Solutions• The ICF and the ECF are homogeneous mixtures of

substances including water, ions, amino acids, disaccharides, triglycerides… called solutions

• Solutions are divided into 2 parts– Solvent

• substance present in greatest amount• the solvent of the body is water

– Solute(s)• substance(s) present in lesser amounts• every other substance in the body is a solute

– ions, carbohydrates, proteins, fats, nucleotides…• Solutions are described in terms of their concentration

– the amount of solutes in a given volume of solution• Units include: molarity, %, weight per volume

ECF vs. ICF• The concentration of individual solutes is different

– the cell membrane transports and separates each solute between the ICF and ECF to maintain optimal working conditions within the ICF

• The total solute concentration of the ECF = ICF– prevents the cell from swelling or shrinking

Concentration (mM)150 100 50 0

Concentration (mM)0 50 100 150

Na +

K +

Mg ++

Cl -

HCO 3-

Protein Phosphates

SO 4--

Org. acids

Major anions:

Major cations:EXTRACELLULAR INTRACELLULAR

Modes of Membrane Transport• Vesicular Transport

– use vesicles to move substances into/out of the cell– transport of substances that are TOO LARGE to move

through a membrane• movement of few very large substances

– cells, bacteria, viruses• movement of very many large molecules

– proteins• Transmembrane Transport

– movement of small molecules through a membrane• plasma membrane, Golgi membrane, ER membrane,

lysosome membrane…– ions, fatty acids, H2O, monosaccharides, steroids,

amino acids…

Vesicles

• “Bubbles” of phospholipid bilayer membrane with substances inside– typically proteins– substances DO NOT contact the cytosol

• Created from a membranous organelle by the “pinching” or “budding” of the membrane

• Can “fuse” (merge) with a phospholipid bilayer membrane

Vesicular Transport - Exocytosis and Endocytosis

• Exocytosis– moves substance out of the cell

• Endocytosis– substances are moved into the cell by trapping them in a

vesicle• cell membrane creates pseudopods (“false feet”)

Exocytosis of Proteins

Exocytosis

Types of Endocytosis• Phagocytosis

– “cell eating” – endocytosis of few very large substances (bacteria,

viruses, cell fragments)– vesicles containing cells fuse with lysosomes which

digest the cells • Pinocytosis

– “cell sipping” – endocytosis of extracellular fluid

• Receptor-mediated endocytosis– endocytosis of a specific substance within the ECF– requires that the substance attaches to the extracellular

portion of an integral membrane receptor protein

Endocytosis – Phagocytosis and Lysosomes

Pinocytosis

Receptor-Mediated

Endocytosis

Transmembrane Transport of Nonpolar vs. Polar Substances

• Nonpolar substances cross a membrane through the phospholipid bilayer– ineffective barrier against the movement of nonpolar

molecules across a membrane• it is impossible to control the movement of nonpolar

molecules through a membrane• Polar substances cross a membrane by moving through

integral membrane transporting proteins– Carriers– Pumps – Channels– Each carrier, pump and channel has a unique tertiary

shape and therefore is designed to transport different substances across a membrane• the cell can control the movement of polar molecules

through a membrane by controlling the activity of the transporting proteins

Carriers and Pumps• Integral membrane proteins that “carry” large polar

molecules (monosaccharides, amino acids, nucleotides…) by changing their shape between 2 conformations – “flip-flop” between being open to the ECF and ICF – transport substances much more slowly across a

membrane compared to channels• the maximum rate at which these proteins can

transport substances across a membrane is limited by how fast they can change shapes

• Pumps hydrolyze a molecule of ATP and use the energy to transport substances across the membrane

• The movement of only one substance across a membrane is called uniport

• The movement of more than one substance across a membrane is called cotransport

Types of Cotransport

• 2 or more substances are simultaneously carried across the cell membrane by a carrier

• Symport – 2 substances are moved across a membrane in the same

direction• Antiport

– 2 substances are moved across a membrane in opposite directions

Channels

• Integral membrane proteins containing a water filled hole (pore) which allows the movement of small polar substances (ions and water)– transport substances very quickly across membrane

• there is virtually no limit to the rate at which substances can cross the membrane through channels

– some channels have “gates” which can be:• open

– allows ion to cross the membrane• closed

– does not allow ion to cross the membrane

Transmembrane Transport• Active

– a transport process that uses an energy source produced by the cell for the movement of a polar substance through a membrane

– substance(s) moves UP a concentration gradient– uses pumps for primary active transport – uses carriers for secondary active transport

