Introduction to
Bioengineering (BBL1020)
Instructor: Dr. Dinesh K. Ahirwar
Department: Bioscience & Bioengineering
Lecture-5
13/08/2024
Recap
Hydrogen bonding
• Polar molecules, like water, have a positive end and a
negative end
• A hydrogen bond forms when the positive end of one
polar molecule is weakly attracted to the negative end
of another polar molecule
• The hydrogen bond is a weaker bond than the ionic or
covalent bonds
• Example: DNA strands are held together with hydrogen
bonds
Van der Waals Interactions
• Even a molecule with nonpolar covalent bonds may have positively and
negatively charged regions
• Electrons are not always evenly distributed; at any instant, they may
accumulate by chance in one part of a molecule or another
• Van der Waals interactions occur between transiently positive and negative
regions of molecules
Additional reading:
Gecko and Van Der Waals adhesion
article on classroom
Molecular shapes and
functions
• Molecular shape is crucial: It determines
how biological molecules recognize and
respond to one another with specificity
Molecular mimicking
• Biological molecules of complementary
shapes often bind temporarily to each
other by forming weak interactions,
• Opiates (drugs derived from opium)
relieve pain and alter mood by weakly
binding to specific receptor molecules on
the surfaces of brain cells
• Basis for designing drug molecules
Chemical Reaction
• Chemical reaction occurs when there is a formation or breaking of
chemical bonds between atoms, ions, molecules, or compounds
• Reactant: substances that enter into the reaction
• Products: substances that result from the reaction
A+B C+D
Reactants Products
Chemical reactions rearrange matter
• All animals depend on photosynthesis for food and oxygen
• In photosynthesis, chemical reactions rearrange atmospheric CO2 and
water molecules to generate food and oxygen
Water and Life
Hydrogen bonds in water
• Hydrogen bond of water, 1/20 as
strong as a covalent bond
• Dynamic in nature, form, break, and
re-form with great frequency
• At any instant, most of the water
molecules are hydrogen-bonded to
their neighbors
Water sustains life
Life-sustaining properties of water
Cohesiveness Moderation of temperature
A raft spider
walking on the
water due to
surface tension
Freezing Versatile Solvent
Cohesiveness
• Attracts the molecule of the same substance, due to hydrogen bonds
• Cohesiveness provides surface tension to the liquid
• Surface tension is a measure of how difficult it is to stretch or break
the surface of a liquid
Cohesiveness
• Cohesion pulls the water molecule
upward
• Adhesion, the clinging of one
substance to another, helps counter
the downward pull of gravity
Moderation of temperature
• Temperature is a measure of the average kinetic energy of all the
molecules
• Energy is the capacity to do work
• Work is the moving of matter
• Anything that moves has kinetic energy, the energy of motion
• The kinetic energy associated with the random movement of atoms or
molecules is called thermal energy
• Thermal energy is related to temperature, but they are different
• Temperature represents the average kinetic energy of the molecules in a
body of matter, regardless of volume
• the thermal energy of a body of matter reflects the total kinetic energy,
thus depends on the volume
Read, Think and Answer
Fill each of the table cells with ‘higher’ or ‘lower’
Jug of boiling water Pond
Temperature
Thermal Energy
Heat
• Thermal energy in transfer from one body of
matter to another is defined as heat
• The specific heat of a substance is defined as
the amount of heat that must be absorbed or
lost for 1 g of that substance to change its
temperature by 1°C
Adopted from foodhero.org
Specific heat of water
• Specific heat of water is 1 calorie per gram and per degree Celsius,
abbreviated as 1 cal/(g . °C)
• Water has high specific heat
• Ethyl alcohol has a specific heat of 0.6 cal/(g # °C)
• Specific heat can be thought of as a measure of how well a substance
resists changing its temperature when it absorbs or releases heat
• Example: the temperature of water inside the pot is less then the
temperature of pot itself
Relevance of water’s high specific heat to life
• It can absorb and store a huge amount of heat from the sun and
release it slowly
• This capability of water serves to moderate air temperatures in
coastal areas
• also tends to stabilize ocean temperatures, creating a favorable
environment for marine life
• Animals are better able to resist changes in their temperature than if
they were made of a liquid with a lower specific heat
The costal region experiences moderate
temperature fluctuations
Evaporative Cooling
• A considerable amount of solar heat absorbed by tropical
seas is consumed during the evaporation of surface water
• Creates rain
• Evaporative cooling of water contributes to the stability of
temperature in lakes and ponds
• Plants and animals cool down using this process
• High humidity on a hot day increases discomfort by
reducing evaporation of sweat
Examples: elephants and buffalo
Freezing
• As the temperature drops to • If the ice sinks, will you be
0℃, more and more of its here today?
molecules are moving too
slowly to break hydrogen
bonds
• The hydrogen bonds keep the
molecules at “arm’s length,”
far enough apart to make ice
about 10% less dense (10%
fewer molecules in the same
volume) than liquid water at
4°C.
Freezing
• As the temperature drops to
0℃, more and more of its
molecules are moving too
slowly to break hydrogen
bonds
• The hydrogen bonds keep the
molecules at “arm’s length,”
far enough apart to make ice
about 10% less dense (10%
fewer molecules in the same
volume) than liquid water at
4°C.
Water: The solvent of life
• The polar nature of water makes it an
excellent solvent
• NaCl ions and regions of the water
molecules are attracted to each other
due to their opposite charges
• As a result, water molecules surround
the individual sodium and chloride
ions, separating and shielding them
from one another
Water: The
solvent of life
• Nonionic polar molecules
dissolve when water
molecules surround each
of the solute molecules,
forming hydrogen bonds
with them
• Example: sugar and
proteins
