Cell Transport

Membrane crossing mechanisms

1. Passive transport (simple and facilitated diffusion)

   1. Does Not require energy

   2. Molecules crossing a membrane

2. Osmosis (facilitated diffusion of water via aquaporins)

   1. Water crossing a membrane

3. Active transport

   1. Energy is needed

   2. Against gradient

4. Endocytosis

5. Exocytosis

Movement

• Molecules are always moving

  • Solids vibrate

  • Gasses are bouncing around off of each other

  • Individual molecules move randomly

  • Heat increases speed of motion and freedom

  • Enough energy breaks hydrogen bonds between molecules (liquid→gas)

  • Temperature increase→molecular movement speed increase

• Groups of molecules have an overall direction of movement

  • Want to go from high concentration to low concentration areas (concentration gradient: area with many to area with less)

    • Doesn’t take energy

    • Always happening

  • Random, spontaneous, overall movement from [hi] to [lo] is called diffusion

    • [hi] is a shortcut for high concentration

    • [lo] is a shortcut for low concentration

  • Concentration gradient is only based on the type of molecule

    • Water concentration gradient is from high to low concentration of water

  • Net movement into a cell, net movement out of a cell is which way the diffusion is going

    • If the net is zero (same number moving and and out) it is called equilibrium

      • Molecules still move from [hi] to [lo] in equilibrium, but they just switch places (maintaining equilibrium)

  • As long as a membrane can pass through the cell membrane, it will go from [hi] to [lo]

    • Nonpolar, small molecules cross membranes easily

      • Small: 1-3 atoms

      • Medium: 4-50 atoms

      • Large: 50+ atoms

    • Hydrophobic membrane makes it hard for polar molecules to cross

    • Oxygen is always diffusing in/out of our lungs

  • Osmosis is a type of diffusion

    • Diffusion of water

    • Dilute: high water concentration

      • Hypotonic

        • Very little solute

    • Concentrated: low water concentration

      • Hypertonic

        • Lots of solute

        • Shrivels cells

        • More concentrated solute

        • Water leaves to try to create equilibrium outside of the cell

    • Equally dilute/concentrated:

      • Isotonic

    • Concentration diagram:

• Water will diffuse to even out the concentration (make it isotonic)

  • Sugar can’t cross the membrane, so water moves to compensate

  • Goes against gravity

• Osmosis happens slowly

  • Can move faster with higher temperature

• Facilitated diffusion is a type of passive transport

  • Diffusion with the help of proteins

    • Proteins form channels or carriers to transport across membranes

      • Called aquaporins

      • Channels are holes through

      • Carriers open one side, close, then open the other side to release

      • Proteins recognize and only allow certain molecules through the membrane

  • Faster than osmosis

  • No energy is needed

• Passive transport is the spontaneous, energy-less movement of molecules from [hi] to [lo]

• Active transport requires energy, moves solutes against the gradient from [lo] to [hi]

  • Sodium tends to be outside the cell

  • Needed more in the cells

    • Has to be moved against the concentration gradient

  • Potassium builds up in cells, needs to be removed

    • Sodium and potassium swap

  • Proton pump:

    • Hydrogen ions move from in the cell to out

    • Adjusts the pH of the cell and the exterior environment

    • Uses ATP to push hydrogen against its gradient

  • Cotransporting:

    • Moves two substances at the same time

    • Cell ‘piggybacks’ on the hydrogen (that’s following its gradient back into the cell)

    • Sucrose is going against the gradient into the cell

• Facilitated diffusion is needed when polar molecules need to cross

• Plant cells prefer hypotonic extracellular environment (turgid)

• Endocytosis

  • Process where cells obtain materials that are too big to fit through the membrane

    • Cell membrane surrounds the molecule

    • Pinches into a vesicle, keeping it inside the cell

• Exocytosis

  • Process where cells get rid of large materials (or in bulk)

    • How they send proteins

    • How cells communicate with other cells

    • Vesicle combines with membrane

      • Breaks open

      • Releases contents into the extracellular matrix

      • That's how nerves work

• Osmoregulation is for organisms living in hypotonic or hypertonic environments

  • Freshwater fish can get rid of extra water

  • Saltwater fish can get rid of extra salt

  • Some species can go from hypotonic to hypertonic

  • Gills help get salts out or retain salt

  • Kidneys do the same thing