2.3 Carbohydrates and Lipids
Essential Idea:
Compounds of carbon, hydrogen and oxygen are used to supply and store energy
Understandings:
- Monosaccharide monomers are linked together by condensation reactions to form disaccharides and polysaccharide polymers
- Fatty acids can be saturated, monounsaturated or polyunsaturated
- Unsaturated fatty acids can be cis or trans isomers
- Triglycerides are formed by condensation from three fatty acids and one glycerol
Applications:
- Structure and function of cellulose and starch in plants and glycogen in humans
- Scientific evidence for health risks of trans fats and saturated fatty acids
- Lipids are more suitable for long-term energy storage in humans than carbohydrates
- Evaluation of evidence and the methods used to obtain the evidence for health claims made about lipids
Skills:
- Use of molecular visualisation software to compare cellulose, starch and glycogen
- Determination of body mass index by calculation or use of a nomogram
2.3.U1 Monosaccharide monomers are linked together by condensation reactions to form disaccharides and polysaccharide polymers.
- Define monosaccharide, disaccharide and polysaccharide.
- List three examples of monosaccharides.
- List three examples of disaccharides.
- List three examples of polysaccharides.
- Use molecular diagrams to draw the formation of maltose from two glucose monomers.
- Explain a condensation reaction connecting two monosaccharides in the formation of a disaccharide.
2.3.U2 Fatty acids can be saturated, monounsaturated and polyunsaturated.
- Describe the differences between saturated and unsaturated (mono- or poly-) fatty acids.
2.3.U3 Unsaturated fatty acids can be cis or trans isomers.
- Describe the differences between cis- and trans- fatty acids.
2.3.U4 Triglycerides are formed by condensation from three fatty acids and one glycerol.
- Outline the difference between fats and oils.
- Explain a condensation reaction connecting fatty acids and glycerol to form a triglyceride..
- State two functions of triglycerides.
2.3.A1 Structure and function of cellulose and starch in plants and glycogen in humans.
- State the structural difference between alpha and beta glucose.
- Contrast the structure and functions of cellulose, amylose, amylopectin and glycogen.
2.3.A2 Scientific evidence for health risks of trans fat and saturated fatty acids.
- Discuss the relationship between saturated fatty acid and trans-unsaturated fat intake and rates of coronary heart disease.
2.3.A3 Lipids are more suitable for long term energy storage in humans than carbohydrates.
- Explain the energy storage of lipids compared to that of carbohydrates.
2.3.A4 Evaluation of evidence and the methods used to obtain the evidence for health claims made about lipids.
- Define evaluation in respect to evidence from and methods of research.
- Outline the manner in which the implications of research can be assessed.
- Outline the manner in which the limitations of research can be assessed.
- Evaluate a given health claim made about lipids.
2.3.S1 Use of molecular visualization software to compare cellulose, starch and glycogen.
- Demonstrate use of JMol to view molecular structures, including changing image size, rotating the image and changing the style of the molecular model.
- Identify carbon, hydrogen and oxygen atoms by color.
2.3.S2 Determination of body mass index by calculation or use of a nomogram.
- Calculate BMI using the formula.
- Determine BMI using a nomogram.
- Outline effects of a BMI that is too high or too low.
2.3.NOS Evaluating claims- health claims made about lipids in diets need to be assessed.
- Describe how the effect of lipids on health can be assessed scientifically.
What are Biomolecules?
Biomolecule is an organic molecule that is produced by living organism. They act as building block of life and perform important functions in living organisms. They are primarily composed of carbon, hydrogen, nitrogen, oxygen, phosphorous and Sulphur. The four common biomolecules are: Proteins, Nucleic Acid, Carbohydrates, and Lipids.
Proteins
Any of a class of nitrogenous organic compounds which have large molecules composed of one or more long chains of amino acids and are an essential part of all living organisms.
The building blocks of proteins are known as Amino Acids. There are 22 naturally occurring amino acids found in nature. Amino Acids are made up of Carbon, Hydrogen, Oxygen and Nitrogen. So, single amino acid is made up of Amino Group, Carboxyl Group, Hydrogen Atom and distinctive side chain, all bonded to alpha-carbon.
Fig.1. Structure of Amino Acid
When Amino Acids are dissolved in water, it exists in solution as the Dipolar Ion or Zwitterion. They can either acts as proton donor or proton acceptor.
Fig. 2. Zwitterion
All Amino Acids are optically active, that is, they can rotate the plane of polarized light. Optically active molecules contain chiral carbon except glycine. A tetrahedral carbon atom with four different constituents are said to be chiral.
