state that glucose, fructose and maltose are reducing sugars and that sucrose is a non-reducing sugar

Carbohydrates, Lipids & Related Biomolecules – Cambridge AS/A‑Level Biology (9700)

Learning Objective (AO1)

State that glucose, fructose and maltose are reducing sugars and that sucrose is a non‑reducing sugar.

1. Cell Structure – Quick Recap (AO1)

• Prokaryotic cells: no nucleus, circular DNA, 70 S ribosomes, cell wall of peptidoglycan.
• Eukaryotic cells: nucleus with nuclear envelope, membrane‑bound organelles (mitochondria, ER, Golgi, lysosomes, peroxisomes, vacuoles).
• Plant cells: rigid cellulose cell wall, chloroplasts, large central vacuole.
• Animal cells: centrioles, usually smaller vacuoles, lack cell wall.

2. Biological Molecules (AO1 & AO2)

2.1 Carbohydrates

General formula: C_n(H₂O)_n. Functions include energy supply, structural support and molecular recognition.

Reducing vs. Non‑reducing Sugars

• Reducing sugar: possesses a free carbonyl group (aldehyde or ketone) that can be oxidised; reduces Cu²⁺ in Benedict’s/Fehling’s reagents.
• Non‑reducing sugar: both anomeric carbons are involved in a glycosidic bond, so no free carbonyl is available.

Why Sucrose Is Non‑reducing

The glycosidic bond joins C‑1 of glucose to C‑2 of fructose, locking both carbonyl carbons. Consequently sucrose cannot donate electrons to Cu²⁺ ions.

2.2 Lipids

• Triglycerides (fats & oils): glycerol + three fatty acids; energy dense (≈9 kcal g⁻¹).
• Phospholipids: glycerol + two fatty acids + phosphate head; form bilayers – basis of cell membranes.
• Steroids: four fused carbon rings (e.g., cholesterol, steroid hormones).

2.3 Proteins

Polymers of α‑amino acids linked by peptide bonds.

2.4 Water (AO1)

• Polarity → extensive hydrogen bonding.
• High specific heat, heat of vaporisation, surface tension – essential for temperature regulation and transport.
• Universal solvent – dissolves ionic & polar substances; medium for biochemical reactions.

3. Enzymes – Core Concepts (AO2)

• Active site: region where substrate binds.
• Lock‑and‑key vs. induced‑fit models.
• Enzyme kinetics: V_max, K_m (Michaelis–Menten equation).
• Factors affecting activity: temperature, pH, substrate concentration, inhibitors (competitive, non‑competitive).

Practical Example (AO3)

Catalase assay – measure O₂ evolution from H₂O₂ breakdown with varying substrate concentrations; plot rate vs. [H₂O₂] to determine K_m.

4. Cell Membranes & Transport (AO1 & AO2)

4.1 Structure – Fluid‑Mosaic Model

4.2 Transport Mechanisms

4.3 Animal Transport (AO1)

• Circulatory system: heart pumps blood; arteries (high pressure), veins (low pressure), capillaries (exchange).
• Gas exchange: alveolar–capillary diffusion; partial pressure gradients of O₂ and CO₂.
• Regulation of blood pH: bicarbonate buffer, chloride shift, Bohr effect.

4.4 Plant Transport (AO1)

• Xylem: upward transport of water & minerals; cohesion‑tension theory.
• Phloem: bidirectional transport of organic solutes (mainly sucrose) – pressure‑flow (mass‑flow) hypothesis.

5. Cell Cycle & Mitosis (AO1)

• Interphase: G₁ (growth), S (DNA synthesis), G₂ (pre‑mitosis).
• Prophase: chromatin → chromosomes, spindle formation, nuclear envelope breakdown.
• Metaphase: chromosomes line up at the metaphase plate.
• Anaphase: sister chromatids separate to opposite poles.
• Telophase & Cytokinesis: nuclear envelopes reform, cytoplasm divides.

