CHAPTER 4

Elements In The Cell

The basic unit of matter is the element. It cannot be broken down to a simpler form by chemical reactions. The elements that are essential to living things include calcium (Ca), phosphorus (P), potassium (K), sulphur (S), chlorine (Cl), sodium (Ng), magnesium (Mg) and iron (Fe). The most common elements in living organisms are hydrogen (H), carbon (C), oxygen (O) and nitrogen (N). 

Chemical Compounds In The cell

The chemical compounds of the cells consist of two major groups; organic compounds and inorganic compounds. Organic compounds are compounds that contain the element called carbon. Examples of organic compounds are carbohydrates, lipids, proteins and vitamins. These compounds are synthesised by the cell.
Inorganic compounds are compounds that do not contain carbon atoms. Examples of inorganic compounds are mineral salts and water. These are the compounds that are not synthesised by the cell. These compounds are obtained from the external environment. 

The Importance of Water in A Cell

About 70% of the human body consists of water. It is an inorganic compound and consists of hydrogen (H) and oxygen (O) atoms. The water molecules are polar molecules due to the uneven distribution of electrons in the molecules where the hydrogen end is more positive and the oxygen end is more negative.

Introduction

Carbohydrates are made up of carbon, hydrogen and oxygen atoms and are commonly known as sugars and starches. Carbohydrates contain a functional group called a carbonyl. The carbonyl group can exist in two forms, as an aldehyde or a ketone. Table below shows the location of the carbonyl group in the carbon chain.

A monosaccharide is a monomer. Monomers are small basic molecules which can be joined together to form a polymer. So, monomers are basic building blocks of carbohydrates. In other words, a monosaccharide is the simplest carbohydrate which cannot be broken down into other simpler forms of carbohydrates.

A monosaccharide is soluble in water and has a characteristic sweet taste. The molecular formula consists of six-carbon atoms and the common monosaccharide is C₆H₁₂O₆

Disaccharides

Disaccharides are known as "complex" or "double" sugars. Like monosaccharides, they have the sweet-taste characteristic. Examples of disaccharides are maltose (malt sugar), sucrose (cane sugar) and lactose (milk & sugar)

Disaccharides are formed when two monomers (monosaccharides) are joined together by the condensation reaction (or the dehydration synthesis). A molecule of water is removed during this process

The molecular formula of disaccharides is C₁₂H₂₂O₁₁. A disaccharide can be broken down to two monosaccharides by a chemical reaction called hydrolysis (or digestion). The process involves the addition of water and requires the presence of specific enzymes.

Polysaccharides

Coriolus versicolor if one of polysaccharide source

Polysaccharides are polymers of monosaccharides or disaccharides formed by the condensation of glucose monomers. The polymers are long-chained molecules (macromolecules) formed by linking thousands of monosaccharide molecules. 

The general formula of polysaccharides is (C₆H₁₀O₅)_n, where n is the number of monomers. Polysaccharides are insoluble in water and do not taste sweet. 

Reducing Sugars and Non-Reducing Sugars

All monosaccharides have reducing properties. Similarly, some disaccharides maltose and lactose ave reducing properties. They are known as reducing sugars. However, sucrose is a non-reducing sugar. There are two common test reagents used to test for reducing sugars:

i.

Benedict's reagent (alkaline solution of CuSO4)

ii.

