Thus there will be the potential for ionic
flux across cell membranes which will be
proportional to the surface area of the
membrane, the difference in concentration
of that ion across the membrane and the
permeability of the membrane to the partic-
ular ion.
Coupled Transport
For many nutrients, e.g. glucose and some
amino acids, transport into a cell is
coupled to sodium entry. The transport
mechanism only allows transport of the
particular nutrient together with a sodium
ion and thus utilizes the large concentra-
tion gradient of sodium ions to drive
uptake of these nutrients from the extra-
cellular water to the intracellular compart-
ment. The sodium ions that are drawn into
the cell in this manner are then pumped
out on the sodium pump.
Adeola et al.(1989) measured Na+,K+-
ATPase activity in muscle of pigs given
diets that varied in protein content such
that the rates of protein synthesis were
altered. They reported an increase in
Na+,K+-ATPase activity in direct proportion
to increases in protein synthesis. Part of
this increase may have been due to
increased uptake of amino acids and
glucose into muscle cells but part may also
have been due to hormonal influences on
pHi, as discussed earlier.
Cell Volume Regulation
Plasma membranes do not provide any
rigidity, and changes in the ionic environ-
ment can cause movement of water across
the cell membrane. Within narrow limits,
cells are able to compensate for this by
altering the intracellular Na+concentration,
thereby changing their osmolarity with
respect to the extracellular environment.
Thus, rates of ion transport are essen-
tially dependent on the hormonal and
ionic environment together with the cell
surface area. In vivo, the ionic environment
is tightly controlled, although in vitrothere
is the potential for it to vary more widely
and it may change appreciably over time in
a closed system (e.g. as end-products of
metabolism accumulate). As cells grow, it
is predicted that the cost of ion pumping
will increase. This is due to both the
hormonal environment necessary to
mediate growth (increased pHi) and the
fact that cell surface area and hence Na+
leakage per cell will also be increasing. The
extent to which this might influence the
rate at which cells grow is considered
below and will depend on the rate at
which energy in the form of ATP can be
produced, i.e. the metabolic rate of the cell.Energy Production
Across species of widely differing body
size (e.g. mouse to elephant), cell size does
not vary appreciably whereas cell number
does. Therefore, species of larger mature
size are characterized by having a greater
number of cells than animals of smaller
mature size. It has long been recognized
that BMR varies with animal size. As
animal size increases so does BMR,
although not in direct proportion to the
increase in size. From experimental observa-
tions, it has been shown that BMR increases
in proportion to body weight raised to the
power 0.73. Therefore as animal size
increases across species, cell number
increases in direct proportion to weight but
metabolic rate increases in proportion to
weight to the power 0.73. Thus metabolic
rate per cell decreases as animal size
increases (cellular metabolic rate will
change in proportion to weight raised to
the power of minus 0.27). Does this mean
that cells from a large animal are incapable
of metabolizing at rates equivalent to those
from a small animal? Studies by Wheatley
and Clegg (1994) suggest that this is not the
case. They compiled data from a number of
sources which compared metabolic rates of
isolated cells or tissue slices taken from
animals varying in size from 0.012 (mouse)
to 780 (horse) kg. Whilst there was some
reduction in metabolic rate measured in
vitro as animal size increased, the156 N.S. Jessop