Grade 10 biology Cell Biology and Biodiversity – Chemicals of Life Notes
Chemicals of Life
Topic: Cell Biology and Biodiversity — Subject: Biology (Age 15, Kenya)
Specific learning outcomes
- a) Describe composition, properties and functions of chemicals of life in organisms.
- b) Investigate presence of carbohydrates, lipids, proteins and vitamin C in food substances.
- c) Investigate presence of enzymes in living tissues.
- d) Determine factors affecting enzymatic reactions in cells (temperature, pH, concentration).
- e) Appreciate the importance of chemical components in cells (health, growth, industry, environment).
1. Overview — What are the chemicals of life?
Cells are made mainly of a few groups of chemicals that determine their structure and activities: carbohydrates, lipids (fats), proteins, vitamins (e.g., vitamin C) and enzymes (special proteins that speed reactions). Below is a short summary of composition, properties and functions.
Carbohydrates
Composition: C, H, O (e.g. glucose C6H12O6). Monosaccharides, disaccharides, polysaccharides (starch, cellulose).
Properties & functions: quick energy (glucose), energy storage (starch in plants, glycogen in animals), structure (cellulose in plant cell walls).
Kenyan examples: ugali (maize starch), cassava, sweet potatoes, rice, beans (contain starch + sugars).
Lipids (Fats & Oils)
Composition: C, H, O (long hydrocarbon chains, glycerol + fatty acids).
Properties & functions: energy storage (high energy per gram), insulation, make up cell membranes (phospholipids), protect organs.
Kenyan examples: avocado, groundnut (peanut) oil, cooking oil, ghee.
Proteins
Composition: C, H, O, N (made of amino acids, some contain S).
Properties & functions: structural (muscles, cell structure), enzymes (biological catalysts), transport (haemoglobin), antibodies, hormones.
Kenyan examples: beans, lentils, eggs, milk, fish, nyama choma (meat).
Vitamin C (Ascorbic acid)
Properties & functions: water-soluble vitamin; antioxidant, helps absorb iron, important for healing and maintaining connective tissue.
Kenyan examples: oranges, mangoes, amaranth/sukuma wiki, tomatoes, pawpaw (papaya).
Enzymes
Nature: mostly proteins that act as catalysts — speed up biological reactions without being used up.
Properties & functions: lower activation energy, are specific (each enzyme works on particular substrate), affected by temperature and pH.
Examples in cells: amylase (breaks starch to sugar), catalase (breaks hydrogen peroxide into water + oxygen), proteases (break proteins).
2. Simple classroom investigations (Practical tests)
Below are clear, safe tests you can do in a school laboratory or classroom with teacher supervision. Use small groups (3–4 learners) and record results in a table.
A. Test for reducing sugars (Benedict's test)
Materials: Benedict's reagent, test tubes, water bath (boiling water), dropper, samples (fruit juice, milk, ugali slurry), controls (glucose solution, distilled water).
Procedure: Place 1 ml sample + 1 ml Benedict's reagent in a test tube, heat in boiling water 2–3 min. Observe color change.
Positive result: green → yellow → orange → red (red = large amount of reducing sugar). No change (blue) = negative.
Kenyan food examples: mango juice, orange juice, sweet potato extract.
Safety: heat carefully; wear eye protection.
B. Test for starch (Iodine test)
Materials: Iodine solution (Lugol's), drops, samples (ugali slurry, potato, bread), white tile or watch glass.
Procedure: Put a drop of iodine on sample. Observe color change.
Positive result: blue-black (starch). Brown/orange = negative.
Safety: iodine stains and is mildly toxic — small amounts, avoid skin and eyes; teacher control.
C. Test for lipids (Sudan III or grease-spot test)
Materials: Sudan III dye (or food oil + paper grease-spot), test tubes, water, samples (avocado, groundnut, milk).
Procedure (Sudan): Add few drops Sudan III to sample in test tube + shake. Lipids stain red and separate.
Grease-spot: Rub sample on filter paper; hold to light — translucent spot = fat.
Safety: dyes can stain; use gloves and avoid ingestion.
D. Test for proteins (Biuret test)
Materials: Biuret reagent (NaOH + CuSO4), test tubes, samples (milk, egg white diluted, beans extract).
Procedure: Add sample + Biuret reagent, mix gently. Positive: purple/lilac. Blue/no change = negative.
Safety: handle reagents with care; teacher supervises.
E. Test for vitamin C (DCPIP titration)
Materials: DCPIP solution (blue), dropper, sample juices (orange, mango, amaranth extract), distilled water, white tile.
Procedure: Add DCPIP dropwise to fixed volume of juice until blue disappears (becomes colorless). Less DCPIP used = more vitamin C.
