Transport
• Affected by:• Size of molecules• Size of pores in a membrane• Temperature• Stirring
Cell Membranes are Semipermeable• Let some molecules pass through and not
others.– Small molecules pass through – ex: water– Large molecules can’t pass through – ex:
proteins and complex carbohydrates
How long does diffusion continue?• Until equilibrium is reached:
– This means there will be an equal distribution of molecules throughout the space – this is why odors smell strong at first, then seem to disappear
Osmosis – A Special kind of Diffusion
• Diffusion of water across a selectively permeable membrane
• Cytoplasm is a solution of water and solutes (stuff dissolved in the water).
• Water moves into and out of cells because of the different concentrations of the solutes.
• Different kinds of cells react differently to osmosis, depending on the solution they are in:
Active Transport – uses carrier molecules to move molecules from an area of lower
concentration to an area of higher concentration
Endocytosis
• Very large molecules or substances must enter the cell by the membrane pinching inward: