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- Alterations in pH, salt concentration, temperature, or other environmental factors can cause a protein to unravel
- This loss of a protein’s native structure is called denaturation
- A denatured protein is biologically inactive and off shaped.

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- Primary: depends on the sequence of amino acids in a linear shape. No polypeptide formed
- Secondary: Primary structures are folded and coiled .It has one polypeptide.
- Tertiary : when two of the secondary structures combine together. It has one polypeptide.
- Quaternary: Two or more tertiary structures combine together. It has two or more polypeptides

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- The specific activities of proteins result from their intricate three-dimensional architecture
- A functional protein consists of one or more polypeptides precisely twisted, folded, and coiled into a unique shape
-The arrangement of the amino acids determine the 3D structure of the protein
- The function of a protein usually depends on
its ability to recognize and bind to some other molecule.
- Protein consists of four levels of structure :-

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-  Amino acids are linked by covalent bonds called peptide bonds
- A polypeptide is a polymer of amino acids
- Each polypeptide has a unique linear sequence of amino acids, with a carboxyl end (C-terminus) and an amino end (N-terminus)

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- A combination of an amino group and a carboxyl group.
- The central carbon is called the alpha carbon it is linked to an amino ,carboxyl, hydrogen and a (R group) side chain. The (R group) side chain determines the characteristics of that articular amino acids and affects the polypeptide.
- A group of non-polar amino acids are counted as hydrophobic,polar amino acids are counted as hydrophilic, acidic amino acids have a negatively charged side chain due to the carboxyl group  whereas basic amino acids have a positively charged side chain. Plus the acidic and basic amino acids are also hydrophilic .

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- DNA molecules have two polynucleotides spiraling around an imaginary axis, forming a double helix
- The backbones run in opposite 5′ → 3′ directions from each other, an arrangement referred to as antiparallel
- One DNA molecule includes many genes
-  Only certain bases in DNA pair up and form hydrogen bonds: adenine (A) always with thymine (T), and guanine (G) always with cytosine (C)
- This is called complementary base pairing
- This feature of DNA structure makes it possible
to generate two identical copies of each DNA molecule in a cell preparing to divide
-  RNA, in contrast to DNA, is single-stranded
- Complementary pairing can also occur between two RNA molecules or between parts of the
same molecule
- In RNA, thymine is replaced by uracil (U), so A and U pair
- While DNA always exists as a double helix, RNA molecules are more variable in form

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- Nucleotides are linked together by a phosphodiester linkage to build a polynucleotide
- A phosphodiester linkage consists of a phosphate group that links the sugars of two nucleotides
- These links create a backbone of sugar-phosphate units with nitrogenous bases as appendages
- The sequence of bases along a DNA or  RNA polymer is unique for each gene.

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-  Quaternary structure results when two or more polypeptide chains form one macromolecule
- Collagen is a fibrous protein consisting of three polypeptides coiled like a rope
- Hemoglobin is a globular protein consisting of
four polypeptides: two α and two β subunits
- Its shape is caused by covalent bonding ,hydrophilic ,ionic, hydrophobic interactions and disulfide bridges .
- Example: collagen and haemoglobin .

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- Tertiary structure, the overall shape of a polypeptide, results from interactions between
R groups, rather than interactions between backbone constituents.
- Its globular shape is caused by covalent bonding ,hydrophilic ,ionic, hydrophobic interactions and disulfide bridges .
- Example : enzymes ( amylase, protease and others

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- The coils and folds of secondary structure result from hydrogen bonds between repeating constituents of the polypeptide backbone
- It is formed by covalent and hydrogen bonding which cause the folding of the structure.
- Example : alpha helix and Beta- pleated sheet.

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- The primary structure of a protein is its sequence of amino acids in a linear shape.
- Primary structure is determined by inherited genetic information.
- It is formed by covalent bond

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- Steroids are lipids characterized by a carbon skeleton consisting of four fused rings
- Particular chemical groups attached to the fused rings differ the steroids.

CHOLESTEROL
- Cholesterol, a type of steroid, is a component in animal cell membranes and a precursor from which other steroids are synthesized such as sex hormones( testosterone and ovum)
- Synthesized in the liver and also from the diet .
- Too much of cholesterol in the body can cause atherosclerosis ( artery problems ).

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- It is formed from 1 glycerol plus 2 fatty acids and 1 phosphate group
- The two fatty acid tails are hydrophobic, but the phosphate group and its attachments form a hydrophilic head.
- When phospholipids are added to water, they
self-assemble into double-layered sheets
called bilayers
- At the surface of a cell, phospholipids are also arranged in a bilayer, with the hydrophobic tails pointing toward the interior and the hydrophilic head pointing outwards so that it can stay in water.
- The phospholipid bilayer forms a boundary between the cell and its external environment.

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- Fats is a large molecules assembled from small molecules which are 1 glycerol plus 3 fatty acid by an ester linkage.The resulting fat is also called triglyceride.
- Glycerol is an alcohol. Each of the carbon has a hydroxyl side chain.
- A fatty acid has a long carbon skeleton.The carbon at the end of the  skeleton is usually a carboxyl group.
- The non polar hydrocarbon bond causes the fats to be hydrophobic therefore it separates  from water.
- Fats  is also act as an energy storage.

