Ch 6

Interactions

Between Cells and

the Extracellular

Environment

Part 1: Transport Mechanisms

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Cells receive nourishment from and release wastes into

the extracellular environment.

Cells communicate with each other by secreting

chemical regulators into the extracellular environment.

3 Body Fluid Compartments

__ fluid __ fluid

_________ fluid plasma

Relatively free

exchange Selective

exchange

Why ?

?? Composition of Body Fluids ??

Cross out the the wrong one

Composition of Body Fluid Compartments

Barrier between ....

...plasma and interstitial fluid: ______________

Allows water, ions and other small molecules to

pass freely whereas larger molecules such as

_______and blood cells cannot.

... ISF and ICF: ________________

What does selectively permeable mean?

In summary:

Membrane Transport Mechanisms

Terminology: • Permeable

• Impermeable

• Selectively permeable

• Passive transport

• Active transport

Categories of Membrane Transport

Diffusion - Noncarrier-mediated

1. Simple diffusion of lipid-soluble molecules

2. Simple diffusion of ions through channels

3. Simple diffusion of water = osmosis

Carrier-mediated

1. Facilitated diffusion

2. Active transports

Energy

Requirements ?

Diffusion requires a ________________

Fig 6.3

Diffusion is Passive

• Small, nonpolar (or uncharged) lipid-soluble

molecules pass through the lipid bilayer of the

membrane. Examples:

_______________________________________

• Movement ......

• Equilibrium = steady state; net movement has

stopped

Fig 6.2

• What about small charged molecules, such as

_____________________________ ?

• Still passive diffusion if movement down

concentration gradient

• Types of Ion Channels:

Non-gated channels

Gated channels

Mostly open

Mostly closed

Fig 6.2 and 6.6

Rate of Diffusion

Measured by the # of diffusing particles per unit of time

Factors influencing it Diffusion Rate

Concentration

Gradient

Temperature

Permeability

Surface Area

surface conc. membrane area gradient permeability

membrane thickness

Diffusion

rate

X X

Fick’s law of Diffusion

Osmosis Definition?

Special channels called __________

Figs 6.7 & 6.8

• Need solute conc. difference across membrane

• Non-penetrating solutes = osmotically active

solutes

Osmotic Pressure

• Force required to stop osmosis

• Can be used to describe the osmotic pull of a solution.

• A higher solute concentration would require a greater osmotic pressure.

• Pure water has an osmotic pressure of zero

Water moves freely in

body until osmotic

equilibrium is reached!

Osmotic Pressure cont.

Osmotic

pressure

Opposes

movement

of water

across

membrane

1. Molarity vs. Osmolarity

In chemistry:

• Mole / L

• Avogadro’s # / L

In Physiology

Important is not # of

molecules / L but

# of particles / L:

osmol/L or Osm

Why?

Osmolarity takes into account the

dissociation of molecules in solution

Convert Molarity to Osmolarity

Osmolarity = # of particles / L of solution

• 1 M glucose = ? Osm glucose

• 1 M NaCl = ? OsM NaCl

• 1 M MgCl2 = ? OsM MgCl2

• Osmolarity of human body 300 mOsm

Terminology: Isosmotic, hyperosmotic, hyposmotic

You are making up 2 solutions in 2 beakers.

Beaker 1 contains 360 g of glucose/L. You are

adding 180 g of fructose and 180g of glucose to the

second beaker which also contains a liter of water.

The solutions in the two beakers are

1. Iso-osmotic

2. Hyper-osmotic

3. Hypo-osmotic

Tonicity

Physiological term describing volume

change of cell if placed in a solution

Always comparative. Has no units. • Isotonic

• Hypertonic

• Hypotonic

Depends not just on osmolarity (conc.) but also on

nature of solutes: Penetrating vs.

nonpenetrating solutes!

Penetrating vs. Nonpenetrating Solutes

• Penetrating solute: can enter cell (e.g.: glucose, urea, glycerol)

• Nonpenetrating solutes: cannot enter or leave cell (e.g.: sucrose, NaCl*)

• Determine relative conc. of nonpenetrating solutes in solution and in cell to determine tonicity.

• Water will move to dilute nonpenetrating solutes • Penetrating solutes will distribute to equilibrium

Tonicity

The fate of red blood cells in

isotonic, hypotonic, and hypertonic solutions

Fig 6.13

Tonicity, examples

1. A membrane only permeable to water separates a

0.3m glucose solution and a 0.15m NaCl solution.

