As Bio Cambridge Ch2 Biological molecules 2025.pptx

Chapter 2
Biological molecules
Cambridge Syllabus

1. Biological molecules As Biology – Chapter 2 Cambridge Syllabus

2. Carbohydrates (Ch 2 0) n Contain C , H , and O where the ratio between H:O is 2:1 . There are three basic types of carbohydrate molecules according to their size: Monosaccharide: a molecule consisting of a single sugar unit, general formula (CH 2 O) n . Disaccharide: a sugar molecule consisting of 2 monosaccharides joined together by a glycosidic bond. Polysaccharide: a polymer consisting of subunits (monosaccharides) joined together by glycosidic bonds. Monosaccharides and disaccharides are also known as sugars , they have names that end in – ose . They have the general formula (CH 2 O) n . triose sugars have 3 carbons, pentose sugars such as ribose and deoxyribose have 5 carbons and hexose sugars such as fructose and glucose have 6 carbons.

3. The structure of Glucose C 6 H 12 O 6 The formula for a hexose can be written as C 6 H 12 O 6 . This is known as the molecular formula . It is also useful to show the arrangements of the atoms, which can be done using a diagram known as the structural formula . 1 2 3 4 5 6

4. The structure of Glucose C 6 H 12 O 6 When dissolved in water linear glucose molecules rearrange to form a ring structure. 1 2 3 4 5 6

5. The structure of Glucose Can you tell the difference between these 2 molecules? The –OH group attached to carbon 1 (C1) might be below the ring ( α- glucose) or above the ring ( β- glucose). α- glucose & β- glucose are called isomers; they have exactly the same number of atoms but have a different 3D arrangement. H A bove ring in α- glucose H B elow ring in β- glucose

6. The structure of Glucose C 6 H 12 O 6 This animation model shows the 3D ring structure of glucose. What do the three colours represent? Green: Red: White: Is this an α- glucose or β- glucose molecule? Give a reason for your choice.

7. Disaccharides Where two monosaccharides bond together by elimination of a water molecule forming a glycosidic bond. This reaction is called a condensation reaction , which is catalysed by an enzyme. Another enzyme can break this bond by adding a water molecule to produce two monosaccharides; this is called a hydrolysis reaction . The diagram on the next slide shows how two monosaccharides can be joined by condensation to produce a disaccharide.

8. Condensation and hydrolysis reactions

9. Some common disaccharides

10. Polysaccharides Long chain molecules made up of many monosaccharides bonded by condensation of (n) molecules with the elimination of (n-1) water molecules. Starch, glycogen are α- glucose polymers while cellulose is a β- glucose polymer.

11. Starch made up of many α- glucose molecules, with the general formula (C 6 H 10 O 5 ) n . Starch is found in plant cells only. It is a mixture of two polysaccharides; amylose and amylopectin. The starch molecule has the advantage of being compact , un-reactive and insoluble which makes it ideal for storage inside the cell. The table on the next page compares both. ⍺ - 1,4 – glycosidic bonds

12. Comparing amylose and amylopectin

13. Test for Starch Add Iodine solution . Iodine molecules fit inside the spiral amylose molecules forming what we call : Iodine–starch complex that is blue-black in colour .

14. Glycogen Found in animal cells (liver & muscles) as well as in fungal & bacterial cells. Have a similar structure to amylopectin but with branches every 8-10 glucose units. The branches can be quickly broken down simultaneously to supply glucose needed by respiring cells e.g. during exercise.

15. Cellulose The most abundant molecule on planet Earth! Has a structural role in plant cell walls. It is a structural polysaccharide made of β- glucose bonded by 1,4-glycosidic linkages. The glucose molecules are arranged so that there is one ring is facing upward while the other is facing downward. This arrangement produces a straight chain and when chains are side-by-side they would form hydrogen bonds along their entire length. A bundle of (60 to 70) cellulose molecules is called a microfibril , a group of microfibrils make up a fibre . Cellulose fibres are arranged like threads in a fabric, running in different directions which give the cell walls their high tensile strength. Other molecules form a glue-like matrix around the fibres, further increases the strength of the cell wall.

16. Animal, bacteria, fungi

17. Lipids Include a wide variety of molecules that contain elements C , H & O where the C-H bonds are much more than that found in carbohydrates. C-H bonds produce large amounts of energy when broken (1gm fat=38KJ). The large number of C-H bonds makes the molecule non-polar which is why they don’t dissolve in water but rather in non-polar (organic) solvents.

18. Lipids Fats and oils contain two types of organic molecules; fatty acids and glycerol . They are combined using ester bonds in a condensation reaction .

