Biology

Key Unit Competences
Explain the important roles of carbohydrates and lipids in the provision and storage of energy and for a variety of other functions.

Learning objectives
By the end of this unit, I should be able to:
–– State the roles of carbohydrates and lipids.
–– Recall the elements that make up carbohydrates and lipids.
–– Explain the proportion of hydrogen in carbohydrates and lipids and relate this to the amount of energy released when oxidized.
–– Define the terms monomer, polymer, macromolecule, monosaccharide, disaccharide and polysaccharide.
–– Describe the ring forms of α-glucose and β-glucose structure.
–– Explain the formation of glycosidic bonds.
–– Describe the structure of phospholipids and relate to their functions in living organisms.
–– Describe the molecular structure and formation of triglycerides and phospholipids, and give their functions in living organisms.
–– Demonstrate that phospholipids have a hydrophilic head and hydrophobic tails using a heterogeneous mixture made up of water and cooking oil.
–– Interpret the charts and illustrations of molecular structure and the formation of maltose and triglycerides.
–– Demonstrate through a process of combustion that sugars and lipids are biological fuel
–– Differentiate between starch and cellulose.
–– Appreciate the importance of carbohydrates and lipids in organisms.
–– Be aware of the other roles of lipids in the formation of soap and with carbohydrates and syrups in medicine

Introductory activity
1. In the previous unit (test for biological molecules), we tested carbohydrates, starch, reducing sugar, lipids, proteins, and vitamins. Where do you classify monosaccharide, disaccharides and polysaccharides in the above tested biochemical compounds?
2. Sometimes people say that fatty persons do not feel cold. What could be the reasons?

7.1 Classes of monomers
Activity 7.1
1. Give the description of the term monomer
2. Where can we find monomers?
3. What is the biological importance of monomers?
A monomer is a molecule that can combine with others of the same kind to form a polymer. A polymer is a large molecule or macromolecule composed of many repeated sub-units (monomers). Because of their broad range of properties, both synthetic and natural polymers play essential and ubiquitous roles in everyday life. Polymers make up many of the materials in living organisms including proteins,
cellulose, and nucleic acids. Glucose molecules for example, are monomers that combine to form the polymer cellulose. The examples of monomers are summarized in the table 7.1.

Table 7.1: Biological molecules and their monomers

Carbohydrates comprise a large group of organic compounds which contain carbon, hydrogen and oxygen. The word carbohydrate suggests that these organic compounds are hydrates of carbon. Their general formula is Cx (H2O) y. In carbohydrates the ration hydrogen-oxygen is usually 2:1. Carbohydrates are divided into three groups including the monosaccharide (single sugars), disaccharides (double sugars) and polysaccharides (many sugars). The most common monosaccharide of carbohydrates is glucose with molecular formula C6H12O6.

Self-assessment 7.1
1. What are some examples of polymers and monomers?
2. How are monomers, polymers and macromolecules related?

7.2 Ring form of α-glucose and β-glucose
Monosaccharides are group of sweet and soluble

Activity7.2
1. Based on the knowledge acquired during the lesson of monomers and further information from books and internet:
a. What are the examples of monosaccharide?
b. Give the molecular formula of each of the monosaccharide stated above
c. Use the books to illustrate the structural formula of each of the monosaccharide stated above

crystalline molecules of relatively low molecular mass. They are named with the suffix –ose. The general formula of a monosaccharide is (CH₂O) n, with n the number of carbon atoms. The simplest monosaccharide has n=3 and it is a triose sugar. When n = 5, this is a pentose sugar, and when n = 6, this is a hexose sugar. The two common pentose sugars are ribose and deoxyribose, while the most known hexose is glucose. Its molecular formula is C6H12O6. It is the most important simple sugar in
human metabolism called simple sugar or monosaccharide because it is one of the smallest units which has the characteristics of this class of carbohydrates.

