Unit 2 – WATER AS BIOCHEMICAL SOLVENT

Water

is the solvent of life. It bathes our cells, dissolves and transports

compounds in the blood, provides a medium for movement of molecules

into and throughout cellular compartments, separates charged

molecules, dissipates heat, and participates in chemical reactions.

(60%) (40%)

Approximately (____) of the total body water is intracellular and (____)

extracellular.

(extracellular water) (interstitial water)

(______) includes the fluid in plasma (blood after the cells have been removed) and (_______) (the fluid in the tissue spaces, lying between cells).

Transcellular water

is a small, specialized portion of extracellular water that includes gastrointestinal secretions, urine, sweat, and fluid that has leaked through capillary walls due to such processes as increased hydrostatic pressure or inflammation.

(hydride of oxygen) (H2O)

Water can be considered a (________) due to its elemental composition (____).

ammonia (NH3), hydrogen fluoride

(HF), and hydrogen sulfide (H2S)

In comparison to

other hydrides that are its nearest neighbors in the periodic table, namely (_______________________________________) water has substantially higher:

(high) (liquid state)

These properties suggest that intermolecular forces of attraction between

H2O molecules are (_____). Furthermore, the maximum density of water is found in the in the (_______), not solid, unlike most matter.

(hydrogen bonds)

These eccentric properties of water are really fascinating but should have an explanation. And the explanation lies in its unrivaled ability to form (____________)

(covalently linked) (bent structure) (O–H) (oxygen) (partial negative) (partial positive)

The 2 hydrogens of water are (____________) to the oxygen atom

giving a non-linear arrangement. This structure of water is known as

(________) (See Figure on the right). In the (_____) bonds of H2O,

(______) is more electronegative than hydrogen, meaning there is

a higher probability that the bonding electrons are closer to the

oxygen. This gives rise to a (______) charge in the oxygen

atom and (_______) charge in the hydrogen atom. The

partial charges are usually depicted us δ+ and δ-, respectively.

(polar bonds) (polar molecules) (water molecule polar) (nonpolar)

Bonds such as this are called (_______). Molecules with polar bonds are (__________), that is why

(__________) is polar. In the case where the electronegativity difference is quite small, such as in the

C–H bond in hydrocarbons, the sharing of electrons is very nearly equal, and the bond is essentially (_______).

(hydrogen bonds) (Hydrogen bond) (hydrogen donor) (hydrogen acceptor) (hydrogen donor) (fluorine (F), oxygen (O), or nitrogen (N) (hydrogen acceptor)

One of the important consequences of the polarity of water molecule is that water molecules attract one another through the formation of (_________). (__________) is a non-covalent interaction (intermolecular force). It is a special case of a dipole-dipole interaction formed between a (__________) and a (__________). A (________) is a hydrogen atom covalently bonded to an electronegative atom, specifically (_____/_______/_______). A (__________) is a lone pair of electrons on any of those mentioned electronegative atoms (F, O, and N).

(hydrogen donors) (hydrogen acceptors) (2 hydrogen atoms) (oxygen atom) (2 lone pairs) (hydrogen acceptors) (4 hydrogen bonds)

Water can then be both (___________) and (_______) (See figure below). The H2O molecule has (__________) bonded to an electronegative oxygen atom which can potentially be hydrogen donors. The (________) of H2O, on the other hand, has (________) of electrons which can potentially be (________). In total, a water molecule has the potential to form (_________) (2 donors and 2 acceptors).

(cooperative)

It must also be pointed out that hydrogen bonding in water is (__________). That is, a hydrogen bonded water molecule serving as an acceptor is a better hydrogen donor that an unbonded molecule. A hydrogen bonded water molecule serving as a donor is also a better hydrogen acceptor. Thus, participation in hydrogen binding by H2O molecule is a phenomenon of mutual reinforcement.

(strong intermolecular attractions) (dissolve many organic biomolecules)

This ability of water for hydrogen bonding is the source of the (__________) that

endow this substance with its anomalously high boiling point, melting point, heat of vaporization, and

surface tension. Hydrogen bonding also enables water to (______________) that

contain functional groups that can participate in hydrogen bonding.

(hydrogen acceptor) (hydrogen acceptors) (donors)

1. The oxygen atoms of aldehydes, ketones, and amides provides lone pairs of electrons that can serve as (__________).

2. Alcohols, carboxylic acids, and amines can serve both as (___________) and (_______) for formation of hydrogen bonds.

hydrophilic

Substances, such as ionic substances and polar substances, that can be dissolved in water, are known as (________).

ion-dipole interaction

Due to the polarity of water molecule, they can align themselves around ionic substances so

that the partially negative oxygen atoms of the water molecules are oriented towards the

cations (positively charged ions) of the ionic substances and the partially positive hydrogen

atoms are oriented towards the anions (negatively charged ions). This intermolecular

interaction is known as (___________)

(dipole-dipole interaction) (intact molecules disperse)

Polar substances can also be hydrated by water in the same manner as ionic substances. The interaction between water and another polar molecule is known as (_____________). Some polar substances may also participate in hydrogen bonding, which enhances solubility. • However, they do not dissociate into ions, rather the solution formed contains (______________) in water. As an example, intact polar methanol molecules are dispersed in water when dissolved. • An increase in number of polar groups in an organic molecule increases its solubility in water.

hydrophobic

Non-polar substances are known as (________) because

of their inability to be dissolved in water.

