Nutrition: Autotrophic and Heterotrophic

🟢 Lite — Quick Review (1h–1d)

Rapid summary for last-minute revision before your exam.

Nutrition is the process by which organisms obtain and utilise food substances for energy, growth, and repair. All living organisms need nutrients — the chemicals found in food.

Two Major Nutritional Types:

1. Autotrophic Nutrition (“Self-feeders”) Organisms that manufacture their own food from simple inorganic substances.

• Photoautotrophs: Use light energy (plants, algae, cyanobacteria)
• Chemoautotrophs: Use chemical energy (certain bacteria, e.g., nitrifying bacteria)

2. Heterotrophic Nutrition (“Other-feeders”) Organisms that cannot make their own food — they must consume other organisms or their products.

• Holophytes: Ingest whole food (animals, fungi)
• Saprophytes/Saprotrophs: Feed on dead/decaying organic matter (fungi, many bacteria)
• Parasites: Feed on living hosts (tapeworm, malaria parasite)

Photosynthesis: The process by which green plants make glucose using light energy.

Word Equation: $$\text{Carbon dioxide} + \text{Water} \xrightarrow{\text{light, chlorophyll}} \text{Glucose} + \text{Oxygen}$$ $$6CO_2 + 6H_2O \xrightarrow{\text{light, chlorophyll}} C_6H_{12}O_6 + 6O_2$$

Two Stages of Photosynthesis:

1. Light-dependent reactions (thylakoid membranes):

   • Chlorophyll absorbs light (mainly red and blue wavelengths)
   • Water is split (photolysis): $2H_2O \rightarrow 4H^+ + 4e^- + O_2$
   • ATP is generated (photophosphorylation)
   • NADPH is produced (electron carrier)

2. Light-independent reactions (stroma):

   • CO₂ is fixed by RuBisCO enzyme
   • Called the Calvin Cycle
   • Uses ATP and NADPH to produce glucose

Factors Affecting Photosynthesis:

• Light intensity (increases rate until saturation point)
• CO₂ concentration (increases rate up to a limit)
• Temperature (peaks at optimum ~25-30°C for most plants)
• Water availability (affects stomatal opening)

⚡ WAEC Tip: If asked about factors affecting photosynthesis, always mention the limiting factor — the one in shortest supply that prevents further increase. In dense Nigerian forests, light is often limiting; in deserts, water is limiting.

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🟡 Standard — Regular Study (2d–2mo)

For students who want genuine understanding of nutrition.

Mineral Nutrition in Plants:

Mineral	Function	Deficiency Symptom
Nitrogen (N)	Leaf growth, chlorophyll synthesis	Yellowing (chlorosis) of older leaves
Phosphorus (P)	Root development, energy transfer	Stunted growth, purple leaves
Potassium (K)	Flower/fruit development, enzyme activation	Brown leaf edges
Magnesium (Mg)	Chlorophyll component	Interveinal chlorosis
Iron (Fe)	Chlorophyll synthesis	Young leaves turn yellow
Calcium (Ca)	Cell wall stability	Deformed new leaves

How Plants Absorb Minerals:

• Active transport (requires ATP) via root hair cells
• Root pressure pushes water up
• Transpiration pull creates negative pressure in xylem
• Mass flow: minerals dissolve in water and move with it

Heterotrophic Modes:

1. Holozoic Nutrition (Animals):

• Ingestion: Taking food into body
• Digestion: Breaking down food (mechanical + chemical)
• Absorption: Nutrients into bloodstream
• Assimilation: Nutrients used for growth, repair, energy
• Egestion: Removing undigested waste

2. Saprophytic Nutrition (Fungi):

• secrete enzymes externally
• enzymes digest dead organic matter
• absorption of soluble products
• Examples: Mucor, Rhizopus (bread mould), Agaricus (mushroom)

3. Parasitic Nutrition:

• Live on/in host
• Absorb nutrients directly from host’s body
• Often reduce host’s fitness
• Examples: Tapeworm (Taenia saginata in cattle, Taenia solium in pigs), malaria parasite (Plasmodium falciparum)

Food and Nutrient Requirements:

Essential Nutrients:

• Carbohydrates: Main energy source (4 kcal/g). Sources in Nigeria: yam, rice, maize, cassava, plantain
• Proteins: Growth and repair (4 kcal/g). Sources: beans, fish, meat, eggs, groundnuts
• Fats: Energy storage, insulation (9 kcal/g). Sources: palm oil, groundnut oil, coconut
• Vitamins: Organic compounds needed in small amounts
• Minerals: Inorganic ions needed for specific functions
• Water: Essential for all metabolic reactions

Vitamins:

Vitamin	Function	Source	Deficiency
A	Vision, immune system	Carrots, palm oil	Night blindness
B1 (Thiamine)	Energy metabolism	Rice, beans	Beriberi
C	Collagen synthesis, immune	Citrus fruits, guava	Scurvy
D	Calcium absorption	Sunlight, fish	Rickets
K	Blood clotting	Green leafy vegetables	Excessive bleeding

Mineral Salts:

Mineral	Function
Iron	Haemoglobin (oxygen transport)
Calcium	Bones, teeth, muscle contraction
Iodine	Thyroid hormones
Sodium	Nerve impulse transmission

⚡ Common Student Mistakes: Students confuse autotrophic with heterotrophic. Autotrophs make their own food; heterotrophs consume others. Also confuse photosynthesis with respiration — photosynthesis MAKES glucose (stores energy); respiration BREAKS DOWN glucose (releases energy). Photosynthesis occurs in chloroplasts; respiration occurs in mitochondria.

