Chapter 1 and 2.pptx

Addis Ababa University
Department of Zoological Sciences
General Biology Program Unit
Course Material for General Biology (Biol. 1012)
2023
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Unit 1: Introduction
 Human being is curious by his nature. This curiosity has driven them to explore the
world around them.
 Over the time, manipulating and controlling nature for the benefit of the mankind has
become an object of exploration.
 Initially the pace of exploration was slow, but with the result of industrial revolution in
the West, the pace of exploration has increased manifold.
 Exploration became a tool for not only modifying and controlling the nature but also
for preserving the natural resources.
 Over the course of human history, people have developed interconnected and
validated ideas about the physical, biological, psychological and social worlds.
 Those ideas have enabled successive generations to achieve an increasingly
comprehensive and reliable understanding of human species and environment.
 Human exploratory activities have resulted in the accumulations of a vast source of
knowledge called ‘natural science’.
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The meaning and scope of biology
 Biological Sciences is the study of life and living organisms. It is also called
as “Biology”.
 The word biology comes from Greek terminology, ‘bios’ means life and ‘logos’
means study.
 In the late 1700s, Pierre-Antoine de Monet and Jean-Baptiste de Lamarck
coined the term biology.
 Earlier study of living things was restricted to the pure sciences like Botany
and Zoology that together comprise the Biology, but as the time passed new
branches evolved.
 New technologies developed in pure subjects as well as in applied fields gave
rise to a very broad science called Biological Sciences.
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The meaning and scope of biology…
• Biological Science is an extensive study covering the minute workings
of chemical substances inside living cells to the broad scale concepts of
ecosystems and global environmental changes.
• It is also concerned with the physical characteristics and behaviors of
organisms living today and long ago.
• How they came into existence, and what relation they possess with each
other and their environments?
• Intimate study of details of the human brain, the composition of our genes,
and even the functioning of our reproductive system are dealt in biological
sciences.
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Some Branches of Biology
• Zoology is the study of animals.
• Botany is the study of plants.
• Anatomy : Study of internal structures of plants and animals after dissection.
• Biochemistry: Study of chemistry of living matter (i.e., chemical
composition, nature, mode of formation, functioning) in relation to life
activities.
• Cytology : Study of the structure and functions of cells and their organelles.
• Ecology : Study of relationship between organisms and environment.
• Embryology : Study of developmental stages of organisms up to hatching or
birth.
• Endocrinology : Study of endocrine glands and hormones action in animals.
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• Evolution : Study of the origin of life and the gradual differentiation or
descent of species.
• Histology : Study of tissues by microscopy.
• Immunology : Study of resistance of organisms to infection.
• Limnobiology : Study of fresh water lakes, ponds and streams.
• Morphology : Study of form and structure of animals.
• Palaeozoology : Study of fossil animals.
• Physiology : Study of functions of various parts within the organisms.
• Taxonomy : Study of classification of organisms and their evolutionary
relationships with other organisms.
• Zoogeography : Study of the distribution of animals over the earth.
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The knowledge of Biological Sciences
 Understanding of basic life processes: Biological sciences help us understand
the basic processes of living organisms, including how they function, reproduce, and
evolve. This knowledge is critical to advancing our understanding of life on Earth.
 Advances in medicine: Medical advancements are dependent on a strong
foundation in biological sciences. Knowledge of biology helps us understand the
causes and treatment of diseases, develop new drugs and therapies, and improve
diagnostics
 Environmental awareness: Studying the natural world provides insight into the
complex relationships between living organisms and their environment.
Understanding these relationships is crucial to maintaining a healthy ecosystem and
mitigating the effects of environmental degradation.
 Agricultural and food production: Biological sciences are key to developing
sustainable farming practices, improving crop yields, and ensuring food security for a
growing global population. Develop scientific outlook.
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The origin and nature of life
Where did the first life come from?
 The origin of life is a fascinating and complex topic that has puzzled scientists
and philosophers for centuries. Many theories have been proposed to explain
how life originated on Earth, but the exact mechanism remains unclear.
