Q: What does IP Combined Science Notes (Lower Sec, Year 1-2): 05) Photosynthesis, Nutrition & Cellular Respiration cover? A: Balance key bioenergetic equations, analyse nutrition experiments, and connect respiration to energy transfer.
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.
These notes align with MOE's Lower Secondary Science syllabus themes commonly taught in IP Sec 1-2, and act as a bridge into upper-secondary Physics, Chemistry, and Biology.
Status: MOE Lower Secondary Science syllabus (current release) checked 2025-11-30 - scope unchanged; remains the reference for these combined science notes.
The core idea is simple: Photosynthesis stores energy; respiration releases energy.
Use it as a working check: Keep the two equations separate, then connect nutrition to what organisms need for growth, repair, movement, and temperature control.
Then go one layer deeper: Example: if bubble count stops increasing when light gets brighter, light is no longer limiting. Carbon dioxide or temperature is probably limiting instead.
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
Photosynthesis - occurs in chloroplasts; stores light energy as chemical energy.
6COX2+6HX2OlightchlorophyllCX6HX12OX6+6OX2
Check this topic from memory
Attempt the matching topic bank before reopening the notes. Use each missed idea to decide what to review next.
Aerobic respiration (plants and animals) - releases approximately 2900kJ per mole of glucose.
CX6HX12OX6+6OX26COX2+6HX2O+Energy
Anaerobic respiration in muscles - produces lactic acid; lower energy yield than aerobic respiration.
CX6HX12OX62CX3HX6OX3+Energy
Anaerobic respiration in yeast - basis of fermentation (brewing, bread-making).
CX6HX12OX62CX2HX5OH+2COX2+Energy
Equation choice checkpoint
Before writing an equation, decide whether the question is about making glucose, releasing energy from glucose, or fermenting without enough oxygen.
Question cue
Process to choose
Key substances to track
Common trap
Plant needs light, chlorophyll, and carbon dioxide
Photosynthesis
Carbon dioxide and water become glucose and oxygen
Writing respiration just because oxygen appears in the sentence.
Cell needs energy for movement, growth, or active transport
Aerobic respiration if oxygen is available
Glucose and oxygen become carbon dioxide, water, and energy
Saying only animals respire.
Muscle works hard and oxygen supply cannot meet demand
Anaerobic respiration in muscles
Glucose becomes lactic acid and releases less energy
Writing ethanol or carbon dioxide as the muscle product.
Yeast makes bread dough rise or ferments sugar
Anaerobic respiration in yeast
Glucose becomes ethanol, carbon dioxide, and energy
Writing lactic acid for yeast.
Worked check: a green plant cell in bright light can photosynthesise and respire at the same time. Photosynthesis makes glucose using light energy, while respiration releases energy from some glucose for cell activities. Do not write that a plant "switches off" respiration in the light.
Misconception check: photosynthesis and respiration are not simply opposites in every detail. They have related overall equations, but they happen in different cell structures and serve different purposes.
Respiration comparison checkpoint
When comparing respiration types, start with oxygen availability, then name the products and energy release. This keeps "respiration" separate from breathing and stops you from mixing muscle and yeast products.
Situation
Oxygen available?
Main products to name
Energy point
Common trap
Normal activity in plants or animals
Yes
Carbon dioxide and water
More energy is released from each glucose molecule.
Saying only animals respire.
Sprinting muscle cells
Not enough for demand
Lactic acid
Less energy is released, so fatigue builds faster.
Writing ethanol for human muscles.
Yeast in fermentation
No
Ethanol and carbon dioxide
Less energy is released than aerobic respiration.
Writing lactic acid for yeast.
Worked check: after a short sprint, a student breathes heavily because muscle cells used some anaerobic respiration when oxygen delivery could not meet demand. Extra oxygen is needed after exercise to help break down lactic acid, so the breathing rate stays high for a while.
Misconception check: breathing is gas movement into and out of the body. Respiration is the chemical process in cells that releases energy from glucose.
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 NaHCOX3).
Temperature (water bath control).
Worked example - Light intensity analysis
If bubble count data show rate increasing with light intensity up to 1200lx then plateauing, conclude light is limiting below 1200lx. Beyond that, another factor (likely COX2 or temperature) becomes limiting.
