Transport Across the Cell Membrane

• The substances that enter and leave the cell may be ions or molecules or, in some cases, microorganisms or other cells.
• Passive Transport: occurs naturally with no energy input from the cell.

  • Low to high
  • e.i. Diffusion, Osmosis, Facilitated diffusion
• Active transport: requires energy input from the cell

  • e.i. endocytosis, exocytosis
• Diffusion: natural movement of particles along their concentration gradient (high to low concentration)

  • passive transport = no added energy is required for it to occur
  • e.i. carbon dioxide leaves the cell by diffusion because the concentration of the gas inside the cell is higher than the outside

  
• Osmosis: movement of water molecules along their concentration gradient

  • high water concentration to low water concentration
  • passive transport
  • If diffusion doesn't occur, osmosis will
• Hypertonic: when a solution has a higher solute concentration than inside the cell

  • water will leave the cell
• Hypotonic: when a solution has a lower solute concentration that inside the cell

  • water will enter the cell
• Isotonic: when a solution has the same solute concentration inside and out of the cell

  • no net movement of water molecules
• Facilitated Diffusion: diffusion across the cell of substances that are soluble in water, but not in lipids, charged, and are too large.

  • Passive transport
  • occurs with the help of carrier proteins or channel proteins
• Channel Proteins: pores or channels through which small water-soluble particles move along the concentration gradient

  • High to low concentration

  
• Carrier Proteins: large molecules change shape and physically move the molecule across the membrane and into the cell

  • once the molecule has been transported, the protein returns to its original shape

  
• Active Transport: movement of particles across the cell membrane against the concentration gradient (low to high concentration) through carrier proteins

  • requires energy
  • necessary to concentrate materials (nutrients) inside the cell or for expulsion of waste particles
  • Carrier proteins work almost as a pump to move molecules or ions across the membrane
• Endocytosis: when a vesicle forms around the particle and the cell membrane pinches around it so that the vesicle is inside the cell

  • occurs when molecules that need to be taken in are too large to pass across with the help of protein carriers
  • active transport --> requires ATP energy
• Exocytosis: when a vesicle surrounds the particle, then fuses with the plasma membrane, releasing its contents into the surroundings

  • occurs when the cell must get rid of large waste particles
  • Active transport --> requires ATP energy
• Large surface area to volume ration = good for cell

  • the rate of nutrient absorption is dependent on its surface area.
  • more surface area = more nutrients through the cell
• Low surface area to volume ratio = bad for the cell

  • when volume increases, it needs more energy, and more molecules will need to be transported across the cell surface
  • materials move slowly; so when the volume increases, the distance any molecule has to travel from the cell will increase
• Surface Area to Volume Ratio Formula: SA/V

  
• Cells tend to be very small because if the surface area-to-volume ratio is too small, the cell will not be able to support itself and will die.