Animal PhysiologyChapter 5Transport

Mechanisms must exist to transport sugars from the open central cavity of a hummingbird’s intestines into its blood following a meal

Transport

• Passive solute transport by simple diffusion• Passive solute transport by facilitated diffusion• Active transport• Diversity and modulation of channels and

transporters• Osmotic pressure and other colligative

properties of aqueous solutions• Osmosis

Figure 5.1 Three focal examples in which transport occurs (Part 1)

Solutions have similar sum-total concentrations of dissolved matter (osmotic pressure) but solute compositions differ

Figure 5.1 Three focal examples in which transport occurs (Part 2)

[ion] differences across gill epithelium are important because it is permeable to ions

Figure 5.1 Three focal examples in which transport occurs (Part 3)

Equilibrium

• The state towards which a system moves – internally-when it has not inputs or outputs of energy or matter

• Further net change is impossible without system inputs or outputs

• A state of minimal capacity to do work under locally prevailing conditions (decreasing work potential)

Equilibrium

• Passive-transport: capable of carrying material only in one direction of equilibrium

• Active-transport: can carry material in a direction opposing equilibrium

Passive Solute Transport by Simple Diffusion

• Transport that arises from the molecular agitation that exists in all systems above absolute zero and from the simple statistical tendency for such agitation to carry more molecules out of regions of relatively high concentration than into such regions

Simple diffusion viewed macroscopically and microscopically

Fick’s Diffusion Equation

J=DC1-C2

X

J=net rate of diffusionD=diffusion coefficientC1=high concentration regionC2=low concentration regionX=distance between C1 and C2

• J is proportional to the difference in concentration (C1-C2)• Rate increase and distance decreases• Solutes diffuse according the their own concentration

gradient, not total solute concentrations• D depends on permeability and temperature

Figure 5.3 Diffusional concentration of a solute in a boundary layer next to an animal or cell

Figure 5.4 Ionic charge separation occurs only within nanometers of membranes

Figure 5.5 Gated ion channels for inorganic ions

• Passive-subject to [chemical] and electrical-charge gradients• Ions do not bind the channel proteins• Selective

Permeability of a cell membrane to a solute

• For solutes such as lipids and O2 that dissolve in the lipid bilayer, molecular size matters.

• For inorganic ions the # of channels per unit of membrane area and the proportion of open channels matters.

• Selectively permeable membranes may have different proportions of certain channels.

• Concentration effect: the influence of the concentration gradient on the ion

• Electrical effect: the influence of the electrical gradient on an ion

• Electrochemical equilibrium: the concentration effect on its diffusion and the electrical effect are equal but opposite.

Figure 5.6 Ion diffusion depends on the dual effects of concentration and electrical gradients

Figure 5.7 An electrochemical view of a typical animal cell

Donnan Equilibrium

• Occurs when a number of ions can cross the membrane but there is a set of nonpermeating ions that are more abundant on one side than the others.

• Animal cells tend toward Donnan equilibrium due to the [high] of nonpermeating anionic materials (e.g. proteins and nucleic acids)

Figure 5.8 The development of an equilibrium in which a voltage difference generated by diffusion exactly opposes the remaining concentration gradient