General Biochemistry

(1)

General

Biochemistry

BIOC 201

CHAPTER II

Water, pH and buffers

(2)

Water, pH

pH

and buffers

(3)

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 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.

(4)

HOMEOSTASIS

(definition)

The dynamic that defines the distribution of water and the maintenance of pH and electrolyte concentrations

The maintenance of a relatively constant

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.

(5)

Water is the universal solvent

Water distribution maintained by:

the

kidneys,

antidiuretic hormone,

hypothalamic thirst response,

respiration

and

perspiration

(6)

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Water

^(cont.,)

Clinically, need to be aware of water depletion caused by:

decreased intake (coma,

decreased intake (coma, wandering the desert)

or increased loss (diarrhea, renal

malfunction, over-exercise)

(7)

Water

^(cont.,)

Be aware of excess body water

due to:

increased intake (too much I.V.)

or decreased excretion (renal failure)

(8)

Water

^(cont.,)

Comprises approx

70% of human mass

45-60% intracellular fluid (ICF)

25% extracellular fluid (ECF)

Plasma: the fluid portion of the

Plasma: the fluid portion of the blood

Interstitial fluid (IF): fluid in spaces

between cells

(9)

Body Fluids

Solutes are broadly classified into:

Electrolytes:

inorganic salts, all acids and bases, and some proteins

Nonelectrolytes:

examples include

Nonelectrolytes:

examples include glucose, lipids, creatinine, and urea

Electrolytes have greater osmotic power than nonelectrolytes

Water moves according to osmotic

gradients

(10)

ECF and ICF

Each fluid compartment of the body has a distinctive pattern of electrolytes

electrolytes

Extracellular fluid- ECF (Na and Cl)

Intracellular fluid- ICF (K, P)

(11)

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

Chloride

Intracellular fluids have low sodium and chloride

Potassium

is the major cation

Phosphate

Electrolytes determine the chemical and physical reactions of fluids

(12)

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

(13)

Hydrogen bond

Lehninger, 4th ed., 2005, Ch 2

(14)

Water Solubility

(15)

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)

(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

(16)

Water Solubility/Hydrophilic

(17)

Water Solubility/Hydrophobicity

Dissolving

hydrophobic compounds

in water produces a measurable

decrease

in entropy.

Water molecules in the nonpolar solute

Water molecules in the nonpolar solute are oriented and form a highly ordered

cagelike shell

around each solute molecule.

(18)

Hydrophobicity Hydrophobicity

(19)

Hydrophobicity Hydrophobicity

(20)

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%)

(21)

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)

(22)

Water Intake and Output

(23)

Acid-Base Balance

Normal pH of body fluids

Arterial blood is 7.4

Venous blood and interstitial fluid

Venous blood and interstitial fluid is 7.35

Intracellular fluid is 7.0

(24)

(25)

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

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.

(26)

Respiratory problems

Respiratory acidosis is a carbonic acid excess (blood CO

₂

is too high)

Respiratory alkalosis is a carbonic acid deficit (blood CO is too low) acid deficit (blood CO

₂

is too low) Compensation would occur

through the kidneys

(27)

Acid-Base Balance

Alkalosis or alkalemia:

arterial blood pH rises above 7.45

7.45

Acidosis or acidemia:

arterial pH drops below 7.35

(physiological acidosis)

(28)

Most hydrogen ions originate from cellular metabolism:

Breakdown of phosphorus containing proteins releases phosphoric acid into the ECF

Anaerobic respiration of glucose

Anaerobic respiration of glucose produces lactic acid

Fat metabolism yields organic acids and ketone bodies

Transporting carbon dioxide as

bicarbonate releases hydrogen ions

(29)

Hydrogen Ion Regulation

Concentration of hydrogen ions is regulated sequentially by:

Chemical buffer systems (act within seconds)

seconds)

The

respiratory center

in the brain stem

(acts within 1-3 minutes)

Renal mechanisms (require hours to

days to effect pH changes)

(30)

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.

(31)

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

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.

(32)

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

(33)

Bicarbonate Buffer System

Bicarbonate buffer system is the only important ECF buffer

A mixture of:

carbonic acid

(H CO ) and its:

carbonic acid

(H

₂

CO

₃

) and its:

salt, sodium bicarbonate

(NaHCO

₃

)

(potassium or magnesium

bicarbonates work as well)

(34)

^(cont.,)

If strong acid is added:

Hydrogen ions released combine with the bicarbonate ions and form carbonic acid (a weak acid)

carbonic acid (a weak acid)

The pH of the solution decreases

only slightly

(35)

^(cont.,)

If strong base is added:

It reacts with the carbonic acid to form sodium bicarbonate (a weak base)

base)

The pH of the solution rises only

slightly

(36)

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

(37)

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