Concept 1: What is cell communication



Concept 1: What is cell communication?

In multicellular organisms, individual cells must communicate and join with one another to create a harmonious organism. Cell junctions can be classified in four functional groups:

1. Tight Junctions (animal cells) – membranes of neighboring cells are actually fused; prevent leakage of extracellular fluid across a layer of epithelial cells.

2. Desmosomes (animal cells) - “anchoring junctions” – function like rivets, fastening cells together in strong sheets (are reinforced by intermediate filaments made of keratin)

3. Gap Junctions (animal cells) - “communicating junctions” provide cytoplasmic channels between adjacent animal cells

4. Plasmodesmata (plant cells) - channels that allow cytosol to pass through and connect the living contents of adjacent cells

Concept 2: What is cell-to-cell recognition?

Cell-to-cell recognition is the cell’s ability to distinguish one type of neighboring cell from another and is crucial to the functioning of a multicellular organism.

← A feature of cells that aids in cell communication is the glycocalyx, which consists of oligosaccharides (small chains of sugar molecules) attached to integral proteins within the plasma membrane.

← The glycocalyx is responsible for contact inhibition, the normal trait of cells to stop dividing when they become too crowded.

Concept 3: What is a signal transduction pathway?

The signal transduction pathway relies on plasma membrane proteins in a multi-step process in which a small number of extracellular signal molecules produce a major cellular response. Essentially, these pathways convert signals on a cell’s surface into cellular responses. Three stages occur in this type of cell signaling:

1. Reception - the signal molecule (from the external environment), commonly a protein that does not enter the cell, binds to a specific receptor protein on the cell surface, causing the receptor molecule to undergo a change in conformation.

2. Transduction - This conformational change in the receptor protein leads to transduction – a change in signal form, where the receptor protein relays a message to a secondary messenger within the cell.

3. Response - This secondary messenger, such as cyclic AMP (cAMP), induces a response within the cell.

Concept 4: What are the three main types of membrane receptors?

There are three main types of membrane receptors

1. G-protein-linked

2. Tyrosine kinases

3. Ion channel

(1) The G-Protein Linked Receptor

Review the animation link below and refer to Figure 11.7 in your textbook on page 211.



A G-protein linked receptor is a membrane receptor that works with the help of a cytoplasmic G protein. Ligand binding activates the receptor, which then activates a specific G-protein, which activates yet another protein in a signal transduction pathway. Ligand is a term used to indicate a small molecule that specifically binds to a larger one – generally causing a conformation change in the shape of the larger molecule. Epinephrine uses this sort of receptor.

(2) The Tyrosine-Kinase Receptor

Review the animation link below and refer to Figure 11.7 on page 212 in your textbook.



Tyrosine-kinase receptors react to the binding of signal molecules by forming dimers (a protein consisting of two polypeptides) and then adding phosphate groups to tyrosines on the cytoplasmic side of the receptor. Relay proteins in the cell can then be activated by binding to different phosphorylated tyrosines, allowing this receptor to trigger several pathways at once. Growth factors commonly use tyrosine-kinase receptors.

(3) Ion Channel Receptors

Review the animation link below and refer to Figure 11.7 on page 213 in your textbook.



In ion channel reception, specific signal molecules cause ligand-gated ion channels in a membrane to open or close, regulating the flow of specific ions. This receptor is a transmembrane protein in the plasma membrane that opens to allow the flow of a specific kind of ion across the membrane when a specific signal molecule binds to the extracellular side of the protein. Ligand-gated ion channels are very important in the nervous system (synapses).

Concept 5: What is the importance of this chapter?

Pathways relay signals from receptors to cellular responses. At each step in the pathway, the signal is transduced into a different form, commonly a conformational change in a protein. Protein phosphorylation (adding a phosphate), a common mode of regulation in cells, is a major mechanism of signal transduction.

Many signal-transduction pathways include phosphorylation cascades, in which a series of protein kinases successively add phosphate groups to the next one in line, activating it.

Certain small molecules and ions are key components of signaling pathways (second messangers), such as cyclic AMP (cAMP) and Ca2+.

In response to a signal, a cell may regulate activities in the cytoplasm or transcription in the nucleus.

Apoptosis (programmed cell death) integrates multiple cell-signaling pathways. Cells that are infected or damaged have reached the end of their functional life and often enter a program of controlled cell suicide. Using signal transduction pathways, the cell components are disposed of in an orderly fashion, without damage to neighboring cells.

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