I. Theory of Special Creation: according to this theory, all the different forms of
life that occur today on planet Earth have been created by God, the almighty.
II. Theory of Spontaneous Generation or Abiogenesis: it assumes that non-living
material in a spontaneous manner gave rise to life.
 Some of several observations supporting this theory were:
 Hair of horse tail dipped in the water gives rise to horsehair worm, gordius.
 Fly larvae develops on rotten meat.
 Van Helmont claimed that he can produce mice from the dirty shirt and handful of
wheat grains kept in dark cupboard in 3 weeks
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The origin and nature of life…
III. Theory of Catastrophism: This is the extension of the theory of special
creation. This theory assumes that life originated by the creation and
followed by catastrophe due to geographical disturbances.
 Each catastrophe destroyed the life completely whereas each creation
forms life different from the previous one. Hence, each round of
catastrophe/creation is responsible for evolving different types of
organisms on earth.
 The criticism of the current theory is same as previous one: No scientific
experiment to support the hypothesis and mostly based on imaginary
concepts.
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IV. Cosmozoic Theory (Theory of Panspermia): The theory assumes that
life was present in the form of resistant spores and appeared on earth from
other planet. Since the condition of earth was supporting the life, these
spores grew and evolved into different organisms. The theory did not gain
any support. This theory lacks evidence, hence it was discarded.
V. Theory of Chemical Evolution: this theory is also known as Materialistic
Theory or Physico-chemical Theory.
– According this theory, origin of life on earth is the result of a slow and gradual
process of chemical evolution that probably occurred about 3.8 billion years ago.
This theory was proposed independently by two scientists - A.I.Oparin, a Russian
scientist in 1923 and J.B.S Haldane, an English scientist, in 1928.
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Nature and characteristics of life
• For centuries, scientists and philosophers have proposed hundreds of definitions of
life, but none of them have been widely accepted.
• But, in biology life can be characterized.
• There are many characteristics that living organisms share. All living organisms:
– respond to their environment
– Grow and change
– reproduce and have offspring
– have complex chemistry
– maintain homeostasis
– are built of structures called cells
– pass their traits onto their offspring
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Living tissues and organisms exhibit:
Irritability: It is also called sensitivity.
• the ability to be excited or detect stimuli and to respond there
to.
It gives consciousness to living organisms to adjust in the
environment.
Living things have the ability to respond to changes in both the
internal and external environments and thus ensure that they
maximize their chances of survival.
Organisms like animals respond to stimuli exhibiting behaviour
and thus leading to orientation or movement known as kineses
or tropism.
It refers to conductivity in higher animals having nervous
system.
Such animals possess receptors of different kinds – photo-,
thermo-, chemo-, thigmo-, etc.
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Movement
Some living beings, such as animals and some bacteria,
have the ability to move from place to place.
Movement from place to place is called locomotion and
seems to be necessary in order to obtain their food, unlike
other living beings, such as plants, which can manufacture
their own food from raw materials obtained in one place.
Locomotion (displacement of animal in space and time) is
movement but all movement (swaying body being
sedentary animal and moving tentacles being sessile
animal) is not locomotion.
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• Growth and reproduction: this consists of the power of
multiplication and duplication, regeneration and
differentiation.
The life span of all living beings is limited, but all of them have
the ability to perpetuate life, thereby ensuring the survival of
the species.
The resulting offspring have the same general characteristics as
the parents, whether such individuals are produced by asexual
or sexual reproduction.
Growth is irreversible increase in mass of an individual.
 A multicellular organism increases its mass by cell division.
In plants, growth continues throughout life as they have
meristematic areas where cell divisions occur regularly.
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• Adaptability: permitting both change and maintenance of
balances (homeostasis)
• Adaptations allow the organisms to overcome seasonal and
other changes in the environment.
• They are of two types, short term adaptations (e.g., hibernation
in most amphibians and reptiles and some mammals) and long
term adaptations (e.g., the claws of different birds are well
adapted to suit their perching habits).
