Unit 6 Chemical basis of life
Key unit competence: Evaluate the presence and relevance of each chemical basis of life.
Key ideas
- Living organisms are made of inorganic substances (water, mineral salts) and organic molecules containing carbon: carbohydrates, lipids, proteins, nucleic acids and vitamins. Large molecules (polymers) are built from smaller units (monomers) by condensation (water released) and broken down by hydrolysis (water added).
- Water (60–90% of living mass) is a polar molecule with hydrogen bonds. Properties and importance: universal solvent (reactions and transport in solution), high specific heat capacity (stable temperature of organisms and lakes), high latent heat of vaporisation (cooling by sweating and transpiration), cohesion and adhesion (water columns in xylem, capillarity), high surface tension, ice less dense than water (floats, insulates lakes), a reactant in photosynthesis and hydrolysis.
- Mineral salts: macronutrients needed in large amounts (Ca, P, K, Na, Cl, Mg, S, N) and micronutrients/trace elements (Fe, I, Zn, Cu, F, Mn). Humans: calcium (bones, teeth, clotting; deficiency rickets/osteoporosis), iron (haemoglobin; anaemia), iodine (thyroxine; goitre), sodium (nerve impulses, fluid balance), phosphorus (bones, ATP, DNA), fluoride (enamel). Plants: nitrogen (proteins; yellowing), magnesium (chlorophyll; chlorosis), phosphorus (roots, ATP), potassium (enzymes, stomata).
- Vitamins are organic substances needed in small amounts. Fat-soluble: A (retinol; night vision; deficiency night blindness), D (calcium absorption; rickets), E (antioxidant), K (blood clotting). Water-soluble: B group (coenzymes in respiration; B1 deficiency beriberi, B3 pellagra, B12 pernicious anaemia, folic acid in pregnancy), C (ascorbic acid; collagen; scurvy: bleeding gums, slow healing).
- Carbohydrates contain C, H and O (ratio Cₙ(H₂O)ₙ). Monosaccharides: trioses, pentoses (ribose, deoxyribose) and hexoses (glucose, fructose, galactose); α-glucose and β-glucose differ in the position of the OH group on carbon 1. Disaccharides form by a glycosidic bond (condensation): maltose = glucose + glucose; sucrose = glucose + fructose; lactose = glucose + galactose.
- Polysaccharides: starch (α-glucose; amylose unbranched helix and amylopectin branched; plant energy store, insoluble, compact), glycogen (α-glucose, highly branched; animal and fungal store in liver and muscles, rapidly hydrolysed), cellulose (β-glucose, straight chains with alternate units inverted, held by hydrogen bonds into microfibrils; plant cell walls, very strong), chitin (fungal walls, insect exoskeleton).
- Lipids (C, H, O, much less O): triglycerides = glycerol + 3 fatty acids joined by ester bonds. Saturated fatty acids (no C=C double bonds; solid animal fats) and unsaturated (C=C bonds; liquid oils). Functions: energy store (about twice the energy of carbohydrate per gram), insulation, protection of organs, waterproofing (waxes), source of metabolic water. Phospholipids form membranes; steroids (cholesterol, sex hormones).
- Proteins are polymers of amino acids (C, H, O, N, often S). Each amino acid has an amino group (−NH₂), a carboxyl group (−COOH) and an R group; there are 20 amino acids. Two amino acids join by a peptide bond (condensation) to form a dipeptide; many form a polypeptide.
- Protein structure: primary (sequence of amino acids), secondary (α-helix or β-pleated sheet held by hydrogen bonds), tertiary (3D folding held by hydrogen, ionic, disulphide bonds and hydrophobic interactions), quaternary (more than one polypeptide, e.g. haemoglobin). Globular proteins (enzymes, antibodies, haemoglobin; soluble) and fibrous proteins (collagen, keratin; strong, insoluble). Heat, extreme pH and heavy metals denature proteins by breaking the bonds that hold their shape.
Common misconceptions
- "Starch, glycogen and cellulose are all made of the same glucose joined the same way." No: starch and glycogen are α-glucose; cellulose is β-glucose, which allows straight chains and microfibrils.
- "Fats give the same energy per gram as carbohydrates." No: lipids release about twice as much energy per gram.
- "Denaturing a protein breaks its peptide bonds." No: denaturation breaks the hydrogen, ionic and disulphide bonds that hold the 3D shape; the primary sequence stays.
- "All vitamins can be stored in body fat." No: only fat-soluble vitamins (A, D, E, K) are stored; water-soluble B and C must be eaten regularly.
Worked examples
Example 1. Explain why glycogen is a suitable storage molecule in animals.
Solution
Glycogen is insoluble, so it does not affect the water potential of cells; it is compact, storing much glucose in little space; it is highly branched, so many ends allow enzymes to hydrolyse it rapidly to glucose when energy is needed.
Example 2. Explain how the structure of cellulose makes it suitable for plant cell walls.
