Membrane Transport

Organ Physiology → specialized cells

Cell Membrane/ Plasma Membrane: allows exchange w/ extracellular environment

  • Composed of phospholipids that isolate the cell and keep its contents concentrated

  • Phospholipids contain…

    • a phosphate group (polar, hydrophilic)

      • Nitrogen-containing choline group

    • Fatty acids (nonpolar, hydrophobic)

      • Bonded to glycerol

  • But they are also amphipathic: a molecule that contains a polar and nonpolar portion

  • Because of this, once exposed to water, it creates a micelle

Not only a bilayer phospholipid in our human cells, but…

  • Peripheral Proteins: can be embedded in one of the leaflets of the cell membrane

  • Integral proteins: can traverse the whole plasma membrane b/c they have a transmembrane domain (lipophilic/nonpolar)

  • Glycoproteins attached to the integral proteins/ Glycolipids attached to the phospholipids that are involved in cell-cell recognition/communication

Fluid Mosaic Model: Phospholipids and proteins can move laterally in the plasma membrane

Raft Domains: concentrated area of proteins and forming clusters (can be enriched with cholesterol)

Anything that has to do with bulk transport → endocytosis

  • Makes invagination → allows the cell to absorb nutrients/ minerals/ or metabolites from the environment

Several forms of Endocytosis

  • Phagocytosis: forms membrane protrusions on where the cell wants to eat

    • Ex: pseudopods in the phagocytic cell create protrusions to engulf the bacterium

  • Pinocytosis: small vesicle formation for lipid droplets or solutes

  • Receptor-mediated endocytosis: adaptor molecules form a vesicle and uncoat themselves off the vesicle → naked vesicle

Viruses hijack the host through endocytic pathways!

  • HIV (AIDS virus)

  • Influenza virus

  • Poliovirus

  • Adenovirus

Anything that has to do with releasing material to the external environment → exocytosis

  • Typically, ER makes proteins and they are packaged by the cisternae of golgi apparatus/complex

  • Secretory vesicles are created and secretion happens

  • A cell that constantly secrete vesicles is doing a process of constitutive exocytosis

  • Regulated Exocytosis takes place only in specialized secretory cells like neurons. 

    • Vesicles do not fuse with the plasma membrane until they receive an appropriate signal.

  • The extracellular environment contains interstitial fluid that is squeezed out of blood vessels to other places.

  • ECM (collagen and elastin fibers) holds the interstitial fluid

  • ECM + fibroblast = connective tissue

Rate of diffusion through a membrane = number of molecules diffusing per unit of time

  • To speed up diffusion, you need to have a high difference of concentration

  • The permeability of membrane

  • Hotter = higher diffusion

  • more surface area means more diffusion

Non-carrier mediated transport: nonpolar molecules passing through a membrane

Carrier mediated transport: channels & transporters

  • Ex: Water specific molecules need aquaporins (channels) to allow facilitated diffusion.

    • Aquaporins have an asparagine core that creates a charge that attracts water molecules

  • Ex: Glucose interacts with the transporter and creates a conformational change to enter the cell

  • Ex: Ion channels, but they can be gated and open after specific stimuli

    • voltage gated channels (K and Na) Membrane potential, ligand gated (receptors), mechanically gated channels (stretched, pressured, sound waves)

Osmosis: net diffusion of water across a membrane that is permeable to water and impermeable to solutes.

Higher concentration of solutes will attract water molecules

Osmotic pressure: is the minimum pressure applied to expansion to stop osmosis

Molarity: is molar concentration: amount of moles per liter

Molality: moles per kilogram