• An organism is considered best adapted to an environment
when it possesses inherited traits that enhance its survival and
ability to reproduce in that environment.
Homeostasis
It is the maintenance of the balance of internal environment of
all living beings to that of external environment.
It is well developed in animals than plants.
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• Metabolism: the transformation of energy and the use of
materials.
• These properties, however, can be retained for a while by
tissues after death of the organism so there is another mystery
of life which we understand only in part.
• Excretion: excretion and osmoregulation regulation are two
important homeostatic processes occurring in living cells,
helping them to maintain a constant internal environment, or
steady state.
• Excretion is the removal from the cell of waste products of
metabolism. In plant cells the major excretory products are
oxygen from photosynthesis and carbon dioxide from cell
respiration.
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Hatching or birth
Every organism at the beginning of their life either emerge
(plants) or hatch (insects) or born (men) after which they start
their life-cycle.
Life-cycle
• Also called life-history
Each organism should have to pass different stages during the
process
• Senescence/ageing and death
All living beings face senescence or ageing during their life-
cycle.
They exhibit death after the completion of their life-history.
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Scientific methods
• What is the scientific method?
– It is a method that is devised to gather knowledge and further develop the field of
science.
– It is also the technique used in the construction and testing of a scientific
hypothesis.
• The scientific method has five basic steps, plus one feedback step:
 Make an observation.
 Ask a question.
 Form a hypothesis, or testable explanation.
 Experimentation
 Test the prediction: data analysis
 Conclusion
 Iterate: use the results to make new hypotheses or predictions.
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Scientific methods…
• Problem/Observation
– Example: I seem to sleep better when I play relaxing music.
• Create hypothesis
– If…then statement
– Example: If there is relaxing music playing, then I will sleep longer.
 Experiment: two parts on experiment.
 Control group: Receives no special treatments, Used as comparison,
The “normal” group
 Experimental Group: Same as control group, but with one difference…
 Independent variable: The one factor that differs from the control
group. The factor you are testing
 Dependent variable: Data gathered
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Control group vs. Experimental Group
Results: Sleep continuously for
6.2 hours
Results: Sleep continuously for 7.5
hours
Dependent Variable
Independent
Variable
Does the data
support the
hypothesis?
Yes. Since all
other factors
are the same,
the music must
have caused the
extra sleep.
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Unit 2: Biological Molecules
• Biological Molecules
 Biomolecules are molecules that are essential to life and are found in living
organisms.
 There are 4 major classes of biological macromolecules viz., carbohydrates,
lipids, proteins, and nucleic acids, and each is an important component of
the cell and performs a wide array of functions. Combined, these molecules
make up the majority of a cell’s mass.
 Biomolecules consists mainly of carbon and hydrogen with nitrogen,
oxygen, sulphur, and phosphorus. Biomolecules are very large molecules of
many atoms, which are covalently bound together.
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Carbohydrates
• Carbohydrates are molecules composed of carbon, hydrogen, and oxygen atoms.
They are important sources of energy for organisms and are also used for
structural purposes. They are common in grains, fruits, and vegetables are all
natural sources. Examples of carbohydrates include sugars, starches, and
cellulose.
• Carbohydrates can be represented by the formula (CH2O)n, where n is the
number of carbon atoms in the molecule. In other words, the ratio of carbon to
hydrogen to oxygen is 1:2:1 in carbohydrate molecules.
• Carbohydrates are classified into three subtypes: monosaccharides,
disaccharides, and polysaccharides.
• Monosaccharides (mono- = “one”; sacchar- = “sweet”) are simple sugars, the most
common of which is glucose.
– In monosaccharides, the number of carbon atoms usually ranges from three to six.
– Depending on the number of carbon atoms in the sugar, they may be known as trioses (three
carbon atoms), pentoses (five carbon atoms), and hexoses (six carbon atoms).
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Carbohydrates…
• Monosaccharides may exist as a linear chain or as ring-shaped molecules; in
aqueous solutions, they are usually found in the ring form.
