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).
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