Monosaccharides can exist as isomers. The isomer is defined as each of two or more compounds with the same formula but a different arrangement of atoms in the molecule and different properties. For example, glucose, fructose and galactose share the same molecular formula which is C₆H₁₂O₆. However, they differ by their structural formulae as follow:

One important aspect of the structure of pentoses and hexoses is that the chain of carbon atoms is long enough to close up on itself and form a more stable ring structure. This can be illustrated using glucose as an example. When glucose forms a ring, carbon atom number 1 joins to the oxygen on carbon atom number 5 (Figure 7.2).

All hexoses sugars can exist as straight-chain structures but they tend to form ring structures. Glucose, fructose, galactose can exist in ring structures (Figure 7.3).

Ring monosaccharides are said to be alpha (α) if the -OH group located on carbon 1 is below the ring and beta (β) when the -OH group is above the ring. The molecule of glucose for example can exist as alpha and beta glucose denoted by α-glucose and β-glucose (Figure 7.4)

Self-assessment 7.2
1. How do we call the monosaccharide with 3, 5 and 6 carbon atoms?
2. Differentiate between α and β glucose
3. What are the properties of glucose?

7.3 Formation and breakdown of glycosidic bonds
Activity 7.3
1. Monomers are joined to form polymers, use a point as a monomer to illustrate how a polymer can be formed
2. How do you call joining structures between atoms?
3. Use books or other sources to show how monosaccharide form a
disaccharide.

7.3.1 Monosaccharides
Monosaccharides may combine together in pairs to give a disaccharide (doublesugar).
The union involves the loss of a single molecule of water and is therefore a condensation reaction. The bond which is formed is called a glycosidic bond. It is usually formed between carbon atom1of one monosaccharide and carbon atom 4 of the other, hence it is called a -1, 4- glycosidic bond. Any two monosaccharides may be linked together to form a disaccharide of which maltose, sucrose and lactose
are the most common.

The addition of water under suitable conditions is necessary if the disaccharide is to be split into its constituent monosaccharide. This is called hydrolysis waterbreakdown or more accurately, breakdown by water.

7.3.2 Disaccharides
These are carbohydrates made of two monosaccharides. They include maltose (glucose + glucose), sucrose or saccharose (glucose +fructose), and lactose (glucose+ galactose). The maltose is the sugar from the germinating seeds, sucrose or saccharose is the common table sugar obtained from sugarcane, while lactose is the sugar from the milk. In addition, sucrose is a non-reducing sugar.
Table 7.2: Types of disaccharides and their monomers

In maltose ring, the two ring of glucose are bonded by the -1, 4-glycosidic bond  while in sucrose the glucose and fructose are bonded by -1, 2-glycosidic bond.

All the disaccharides are non-reducing sugar, except maltose which behaves in the same as a reducing sugar with benedict’s solution. All monosaccharides and disaccharides have the following characteristics: sweet taste, soluble in water and lower molecular mass.
Self-assessment 7.3
1. Write the molecular structure of sucrose
2. How is the glycosidic link is formed
3. Sucrose is formed when two monosaccharide are assembled together:
a. Name those two monosaccharides.
b. Using the ring form of these monosaccharide named above to explain and
show sucrose formation?

7.3.4 Polysaccharides: starch, glycogen and cellulose
Activity 7.4
1. Based on the meaning of monosaccharide, what is a polysaccharide?
2. Classify the following compound into polysaccharide, monosaccharide and disaccharide
a. Glucose, fructose and galactose
b. Lactose, sucrose, and maltose
c. Starch, cellulose and glycogen
3. Use glucose to form any polysaccharide of your choice In the same way that two monosaccharides may combine in pairs to give a disaccharide, many monosaccharides may combine by condensation reactions to form a polysaccharide.