– If the sugar has an aldehyde group (the functional group with the structure R-CHO), it
is known as an aldose, and
– if it has a ketone group (the functional group with the structure RC(=O)R'), it is
knownse as a ketose.
– Aldoses are reducing sugars: common in plants
• Examples of aldose sugars are glycolaldehyde, glyceraldehyde, erythrose, threose, ribose,
arabinose, xylose, lyxose, allose, altrose, glucose, mannose, gulose, idose, talose, and
galactose
– Ketoses are non-reducing sugars. Common in processed foods
• E.g. fructose, ribulose and xylulose, erythrulose, tagatose, sorbose, and psicose.
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Carbohydrates…
Glucose versus galactose
• Galactose (part of lactose, or milk sugar) and glucose (found in
sucrose, glucose disaccharride) are other common
monosaccharides.
• The chemical formula for glucose and galactose is C6H12O6; both
are hexoses, but the arrangements of the hydrogen atoms and
hydroxyl groups are different at position C4. Because of this small
difference, they differ structurally and chemically (and are known
as chemical isomers) because of the different arrangement of
functional groups around the asymmetric carbon; both of these
monosaccharides have more than one asymmetric carbon
Fructose versus both glucose and galactose
• When looking at glucose, galactose, and fructose, all three are
hexoses, but there is a major structural difference between glucose
and galactose versus fructose: the carbon that contains
the carbonyl (C=O).
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Carbohydrates…
• In glucose and galactose, it is on the C1 carbon, forming an aldehyde group; in
fructose, it is on the C2 carbon, forming a ketone group. The former are
called aldoses based on the aldehyde group that is formed; the latter are designated
as ketoses based on the ketone group.
• Again, this difference gives fructose different chemical and structural properties from
those of the aldoses, glucose, and galactose, although fructose, glucose, and
galactose all have the same chemical composition: C6H12O6.
ii. Disaccharides
• Disaccharides are two monosaccharides that have been joined to one another
through a glycosidic bond, and occurs between hydroxyl groups on each of the
individual monosaccharides.
• Disaccharides are a major source of energy in the diet and the most important of
them are sucrose, lactose, and maltose.
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Carbohydrates…
• Sucrose is the most abundant and consists of a molecule of α-
glucose and β-fructose linked together. They are constituent of fruits,
vegetables, and sweetener
• Lactose is found in milk and dairy products and consists of
galactose and glucose linked by a β-1,4-glycosidic bond.
• Maltose is mainly produced by partial hydrolysis of starch and
consists of two glucose units linked by an α-1,4-glycosidic bond.-
Constituent of starch.
• Other groups of diasaccharides:
– Trehalose: Found in fungi, plants, and insects.
– Lactulose: formed from galactose and fructose. It is helpful for
the treatment of constipation and liver diseases.
– Cellobiose: made up of two glucose molecules which are also
arranged differently. These can be seen in bacteriology which is
a form of chemical analysis.
– Chitobiose: comprises two glucosamine molecules which are
linked. Seen in some bacteria, exoskeletons of insects and is
also found in fish, octopus, and squid.
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Carbohydrates…
• Polysaccharides
 They are long chains of carbohydrate molecules, composed of several smaller
monosaccharides.
 These complex bio-macromolecules functions as an important source of energy
in animal cell and form a structural component of a plant cell.
 It can be a homopolysaccharide or a heteropolysaccharide depending upon the
type of the monosaccharides.
 Polysaccharides can be a straight chain of monosaccharides known as linear
polysaccharides, or it can be branched known as a branched polysaccharide.
Polysaccharides have the following properties:
 They are not sweet in taste; many are insoluble in water; are hydrophobic in
nature; They do not form crystals on desiccation; can be extracted to form a
white powder; are high molecular weight carbohydrates; Inside the cells, they are
compact and osmotically inactive; consist of hydrogen, carbon, and oxygen. The
hydrogen to oxygen ratio being 2:1.
