Transport Across

Membrane

The selective permeability of

biological membranes to small

molecules allows the cell to control

and maintain its internal composition

Transport of molecules:

1. Passive diffusion

2. Facilitated diffusion

3. Active transport

PASSIVE DIFFUSION

• Molecules simply dissolves in the phospholipid

bilayer, diffuses across it and then dissolve in the

aqueous solution at the other side of the membrane

• No membrane proteins are involved

• Direction of the transport is determined simply by the

relative concentration of the molecules inside and

outside of the cell

• Net flow of molecules is from high concentration

to low concentration

Thus passive diffusion is non-selective, any

molecule that is dissolve in phospholipid bilayer is

able to cross across until equilibrium between

inside and outside is maintained.

Only small uncharged molecules can diffuse freely through

phospholipid bilayers.

e.g. hydrophobic molecules - O2 and CO2, benzene

Small uncharged polar molecules - H2 O, ethanol

but large uncharged polar molecules, such as glucose, fructose

cannot.

Large polar molecules and ions Small uncharged molecules

Charged molecules, such as charged ions – Na+, H+,

K+, and Ca+2 are unable to diffuse through a

phospholipid bilayer.

These molecules pass across membranes

through specific transmembrane proteins

which acts as transporters (carrier and

channel proteins).

FACILITATED DIFFUSION

• Like passive diffusion, the movement of molecules

takes place from high to low concentration

• No energy is required

• In facilitated diffusion, the transport of molecules do

not dissolve in the phospholipid bilayer, instead the

passage is mediated by proteins.

• It allows polar and charged molecules such

as carbohydrates, amino acids, nucleosides

and ions.

• Two classes of protein has been classified:

(i) Carrier Protein

(ii) Channel Protein

Carrier Protein

Carrier Proteins bind specific molecules to be transported and

undergo confirmation change that allow the molecule to pass

through the membrane and released on the other side.

Carrier protein facilitate diffusion of sugars, amino acids and

nucleosides.

Example: Glucose transporter

Glucose transporter:

It is a 55 kDa protein in human RBC in which it represent

approximately 5% of total membrane protein.

The glucose transporter has 12 transmembrane α helices. Polar amino

acid residues located within the phospholipid bilayer (indicated as

dark purple circles) and are binding site of glucose in the interior of

the protein.

(A)Glucose binds to a site exposed on the outside of the plasma

membrane.

(B)The transporter then undergoes a conformational change such that the

glucose-binding site faces the inside of the cell and glucose is released

into the cytosol.

(C) The transporter then returns to its original conformation.

Channel Protein

It simply form open pores in the membrane allowing

small molecules of the appropriate size and charge

to pass freely through the lipid bilayer.

Example: Porins

• It also allow the passage of molecules between

the cells connected at the gap junctions.

• Three properties of ion channels are central to

their function:

(1) Transport through channel is extremely rapid.

(2) Ion channel is highly selective because narrow pores

in the channel restrict the passage to ions of appropriate size

and charge. Thus specific channel protein allows passage of

Na+, K+ and Ca+2 across the membrane

(3) Most channel are not permanently open, rather they

open in response to specific stimuli.

example: opening in response to binding to

neurotransmitter (ligand gated channels) or in response to

electric potential (voltage gated channel).

K+

Na+

Ca2+

K+

Na+

Ca2+

5 mM

100 mM

145 mM

10-20 mM

2.5-5 mM

0.0001 mM

Mammalian cell

outside

inside

1. The ability of cell to generated such steep concentration gradients

across it plasma membrane is maintained by Active transport.

2. Like facilitated diffusion, active transport depend on integral

membrane protein i.e., binding – confirmation change – transfer to

other side.

3. However, movement of solute is against gradient and required energy.

4. The movement of ions or other solutes across the membrane against a

concentration gradient is coupled to an exergonic process such as:

(i) hydrolysis of ATP

(ii) Absorption of light

(iii) Transport of electrons

(iv) flow of other substances down a gradient.

ACTIVE TRANSPORT

Active transport driven by ATP hydrolysis:

1. In this case, the ion pump responsible for

maintaining the gradients across the membrane is

Na+- K+ ATPase or Na+- K+ pump.

2. Na+- K+ ATPase uses energy derived from ATP

hydrolysis to transport Na+ and K+ against their

electrochemical gradient.

3. This process is a result of ATP driven conformation

changes in the pump.

i. In this process 3 Na+

bind to site sites

exposed inside the cell

ii. Binding of Na+

stimulates ATP

dependent

phosphorylation of

pump.

iii. Phosphorylation expose

the Na+ binding sites to

the cell surface and

lower their binding

affinity so Na+ is

released outside cell.

iv. At a same time 2 K+

bind to high affinity site

exposed on the cell

surface.

v. The binding of K+

stimulates

dephosphorylation of

the pump.

vi. The pump then release

K+ and return to original

shape.

Active transport driven by ion gradient or Co-transport:

1. The epithelial cells lining the intestine provide a good example of

active transport driven by the Na+ gradient.

2. The movement of glucose across the apical plasma membrane

against the concentration gradient occurs by cotransport called as

Na+/glucose cotransport

3. The Na+ gradient established by Na+ K+ pump provide a source of

energy to power active transport of sugar, amino acid and ions in

mammalian cells.

4. The transport of glucose is carried out by a transporter that

coordinately transport 2 Na+ and 1 glucose into the cell.