The number of monosaccharides that combine is variable and the chain produced may be branched or unbranched. Polysaccharide are many but the most known are starch, glycogen and cellulose.

a. Starch
Starch is made up of two components: amylose and amylopectin. Amylose is a linearunbranched polymer of 200 to 1500 α-glucose units in a repeated sequence of α-1,4-glucosidic bonds. The amylose chain coils into helix held by hydrogen bonds formed between hydroxyl groups. A more compact shape is formed. The amylose helices are entangled in the branches of amylopectin to form a complex compact
three dimensional starch molecule.

Amylopectin is a branched polymer of 200 to 200,000 α-glucose units per starch molecule. The linear chains of α-glucose units are held together by α-1, 4-glucosidic bonds. Branches occur at intervals of approximately 25 to 30 where α-1, 6-glucosidic bonds occur. Starch grains are found in chloroplast, potato tubers, cereals and legumes. Starch is insoluble in cold water. It is digested by salivary amylase and pancreatic amylase into maltose and the latter is hydrolyzed by maltase enzyme to form glucose. Therefore, diabetic people should avoid tubers since they are rich in starch which in turn gives glucose (Figure 7.8).

b. Glycogen
Glycogen is often called animal starch because it is a major polysaccharide storage material in animals and fungi. The brain and other tissues require constant supply of blood glucose for survival. Some tissues particularly the liver and skeletal muscles store glycogen in the form that can be rapidly mobilized to form glucose. Like starch, glycogen is made up of α-glucose and exists as granules. It is similar to amylopectin in structure but it has shorter chains (10-20 glucose unit) and is more highly branched.

c. Cellulose
Cellulose is the structural polysaccharide in plant cell wall. It is found in vegetables and fruits but it cannot be hydrolyzed by enzymes in the human digestive system. Cellulose is composed of long unbranched chains of up to 10,000 β-glucose units linked by β-1,4-glucosidic bonds. Each β-glucose unit is related to the next by a rotation of 180 ͦ C with OH groups projecting outwards on either side of the chain.

Cellulose chains run parallel to one another. Unlike amylopectin and glycogen molecules, there are no side chains (no branch) in the cellulose. This allows the linear chains to lie close together. Many H-bonds are formed between the OH groups of adjacent chains. The chains group together to form microfibrils arranged in larger bundles of macrofibrils. The fibrils give the plant cell their high tensile strength and rigidity. The layers of fibrils are permeable to water and solutes.

Cellulose is formed from ß - glucose units linked by 1,4 glycosidic bonds. The hydroxyl groups alternate on either side of the molecule forming straight chains giving cellulose a fibrous structure. Cellulose are strengthened further by hydrogen bonds that link adjacent chains.

d. Chitin
Chitin is one of naturally occurring Polymers. It forms a structural component of many animals such as exoskeleton in arthropods. Chitin is a polymer of glucose although in its structure a molecule of amino acid is added to each glucose. The digestion of chitin yields simple sugars and ammonia.

Self-Assessment 7.4
1. What type of reaction is involved in the formation of glucose from starch?
2. Use the type of reaction above to form glucose from sucrose molecule
3. What are the 2 main components of starch? Give the difference between them

7.3.5 Lipids
Activity 7.5
1. List the monomers that are present in lipids
2. Where can we find lipids?
3. Discuss the reasons why animals like pig do not like hot weather.
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. Lipids are grouped into fats which are solid at room temperature and oils which are liquid at room temperature. Lipids are made by carbon, hydrogen and oxygen, but the amount of oxygen in lipids is much smaller than in carbohydrates. Lipids are made by two components namely glycerol and fatty acids. The chemical formula for glycerol is C₃H₈O₃ with structural formula as shown in the figure 7.11

In all lipids glycerol do not show any variation while fatty acids vary. Therefore, the nature of lipid depends on the fatty acid it contains. There are two types of fatty acids: unsaturated fatty acid characterized by the chain of hydrocarbon containing one or more double and triple bonds; and saturated fatty acid characterized by the chain of hydrocarbon without any double or triple bond.