Membrane Transport - Clinical Tree

Cellular Transport: Passive and Active Mechanisms

Ions and solutes move through several different types of carrier proteins and channels that allow solute movement through the plasma membrane in several different ways. The carriers and channels include:

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Ion channels and pores that allow diffusion of solutes between compartments.
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Uniporters , which are membrane transport proteins that recognize specific molecules, such as fructose.
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Symporters that transport a cation (or cations) down its concentration gradient with another molecule (either another ion or a sugar, amino acid, or oligopeptide).
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Antiporters that transport an ion down its concentration gradient while another substance is transported in the opposite direction. This type of transport is often associated with Na ⁺ transport, or it may be dependent on gradients of other ions, as in the case of the HCO ₃ ⁻ /Cl ⁻ exchanger.

Transporters (or carrier proteins) can be energy dependent or independent. Movement through channels and uniporters follows the concentration gradient of the molecule or the electrochemical gradient established by movement of other ions. However, most movement in nonexcitable cells occurs though some expenditure of energy, either by primary or secondary active transport.

Passive Transport

Regardless of the type of carrier or channel involved, if no energy is expended in the transport process, it is considered passive transport . Passive transport can occur via simple or facilitated diffusion .

Simple Diffusion

If a substance is lipid soluble (which is a property of gases, some hormones, and cholesterol), it will move down its concentration gradient through the cell membrane by simple diffusion ( Fig. 2.1 ). This movement is described by Fick's law .

\begin{matrix} Fick's Law \\ J_{i} = D_{i} \times A (1 / X) \times (C_{1} - C_{2}) \end{matrix}

$\begin{matrix} Fick's Law \\ J_{i} = D_{i} \times A (1 / X) \times (C_{1} - C_{2}) \end{matrix}$

Where:

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J _i represents net flux
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D _i is the coefficient of diffusion
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A is the area
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X is the distance through the membrane
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(C ₁ − C ₂ ) is the difference in concentration across the membrane

Thus passive diffusion of a molecule across a membrane will be directly proportional to the surface area of the membrane and the difference in concentration of the molecule and inversely proportional to the thickness of the membrane.

Figure 2.1, Diffusion Through a Semipermeable Membrane

Facilitated Diffusion

Facilitated diffusion can occur through either gated channels or carrier proteins in the membrane. Gated channels are pores that have “doors” that can open or close in response to external elements, regulating the flow of the solute ( Fig. 2.2, A ). Examples include Ca ²⁺ , K ⁺ , and Na ⁺ . This type of gated transport into and out of the cell is critical to most membrane potentials, except the resting potential (see Chapter 3 ). When facilitated diffusion of a substance involves a carrier protein, binding of the substance to the carrier results in a conformational change in the protein and translocation of the substance to the other side of the membrane ( Fig. 2.2, B ).

Simple and facilitated diffusion do not require expenditure of energy, but they do depend on the size and composition of the membrane and the concentration gradient for the solute. The following key differences exist between these two types of diffusion:

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Simple diffusion occurs over all concentration ranges at a rate linearly related to the concentration gradient; if the concentration gradient increases, the rate of diffusion from the compartment with high concentration to the compartment with low concentration will increase.
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Facilitated diffusion is subject to a maximal rate of uptake (V _max ) . The rate of facilitated diffusion is greater than that of passive diffusion at lower solute concentrations. However, at higher solute concentrations the rate of facilitated uptake reaches its V _max (i.e., the carrier is saturated), whereas the rate of passive diffusion is not limited by a carrier. Another characteristic of facilitated diffusion is that the V _max can be increased by adding transport proteins to the membrane, which is a key regulatory aspect of the transport process.

Active Transport

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