Flashcards in BIOL 0800 Reading- Chapter 3 Deck (59):
List four functions of the plasma membrane.
To regulate the passage of substances into and out of cells and between cell organelles and cytosol; Detect chemical messengers arriving at the cell surface; Link adjacent cells together by membrane junctions; Anchor cells to the extracellular matrix
What is one of the advantages of restricting the movements of molecules across membranes?
Confining the products of chemical reactions to specific cell organelles
What are the major membrane lipids?
What is a phospholipid?
Has one end that's charged/polar, and the other end that's two long fatty acid chains that is nonpolar
What are the polar heads of phospholipids oriented toward the surface of the membrane?
Because they are attracted to the polar water molecules in the ECF and cytosol
Do chemical bonds usually link the phospholipids together?
Which membrane type contains cholesterol, intracellular or plasma membrane?
The plasma membrane: increases fluidity
Is cholesterol amphipathic?
Yup: single polar hydroxyl group on its nonpolar ring structure
What role does cholesterol play in the plasma membrane?
Associates with certain classes of plasma membrane phospholipids and proteins to form organized clusters that work together to pinch off portions of the plasma membrane to form vesicles that deliver their contents to various intracellular organelles
What are the two classes of membrane proteins?
Integral and peripheral
What are integral membrane proteins?
Associated with membrane lipids and can't be extracted; amphipathic (polar amino acid side chains on one side, and nonpolar side chains clustered together in a separate region); not all can move freely in the membrane, some are anchored to peripheral proteins
What are most integral proteins referred to as, if they span the entire membrane?
Transmembrane proteins: often cross the lipid bilayer several times
What is a peripheral membrane protein?
Not amphipathic, don't associated with the nonpolar regions of the lipids in the interior of the membrane; located at membrane surface, bound to polar regions of the integral membrane proteins; mostly on cytosolic surface of the plasma membrane, associated with cytoskeletal elements that influence cells shape and motility
Why is glycocalyx?
The layer of short, branched chains of monosaccharides that extend from the cell surface into the ECF; carbohydrates covalently linked to some membrane lipids and proteins
What is the glycocalyx for?
Enabling cells to identify and interact with each other
What are two examples of a region in the plasma membrane that doesn't conform to the fluid-mosaic model?
Lipid rafts (cholesterol-rich regions of reduced membrane fluidity that serve as organizing centers for the generation of complex intracellular signals); also membrane junctions (proteins linked in special patches)
What are lipid rafts?
Cholesterol-rich regions of reduced membrane fluidity that serve as organizing centers for the generation of complex intracellular signals
What are integrins?
Certain transmembrane proteins in the plasma membrane that bind to specific proteins in the extracellular matric and link the to membrane proteins on adjacent cells
What are desmosomes?
Firmly attaches two cells together; regions between two adjacent cells where the apposed plasma membranes are separated by about 20 nm; characterized be accumulations of protein known as "dense plaques" along the cytoplasmic surface that serve as anchoring point for cadherins (proteins that extend from the cell into the extracellular space to link up and bind with cadherins in an adjacent cell)
What are dense plaques?
Proteins that accumulate along the cytoplasmic surface along desmosomes that are anchor points for cadherins
What are cadherins?
Proteins that extend from the cell into extracellular space and bind to cadherins from the next cell; anchored to dense plaques for desmosomes
What are tight junctions?
When the extracellular surfaces of two adjacent plasma membranes join together so that no extracellular space remains between them; occurs in a band around the entire circumference of the cell
What kind of cell type is most often joined by tight junction?
Epithelial cells, usually on the apical surfaces
What is one of the advantages of having tight junctions in, say, the epithelial cells of the intestinal lumen?
Forces molecules to pass through the cells rather than between them; allows the selective barrier to control the types and amounts of substances absorbed
What is a gap junction?
Protein channels linking the cytosols of adjacent cells; connexin proteins from both cells join up and form small protein-lined channels; smaller enough to allow small molecules and ions, but not large proteins
What are connexins?
Proteins involved in linking the cytosols of two adjacent cells in gap junction connections
What is one example of where gap junctions are used?
The muscle cells of the heart, where they transmit electrical activity between cells
What is an example of where desmosomes are used?
