physiology-membrane transport

The membrane transport

there are different types of basic membrane transport which can help to regulate the solutes such as ions and small molecules through biological membranes of cell and organelles inside the cell.

• Simple Diffusion
• Facilitated Diffusion
• Active Transport
• Endocytosis and Exocytosis

simple diffusion

movement of solutes from higher concentration to lower concentration. (down the concentration gradient only)

Rate of diffusion depends on

• The concentration gradient
• Charge on the molecule
• Size
• Lipid solubility
• Temperature

Facilitated Diffusion

• Carrier-mediated diffusion
• Channel-mediated diffusion

carrier-mediated diffusion

In simple diffusion,flux rate is limited only by the concentration gradient. however, In carrier-mediated transport, the number of available carriers places an upper limit on the flux rate.

Figure 2.  Carrier mediated transport and simple diffusion, the maximal flux difference

The solute acts as a ligand that binds to the transporter protein. and then a subsequent shape change in the protein releases the solute on the other side of the membrane.

Figure 3. Carrier-mediated diffusion

Characteristics of carrier-mediated  diffusion:

net movement always depends on the concentration gradient

• Specificity
• Saturation
• Competition

channel-mediated diffusion

The opening and closing of ion channels results from conformational changes in integral proteins.

Characteristics of ion channels

• Specificity
• Gating

Figure 4. Channel-mediated diffusion

difference between channel, carrier and pores 

Table 1. comparison of channel, carrier and pores

Active transport

• Primary active transport
• Secondary active transport

Primary Active Transport

making direct use of energy derived from ATP to transport the ions across the cell membrane.

Figure 5. Na-K pump cycle demonstrating an example of primary active transport

Other primary active transport

• Primary active transport of calcium
• Primary active transport of hydrogen ions (proton pump in the stomach)

Secondary Active Transport

The ion gradients established by primary active transport permits the transport of other substances against their concentration gradients.

Cotransport

the ion and the second solute cross the membrane in the same direction.

(e.g. Na+-glucose, Na+-amino acid cotransport)

Countertransport

the ion and the second solute move in opposite directions

(e.g. Na+-Ca2+, Na+-H+ exchange)

Figure 6. cotransport and countertransport in secondary active transport

comparison of different types of membrane transport

Figure 7. comparison of membrane transports

Exocytosis and endocytosis

large scale movements of molecules. Exocytosis is a form of active transport and large-scale transport in which transportation of molecules to out of cell occurs. Endocytosis is also a form of active transport in which molecules are brought into the cell.

Figure 8.Exocytosis and endocytosis

Figure 9. Types of endocytosis