activity associated with eating and pro-
cessing of food within the digestive tract.
Other maintenance functions include
operation of the immune system and
fighting infection, as well as costs of
thermoregulation should environmental
factors result in the rate of heat loss from
an animal’s body exceeding the rate of
heat production. In such cases, heat pro-
duction is increased by shivering or non-
shivering thermogenesis to maintain body
temperature, resulting in increased energy
usage.
Components of Cellular Energy
Requirements
In order to understand the requirements for
energy and how they change across an
animal’s life, or between animals of different
sexes or different species, we need to
consider energy requirements at the level
of the cell. The overall requirement of an
animal is the sum of the requirements of all
individual cells. By considering such
processes at a cellular level, we can begin
to understand what factors influence these
requirements.
Maintenance costs have been
estimated by many authorities (e.g. AFRC,
1993; NRC, 1996) to be a function of the
weight of an animal. Since body composi-
tion can vary widely and since adipose
tissue is metabolically relatively inactive
when compared with other tissues, fat-free
mass or protein weight is a better deter-
minant (Emmans, 1994). Table 7.1 shows
early proposals of how maintenance costs
can be subdivided further, although this
relates principally to subdivision of BMR.
The broad division is in terms of energetic
costs of organ systems, termed service
functions, and those of individual cells.
However, the former constitute the addi-
tional cellular costs of a particular organ or
tissue. For example, circulation or heart
work is the additional energy requirement
of the muscle cells of the heart above those
termed cell maintenance. At a cellular
level, many experiments have supported
these proposals.
Ion Transport
These estimates come from experiments
performed in vitroin which the proportion
of a cell’s rate of oxygen use required for
sodium pump (Na+,K+-ATPase, EC 3.6.1.3)
activity is measured. This enzyme is pre-
sent in the plasma membrane of all animal
cells and serves to maintain the observed
ionic gradient of Na+between intra- and
extracellular space at the expense of ATP
(see also Chapters 1 and 6). Such measure-
ments make use of the specific inhibitor of
the sodium pump, ouabain. Isolated cells
are suspended in appropriate incubation
medium in a closed and temperature-
regulated chamber of a Clark-type oxygen
electrode. The oxygen content of the
medium is recorded continuously and falls
as the cells consume oxygen. The rate of
oxygen use is linear and depends on the
type and quantity of tissue being studied.
During the course of the incubation (after a
sufficient period of time that allows the
rate of oxygen use to be measured),
ouabain is added to the medium. Sodium
pump activity is inhibited and thus ATP,
and hence oxygen, usage declines. The pro-
portional decrease in the rate of oxygen
consumption is assumed to represent the
ATP, and hence oxygen, use of the sodium
pump.
The activity of the sodium pump is
responsive to the intracellular concentration150 N.S. Jessop
Table 7.1.Apportionment of BMR (from Baldwin et
al., 1980).Function % BMRService functions
Kidney work 6–7
Heart work 9–11
Respiration 6–7
Nervous functions 10–15
Liver functions 5–10
Total 36–50
Cell maintenance
Ion transport 30–40
Protein synthesis 9–12
Lipid synthesis 2–4
Total 41–56