222 Cell Biology 1

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Lecture 3

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Cellulose

It is a polymer of glucose that forms Plant Cell WallsPlant Cell Walls cellulose is the most abundant organic molecule on

Earth

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Chitin

Is a PolysaccharidePolysaccharide used by insects and crustaceans to build an exoskeleton

Found in cell wall of fungicell wall of fungi

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Hyaluronic acid

Found in connective tissuesconnective tissues

1,3 glycosidic bond 1,4 glycosidic

bond

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Inulin

Found in some plantes, formed of fructose units (b 1,2 glycosidic bond)

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Glycogen

Is a storage storage polysaccharide polysaccharide composed of glucoseglucose, which is hydrolyzed by animals when glucose is needed

Highly branched molecules

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Starch

Is a storage polysaccharide

composed of glucose glucose monomersmonomers

and found in plantsplants

Starch helix may be unbranched or branched

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Other polysaccharides

Formed of Mannose Mannose unitsunits

Found in Yeast (branched) and in Some plantes ( non-branched)

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Paramylum

Homopolsaccharides, found in Euglina

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Dextrin

Branched polysaccharides , produced by some bacteria

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Complex polysaccharides

Polysaccharides can combine with other classes of classes of macromoleculesmacromolecules to form complex polysaccharides: GlycoproteinsGlycoproteins:: polysaccharides + proteins GlycolipidsGlycolipids:: polysaccharides + lipids Cellular functionsCellular functions

Cell-surface receptor molecules; typically reside on external surfaces of the membrane

Glycolipids important in cell walls of gram-negative bacteria

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PROTEINS Are a polymer built from various combinations of 20 amino acid 20 amino acid Monomers Are the most abundant macromolecules in cells found throughout cell Have an important structural and enzymatic rolesstructural and enzymatic roles Transmit information between cells (e.g : protein hormones) Provide a defense against infection (e.g antibodies) Account for more than 50% of the dry mass of cells.

Proteins divided into two groups based on its function in the cells:

Structural proteins : Integral parts of cellular structures (Fibrous proteins (Collagen) , cartilage, skin and bone (keratin .(tubulin, actin like protiens, microtubules and ,Microfilaments

Dynamic proteins : Catalytic proteins; catalysts for chemical reactions, cell metabolism (hormones , insulin, erthrproetin and thyroxine, hemoglobin-hemocyanin-myoglobin

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PROTEINS Proteins can be divided into three main classes, which

correlate with typical tertiary structures:

1-1- Globular proteins: Globular proteins: Almost all are soluble and many are enzymes.

2- Fibrous proteins: 2- Fibrous proteins: Fibrous proteins are often structural, such as collagen, the major component of connective tissue, or keratin, the protein component of hair and nails

3- Membrane proteins: 3- Membrane proteins: Membrane proteins often serve as receptors or provide channels for polar or charged molecules to pass through the cell membrane.

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Amino Acids Most consist of carbon, hydrogen, oxygen, and nitrogen; 2 of 22

contain sulfur, 1 contains selenium All contain two important functional groups

Carboxylic acid group (-COOH)Carboxylic acid group (-COOH) Amino group (-NHAmino group (-NH22))

Amino acid monomers held together by covalent bonds Peptide Peptide bonds.bonds.

Polypeptides: thousands or millions of amino acids Have two distinct ends: one terminating in an amino group (the amino- or N- terminus) and the other is an carboxylic group (carboxyl or C-terminus)

Protein consists of a specific amino acid sequence The amino acid sequence of protein determines its three

dimensional structure and its chemical reactivity. Side chains of amino acids impart its chemical properties

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Peptide bond

The amino acids in a polypeptide chain are linked by Peptide BondsPeptide Bonds

This bond links the carboxyl group of one amino acid to the amino group of the next amino acid

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Conjugated proteins

A conjugated protein is a protein that functions in interaction with other chemical groups

o Nucleoproteins:Nucleoproteins: chromosomeo Chromoprotein:Chromoprotein: haemoglobino Phosphproteins:Phosphproteins: found in milko Glycoproteins : Glycoproteins : contain oligosaccharide chains

covalently attached to their polypeptide side-chains, they play a role in cell-cell interactions

o Lipioproteins:Lipioproteins: enzymes, transporters

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Levels of protein structure

Protein can have four levels of structure: Protein can have four levels of structure: 1- Primary structure2- Secondary structure3-Tertiary structure4-Quaternary structure

Primary structure Primary structure Primary structure: sequence of amino acids in a polypeptide-The correct amino acid sequence is determined by the cell’s genetic

information-The slightest change in this sequence affects the protein’s ability to

function-Sickle cell disease is manifested by an inability of hemoglobin in red blood cells to carry oxygen, the primary function of hemoglobin. This blood disorder is the result of change in a single amino acid

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Four levels of protein structure

Four Levels of Protein Structure

Amino acids

Primary structure

Alpha helix

Hydrogenbond

Secondary structure

Pleated sheet

Polypeptide(single subunitof transthyretin)

Tertiary structure

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2 -Secondary structure

Secondary structure: Secondary structure: the regular arrangement of amino acids within localized regions of the polypeptide

Folds in polypeptide that form a more stable structure, often involving hydrogen bonding between R groups

There are two types of secondary structure:

Helical structure called an alpha helix ((α-helix)α-helix) (region of polypeptide chain coils around itself

Pleated sheet (β sheet): (β sheet): two parts of polypeptide chain lie side by side with hydrogen bonds between them.

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Tertiary structure

Tertiary structure: Tertiary structure: additional folding of polypeptide to result in greater stability and unique three-dimensional shape Forms exposed regions or grooves in the molecule that are important for binding

other molecules Disulfide bonds:Disulfide bonds: bonds between -SH groups from two different amino acids

In most proteins combination of alpha helix and beta sheets connected by loop regions of polypeptide chain, fold into compact globular structures called Domains Domains (the basic unit of tertially structure)

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Quaternary structure

Quaternary structure: Quaternary structure: occurs in proteins composed of two or more polypeptides Subunit:Subunit: each polypeptide

in the protein, held together by either/both covalent and noncovalent linkages

Homodimer:Homodimer: protein containing two identical subunits

Heterodimer:Heterodimer: protein containing two nonidentical subunits

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