19. Lipids Fatty acids are long hydrocarbon chains with a carboxylic group (COOH) at one end. Fatty acids may vary in two ways: The length of the hydrocarbon chain (usually 15-17 carbon atoms long) The fatty acid might be a saturated fatty acid or an unsaturated fatty acid .

20. Fatty acids with double bonds in their structure are form unsaturated triglycerides also known as oils, which are liquid at room temperature. While fatty acids with no double bonds form saturated triglycerides, which are normally solid at room temperature such as butter, cream, margarine & ghee . This is due to the fact that saturated chains are straight which allows the triglycerides to come closer together forming a tight arrangement of molecules as in solids while unsaturated chains are askew which prevents the molecules from coming that close together.

21. Types of Lipids Lipids include: Triglycerides(fats and oils) Phospholipids (main component of plasma membrane) Steroids Triglycerides : a molecule of glycerol when bonded to three fatty acids is called a triglyceride. The reaction is also a condensation reaction where three water molecules are removed one from each fatty acid that bonds. The resulting bond is called an ester bond since glycerol is considered and alcohol and fatty acids are acids!

22. Functions of triglycerides Energy source; can be broken down to release energy. Energy store; contain much more energy/gm due to many C-H bonds. Thermal insulation; fat under skin (adipose tissue) is a poor conductor of heat which reduces heat loss from the body. Electrical insulation; myelin sheath covering axons of nerve cells act as an electrical insulator and helps nerve impulses to pass quickly. Water-proofing surfaces; due to their hydrophobic nature. Have a low density; providing buoyancy to aquatic animals e.g. whales Metabolic source of water; when oxidized in respiration TG release many water molecules which is important for animals living in dry habitats e.g. kangaroo rat.

23. Phospholipids form the basic structure of cell membranes. In this molecule, a phosphorous group reacts instead of one of the fatty acids. The phosphorous group ionizes to become – vely charged, this creates a polar side along with the glycerol part of the molecule. We call this part the head and describe it as hydrophilic “water-loving”, the rest of the molecule consists of the long two fatty acid chains which carry no charges, they are named tails and are hydrophobic “water-hating”.

24. Proteins Contain elements C, H, O, N and sometimes S and P. Proteins are polymers of many amino acids linked together (again by condensation!) forming what we call peptide bonds . R= residual group which changes from one amino acid to another. If polar the amino acid is hydrophilic (water soluble) if non-polar the amino acid is hydrophobic (water insoluble). In some proteins hydrophilic R-groups are arranged so that they face the outer side of the protein, while hydrophobic R-groups face inwards, thus the protein molecule is water soluble. R- groups are also involved in formation of cross links which gives the protein its 3D shape.

25. Types of amino acids

26. Adding one more amino acid gives a tripeptide, more than three amino acids in a chain is called a polypeptide. Proteins can contain one polypeptide chain or more. Formation of peptide Bond between amino acids

27. Functions of proteins: Type Example Function Occurrence Hormones Insulin, Glucagon Regulate blood sugar level Pancreas Contractile fibres Actin, Myosin Muscle contraction Muscles Storage Ferritin Stores iron Liver Enzyme Lipase, amylase Digestion Pancreas Structural Collagen, Keratin Provide strength Skin, hair, nails Protective Antibodies Fight foreign organism WBCs 1. All enzymes are made of proteins. 2. Proteins are essential components of cell membranes. 3. Storage products e.g. casein in milk and ovalbumin in egg white.

28. Protein structure Primary structure : there are only 20 different amino acids in our world, but the number of different proteins known to us is increasing every day. That is because there are infinite number of ways you could arrange the different amino acids to produce a protein. The primary structure refers to the type and sequence of amino acids in a polypeptide chain. This determines the overall shape of the protein and thus its function. The 1ry structure is determined by the code on the DNA which is translated by ribosomes that join the amino acids together forming the protein. Primary Structure

29. Protein structure Secondary structure: the polypeptide often twists into either a spiral ( α- helix) or a bent sheet ( β- pleated sheet) due to hydrogen bonds forming between –NH group of one amino acid residue and –C=O group of another. The secondary structure is how the polypeptide molecules twists forming an α– helix or β– pleated sheets stabilized by H- bonds. α- helix : most common type where the polypeptide is coiled into a spiral and stabilized by H-bonds. β- pleated sheet : a flat structure where two or more polypeptide chains lie side by side and the H-bonds are formed between them. Secondary Structure

30. Protein structure Tertiary structure : do you remember trying to set your phone cord straight? The polypeptide can coil further like your entangled phone cord to form a 3D shape which is stabilized by one or more of these different bonds between the different R- groups: Hydrogen bonds between slightly +ve and slightly –ve R-groups. Disulphide bridges (covalent bond) between two SH-groups in cysteine residues. Ionic bonds between two oppositely charged ions in R-groups Hydrophobic attractions between non-polar R- groups. Tertiary Structure

