Water Structure and Properties

WATER:

STRUCTURE

AND

PROPERTIES

Why do we study water?

• Because water is the substance that makes possible life

• Life on Earth began in water and evolved there for 3 billion years before spreading onto land.

• Even terrestrial organisms are tied to water.

– cells are about 70-95% water.

Nature of water

•Polar molecule

two hydrogen atoms form

single polar covalent bonds

with an oxygen atom.

the region around oxygen

has a partial negative charge.

the region near the two hydrogen atoms has a

partial positive charge.

The slightly negative

regions of one

molecule are attracted

to the slightly positive

regions of nearby

molecules, forming a

hydrogen bond.

Water

Each water

molecule

can form hydrogen

bonds with up to

four neighbors.

Hydrogen bond is a

weak bond. It is 1/20th as

strong as covalent bonds.

H-bond continually

forms, break ups and re-

forms

At any instant, a

substantial percentage of

all water molecules are

bonded to their neighbors,

making water more

structured than most other

liquids

Water

It is responsible for many

of the unusual physical

properties of water

Properties of water

1. Water is cohesive.

Water that evaporates from

a leaf is replaced by water

from vessels in the leaf.

Hydrogen bonds cause

water molecules leaving the

veins to tug on molecules

further down.

This upward pull is

transmitted to the roots.

Cohesion among water molecules plays a key role in the transport of water against gravity in plants.

Properties of water

2. Water exhibits adhesion or attraction to a solid phase.

Adhesion- attraction

between unlike

molecules

Adhesion is also due

to hydrogen bonding

Adhesion of water to the walls of the vessels helps

counter the downward pull of gravity during water

transport

Properties of water

3. Water exhibits high surface tension

Surface tension – a force exerted by water

molecules at the air-water interface resulting from

the cohesion properties of water

As a result of unequal attraction, an air-water

interface minimizes the surface area of water

Some animals can stand, walk, or run on water without

breaking the surface.

surface tension

Influence the

shape of the surface

Create a pressure

in the rest of the

liquid; at the

evaporative surfaces

of the leaves

generates the

physical forces that

pull water through

the plant’s vascular

system

Capillarity

Movement of water for a small distance up a

glass capillary tube or within a cell wall, due to

water’s cohesion, adhesion and surface tension

The smaller

the tube the

higher the

capillary rise

Properties of water

4. Water has high specific heat

specific heat of a substance is the amount of heat

energy required to raise the temperature of a

substance by a specific amount.

Water’s high specific heat is due to hydrogen

bonding. Heat must be absorbed to break

hydrogen bonds and is released when hydrogen

bonds form.

Investment of heat causes relatively little change

to the temperature of water because much of the

energy is used to disrupt hydrogen bonds, not

move molecules faster.

4. Water has high specific heat

The water that dominates the composition of biological organisms moderates changes in temperature better than if composed of a liquid with a lower specific heat.

Water moderates temperatures on earth

Properties of water

• H2O resists changes in temperature – High specific heat

• takes a lot to heat it up • takes a lot to cool it down

– High thermal conductivity – rapidly conducts heat away from point of application • Localized overheating in a cell due to the heat

of a biochemical reaction is largely prevented because the heat is quickly dissipated throughout the cell

Properties of water

5. Water exhibits high heat of vaporization

Heat of vaporization is the quantity of heat that a liquid must absorb for it to be converted from the liquid to the gaseous state at constant temperature.

Hydrogen bonds must be broken before a water molecule can evaporate from the liquid.

Evaporative cooling moderates temperature in

lakes and ponds and prevents terrestrial

organisms from overheating.

Properties of water

6. Solid water floats

Water is unusual because it is less dense as a

solid than as a liquid.

When water reaches 0oC, water becomes

locked into a crystalline lattice with each

molecule bonded to the maximum of four

partners.

As ice starts to melt, some of the hydrogen

bonds break and some water molecules can

slip closer together than they can while in the

ice state.

Properties of water

Important consequences for life of this

property of water:

allows life to exist under the frozen surface

Properties of water

Many of the solutes of importance to plants are

charged

Water is the medium for movement of molecules

within and between cells

Forms the environment in which most of the

biochemical reactions of the cell occur (oxidation,

reduction,hydrolysis)

Cells are made up of 70-95% water

7. Water is the solvent of life

Water is the solvent of life

• Polarity makes H2O a good solvent

– polar H2O molecules surround + & – ions

– solvents dissolve solutes creating solutions

Properties of water

8. Water has a high tensile strength

Tensile strength – maximum tension that an

uninterrupted column of any material can withstand

without breaking

the ability to resist a pulling force

TRANSPORT

PROCESSES

Diffusion

• Movement of molecules along a concentration

gradient by random thermal agitation

• Described by Fick’s equation:

• Js = -Ds ΔCs where Js = rate of transport

• Δ x s = substance

• D = diffusion coefficient

• ΔC = concentration

• gradient

• Δ x = distance

DIFFUSION

The net movement of molecules from regions of high

concentration to regions of low concentration through

random thermal motion of individual molecules

at dynamic equilibrium:

