This chapter discusses cell membranes, epithelia, enzymes, and cell signaling. It describes the structure of cell membranes using phospholipid molecules and membrane proteins. Epithelia form tissue sheets that line body cavities and organs. There are simple, tubular, and glandular epithelia with different cell shapes. Enzymes catalyze metabolic reactions and have specific structures and kinetic properties that determine their reaction rates. Cooperative binding and allosteric regulation allow enzymes to be activated or inhibited. The chapter presents diagrams and examples to illustrate these key concepts in animal physiology.

1. Animal Physiology
Chapter 2

2. Chapter 2 Opener Two slow-moving predators that use molecular weapons to capture fast-moving
prey

3. Four Topics in Chapter 2
1. Cell membranes and intracellular membranes
2. Epithelia-the sheets of tissue that line body
cavities and form the outer surfaces of
organs
3. Enzyme function, diversity and evolution
4. Mechanisms by which cells receive and act
on signals

4. Cell membranes and
intracellular membranes

5. Figure 2.1 The structure of a cell membrane

6. Terminology
• Polar – electrons are unevenly distributed
• Nonpolar – electrons are evenly distributed,
no charge imbalance
• Amphipathic-each molecule consists of a
polar part and a nonpolar part (head group of
phospholipid)
• Leaflets – two layers of the membrane

7. Figure 2.2 The structure of membrane phospholipid molecules

8. Figure 2.2 The structure of membrane phospholipid molecules

9. Membrane Fluidity
• RBC’s have >150 kinds of phospholipids
• Phospholipids can travel (by diffusion) around
the whole cell in a matter of minutes
• Temperature: lowerlower fluidity
• Fluidity depends on chemical saturation of the
hydrocarbons in the tails
– Saturated-non double bonds
– Unsaturated – one or more double bonds, creates
bends  higher fluidity

10. Figure 2.3 The degree of chemical unsaturation of the hydrocarbon tails of brain phospholipids in
fish varies with habitat temperature
Arctic fish have
high levels of
unsaturated
phospholipids

11. Proteins Endow Membranes
with Numerous Functional
Capacities

12. Two structural designations for membrane
proteins
1. Integral – parts of the membrane, can’t be
removed, usually transmembrane (span the
whole membrane)
2. Periperal – can be removed, bonded
noncovalently to membrane components, on
one side of the membrane or the other

13. Five functional designations for membrane proteins

14. Fluid Mosaic Model of Membranes
A membrane consists of a mosaic of protein and
lipid molecules, all of which move about in
directions parallel to the membrane faces
because of the fluid state of the lipid matrix

15. Box Extension 2.1 Figure A The structural hierarchy of proteins

16. Box Extension 2.1 Figure B Three ways to diagram the tertiary structure of one protein. All three
diagrams represent lysozyme

17. Box Extension 2.1 Figure C Types of weak, noncovalent bonds that are important in protein
structure

18. Figure 2.4 The structure of a transmembrane protein—a voltage-gated Na+ channel—illustrating
several modes of presentation

19. Four Topics in Chapter 2
1. Cell membranes and intracellular membranes
2. Epithelia-the sheets of tissue that line body
cavities and form the outer surfaces of
organs
3. Enzyme function, diversity and evolution
4. Mechanisms by which cells receive and act
on signals

20. • Epithelia compartmentalize the body by
forming boundaries between body regions
• Also form a boundary between an animal and
its external environment

21. Figure 2.5 Simple epithelia-single layer of cells (squamous, cuboidal, columnar)

22. Figure 2.6 Tubules and follicles formed by simple epithelia (Part 1)

23. Figure 2.6 Tubules and follicles formed by simple epithelia (Part 2)
The cuboidal cells
of this epithelium
display two
signature
properties of high
metabolic activity:
microvilli on the
apical surfaces and
abundant
mitochondria

24. Figure 2.6 Tubules and follicles formed by simple epithelia (Part 3)
Squamous cells (no
microvilli), can’t
see all nuclei in a
cross section since
cells are quite long

