Molecules of life

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Molecules of life

Four categories of molecules called CHOs, Lipids, Proteins, and nucleic acid are unique to cells.

In each category the molecules are composed of more than one subunit:

Category example subunits

_________ __________ _________

Lipids Fat Glycerol & fatty acids

CHOs Polysaccharide Monosaccharide

Proteins Polypeptide Amino acid

Nucleic acid DNA, RNA Nucleotide


When a cell constructs a macromolecule, a molecule that contains subunits, it uses a dehydration reaction.

During a dehydration, an –OH (hydroxyl group) and an –H (hydrogen atom), the equivalent of a water molecule are removed (synthesis of macromolecules in cells occurs when subunits bond following a dehydration reaction – removed of H2O).

To degrade macromolecules, the uses a hydrolysis reaction in which the components of water are added (degradation of macromolecules in cell occurs when addition of H2O).

Carbohydrates

CHOs like all organic molecules always contain carbon (C) and hydrogen (H) atoms. However , CHOs molecules are characterized by presence of the atomic grouping H-C-OH in which the ratio of (H) atoms to oxygen atom (O) is 2:1, like the ratio in water the name "hydrates of carbon" seems appropriate , human foods are rich in CHOs.

Simple CHOs

If the number of carbon atoms in a CHO is low (from 3 to 7), it is called a simple sugar or monosaccharide e.g. pentose (5-carbon sugar), hexose (6-carbon sugar). Glucose (C6H12O6), the hexose our bodies use as an immediate source of energy. Other common hexoses are fructose , found in fruits & galactose a constituent of milk.

A disaccharide is made by joining only two monosuccharide together by a dehydration reaction e.g. Maltose = glucose + glucose

Sucrose = glucose + fructose

Complex CHO2 (polysaccharides)

Macromolecules such as starch, glycogen, and cellulose are polysaccharides that contain many glucose units.


Starch and Glycogen

They are stored forms of glucose in plants and animals. Some macromolecules in starch are long chains of up to 4000 glucose units. Starch has fewer side branches of glucose that branch off from the main chain than does glycogen.

After we eat starchy foods such as bread (flour grinding wheat & use for baking) is high in starch potatoes & cake, glucose enters the blood stream and normally the liver stores glucose as glycogen.

In between eating the liver release glucose so that the blood glucose concentration is always about 0.1 %.

Starch is the storage from of glucose in plants.

Glycogen is the storage from of glucose in humans.

Cellulose

The polysaccharide cellulose is found in plant cell walls. In cellulose the units are joined by a slightly different type of linkage than that in starch or glycogen.

Actually it is important because we are unable to digest foods containing this type of linkage; therefore cellulose largely passes through our digestive tract as fiber or roughage .It is believed that fiber in the diet is necessary to good health & some have suggested it may even help prevent colon cancer.

Cells usually use the monosaccharide glucose as an energy source. The polysaccharides starch and glycogen are storage compounds in plant and animal cells, respectively and the polysaccharide cellulose found in plant cell walls is dietary fiber.

Lipids

It contains more energy per gram than other biological molecules and some function well as energy storage molecules in organisms. Others form a membrane so that the cell is separated from its environment and has inner compartments as well.

Steroids are a large class of lipids that includes other molecules like sex hormones.

Lipids are diverse in structure & function, but they have a common characteristic they don't dissolve in water.

Their low solubility in water is due to an absence of polar groups .They contains little oxygen & consists mostly of carbon & hydrogen atoms.

Fats and Oil

The most familiar lipid are those found in fats & oils.

Fats which are usually of animal origin e.g. lard & butter, are solid at room temperature.

Oils which are usually of plant origin e.g. corn oil & soy bean oil, are liquid at room temperature.

Fat has several functions in the body, it is used for long term energy storage, it insulates against heat loss, and it forms a protective layer around major organs.

Synthesis and degradation of a fat molecule

Fats and oils from when one glycerol molecule reacts with three fatty acid molecules. A fat is some time called a triglyceride because of its three- part structure or the term neutral fat can be used because the molecule is non polar are carries no charges.


Emulsification

Emulsifier can cause fats to mix with water .They contain molecules with a non polar end and a polar end .The molecules position themselves about an oil droplet so that their polar ends project out ward. Now the droplet disperses in water which means that emulsification has occurred.

Emulsification take place when dirty clothes are washed with soaps or detergents .Also prior the digestion of fatty foods , fats are emulsified by bile .The gallbladder stores bile for emulsifying fats prior to digestive process.

Saturated and unsaturated fatty acids

A fatty acid is carbon –hydrogen chain that ends with the acidic group-COOH .Most of fatty acids in the cells contains 16 or 18 carbon atoms per molecule.

Fatty acids are either saturated or unsaturated

Saturated fatty acids have only single covalent bonds because the carbon chain is saturated. Saturated fatty acids account for the solid nature of fats such as lard & butter at room temperature.

Unsaturated fatty acids have double bonds between carbon atoms wherever fewer than two hydrogen are bonded to a carbon atom. Unsaturated fatty acid account for the liquid nature of vegetable oils at room temperature. Hydrogenation of vegetable oils can convert those to so-called trans- fats often found in processed foods.

Phospholipids

Phospholipids have a phosphate group .They are constructed like fats , except that in place of the third fatty acid , there is a phosphate group or a grouping that contain both phosphate & nitrogen . These molecules are not electrically neutral as are fats because the phosphate & nitrogen –containing groups are ionized. They form the so called polar head of the molecule , while the rest of the molecule becomes the hydrophobic tails ,phospholipids are components of cellular membranes ;they spontaneously form a bilayer in which the hydrophilic heads face out ward toward watery solutions and the tails form the hydrophobic interior.