Peptide is a compound consisting of two or more amino acids. When two amino acids are linked together via peptide bond, they are said to form a dipeptide. Three Amino Acids joined to form tripeptide etc. Peptide chains of 12 to 20 amino acids form oligopeptide. When many amino acids are joined, they form polypeptide. The first amino acid is known as N Terminal or Amino Terminal and Last Amino Acid is said to be C terminal or carboxyl terminal.
Fig. 3. Formation of peptide bond
Protein Structure
Proteins have four levels of protein organization. They are as follows:
Primary Structure of the protein is its sequence of amino acids. Amino Acids are joined by a peptide bond.
Secondary Structures are higher level of protein organization which includes- alpha helix and beta sheets. Both of them are stabilized by hydrogen bonds between carbonyl and N-H groups in a polypeptide backbone.
Alpha Helix is a rigid, rod like structure that forms when a polypeptide chain twists into a helical conformation.
Beta Pleated Sheets are formed when two or more polypeptide chain segment line up side by side. Each individual segment is known as Beta Strand.
Tertiary Structures refer to the three-dimensional conformations that a protein assumes as a consequence of the interactions between the side chains in their primary structure. They are stabilized by hydrophobic interactions, electrostatic interactions, hydrogen bonds, van der waal forces and covalent bonds.
Fig. 4. Levels of protein organization
Quaternary Structures are composed of two or more polypeptide chains. Polypeptides are held together via hydrophobic interactions, electrostatic interactions, and hydrogen bonds etc. For Example: Hemoglobin.
Fibrous and globular proteins
Fibrous are long, rod shaped molecules which are insoluble in water. They are generally protective and structural in nature. Globular proteins are compact spherical molecules that are usually water soluble.
Nucleic Acids
Nucleic Acid was first discovered by Friedrich Miescher from the nuclei of pus cells. There two types of Nucleic Acids- Deoxyribonucleic Acids and Ribonucleic Acids.
The monomeric unit of nucleic acid is known as Nucleotides. Nucleotide is made up of three components: A Nitrogenous Base, A Five-Carbon Sugar and Phosphoric Acid.
Fig. 5. Structure of Nucleotide
Nitrogenous bases are heterocyclic, aromatic molecules. There are basically two types of Nucleic Acids: Purines and Pyrimidines.
Purines are of two types – Adenine and Guanine whereas pyrimidines can be thymine, cytosine, or uracil.
Fig. 6. Structure of purines and pyrimidines
The 5- carbon sugar found in DNA is deoxyribose and for RNA is ribose. Nucleotide without phosphate is known as nucleoside.
Structure of Double Stranded DNA
DNA are of different types such as:
B DNA
- Two long polynucleotide strands are coiled around the axis.
- Strands are antiparallel to each other.
- Guanine base pairs with cytosine and adenine base pairs with thymine.
- Guanine forms 3 hydrogen bonds with cytosine whereas adenine forms two hydrogen bonds with thymine.
Z DNA
- They are thinner than B DNA.
- They have alternating purine and pyrimidine bases.
- They are stabilized by high salt concentration.
Denaturation of DNA
When DNA is subjected to varying pH, temperature etc, two DNA strands separate out. That is the DNA is said to be denatured. If the denaturant such as temperature, pH is removed, the DNA regains its original or native structure. This is known as Renaturation.
Fig. 7. Process of denaturation
RNA
RNA known as Ribonucleic Acid. It usually exists as single strand. RNA exists in cells in various forms as explained below-
Messenger RNA carries genetic information from DNA in the form of codons (three nucleotides forms a codon), which codes for amino acids or protein.
Transfer RNA plays an important role during protein synthesis. They act as an interface between nucleic acid language and protein language. They are fund in the cytosol of the living cell.
Ribosomal RNA is a component of ribosomes. They have important roles during protein synthesis in eukaryotes and prokaryotes.
Carbohydrates
They are defined as the polyhydroxy aldehydes or polyhydroxy ketones. They consist of hydrogen, carbon, and oxygen. The simplest carbohydrates are known as Monosaccharides, such as glucose. Two monosaccharides joined to form disaccharides. Polymers of 2 to 10 units of monosaccharides is known as Oligosaccharides. When hundreds to thousands of monosaccharides are joined, they are known as Polysaccharides.
Monosaccharides with aldehyde group is known as Aldoses. And monosaccharides with ketone group is known as Ketoses. Trioses are the simplest monosaccharides.
Some common saccharides
Starch is the storage form of energy is plants. It is a branched polysaccharide made up of glucose units. It is mixture of two different polymers amylose and amylopectin. Amylose is the unbranched polymer of glucose units whereas amylopectin is the branched polymer of glucose units.
