Transport of Substances Throughthe Cell Membrane

Molecular Gradients

Na+

K+

Mg2+

Ca2+

H+

HCO3-

Cl-

SO42-

PO3-

protein

inside(in mM)

1414020

10-4

(pH 7.2)10

52

75

40

outside(in mM)

14241-21-2(pH 7.4)2811014

5

• barrier to water and water-soluble substances

ions glucose H2Ourea

CO2

O2N2

Lipid Bilayer:

•Channel proteins allow free movement of water and selected ions•Carrier proteins bind with molecules or ions that are to be transported •Both channel proteins and the carrier proteins are highly selective in the types of molecules

K+

Channel protein carrier proteins

Proteins:

Diffusion Active Transport• occurs down a concn. gradient• no mediator or involves a “channel” or “carrier”• no additional energy

• occurs against a concn. gradient• involves a “carrier”

• requires ENERGY

Simple diffusion can occur through the cell membrane by two pathways (a) Through lipid bilayer if the diffusing substance is lipid

soluble(b) Water-soluble molecules cross via channels or pores

(a) (b)

Simple Diffusion

ungated• determined by size, shape, distribution of charge, etc.

Characteristics:

Na+

in

outNa+ and other ions

gated• voltage (e.g. voltage-dependent Na+ channels)• chemically (e.g. acetylcholine receptor channels)

Ion Channels

Rate of diffusion is limited by

− Vmax of the carrier protein

− the density of carrier proteins in the membrane

Facilitated Diffusion

(also called carrier mediated

diffusion)

rate of diffusion

Concn of substance

simple diffusion

Simple vs. Facilitated

Tm

facilitated diffusion

What limits maximum rate of facilitated diffusion?

Vmax

Factors that affect the net rate of diffusion:

1. Concentration difference (Co- Ci)

net diffusion (Co- Ci)

-

2. Electrical potential (EMF)

The Nernst potential (equilibrium potential) is the theoretical intracellular electrical potential that would be equal in magnitude but opposite in direction to the concentration force.

EMF (mV) = ±61 log (Co / Ci)

-

--

- -

-

- -

- -

-

- -

- -

-

- -

- -

-

--

- - -

--

-

-

--

--

-

+

-

-

---

-

-

-

- -

-

-

--

-- -+ When will the

negatively charged molecules stop entering the cell?

-

3. Pressure difference

• Higher pressure results in increased energy available to cause net movement from high to low pressure.

Osmosis:- Net diffusion of water -Osmosis occurs from pure water toward a water/salt solution. Water moves down its concn gradient.

Osmotic Pressure:

the amount of pressure required to counter osmosis

Osmotic pressure is attributed to the osmolarity of a solution

Relation between osmolarity and molarity

mOsm (millisomolar) = index of the concn or mOsm/L of particles per liter solution

mM (millimolar) = index of concn of or mM /L molecules per liter solution

150 mM NaCl =

300 mM glucose =

300 mOsm

300 mOsm

Estimating Plasma Osmolarity

Dominated by [Na+] and the associated anions

Under normal conditions, ECF osmolarity can be roughly estimated as:

POSM = 2.1 x [Na+]p 270-290 mOSM

Active Transport

Primary Active Transport• molecules are “pumped” against a concentration

gradient at the expense of energy (ATP) – direct use of energy

Secondary Active Transport• transport is driven by the energy stored in the

concentration gradient of another molecule (Na+) – indirect use of energy

Primary Active Transport

• carrier protein located on the plasma membrane of all cells

• plays an important role in regulating osmotic balance by maintaining Na+ and K+ balance

• requires one to two thirds of cells energy

1. Na+/K+ ATPase

2. Ca2+ ATPase• present on the cell membrane and the sarcoplasmic reticulum• maintains a low cytosolic Ca2+ concentration

3. H+ ATPase• found in parietal cells of gastric glands (HCl secretion) and intercalated cells of renal tubules (controls blood pH)

Secondary Active Transport

1. Co-transport (co-porters): substance is transported in the same direction as the

“driver” ion (Na+) Examples:

inside

outside

Na+ AA Na+ gluc 2 HCO3-Na+

- co-transport and counter-transport -

2. Counter-transport (anti-porters): substance is transported in the opposite direction as the

“driver” ion (Na+)

Examples:

Na+

Ca2+

Na+

H+ Cl-/H+

Na+/HCO3-

outside

inside