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🔴 Extended — Deep Study (3mo+)

Comprehensive theory for thorough preparation.

The Calvin Cycle (C₃ Pathway):

1. Carbon Fixation: CO₂ + RuBP → 2 × 3-phosphoglycerate (3-PGA)

   • Enzyme: RuBisCO (ribulose-1,5-bisphosphate carboxylase/oxygenase)

2. Reduction: 2 × 3-PGA + 2 ATP + 2 NADPH → 2 × glyceraldehyde-3-phosphate (G3P)

3. Regeneration: G3P + ATP → RuBP

Net: 3 CO₂ + 6 NADPH + 9 ATP → G3P (half glucose molecule)

C₄ and CAM Plants:

C₄ Plants (maize, sugar cane, millet):

• Fix CO₂ into 4-carbon compound (oxaloacetate) in mesophyll cells
• Transported to bundle sheath cells where CO₂ is released
• Reduces photorespiration
• More efficient in hot, bright conditions
• Common in Nigeria’s savanna regions

CAM Plants (cacti, pineapple):

• Crassulacean Acid Metabolism
• Fix CO₂ at night → stored as malic acid
• Release CO₂ during day for photosynthesis
• Reduces water loss (stomata open at night)
• Adaptations to arid environments

Chemosynthesis: Organisms that use chemical energy to make food:

• Nitrifying bacteria: $2NH_3 + 3O_2 \rightarrow 2HNO_2 + 2H_2O$ (Nitrosomonas)
• $2HNO_2 + O_2 \rightarrow 2HNO_3$ (Nitrobacter)
• Sulfur bacteria: $H_2S + \frac{1}{2}O_2 \rightarrow H_2O + S$

Energy and Food Production:

Energy Release in Cells (Aerobic Respiration): $$C_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O + \text{energy (ATP)}$$

• Occurs in mitochondria
• 38 ATP produced per glucose (theoretical yield)
• Actual yield ~30-32 ATP (some lost)

Anaerobic Respiration (Fermentation):

In plants and yeast: $$C_6H_{12}O_6 \rightarrow 2C_2H_5OH + 2CO_2 + \text{energy (2 ATP)}$$

In muscles (animals): $$C_6H_{12}O_6 \rightarrow 2\text{lactic acid} + \text{energy (2 ATP)}$$

Photosynthesis vs Respiration:

Feature	Photosynthesis	Respiration
Location	Chloroplasts	Mitochondria
Raw materials	CO₂ + H₂O	Glucose + O₂
Products	Glucose + O₂	CO₂ + H₂O
Energy	Stores energy	Releases energy
Role	Anabolic (builds)	Catabolic (breaks down)
Equation	6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂	C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O

Gaseous Exchange:

Gas	Photosynthesis	Respiration
CO₂	Absorbed	Released
O₂	Released	Absorbed

Balanced Ecosystem: In a balanced ecosystem, the CO₂ produced by respiration equals CO₂ used in photosynthesis, and O₂ used in respiration equals O₂ produced by photosynthesis.

Nitrogen Cycle:

1. Nitrogen fixation: N₂ → NH₃ (by Azotobacter in soil, Rhizobium in legume nodules)
2. Nitrification: NH₃ → NO₂⁻ → NO₃⁻ (by Nitrosomonas and Nitrobacter)
3. Assimilation: Plants absorb NO₃⁻ → used to make amino acids, proteins
4. Ammonification: Dead organisms → NH₃ (by decomposers)
5. Denitrification: NO₃⁻ → N₂ (by Pseudomonas in anaerobic conditions)

Food Security in Nigeria:

• Staple foods: Yam, cassava, rice, maize, sorghum, millet
• Protein sources: Fish (coastal communities), beans, groundnuts
• Nigeria’s Challenge: Population growing faster than food production
• Solutions: Improved seeds, irrigation, reduced post-harvest losses, agricultural education

⚡ WAEC Examination Patterns: Write the word and balanced chemical equations for photosynthesis. Explain the two stages of photosynthesis. Describe how to test a leaf for starch. Explain factors affecting photosynthesis rate. Differentiate between autotrophic and heterotrophic nutrition. Draw the nitrogen cycle. Describe the carbon cycle.