transition state

Examples of transition state in the following topics:

• Activation Energy

  • However, to get them into a state that allows the bonds to break, the molecule must be somewhat contorted.
  • A small energy input is required to achieve this contorted state, which is called the transition state: it is a high-energy, unstable state.
  • For this reason, reactant molecules don't last long in their transition state, but very quickly proceed to the next steps of the chemical reaction.
  • It also moves atoms and bonds within the molecule slightly, helping them reach their transition state.
  • Once reactants have absorbed enough heat energy from their surroundings to reach the transition state, the reaction will proceed.

• Enzyme Active Site and Substrate Specificity

  • The enzyme will always return to its original state at the completion of the reaction.
  • The enzyme contorts the substrate into its transition state, thereby increasing the rate of the reaction.

• Regulator Molecules of the Cell Cycle

  • In the active, dephosphorylated state, Rb binds to proteins called transcription factors, most commonly to E2F .
  • When Rb is bound to E2F, production of proteins necessary for the G1/S transition is blocked.
  • Rb releases E2F, which can now turn on the gene that produces the transition protein and this particular block is removed.
  • Also, note the sharp decline of cyclin levels following each checkpoint (the transition between phases of the cell cycle) as cyclin is degraded by cytoplasmic enzymes.

• Genetic Control of Flowers

  • the plant must pass from sexual immaturity into a sexually mature state
  • The transition to flowering is one of the major phase changes that a plant makes during its life cycle.
  • The transition must take place at a time that is favorable for fertilization and the formation of seeds, hence ensuring maximal reproductive success.
  • The first genetic change involves the switch from the vegetative to the floral state.

• Regulation of the Cell Cycle at Internal Checkpoints

  • These checkpoints occur near the end of G1, at the G2/M transition, and during metaphase .
  • This transition, as with all of the major checkpoint transitions in the cell cycle, is signaled by cyclins and cyclin dependent kinases (CDKs).

• What constitutes a biome?

  • Instead, there is a transition zone called an ecotone, which contains a variety of plants and animals.
  • For example, an ecotone might be a transition region between a grassland and a desert, with species from both.

• Epithelial Tissues

  • Transitional (or uroepithelial) cells appear only in the urinary system, primarily in the bladder and ureter.
  • In other words, the tissue transitions from thick to thin.

• The Fossil Record and the Evolution of the Modern Horse

  • The fossil record of horses in North America is especially rich and contains transition fossils: fossils that show intermediate stages between earlier and later forms.
  • The sequence of transitional fossils was assembled by the American Museum of Natural History into an exhibit that emphasized the gradual, "straight-line" evolution of the horse.
  • Although some transitions were indeed gradual progressions, a number of others were relatively abrupt in geologic time, taking place over only a few million years.

• Biodiversity Change through Geological Time

  • The fossil record of the mass extinctions was the basis for defining periods of geological history, so they typically occur at the transition point between geological periods.
  • The transition in fossils from one period to another reflects the dramatic loss of species and the gradual origin of new species .
  • The transitions between the five main mass extinctions can be seen in the rock strata.

• Pre-Cambrian Animal Life

  • These fossils from South Australia date back 650 million years, actually placing the putative animal before the great ice age extinction event that marked the transition between the Cryogenian period and the Ediacaran period.