movement through cell membranes

• diffusion is the passive movement of molecules from areas of high concentration to areas of low concentration
• the difference in concentration between two areas is called a concentration gradient
• diffusion is a passive process
• diffusion stops when there is no difference in concentration between two areas
• examples of diffusion:

  • alveoli: gases exchange between the capillaries and air (CO2 ou and O2 in)
  • synapse: neurotransmitters move across the synapse to the target cell
  • stomata and lenticels: gas exchange in plants happen here
• osmosis is the movement of water from an area of high water concentration to an area of low water concentration, across a semi-permeable membrane
• osmosis is a special form of diffusion. in terms of solutes, osmosis involves the movement of water from a dilute region to a concentrated region. osmosis is also a passive process
• the cell membrane is a selectively permeable membrane
• proteins in cell membranes select or regulate what kind of molecules can move in and out of cells
• membrane controls permeability
• examples of osmosis:

  • amoeba: a contractile vacuole regulates amoebas water content
  • nephron: water reabsorption is done by osmosis
  • plant roots: water enters the root hair by osmosis
  • stomata: guard cells take in water, causing the opening of the stomata
• active transport requires energy (ATP)
• in active transport, substances are moved against their concentration gradient