Is the 0.15 m NaCl solution

• hyperosmotic, hyposmotic, or isosmotic?

• hypertonic, hypotonic, or isotonic?

2. RBCs are placed in a 0.3m solution of urea. (Note:

urea is small and lipophilic.) Is this solution

• hyperosmotic, hyposmotic, or isosmotic?

• hypertonic, hypotonic, or isotonic?

IV Fluids – are for 2 different purposes

You must decide if your patient needs IV Fluid

therapy to....

• ...get fluid into dehydrated cells or

• ...keep fluid in extra-cellular compartment

Regulation of Blood Osmolarity

• Osmolarity of EC

fluid must be

maintained, or

neurons and other

cells will be

damaged.

• Hypothalamic

Osmoreceptors

Fig 6.14

Carrier-Mediated Transport

• Large or polar molecules (e.g.,___________,

__________) cannot diffuse directly across the

membrane

• Require carrier proteins

• Characteristics:

• Specificity (e.g.: GLUT transporters for hexoses)

• Competition (competitive inhibition applied in medicine, e.g.: gout)

• Saturation transport maximum (numbers of carriers can be adjusted)

Fig 6.15

competition

saturation

Facilitated Diffusion

Carrier mediated, _________transport

Net movement from high to low conc.

Transport proteins

may always exist in

plasma membrane or

be inserted when needed

Fig 6.16

Example: GLUT Transporters Four Isoforms:

• GLUT1 – CNS

• GLUT2 – pancreatic beta

cells & hepatocytes

• GLUT3 – neurons

• GLUT4 – adipose tissue &

skeletal muscles. Insertion

regulated by exercise

Cells avoid reaching

glucose equilibrium

Why ?

How ?

Summary: Passive Transport

= Diffusion (Def?) – 3 types:

1. Simple diffusion

2. Osmosis

3. Facilitated diffusion (= mediated

transport)

Insertion of Carrier Proteins into the

Plasma Membrane

Fig 6.17

Active Transport

Movement from low to high conc. (move uphill)

Requires ATP

Creates state of ____ equilibrium

Two types:

1. Primary active transport: ATPases or

“pumps” (uniport and antiport) – examples?

2. Secondary (or coupled) active transport

Symport or antiport

Primary Active Transport

• Hydrolysis of ATP directly

fuels transport.

• Transport protein is also

an ATPase enzyme that

will hydrolyze ATP

• Pump activated by

phosphorylation using a Pi

from ATP.

Fig 6.18

Na+/K+ Pump

• Ubiquitous

• uses up to 30% of cell’s ATP

• ATPase enzyme pumps _____out of the cell and

_____ into the cell

• Maintains ionic imbalance of these two ions across

cell membranes

• Sodium concentration gradient is Epot. and can be

harnessed for other cell functions, e.g.:

• Coupled transport of other molecules

• Electrochemical impulses in neurons and

muscles cells

Mechanism of the Na+/K+-ATPase

Compare

to Fig 6-19

Secondary Active Transport

• Indirect ATP use:

uses Epot. stored in conc. gradient

• Also called coupled transport.

Coupling of Ekin of one molecule

with movement of another.

• Energy needed to move molecules

against their concentration gradient is acquired by

moving sodium back into the cell.

• Since the sodium was originally pumped out of the

cell using ATP, this is considered active transport.

Example: SGLT

Distinguish from GLUT!

Fig 6.20

Sodium /Glucose transporters

1:1 ratio in kidneys

2:1 ratio in GI tract

Uniport vs. Cotransport

Symport Molecules are

carried in same

direction

Examples:

Glucose

and Na+

Antiport Molecules are

carried in

opposite direction

Examples:

Na+/K+

pump

• Uses combination of active and passive transport

• Maybe transcellular or paracellular

• Molecules have to cross two phospho- lipid bilayers

• Polarity of epithelial cells: Different transport proteins on • Apical membrane vs.

• basolateral membrane

Transport Across Epithelial Membranes

(Trans)Epithelial Transport

Absorption from GI tract and

reabsorption from urinary filtrate

Fig 6.21

Bulk Transport

Many molecules moved simultaneoulsy

Large molecules (proteins, hormones, NTs) are

secreted via exocytosis or taken up into the cells

via ______________.

Involves fusion of a

vesicle with the plasma

membrane.

Requires ATP

Again polarity!

Fig 6.23