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Carbohydrates…
• Homopolysaccharides: a polysaccharide that contains the same type of
monosaccharides. Some of the important homopolysaccharides are:
– Glycogen: It is made up of a large chain of molecules. It is found in animals and
fungi.
– Cellulose: The cell wall of the plants is made up of cellulose. It comprises long
chains of β-D-glucan.
– Starch: formed by the condensation of amylose and amylopectin. It is found
largely in plants, fruits, seeds, etc.
– Inulin: made up of a number of fructofuranose molecules linked together in
chains. It is found in plants such as agave, wheat, onion, bananas, garlic, asparagus,
and chicory.
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Types Of Polysaccharides: Homopolysaccharides and Heteropolysaccharides.

Carbohydrates…
• Heteropolysaccharides: a polysaccharide that contains different types of
monosaccharides. Examples of heteropolysaccharides are:
– Hyaluronic Acid: It is made up of D-glucuronic acid and N-acetyl-
glucosamine. It is found in connective tissues and skin.
– Heparin: It is made up of D-glucuronic acid, L-iduronic acid, N-sulfo-D-
glucosamine and is largely distributed in mast cells and blood.
– Chondroitin-4-sulfate: Its component sugars are D-glucuronic acid and N-
acetyl-D-galactosamine-4-O-sulfate. It is present in the cartilages.
– Gamma globulin: N-acetyl-hexosamine, D-mannose, D-galactose are the
component sugars of this polysaccharide. It is found in the blood.
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Lipids
• Lipids are a broad group of naturally-occurring molecules which
include fats, waxes, sterols, fat-soluble vitamins (such as
vitamins A, D, E and K), monoglycerides, diglycerides, phospholipids, and others.
• Fats and oils are esters made up of glycerol and fatty acids.
– Glycerol is a small organic molecule with three hydroxyl (OH) groups, while a fatty acid
consists of a long hydrocarbon chain attached to a carboxyl group
• Fats are materials that are solid at ambient temperature and oils are those liquid at
ambient temperature.
• Fats are solid triglycerides.
– In the human body, triglycerides are primarily stored in specialized fat cells, called
adipocytes, which make up a tissue known as adipose.
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Saturated and unsaturated fatty acids
• Fatty acid chains may differ in length as well as in their degree of unsaturation.
• If there are only single bonds between neighboring carbons in the hydrocarbon chain, a
fatty acid is said to be saturated. (The thing that fatty acids are saturated with is
hydrogen; in a saturated fat, as many hydrogen atoms as possible are attached to the
carbon skeleton.)
• When the hydrocarbon chain has a double bond, the fatty acid is said to
be unsaturated, as it now has fewer hydrogen. If there is just one double bond in a
fatty acid, it’s monounsaturated, while if there are multiple double bonds,
it’s polyunsaturated.
– Saturated fats, like butter or bacon grease, solidify at room temperature, while
unsaturated fatty acids, like olive oil, tend to be liquid at room temperature.
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Lipids
• General Properties of Lipids
– They are soluble in non-polar solvents, such as ether, alcohol, chloroform, acetone,
and benzene;
– insoluble in water;
– Lipid molecules have no ionic charges;
– Pure fats and oils are colorless, odorless, and tasteless.
– lipids are considered hydrophobic molecules.
• Importance of lipids
– The functions of lipids include storing energy, protection, signaling, and acting as
structural components of cell membranes.
– Lipids have applications in the cosmetic and food industries, and in nanotechnology.
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Proteins
• Proteins are large macromolecules that comprise one or more long chains of amino
acid residues.
• Are found in every cell in the body
• Are involved in most of the body’s functions and life processes
Structure of Proteins
• Made up of chains of amino acids; classified by number of amino acids in a chain
– Peptides: fewer than 50 amino acids
• Dipeptides: 2 amino acids
• Tripeptides: 3 amino acids
• Polypeptides: more than 10 amino acids
– Proteins: more than 50 amino acids
• Typically 100 to 10,000 amino acids linked together
• Chains are synthesized based on specific bodily DNA
• Amino acids are composed of carbon, hydrogen, oxygen, and nitrogen
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Amino Acids - Protein building blocks
• Amino acids are small compounds that are made of carbon, nitrogen, oxygen,
hydrogen, and sometimes sulfur.