(hydrophobic interaction) (hydrophobic effect)

Non-polar substances are incapable of forming dipole-dipole interaction or hydrogen bond with water, but can interact with each other through (____________________). When non polar substances are mixed with water are mixed, water molecules tend to interact with other water molecules rather than with non-polar molecules. Consequently, water molecules exclude non-polar substances forcing them to associate with each other. This is known as (____________).

(fatty acids) (phospholipids) (amphipathic) (amphiphiles)

There are molecules, such as (______) (right) and (________) (left), that are both hydrophilic and hydrophobic. They usually have a non-polar hydrocarbon tail and an ionic or polar end. These substances are said to be (_________) and are known as (________).

(hydrophilic head (polar) (excluded) (structurally ordered aggregates)

When amphiphiles are dispersed in water, the (_________) tends to be hydrated, while the hydrophobic tail (non-polar) tends to be (________). This results to the formation of (_________________).

(micelles) (amphipathic substances) (bilayer) (hydrophobic)

An example of these aggregates are (______), which are

globules of up to several thousand (_______________)

arranged so that hydrophilic head at the globule surface can

interact with the aqueous environment, while the non-polar tails

associate with one another in the center of the structure

minimizing contact with water. Alternatively, amphipathic

molecules may arrange themselves to form (_____), which are

sheets in which the polar groups face the aqueous phase. In

both micelles and bilayers, the aggregate is stabilized by

(_________) effect.

(carboxylic acids and amines) (acidic) (basic)

Biological molecules, such as proteins and nucleic acids, have numerous functional groups that acts as

acids or bases – for example, (____________), respectively. How exactly are these functional

groups considered to be (_______) and (______)?

(acids) (bases) (Johannes Bronsted) (Thomas Lowry) (Bronsted-Lowry definition)

There are plenty of theories that can be used to describe the nature of (_____) and (_____). For describing

functional groups in organic compounds and biomolecules, the most important is the one formulated by

(______________) and (_________) in 1923 known as the(_______________). These states

that an acid is a substance that can donate a proton (H+), and a base is a substance that can accept a proton (H+).

In the previous general reaction, an acid (HA) reacts with a base (B) to form the conjugate base of the acid (A-) and the conjugate acid of the base (BH+). A conjugate base is the one that is left when a H+ leaves an acid, and conjugate acid is the one that is formed when a base accepts a H+.

(Acetic acid) (Acetate) (acetic acid) (hydronium ion)

o (_______) donates H+ to water; thus, it’s an acid. Conversely, water accepts H+ and is therefore a base.

o (_____) is what is left when H+ leaves (_______); thus it is the conjugate base. On the other hand, the (_______) is formed when water accepts H+ and is therefore the conjugate acid.

(Ammonia) (Ammonium) (ammonia) (conjugate acid). (hydroxide) (conjugate base).

o (_____) accepts H+ from water; thus, it’s a base. Conversely, water donates the H+ and is

therefore an acid

o (_____) is formed when (______) accepts H+; thus it is the conjugate acid. On the other

hand, the (______) is what is left when H+ leaves water and is therefore the conjugate base.

(water) (acid) (base) (amphoteric substances)

Notice how (_____) can act as (_____) or (____) in the examples above. Substances, such as water, that can act

as an acid in the presence of a base or act as a base in the presence of an acid are referred to as

(___________).

(double-sided arrow) (reversible) (equilibrium) (equilibrium expression) (reactant) (product)

Notice that the chemical equation for the auto-ionization of water uses a (________). This means

this reaction is (________) and in (______). For reactions in equilibrium, we can often describe it by an

(_____________) in which the concentration of the (_____) is in the denominator and the

concentration of the (_______) is in the numerator

(hydrogen ion concentration) (1909) (Søren Sørensen) (pH (power of hydrogen) (negative logarithm of the hydrogen ion concentration)

The values of (______) ([H+]) for most solutions that we will encounter in biochemistry are conveniently small and thus difficult to compare. A more practical quantity, which was devised in (_____) by Danish biochemist (__________), can be used. This is known as (_______) which is defined as the (____________________________)

Just like that of hydrogen ion concentrations [H+], the values of Ka are also inconveniently small and difficult to compare. So we could also express it in terms of its negative logarithm known as pKa, that is: pKa = -log (Ka) Because pKa is the negative logarithm of Ka, pKa has an inverse relationship with acid strength, i.e. the lower the pKa the stronger the acid. This is the reverse of the relationship between Ka and acid strength. The table above also list the corresponding pKa values of the common weak acids.

(leftmost) (CO2) (H2O) (Respiratory acidosis) (right) (Respiratory alkalosis) (left)

Let’s consider the (______) side of the buffer first (interaction between (___) and (____). Recall,

that this occurs in the lungs. Acidosis or alkalosis caused by disturbance of this side of buffer

is considered respiratory.

o (____________)

§ Increase of CO2 resulting to a shift of the buffer reaction to the (____) which

produces more H+ and lowers blood pH.

o (_____________)

§ Decrease of CO2 resulting to a shift of the buffer reaction to the (___) which

lowers H+ and increases blood pH.

(rightmost) (Metabolic acidosis) (Metabolic alkalosis)

• Let’s consider the (____) side of the buffer first (interaction between H+ and HCO3).

Recall, that this occurs in the kidneys. Acidosis or alkalosis caused by disturbance of

this side of buffer is considered metabolic.

o (_________)

§ Decrease of HCO3- resulting to a shift of the buffer reaction to the right

which produces more H+ and lowers blood pH.

o (__________)

§ Increase of HCO3- resulting to a shift of the buffer reaction to the left

which lowers H+ and increases blood pH.