Each of the small
molecules can serve as a unit of a large organic molecule, often called a
macromolecule. A unit is called a monomer, and the macromolecule is
referred to as a polymer (from the Greek words "polys" meaning
"many" and "meros" meaning "part"). Cells contain
four classes of macromolecules: polysaccharides, lipids, proteins, and
nucleic acids. These macromolecules exhibit great variety and, therefore,
play different roles in cells, ranging from structural functions to metabolic
processes and energy storage.
Water: The Essential
Component of Life
Importance
of Water for Life
Water is a vital
component of all living organisms. Life first originated in water, and no life
can exist on Earth without it. All living things are composed of 70% to 90%
water. Water provides a suitable environment for the organisms that live in
it and is also essential for metabolic activities in all cells. It serves as a
solvent, a temperature stabilizer, and a medium for biochemical reactions.
Water
as a Polar Molecule
Water molecules
exhibit polarity due to the arrangement of their atoms. Oxygen has a partial
negative charge, while hydrogen atoms have a partial positive charge.
As a result, water molecules interact with each other via hydrogen bonds,
giving water its unique properties.
Biological
Importance of Water
Water constitutes
65% to 90% of different organisms. The water content in various human
tissues varies significantly; for example, bone cells contain 20% water,
while brain cells contain 85% water. This high water content allows
biological functions to proceed efficiently.
Hydrogen
Bonds in Water
Hydrogen bonds
are weak individually, but collectively, they impart crucial properties to
water:
- They make water a liquid at
room temperature, as opposed to a gas.
- Without hydrogen bonding, water
would boil at 80°C and freeze at 100°C, which would make
life impossible. However, due to hydrogen bonding, water boils at 100°C
and freezes at 0°C.
- Hydrogen bonding is also
responsible for the unique behavior of water in different temperature
ranges.
Solvent
Properties of Water
Water is an
excellent solvent for various substances, including ionic compounds (salts)
and polar molecules (sugars). This property is crucial for biological
functions because:
- Chemical reactions in cells take place in aqueous
solutions.
- Transport of nutrients and
waste occurs
efficiently in living organisms (e.g., blood circulation, excretion).
- Hydrophobic interactions are significant in the
formation of cell membranes and proteins.
Thermal
Properties of Water
- High Heat Capacity: Water can absorb and store
large amounts of heat with minimal temperature change. This prevents
organisms from experiencing rapid temperature fluctuations.
- High Heat of Vaporization: Water requires a significant
amount of heat to evaporate. This property is beneficial for cooling
mechanisms, such as sweating in animals.
- High Heat of Fusion: Water requires substantial
energy to freeze or melt, reducing the risk of cell contents freezing in
colder environments.
Density
and Freezing Properties
- Water expands upon freezing,
making ice less dense than liquid water. This causes ice to float,
creating an insulating layer that protects aquatic life from freezing
temperatures.
Cohesion
and Adhesion
- Cohesion: Water molecules stick
together due to hydrogen bonding, allowing water to flow smoothly.
- Adhesion: Water molecules stick to
surfaces, assisting in capillary action, which is essential for
water movement in plants.
Surface
Tension
Water molecules
exhibit high surface tension due to cohesive forces. This allows certain
insects, such as water striders, to walk on water without sinking.
Water
as a Reactant
Water plays an
essential role in metabolism:
- Photosynthesis, where water provides hydrogen
atoms.
- Hydrolysis reactions, where water breaks down
larger molecules into smaller units.
Ionization
of Water
Water can ionize
into hydrogen ions (H⁺) and hydroxide ions (OH⁻):
H₂O → H⁺ + OH⁻
This reaction is
crucial in maintaining pH balance in cells.
Condensation
and Hydrolysis Reactions
- Condensation: Two monomers join together,
releasing a water molecule.
- Hydrolysis: The addition of water breaks
down polymers into monomers.
Carbon: The Backbone
of Life
Unique
Properties of Carbon
Carbon is a
fundamental element in all organic molecules. It has four valence electrons,
allowing it to form four covalent bonds with other elements such as
hydrogen, oxygen, nitrogen, and other carbon atoms. These versatile bonding
capabilities make carbon the basis of life on Earth.
Multiple
Bonds in Carbon Compounds
Carbon can form:
- Single bonds (C-C)
- Double bonds (C=C, C=O, C=N)
- Triple bonds (C≡C, C≡N)
These bonds allow
the formation of a vast array of molecules with different properties and
functions.
Classification
of Carbon Compounds
Carbon compounds
are categorized as:
- Saturated compounds: Contain only single bonds.
- Unsaturated compounds: Contain one or more double or
triple bonds.
Importance
of Carbon in Biological Molecules
- Carbon forms the backbone of
organic molecules.
- Carbon-based molecules can
store high amounts of energy (e.g., ATP, glucose).
- Carbon-based structures can be stable
or reactive, depending on their molecular arrangement.
Macromolecules and
Their Functions
Biological
macromolecules fall into four major categories: proteins, carbohydrates,
lipids, and nucleic acids.
Proteins:
The Building Blocks of Life
Proteins are
composed of amino acids joined by peptide bonds. They serve
multiple functions, including:
- Structural roles (e.g., collagen in connective
tissues, keratin in hair and nails).
- Enzymatic functions, where proteins act as
catalysts in biochemical reactions.
- Transport and communication (e.g., hemoglobin in blood,
receptor proteins in membranes).
Carbohydrates:
Energy Providers
Carbohydrates
include monosaccharides, disaccharides, and polysaccharides. They play
key roles in energy storage and structural support:
- Monosaccharides (simple sugars) serve as quick
energy sources (e.g., glucose, fructose).
- Polysaccharides like starch (plants)
and glycogen (animals) store energy.
- Cellulose provides structural support in
plant cell walls.
Lipids:
Energy Storage and Membrane Components
Lipids are hydrophobic
molecules that include fats, phospholipids, steroids, and waxes. They
function in:
- Long-term energy storage.
- Cell membrane formation (phospholipids).
- Hormone production (steroids like cholesterol).
Nucleic
Acids: Genetic Information Carriers
Nucleic acids, DNA
and RNA, store and transmit genetic information, guiding the synthesis of
proteins in living cells.
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