Transport across the cell membrane

Learning objectives

Body compartments

ECF and ICF Constituents

• Barrier to water and water-soluble substances

ions glucoseH2O

urea

Lipid Bilayer:

CO2

O2N2

halothane

Cell Membrane

ions

glucose

H2O

urea

… but, other molecules still get across!

CO2

O2N2

halothane

Molecular Gradients

Na+

K+

Mg2+

Ca2+

H+

HCO3-

Cl-

SO42-

PO3-

protein

inside

(in mM)

14

140

0.5

10-4

(pH 7.2)

10

5-15

2

75

40

outside

(in mM)

142

4

1-2

1-2

(pH 7.4)

28

110

1

4

5

Types of Transport

• Passive and Active

• Passive

• Simple Diffusion

• Facilitated Diffusion

• Active

• Primary

• Co-Transport

• Counter Transport

• Secondary

(a) lipid-soluble molecules move readily across the membrane

(rate depends on lipid solubility)(b) water-soluble molecules cross via channels or pores

(a) (b)

Simple Diffusion

Fick's law of Diffusion

Several factors affect the rate of net diffusion across the membrane

• The magnitude of the concentration gradient (∆C)

• The permeability of the membrane to substance (P)

• Surface area of the membrane (A)

• Molecular weight of the substance (MW)

• Distance (thickness) ∆X

• Net rate of Diffusion Q = ∆C .P.A

MW. ∆X

Restated=

Q ∆C.A.D P =Diffusion coefficient (D)

∆X √ MW

Factors that affect net rate of diffusion……. cont'd

1. Membrane electrical potential-

Nernst Potential

1. Pressure difference across the

membrane

Nernst Potential:

At normal body temperature, the electrical difference

that will balance a given concentration difference of univalent ions-

such as Na ions- can be determined from the formula

called Nernst equation

EMF (in millivolts) = ±61 log C1/C2

Diffusion Through Protein Channels and “Gating” of These Channels

• Pores Integral cell membrane proteins that form open tubes through the membrane and are always open

Characteristics of protein channels:

• Selective Permeability( due to diameter, shape & charge on channel)

• Gating

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. nicotinic ACh receptor channels)

Ion Channels

Selective permeability of protein channels

Gating of protein channels

Facilitated Diffusion

• Carrier mediated diffusion• A carrier facilitates diffusionof substance to the other sidedownhill.• The rate of diffusion

approaches a maximum called Vmax, as conc of substance increases

Carrier mediated transport system display three important characteristics

1. Specificity - Transport selectivity, Defective transport system for cysteine-

an inherited disease -cysteine urea

2. Saturation –Limited number of carrier binding sites- transport maximum (Tm)

3. Competition- Closely related substances may compete for a ride across

the membrane on the same carrier

Osmosis

The process of net movement of water caused by a concentrationdifference of water, across a selectively permeable membrane is called osmosis

Osmotic pressure

The amount of pressure required to stop osmosis is called osmotic pressure of any particular solution

• Osmotic pressure is determined by number of particles per unit volume of fluid, not by the mass of particles

• 1 osmole is 1 gram molecular weight of osmotically active solute ( 180 gof glucose which is 1 g molecular weight of glucose is 1 Osmole

Osmolality

• Osmolality given in Osmoles

• One osmole is 1 gram mol. wt. of undissociated solute

• One osmole of Glucose (180 gm)

• Two osmoles NaCl (58.5 gm) (2 ions)

• Osmolality is Osmoles/1kg of solvent

• Osmolarity is Osmoles/1L of solution

Active Transport

When a cell membrane moves molecules or ions “uphill” against the concentration gradient ( or uphill against an electrical or pressuregradient), the process is called active transport.

Examples:Sodium, potassium, Calcium, hydrogen, chloride , urate ,some amino acids etc

• Energy is required for active transport

Active transport is divided into two types according to the source of theenergy used

❑ Primary active transport

❑ Secondary active transport

Primary Active Transport

Energy is derived directly from breakdown of ATP (Adenosine triphosphate)

➢ Sodium potassium pump ➢ Substances such as sodium, potassium, calcium, hydrogen, chloride

and few other ions are transported by primary active transport

Na-K PUMP

Functions of sodium potassium pump

Control volume of each cellWithout this function , most cells would swell until they burst.

Electrogenic nature of Na-K pump:It causes negativity inside and positivity outside- cause an electric potential Across the cell membrane

Primary active transport of Calcium ionsTwo Calcium pumps- one in cell membrane and other in intracellular

organelle (sarcoplasmic reticulum in muscle cell) maintain very lowintracellular calcium concentration

Primary active transport of Hydrogen ions:• Gastric glands of the stomach• Late distal tubules and cortical collecting ducts of the kidney

Secondary Active Transport

In secondary active transport, the energy stored due to

an ion gradient is used to transport another substance.

e.g

A large gradient (storehouse of energy) for sodium is created when it

moves to outside the cell membrane by primary active transport.

Types of active transport

Co- transport

Co transport of glucose and

amino acid along with

Sodium ions in

• Epithelial cells of the

intestinal tract

• Renal tubular epithelial cells

Counter Transport

• Sodium- Calcium counter-transport

occurs through all or almost all

Cell membranes

• Sodium- Hydrogen counter transport

in proximal tubules of kidneys

Active transport through cellular sheets

Substances must transfer through

cellular sheets.

• Intestinal epithelium

• Epithelium of renal tubules

• Epithelium of all exocrine glands

• Epithelium of gall bladder

• Membrane of choroid plexus of

brain