1. movement is still taking place from one

area to the other

2. the concentrations in the 2 areas are

equal

Diffusion – movement of molecules or ions from

one location to another

-continues even at equilibrium

Net diffusion – direction of greatest number of

molecules

DIFFUSION

Factors affecting the rate of diffusion

Temperature

an increase results in an

increase in the activity of

molecules; thus, increase

in speed of diffusion

Concentration gradients

The steeper the gradient,

the faster the rate of diffusion


Concentration gradients

influenced by the

distance between

the 2 regions

External Forces

The greater the

force, the faster the

rate of diffusion


Size of molecules

The larger the molecule, the slower is the rate of

diffusion

For gases Graham’s law of diffusion states that

the rates of diffusion are inversely proportional to

the square roots of their densities

Size of molecules

r1 (HCl) = √d2 (NH3) = √17 = 4

r2 (NH3) √d1 (HCl) √36 6

Solubility in diffusion medium

the more soluble a substance is in the diffusion medium, the faster it will diffuse (unless the diffusion medium is concentrated)

Presence of other molecules

Decreases rate of diffusion because of

additional collisions that occur

Direction of net diffusion is not influenced by the presence of other types of molecules


Osmosis

movement (net diffusion) of water through

a differentially permeable membrane from

a region of high water concentration to a

region of low water concentration

Special example of net diffusion

The solution with the higher concentration

of solutes is hypertonic.

The solution with the lower concentration

of solutes is hypotonic.

These are comparative terms. • Tap water is hypertonic compared to distilled water

but hypotonic when compared to sea water.

Solutions with equal solute concentrations are isotonic.

direction of osmosis is determined only by a

difference in total solute concentration.

-The kinds of solutes in the solutions do not

matter

water molecules move at equal rates from one

to the other, with no net osmosis.

Osmosis

Animal cell

in an isotonic environment

experiences no net movement

of water across its plasma

membrane.

Volume of the cell is stable

Water balance of living cells

Animal cell

• in a hypertonic environment will loose

water, shrivel, and probably die.



A cell in a hypotonic solution will gain

water, swell, and burst.

Animal cell


Water balance between cell and its environment

is crucial to organisms

cells without walls cannot tolerate too much

uptake or loss of water

Animal cell

Unless it has a special adaptation for to offset

the osmotic uptake or loss of water, an animal cell

fares best in an isotonic environment

Plant cell

have walls that contribute to the cell’s water

balance.

in a hypotonic solution will swell until the elastic

wall opposes further uptake.

turgid, a healthy state for

most plant

cells.


Plant cell

If a cell and its surroundings are isotonic, there is no movement of water into the cell and the cell is flaccid and the plant may wilt.


Plant cell

In a hypertonic solution, a cell wall has no

advantages.

As the plant cell looses water, its volume shrinks.

Eventually, the plasma membrane pulls away

from the wall.

plasmolysis

is usually

lethal.


Plasmolysis

Common examples

1. “Burning” of plants after spraying with insecticides

2.Excessive addition of chemical fertilizers

3.Salting of meat and fish

4.Jams and jellies

5.Undesirable plants

Osmosis

The movement of water in osmosis cannot be

accurately explained in terms of differences in

concentration

Movement of water through a differentially

permeable membrane from an area of high free

energy to an area of low free energy of water

Free energy-useful or available energy; the

capacity to do work

In the osmometer,

equilibrium was reached

even though the

concentrations on

opposite sides of the

membrane were not

equal

Osmosis

Water potential

• Free energy of water is affected by:

-presence of solutes

-external force (hydrostatic pressure, wall pressure/turgor pressure)

• combined effect of these factors are included in a single measurement called water potential (ψ)

• potential in water potential refers to the capacity to do work when water moves from an area of higher ψ to an area of lower ψ

Water potential

• Measure of the free energy of water/unit volume (J m-3 )

• Express in pressure units

– Bars

– Atmospheres ( 1 bar= 0.987 atm)

– Pounds/square inches ( 1 bar = 14.7 lb/ in2 )

– Milimeters of mercury (1 bar = 750 mm Hg)

– Pascals = J m-3

– Megapascals= Pa/ 106 ( 1MPa=10 bars)

Ψw= Ψs + Ψp + Ψm + Ψg

--the reference standard is pure water; water potential equal to 0 MPa.

Ψs –osmotic potential, the amount by which water potential is reduced as a result of the presence of solutes

--negative values

Water potential

Osmotic potential

Ψs = -CiRT

• Ψs - osmotic potential

• C – concentration of the solute expressed as molality (moles solute/ kg H2O)

• i- ionization constant

• R- gas constant T- absolute temperature (C + 273)

Water potential

Ψp – hydrostatic pressure/ pressure potential

- 0 or positive

--the positive pressure operating in plant cells is the wall pressure or turgor pressure; in the osmometer it is the hydrostatic pressure

Ψm – matrix potential -- the component of water potential influenced

by the presence of a matrix (surfaces to which water molecules are adhered)

Water potential

• Ψ g : Gravity - causes water to move downwards

unless opposed by an equal and opposite force

• Ψg = ρwgh

– ρw - density of water

– g- acceleration due to gravity

– h – height of water above the reference-state water

– Ρwg has value of 0.01MPa m-1 at standard state

– generally omitted at cell level

Ψw (sol’n) = 0 bars

Cell original

condition:

Limp cell, Ψs

(cell) =-10 bars

Cell after

equilibrium

Ψw =?