Note: If DCPIP is unavailable, simple qualitative: add DCPIP drop — if color is lost immediately → vitamin C present.
F. Investigate enzymes in living tissues
1) Catalase test (breakdown of hydrogen peroxide)
Materials: Fresh potato or liver, 3% H2O2, test tubes, knife, stopwatch, marker.
Procedure: Cut small piece of potato or liver, place in test tube, add 5 ml H2O2. Observe bubbling (oxygen). Measure amount or vigour of bubbling (use covered measuring tube to collect gas if available).
Interpretation: Bubble = catalase present, which decomposes H2O2 → H2O + O2.
2) Amylase test (starch digestion)
Materials: Cooked starch solution (starch + water), saliva (from volunteer), iodine, test tubes, timer.
Procedure: Mix starch solution with saliva, incubate at room temperature. Take samples every few minutes and add iodine. Observe loss of blue-black colour as starch is broken down by amylase in saliva.
Interpretation: Disappearance of blue-black = starch digested by amylase (present in saliva).
Safety & ethics: Saliva tests involve personal fluids — use gloves, do not mix samples from different students without consent; teacher to manage sampling.
3. Factors affecting enzymatic reactions — investigation ideas
Enzymes are affected by temperature, pH and substrate concentration. Use catalase (from potato/liver) or amylase (saliva) for simple experiments.
A. Effect of temperature (example with catalase)
Prepare identical potato samples and place into water baths at different temperatures: cold (5°C, fridge), room temperature (~25°C), warm (37°C), hot (60°C). Add H2O2 and record rate/height of bubbling.
Expected: rate increases from cold to an optimum (~35–40°C), then decreases at higher temperature because enzyme denatures.
B. Effect of pH
Use buffered solutions (vinegar — acidic, water — neutral, baking soda solution — alkaline) and test enzyme activity (e.g., amylase on starch or catalase on H2O2). Observe which pH gives the highest activity.
Expected: each enzyme has an optimum pH (amylase ~pH 6–7; stomach enzyme pepsin ~pH 2).
C. Effect of substrate concentration
Keep enzyme amount constant (same potato extract) and vary concentration of H2O2 — record reaction rate. Rate increases with substrate until enzyme becomes saturated (plateau).
Simple visual: rate vs temperature (schematic)
(Schematic only — shows increase to optimum then fall due to denaturation.)
4. Appreciation — Why these chemicals matter
- Energy and survival: Carbohydrates and fats supply energy for school, play and farming activities.
- Growth and repair: Proteins build muscles, enzymes and tissues — important during adolescence.
- Health and immunity: Vitamins like vitamin C support healing and resistance to infections (important in communities with seasonal illnesses).
- Cell function: Enzymes allow digestion and all metabolic reactions to occur quickly at body temperatures.
- Agriculture & industry: Understanding enzymes helps in food processing (e.g., fermentation) and crop storage.
5. Suggested learning experiences (Kenyan context, age 15)
- Lab practicals in small groups to perform the tests above; record results in tables and draw conclusions.
- Field/data activity: Collect samples of common Kenyan foods (ugali + sukuma wiki, milk, beans, avocado, orange) and test for macromolecules and vitamin C. Present findings as posters.
- Project: Investigate how storage affects vitamin C in fruits (compare fresh mango vs stored for several days).
- Community link: Visit a local food market and identify foods rich in carbohydrates, proteins, lipids and vitamin C; discuss nutrition in households.
- Group investigation: Design experiments to find optimum temperature for catalase in potato and present a short report explaining results with graphs.
- Cross-curricular: Maths — calculate percentage concentration needed for substrate experiments; Kiswahili/English — write report and present findings.
6. Practical tips, safety & assessment
- Always wear goggles and gloves when handling reagents (iodine, DCPIP, Biuret, H2O2).
- Dispose of biological waste (egg, saliva) according to school rules. Clean work surfaces after experiments.
- Encourage careful observation: record time, color, temperature and repeat tests for reliability.
- Assessment suggestions: practical report, poster, short test (definitions & functions), and group presentation.
7. Key terms (quick revision)
Monosaccharide, polysaccharide, starch, cellulose, lipid, triglyceride, amino acid, peptide, protein, vitamin C (ascorbic acid), enzyme, catalyst, denaturation, active site, substrate.
8. Short summary
Carbohydrates, lipids, proteins, vitamins and enzymes are essential chemicals in cells. Simple classroom tests allow learners to identify these chemicals in common Kenyan foods. Practical experiments on enzymes teach how temperature, pH and concentration affect life processes — knowledge important for health, agriculture and everyday life.