Cell Signaling Cell Membrane Structure Consists of lipids

Cell Signaling

Cell Membrane Structure • Consists of lipids, proteins, and carbohydrates. • Fluid mosaic model • Phospholipid bilayer

The Phospholipid Bilayer • Hydrophilic heads that associate with water molecules • Hydrophobic nonpolar fatty acid “tails” that do not dissolve in water

Membrane Proteins • Peripheral membrane proteins lack hydrophobic groups and are not embedded in the bilayer. • Integral membrane proteins have lipid components and are partly embedded in the phospholipid bilayer. A transmembrane protein extends through the bilayer on both sides, and may have different functions in its external and transmembrane domains.

Membrane Carbohydrates Plasma membrane carbohydrates are located on the outer membrane and can serve as recognition sites. • Glycolipid—a carbohydrate bonded to a lipid • Glycoprotein—a carbohydrate bonded to a protein

Selective Permeability Some substances can pass through, others cannot. Two processes of transport: • Passive transport does not require energy. • Active transport requires energy.

Passive Transport • Simple diffusion through the phospholipid bilayer • Facilitated diffusion through channel proteins or aided by carrier proteins

Diffusion is the process of random movement toward equilibrium. Speed of diffusion depends on three factors: • Diameter of the molecules—smaller molecules diffuse faster • Temperature of the solution—higher temperatures lead to faster diffusion

Diffusion • The concentration gradient in the system—the greater the concentration gradient in a system, the faster a substance will diffuse A higher concentration inside the cell causes the solute to diffuse out, and a higher concentration outside causes the solute to diffuse in, for many molecules.

Simple Diffusion A molecule that is hydrophobic and soluble in lipids can pass through the membrane. Polar molecules do not pass through— they are not soluble in the hydrophilic interior and form bonds instead in the aqueous environment near the membrane.

Osmosis is the diffusion of water across membranes. It depends on the concentration of solute molecules on either side of the membrane. Water passes through special membrane channels.

Osmotic Solutions When comparing two solutions separated by a membrane: • A hypertonic solution has a higher solute concentration. • Isotonic solutions have equal solute concentrations. • A hypotonic solution has a lower solute concentration.

Turgor Pressure Cell walls (in plants) limit the volume that can be taken up. Turgor pressure is the internal pressure against the cell wall—as it builds up, it prevents more water from entering.

Facilitated Diffusion Channel proteins are integral membrane proteins that form channels across the membrane. Substances can also bind to carrier proteins to speed up diffusion.

Facilitated Diffusion Ion channels are a type of channel protein—most are gated, and can be opened or closed to ion passage. A gated channel opens when a stimulus causes the channel to change shape. • Chemical stimuli are called ligands. • A stimulus may also be a change in electrical charge.

Active Transport Active transport requires the input of energy to move substances against their concentration gradients. • Primary active transport involves hydrolysis of ATP for energy. • Secondary active transport uses the energy from an ion concentration gradient, or an electrical gradient.

Sodium-Potassium Pump The sodium–potassium (Na+– K+) pump is an integral membrane protein that pumps Na+ out of a cell and K+ in. One molecule of ATP moves two K+ and three Na+ ions.

Use of Vesicles • Macromolecules are too large or too charged to pass through instead pass through vesicles. • Endocytosis • Exocytosis

Endocytosis • Phagocytosis (cellular eating)phagosome is formed and fuses with a lysosome • Pinocytosis (cellular drinking) • Receptor–mediated endocytosisreceptors on integral proteins bind to specific ligands in regions called coated pits.

Exocytosis moves materials out of the cell in vesicles. The vesicle membrane fuses with the plasma membrane and the contents are released into the cellular environment.

Signal Transduction Pathways A signal transduction pathway is a sequence of molecular events and chemical reactions that lead to a cellular response, following the receptor’s activation by a signal.

Types of Signals • Autocrine signals affect the same cells that release them. • Paracrine signals diffuse to and affect nearby cells. • Hormones travel to distant cells.

Allosteric Regulation This involves an alteration in a protein’s shape as a result of a molecule binding to it.

Binding Ligands are generally not metabolized further, but their binding may expose an active site on the receptor. Binding is reversible and the ligand can be released, to end stimulation. An inhibitor, or antagonist, can bind in place of the normal ligand.

Types of Receptors for Binding Cytoplasmic receptors have ligands, such as estrogen, that are small or nonpolar and can diffuse across the membrane. Membrane receptors have large or polar ligands, such as insulin, that cannot diffuse and must bind to a transmembrane receptor at an extracellular site.

Types of Receptors for Binding Receptors are also classified by their activity: • Ion channel receptors • Protein kinase receptors • G protein–linked receptors

Ion Channel Receptors Ion channel receptors, or gated ion channels, change their threedimensional shape when a ligand binds. The acetylcholine receptor, a ligandgated sodium channel, binds acetylcholine to open the channel and allow Na+ to diffuse into the cell.

Protein Kinase Receptors Protein kinase receptors change their shape when a ligand binds. Protein kinases catalyze the following reaction: ATP + protein ADP + phosphorylated protein Each protein kinase has a specific target protein, whose activity is changed when it is phosphorylated.

G Protein Linked Receptors Ligands binding to G protein–linked receptors expose a site that can bind to a membrane protein, a G protein. The G protein is partially inserted in the lipid bilayer, and partially exposed on the cytoplasmic surface.

G Proteins Many G proteins have three subunits and can bind three molecules: • The receptor • GDP and GTP, used for energy transfer • An effector protein to cause an effect in the cell

G Protein Activity The activated G protein–linked receptor exchanges a GDP nucleotide bound to the G protein for a higher energy GTP. The activated G protein activates the effector protein, leading to signal amplification.

The Fight-or-Flight Response Epinephrine (adrenaline) activates the liver enzyme glycogen phosphorylase. The enzyme catalyzes the breakdown of glycogen to provide quick energy. The signal cascade begins when epinephrine stimulates a G protein– mediated protein kinase pathway.

The Fight-or-Flight Response Epinephrine binds to its receptor and activates a G protein. c. AMP is produced and activates protein kinase A—it phosphorylates two other enzymes, with opposite effects: • Inhibition • Activation

The Fight-or-Flight Response • Inhibition—protein kinase A inactivates glycogen synthase through phosphorylation, and prevents glucose storage. • Activation—Phosphorylase kinase is activated when phosphorylated and is part of a cascade that results in the liberation of glucose molecules.

Signal Transduction Ends After the cell responds—enzymes convert each transducer back to its inactive precursor. Cells can alter the balance of enzymes in two ways: • Synthesis or breakdown of the enzyme • Activation or inhibition of the enzymes by other molecules