BASIC MECHANISM OF TRANSPORT

BASIC MECHANISM OF TRANSPORT

Two types of basic mechanisms are involved in the transport of substances across the cell membrane:

1. Passive transport mechanism

2. Active transport mechanism.

PASSIVE TRANSPORT

Passive transport is the transport of substances along the concentration gradient or electrical gradient or

both (electrochemical gradient). It is also known as diffusion or downhill movement. It does not need

energy. Passive transport is like swimming in the direction of water flow in a river. Here, the substances

move from region of higher concentration to the region of lower concentration. Diffusion is of two types, namely simple diffusion and facilitated diffusion. Simple diffusion of substances occurs either through lipid layer or protein layer of the cell membrane. Facilitated diffusion occurs with the help of the carrier

proteins of the cell membrane. Thus, the diffusion can be discussed under three headings:

1. Simple diffusion through lipid layer

2. Simple diffusion through protein layer

3. Facilitated or carrier-mediated diffusion.

SIMPLE DIFFUSION THROUGH LIPID LAYER

Lipid layer of the cell membrane is permeable only to lipid-soluble substances like oxygen, carbon dioxide and alcohol. The diffusion through the lipid layer is directly proportional to the solubility of the substances in lipids.

SIMPLE DIFFUSION THROUGH PROTEIN LAYER

Protein layer of the cell membrane is permeable to water-soluble substances. Mainly, electrolytes diffuse

through the protein layer.

Protein Channels or Ion Channels

Throughout the central lipid layer of the cell membrane, there are some pores. Integral protein molecules of protein layer invaginate into these pores from either surface of the cell membrane. Thus, the pores present in the central lipid layer are entirely lined up by the integral protein molecules. These pores are the hypothetical pores and form the channels for the diffusion of water, electrolytes and other substances, which cannot pass through the lipid layer. As the channels are lined by protein molecules, these are called protein channels for water-soluble substances.

Types of Protein Channels or Ion Channels

Characteristic feature of the protein channels is the selective permeability. That is, each channel can permit only one type of ion to pass through it. Accordingly, the channels are named after the ions which diffuse through these channels such as sodium channels, potassium channels, etc.

Regulation of the Channels

Some of the protein channels are continuously opened and most of the channels are always closed. Continuously opened channels are called ungated channels. Closed channels are called gated channels.

Gated Channels

Gated channels are divided into three categories:

i. Voltage- protein layer invaginate into these pores from either surface of the cell membrane. Thus, the pores present in the central lipid layer are entirely lined up by the integral protein molecules. These pores are the hypothetical pores and form the channels for the diffusion of water, electrolytes and other substances, which cannot pass through the lipid layer. As the channels are lined by protein molecules, these are called protein channels for water-soluble substances.

Types of Protein Channels or Ion Channels

Characteristic feature of the protein channels is the selective permeability. That is, each channel can permit only one type of ion to pass through it. Accordingly, the channels are named after the ions which diffuse through these channels such as sodium channels, potassium channels, etc.

Regulation of the Channels

Some of the protein channels are continuously opened and most of the channels are always closed. Continuously opened channels are called ungated channels. Closed channels are called gated channels.

Gated Channels

Gated channels are divided into three categories:

i. Voltage-gated channels

ii. Ligand-gated channels

iii. Mechanically gated channels.

i. Voltage-gated channels

Voltage-gated channels are the channels which open whenever there is a change in the electrical potential. For example, in the neuromuscular junction, when action potential reaches axon terminal, the calcium channels are opened and calcium ions diffuse into the interior of the axon terminal from ECF.

Similarly, in the muscle during the excitation-contraction coupling, the action potential spreads through

the transverse tubules of the sarcotubular system. When the action potential reaches the cisternae, large number of calcium ions diffuse from cisternae into sarcoplasm.

ii. Ligand-gated channels

Ligand-gated channels are the type of channels which open in the presence of some hormonal substances. The hormonal substances are called ligands and the channels are called ligand-gated channels. During the transmission of impulse through the neuromuscular junction, acetylcholine is released from the vesicles. The acetylcholine moves through the presynaptic membrane (membrane of the axon terminal) and reaches the synaptic cleft. Then, the acetylcholine molecules cause opening of sodium channels in the postsynaptic membrane and sodium ions diffuse into the neuromuscular junction from ECF.

iii. Mechanically gated channels

Mechanically gated channels are the channels which are opened by some mechanical factors. Examples are, channels present in the pressure receptors (Pacinian corpuscles) and the receptor cells (hair cells).