1

Intracellular vs. extracellular concentrations

Note:Na+, K+, Cl-, phosphate,- & protein-

2

[IC] vs. [EC] important points

*Intracellular cations = Intracellular anions (mEq/L)

*Extracelluar cations = Extracellular anions (mEq/L)

*miniscule, unmeasurable differences

Intracellular particles = Extracellular particles

i.e. IC osmolality = EC osmolality

3

Membrane transport overview

No carrier:

simple diffusion (lipid soluble substances)

diffusion through ion channels

diffusion through water channels

Carrier mediated transport

facilitated diffusion (passive)

primary active transport (active, uses ATP)

secondary active transport (active, uses ion gradient)

Endocytosis & exocytosis

4

Simple diffusion

Through phospholipid bilayer

Lipid soluble substances

e.g. O2, CO2, NH3, N2, fatty acids, steroids, ethanol,

Passive (down concentration gradient)

No carrier ( no saturation, competition)

5

Simple diffusion

fig 4-2

6

Simple diffusion (flux)

At equilibrium:compartment 1 concentration = compartment 2 concentrationone-way flux (left right) = one-way flux (right left)net flux = 0

fig 4-3

7

Simple diffusion (graph of Ci vs. time)

Graph shows that transport is passivei.e. over time Ci will reach, but never exceed Co

fig 4-4

8

Simple diffusion (graph of rate vs. concentration)

Graph shows that transport is not carrier mediated;because no saturation of transport rate

9

Transport through ion channels

fig 4-7

10

Properties of ion channels

Usually (not always) highly specific for the ion

Ion transport is passiveions are chargedtherefore, gradient depends on concentration & chargecombination is “electrochemical gradient”

Channels open and close spontaneously

Percentage of “open time” can be regulated (gating)

Open time regulated by:binding of ligands to the channels (ligand gating)voltage difference across membrane (voltage gating)stretch of membrane (mechanical gating)covalent alteration of channel protein

11

Facilitated diffusion

fig 4-8

12

Facilitated diffusion (properties)

Passive, carrier mediated

Examples: glucose into most cells (not luminal membrane of kidney or intestine), urea, some amino acids

Kinetics:

shows: passive shows: carrier mediated

13

Non-mediated vs. mediated transport

fig 4-9

14

Primary active transport (Na+/K+ ATPase pump)

3 Na+’s out, 2 K+’s in, 1 ATP hydrolyzed

fig 4-11

15

Primary active transport properties

Active (energy from direct hydrolysis of ATP)

Carrier mediated

Used when:many ions moved (e.g. 5 for Na+/K+ ATPase pump)ions moved against steep gradient (Ca++ ATPase in muscle, H+/K+ ATPase in stomach, H+ ATPase in kidney)

16

Primary active transport kinetics

shows active transport shows carrier mediated

17

Effect of Na+/K+ ATPase pump

fig 4-12

18

Secondary active transport

fig 4-13

19

Secondary active transport properties

Active (energy from ion gradient, usually Na+)

Carrier mediated

Can be cotransport (symport) or countertransport (antiport)

Examples (many):Na+/amino acids, Na+/glucose (luminal membrane kidney, GI tract), *Na+/H+ kidney, *Ca++/3Na+ muscle, *Cl-/HCO3

- red cell. (* = countertransport)

Kineticssee primary active transport graphs

20

Transport, the big picture

fig 4-15

21

Table 4-2

22

Water transport (aka osmosis)

Water moves through aquaporin channels

Water moves passively down its own concentration gradient

Dissolving solute in water reduces the water concentration

Water therefore moves from a dilute solution to a more concentrated solution

The “solute concentration” depends on the number of particles

The number of particles is called “osmolarity” (?osmolality?)

The units of osmolarity are milliosmoles/L (mOsm/L)

23

The osmolarity of a 100 mM glucose solution is 100 mOsm/L

A 100 mM NaCl solution dissociates into 100 mM Na+ and 100 mM Cl-; its osmolarity is therefore 200 mOsm/L

Assuming complete dissociation, calculate the osmolarity of the following solutions:

1. 100 mM NaCl, 50 mM urea

2. 200 mM glucose, 30 mM CaCl2

Calculation of osmolarity

Answer: 250 mOsm/L

Answer: 290 mOsm/L

24

Red cells in solution

Notes: nonpenetrating solutes, cell osmolarity ~300 mOsm/L

fig 4-19

25

Crenated red cells

26

Osmolarity and tonicity

Osmolarity is a measure of the total number of particles

Tonicity is a measure of the solute particles which do not cross the cell membrane “non-penetrating solutes”

Tonicity therefore depends on the properties of the solute and the cell membrane

For example, urea crosses most cell membranes, and will enter the cell down its concentration gradient

A solution of 300 mM urea is isosmotic to red cells but is hypotonic

27

Osmolarity and tonicity problems

1. Consider a solution of 100 mM NaCl and 200 mM urea. How does its osmolarity and tonicity compare with red cells having an osmolarity of 300 mOsm/L?

Answer: hyperosmolar and hypotonic

2. Consider a solution of 125 mM NaCl and 50 mM urea. How does its osmolarity and tonicity compare with red cells having an osmolarity of 300 mOsm/L?

Answer: isosmolar and hypotonic

28

Osmolarity (important concept)

Because cells contain abundant aquaporin channels, water rapidly equilibrates across the cell membrane

Therefore, the osmolarity of virtually all body cells is equal, and equal to the osmolality of extracellular fluid

29

Drinking water

30

Endocytosis and exocytosis

fig 4-20

31

Endocytosis and exocytosis properties

Endocytosis:

pinocytosis, phagocytosis

specificity conferred by receptor mediated endocytosis

route: see next slide

Exocytosis:

release of neurotransmitters, hormones, digestive enzymes

route: rough er Golgi secretory vesicles

release usually triggered by cytosolic [Ca++]

insertion of glucose transporters (insulin),

insertion of water channels (ADH)

32

Endocytosis route

fig 4-21

33

Epithelial transport (Na+)

fig 4-22

34

Epithelial transport (water)

fig 4-24

35

Epithelial transport (glucose in kidney, GI tract)

fig 4-23