A.P1 – Explain the structure of biological molecules in living organisms
A.M1 – Explain the links between the structure and function of biological molecules and their role in
living organisms
A.D1 – Evaluate the effects of disruption of biochemical processes in living organisms
The human body is composed of millions of cells that collaborate to perform the essential functions
of living organisms. All living cells are made of 4 main types of biological molecules known as: Water,
Carbohydrates Proteins and Lipids. They are essential molecules for living organisms because they
allow them to grow, sustain themselves, and reproduce. By interacting with one another, they
contribute to the formation of organisms ranging from single cells to complex molecules. Because of
their various shapes and structures, they can perform a wide range of functions. (Singh, 2021)
(Biological Molecules (A Level) — the science hive, n.d.)
WATER:
60% of our body weight is made of water (Jéquier and Constant, 2009). There are three major fluid
compartments: intravascular fluid (blood volume) interstitial fluid (fluid found in the spaces around
the cells) and intracellular fluid (which is the liquid found inside cells). (15.2 BASIC FLUID AND
ELECTROLYTE CONCEPTS, n.d.) (NCI Dictionary of Cancer Terms, n.d.) (Intracellular fluid, n.d.)
(Linklater and R. Hanson, n.d.). The function of water varies throughout the different locations in the
body. Their main function is to:
Regulate body temperature – when body becomes too hot water it is lost through sweat and
evaporation from the skin reducing body temperature. It maintains water during cold
temperatures because as temperature decreases the water molecules lose kinetic energy
which allows the water to form more stable polar covalent bonds.
Moisten tissues in the eyes nose and mouth – water transports oxygen and nutrient to cells
Protect internal organs and tissues – water help the organs to stay soft and moist
Transports nutrients and oxygen to cells – allows nutrients to pass through the capillaries to
the circulatory system
Lubricate joints – reduces friction between joints
Maintain blood pressure – dilutes the plasma in the blood vessels
Help in the kidneys – excrete waste
Use in the stomach for metabolic reactions – helps breakdown food so nutrients can be
absorbed
Water is one of the most essential molecules used in all the metabolic processes in the body. It is a
polar made up of 2 hydrogen atoms and a single oxygen atom by covalent bonds. The liquid state of
H2O is referred to as water, while the solid state is referred to as ice or snow, and the gaseous state
as vapours. The oxygen present in water has a high electron affinity and electronegativity. This
causes the oxygen to attract electrons from the hydrogen. Because oxygen is more electronegative it
has a greater pull on shared electrons. As a result the water molecule becomes dipole with negative
charges centred around oxygen, forming a negative pole, and the protons of hydrogen atoms
forming positive pole. Overall the oxygen is partially negatively charged and hydrogen is partially
positively. The bond angle of water molecules is 104.5°C. Because of oxygen's high electronegativity
and the presence of two lone pairs in the water molecule, it is ideal for forming hydrogen bonds.
, (Water | Facts, Properties, Structure, Compounds & Summary, n.d.). Hydrogen bonds give water a
high stability. This means that a large amount of energy is required to raise the temperature of
water indicating that it has a high heat capacity. This implies that it takes a lot of heat to evaporate
water making it great for cooling also known as a high heat of vaporisation. As the temperature
drops, the kinetic energy of the water molecules decreases, making it less capable of breaking the
hydrogen bonds. This causes the formation of a semi-crystalline structure that separates the water
molecules, making ice less dense than liquid water and allowing it to float. This means that it
insulates the water beneath it, allowing organisms to survive in it. Water is also a great solvent for
other polar molecules because it can interact which charged regions of other molecule to dissolves
them. It also useful for ionic solvents as the water separates the ions allowing them to dissolve. This
property combined with water being a liquid at various temperatures makes it a suitable medium for
metabolic reactions as well as a transport vehicle. It is the primary blood transport medium for
glucose and other nutrients, as well as gases. It is also used as a transport vehicle for both
intracellular and extracellular molecule transport. (Water | A Level Notes, n.d.) (Lesson summary:
Water and life (article) | Khan Academy, n.d.) (The Role of Water in Living Organisms (2.3.8) | CIE AS
Biology Revision Notes 2019, n.d.) (Water | Facts, Properties, Structure, Compounds & Summary,
n.d.)
CARBOHYDRATES:
Carbohydrates are the main energy supply to living organisms and is the body’s primary source of
ATP production in aerobic respiration. Carbohydrates are polymers that are made up of monomers
called monosaccharides. They are bonded together by glyosidic bonds to form disaccharides (when 2
monosaccharides join) and polysaccharides (when more than 2 monosaccharides join together).
When they are formed the hydrogen from one monosaccharide bonds to a hydroxyl group from
another monosaccharide releasing a water molecule. This process is called a condensation reaction.
These molecules can also be broken by the reverse process called hydrolysis which means adding a
water molecule. Carbohydrates include starch, sugars and fibres and are symbolized by the formula
(CH2O)n. from this we can see that carbohydrates are made up of carbon hydrogen and oxygen in
the ratio 1:2:1 (Carbohydrate Polymers | A-Level Biology Revision Notes, n.d.). There are 3 main
types of carbohydrates that work in both plants and animals: starch, glycogen, and cellulose. Plants
gain their energy from the breakdown of starch into glucose. They store excess glucose as starch so
it can be used by the plant when it is needed. Starch is made up of two polysaccharides of alpha-
glucose, amylose and amylopectin. Amylose is long unbranched chain. The angles of the glyosidic
bonds gives amylose a coiled structure which makes it compact and useful for storage of long
polypeptide chains. Amylopectin is the opposite in structure as it a branched chain of alpha-glucose.
The branches allow the enzymes to easily access the glyosidic bonds. This allows glucose to be
released quickly when it is needed. Glycogen is the energy storage used in animals and is also
polysaccharide of alpha-glucose. Animals also receive energy in the form of glucose but store excess
glucose as glycogen. It has a similar structure to amylopectin however it has a lot more branches.
The increased number of branches allows it to be quickly released when it is needed. It is also
compact which make it useful for storage. Unlike than the other two types, cellulose is a long beta-
glucose chain. It is a branched chain that is linear when the molecules are bonded together. They are
linked by hydrogen bonds to form fibres called microfibrils. (Fielding and Anderson, n.d.). Microfibrils
provide tensile strength to the cell wall which help the cell maintain its structure. Microfibrils are
elastic which helps the cell wall withstand the pressure of substance in out the cell. (Alberts et al.,
2002) (Fielding and Anderson, n.d.)
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