tonic activity

Examples of tonic activity in the following topics:

• Transduction of Light

  • Photoreceptors in the retina continuously undergo tonic activity.
  • That is, they are always slightly active even when not stimulated by light.
  • In neurons that exhibit tonic activity, the absence of stimuli maintains a firing rate at an equilibrium; while some stimuli increase firing rate from the baseline, other stimuli decrease firing rate.
  • Thus, the visual system relies on changein retinal activity, rather than the absence or presence of activity, to encode visual signals for the brain.
  • When light strikes rhodopsin, the G-protein transducin is activated, which in turn activates phosphodiesterase.

• Osmoregulation

  • Tonicity is the ability of a solution to exert an osmotic pressure upon a membrane.
  • There are three types of tonicity: hypotonic, hypertonic, and isotonic.
  • Tonicity is a concern for all living things.
  • These fish actively take in salt through their gills and excrete diluted urine to rid themselves of excess water.
  • The turgor pressure within a plant cell depends on the tonicity of the solution in which it is bathed.

• Tonicity

  • Tonicity, which is directly related to the osmolarity of a solution, affects osmosis by determining the direction of water flow.
  • Tonicity is the reason why salt water fish cannot live in fresh water and vice versa.
  • Tonicity describes how an extracellular solution can change the volume of a cell by affecting osmosis.
  • A solution's tonicity often directly correlates with the osmolarity of the solution.

• Plasma Membrane Hormone Receptors

  • Binding of these hormones to a cell surface receptor results in activation of a signaling pathway; this triggers intracellular activity to carry out the specific effects associated with the hormone.
  • The activated G protein in turn activates a membrane-bound enzyme called adenylyl cyclase.
  • These activated molecules can then mediate changes in cellular processes.
  • The binding of a hormone at a single receptor causes the activation of many G-proteins, which activates adenylyl cyclase.
  • Hormone binding to receptor activates a G protein, which in turn activates adenylyl cyclase, converting ATP to cAMP. cAMP is a second messenger that mediates a cell-specific response.

• Secondary Active Transport

  • In secondary active transport, a molecule is moved down its electrochemical gradient as another is moved up its concentration gradient.
  • Unlike in primary active transport, in secondary active transport, ATP is not directly coupled to the molecule of interest.
  • Both antiporters and symporters are used in secondary active transport.
  • Secondary active transport brings sodium ions, and possibly other compounds, into the cell.
  • An electrochemical gradient, created by primary active transport, can move other substances against their concentration gradients, a process called co-transport or secondary active transport.

• Cancer and Transcriptional Control

  • Increased transcriptional activation of genes result in alterations of cell growth leading to abnormal gene expression, as seen in cancer.
  • This could lead to increased transcriptional activation of that gene that results in modified cell growth.
  • Researchers have been investigating how to control the transcriptional activation of gene expression in cancer.
  • The EGFR pathway activates many protein kinases that, in turn, activate many transcription factors that control genes involved in cell growth.
  • New drugs that prevent the activation of EGFR have been developed and are used to treat these cancers.

• Activation Energy

  • This small amount of energy input necessary for all chemical reactions to occur is called the activation energy (or free energy of activation) and is abbreviated EA.
  • However, the measure of the activation energy is independent of the reaction's ΔG.
  • The activation energy of a particular reaction determines the rate at which it will proceed.
  • The higher the activation energy, the slower the chemical reaction will be.
  • This figure implies that the activation energy is in the form of heat energy.

• Control of Metabolism Through Enzyme Regulation

  • This prevents the enzyme from lowering the activation energy of the reaction, and the reaction rate is reduced.
  • Allosteric activators can increase reaction rates.
  • Cells have evolved to use feedback inhibition to regulate enzyme activity in metabolism, by using the products of the enzymatic reactions to inhibit further enzyme activity.
  • However, while ATP is an inhibitor, ADP is an allosteric activator.
  • In contrast, allosteric activators modify the active site of the enzyme so that the affinity for the substrate increases.

• Cell Signaling and Cellular Metabolism

  • Metabolic regulation also allows organisms to respond to signals and interact actively with their environments.
  • Firstly, the regulation of an enzyme in a pathway is how its activity is increased and decreased in response to signals.
  • Secondly, the control exerted by this enzyme is the effect that these changes in its activity have on the overall rate of the pathway.
  • Cyclic AMP activates PKA (protein kinase A), which in turn phosphorylates two enzymes.
  • The first enzyme promotes the degradation of glycogen by activating intermediate glycogen phosphorylase kinase (GPK) that in turn activates glycogen phosphorylase (GP), which catabolizes glycogen into glucose.

• Enzyme Active Site and Substrate Specificity

  • Enzymes catalyze chemical reactions by lowering activation energy barriers and converting substrate molecules to products.
  • The enzyme's active site binds to the substrate.
  • The positions, sequences, structures, and properties of these residues create a very specific chemical environment within the active site.
  • Environmental conditions can affect an enzyme's active site and, therefore, the rate at which a chemical reaction can proceed.
  • If the enzyme changes shape, the active site may no longer bind to the appropriate substrate and the rate of reaction will decrease.