The Cell is the basic unit of life.
All animals are composed of cells.
All cells originate from pre-existing cells.
All cells exist in an aqueous (fluid) environment.
Our primordial unicellular eukaryotic ancestor presumably
[see Figures of exchanges between cells and their environment]
[see Fig. 1-1]
HOMEOSTASIS
HOMEOSTASIS (the tendency to maintain a relatively constant
Although he did not coin the word, the concept was
"All the vital mechanisms, varied as they are, have only one
Homeostasis requires a control system of some sort.
The body must be able to detect a change in the environment
They are called closed loop systems and they depend on
[see Fig. 1-4 and 1-5]
[see Exchanges of matter figure]
GENERAL FUNCTIONS
SPECIALIZED FUNCTIONS
WATER is the major component of a cell
More than 99% of the molecules in our body
The human body averages about 60% water by weight
Females, on average, has a slightly lower water content
[see Figure of water distribution]
Phospholipids are amphipathic (i.e. thay have a hydrophilic
[see Fig. 3-9]
The plasma membrane performs a number of essential functions:
[see Figs. 3-7, 4-5 a,b,c, 4-7]
MEMBRANES
Biological membranes have been viewed for a long time as
Recent studies suggest, however, that by virtue of their acyl chain
As the term "Fluid-Mosaic Model" suggests, the plasma membrane
HOMEOVISCOSITY = the tendency to maintain a relatively constant
Cholesterol contributes to membrane fluidity by blocking closs
Cholesterol actually binds weakly to adjacent phospholipid
There are two other important factors, however: temperature
The fluidity of lipids is strongly dependent upon temperature.
Humans, like other mammals (and birds) are HOMEOTHERMIC
In light of these large fluctuations, how can animals maintain
Changing cholesterol content is one way (more cholesterol =
At any given temperature, unsaturated fatty acids are more
[see figure of fatty acids]
Cellular metabolism is strongly dependent on membrane
Omega-3 polyunsaturated docosahaenoic acid (DHA) appears to
[see Figure of DHA & metabolism in relation to body mass]
The current view suggests that death from old age is the
Long chain polyunsaturated acyl molecules (DHA and others) are
[see Figure of longevity vs. peroxidizability]
associations of prokaryotic cells.
lived in a salty, nutrient rich, watery environment capable
of supplying all that it needed for survival
internal environment) is, arguably, the most important and
central concept in all of physiology.
developed by Claude Bernard.
object, that of preserving constant the conditions of life in
the internal environment" Claude Bernard, 1876
and to make the needed compensatory response. Homeostatic
mechanisms monitor the INTERNAL ENVIRONMENT.
NEGATIVE FEEDBACK (i.e. the deviation produces a corrective
response in the opposite direction).
Enzyme synthesis
Synthesis of proteins, lipids, carbohydrates, etc.
ATP production
Cellular respiration
Ionic regulation
Reproduction (most, but not all, cells)
etc.
Gas transport (erythrocytes)
Contraction (muscle cells)
Energy storage (fat cells)
Hormone production (endocrine cells)
Transmission of action potentials (nerve cells)
etc.
(and of an entire organism).
are water molecules!
(about 70 - 74% fat-free weight)
than do males.
Water content decreases with increasing age
[see Figure 1-2]
[see text Figures 3-4, 3-5, 3-6A
[see Figure of classic (Danielli-Davson) model
[polar or ionized] group at one end and a hydrophobic
[nonpolar] group at the other).
of a typical cell: Many, if not most, of these are phospholipids
[see Figures of Fluid Mosaic model]
1. It regulates the movements of substances into and out of cells
2. Specialized membrane proteins (receptors) can recognize
and respond to specific chemical messengers at the cell surface
3. Specialized regions or structures can link adjacent cells
4. By anchoring various intra- and extracellular proteins,
membranes can contribute to the generation and transmission
of force
relatively passive components of living cells.
composition, membrane bilayers may significantly impact the
thermal biology, the metabolic rate, and even the aging process
of animals.
is a fluid rather than a rigid structure.
In order for membranes to function properly, they must be neither
too fluid nor too rigid
fluidity (viscosity) of the plasma membrane
association of the long-chain fatty acids.
molecules, which makes the lipid bilayer stronger, but markedly
less fluid,
and the composition of the fatty acid chains.
As temperature increases, lipids become more fluid
(i.e. less viscous).
(i.e. we maintain a relatively constant internal temperature).
Therefore, temperature-induced changes in membrane fluidity
are not of major importance. However, most animals are
POIKILOTHERMIC (i.e. their body temperature fluctuates
with the temperature of their environment). Some animals
live in very hot and others in very cold environments.
Many animals experience large seasonal fluctuations in
environmental (and hence body) temperature.
the appropriate membrane viscosity?
less fluid). However, the composition of the fatty acid
chains of the phospholipids is also very important.
fluid than are chemically similar saturated fatty acids.
[see figure of HOMEOVISCOUS ADAPTATION]
associated processes. This has given rise to the MEMBRANE
PACEMAKER theory of metabolism. This theory suggests that
metabolism is strongly dependent on the physical properties of
specific acyl chains.
be a particularly important membrane bilayer constituent.
[Incidentally, DHA is one of the major lipids in fish oil]
result of accumulated damage from reactive oxygen species.
very susceptible to oxidative attack (i.e. they have a high
peroxizability index).