To quantify, plot rate vs distance21 because light intensity ∝d21.
Photosynthesis experiment checkpoint
For aquatic-plant experiments, separate the variables before interpreting the graph.
Part of setup
What to keep or change
Why it matters
Independent variable
Change one factor only, such as lamp distance or NaHCOX3 concentration.
This makes the rate change traceable to one factor.
Dependent variable
Count bubbles per minute or measure oxygen volume per minute.
This estimates the rate of photosynthesis.
Controlled variables
Keep plant length, temperature, measuring time, and carbon dioxide supply constant unless one is being tested.
Uncontrolled factors can create a false limiting-factor conclusion.
Worked check: if the lamp distance is halved from 40cm to 20cm, the light intensity is about four times greater because intensity follows d21. If bubble count barely increases after that, light is probably no longer the limiting factor.
Common trap: do not conclude "the plant stopped photosynthesising" just because the graph plateaus. A plateau usually means another factor, such as COX2 concentration or temperature, is limiting the rate.
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.
Nutrient-function symptom checkpoint
When a nutrition question gives a symptom, match the symptom to the nutrient's job before naming the deficiency or excess. This keeps diet answers from becoming memorised lists.
Question clue
Nutrient group to consider
Function link to write
Common trap
Tiredness after too little food intake
Carbohydrates or fats
These are energy sources, so too little intake can reduce available energy for activity.
Naming protein first when the clue is mainly energy.
Poor growth or slow repair
Proteins
Proteins are needed to build new tissues and repair damaged tissues.
Saying protein is only an energy source.
Frequent constipation
Fibre and water
Fibre adds bulk to food, while water helps move material through the gut.
Treating fibre as a nutrient that is digested for energy.
Weak bones or poor oxygen transport
Minerals
Calcium is needed for bones; iron is needed for haemoglobin in red blood cells.
Mixing up iron with calcium because both are minerals.
Excess body mass from regular overeating
Carbohydrates and fats
Extra energy intake can be stored by the body, especially when activity is low.
Blaming one food group without comparing intake with energy use.
Worked check: if a student has tiredness, pale skin, and shortness of breath during exercise, connect the clue to iron first. Less iron can mean less haemoglobin, so less oxygen is transported for aerobic respiration.
Misconception check: a balanced diet is not "equal amounts of every nutrient". It means enough of each nutrient for the body's needs, with energy intake matched to activity level.
4 Calorimetry & energy values
Food calorimetry setup checkpoint
Before calculating energy per gram, separate the heat gained by water from the energy content of the food. The experiment estimates food energy indirectly through the water temperature rise.
Calculation step
What to use
Why it matters
Common trap
Heat gained by water
Q=mcΔT using the water mass, water's specific heat capacity, and temperature rise
The thermometer measures the water's temperature change, not the food's temperature.
Using the food mass inside Q=mcΔT.
Unit conversion
Convert g of water to kg if c is in kJ⋅kg−1⋅∘C−1
The mass unit must match the value of c.
Mixing grams with a per-kilogram specific heat capacity.
Energy per gram of food
Divide the heat gained by water by the mass of food burned
This compares different foods fairly.
Dividing by the mass of water instead of the mass of food.
Evaluation
Name where heat was lost or absorbed
The calculated value is usually lower than the true food energy.
Saying the food had less energy without discussing heat loss.
Worked check: if 0.80g of food heats 50.0g of water by 20.0∘C, then the water gains 0.0500×4.18×20.0=4.18kJ. The estimated energy value is 4.18/0.80=5.23kJ⋅g−1.
Misconception check: the water is not the fuel. Use water data to estimate heat transferred, then divide by the food mass to find energy per gram.
Worked example - Food energy calculation
A peanut (mass 1.20g) is burned under 100g water. Temperature rises from 24∘C to 58∘C.
Heat gained by water:
Q=mcΔT=0.100×4.18×(58−24)=14.3,kJ.
Energy per gram:
E=1.2014.3=11.9,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 50.0g water and records a 15.0∘C temperature increase when 0.85g of food is burned. Calculate the energy content per 100g of the food.
