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e-book Genes and Membranes: Transport Proteins and Receptors

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According to this principle, a dissolved substance diffuses down a concentration gradient; that is, given no energy from an outside source, it moves from a place where its concentration is high to a place where its concentration is low.

Diffusion continues down this gradually decreasing gradient until a state of equilibrium is reached, at which point there is an equal concentration in both places and an equal, random diffusion in both directions. In performing the work of diffusion, the solute loses free energy, so that, when it reaches equilibrium at a lower concentration, it is unable to return spontaneously under its own energy to its former high concentration.

Membrane protein

However, by the addition of energy from an outside source through the work of an ion pump , for example , the solute may be returned to its former concentration and state of high free energy. See above Coupled chemical reactions. For most substances of biological interest, the concentrations inside and outside the cell are different, creating concentration gradients down which the solutes spontaneously diffuse, provided they can permeate the lipid bilayer.

Membrane channels and diffusion facilitators bring them through the membrane by passive transport; that is, the changes that the proteins undergo in order to facilitate diffusion are powered by the diffusing solutes themselves. For the healthy functioning of the cell, certain solutes must remain at different concentrations on each side of the membrane; if through diffusion they approach equilibrium, they must be pumped back up their gradients by the process of active transport.

Those membrane proteins serving as pumps accomplish this by coupling the energy required for transport to the energy produced by cell metabolism or by the diffusion of other solutes. Permeation is the diffusion, through a barrier, of a substance in solution. The rates at which biologically important molecules cross the cell membrane through permeation vary over an enormous range.

Table of Several Ion Channel Family Members

Proteins and sugar polymers do not permeate at all; in contrast, water and alcohols permeate most membranes in less than a second. This variation, caused by the lipid bilayer, gives the membrane its characteristic permeability. Permeability is measured as the rate at which a particular substance in solution crosses the membrane.


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The consistency of this parallel has led researchers to conclude that permeability is a function of the fatty acid interior of the lipid bilayer, rather than its phosphoryl exterior. This property of dissolving in organic solvents rather than water is given a unit of measure called the partition coefficient. The greater the solubility of a substance, the higher its partition coefficient, and the higher the partition coefficient, the higher the permeability of the membrane to that particular substance.

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For example, the water solubility of hydroxyl, carboxyl, and amino groups reduces their solubility in organic solvents and, hence, their partition coefficients. Cell membranes have been observed to have low permeability toward these groups.

The human ATP-binding cassette (ABC) transporter superfamily

In contrast, lipid-soluble methyl residues and hydrocarbon rings, which have high partition coefficients, penetrate cell membranes more easily—a property useful in designing chemotherapeutic and pharmacological drugs. For two molecules of the same partition coefficient, the one of greater molecular weight , or size, will in general cross the membrane more slowly. PDE is always present in the cell and breaks down cAMP to control hormone activity, preventing overproduction of cellular products. The specific response of a cell to a lipid insoluble hormone depends on the type of receptors that are present on the cell membrane and the substrate molecules present in the cell cytoplasm.

Cellular responses to hormone binding of a receptor include altering membrane permeability and metabolic pathways, stimulating synthesis of proteins and enzymes, and activating hormone release. Hormones cause cellular changes by binding to receptors on target cells. The number of receptors on a target cell can increase or decrease in response to hormone activity. Hormones can affect cells directly through intracellular hormone receptors or indirectly through plasma membrane hormone receptors.

The hormone is called a first messenger and the cellular component is called a second messenger.


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  • G-proteins activate the second messenger cyclic AMP , triggering the cellular response. Response to hormone binding is amplified as the signaling pathway progresses. Cellular responses to hormones include the production of proteins and enzymes and altered membrane permeability. Skip to content Increase Font Size. Unit 4: Animal Structure and Function. Learning Objectives By the end of this section, you will be able to: Explain how hormones work Discuss the role of different types of hormone receptors.

    Intracellular Hormone Receptors. Figure Upon hormone binding, the receptor dissociates from the heat shock protein and translocates to the nucleus. In the nucleus, the hormone-receptor complex binds to a DNA sequence called a hormone response element HRE , which triggers gene transcription and translation. The corresponding protein product can then mediate changes in cell function. Plasma Membrane Hormone Receptors.

    https://senjouin-kikishiro.com/images/bakotud/1320.php Summary Hormones cause cellular changes by binding to receptors on target cells. Exercises A new antagonist molecule has been discovered that binds to and blocks plasma membrane receptors. What effect will this antagonist have on testosterone, a steroid hormone? It will block testosterone from binding to its receptor. It will block testosterone from activating cAMP signaling. It will increase testosterone-mediated signaling. It will not affect testosterone-mediated signaling. What effect will a cAMP inhibitor have on a peptide hormone-mediated signaling pathway?

    It will prevent the hormone from binding its receptor. It will prevent activation of a G-protein. It will prevent activation of adenylate cyclase. It will prevent activation of protein kinases. Name two important functions of hormone receptors. How can hormones mediate changes?

    Answers D D The number of receptors that respond to a hormone can change, resulting in increased or decreased cell sensitivity. The number of receptors can increase in response to rising hormone levels, called up-regulation, making the cell more sensitive to the hormone and allowing for more cellular activity. The number of receptors can also decrease in response to rising hormone levels, called down-regulation, leading to reduced cellular activity. Depending on the location of the protein receptor on the target cell and the chemical structure of the hormone, hormones can mediate changes directly by binding to intracellular receptors and modulating gene transcription, or indirectly by binding to cell surface receptors and stimulating signaling pathways.

    Glossary adenylate cyclase an enzyme that catalyzes the conversion of ATP to cyclic AMP down-regulation a decrease in the number of hormone receptors in response to increased hormone levels first messenger the hormone that binds to a plasma membrane hormone receptor to trigger a signal transduction pathway G-protein a membrane protein activated by the hormone first messenger to activate formation of cyclic AMP hormone receptor the cellular protein that binds to a hormone intracellular hormone receptor a hormone receptor in the cytoplasm or nucleus of a cell phosphodiesterase PDE enzyme that deactivates cAMP, stopping hormone activity plasma membrane hormone receptor a hormone receptor on the surface of the plasma membrane of a cell up-regulation an increase in the number of hormone receptors in response to increased hormone levels.

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