Cell Membrane Transport

 

Transport Across Membranes

•        What membranes do

•         Separate material: ICF / ECF

•         Allow exchange of material: ICF / ECF

•        Why transport is important

•         Obtaining O2 and nutrients

•         Getting rid of waste products

 

 

Membranes Are Selective

•        Selectively permeable

•         Permeable = to pass through

•         Selective = restrictive

•        Membranes allow the transport of some substances, but not others

•        Non-polar and small polar molecules

•         Easily transported across membrane

•         Examples: O2, CO2, fatty acids, water

•        Large polar molecules and ions

•         Normally not transported

•          Examples: Glucose, proteins, Na+

 

 

Energy of Solutions

 

•        Change in free energy

•           Negative

•       Transport is spontaneous

•           Positive

•       Transport is not spontaneous

 

 

Types of Transport

•        Passive

•         Spontaneous

•         Downhill movement

•        Active

•         Not spontaneous

•         Uphill movement

 

Driving Forces

•        Forces which act to move particles

•         Force = push or pull

•         Particle = atom, ion, or molecule

•        Include chemical and electrical forces

•        Both forces = electrochemical force

•         Passive

•       Movement down the force

•         Active

•       Movement against the force

 

Chemical Driving Force

•        Characteristics

•         Concentration gradient = DC

•         Gradient “pushes” particles from higher to lower concentration

•         Force acts from higher to lower concentration

•        Direction of chemical driving force

•         Down the chemical gradient

•         From higher to lower concentration

•        Examples: Direction of chemical force

•         If: Co = 21 mM/L; Ci = 83 mM/L; then direction is out of cell (-)

•         If: Co = 145 mM/L; Ci = 15 mM/L; direction is into cell (+)

 

 

Chemical Driving Force

Magnitude of chemical driving force proportional to concentration gradient

 

 

Electrical Driving Force

•        Terminology

•         Ion = a charged particle

•         Cation = particle with a (+) charge

•         Anion = particle with a (-) charge

 

•        Membrane potential (Vm)

•         Is a force

•         Due to unequal distribution of anions and cations across cell membrane

•         Charge separation = source of energy

•        Magnitude of Vm

•         Strength of force

•         Usually measured in millivolts

•         Has a polarity (reference is ICF)

•        Principles

•         Opposite charges attract

•         Like charges repel

•        Direction of force depends on

•         Polarity charge on particle

•        Magnitude of force depends on

•         Strength of Vm

•         Amount of charge on particle

 

 

Electrical Driving Force

 

 

Electrochemical Driving Force

•        Total force acting on particles

•        Sum of chemical and electrical forces

•        If chemical and electrical forces act in same direction

•         Electrochemical force acts in the direction of each

•         Magnitude = sum of chemical force and the electrical force

•        If chemical and electrical forces act in opposite directions

•         Electrochemical force acts in direction of the stronger force

•         Magnitude = larger force minus smaller

•        Passive transport

•         Particles move from high to low force

•         Is spontaneous

•        Active

•         Particles are moved from low to high force

•         Not spontaneous and requires energy

 

Electrochemical Driving Force

•        If a chemical gradient existed, under what conditions would a particle not be

transported across a membrane even if it is permeable to that particle?

•        When the electrical force is equal to, but opposite in direction to the chemical force

Equilibrium Potential

•        Membrane potential (Vm) when

•         Electrical force = chemical force

•         Electrical and chemical act in opposite directions

 

 

 

Passive Transport

•        Spontaneous

•        No cell energy is required

•        Downhill movement

Types of Passive Transport

•        Simple diffusion

•        Facilitated diffusion

•        Diffusion through channels

Simple Diffusion

•        No membrane proteins are needed

•        Transport is through the bilipid layer

•        Basis for simple diffusion

•        Factors affecting the rate

•         Magnitude of the driving force

•         Membrane surface area

•         Membrane permeability

•        Membrane permeability factors

•         Lipid solubility of diffusing substance

•         Size and shape of diffusing particle

•         Temperature

•         Thickness of membrane

 

Simple Diffusion—Rates

 

 

Facilitated Diffusion

•        Passive transport through a carrier

•        Characteristics of a carrier

•         Transmembrane protein

•         Has binding sites for specific particles

•         Binding occurs one side at a time

•         Random conformational changes

•        Factors affecting rate of transport

•         Rate of transport of each carrier

•         Number of carriers in membrane

•         Concentration gradient

 

