Cellular Communications and Junctions - AP Biology

The membrane of the cell is a phospholipid bilayer, which allows hydrophobic molecules to diffuse through it. Small, hydrophobic ligands are able to diffuse through the plasma membrane. Nitric oxide is lipophilic, readily dissolving in lipids, and can diffuse across the plasma membrane. Steroid hormones are hydrophobic, and can thus diffuse across the plasma membrane. Water-soluble ligands cannot diffuse across the plasma membrane to enter a cell.

Calcium is a widely used second messenger of signal transduction. Calcium ions can function as a second messenger because its concentration within cell cytosol is much lower than outside the cell, and it is actively transported out of the cell by protein pumps. Modulation in calcium levels is used to transmit signals from both G protein and receptor tyrosine kinase signaling cascades and is involved in such functions as muscle contractions and synaptic signaling.

The other common second messenger molecule is cAMP.

After a ligand binds to receptor tyrosine kinases, the receptors need to form a dimer to foster activation of their tyrosine kinase activity. After dimerization, a phosphate is transferred from ATP to the amino acid tyrosine at specific sites on the cytoplasmic region of receptor. The phosphorylation of the tyrosines provides a site where other cellular proteins can bind and further relay the signal from the receptor to the cell.

Ligands are molecules that form complexes with other molecules, proteins, or genetic material. The binding of a ligand causes a conformational change to the complex that leads to a response. This response by associated molecules is not limited to activation, but also includes inhibition, partial activation, and inhibition of constitutive activity. Ligands can be either selective or non-selective, meaning that they can bind to only specific molecules or to many receptors, respectively.

Kinases are enzymes that phosphorylate the substrates they bind to, meaning that the kinase transfers a phosphate group to the substrate. ATP donates the phosphate group used in this process. Phosphorylation by kinases can lead to a variety of effects including: activation, inhibition, stabilization, destabilization, and localization. Kinases are very important in cellular activities and metabolic processes.

The Golgi apparatus is an organelle that modifies and packages proteins for export. The Golgi receives proteins in vesicles from the endoplasmic reticulum on the cis face. Protein modification by enzymatic activity occurs within the Golgi. The modified proteins are then packaged into vesicles on the trans face, some of which are destined for extracellular transport via exocytosis.

Glycoproteins are attached to the cell membrane's lipid bilayer. They serve a unique role at the cellular level to help with cell to cell recognition. Glycoproteins serve as unique "markers" allowing nearby cells to know they have reached their destination. Cholesterol is embedded in the cell membrane to maintain fluidity.

Cardiac myocytes (muscle cells) need to be able to contract in unison in order to ensure proper pumping of blood. This is accomplished by specialized junctions called gap junctions that allow cells to communicate with one another very quickly. These junctions join the cytoplasms of adjacent cells, allowing ions to flow between them. When one cardiac cell is depolarized, the gap junctions found in intercalated discs allow the depolarization to jump from cell to cell.

Plasmodesmata are structurally analogous to gap junctions, but are located in plant cells.

Gap junctions form pores connecting neighboring cells and allowing the mixture of cytoplasm and small solutes including ions. Desmosomes are specialized for cell-cell adhesion, and hemidesmosomes are specialized for cell-extracellular matrix adhesion. Tight junctions (occluding junctions) form a seal across a layer of cells and is virtually impermeable.

Gap junctions are protein channels that span the intercellular space that connect two cells. They allow cytoplasmic exchange in animal cells. The diameter of gap junctions limits what is able to travel though them from one cell to another. Ions, amino acids, and small molecules can flow through gap junctions; however, proteins, polysaccharides, and nucleic acids cannot. Gap junctions allow the transfer of small molecules to direct communication and cellular activities.

Plasmodesmata are the plant structures that are analogous to gap junctions in animal cells. Plasmodesmata are protein channels between the cell walls of plant cells. They facilitate communication and the transport of solutes and small proteins between plant cells.

Gap junctions are cellular junctions that attach two or more cells together but also allow the exchange of products through an opening. Tight junctions, desmosomes, and hemidesmosomes do not allow direct communication among cells.

Endocrine signals are signals from distance cells that move using the bloodstream, paracrine signals are signals used to communicate between cells in close proximity, and autocrine signals are signals that are received by the same cell in which the signal originated. Paracrine signals are signal are short-lasting, whereas endocrine signals are long-lasting.

Endocrine signals are signals from distance cells that move using the bloodstream, paracrine signals are signals used to communicate between cells in close proximity, autocrine signals are signals that are received by the same cell in which the signal originated, and direct signaling occurs across gap junctions through the movement of small molecules (such as Calcium ions).

G-protein linked receptors are a type of cell-surface receptor that, when unbound by a ligand, consists of an alphaGDP subunit and a beta gamma subunit. When a ligand binds, GDP is exchanged for GTP, which causes the alphaGTP subunit to dissociate from the receptor and the beta gamma subunit. Then, the alphaGTP and beta gamma subunits can activate other molecules in the cell.

Endocrine signals are signals from cells that move using the bloodstream and signal to distant cells, paracrine signals are signals used to communicate between cells in close proximity, autocrine signals are signals that are received by the same cell in which the signal originated, and direct signaling occurs across gap junctions through the movement of small molecules (such as Calcium ions).

Intracellular receptors are found in the cytoplasm of the cell. Ligands for intracellular receptors are usually small molecules that can pass through the cell membrane, and include substances such as steroid hormones. Upon binding and activation, intracellular receptors bind specific DNA motifs in the nucleus and function as transcription factors, directly changing expression of genes.

In contrast, transmembrane receptors are embedded in the plasma membrane and bind extracellular ligands to mediate intracellular responses. Ligand binding to transmembrane receptors often initiates a signal cascade or mediates channel activity within the membrane of the cell.

Seeing as peptide hormones are generally large, water-soluble molecules, they cannot transverse the phospholipid membrane. Instead, they must act through a membrane-bound protein receptor. Steroid hormones are generally small, fat-soluble organic molecules that can easily travel through the phospholipid membrane and the nuclear membrane. They can then act on transcription factors or interact directly with DNA. Both peptide and steroid hormones initiate changes within the cell; they simply do so by different mechanisms.

Cell signaling is the process used by cells to communicate and control cellular activities. It can occur both within and between cells. The correct sequence of events that takes place during cell signaling is as follows: reception, transduction, and response. The reception stage is the detection of a signal, typically by a receptor on the cell surface. Next, transduction is characterized by the transmission of signals from the cell’s exterior to its interior by way of proteins. Finally, the response is the subsequent cellular reaction to the signaling. Cell signaling is critically important in normal cell function and widely diversified.