In areas that undergo considerable stretching, like in the skin
What is a ligand?
Any molecule of ion that is bound to a protein by 1) electrical attractions between opposite charge/polarized groups on the ligand/protein; 2) weaker attractions due to hydrophobic forces between the nonpolar regions on the two molecules
What are the two ways a ligand is bound to a protein?
First, by electrical attraction (opposite charge/polarization) or Second, weaker attractions due to hydrophobic forces between the nonpolar regions on the two molecules
What is the significant characteristic of the bonding between ligand and protein?
Covalent: aka reversible
What is the binding site?
The region of a protein to which a ligand binds
What typically happens when a ligand binds to a protein?
The protein's conformation changes, activating/inhibiting it's specific function
What must occur first for the ligand to have any chance of binding to a protein?
They need to be close enough for the force of attraction between them to be significant at all.
What is chemical specificity?
The ability of a protein-binding site to bind specific ligands
Are all protein receptors very specific?
Not particularly: some can bind to a variety of related ligands, as long as the portion of the ligand is complementary to the shape of the binding site; bond strength affected by affinity
What is affinity?
The strength of a ligand-protein binding; determines how likely is it that a bound ligand will leave the protein surface and return to its unbound state
What is the difference between chemical specificity and affinity?
Chemical specificity depends on the shape, but affinity depends on the strength of the attraction
Can shape influence affinity?
Yes: the more closely the ligand shape matches the binding site, the higher the affinity
Why is affinity important in physiology and medicine?
If a ligand and binding site have high affinity, less of the ligand is needed to bind to the protein; less drug needed can reduce side effects
What is saturation?
The fraction of total binding sites that are occupied at any given time
What does the percent saturation of a binding site depend on?
The concentration of unbound ligand in the solution AND the affinity of the binding site for the ligand
What is the relationship between percent saturation and ligand concentration?
What is competition among ligands?
The presence of multiple ligands able to bind to the same site affects the percentage of binding sites occupied by any one ligand
What are two ways of controlling protein activity?
Changing protein shape, which alters binding of ligands; AND regulating protein synthesis and degradation, determining the types and amounts of proteins in the cell
What are two mechanisms that alter protein shape?
Allosteric modulation and covalent modulation
What is allosteric modulation?
When the noncovalent binding of a ligand to an alternate binding site alters the shape of the second binding site, which is meant for the proper ligand, changing the binding characteristics of the ligand-binding site
What are the two names for the different sites involved in allosteric modulation?
The functional site (for the ligand to carry out the physiological function) and the regulatory site (to bind to the modulator molecule and allosterically modulate the protein's shape)
What is one advantage of allosteric modulation?
Changes activity of the protein without altering the concentration of the protein or the functional ligand
Does allosteric modulation always involve interaction between a functional site and a regulatory site?
No, can involve two functional sites in certain proteins: when proteins are composed of more than one polypeptide chain (multimeric) held together by an electrical attraction; may be only one binding site on each chain, and the binding of a functional ligand to one of the chains can alter the functional binding site on the other chains = cooperativity
What is cooperativity in allosteric modulation?
With a multimeric protein, when the binding of a functional ligand to one of the amino acid chains' functional site alters the functional site of one of the other amino acid chains
What is an example of cooperativity in allosteric modulation?
Hemoglobin: composed of four polypeptide chains; when oxygen binds to the first binding site, increases affinity for oxygen on other sites
What is covalent modulation?
Covalent bonding of charged chemical groups to some of the protein's side chains; usually involves covalent attachment of a phosphate group through phosphorylation (adds a negative charge, which alters the distribution of electrical forces in the protein and changes its conformation)
What is the main difference between allosteric and covalent modulation?
Covalent modulation requires chemical reactions involving covalent bonds
What are most chemical reactions mediated by?
What is a protein kinase?
An enzyme that mediates protein phosphorylation; catalyze the transfer of phosphate from a molecule of ATP to a hydroxyl group present on the side chain of certain amino acids
What are phosphoprotein phosphatases?
A second class of enzymes that dephosphorylates the protein, returning it to its original shape
Which is more specific, protein kinases or phosphoprotein phosphatases?
Protein kinases; phosphoprotein phosphotases can dephosphorylate muleple phosphorylated proteins