 

Facilitated Diffusion

Carriers demonstrate saturation

 

Diffusion Through Channels

•        Passive transport through a channel

•        Characteristics of a channel

•         Transmembrane protein

•         Functions like a passageway or pore

•         Substance specific

 

 

Types of Channels

•        Aquaporins

•        Ion Channels

•         Leak channels

•         Gated channels

Ion Channels

•        Factors affecting rate of transport

•         Transport rate of each channel

•         Number of channels in membrane

Active Transport

•        Non-spontaneous

•        Requires cell energy

•        Involves a pump (membrane protein)

•        Movement is uphill

 

 

Characteristics of a Pump

•        A type of membrane protein

•        Function as transporter and enzyme

•        Can harness energy

•        Have specific binding sites

•        Demonstrate saturation

Types of Active Transport

•        Primary active transport

•        Secondary active transport

Primary Active Transport

•        Energy from high energy compound

•        Energy is usually from ATP hydrolysis

Example: Sodium–Potassium Pump

 

 

Primary Active Transport

 

Secondary Active Transport

•        Energy released from ion diffusion

•        Energy drives a pump

•        Diffusion results from previous active transport of ion

 

•        Sodium linked glucose pump

•         Secondary active transport of glucose

•         Diffusion of Na+ provides energy

•         Pump actively transports glucose

•        Sodium linked proton pump

•          Secondary active transport of H+

•         Diffusion of Na+ provides energy

•         Pump actively transports hydrogen ions

Active–Passive Coordination

•        ICF / ECF concentrations differ because of active transport

 Osmosis

  1. Diffusion is a physical process where molecules move down a concentration gradient from higher to lower concentration until equally distributed.
  2. Diffusion applies to any type of molecule; for example, dye crystals are a solute that diffuse in water, the solvent.
  3. Lipid-soluble molecules (alcohols) pass easily through the lipid membrane.
  4. Gases such as oxygen and carbon dioxide diffuse easily through alveoli, capillaries, and blood plasma membranes. 
  5. Implanted timed-release capsules rely on diffusion to distribute medication.
  6. Water probably passes through protein channels in the plasma membrane.

 

  1. Osmosis is a net movement of water molecules from a region of greater concentration to a region of lesser concentration across a differentially permeable membrane. For example, a thistle tube with a differentially permeable membrane separates different concentrations of sugar and water. As water enters the thistle tube, the hydrostatic pressure builds up and the net movement of water ceases. The hydrostatic pressure is equivalent to osmotic pressure of the solution inside the thistle tube.
  2. Due to osmosis, water is absorbed from human large intestine and taken up by blood.

  1. Tonicity refers to the strength of a solution in relationship to osmosis.

 

PCT Basolateral Membrane:
Active Transport

Transport Via Compartments

•        Transport of macromolecules

•        Uses membrane compartments

•         Endocytosis

•         Secretory vesicles

•         Exocytosis

 

Endocytosis

•        Phagocytosis

•        Pinocytosis

•        Receptor-mediated transport

Peptide Hormones: Insulin

Transport Across Epithelium

•        Transport across cell

•        Involves two membranes

 

Epithelial Structure

•        Tight junctions

•        Apical membrane

•        Basolateral membrane

Epithelium: Solute Transport

•        Basolateral membrane

•         Na+ / K+ pump

•         K+ channel

•         Glucose carrier

Epithelium: Solute Transport

•        Apical membrane

•         Na+ channel

•         Na+ linked glucose pump

Epithelium: Na+ Transport

•         Low intracellular Na+

•         Caused by outward pumping basolateral

•         Creates inward gradient

•         Na+ leaks across apical

•        Na+ transported across cell

Epithelium: Glucose Transport

•        Secondary active transport: apical

•        Facilitated diffusion: basolateral

•        Glucose transported across cell

 

Epithelium: Water Transport

•        Solutes pumped into interstitial fluid

•        Osmotic pressure of interstitial fluid increases

•        Water osmotically pulled across cell

•        Water is transported lumen to interstitial fluid

Epithelium: Water Transport

Cystic Fibrosis

•        Thick respiratory mucus

•        Defect in transport mechanism

 

Transcytosis

•        Transport of macromolecules

•        Molecules transported across cell