– All amino acids share the same general structure.
– Amino acids have a central carbon atom. Carbon can form four covalent bonds.
– One of those bonds is with hydrogen. The other three bonds are with an amino
group (–NH2), a carboxyl group (–COOH), and a variable group (–R). The
variable group makes each amino acid different.
– There are 20 different variable groups, and proteins are made of different
combinations of all 20 different amino acids.
– Several covalent bonds called peptide bonds join amino acids together to form
proteins.
– When only two amino acids combine by peptide bond, it is called dipeptide,
when amino acids involved in the bond formation become 3, 4, 5 they are
named as tri-, tetra-, and penta- peptides respectively.
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Protein classification
• Based on their shape proteins can be classified as fibrous proteins and globular
proteins
• Proteins have four levels of structure
 Primary structure: the sequence of amino acids in a polypeptide chain;
 Secondary structure: the folding of short(3-30 residues) contiguous segment of
polypeptide into geometrically order units
 Tertiary structure: the three dimensional assembly of secondary structural units
to form larger functional units such as the mature polypeptide and its
components domains;
 Quaternary structures: number and types of polypeptide units of oligomeric
proteins and their spatial arrangement.
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Function of Proteins
• used to construct or build our body
• catalyze biochemical reactions as an enzyme
• regulate body metabolism as hormones
• protect our body from foreign body attack as an antibody and components of
complement
• maintain osmotic pressure in plasma
• Transport different lipids, minerals, hormones, vitamins etc as hemoglobin,
apolipoprotein, albumin etc
• assist to arrest bleeding and maintain homeostasis as coagulation factor
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Nucleic acids
• Nucleic acids are complex macromolecules that store and transmit genetic
information.
– Nucleic acids are made of smaller repeating subunits called nucleotides.
– Nucleotides are composed of carbon, nitrogen, oxygen, phosphorus, and
hydrogen atoms.
• Nucleotides have 3 parts
– nitrogen base (C-N ring)
– pentose sugar (5 C)
• ribose in RNA
• deoxyribose in DNA
– phosphate (PO4) group
 The nitrogenous base can be adenine, guanine, cytosine, or thymine in DNA, and
uracil in RNA.
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Nucleic Acids
• There are two types of nucleic acids:
 Deoxyribonucleic acid (DNA)
 Ribonucleic acid (RNA)
• RNA
– single nucleotide chain
– The pentose sugar is Ribose (has a hydroxyl group in the 3rd carbon---OH)
• DNA
– double nucleotide chain
• N bases bond in pairs
across chains
– The pentose sugar is Deoxyribose (has just an hydrogen in the same place--- H)
Deoxy = “minus oxygen”
– spiraled in a double helix
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Vitamins
• Vitamins are organic components in food that are needed in very small amounts for
growth and for maintaining good health.
• They cannot be synthesized by the body and, therefore, must be supplied in the diet.
• Classification: on the basis of their solubility vitamins are mainly 2 types- Fat soluble and
water soluble vitamins:
– Fat soluble vitamins: vitamins that dissolve in fat.
• Because fat is easily stored on our body, fat-soluble vitamins can be stored within our
fat. This means they can accumulate and be saved for later use.
• The fat-soluble vitamins are A, D, E and K. –
• Toxicity can result from excessive intake of these vitamins (A and D).
– Water soluble vitamins: Vitamins that dissolve in water.
• They travel in the blood and are stored in limited amounts.
• They are readily excreted from the body through urine.
• Water-soluble vitamins include the B-complex vitamins and vitamin C.
• There are eight B vitamins, including vitamin B1, B2, B3, B5, B6, B7, B9 and B12.
• Uses: required for growth, maintenance, reproduction, lactation and serves as
coenzymes.
