Active transport (Pumps, cotransport, Exocytosis, Endocytosis)

• Requires energy because it moves a solute against its concentration gradient (EG: Pushing two positive magnets together)

  • Using proteins ≠ requires energy (facilitated diffusion).

• Uses carrier proteins

  • Facilitated diffusion used channel proteins. 

ATP: Adenosine triphosphate (Adenosine + 3 phosphate groups)

• An “energy source” for the cells. 

• Comes from breaking part sugars (EG glucose)

• Used by active transport

• Is broken apart by the cells for the release of energy

  • Transfers the terminal (last) phosphate group which changes the shape of a transport protein to help it better move.

    • Converts it to ADP

    • STRUCTURE = FUNCTION

      • Proteins are easily influenced by different charges and can easily change their shape depending on that. 

Pumps: Maintain membrane potential

• Membrane potential is the electrochemical (unequal) charge, which results in an electrical charge

  • Not allowing the gradient to balance out. 

  • Cytoplasm is relatively (slightly, but compared to everything else it is) negative in comparison to the extracellular fluid. 

    • Cell environment = negative / Outside = positive most of the time

  • Stimulus outside of the cell can elicit responses. 

• Electrogenic pumps: Proteins that generate voltage across the membrane, later can release voltage for energy.

  • Sodium Potassium pump: Maintains resting membrane potential

    • Both sodium and potassium have positive charges.

    • Three sodiums + phosphate enter on the inside, makes protein change shape and release it to the outside. Two potassium ions enter from the outside, which causes it to change shape and allow it in.

      • Sodium can’t come back in but potassium can. 

    • Low → High concentration.

    • Takes out 3 positive charges, only brings in two to help keep the resting potential negative. 

      • +1 charge outside the cell / -1 inside the cell

    • Proton pumps

      • Used by plants, fungi, and bacteria

      • Pumps hydrogen+ ions out of the cell

Cotransport: Having a substance that can get across the membrane w/ another substance that would struggle (moving against the gradient, etc) to get across it work together.

• Plants use this for sugars and amino acids (EG: Sucrose-H+ cotransporter)

  • Hydrogen proton is naturally diffusing down the concentration gradient, sucrose hitches a ride. 

• Active because H+ needs to exit the cell, which requires energy to perform. Molecules coming in via active transport do not use ATP because it’s “following” the concentration gradient.

Large molecules use exocytosis and endocytosis. 

Exo out, endo in. 

• Exocytosis: Secretion of molecules via vesicles that fuse to the membrane

  • Done by forming a bilayer. 

  • Once fused, the contents of the vesicle get released outside. 

  • EXAMPLE: Neurons

• Endocytosis: The uptake of molecules from vesicles fused to the plasma membrane. 

  • Phagocytosis (immune system second line of defense)

    • When a cell engulfs particles to be digested by lysosomes.

    • Phago - Greek for eat

    • Cell surrounds particle with pseudopodia

      • Pseudo = kind of; podia  

    • Packages into a food vacuole 

  • Pinocytosis

    • Nonspecific uptake of extracellular fluid

      • Takes dissolved molecules in a protein coated vesicle

  • Extracellular vesicles (exosomes) 

  • Receptor mediated endocytosis

    • Specific uptake of molecules via solute binding 

    • Glycoproteins bind to a substrate to “know” to let molecules in. 

  • Phago/Pinocytosis are nonspecific and will envelope foreign material in general.