SIG - Illustrative Examples of Signal Transduction [LESSON]
Illustrative Examples of Signal Transduction
There are many, many illustrative examples of signaling pathways scattered throughout the six kingdoms of life. Here are a few of the most significant examples. You do NOT need to memorize all of the details of each pathway. Rather, focus on learning where and how the signaling is performed.
Nervous System
Watch the Neurons videos below to learn about the basics of nervous system signaling.
Check your knowledge by filling in the blanks for the Nervous System learning object.
Immune System
Watch the Immune System videos below to learn about the basics of immune system signaling.
Check your knowledge by filling in the blanks for the Immune System learning object.
Plant signaling
The sensing of light in the environment is important to plants; it can be crucial for competition and survival. The response of plants to light is mediated by different photoreceptors: a protein covalently-bonded to a light-absorbing pigment called a chromophore; together, called a chromoprotein. The chromophore of the photoreceptor absorbs light of specific wavelengths, causing structural changes in the photoreceptor protein. The structural changes then elicit a cascade of signaling throughout the plant.
The phytochromes are a family of chromoproteins. Phytochromes have two photo-interconvertible forms: Pr and Pfr. Pr absorbs red light (~667 nm) and is immediately converted to Pfr. Pfr absorbs far-red light (~730 nm) and is quickly converted back to Pr. Absorption of red or far-red light causes a massive change to the shape of the chromophore, altering the conformation and activity of the phytochrome protein to which it is bound. Pfr is the physiologically-active form of the protein; exposure to red light yields physiological activity in the plant. Exposure to far-red light converts the Pfr to the inactive Pr form, inhibiting phytochrome activity. Together, the two forms represent the phytochrome system.
The phytochrome system acts as a biological light switch. It monitors the level, intensity, duration, and color of environmental light. The effect of red light is reversible by immediately shining far-red light on the sample, which converts the chromoprotein to the inactive Pr form. Additionally, Pfr can slowly revert to Pr in the dark or break down over time. In all instances, the physiological response induced by red light is reversed. The active form of phytochrome (Pfr) can directly activate other molecules in the cytoplasm, or it can be trafficked to the nucleus, where it directly activates or represses specific gene expression.
This system is used by plants to monitor changes to the amount and intensity of light. For example, as winter approaches, the daylight hours shorten and the plant is able to signal for certain responses. Likewise, as the daylight hours increase as spring approaches, the plant will know that it’s time to produce flowers and pollen as a cellular response.
Plants also use signaling to detect proximity to a light source. Phototropism is the growth of plants in response to light. When the sun is positioned almost directly over the plant, the hormone auxin in the plant stem is evenly distributed. As the sun moves, the auxin is repositioned on the other side of the plant. This overload of auxin next to these cells causes them to start to grow or elongate, tipping the growth of the stem toward the light.
Another tropism is called gravitropism. Whether or not they germinate in the light or in total darkness, shoots usually sprout up from the ground, while roots grow downward into the ground. A plant laid on its side in the dark will send shoots upward when given enough time. Gravitropism ensures that roots grow into the soil and that shoots grow toward sunlight. Growth of the shoot apical tip upward is called negative gravitropism, whereas growth of the roots downward is called positive gravitropism.
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