Solution
Cellulose is made of β-glucose joined by 1,4 glycosidic bonds, with every other unit inverted, forming long straight unbranched chains. Many chains lie side by side, linked by hydrogen bonds, forming microfibrils with high tensile strength; layers of microfibrils make a strong, fully permeable wall.
Practice questions
State five properties of water and their importance to living organisms.
Show answer
Solvent (transport, reactions); high specific heat capacity (stable temperature); high latent heat of vaporisation (cooling by sweat/transpiration); cohesion and adhesion (xylem transport); ice less dense (insulates aquatic life); reactant in photosynthesis and hydrolysis.
Common mistake: Listing properties without importance.
Name the source, function and deficiency disease of: iron, calcium, iodine.
Show answer
Iron: liver, beans, green vegetables; haemoglobin; anaemia. Calcium: milk, small fish; bones, teeth, clotting; rickets/osteoporosis. Iodine: iodised salt, fish; thyroxine; goitre.
Common mistake: Linking iodine to anaemia.
Compare starch, glycogen and cellulose.
Show answer
Starch: α-glucose, amylose (unbranched) and amylopectin (branched), plant store. Glycogen: α-glucose, highly branched, animal store. Cellulose: β-glucose, straight chains forming microfibrils, plant cell walls.
Common mistake: Saying cellulose is α-glucose.
Describe the formation and breakdown of a peptide bond.
Show answer
The carboxyl group of one amino acid reacts with the amino group of another, releasing water (condensation) and forming a peptide bond (−CO−NH−). Hydrolysis adds water to break it.
Common mistake: Calling it a glycosidic bond.
Describe the four levels of protein structure.
Show answer
Primary: amino acid sequence. Secondary: α-helix or β-sheet held by hydrogen bonds. Tertiary: 3D folding held by hydrogen, ionic, disulphide bonds and hydrophobic interactions. Quaternary: two or more polypeptides together (haemoglobin).
Common mistake: Saying primary structure is the 3D shape.
State four functions of lipids.
Show answer
Energy store; thermal insulation; protection of organs; waterproofing (waxes); membrane structure (phospholipids); hormones (steroids); metabolic water.
Common mistake: Saying lipids are enzymes.
Distinguish between saturated and unsaturated fats.
Show answer
Saturated: no C=C double bonds, usually solid at room temperature, mainly animal fats. Unsaturated: one or more C=C double bonds, usually liquid oils, mainly from plants.
Common mistake: Reversing them.
Explain what happens when a protein is denatured.
Show answer
Heat, extreme pH or heavy metals break the hydrogen, ionic and disulphide bonds holding the tertiary structure; the protein loses its shape and function (e.g. enzyme active site lost); peptide bonds stay intact.
Common mistake: Saying the amino acid sequence changes.
Matching and drag-and-drop
Matching
Match each item in column A with the correct answer in column B. Write the letter next to each number.
Column A Column B 1. A triglyceride is made of: a. primary structure 2. Cellulose is made of: b. thyroxine 3. The sequence of amino acids in a polypeptide is its: c. glycerol and three fatty acids 4. Iodine is needed to make: d. β-glucose Show answer
1 → c (glycerol and three fatty acids) · 2 → d (β-glucose) · 3 → a (primary structure) · 4 → b (thyroxine)
- Joined by ester bonds.
- Alternate units are inverted.
- It determines the final shape.
- Deficiency causes goitre.
Drag and drop
Choose the correct word or phrase from the box for each gap. One word in the box is not needed.
glycosidic bond · disulphide bond · two glucose molecules · carbon-carbon double bonds- Maltose is formed from .
- Unsaturated fatty acids contain .
- The bond between two monosaccharides is a .
Show answer
1: two glucose molecules · 2: carbon-carbon double bonds · 3: glycosidic bond
- Joined by a glycosidic bond.
- They are usually liquid oils.
- Formed by condensation.
Matching
Match each item in column A with the correct answer in column B. Write the letter next to each number.
Column A Column B 1. Amino acids are joined by: a. peptide bonds 2. Night blindness is caused by lack of vitamin: b. A 3. The storage polysaccharide in animals is: c. polar 4. Water is a good solvent because its molecules are: d. glycogen Show answer
1 → a (peptide bonds) · 2 → b (A) · 3 → d (glycogen) · 4 → c (polar)
- Formed by condensation.
- Vitamin A forms visual pigment.
- Stored in liver and muscle.
- Ions and polar substances dissolve.
Drag and drop
Choose the correct word or phrase from the box for each gap. One word in the box is not needed.
anaemia · night vision · blood clotting · C- Vitamin K is needed for .
- Lack of iron in the diet causes .
- Scurvy is caused by lack of vitamin .
Show answer
1: blood clotting · 2: anaemia · 3: C
- It is made partly by gut bacteria.
- Iron is part of haemoglobin.
- Gums bleed and wounds heal slowly.