Cambridge International A Level Biology Revision Guide

Cambridge International A Level Biology
The black bear’s big sleep
It is metabolically expensive for a mammal to maintain
a constant, warm body temperature in long winters
when it is very cold and food is hard to find. Black
bears feed well during the summer to build up stores
of energy-rich fat (Figure 14.1). In the autumn and
early winter, the bears dig dens for themselves, or
find a ready-made one in somewhere like a cave,
curl up and sleep until the weather improves. Their
metabolism adjusts for this lengthy period of inactivity
when they do not eat, drink, urinate or defecate.
Their stores of fat and some muscle protein provide
energy. The waste product of protein breakdown is
urea, which is filtered from the blood by the kidneys.
The kidneys continue to produce urine, but it is all
reabsorbed by the bladder. The urea cannot be stored;
instead it is recycled by the bear’s gut bacteria. These
break it down to ammonia and carbon dioxide,
which are absorbed into the blood. Carbon dioxide is
breathed out and ammonia combined with glycerol
from the breakdown of fat to make amino acids.
The amino acids are used to synthesise the enzymes
that are needed in larger quantities for the increased
hydrolysis of fat during the bear’s hibernation.
Figure 14.1 During the summer, black bears build up stores
of fat for survival during the seven months or so when they do
not eat.
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To function efficiently, organisms have control systems to
keep internal conditions near constant, a feature known as
homeostasis. This requires information about conditions
inside the body and the surroundings, which are detected
by sensory cells. Some of the physiological factors
controlled in homeostasis in mammals are:
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core body temperature
metabolic wastes, particularly carbon dioxide and urea
blood pH
blood glucose concentration
water potential of the blood
the concentrations in the blood of the respiratory gases,
oxygen and carbon dioxide.
First, we will look at the need for mammals to maintain a
stable internal environment, and then consider how they
maintain a constant core body temperature.
Internal environment
The internal environment of an organism refers to all
the conditions inside the body. These are the conditions
in which the cells function. For a cell, its immediate
environment is the tissue fluid that surrounds it.
Many features of the tissue fluid influence how well the
cell functions. Three features of tissue fluid that influence
cell activities are:
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temperature – low temperatures slow down metabolic
reactions; at high temperatures proteins, including
enzymes, are denatured and cannot function
water potential – if the water potential decreases, water
may move out of cells by osmosis, causing metabolic
reactions in the cell to slow or stop; if the water
potential increases, water may enter the cell causing it
to swell and maybe burst
concentration of glucose – glucose is the fuel for
respiration, so lack of it causes respiration to slow or
stop, depriving the cell of an energy source; too much
glucose may cause water to move out of the cell by
osmosis, again disturbing the metabolism of the cell.
In general, homeostatic mechanisms work by controlling
the composition of blood, which therefore controls the
composition of tissue fluid. See page 164 to remind
yourself how this happens. There are control mechanisms
for the different aspects of the blood and tissue fluid. These
include the three physiological factors listed above.