6. Nucleic Acids & Protein Synthesis (AO1 & AO2)

6.1 DNA Structure

• Double helix; antiparallel strands; deoxyribose sugar; phosphate backbone.
• Base pairing: A–T (2 H‑bonds), G–C (3 H‑bonds).

6.2 RNA Structure

• Single‑stranded; ribose sugar; uracil replaces thymine.

6.3 Central Dogma Flowchart (AO2)

6.4 Key Processes

• Transcription (DNA → mRNA) – RNA polymerase, promoter, terminator; occurs in nucleus.
• Translation (mRNA → polypeptide) – ribosome (large & small subunits), tRNA anticodon, peptide‑bond formation; occurs in cytoplasm.
• Genetic code: 64 codons; start codon AUG; stop codons UAA, UAG, UGA.

7. Energy & Respiration (A‑Level – AO1 & AO2)

• Four stages of aerobic respiration:

  1. Glycolysis (cytoplasm) – 2 ATP net, 2 NADH, 2 pyruvate.
  2. Link reaction (mitochondrial matrix) – pyruvate → acetyl‑CoA, 2 NADH.
  3. Krebs cycle (matrix) – 2 ATP (GTP), 6 NADH, 2 FADH₂, CO₂.
  4. Electron transport chain & oxidative phosphorylation (inner mitochondrial membrane) – ≈34 ATP, H₂O formed.

• ATP – energy‑currency; produced by substrate‑level phosphorylation and chemiosmotic coupling.
• Respiratory quotient (RQ): RQ = CO₂ produced / O₂ consumed. RQ ≈ 1 for carbohydrates, ≈0.7 for fats.

Practical Idea (AO3)

Respirometer with germinating beans: measure volume of CO₂ released over time; calculate RQ by also measuring O₂ uptake with an O₂ sensor.

8. Photosynthesis (A‑Level – AO1 & AO2)

• Overall equation: 6 CO₂ + 6 H₂O + light → C₆H₁₂O₆ + 6 O₂.
• Light‑dependent reactions (thylakoid membranes):

  • Photon absorption by chlorophyll a (peak ≈ 680 nm) and accessory pigments.
  • Water splitting → O₂, H⁺, electrons.
  • Electron transport chain → formation of a proton gradient.
  • ATP synthase produces ATP (photophosphorylation).
  • NADP⁺ reduced to NADPH.

• Calvin cycle (light‑independent) (stroma):

  • CO₂ fixation by Rubisco → 3‑phosphoglycerate.
  • Reduction phase: ATP + NADPH convert 3‑PGA to G3P.
  • Regeneration of RuBP using ATP.

Practical Example (AO3)

Starch‑iodine test on leaves kept under different light intensities: darker leaves retain more starch (positive iodine colour) indicating lower photosynthetic activity.

9. Homeostasis (A‑Level – AO1 & AO2)

• Negative feedback – the most common regulatory mechanism.
• Blood glucose control:

  • High glucose → insulin release → uptake by liver, muscle, adipose; glycogen synthesis.
  • Low glucose → glucagon release → glycogenolysis & gluconeogenesis.

• Temperature regulation:

  • Thermoreceptors → hypothalamus → vasodilation/vasoconstriction, sweating, shivering.

• Osmoregulation – ADH control of kidney water re‑absorption.

10. Control & Coordination (A‑Level – AO1 & AO2)

10.1 Nervous System

• Neuron structure: dendrite, soma, axon, myelin sheath, synaptic terminal.
• Action potential: depolarisation (Na⁺ influx) → repolarisation (K⁺ efflux).
• Synaptic transmission – neurotransmitter release, receptor binding, removal.

10.2 Endocrine System

• Hormones travel via bloodstream; act on target cells with specific receptors.
• Examples: insulin, glucagon, adrenaline, thyroid hormones.
• Feedback loops (negative & positive) regulate hormone levels.