Fig. 8. Structure of starch
Glycogen is the storage form of energy in animals. It is stored in muscle and liver. It is also a highly-branched polymer made up of amylose and amylopectin.
Fig. 9. Structure of glycogen
Cellulose is an unbranched polymer of glucose units. It is a structural polysaccharide of plant cells. It is the most abundant organic molecule in the biosphere. It is found in plant cells and provide strength and rigidity to the cell.
Fig. 10. Structure of cellulose
Chitin is a linear polysaccharide composed of N-acetyl-D-glucosamine residues. It is found in the exoskeleton of insects and crustaceans.
Reducing and Non-Reducing Sugar
Sugars capable of reducing ferric or cupric ion are called reducing sugar. All monosaccharides are reducing sugars.
Non-reducing sugars are not capable of reducing ferric or cupric ion. For Example: Sucrose is a Non-Reducing Sugar.
Lipids
They are insoluble or poorly insoluble in water. They are soluble in non-polar solvents such as ether, chloroform or benzene.
Biological functions of Lipids
- They serve as storage form of energy.
- They are major component of membranes.
- They are protective in function such as in bacteria, plants, insects, and vertebrates.
Fatty Acids
They are the simplest form of lipids. They are composed of long chains of hydrocarbons with one carboxyl group. The alkyl chain may be saturated or unsaturated. Unsaturated fatty acids may contain one or more double bonds. They have both polar and non-polar ends. Mammals are unable to synthesize some fatty acids such as linoleate. Such fatty acids must be obtained from the diet, they are known as Essential Fatty Acids. Some Fatty Acids can be synthesized endogenously, they are known as Non-Essential Fatty Acids.
Fig. 11. Structure of Fatty Acids
Triacylglycerol also known as Triglycerides are esters of fatty acids and glycerol. They are non-polar, hydrophobic in nature.
Topic 2.3: Carbohydrates and lipids
In the Basics of Biochemistry unit students are introduced to the major classes of biologically important molecules and the types of reactions used to build and break apart those molecules. The structure and function of water, as the medium of life, is also a focus.
The unit is planned to take 2 school days.
- Compounds of carbon, hydrogen and oxygen are used to supply and store energy.
- Evaluating claims—health claims made about lipids in diets need to be assessed. (5.2)
- Describe how the effect of lipids on health can be assessed scientifically.
2.3.U.1 Monosaccharide monomers are linked together by condensation reactions to form disaccharides and polysaccharide polymers
- Define monosaccharide, disaccharide and polysaccharide.
- List three examples of monosaccharides.
- List three examples of disaccharides.
- List three examples of polysaccharides.
- Use molecular diagrams to draw the formation of maltose from two glucose monomers.
- Explain a condensation reaction connecting two monosaccharides in the formation of a disaccharide.
Carbohydrates (also called saccharides) are molecular compounds made from just three elements: carbon, hydrogen and oxygen. Monosaccharides (e.g. glucose) and disaccharides (e.g. sucrose) are relatively small molecules. They are often called sugars. Other carbohydrate molecules are very large (polysaccharides such as starch and cellulose).
Carbohydrates are:
- a source of energy for the body e.g. glucose and a store of energy, e.g. starch in plants
- building blocks for polysaccharides (giant carbohydrates), e.g. cellulose in plants and glycogen in the human body
- components of other molecules eg DNA, RNA, glycolipids, glycoproteins, ATP
monosaccharides: | disaccharides | polysaccharides |
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Condensation reactions make bonds.
- condensation synthesis reactions link two monosaccharide monomers
- forming one disaccharide molecule and one H2O molecule
- repeated additions of monosaccharides produces a polysaccharide
Hydrolysis bonds break these bonds.
- a polysaccharides can be broken down into monosaccharides
- H2O molecules used as a sources of -H and a -OH groups
- catalyzed by enzymes
2.3.U.2 Fatty acids can be saturated, monounsaturated or polyunsaturated.
- Describe the differences between saturated and unsaturated (mono- or poly-) fatty acids.
Fats are a group of chemical compounds that contain fatty acids. Energy is stored in the body mostly in the form of fat. Fat is also needed in the diet to supply essential fatty acids that are substances essential for growth but not produced by the body itself
There are three main types of fatty acids: saturated, monounsaturated and polyunsaturated. All fatty acids are chains of carbon atoms with hydrogen atoms attached to the carbon atoms.
Saturated fatty acid
- Hydrogen atoms attached to every carbon atom.
- All of the carbons are attached to each other with single bonds.