25. Figure 2.7 Types of junctions between cells

26. Figure 2.8 The organization of epithelial cells into apical and basolateral regions

27. Figure 2.9 Transcellular and paracellular paths across an epithelium

28. Four Topics in Chapter 2
1. Cell membranes and intracellular membranes
2. Epithelia-the sheets of tissue that line body
cavities and form the outer surfaces of
organs
3. Enzyme function, diversity and evolution
4. Mechanisms by which cells receive and act
on signals

29. Metabolism
• The set of processes by which cells and
organisms acquire, rearrange and void
commodities in ways that sustain life.
• Can be described by commodities – nitrogen
metabolism
• Or energy metabolism-how energy is
acquired, transformed, channeled and
dissipated.
• Mediated by enzymes

30. Metabolism-type of transformation
• Catabolism-complex chemical compounds are
broken down to release energy, create smaller
building blocks or prepare chemical
constituents for elimination
• Anabolism – synthesize larger or more
complex chemical compounds from smaller
building blocks

31. Figure 2.10 Two amphibians with different jumping capabilities based in part on different levels of a
key enzyme, lactate dehydrogenase
Amphibians can’t make a
lot of ATP by aerobic
mechanisms due to
simple lungs. LDH lets
the frog use anaerobic
mechanisms

32. Figure 2.11 The reaction catalyzed by lactate dehydrogenase (LDH)
Substrate Product
Cofactor

33. Enzyme Catalysis
E + S ↔ E-S complex ↔ E-P complex ↔ E + P

34. Kinetics
• Kinetics: velocity properties of rxns
• Saturation kinetics: limited to a maximum
velocity because of a limited supply of a
molecule (enzyme)
– Hyperbolic (Michaelis-Menten): enzyme has one
substrate binding site or multiple sites that behave
independently
– Sigmoid: Multiple substrate binding sites that
influence each other

35. Figure 2.12 Reaction velocity as a function of substrate concentration
V= Vmax [S]
[S] + Km
V: velocity
[S]: substrate conc.
Vmax: max velocity
Km: Michaelis constant

36. Figure 2.12 Reaction velocity as a function of substrate concentration

37. Catalytic Effectiveness of an Enzyme
• Kcat: turnover number – number of substrate molecules
converted to product per second by each enzyme molecule
when saturated
• Depends partly on activation energy
– Substrate must pass through a transition state to form a product
– Must gain energy by random collisions with other molecules
– Enzymes lower activation energy
– Also depends on how fast an enzyme can go through its
conformational change

38. Figure 2.13 Enzymes accelerate reactions by lowering the needed activation energy

39. Enzyme-substrate affinity affects the
reaction velocity
The enzyme and substrate might bounce apart
when they collide or form the necessary
complex (enzyme-substrate affinity)

40. Figure 2.14 The approach to saturation depends on enzyme–substrate affinity

41. Figure 2.14 The approach to saturation depends on enzyme–substrate affinity
Michaelis constant
Km=1/2Vmax:
[substrate] required
to attain one-half of
the maximal
reaction velocity
Low affinity=higher
Km

42. Reaction Velocities in Cells Therefore
Depend On:
1. Number of active enzymes present
2. Catalytic effectiveness of each enzyme
3. Affinity of enzyme for its substrate

43. Molecular flexibility is important for enzyme function
Active Site
for
substrate
binding
Ligand: any molecule that binds an enzyme (substrate or nonsubstrate)

44. Cooperativity
Positive cooperativity: ligand binding at
one site facilitates the binding of other
sites on the same molecule
Negative cooperativity: binding at one site
inhibits binding at other sites on the same
molecule

45. Cooperativity continued
Homotropic cooperativity: the binding of
a ligand facilitates or inhibits binding of
other molecules of the same ligand to the
same enzyme molecule
Heterotropic cooperativity: the binding of
one type of ligand to an enzyme molecule
influences the binding of other types of
ligands

46. Cooperativity continued
• Allosteric modulation: the modulation
of catalytic properties of an enzyme by
the binding of nonsubstrate ligands
(allosteric to regulatory/allosteric sites

47. Cooperativity continued
• Allosteric activation: the binding of an
allosteric modulator increases the affinity
of the active site for its substrate or
increase catalytic activity of the enzyme
• Allosteric inhibition: the binding of an
allosteric modulator impairs catalytic
activity of an enzyme