Fehling's reagent (alkaline solution of CuSO4) Reducing sugars reduce Cu2+ (a blue solution) to Cu+ (a reddish brown precipitate) in both Benedict's and Fehling's reagents. So, the reddish brown precipitate indicates the presence of a reducing sugar. But a non-reducing sugar (sucrose) cannot reduce Benedict's reagent to the reddish brown precipitate because the carbonyl groups of the glucose and fructose units are locked up in the glycosidic bond Introduction Proteins are complex molecules. Examples of proteins are enzymes, antibodies and hormones. Proteins are important for growth and metabolism in the body. Proteins are also responsible for attacking and destroying invading pathogens. The basic building units (monomers) of proteins are amino acids. Proteins or the polypeptide chains are formed when amino acids join each other in a particular sequence. Polypeptides have many monomers ranging from a few to hundreds and thousands Amino acids contain an amino group (-NH2) which is basic and a carboxyl group (-COOH) which is acidic. The general formula for amino acids is shown in figure below. Amino acids are not soluble in organic solvents but are soluble in water. Types of Amino Acids There are two types of amino acids, essential amino acids and non-essential amino acids. Essential Amino Acids Amino acids can be divided into two groups, essential amino acids and non-essential amino acids. Essential amino acids are amino acids that cannot be synthesised by the human body but are needed by the body. An example of an essential amino acid is leucine. Amino acids can only be obtained from the food we eat. There are altogether nine essential amino acids. Non-essential Amino Acids Non-essential amino acids are amino acids that can be synthesised by the human body. They are derived from other amino acids. There are altogether 11 non-essential amino acids. Table below shows the essential amino acids and non-essential amino acids. The Formation and Breakdown of Dipeptides When joined together, two amino acid molecules form a dipeptide. The process of this combination is called the condensation reaction. In the reaction, a water molecule is removed. The resulting bond attaching the two amino acids is called the peptide bond. The hydrolysis reaction is the process of breaking down the proteins with the addition of a water molecule. This process involves the presence of specific enzymes. The Formation ant Breakdown of Polypeptides When three or more amino acids are linked together by peptide bonds with the removal of water a polypeptide is formed. A polypeptide chain can be hydrolysed to form individual amino acids with the addition of water molecules. The formation and breakdown of polypeptides A protein molecule is made of one or more polypeptides linked together by various chemical bonds. The process of breaking up large and complex protein molecules is via the hydrolysis reaction which involves the addition of a water molecule. This process also involves the presence of specific enzymes. The peptides bond can also be broken by hydrolysis with heat dilute acids or enzymes. The loss of the three-dimensional structures of a protein molecule is known as denaturation. Changes in temperature, pH and salt concentration can also cause denaturation.  Various Structure of proteins The protein structure is classified into four levels: i. the primary structure ii. the secondary structure iii. the tertiary structure iv the quaternary structure The primary structure is the linear sequence of amino acids in a protein molecule. The secondary structure is formed when the peptide chain (a chain of amino acids) becomes folded or twisted forming a helix or pleated sheet. The tertiary structure is formed when the polypeptide helix (secondary-structured protein chain) is bent and twisted into a compact structure. Examples of proteins with tertiary structures are enzymes, hormones and antibodies. The quaternary structure is formed when different polypeptide chains (tertiary-structure protein molecules) combine with associated non-protein groups to form a large complex protein molecule. Lipids Lipids are organic compounds containing carbon, hydrogen and oxygen. Some lipids also contain other elements such as nitrogen and phosphorus. Lipids are non-polar hydrophobic compounds that are insoluble in water but soluble in organic solvents (lipophilic) such as ether and benzene. Lipids are important as a source of energy, as storage of long-term energy and they form the main component in the structure of cell membranes. In terms of size, lipids are medium-sized molecules compared to the macromolecules of polysaccharides, proteins and nucleic acids.  Types of Lipids  There are four main types of lipids. They are: i. fats and oils ii. waxes iii. phospholipids iv. steroids Fats and Oils At room temperature, fats are solids while oils are liquids. They are almost similar chemically. To form fats and oils, two different kinds of organic molecules are needed – glycerol and fatty acids. Glycerol is a three-carbon alcohol with a three hydroxyl group (-OH). Glycerol is a colourless, odourless, sweet-tasting and syrupy liquid. A fatty acid is an organic acid. Its molecular structure consists of a long hydrocarbon tail with a carboxyl group (-COOH) at one end. Different fatty acids have different hydrocarbon tails. Glycerol plays the role of a "holder" molecule which bonds with fatty acids at its hydroxyl group to form a lipid molecule. The properties of a lipid depend on the types of its fatty acids. The glycerol in any lipid is always the same (it is the fatty acids that change, giving the many different types of lipids). Formation and Breakdown of Triglyceride Each molecule triglyceride is formed by the condensation reaction between a glycerol and three fatty acids. The bonds formed are called the ester bonds. Three molecules of water are removed in the condensation reaction The breakdown of triglycerides involves the hydrolysis reaction. The hydrolysis occurs in the presence of enzymes and the triglycerides are broken down into glycerol and fatty acids. Saturated and Unsaturated Fatty Acids Fats often contain only saturated fatty acids but oils usually contain unsaturated fatty acids. Introduction Enzymes are protein molecules which act as biological catalysts to speed up a biochemical reaction in the cell. However, they themselves remain unchanged at the end of the reaction. The substance acted on by the enzyme is called the substrate and the products of the reaction are called the products. Metabolism The biomedical reaction that occurs in a cell of a living organism is called metabolism. As is already known, metabolism requires the action of specific enzymes to take place and it involves the transformation of one type of substance into another. Metabolism consists of anabolism and catabolism. i. Anabolism the synthesis of a complex molecule from simple molecules and the energy-consuming (endergonic) process ii. Catabolism the breaking down of a complex molecule into simple molecules and energy-releasing (exergonic) process. Examples of catabolism are digestion and decomposition. The activation energy is the amount of energy required to activate a reaction. The reaction can take place faster because enzymes can lower the activation energy.  The activation energy is the amount of energy required to activate a reaction. The reaction can take place faster because enzymes can lower the activation energy.  Carbohydrates, proteins, lipids and enzymes are essential for the survival of the cells. The deficiency of any one of the chemical components will affect the function of the cells.