SATURATED FATS
- It is single bonded between the carbon and hydrogen in a carbon skeleton therefore it is flexible.
-Saturated fats appear as solid at room temperature.
-Example: butter and lard.
- A diet rich in saturated fats is bad for health as well it is because the fat molecule is hard to be completely broken due to its single bonded hydrocarbon and it will cause blockage in the arteries and high blood pressure.

UNSATURATED FATS
- It consists of a double bond between the hydrocarbon skeleton.
- Appears as liquid at room temperature due to the cis double bond that prevent the molecule to pack together in order to solidify.
- Example: vegetable oil and cod liver oil.
- "HYDROGENATED VEGETABLE OIL" means that the unsaturated has been synthesized to saturated fats by adding a hydrogen and also unsaturated trans double bond fats , therefore the vegetable oil can be solid at room temperature .Example: peanut butter and margarine.
- Trans double bond fat is bad for health ,it causes coronary heart disease

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- Large biological molecules that does not include true polymer
- Lipids mix poorly with water due to its hydrophobic condition
- The most biologically important lipids are fats, phospholipids, and steroids

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- The polysaccharide cellulose is a major component of the tough wall of plant cells.
- Like starch, cellulose is a polymer of glucose, but the glycosidic linkages differ

- The difference is based on two ring forms for glucose: alpha (α) and beta (β)

- Starch (α configuration) is largely helical

- Cellulose molecules (β configuration) are straight and unbranched

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- Both animals and plants store sugars for later use.

STARCH
-Plants store starch, a polymer of glucose monomers.
Starch represents stored energy. The sugar can later be withdrawn by the plant by hydrolysis which breaks the bonds of glucose molecules
- Example: potato,corn, wheat and rice.

GLYCOGEN
- Animals store polysaccharides called glycogen.
-Glycogen is mainly stored in liver and muscle cells. When the demand of sugar increases , the glycogen will convert into glucose by hydrolysis and vice versa for glucose to glycogen when the demand of glucose decreases.
- This stored fuel cannot sustain an animal for long unless it is replenish by food daily.

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- Polysaccharides are macromolecules, polymers  with a few hundred to a few thousand monosaccharides joined by glycosidic linkages.
- Some polysaccharides serve as storage,material ,hydrolyzed and others serve as building materials for the cell for protection.

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- Monosaccharide carbohydrate
- Formula : C₆H₁₂O 6
- Consists of multiple hydroxyl group and the placement of  one carbonyl group which help to separate the sugar into ALDOSES(Aldehyde sugars) and KETOSES( ketone sugars)

- The carbonyl group of aldoses will be at the end of the carbon skeleton whereas for ketoses the carbonyl group will be in between the carbon skeleton.

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- All living things  are made up of 4 classes of biological molecules (carbohydrates, proteins, lipids and nucleic acids)
- Macromolecules are polymers build from monomers.
- Polymer is a long chain  of  monomers repeating itself.
- (carbohydrates, proteins, lipids and nucleic acids) are polymers from monomers of ( glucose, amino acids, 3 triglycerides plus one fatty acid and 1 pentose sugar plus 1 phosphate group plus 1 nitrogeneous base) respectively.
- Monomers go through a process called dehydration to form polymers whereas water will be taken out from the molecule, in addition polymers go through a process called hydrolysis whereas water will be added back to the molecules to form back the monomers.

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- The two types of nucleic acids, deoxyribonucleic acid(DNA) and ribonucleic acid ( RNA) , enable living organisms to reproduce their complex components from one generation to the next. Unique among molecules , DNA provides directions for its own replication . DNA also directs RNA synthesis and through RNA , controls protein synthesis ; this entire process is called gene expression .

- Nucleic acids are polymers called polynucleotides which are from nucleotides( monomers)

- Each nucleotide consists of a nitrogenous base, a pentose sugar, and one or more phosphate groups
- The portion of a nucleotide without the phosphate group is called a nucleoside which is  nitrogenous base + sugar

There are two families of nitrogenous bases

- Pyrimidines
(cytosine, thymine, and uracil)
have a single six-membered ring
- Purines
(adenine and guanine) have a six-membered ring fused to a five-membered ring
- In DNA, the sugar is deoxyribose; in RNA, the sugar is ribose
- Nucleotide = nucleoside + phosphate group
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- Proteins account for more than 50% of  dry mass of most cells
- Some proteins speed up chemical reactions
- Other protein functions include defense, storage, transport, cellular communication, movement, and structural support
- Enzymes are proteins that act as catalysts to speed up chemical reactions
- Proteins are all constructed by the same set of 20 amino acids by a bond called peptide bond .
- The polymer of amino acids are  polypeptides, thus a protein can be form by more than one polypeptide and coiled into   a specific three-dimensional structure.

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- Carbohydrates serve as fuel and building material for living things like a source of energy.
-  Carbohydrates include sugars and the polymers of sugars.

- The simplest carbohydrates are monosaccharides, or simple sugars and a disaccharide is where two monosaccharides join together by glycosidic linkage which is a covalent bond between two monosaccharides by dehydration reaction. For example : Maltose is formed by (glucose plus glucose)