<< Chapter < Page Chapter >> Page >

Signaling molecules

Produced by signaling cells and the subsequent binding to receptors in target cells, ligands act as chemical signals that travel to the target cells to coordinate responses. The types of molecules that serve as ligands are incredibly varied and range from small proteins to small ions like calcium (Ca 2+ ).

Small hydrophobic ligands

Small hydrophobic ligands can directly diffuse through the plasma membrane and interact with internal receptors. Important members of this class of ligands are the steroid hormones. Steroids are lipids that have a hydrocarbon skeleton with four fused rings; different steroids have different functional groups attached to the carbon skeleton. Steroid hormones include the female sex hormone, estradiol, which is a type of estrogen; the male sex hormone, testosterone; and cholesterol, which is an important structural component of biological membranes and a precursor of steriod hormones ( [link] ). Other hydrophobic hormones include thyroid hormones and vitamin D. In order to be soluble in blood, hydrophobic ligands must bind to carrier proteins while they are being transported through the bloodstream.

The molecular structures of estradiol, testosterone, and cholesterol are shown. All three molecules share a four-ring structure but differ in the types of functional groups attached to it.
Steroid hormones have similar chemical structures to their precursor, cholesterol. Because these molecules are small and hydrophobic, they can diffuse directly across the plasma membrane into the cell, where they interact with internal receptors.

Water-soluble ligands

Water-soluble ligands are polar and therefore cannot pass through the plasma membrane unaided; sometimes, they are too large to pass through the membrane at all. Instead, most water-soluble ligands bind to the extracellular domain of cell-surface receptors. This group of ligands is quite diverse and includes small molecules, peptides, and proteins.

Other ligands

Nitric oxide (NO) is a gas that also acts as a ligand. It is able to diffuse directly across the plasma membrane, and one of its roles is to interact with receptors in smooth muscle and induce relaxation of the tissue. NO has a very short half-life and therefore only functions over short distances. Nitroglycerin, a treatment for heart disease, acts by triggering the release of NO, which causes blood vessels to dilate (expand), thus restoring blood flow to the heart. NO has become better known recently because the pathway that it affects is targeted by prescription medications for erectile dysfunction, such as Viagra (erection involves dilated blood vessels).

Section summary

Cells communicate by both inter- and intracellular signaling. Signaling cells secrete ligands that bind to target cells and initiate a chain of events within the target cell. The four categories of signaling in multicellular organisms are paracrine signaling, endocrine signaling, autocrine signaling, and direct signaling across gap junctions. Paracrine signaling takes place over short distances. Endocrine signals are carried long distances through the bloodstream by hormones, and autocrine signals are received by the same cell that sent the signal or other nearby cells of the same kind. Gap junctions allow small molecules, including signaling molecules, to flow between neighboring cells.

Internal receptors are found in the cell cytoplasm. Here, they bind ligand molecules that cross the plasma membrane; these receptor-ligand complexes move to the nucleus and interact directly with cellular DNA. Cell-surface receptors transmit a signal from outside the cell to the cytoplasm. Ion channel-linked receptors, when bound to their ligands, form a pore through the plasma membrane through which certain ions can pass. G-protein-linked receptors interact with a G-protein on the cytoplasmic side of the plasma membrane, promoting the exchange of bound GDP for GTP and interacting with other enzymes or ion channels to transmit a signal. Enzyme-linked receptors transmit a signal from outside the cell to an intracellular domain of a membrane-bound enzyme. Ligand binding causes activation of the enzyme. Small hydrophobic ligands (like steroids) are able to penetrate the plasma membrane and bind to internal receptors. Water-soluble hydrophilic ligands are unable to pass through the membrane; instead, they bind to cell-surface receptors, which transmit the signal to the inside of the cell.

Art connections

[link] HER2 is a receptor tyrosine kinase. In 30 percent of human breast cancers, HER2 is permanently activated, resulting in unregulated cell division. Lapatinib, a drug used to treat breast cancer, inhibits HER2 receptor tyrosine kinase autophosphorylation (the process by which the receptor adds phosphates onto itself), thus reducing tumor growth by 50 percent. Besides autophosphorylation, which of the following steps would be inhibited by Lapatinib?

  1. Signaling molecule binding, dimerization, and the downstream cellular response.
  2. Dimerization, and the downstream cellular response.
  3. The downstream cellular response.
  4. Phosphatase activity, dimerization, and the downsteam cellular response.

[link] C. The downstream cellular response would be inhibited.

Got questions? Get instant answers now!

Questions & Answers

Simple term of science
Palesa Reply
what is the biology? what do you know about biology
Phathu Reply
biology is the study of living organisms, divided into many specialized fields that cover their morphology, physiology, anatomy, behavior, origin, and distribution.
Julia
The study of all aspects of life. The study of all living organisms (such as animal cells and plant cells) in greater detail (their structure and how they function). It's a very broad science.
juanita
what is prokaryotic
Bhaskar Reply
what is pathogens
Bhaskar
transistion metals....
Wasik Reply
Why study ecology
Amos Reply
What name is given to group 8metals on a periodic table
Amos
what is evolution
Elia Reply
Is the gradual change of something it can be either organisms
Amos
which of the following statements about the moss life cycle is false?
Israel Reply
posterior lobe of pitutary contains what?
MR Reply
What if vincristine and colchicibe disrupt mitosis by binding to tubulin
Rohith Reply
A plant in the understory of a forest displays a segmented stem and slender leaves arranged in a whorl. It is probably....
Israel Reply
How did the development of a vascular system contribute to the increase in size of plants?
Israel
If a flower lacked a megasporangium and microsporangium, what type of gametes would not form?
Israel
Seed plants are.. A. all homosporous B. mostly homosporous with some heterosporous C. mostly heterosporous with some homosporous D. all heterosporous
Israel
Besides the seed, what other major structure diminishes a plant's reliance on water for reproduction?
Israel
what role did the adaptations of seed and pollen play in the development and expansion of seed plants?
Israel
Some cycads are considered endangered species and their trade is severely restricted. Customs officials stop suspected smugglers who claim that the plants in their possession are palm trees, not cycads. How would a botanist distinguish between the two types of plants?
Israel
What are the two structures that allow angiosperms to be the dominant form of plant life in most terrestrial ecosystems?
Israel
how are carbohydrates,proteins and fats formed from triose phosphate
fonyuy Reply
Why does the actin filament only move in one direction? Describe in great detail.
Lashonda Reply
Two events happen when calcium binds to troponin.
Lashonda
which of the following statements about plant division is false?
Israel Reply
😂😂😂 i dont know how i reached here
esther
What are the events that occur in each phase of interphase
Hazey Reply
what is photosynthesis
Victor Reply
The process plants use to convert sunlight into food (energy).
juanita
Some other organisms use
juanita
types of photosynthesis
Victor
ps1 and ps2
smritilekha

Get the best Biology course in your pocket!





Source:  OpenStax, Biology. OpenStax CNX. Feb 29, 2016 Download for free at http://cnx.org/content/col11448/1.10
Google Play and the Google Play logo are trademarks of Google Inc.

Notification Switch

Would you like to follow the 'Biology' conversation and receive update notifications?

Ask