# Chapter 5 Study Guide

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CHAPTER 5
Mass Balance and Homeostasis
1) law of mass balance says that if the amount of substance in the body is to remain
constant, any gain must be offset by an equal loss. It is summarized by the following
equation:
Total amount (or load) of substance x in the body = intake + production ± excretion ±
metabolism
o To maintain constant level, output must equal input.
o Materials enter the body by various routes or are produced through metabolism
o Materials leave the body either by excretion or by metabolism
o Excretion is defined as the elimination of material from the body, and it usually
takes place through the urine, feces, lungs, or skin. Xenobiotics (foreigner), like
drugs or artificial food additives, are excreted by the liver and kidneys
o Metabolism converts the original nutrient to a different substance but in doing so
creates a new mass balance disturbance by adding more of the metabolite ( any
product created in a metabolic pathway) to the body
Excretion Clears Substances from the Body
2) Clearance is the rate at which a molecule disappears from the body by excretion,
metabolism, or both. The major organs that are involved are kidney, liver, and lungs.
3) Clearance is usually expressed as a volume of blood plasma cleared of substance x per
unit of time. It is therefore only an indirect measure of the movement of substance x.
o mass flow ± rate of intake, production, or output of x:
mass flow (amount x/min) = concentration (amount x/vol) * volume flow
(vol/min)
a more direct measure
applies to movement of substances from one compartment in the body to
another
Homeostasis Does Not Mean Equilibrium
5) homeostasis refers to the stability of the extracellular fluid
6) dynamic disequilibrium??
7) Water is the only molecule that moves freely between most cells and the extracellular
fluid
8) Osmotic equilibrium is a state in which the total amount of solute per volume of fluid is
equal on the two sides of the cell membrane. Extracellular and intracellular compartments
can reach this state due to the free movement of water
9) A chemical disequilibrium is a state in which the major solutes are more concentrated in
one of the two body compartments (extra or intracellular) than in the other.( e.g., sodium
and calcium are more concentrated in the extracellular fluid where K+ ions are more
concentrated inside the cell)
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10) The concentration differences of chemical disequilibrium are a hallmark of a living
organism as only the continual input of energy keeps the body in this state. If solutes leak
across the cell membrane dividing the intra and extracellular compartments, energy is
required to return them to the compartments they left ( e.g., sodium-potassium needs to
ATP to pump ions across cell membrane). When cells die and cannot use energy, they
obey the second law of thermodynamics and return to a state of randomness that is
marked by a loss of chemical disequilibrium.
11) The inside of cells is slightly negative relative to the extracellular fluid due to the few
extra negative ions. This electrical difference creates a state of electrical disequilibrium
- the osmotic equilibrium and the two disequilibria are dynamic steady states. The modifier
steady indicates that materials are constantly moving back and forth between the two
compartments, but in a steady state, there is no net movement of materials between the
compartments
Diffusion
12) selectively permeable ± lipid and protein composition of a given cell membrane
determines which molecules will enter the cell and which will leave. If a membrane
allows a substance to pass through it, the membrane is said to be permeable to that
substance. If a membrane does not allow a substance to pass, the membrane is said to be
impermeable to that substance
13) the size of the molecule and its lipid solubility are the two properties of a molecule that
influence its movement (very small and those that are lipid soluble can cross directly
through the phospholipid bilayer
14) passive transport does not require input of energy whereas active transport requires the
input of energy from an outside source (ATP)
15) a molecule can move across a membrane through simple diffusion (molecule goes
through lipid bilayer), facilitated diffusion, primary active transport, secondary active
transport ( mediated transport requires a membrane protein), endocytosis, exocytosis and
phagocytosis (uses a membrane bound vesicle) Æ refer to Fig. 5-4 on pg 132
16) Diffusion is the movement of molecules from an area of higher concentration of the
molecules to an area of lower concentration of the molecules
17) Seven properties of diffusion are:
General Properties of diffusion
o Diffusion uses the kinetic energy of molecular movement and does not require an
outside energy source
o Molecules diffuse from an area of higher concentration to an area of lower
concentration
o Diffusion continues until concentrations come to EQ. Molecular movement
continues, however, after
EQ has been reached
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o Diffusion is faster
- Over shorter distances ( if distance increased from 1 to 2, then the time will
increase from 1^2 to 2^2)
- At higher temperatures
- For smaller molecules
o Diffusion can take place in an open system or across a partition that separates two
systems
Simple Diffusion across a membrane
o The rate of diffusion through a membrane is faster if
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The membrane is thinner
The membrane is more permeable to the molecule
o Membrane permeability to a molecule depends on
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The lipid composition of the membrane
- ion movement is influenced by electrical gradients due to the attraction of opposite
charges and repulsion of like charges. They move in response to combined electrical and
18) Factors affecting rate of simple diffusion are:
o The rate of diffusion depends on the ability of the diffusing molecule to dissolve
in the lipid layer of the membrane. Molecules that are lipid-soluble (lipophilic)
can traverse the central lipid core of the membrane. The small size of the water
molecule allows it to slip between the lipid tails in some membranes. Membranes
with a high cholesterol content are less permeable to water than those with a low
cholesterol content, presumably because the cholesterol molecules fill the spaces
between the fatty acid tails of the lipid bilayer and thus exclude water
o The rate of diffusion across a membrane is directly proportional to the surface
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molecules can diffuse across per unit time
o Rate of diffusion across a membrane is inversely proportional to the thickness of
the membrane. The thicker the membrane, the slower the rate at which diffusion
takes place
19) rate of diffusion is proportional to (surface area X concentration gradient X membrane
permeability) / (membrane thickness)
20) membrane permeability is influenced by (1) the size of the diffusing molecule, (2) the
lipid-solubility of the molecule and (3) the composition of the lipid bilayer across which
it is diffusing. As molecular size increases, membrane permeability decreases. As lipid
solubility of the diffusing molecule increases, membrane permeability to the molecule
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