This document discusses various mechanisms of transport in animal physiology, including: 1) Passive transport mechanisms like simple diffusion and facilitated diffusion, as well as active transport mechanisms. 2) The concepts of equilibrium, passive transport which moves with equilibrium, and active transport which moves against equilibrium. 3) Specific passive transport mechanisms like simple diffusion, facilitated diffusion through ion channels, and the factors that determine permeability. 4) How concentration gradients, electrical gradients, and electrochemical equilibrium interact to drive ion transport across membranes.

1. Animal Physiology
Chapter 5
Transport

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

3. 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

4. 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

5. 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

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

7. 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)

8. Equilibrium
• Passive-transport: capable of carrying
material only in one direction of equilibrium
• Active-transport: can carry material in a
direction opposing equilibrium

9. 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

10. Simple diffusion viewed macroscopically and microscopically

11. Fick’s Diffusion Equation
J=D
C1-C2
X
J=net rate of diffusion
D=diffusion coefficient
C1=high concentration region
C2=low concentration region
X=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

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

14. Figure 5.4 Ionic charge separation occurs only within nanometers of membranes

15. 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

16. 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.

17. • 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.

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

19. Figure 5.7 An electrochemical view of a typical animal cell

20. 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)

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