11. Inheritance (A‑Level – AO1 & AO2)

• Meiosis – reductional division (2 → 1 chromosome set); produces four genetically distinct gametes.
• Mendelian genetics:

  • Monohybrid crosses – dominant/recessive, genotype ratios 1 : 2 : 1.
  • Di‑hybrid crosses – independent assortment, 9 : 3 : 3 : 1 phenotype ratio.

• Linkage & recombination – genes on the same chromosome may be inherited together; crossing‑over creates new combinations.
• Sex determination – XY system (mammals); other systems (ZW, XO) briefly noted.
• Pedigree analysis – tracing inheritance of autosomal dominant, autosomal recessive, X‑linked traits.

12. Selection & Evolution (A‑Level – AO1 & AO2)

• Natural selection – variation, differential survival/reproduction, inheritance of favourable traits.
• Genetic drift – random changes in allele frequencies (bottleneck, founder effect).
• Speciation – allopatric (geographic isolation) and sympatric (reproductive isolation) mechanisms.
• Phylogenetic trees – diagrammatic representation of evolutionary relationships.

13. Classification, Biodiversity & Conservation (A‑Level – AO1)

• Three‑domain system: Bacteria, Archaea, Eukarya.
• Five‑kingdom system (used in exam contexts): Monera, Protista, Fungi, Plantae, Animalia.
• Criteria for classification – cell structure, mode of nutrition, reproduction, molecular data.
• Conservation issues – habitat loss, invasive species, climate change; case study: coral‑reef bleaching.

14. Genetic Technology (A‑Level – AO2 & AO3)

• Recombinant DNA – restriction enzymes, ligases, plasmid vectors.
• Polymerase Chain Reaction (PCR) – denaturation, annealing, extension; exponential amplification of a target DNA fragment.
• DNA sequencing – Sanger method, next‑generation sequencing.
• CRISPR‑Cas9 – targeted genome editing using guide RNA.
• Practical application – production of insulin, gene therapy, forensic DNA profiling.

15. Typical Laboratory Test for Reducing Sugars (AO3)

1. Prepare a 1 % (w/v) solution of the carbohydrate sample.
2. Add an equal volume of freshly prepared Benedict’s reagent.
3. Heat in a boiling water bath for 2–3 minutes.
4. Observe the colour change:

   • Blue → green → yellow → orange → brick‑red = positive (reducing sugar).
   • Remains blue = negative (non‑reducing sugar, e.g., sucrose).

Key Points to Remember (AO1)

• Glucose, fructose and maltose each have a free carbonyl group → they are reducing sugars.
• Sucrose’s glycosidic bond involves both anomeric carbons → non‑reducing.
• Reducing sugars give a positive Benedict’s/Fehling’s test; non‑reducing sugars do not.
• Carbohydrates, lipids, proteins and water together satisfy the major biological‑molecule requirements of the syllabus.
• Enzyme activity follows lock‑and‑key/induced‑fit models and is quantified by V_max and K_m.
• Cell membranes are fluid mosaics; transport occurs by diffusion, facilitated diffusion, active transport, osmosis and vesicular mechanisms.
• The mitotic cell cycle ensures accurate chromosome segregation.
• DNA → RNA → protein is the central flow of genetic information; transcription in the nucleus, translation in the cytoplasm.
• In plants, xylem conducts water upward; phloem conducts sugars via the pressure‑flow mechanism.
• Aerobic respiration yields ~38 ATP per glucose; photosynthesis stores solar energy as chemical energy.
• Homeostatic control relies on negative feedback loops (e.g., blood glucose, temperature).
• Nervous and endocrine systems coordinate rapid and long‑term responses respectively.
• Inheritance follows Mendelian principles, meiosis, and can be modified by linkage and genetic drift.
• Evolution is driven by natural selection, genetic drift, and speciation events.
• Modern genetic technologies enable manipulation and analysis of DNA for medicine and research.