Ψs = ?

Ψp = ?

Cell orig condition

Ψs = -10 bars

Ψp = 0

Ψw = -10 bars

Ψs = -2 bars

Ψw = -2 bars

solution

Cell after

equilibrium

Ψw =?

Ψs = ?

Ψp = ?

Facilitated diffusion

The passive movement of molecules down its

concentration (uncharged)/electrochemical (ions)

gradient via a transport protein

Types of transport protein

1. Channel protein

simply provide corridors allowing a water or

specific ion to cross the membrane.

Some are gated channels e.g. K+ gates in

guard cell membrane


1.Channel protein

Involved whenever large quantities of solutes must

cross the membrane rapidly

Very rapid process- ~ 108 ions/sec through each

channel protein

K+, Cl-, Ca+ channels


2.Transfer /carrier proteins

Selectively bind to a solute on one side of the

membrane and releasing the solute on the other

side

Involves conformational change of the transport

protein

Much slower -100-1000

ions/sec

Transport proteins have

much in common with

enzymes.

They have specific binding

sites for the solute.

While transport proteins do not usually catalyze

chemical reactions, they do catalyze a physical

process, transporting a molecule across a membrane

that would otherwise be relatively impermeable to the

substrate.

Transport proteins can become saturated

How do water molecules actually cross the

membranes?

How do water molecules actually cross the

membranes?

Lipid bilayer

Because water molecules

are so small, they move

relatively freely across the

lipid bilayer of membranes

even though the middle

zone of that bilayer is

hydrophobic

Aquaporins- selective protein

channels

Facilitate water diffusion

Active transport

Pumping of solutes against their gradients

Requires the cell to expend its own metabolic

energy, usually but not always, hydrolysis of

ATP

Active transport is critical for a cell to maintain

its internal concentrations of small

molecules that would otherwise diffuse

across the membrane.

Active transport is performed by specific

proteins embedded in the membranes.

Active transport

• Primary active transport-coupled directly to

a source of energy (e.g. ATP hydrolysis,

oxidation-reduction reaction)

pump

Active transport • Electroneutral transport- no net movement

of charge

– e.g. H+ /K+ -ATPase in gastric mucosa of

animals

• Pumps one H+ out of the cell for every one K+ in

Active transport

• Electrogenic transport- ion transport involving

a net movement of charge across the

membrane

• Uniport- transport of a single species in one

direction

It hydrolyzes ATP and uses the released energy

to pump hydrogen ions (H+) out of the cell.

This creates a proton gradient because the H+

concentration is higher outside the cell than

inside.

It also creates a membrane potential or voltage

because the proton pump moves positive

charges (H+) outside the cell, making the inside

of the cell negative in charge relative to the

outside.

Active transport

Both the concentration gradient and the

membrane potential are forms of potential

(stored) energy that can be harnessed to perform

cellular work.

These are often used to drive the transport of

many different solutes.

by root cells.

Na+ / K+ -ATPase of animal cells

Active transport

• Secondary active transport- uses the

energy stored in electrochemical-potential

gradients to drive the transport of other

substances against their gradient of

electrochemical potentials

– 2 types:

• Symport

• antiport

The proton gradient also functions in

cotransport, in which the downhill passage of

one solute (H+) is coupled with the uphill passage

of another, such as NO3- or sucrose.

Summary of transport processes

e.g., O2, CO2,NH3

• Diffusion over short distances is rapid: about 2.5 s in a cell size of 50um

• Diffusion over long distances is far too slow for mass transport: – Average time for a particle to diffuse = L2 /Ds

• L2 -distance

• Ds – diffusion coefficient; depends on identity of the particle and diffusing medium

• Ds -for glucose in water 10-9 m2 s-1

• Diffusion over long distance – 32 years!

Pressure-driven bulk flow drives long-distance

water transport

• Bulk flow:

– Concerted movement of groups of molecules en

masse, most often in response to a pressure

gradient.

• Independent of solute concentration gradients

– So different from diffusion

• Common examples of bulk flow: water moving through

a hose, a flowing river and rain falling

Long distance water transport in plants

• Relation described by the Poiseuille equation •

• Volume flow rate = πr4 ΔP

• 8ή Δx

• ή = viscosity of liquid

• ΔP/ Δx = pressure gradient

• Sensitive to the radius of the container such as xylem

• This is the main method for water movement in

Xylem, Cell Walls and in the soil.

• Dependent on the radius of the tube that water is

traveling in.

– Double radius – flow rate increases 16

times!!!!!!!!!!

Documents

Water Structure and Properties