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Minerals
 Minerals are inorganic elements that are essential for the body’s metabolic
processes.
 Only about 4% of an animal’s total body weight comprises mineral matter;
however, like the vitamins, the presence of these elements is essential for life.
 Based on the requirement of the body, minerals in food are classified into two
types:
 Macrominerals are those minerals which are required in relatively large
doses. Therefore, they are also called major minerals.
 They include sodium, calcium, chloride, magnesium, potassium, phosphorus, and
sulfur.
 Microminerals, also called trace minerals, are minerals which are required in
small amounts. Therefore, they are also called minor minerals.
 Trace minerals include iron, copper, iodine, zinc, manganese, fluoride, cobalt and
Selenium.
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Functions of Minerals
 Function to promote growth in plants. E.g. Mg+2 is an important component of the green
pigment, chlorophyll, involved in photosynthesis.
 Minerals give the human body structure (formation of bones, teeth); they are needed
for growth and development, and are components of organs and fluids; and they
serve as cofactors for enzymes.
 Minerals also participate in regulating nerve impulses, acid–base balance in fluids, and
body fluids and contraction and relaxation of muscles.
 Mineral deficiencies or excesses in humans may result in major health problems.
 Long-term deficiency in calcium intake is known to be a major contributor to osteoporosis,
often called “brittle bone” disease.
 Deficient intake of iron commonly results in iron-deficiency anemia.
 An excess of sodium intake has been associated with high blood pressure in some people.
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Water
• Water is a substance made up of two H atoms and one O atom (H2O), and can
be found in solid state as ice, in gaseous state as water vapor, and in liquid
state as water.
• It is a transparent, tasteless, odorless, and nearly colorless chemical substance.
• The substance that make possible life on earth; it is the main constituent
of Earth's hydrosphere and occupies most of a cell’s volume of all known living
organisms.,
– E.g. Water covers about 71% of the Earth's surface and it makes up 60-75% of
human body weight. →a loss of 15% can be fatal.
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Water chemistry: polarity
• The 3 atoms are held together by polar covalent bonds.
• When the H atoms bind to the O, it creates an asymmetrical molecule with
positive charge on one side and negative charge on the other side.
• This charge differential is called polarity, and this causes water a polar
molecular, where the –ve end of one water molecule to be attracted to the +ve
end of a different water molecule.
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The structure of water is the basis for its unique properties

Properties of water
1. Polarity: the polarity of the water molecule makes it to be “Universal Solvent”
2. Water has lower density on freezing: Gas, Liquid, and Solid: When water
freezes and becomes ice, the polarity causes hydrogen bonds to form but there
is air in between, causing expansion and ice floats in water. Thus, ice floats on
top and insulates water below- fish can live. But, if this didn’t happen all the
icebergs would sink- water levels would rise.
3. Water is highly cohesive and adhesive: Due to cohesion and adhesion,
seeds swell and germinate; ascent of sap and capillary movement of water
takes place.
4. Water has high tension: This is due to cohesion of water molecules. Due to
this property, small organism float or walk on water surface.
5. Water has high boiling point and high thermal conductivity: Because of these
properties water store and spreads heat and prevent from overheating
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Properties of water…
Water has high heat of fusion: Water requires a loss of lot of heat to freeze. This
prevents freezing and ice formation in the protoplasm even when exposed to very
low temperature.
Water has high heat capacity. It takes a lot of energy to raise the temperature of a
certain amount of water by a degree, so water helps with regulating temperature in
the environment.
Water is transparent: This allows light to penetrate 200 m depth in sea and lakes. So
plants carry on photosynthesis under submerged conditions.
Water is incompressible: It helps organisms to tolerate pressure and compression.
Because of this in earthworm water acts as hydro-skeleton. Loss or gain of water
causes various plant movements like stomatal movement,
Water organizes non-polar molecules. - hydrophilic: “water-loving”; -hydrophobic:
“water-fearing”; Water causes hydrophobic molecules to aggregate or assume
specific shapes.
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