• Passive– a transport process that DOES NOT use an energy

source produced by the cell for the movement through a membrane

– substance(s) moves DOWN a concentration gradient • Diffusion

– uses channels or carriers to transport polar substances through a membrane

Concentration Gradients• The difference in the amount of a substance between 2

locations – region of greater amount vs. region of lesser amount– the difference may be LARGE or very small – may exist across a physical barrier (membrane)– may exist across a distance without a barrier

• Form of mechanical energy– location of greater amount provides more collisions– collisions cause substances to move away from one

another• net movement from location of greater amount to

locations of lesser amounts

Concentration Gradients

Concentration Gradients and Transmembrane Transport

• The movement of a substance from a region of lesser amount to a region of greater amount:– is an “uphill” movement– requires an energy source

• Active Transport– causes the gradient to increase

• The movement of a substance from a region of greater amount to a region of lesser amount:– is a “downhill” movement– releases energy during transport

• Passive Transport– causes the gradient to decrease

Transmembrane Concentration Gradients and Equilibrium

• Equilibrium– a condition that is met when substances move down a

gradient until there is NO gradient • equal concentrations of a substance between 2

locations• no net movement of substances from one location to

another – substances do continue to move due to heat energy

– Equilibrium of substances across the various membranes in the cells of the body = DEATH • your body is in a constant battle to ensure equilibrium

of solutes across the membranes is never reached

Transmembrane Concentration Gradients and Steady State

• Steady State– a condition where a gradient is PRESENT and is

MAINTAINED by the constant expenditure of energy by the cell

– unequal concentrations of a substance between 2 locations

– no net movement of substances from one location to another • one substance moving passively down the gradient, is

exactly balanced by one substance moving actively up the gradient

– Life depends upon the ability of the organism to exist in a steady state

Passive Transmembrane Transport – Diffusion• Movement a substance to move DOWN its gradient• Releases energy as a result of the movement• Does not occur if there is no gradient

– Equilibrium

• Nonpolar molecules diffuse through the nonpolar phospholipid bilayer in a process called simple diffusion

• Polar molecules require the help (facilitation) of integral membrane proteins (channels or carriers) to cross the bilayer in a process called facilitated diffusion

Facilitated Diffusion through a Carrier

Factors Affecting the Rate of Diffusion• The rate at which a substance moves by way of diffusion is

influenced by 3 main factors– Concentration gradient

• a large concentration gradient, results in a high rate of diffusion

– Temperature • a high temperature, results in a high rate of diffusion

– heat causes motion– Size of the substance

• a large substance, has a low rate of diffusion– larger objects move more slowly

Osmosis• The diffusion of water (solvent) across a selectively

permeable membrane– requires “water channels” called aquaporins

• Water diffuses toward the location with the greatest amount of solutes (region of less water)

• Solutes generate a force that “pulls” water molecules causing them to move toward the solutes – osmotic pressure

• the greater the osmotic pressure, the greater the amount of water movement

• Water in a container (such as a cell) exerts a pressure on the walls of the container – hydrostatic pressure

• the greater the amount of water in a container, the greater the hydrostatic pressure

• Osmotic pressure is the amount of pressure which must be applied to the membrane in order to opposeosmosis.

due to the force generated by solutes which attracts water molecules towards them.

Tonicity• The ability of the solutes in the ECF to cause osmosis across

the plasma membrane• Isotonic (same)

– solute concentration in ECF = ICF– water does not move into or out of cell causing no change

in the cell volume• no change in intracellular hydrostatic pressure

• Hypertonic (more than) – solute concentration in ECF > ICF– water diffuses out of the cell causing the cell to shrink

(crenate)• intracellular hydrostatic pressure decreases

• Hypotonic (less than) – solute concentration in ECF < ICF– water diffuses into the cell causing the cell to swell

• intracellular hydrostatic pressure increases

Cell in a Hypotonic Solution.

• Cell swells• Increases hydrostatic pressure

Osmosis and cell volume changes

Primary Active Transport• Integral membrane pumps use the energy stored in a

molecule of ATP to transport substances across a membrane UP a concentration gradient – The pump:

• hydrolyzes the high energy bond in a molecule of ATP (releasing energy)

• uses the energy of ATP hydrolysis to “flip-flop” between conformations while moving substances UP a concentration gradient

• This process converts chemical energy (ATP) to mechanical energy (gradient across the membrane)– the gradient can be used if necessary by the cell as a form

of energy to do work

Examples of Primary Active Transport

• Na+,K+-ATPase (Na+,K+ pump) – located in the plasma membrane– actively cotransports:

• 3 Na+ from the ICF (lesser amount) to the ECF (greater amount)

• 2 K+ from the ECF (lesser amount) to the ICF (greater amount)