Steroids

Steroids are lipids that have an entire different structure from those of fats. Steroids molecules have a backbone of four fused carbon rings. Each one differs primarily by the functional groups attached to the rings.

Cholesterol is a component of an animal cell's plasma membrane and is the precursor of several other steroids such as the sex hormones (estrogen & testosterone).

Proteins

Proteins perform many functions .Proteins such as keratin which makes up hair and nails; and collagen which lends support to ligaments, tendons and skin are structural proteins.

The Proteins actins &myosin account for the movement of cells and the ability of our muscles to contract .Many hormones which are messengers that influence cellular activity are also proteins. Some proteins transport molecules in the blood ;hemoglobin is a complex protein in our blood that transport oxygen .Antibodies in the blood & other body fluids are proteins that combine with foreign substances preventing them from destroying cells & up setting homeostasis. Enzymes are proteins that control nearly all the chemical reactions in the body because they allow reactions to occur under relatively mild conditions without enzymes most reactions would not occur in the body.

Amino acids

Proteins are macromolecules with amino acid sub units .An amino acid has a central carbon atom bonded to a hydrogen atom and three groups. The name of the molecule is appropriate because one of these groups is an amino group (-NH2) and another is an acidic group (-COOH). The third group is called an R group because it is the remainder of the molecule.Amino acids differ from one another by their R group; the R group varies from having a single carbon to being a complicated ring structure.

Levels of protein organization

The structure of a protein has four levels:

Primary structure is linear sequence of amino acids joined by peptide bonds.

A polypeptide is a single chain of amino acids .The atoms associated with the peptide bond O,C,N,and H share electrons in such away that the oxygen has a slight negative & the H has a slight positive charge .


Therefore, the peptide bond is polar.

Secondary structure of a protein comes about when the polypeptide takes on a certain orientation in space .A coiling of the chain results in a helix or a right -handed spiral and a folding of the chain results in a pleated sheet .Hydrogen bonding between peptide bonds holds the shape in place.

Tertiary structure of a portion is its final three dimensional shape .In muscles, myosin molecules have rod shape ending in globular (globe- shaped head).

Quaternary structure: some proteins have only one polypeptide and others have more than one polypeptide each with it is own primary, secondary and tertiary structures.

These separate polypeptides are arranged to give some proteins a fourth level of structure .Hemoglobin is a complex protein having a quaternary structure such as enzyme.

Nucleic acids

The two types of nucleic are DNA (deoxyribonucleic acid) and RNA (ribonucleic acid).

DNA storage genetic information in the cell and in the organism, further it replicates and transmits this information when a cell reproduces and an organism reproduces.

DNA codes for the order in which amino acids are to be joined to form a protein .RNA is intermediaries that convey DNA instructions regarding the amino acid sequence in a protein.

Structure of DNA and RNA

Both DNA & RNA are polymers of nucleotides .Every nucleotide is a molecular complex of three types of subunit molecules – phosphate (phosphoric acid) a pentose sugar and nitrogen – containing base.


The nucleotides in DNA contain the sugar deoxyribose & the nucleotides in RNA contain the sugar ribose; this difference accounts for their respective name.

There are four different types of bases in DNA, adinine (A), thymine (T), guanine (G), and cytosine (C). The base can have two rings ( adinine or quanine) or one ring ( thymine or cytosine ) .In RNA the base uracil (U) replaces the base thymine .These structures called bases because their presence raises the PH of a solution .

The nucleotides form a linear molecule called a strand which has a backbone made up of phosphate –sugar –phosphate –sugar with bases projecting to one side of the backbone .Since the nucleotides occur in a definite order so do the bases. The sequence of the bases in human –DNA (human genome) .The breaks through is expected to lead to improved genetic counseling, gene therapy, and medicines to treat the cause of many human illnesses.

DNA is double -stranded with the two strands twisted about each other in the form of double helix .In DNA the two strands are held together by hydrogen bonds between bases ,when unwound DNA resembles a stepladder .The up rights (sides) of the ladder are made entirely of phosphate and sugar molecules and the rungs of the ladder are made only of complementary paired bases .Thymine always pairs with adinine , quanine always pairs with cytosine .Complementary bases have shapes that fit together Complementary bases pairing allows DNA to replicate in a way that ensures the sequences of bases will remain the same .This sequence of the DNA bases contains a code that specifies the sequence of amino acids in the proteins of cell. RNA is single – stranded and when it forms complementary bases pairing with one DNA strand passes this information on to RNA.

ATP (Adenosine Triphosphate)

In addition to being monomers of nucleic acids nucleotides have other metabolic functions in cells when adenosine ( adenine plus ribose) is modified by the addition of three phosphate groups instead of one it becomes ATP an energy carrier in cells ,a glucose molecule contain too much energy to be used as a direct energy source in cellular reactions .Instead the energy of glucose is converted to that of ATP molecules .ATP contains an amount of energy that makes it usable to supply energy for chemical reactions in cells .

ATP is a high –energy molecule because the last two phosphate bonds are unstable and easily broken .Usually in cells , the terminal phosphate bond is hydrolyzed ,leaving the molecule ADP(adenosine diphosphate)and a molecule of phosphate (P) .The energy released by ATP break down is used by the cell to synthesize macromolecules such as CHOs and proteins .In muscle cells the energy is used for muscle contraction ,and in nerve cells it is used for conduction of nerve impulses .After ATP down it is rebuilt by the addition of (P) to ADP.