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General
Biochemistry
BIOC 201
CHAPTER II
Water, pH and buffers
Water,
pH
and buffers
Objectives:
The main objective of this chapter is getting
the student to understand the basis of
homeostasis process.
This chapter is aimed to familiarize the
student with the dynamic of water movement
and solubility in living cells.
To provide the student with the principal
information about hydrogen ion concentration
and buffers composition and functions.
HOMEOSTASIS (definition)
The dynamic that defines the distribution
of water and the maintenance of pH and
electrolyte concentrations
The maintenance of a relatively constant
internal environment in the bodies of
higher animals by means of a series of
interacting physiological and biochemical
processes.
Water is the universal solvent
Water distribution maintained by:
kidneys,
antidiuretic hormone,
hypothalamic thirst response,
respiration
and perspiration
the
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Water (cont.,)
Clinically, need to be aware of water
depletion caused by:
decreased
intake (coma,
wandering the desert)
or increased loss (diarrhea, renal
malfunction, over-exercise)
Water (cont.,)
Be aware of excess
body water
due to:
increased intake (too much I.V.)
or decreased excretion (renal failure)
Water (cont.,)
Comprises approx 70%
of human mass
45-60%
intracellular fluid (ICF)
25% extracellular fluid (ECF)
Plasma: the fluid portion of the
blood
Interstitial fluid (IF): fluid in spaces
between cells
Body Fluids
Solutes are broadly classified into:
Electrolytes: inorganic salts, all acids
and bases, and some proteins
Nonelectrolytes: examples include
glucose, lipids, creatinine, and urea
Electrolytes have greater osmotic power
than nonelectrolytes
Water moves according to osmotic
gradients
ECF and ICF
Each fluid compartment of the
body has a distinctive pattern of
electrolytes
Extracellular fluid- ECF (Na
and Cl)
Intracellular fluid- ICF (K, P)
ECF and ICF (cont.,)
Extracellular fluids are similar (except for the
high protein content of plasma)
Sodium is the major cation
Chloride is the major anion
Intracellular fluids have low sodium and
chloride
Potassium is the major cation
Phosphate is the major anion
Electrolytes determine the chemical and
physical reactions of fluids
Water and Hydrogen bond
Dipolar: partial negative charge on
oxygen, partial positive charge on
hydrogens
dipolar nature leads to formation of
many low energy hydrogen bonds
Hydrogen bond
Lehninger, 4th ed., 2005, Ch 2
Water Solubility
Entropy Increases as Crystal
Substances Dissolve
As a salt such as NaCl dissolves, the Na
and Cl ions leaving the crystal lattice
acquire far greater freedom of motion
(Fig. 2–6)
The resulting increase in entropy
(randomness) of the system is largely
responsible for the ease of dissolving salts
such as NaCl in water
Hydrophilic
Water Solubility/Hydrophilic
Lehninger, 4th ed., 2005, Ch 2
Water Solubility/Hydrophobicity
Dissolving hydrophobic
compounds in
water produces a measurable decrease
in entropy.
Water molecules in the nonpolar solute
are oriented and form a highly ordered
cagelike shell around each solute
molecule.
Hydrophobicity
Lehninger, 4th ed., 2005, Ch 2
Hydrophobicity
Lehninger, 4th ed., 2005, Ch 2
Water Balance and ECF
Osmolality
To remain properly hydrated, water
intake must equal water output
Water intake sources
Ingested
fluid (60%)
Solid food (30%)
Metabolic water or water of oxidation
(10%)
Water Balance and ECF
Osmolality
Water output
Urine (60%)
Feces (4%)
Insensible losses (28%)
sweat (8%)
Increases in plasma osmolality trigger
thirst and release of antidiuretic hormone
(ADH)
Water Intake and Output
Acid-Base Balance
Normal pH of body fluids
Arterial blood is 7.4
Venous blood and interstitial fluid
is 7.35
Intracellular fluid is 7.0
pH imbalances
The normal blood pH range is 7.35 – 7.45
Any pH below this range is considered a
condition of ACIDOSIS
Any pH above this range is considered a
condition of ALKALOSIS
The body response to acid-base imbalance is
called compensation which may be complete
if the blood pH is brought back to normal, or
partial if it is still outside the norms.
Respiratory problems
Respiratory acidosis is a carbonic
acid excess (blood CO2 is too high)
Respiratory alkalosis is a carbonic
acid deficit (blood CO2 is too low)
Compensation would occur
through the kidneys
Acid-Base Balance
Alkalosis or alkalemia:
arterial
blood pH rises above
7.45
Acidosis or acidemia:
arterial pH drops below 7.35
(physiological acidosis)
Most hydrogen ions originate
from cellular metabolism:
Breakdown of phosphorus containing
proteins releases phosphoric acid into
the ECF
Anaerobic respiration of glucose
produces lactic acid
Fat metabolism yields organic acids and
ketone bodies
Transporting carbon dioxide as
bicarbonate releases hydrogen ions
Hydrogen Ion Regulation
Concentration of hydrogen ions is
regulated sequentially by:
Chemical
buffer systems (act within
seconds)
The
respiratory center in the brain
stem (acts within 1-3 minutes)
Renal
mechanisms (require hours to
days to effect pH changes)
Buffers
Cells and organisms maintain a specific
and constant cytosolic pH, keeping
biomolecules in their optimal ionic state,
usually near pH 7.
In multicellular organisms, the pH of
extracellular fluids is also tightly
regulated.
Constancy of pH is achieved primarily by
biological buffers: mixtures of weak
acacids and their conjugate bases.
Buffers (cont.,)
Buffers are aqueous systems that tend to
resist changes in pH when small amounts
of acid (H) or base (OH) are added.
A buffer system consists of a weak
acid
(the proton donor) and its conjugate
base (the proton acceptor).
Biological buffering is illustrated by the
phosphate and carbonate buffering
systems of human.
Physiological Buffers
Three major physiological buffer
systems
Bicarbonate buffer system
Phosphate buffer system
Protein buffer system
Any drifts in pH are resisted by the
entire chemical buffering system
Bicarbonate Buffer System
Bicarbonate buffer system is the only
important ECF buffer
A mixture of:
carbonic
acid (H2CO3) and its:
salt, sodium bicarbonate (NaHCO3)
(potassium or magnesium
bicarbonates work as well)
Bicarbonate Buffer System (cont.,)
If strong acid is added:
Hydrogen
ions released combine
with the bicarbonate ions and form
carbonic acid (a weak acid)
The pH of the solution decreases
only slightly
Bicarbonate Buffer System (cont.,)
If strong base is added:
It
reacts with the carbonic acid to
form sodium bicarbonate (a weak
base)
The pH of the solution rises only
slightly
Phosphate Buffer System
Phosphate Buffer system is an effective
buffer in urine and intracellular fluid
Its components are:
Sodium
salts of dihydrogen phosphate
(H2PO4‾), a weak acid
Monohydrogen phosphate (HPO42‾), a
weak base
Protein Buffer System
Plasma and intracellular proteins are the
body’s most plentiful and powerful
buffers
Some amino acids of proteins have:
Free organic acid groups (weak acids)
Groups that act as weak bases (e.g.,
amino groups)
Amphoteric molecules are protein
molecules that can function as both a
weak acid and a weak base