– creates/maintains a Na+ gradient across the cell membrane

– creates/maintains a K+ gradient across the cell membrane

Sodium, Potassium ATPase

Secondary Active Transport• Integral membrane carriers move at least 2 different

substances across a membrane – One substance moves across a membrane UP a

concentration gradient – One substance moves across a membrane DOWN a

concentration gradient– The movements of the substance DOWN a concentration

gradient releases energy which the carrier uses to “flip-flop” between conformations and move a second substance UP a concentration gradient • “piggyback” transport

• This type of transport is called secondary because this process is driven by a gradient that is created by a previous occurring primary active transport process

Example of Secondary Active Transport

• Na+, glucose cotransporter – located in the plasma membrane– cotransports:

• Na+ DOWN its concentration gradient – from the ECF (greater amount) into the ICF (lesser

amount) • gradient created by the Na+,K+-ATPase

– releasing energy that is used by the cotransporter to transport:

• glucose UP its concentration gradient– from the ECF (lesser amount) into the ICF (greater

amount)

Example of Secondary Active Transport

• Na+, glucose cotransporter – located in the plasma membrane– cotransports:

• glucose UP its concentration gradient from the ECF into the ICF driven by the energy released from the movement of:

• Na+ DOWN its concentration gradient (created by the Na+,K+-ATPase) from the ECF into the ICF

Na+, Glucose Cotransporter

Na+, Glucose Cotransporter

Membrane Potential• Although the total solute concentration in the ICF and ECF

are equal, there is an uneven distribution of charged substances across the cell membrane of every cell in the body– creates an electrical potential (energy) between the ICF

and ECF– measured as a voltage

• in millivolts (mV)– causes the cell membrane to be polarized

• a measurable charge difference between the ICF and ECF

• The ICF is negatively charged compared to the ECF– a typical membrane potential is –70 mV

• In an UNSTIMULATED (resting) cell this potential remains constant and is referred to as the resting membrane potential (RMP)

Resting Membrane Potential

Basis of the Resting Membrane Potential• Due to the permeability characteristics of the plasma

membrane to charged (polar) substances– permeability is the ease in which one substance can move

through another substance• Permeable charged substances

– K+ – Na+

Basis of the Resting Membrane Potential• In a resting cell, Na+ and K+ are constantly pumped across

the cell membrane by the Na+,K+-ATPase maintaining: – a high Na+ concentration in the ECF– a low Na+ concentration in the ICF– a high K+ concentration in the ICF – a low K+ concentration in the ECF

Basis of the Resting Membrane Potential

Diffusion of Na+ and K+

• There is a constant diffusion of Na+ into the cell by:• Na+ channels that are always open (leaky)

• There is a constant diffusion of K+ out of the cell by:• open K+ channels that are always open (leaky)

• The permeability of the cell membrane in a resting cell to potassium is approximately 40 times greater than the permeability to sodium– due to a much larger number of potassium leak channels

compared to sodium leak channels

• When a cell is at rest, the pumping of the Na+,K+-ATPase, exactly equals the diffusion of Na+ and K+

– results in a steady state condition

Contribution of Na+ to the RMP• If the cell membrane were permeable only to sodium then

sodium would diffuse into the cell– as sodium diffuses into the cell it causes the inside of the

cell to become positively charged (it only takes a few ions because each ion has a large charge) which begins to reduce additional sodium ion entry (due to repulsion)

• Sodium diffusion stops when the inside of the cell has 58 more mV of charge compared to outside (membrane potential = +58 mV)– at this potential, the concentration gradient moving Na+

into the cell exactly balances the positive electric charge repelling Na+ out of the cell

– Equilibrium potential for Na+ (ENa)

Contribution of K+ to the RMP• If the cell membrane were permeable only to potassium then

potassium would diffuse out of the cell– as potassium diffuses out of the cell it causes the inside

of the cell to become negatively charged (it only takes a few ions because each ion has a large charge) which begins to reduce additional potassium ion exit (due to attraction)

• Potassium diffusion stops when the inside of the cell has 90 less mV of charge compared to outside (membrane potential = -90 mV)– at this potential, the concentration gradient moving K+

out of the cell exactly balances the negative electric charge attracting K+ into the cell

– Equilibrium potential for K+ (EK)

RMP• Note that the RMP is neither equal to ENa or EK, but is

somewhere between these 2 values• If the permeability of these 2 ions through the cell

membrane were exactly the same, the RMP would be exactly between the values of ENa and EK, or -16 mV.

• However, the permeability of the cell membrane to potassium is approximately 40 times greater than that of sodium due to a much greater number of potassium leak channels.

• This causes potassium to have a much greater influence on the RMP compared to sodium, which is why at -70 mV the RMP is closer to -90 mV than +58 mV.