How do individual neurons establish signals at chemical synapses. Excitatory neurotransmitters, such as acetylcholine and maintain their characteristic and glutamate, open transmitter-gated cation channels and thereby depolarize the intrinsic firing properties?
postsynaptic membrane toward the threshold level for firing an action potential. Inhibitory neurotransmitters, such as GABA and glycine, open transmitter-gated
Even organisms with very simple Cl– or K+ channels and thereby suppress firing by keeping the postsynaptic mem-nervous systems have dozens of brane polarized. A subclass of glutamate-gated ion channels, called NMDA-recep-different K+ channels. Why is it tor channels, is highly permeable to Ca2+, which can trigger the long-term changes important to have so many? in synapse efficacy (synaptic plasticity) such as LTP and LTD that are thought to be involved in some forms of learning and memory. • Why do cells that are not electrically Ion channels work together in complex ways to control the behavior of electri-active contain voltage-gated ion cally excitable cells. A typical neuron, for example, receives thousands of excitatory channels? and inhibitory inputs, which combine by spatial and temporal summation to produce a combined postsynaptic potential (PSP) at the initial segment of its axon. The • How are memories stored for so magnitude of the PSP is translated into the rate of firing of action potentials by a many years in the human brain? mixture of cation channels in the initial segment membrane.
Which statements are true? explain why or why not.
11–1 Transport by transporters can be either active or b. Does the linked action of these two pumps cause passive, whereas transport by channels is always passive. imbalances in either the K+ concentration or the membrane potential? Why or why not?
11–2 Transporters saturate at high concentrations of 11–7 Microvilli increase the surface area of intestinal the transported molecule when all their binding sites are cells, providing more efficient absorption of nutrients.
occupied; channels, on the other hand, do not bind the Microvilli are shown in profile and cross section in Figure ions they transport and thus the flux of ions through a Q11–1. From the dimensions given in the figure, estimate channel does not saturate. the increase in surface area that microvilli provide (for 11–3 The membrane potential arises from movements the portion of the plasma membrane in contact with the of charge that leave ion concentrations practically unaf-lumen of the gut) relative to the corresponding surface of a fected, causing only a very slight discrepancy in the num-cell with a “flat” plasma membrane. ber of positive and negative ions on the two sides of the membrane.
Discuss the following problems.
11–4 Order Ca2+, CO2, ethanol, glucose, RNA, and H2O according to their ability to diffuse through a lipid bilayer, beginning with the one that crosses the bilayer most readily. Explain your order.
11–5 How is it possible for some molecules to be at 1 µm 0.1 µm equilibrium across a biological membrane and yet not be at the same concentration on both sides?