Department of Health, Nutrition, and Exercise Sciences Chapter 3: Cells Plasma membrane: structure Plasma membrane: transport Resting membrane potential Cell-environment interactions Cytoplasm Nucleus Cell growth & reproduction Extracellular materials, developmental aspects Department of Health, Nutrition, and Exercise Sciences

Resting membrane potential All living cells at rest Voltage inside is negative relative to outside Ranges from –50 to –100 mV in different cells Results from separation of oppositely charged particles (ions) across the membrane A form of stored (i.e. potential) energy Energy comes from active transport of ions (mainly pumping Na+ out of cells and K+ into cells) Human adipocyte. Yeh & Shi (2010), WIREs Nanomed Nanobiotechnol 2: 176–188. Department of Health, Nutrition, and Exercise Sciences

Generation and maintenance of resting membane potential Na+-K+ pump continuously pumps Na+ out, K+ in Resting membrane virtually impermeable to Na+, slightly permeable to K+ Some K+ continually diffuses down its concentration gradient, out of cell, through K+ channels Membrane interior becomes negative (relative to exterior) because anions can’t follow the K+ out: Vin<0

Generation and maintenance of resting membrane potential Concentration gradient (chemical force) pushes K+ out. Negative intracellular voltage (electrical force) pulls K+ in. Net force is the electrochemical gradient. When intracellular voltage is negative enough that electrical and chemical forces on K are (approximately) balanced, there is hardly any net force on K+, and that is the RMP. Steady state is maintained because active transport of Na+ out and K+ in is equal and opposite to the residual leakage of Na+ in and K+ out.

Generation and maintenance of R.M.P. K+ diffuses “down” its steep concentration gradient (out of cell) via leakage channels. Loss of K+ results in a net negative charge, and therefore negative voltage, inside cell. 1 Extracellular fluid K+ also moves into cell because it is attracted to negative charge inside cell. 2 A stable negative membrane potential (-50 to -100 mV) is established when K+ movement out of cell equals K+ movement into cell. At this point, chemical concentration gradient for K+ exit is equal* & opposite to electrical gradient for K+ entry. 3 Potassium leakage channels *Not exactly equal. Actual RMP is slightly more + than the voltage that would put K+ at equilibrium, so there is a small residual outflow of K+. This is counteracted by the Na-K pump. Protein anion (unable to follow K+ through the membrane) Cytoplasm Figure 3.15

Involves glycoproteins and proteins of glycocalyx Cell-Environment Interactions: How a cell connects to its surroundings, both literally and figuratively Involves glycoproteins and proteins of glycocalyx Cell adhesion molecules (CAMs) Membrane receptors

Roles of Cell Adhesion Molecules Anchor cells to extracellular matrix or to each other Assist in movement of cells past one another CAMs of blood vessel lining attract white blood cells to injured or infected areas Stimulate synthesis or degradation of adhesive membrane junctions Transmit intracellular signals to direct cell migration, proliferation, and specialization

Roles of Membrane Receptors Contact signaling touching and recognition of cells receptors on one or both cells recognize proteins/glycoproteins on other cell’s surface regulates development, growth, immunity, etc. Chemical signaling receptor recognizes chemical (ligand*) released by another cell neurotransmitters, hormones, etc. G protein–linked receptors important class of membrane receptor proteins for chemical signaling receptor activation turns G protein on or off change in G protein causes change in intracellular concentration of a second messenger such as Ca or cyclic AMP (cAMP) * Ligand = molecule that binds to a receptor

G Protein relay between extracellular first messenger and intracellular second messenger