Unit 1.2 - Cell Membrane

6 Basic Transport Mechanisms

diffusion, osmosis, facilitated diffusion, active transport, cotransport, and countertransport

Diffusion

movement of a solute within a medium and membrane in biological system

by concentration gradient (from concentration to dilute)

Osmosis

movement of solvent through a membrane by means of a concentration gradient

water moves from dilute (low osmolality) to concentrated (high osmolality)

requires water channels (aquaporins) inserted within the semi-permeable membrane

Facilitated Diffusion

carrier-mediated transport (carrier proteins) from higher to lower concentration

transport of large or electrically charged molecules (e.g. glucose, amino acids, HCO3, etc)

Active Transport

movement against concentration gradient

energy dependent carrier mediated, requires ATP

Cotransport (Symport)

carrier mediated simultaneous movement of 2 or more solutes in the same direction

Countertransport (Antiport)

carrier mediated simultaneous movement of 2 or more solutes in opposite directions

Categories of Transport Processes

bulk flow, diffusion, protein-mediated transport, and vesicular transport

Bulk Flow

requires pressure gradient

flow from high to low pressure (e.g. blood flow within a blood vessel, air moving in and out of the lungs)

Diffusion

requires concentration gradient such as chemical, electrical, or both (electrochemical gradient)

flow from high to low concentration - can take place in an open system or through a plasma membrane

passive process which requires no input or energy

finish when dynamic equilibrium is established - equal gradient in both regions

rate of diffusion through membrane is dependent on: membrane surface area, molecular size, concentration gradient, membrane composition, and lipid solubility

Lipid Solubility

ability of a molecule to dissolve into the lipid bilayer plasma membrane is dependent on:

chemical nature of the molecule - only non-polar lipophilic molecule can dissolve in the central lipid region of the membrane (e.g. lipids, steroids, and small lipophilic molecules)

cholesterol content within the membrane

Protein-Mediated Transport

4 major groups of the membrane proteins with their respective physiological functions:

membrane transporters, membrane enzymes, membrane receptors, and structural proteins

majority of solutes transport across the membrane are in the form of this transport because they are lipophobic or electrically charged

could be passively transported by moving through channels down the gradient (facilitated diffusion) or move through channels against the gradient with the use of ATP (active transport)

Protein Mediated Transport Subgroups

channel transporters - channels with fluid filled chambers that are directly linked between the intercellular and extracellular compartments
allows very rapid transports across the membrane
confined to the transport of smaller size ions and water

carrier transporters - no direct contacts of the substrate between the intercellular and extracellular compartments at a given moment (one end is always closed)

Channel Transporters

could have a cluster of tunnels or pores
can be divided into 2 types: open and gated channels

Open Channels (Leak Channels)

with pores open at all times and allow molecules moving in and out without restriction

the only contributing factor for the direction of flow is the gradient

Gated Channels

in a closed state most of the time

opening of gates regulated by:
intercellular messenger molecules or binding of ligands to the extracellular receptors (chemically gated channels)
electrical state of the membrane changes (voltage-gated channels)
simulation to the membrane in the form of pressure (mechanically gated channels)

Carrier Transporters

transport of a substance across the channels is initiated by the binding of that substance to the carrier proteins of the channels

it requires conformation changes to the carrier protein in order to transport the substrate - slower than channel transporters

uniport carriers - moves only 1 kind of substrate
symport carriers - moves multiple substrates in the same direction
antiport carriers - moves multiple substrates in the opposite direction

Primary (Direct) Actice Transport

requires energy in the form of ATP to create a higher concentration gradient between the intercellular and the extracellular compartments

primary active transporters also know as ATPase

Secondary (Indirect) Active Transport

uses the potential energy created by the primary active transport and converts it into kinetic energy for the transport of a substrate against the concentration gradient