Monounsaturated fatty acid
- A pair of hydrogen atoms in the middle of a chain is missing, creating a single gap that leaves two carbon atoms connected by a double bond rather than a single bond.
Polyunsaturated fattyacid.
- A pair of hydrogen atoms in the middle of a chain is missing, having more than one gap that leaves two carbon atoms connected by a double bond rather than a single bond.”
While a saturated fatty acid is a straight molecule on the average, the double bond in an unsaturated fatty acid produces a kink in the molecule. This is because a double bond cannot rotate. The bend in the carbon chain is much more pronounced in the cis isomer compared to the trans isomer. For this reason, cis fatty acids (and triacylglycerols made from them) do not solidify as readily as trans fatty acids. Due to the larger bend, the cis isomers cannot line up next to one another in as ordered a fashion as the trans isomers.
The configuration of the double bond in an unsaturated fatty acid can take two forms (or, to chemists, isomers): the naturally predominant cis form, in which both of the hydrogen atoms are on the same side of the chain; and the uncommon-in-nature trans isomer, in which the hydrogen atoms are on opposite sides. The trans form is (in most cases) best thought of as ‘damaged’.
2.3.U.4 Triglycerides are formed by condensation from three fatty acids and one glycerol.
- Outline the difference between fats and oils.
- Explain a condensation reaction connecting fatty acids and glycerol to form a triglyceride.
- State two functions of triglycerides.
2.3.A.1 Structure and function of cellulose and starch in plants and glycogen in humans.
- State the structural difference between alpha and beta glucose.
- Contrast the structure and functions of cellulose, amylose, amylopectin and glycogen
2.3.A.2 Scientific evidence for health risks of trans fats and saturated fatty acids.
- Discuss the relationship between saturated fatty acids and/or trans fat intake and rates of coronary heart disease.
2.3.A.3 Lipids are more suitable for long-term energy storage in humans than carbohydrates.
- Explain the energy storage of lipids compared to that of carbohydrates.
2.3.A.4 Evaluation of evidence and the methods used to obtain the evidence for health claims made about lipids.
- Define evaluation in respect to evidence from and methods of research.
- Outline the manner in which the implications of research can be assessed.
- Outline the manner in which the limitations of research can be assessed.
- Evaluate a given health claim made about lipids.
Most research looking at the health claims of diets or nutritional components in a diet are based on observational studies. Outlined below are the differences between observational studies and experimental tests and considerations that need to be taken when making any conclusions regarding observational studies
Observational Studies:
- these studies only give you correlations and do not allow for establishing the causal relationship between the phenomena studied,
- the research observes what happens to people under exposure conditions that have been self-selected or have been determined by influences outside the control of the researcher
- these studies, regardless of their size or number, only provide hypothesis generating data.regardless of their size or number, only provide hypothesis given have to be rigorously generating data
Experimental Tests
- uses hypothesis then have to be rigorously tested using the scientific process
- the research can choose what conditions to study by controlling all key factors except those being test
- the researcher observes what happens to people under exposure conditions that have been self-selected or have been determined by influences outside the control of the research
2.3.S.1 Use of molecular visualization software to compare cellulose, starch and glycogen. (Oxford Biology Course Companion page 75).
- Demonstrate use of JMol to view molecular structures, including changing image size, rotating the image and changing the style of the molecular model.
- Identify carbon, hydrogen and oxygen atoms by color.
2.3.S.2 Determination body mass index by calculation or use of a nomogram.
- Calculate BMI using the formula.
- Determine BMI using a nomogram.
- Outline effects of a BMI that is too high or too low.
Hover over the picture to access the BMI Calculator
Body mass index, or BMI, is used to determine whether you are in a healthy weight range for your height.
It is useful consider BMI alongside waist circumference, as increases or decreases in weight outside the healthy range may increase your health risks.
BMI compares your weight to your height, and is calculated by dividing your weight in kilograms by your height in metres squared.
It gives you an idea of whether you’re underweight, a healthy weight, overweight, or obese for your height. BMI is one type of tool to help health professionals to assess risk for chronic disease. Another important tool is waist circumference. It is also important to understand your other risk factors.
The Body Mass Index (BMI) Nomogram is a graph that shows a person’s Body Mass Index as the point on the chart nearest the dashed line (representing the Body Mass Index) where height (in inches or centimetres) and weight (in pounds or kilograms) intersect.
Height is shown on the x-axis in centimetres or inches, and weight is shown on the y-axis in kilograms or pounds.
Dashed lines, representing the Body Mass Index, are displayed on the graph as calculated by the formula of weight (in kilograms) divided by height squared (in metres).