IP Combined Science Notes (Lower Sec, Year 1-2): 05) Photosynthesis, Nutrition & Cellular Respiration
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Energy flow in living systems depends on both photosynthesis (energy capture) and respiration (energy release). Pair these processes with nutrition to explain growth, maintenance, and activity.
Learning targets
- State balanced equations for photosynthesis and aerobic/anaerobic respiration.
- Explain limiting factors of photosynthesis with experimental evidence.
- Describe components of a balanced human diet and consequences of deficiency/excess.
- Interpret calorimetry experiments and calculate energy released per unit mass.
1. Core equations
| Process | Balanced equation | Notes |
| Photosynthesis | \( \ce{6CO2 + 6H2O ->[light][chlorophyll] C6H12O6 + 6O2} \) | Occurs in chloroplasts; stores light energy as chemical energy. |
| Aerobic respiration (plants & animals) | \( \ce{C6H12O6 + 6O2 -> 6CO2 + 6H2O + Energy} \) | Releases \( \pu{~ 2900 kJ} \) per \( \pu{mol} \) glucose. |
| Anaerobic respiration (in muscles) | \( \ce{C6H12O6 -> 2C3H6O3 + Energy} \) | Produces lactic acid; lower energy yield. |
| Anaerobic respiration (yeast) | \( \ce{C6H12O6 -> 2C2H5OH + 2CO2 + Energy} \) | Basis of fermentation. |
2. Limiting factors of photosynthesis
Common investigations use aquatic plants to measure oxygen output (bubble count or dissolved oxygen). Key factors:
- Light intensity (distance from lamp, use neutral density filters).
- Carbon dioxide concentration (adding \( \ce{NaHCO3} \)).
- Temperature (water bath control).
Worked example — Light intensity analysis
If bubble count data show rate increasing with light intensity up to \( \pu{1200 lx} \) then plateauing, conclude light is limiting below \( \pu{1200 lx} \). Beyond that, another factor (likely \( \ce{CO2} \) or temperature) becomes limiting.
To quantify, plot \( \text{rate} \) vs \( \dfrac{1}{\text{distance}^2} \) because light intensity \( \propto \dfrac{1}{d^2} \).
3. Human nutrition basics
| Nutrient | Function | Deficiency / Excess |
| Carbohydrates | Primary energy source. | Low: fatigue. Excess: obesity. |
| Proteins | Growth, repair, enzyme production. | Deficiency: kwashiorkor. |
| Fats | Energy storage, insulation. | Excess increases cardiovascular risk. |
| Vitamins (e.g. C, D) | Co-factors for metabolic reactions. | Lack of vitamin C causes scurvy. |
| Minerals (e.g. iron, calcium) | Components of haemoglobin/bones. | Iron deficiency causes anaemia. |
| Water | Transport medium, temperature regulation. | Dehydration impairs enzyme function. |
| Fibre | Maintains bowel health. | Low fibre causes constipation. |
4. Calorimetry & energy values
Worked example — Food energy calculation
A peanut (mass \( \pu{1.20 g} \)) is burned under \( \pu{100 g} \) water. Temperature rises from \( \pu{24 ^\circ C} \) to \( \pu{58 ^\circ C} \).
- Heat gained by water:
\[ Q = mc\Delta T = 0.100 \times 4.18 \times (58 - 24) = 14.3 , \pu{kJ}. \]
- Energy per gram:
\[ E = \frac{14.3}{1.20} = 11.9 , \pu{kJ.g-1}. \]
Discuss energy losses (e.g. heat lost to surroundings) and suggest improvements (insulation, using a bomb calorimeter).
Try it yourself
- State two structural adaptations of chloroplasts that support photosynthesis.
- Explain why athletes breathe heavily after a sprint in terms of oxygen debt and lactic acid removal.
- A calorimetry experiment uses \( \pu{50.0 g} \) water and records a \( \pu{15.0 ^\circ C} \) temperature increase when \( \pu{0.85 g} \) of food is burned. Calculate the energy content per \( \pu{100 g} \) of the food.
Continue with transport systems at https://eclatinstitute.sg/blog/ip-combined-sciences-lower-sec-notes/IP-Combined-Science-Lower-Sec-06-Transport-Systems-in-Humans-and-Plants.