31. Protein structure Quaternary structure : picture a group of entangled necklaces made of beads. A protein may be made of more than one polypeptide. The way the polypeptides are held together to form a 3D shape is called the quaternary structure . They are held in place by the same four bonds stated in the 3ry structure. Example of a protein with a 4ry structure is haemoglobin . Quaternary Structure

32. Globular and fibrous proteins.

33. Haemoglobin This protein is made up of four polypeptide chains, two identical α– chains and two identical β– chains. Each chain is folded into a sphere and they are held together by disulphide bonds. Each polypeptide holds a non-protein prosthetic group called the haem group. It is to the Fe+2 at the centre of the haem group the Oxygen molecule binds. This makes a haemoglobin molecule capable of carrying four O 2 molecules.

34. Collagen It is present in tendons, ligaments, blood vessels, bones and skin. It is the most common structural protein found in animals (up to 35% of the proteins in your body is collagen, with a tensile strength similar to that of steel!) The primary structure of -glycine-X-Y-glycine-X-Y- glycine-X-Y-(other amino acids often are proline & hydroxyproline) allows the polypeptide molecule to have a stretched-out helical shape (not an α – helix). Its quaternary structure has three polypeptide chains, each up to 1000 amino acids long, the three identical polypeptides are intertwined forming a triple helix held together by a very large number of H-bonds . The three stranded molecule interacts with other collagen strands by covalent bonds forming fibres that can be up to several millimetres long.

35. water This very important molecule has many roles: 1. It is a major component of cells 2. Provides a habitat for aquatic animals 3. It is involved in many metabolic reactions e.g. photosynthesis and hydrolysis 4. It helps to provide strength to plants by keeping their cells turgid 5. It helps to cool body of living things by evaporation 6. Helps gas exchange at respiratory surfaces 7. Acts as a solvent for ionic and polar molecules which makes it a good transport medium The unusual physical and chemical properties of water are due to the small size of the water molecules & due to their dipolar quality. Dipoles occur in many different molecules, particularly whenever there is an –OH, –C=O or N-H. Hydrogen bonds can form between these groups. These bonds are very important in the structure and properties of carbohydrates and proteins.

36. Details H 2 O is dipolar : oxygen is slightly negative while hydrogen is slightly positive. Each pole can attract ions of an opposite charge or other polar molecules Uses to living things Main component of the cytoplasm , providing a good medium for dissolved substances to react. A good transport system e.g. in blood plasma and phloem vessels. Properties of water and its importance to sustain life 1. Solvent

37. Details Hydrogen bonds between water molecules makes the water surface appear as a film Uses to living things Can support small animals to walk on the surface of the water (pond skaters) Properties of water and its importance to sustain life 2. Cohesion (surface tension )

38. Forces between water molecules and other surfaces Allows water to travel up capillary tubes such as xylem vessels Details Uses to living things Properties of water and its importance to sustain life 3. Adhesion

39. Details Water is slow to absorb and release heat. Water needs a relatively large amount of energy to raise its temperature which means that areas with large water composition can keep a fairly constant temperature even when their surrounding temperatures fluctuate. Uses to living things Helps to sustain aquatic life in seas, lakes and rivers. This keeps the body temperature constant (living things are mostly water); this helps in body temperature regulation . Properties of water and its importance to sustain life 4. High specific heat capacity

40. Details Hydrogen bonds between water molecules have to be broken before they can evaporate which requires more heat energy. Uses to living things Helps to cool down body temperature as a small volume of water will use up a considerable amount of body heat to evaporate, so not much water is lost when cooling the body. Properties of water and its importance to sustain life 5. High latent heat of vaporisation

41. Details When solid; water molecules arrange in a way, so they are further apart making ice less dense than liquid water. Uses to living things Ice floats on the surface of lakes/ rivers/ sea insulating the water beneath to support aquatic life. As density of water changes with temperature, this creates water currents that can circulate dissolved gases and nutrients. Properties of water and its importance to sustain life 6. Density

42. Details Viscosity is the resistance to flow. Water has a low viscosity compared to other liquids such as ethanol, glycerol, oil. Uses to living things Allows blood to flow easily in blood vessels. Allows aquatic animals to swim easily in water Properties of water and its importance to sustain life 6. Low viscosity

43. Useful videos to watch Linear and cyclical forms of glucose: https://youtu.be/-Aj5BTnz-v0 Carohydrates : https://youtu.be/rQyWJIn1HYE Lipids: https://youtu.be/ebScOnAJdu0?si=cZBh7zs7VqmqTGPU Proteins: https://youtu.be/kMg517MHDJs?si=5CRywLXklzvchHbT