Membrane Structure

The plasma membrane contains two sheets of phospholipids that are diverse and asymmetrical. It is a fluid (mosaic) structure.

The types of lipids found in the plasma membrane include phospholipids, glycerophospholipids (with a glycerol backbone), sphingolipids (with a sphingosine backbone), and cholesterol.

Diffusion is the movement of particles down a concentration gradient. It depends on factors such as size, shape, solvent viscosity, and temperature.

Integral membrane proteins are embedded in the plasma membrane. They can be subclassified as monotopic (one side of the bilayer) or ditopic (transmembrane).

Seven-transmembrane receptors contain seven transmembrane α helices that span the membrane. They are involved in cell signaling and can include examples such as integrins and aquaporins.

Peripheral membrane proteins are associated with the surface of the membrane and can be linked through a hydrophobic anchor. They can be dissociated from the membrane by changes in pH, salt, urea, or guanidinium hydrochloride.

Transporters are methods for molecules to pass in and out of the cell or organelle. The three systems involved are uniporter, symporter, and antiporter.

Active transport requires energy and moves molecules up a concentration gradient, while passive transport moves molecules down a concentration gradient and can occur through channels.

Lipid rafts are structures composed of cholesterol and sphingolipids that aggregate in the plasma membrane. They are involved in cell signaling and tethering cell membranes.

Osmosis is the movement of solvent through a semipermeable barrier from a region of low to high solute concentration.

Membranous structures can either fuse together when a hormone or neurotransmitter is secreted from the cell, or they can bud or bleb off from an existing structure.

The components of cell membranes depicted in Figure 5.3 include membrane proteins and the plasma membrane.

The main components of cell membranes are lipids and proteins. Polar lipids are significant in membrane formation because they are amphipathic, meaning they have both polar and nonpolar components, which is important in forming a membrane.

The lipids found in cell membranes have two acyl chains (long-chain fatty acids) esterified to a glycerol backbone. The structure of these lipids contributes to membrane fluidity, which is enhanced at higher temperatures and affected by the composition of the bilayer.

Membranes are often seen as something static, but in reality, they are dynamic, fluid, and only a few molecules thick.

Lipids are hydrophobic molecules that can contain water-soluble functional groups. Their role in cell membranes is significant, as polar lipids are amphipathic and contribute to the structure and function of membranes.

Cellular communication involves the generation of a signal by a cell, reception of the signal by a target cell, transduction of the signal across the target cell membrane, response by the target cell, and termination of the signal. Each step is crucial for the successful communication and coordination of biological processes within and between cells.

Signal transduction involves a chemical signal binding to a receptor, leading to a conformational change and the production of second messengers, which amplify the signal and activate or inhibit protein kinases. The advantages of this system include its ability to be both general and highly specific, allowing one signal to have multiple different outcomes in different tissues based on the types of receptor and downstream signaling pathways activated, and enabling multiple chemical signals to bind to a single receptor and elicit the same response.

The fundamental process depicted in Figure 5.8 'Signal Transduction Diagram' illustrates the chemical signal binding to a receptor, leading to a conformational change, the production of second messengers, and the activation or inhibition of protein kinases, which ultimately regulate biological processes within the cell.

Signaling pathways act to regulate biological processes by coordinating and controlling various cellular activities. They can impact gene expression, cell growth, differentiation, and survival, as well as influence responses to environmental stimuli. Signaling pathways play a critical role in maintaining cellular homeostasis and coordinating complex physiological processes.

Protein Kinase A (PKA) is involved in various cellular pathways including lipolysis, glycogen metabolism, and neurotransmission. It is a heterotetrameric enzyme complex with two catalytic and two regulatory subunits, containing two cAMP binding sites in the regulatory subunits. The PKA pathway is regulated by GTP hydrolysis, requiring the Gα subunit to reassociate with the Gβγ dimer and a ligand-bound receptor for reactivation.

The Gα subunit, bound to GTP, activates adenylate cyclase, which then catalyzes the formation of cyclic AMP (cAMP) from ATP, illustrating signal amplification.

Insulin is a pancreatic hormone that plays a crucial role in glucose metabolism, diabetes, and gene expression, acting as a receptor tyrosine kinase (RTK). The insulin receptor is a kinase that phosphorylates tyrosine residues, and it binds to and phosphorylates the insulin receptor substrate (IRS-1), a scaffolding protein. The insulin signaling pathway activates the Erk proteins of the MAP kinase cascade, affecting gene expression, cell growth, and differentiation.

The phosphoinositide (PI) cascade involves the phosphorylation of phosphatidylinositol, ultimately affecting cellular signaling and regulating cross talk between signaling pathways. PIP3 activates phosphoinositide dependent kinase 1 (PDK1), which phosphorylates and activates the kinase Akt, leading to the translocation and fusion of Glut4-coated vesicles to the plasma membrane, increasing glucose transport.

Akt increases protein expression and production via a signaling cascade involving the protein complex mTOR.

AMP Kinase is a cytosolic kinase activated by binding of AMP or phosphorylation by kinases, acting as an energy sensor and being regulated by PKA, insulin, and calmodulin kinase kinase.

cAMP, or cyclic AMP, serves as a second messenger that activates PKA by binding to its regulatory subunits. This binding causes the release of the catalytic subunits, which then phosphorylate target proteins, leading to the regulation of various cellular pathways such as lipolysis, glycogen metabolism, and neurotransmission.

Insulin activates its receptor, a tyrosine kinase, leading to the phosphorylation of tyrosine residues on the insulin receptor substrate (IRS-1). This phosphorylation initiates downstream signaling cascades, ultimately impacting glucose metabolism, diabetes, and gene expression.

Activation of the insulin signaling pathway leads to the phosphorylation and activation of Akt, which in turn promotes the translocation and fusion of Glut4-coated vesicles to the plasma membrane. This process increases the uptake of glucose into the cells.

The phosphorylation of phosphatidylinositol in the PI cascade plays a crucial role in modulating cellular signaling and facilitating cross talk between different signaling pathways, thereby influencing various cellular processes.

AMP Kinase serves as an energy sensor and is activated by the binding of AMP or phosphorylation by kinases. It is regulated by multiple kinases, including PKA, insulin receptor kinase, and calmodulin kinase kinase, allowing it to integrate signals related to cellular energy status.