Cell Communication Chapter 11

Cellular Messaging Cell-to-cell communication is essential for both multicellular and unicellular organisms. Cells can signal to each other and interpret the signals they receive from other cells and the environment. Signals include light, touch, and chemicals. Biologists have discovered some universal mechanisms of cellular regulation. The same small set of cell-signaling mechanisms shows up again and again in diverse species. The fight-or-flight response is triggered by a signaling molecule called epinephrine

11.1 External signals are converted to responses within the cell

Evolution of Cell Signaling: S. cerevisiae The yeast, Saccharomyces cerevisiae, has two mating types, a and . Cells of different mating types locate each other via secreted factors specific to each type. Each type secretes a specific factor that binds to receptors only on the other type of cell. When exposed to each other’s mating factors, a pair of opposite cell types fuse (mate). Offspring contains a combination of genetic material that is advantageous.

Figure 11.2  factor Receptor 1 Exchange of mating factors Once received by the yeast cell surface receptor, a mating signal is changed, or transduced, into a form that brings about the cellular response of mating. A signal transduction pathway is a series of steps by which a signal on a cell’s surface is converted into a specific cellular response. a  Each cell type secretes a mating factor that binds to receptors on the other cell type. a factor Yeast cell, mating type a Yeast cell, mating type  2 Mating Binding of the factors to receptors induces changes in cells that leads to their fusion. a  Figure 11.2 Communication between mating yeast cells. 3 New a/ cell a/

Evolution of Cell Signaling: Bacteria Bacterial cells secrete molecules that can be detected by other bacterial cells. Sensing the concentration of such signaling molecules allows bacteria to monitor the local density of cells, called quorum sensing. Quorum sensing allows bacterial populations to coordinate their behaviors in activities that require a given number of cells acting simultaneously. Example: biofilm formation.

Local and Long-Distance Signaling Cells in a multicellular organism communicate by chemical messengers. Animal and plant cells have cell junctions that directly connect the cytoplasm of adjacent cells. In local signaling, animal cells may communicate by direct contact, or cell-cell recognition.

In many other cases, animal cells communicate using local regulators, messenger molecules that travel only short distances.

Long-distance signaling Endocrine cell Blood vessel In long-distance signaling, plants and animals use chemicals called hormones. The ability of a cell to respond to a signal depends on whether or not it has a receptor specific to that signal. Hormone travels in bloodstream. Target cell specifically binds hormone. (c) Endocrine (hormonal) signaling

The Three Stages of Cell Signaling: A Preview Earl W. Sutherland discovered how the hormone epinephrine acts on cells. Sutherland suggested that cells receiving signals went through three processes: Reception Transduction Response Sutherland and his colleagues at Vanderbilt University were investigating how the animal hormone epinephrine stimulates the breakdown of the storage polysaccharide glycogen within liver cells and skeletal muscles. The effect of epinephrine is the mobilization of fuel reserves. Glycogen is broken down into glucose (which goes to the bloodstream) or an early intermediate of glycolysis. Experiment Epinephrine added to test tube with glycogen and glycogen phosphorylase results in no glycogen breakdown. Glycogen phosphorylase could be activated by epinephrine only then the hormone was added to intact cells in a solution. Conclusion Epinephrine does not interact directly with the enzyme responsible for glycogen breakdown. The plasma membrane itself is necessary for transmission of the signal to take place.

Relay molecules in a signal transduction pathway EXTRACELLULAR FLUID CYTOPLASM Plasma membrane 1 Reception 2 Transduction 3 Response Receptor Activation of cellular response Relay molecules in a signal transduction pathway Reception. Reception is the target cell’s detection of a signaling molecules coming from outside the cell. A chemical signal is “detected” when the signaling molecule binds to a receptor protein located at the cell’s surface. Transduction. The binding of the signaling molecule changes the receptor protein in some way, initiating the process of transduction. The transduction stage converts the signal to a form that can bring about a specific cellular response. This stage usually requires a sequence of changes in a series of different molecules. Response. The transduced signal finally triggers a specific cellular response. Signaling molecule

Concept Check Explain how signaling is involved in ensuring that yeast cells fuse only with cells of the opposite mating type. In liver cells, glycogen phosphorylase acts in which of the three stages of the signaling pathway associated with an epinephrine-initiated signal? The two cells of opposite mating type each secrete a certain signaling molecule, which can only be bound by receptors carried on cells of the opposite mating type. Therefore, the a mating factor cannot bind to another a cell. Third stage, response.