BIOL125 Lecture Notes - Lecture 2: Cardiac Muscle, Exocytosis, Endocytosis
Membrane transport mechanisms
Body fluid
- Large amount of fluid (solution) in cells (ICF – intracellular fluid) and around cells
(ECF – extracellular fluid)
- Water is a high % of the fluid both inside and outside the cell but the tonicity is
similar
- Solutions (eg: cytoplasm, blood or lymph) in the body contain:
oSolutes molecule in smaller amount (molecules eg: plasma proteins,
lipoproteins; ions eg: Na+, K+)
oSolvent substance in larger amount (important solvent in the human body
is water)
- A solute dissolves in a solvent to form a solution
- Solutes, solvents and solutions may be liquids or gases (two solids combined form a
mixtures)
- Solute concentration can be very different intracellularly vs extracellularly
Plasma membrane
- Separates cytoplasm from extracellular fluid (ECF)
- Permits cells to control movement (traffic) of materials in and out of cytoplasm
- Uneven distribution of ions – ECF contains higher amounts of sodium and chloride
ions and lower amounts of potassium ions than ICF
- Maintains concentration differences resting membrane potential
- Tends to be slow drift of potassium ions to the outside of the cell
- Uneven distribution of ions means outside is positively charged and inside is
negatively charged
- Selectively permeable
- Dynamic membrane a lot of activity happening
- Cholesterol also confers some fluidity to the membrane
- Internal cytoskeleton providing support and linkage to neighbouring cells
- Proteins penetrate through the membrane and can act as channel proteins or carrier
proteins
Organization of the plasma membrane
- Phospholipid bilayer (schematic representation)
- Hydrocarbon tails creates hydrophobic region
- Polar head which is hydrophilic charged
The fluid mosaic model of plasma membrane structure
- Phospholipid bilayer + proteins + cholesterol + glycolipids/proteins
Membrane traffic/transport
- The plasma membrane is a barrier, but
oNutrients must get in
oProducts and wastes must get out
- Permeability determines what moves in and out of a cell
oImpermeable – to large, polar substances
oPermeable – to non-polar substances, lipid molecules
oSelectively permeable
Selective permeability
find more resources at oneclass.com
find more resources at oneclass.com
- Restrict
s
materials based on
oSize
oElectrical charge (membrane potential)
oMolecular shape
oSolubility (lipid/water) lipid-soluble substances can get through membrane
very readily
Routes for membrane traffic
- Two-way flow of molecules and ions via
oLipid bilayer very small, uncharged (nonpolar), lipid soluble (dissolve in fat)
eg: CO2, O2, hydrocarbons, alcohol
oTransport proteins (transmembrane) larger size/water soluble/charged
(polar) eg: glucose, sugars, waters, ions
oVesicular transport bulk fluid and solutes bought in by vesicular transport
Transport proteins
- Are specific for the substance passing across
- Two types:
oCarrier proteins blind their passengers and change shape to allow passage
Eg: glucose transporter proteins glucose picked up by ECF, change to
configuration, allowing glucose to be taken into the cell
oChannel proteins pores/tunnels for ions and water-soluble molecules
Open channels eg: Aquaporin water channels, passive ion channels
Gated channels (ie. Controlled channels)
Voltage gated ion channels open and closes in response to
membrane potential/voltage when there’s a stimulus, like
action potential, the gate can open and close
Ligand gated channels gates open and closes in response to
chemical signal
Mechanical gated channels open and closes in response to
mechanical stimulation eg: touch or tactile stimulus, tissue
stretching, vibrations, etc. channel closes when pressure
(mechanical stimulus) is removed
Membrane transport process
- Transport through a plasma membrane can be
oPassive (no metabolic energy required)
oActive (requiring energy – ATP from the cell)
- The different types of transports are
find more resources at oneclass.com
find more resources at oneclass.com
oSimple diffusion (passive), including osmosis
oCarrier-mediated transport (passive or active)
oVesicular transport (active)
Diffusion
- Passive transport
oSpontaneous process (molecule bumps around due to kinetic energy)
oNo energy input ie. Doesn’t use cellular energy (ATP)
oDriven by the concentration/electrochemical gradient of the individual
substance/ion
oNet movement stops when there is no driving gradient force
oCannot create or maintain concentration/electrochemical gradient
o“downhill” movement of particles – ie. Higher to lower concentration
Factors influencing diffusion
- Distance the particle has to move rate of diffusion is faster across smaller distances
- Molecule size larger molecules diffuse slower
- Temperature increased temperature (more kinetic energy) causes an increase in
the rate of diffusion
- Concentration gradient if there is a larger differences between high concentration
and low concentration, diffusion will occur faster than if there is a limited difference
between high and low concentration
- Electrical forces an increase in electrical forces causes diffusion to occur faster
Simple diffusion
- Occurs through
oThe lipid bilayer (the substance must be lipid soluble)
oExamples:
Steroid hormones, fatty acids
CO2, O2
Facilitated diffusion
- Passive process using transport proteins
oChannel proteins
oCarrier proteins when substances of the ECF bind to the carrier protein, the
protein changes shape to move the solute into the cytoplasm inside the
plasma membrane
Osmosis: A special case of facilitated diffusion
- Diffusion of water molecules across a membrane
- Through water channels (aquaporins)
- From [high water] to [low water] and [low solute] to [high solute]
- Osmotic pressure ?? – oncotic pressure??
- In the body, water distributes itself depending on concentration of ions and proteins
Tonicity
- Concentration of solute in a solution
- Hypotonic = dilute
- Hypertonic = concentrated
- Isotonic = same amount of solute and solvent
- Isotonic, hypotonic, hypertonic ECF solutions
- If a semipermeable membrane is present, water can pass across the membrane but
the solute may not (non-penetrating solutes)
find more resources at oneclass.com
find more resources at oneclass.com
Document Summary
Large amount of fluid (solution) in cells (icf intracellular fluid) and around cells (ecf extracellular fluid) Water is a high % of the fluid both inside and outside the cell but the tonicity is similar. A solute dissolves in a solvent to form a solution. Solutes, solvents and solutions may be liquids or gases (two solids combined form a mixtures) Solute concentration can be very different intracellularly vs extracellularly. Permits cells to control movement (traffic) of materials in and out of cytoplasm. Uneven distribution of ions ecf contains higher amounts of sodium and chloride ions and lower amounts of potassium ions than icf. Maintains concentration differences resting membrane potential. Uneven distribution of ions means outside is positively charged and inside is. Tends to be slow drift of potassium ions to the outside of the cell negatively charged. Dynamic membrane a lot of activity happening. Cholesterol also confers some fluidity to the membrane.
