2 USMLE Road Map: Physiology surface of the plasma membrane are known, respectively, as peripheral or lipid-anchored membrane proteins a. The majority of integral membrane proteins span the bilayer through the formation of a-helices, a group of 20-25 amino acids twisted to ex- pose the hydrophobic portion of the amino acids to the lipid environment in the membrane( Figure 1-1) b. Protein content of membranes varies from less than 20% for myelin,a substance that helps the propagation of action potentials, to more than 60%in liver cells, which perform metabolic activities C. Cellular proteins act as receptor sites for antibodies as well as hormone -, neurotransmitter-,and drug-binding sites d. Enzymes bound to the cell membrane are often involved in phosphoryla- tion of metabolic intermedi Carrier proteins in the membrane transport materials across the cell mem- f. Membrane channels allow polar charged ions(Na, K, Cl, and Ca")to flow across the plasma rane.lon channel gates regulate ion passage and are controlled by voltage (voltage gated, ligands(ligand gated) mechanical means(mechanically gated D. The plasma membrane acts as a selective barrier to maintain the composition of the intracellular environment 1. Passive transport, or diffusion, involves transport of solutes across the plasma membrane due to the substance's concentration gradient a. The term passive implies that no energy is expended directly to mediate the transport process. b. Passive transport is simple diffusion of substances that can readily pene trate the plasma membrane, as is the case for O2 or CO C. Passive transport is the only transport mechanism that is not carrier medi- egral membrane Figure 1-l. Membrane proteins.2 USMLE Road Map: Physiology N Integral membrane protein Peripheral Cholesterol membrane protein Figure 1–1. Membrane proteins. surface of the plasma membrane are known, respectively, as peripheral or lipid-anchored membrane proteins. a. The majority of integral membrane proteins span the bilayer through the formation of
-helices, a group of 20–25 amino acids twisted to ex￾pose the hydrophobic portion of the amino acids to the lipid environment in the membrane (Figure 1–1). b. Protein content of membranes varies from less than 20% for myelin, a substance that helps the propagation of action potentials, to more than 60% in liver cells, which perform metabolic activities. c. Cellular proteins act as receptor sites for antibodies as well as hormone-, neurotransmitter-, and drug-binding sites. d. Enzymes bound to the cell membrane are often involved in phosphoryla￾tion of metabolic intermediates. e. Carrier proteins in the membrane transport materials across the cell mem￾brane. f. Membrane channels allow polar charged ions (Na+ , K+ , Cl− , and Ca2+) to flow across the plasma membrane. Ion channel gates regulate ion passage and are controlled by voltage (voltage gated), ligands (ligand gated), or mechanical means (mechanically gated). D. The plasma membrane acts as a selective barrier to maintain the composition of the intracellular environment. 1. Passive transport, or diffusion, involves transport of solutes across the plasma membrane due to the substance’s concentration gradient. a. The term passive implies that no energy is expended directly to mediate the transport process. b. Passive transport is simple diffusion of substances that can readily pene￾trate the plasma membrane, as is the case for O2 or CO2. c. Passive transport is the only transport mechanism that is not carrier medi￾ated. 5506ch01.qxd_ccII 2/17/03 2:08 PM Page 2