Photosynthesis

IB Biology SL· 25 min read

1. Photosynthesis Core Overview★★☆☆☆⏱ 5 min

Photosynthesis is the anabolic process used by plants, algae and cyanobacteria to convert light energy from the sun into chemical energy stored in carbohydrate molecules. All heterotrophic life on Earth relies directly or indirectly on this reaction for food supply.

📘 Definition

Net Photosynthesis Equation

The simplified balanced summary of all inputs and outputs of the full photosynthesis pathway

6CO_2 + 6H_2O \xrightarrow{light} C_6H_{12}O_6 + 6O_2

📐 Worked Example

Identify which of the following molecules is the source of the oxygen gas released during photosynthesis: carbon dioxide, water, glucose, or chlorophyll

1
Recall the process of photolysis in the light-dependent reaction
2
Water molecules are split to release electrons, hydrogen ions, and pure oxygen as a waste product
3
No oxygen from CO2 is released, as all carbon and oxygen from CO2 is incorporated into glucose in the Calvin cycle
4
The correct answer is water

✓ Quick check

What energy transformation occurs during photosynthesis?
- Chemical to light
- Light to chemical
- Heat to electrical
- Electrical to heat
Reveal answer
Light to chemical —
Photosynthesis captures light photon energy and stores it in the bonds of glucose

2. Light-Dependent Reactions★★★☆☆⏱ 7 min

This first stage of photosynthesis takes place across the thylakoid membranes of the chloroplast, where chlorophyll pigments absorb light energy to excite electrons. Excited electrons move down the electron transport chain to generate ATP via chemiosmosis.

Chlorophyll absorbs red and blue light photons to excite electrons
Water is split via photolysis to replace lost electrons, releasing oxygen
Excited electrons pass through the electron transport chain to pump protons
Proton gradient drives ATP synthesis, and electrons are used to reduce NADP+ to NADPH

📐 Worked Example

Explain why a decrease in light intensity will immediately stop the production of NADPH in isolated chloroplasts

1
NADPH is only produced when excited electrons from chlorophyll are available to reduce NADP+
2
No light means no photons are absorbed by chlorophyll to excite electrons
3
No new electrons can be generated from photolysis if no energy is available to split water
4
Without a supply of excited electrons, NADP+ cannot be reduced to form NADPH

Exam tip:

IB SL markers will deduct marks if you state oxygen comes from CO2, always link oxygen production directly to photolysis of water.

3. Light-Independent Reactions (Calvin Cycle)★★★☆☆⏱ 7 min

This stage occurs in the fluid stroma of the chloroplast, and does not require direct light to proceed, as long as a supply of ATP and NADPH from the light-dependent reaction is available. The cycle is catalysed by the enzyme rubisco, the most abundant protein on Earth.

📘 Definition

Carbon fixation

Rubisco enzyme

The attachment of inorganic CO2 molecules to 5-carbon RuBP molecules to form 3-carbon glycerate 3-phosphate

📐 Worked Example

A student places a plant in a dark cupboard for 24 hours. Explain why the Calvin cycle stops after a few minutes, even though no light is required for its reactions

1
When the plant is placed in the dark, the light-dependent reaction stops producing new ATP and NADPH
2
Any existing ATP and NADPH in the stroma are quickly consumed by the Calvin cycle to reduce GP to TP
3
Without new ATP and NADPH, the cycle cannot regenerate RuBP to accept new CO2 molecules
4
All reactions of the Calvin cycle halt once the energy carrier supply is exhausted

4. Limiting Factors of Photosynthesis★★★☆☆⏱ 6 min

At any given time, only one factor will be limiting the maximum possible rate of photosynthesis. The three main limiting factors for C3 plants are light intensity, carbon dioxide concentration, and temperature.

Limiting Factor	Effect of increase	Optimum for C3 plants
Light intensity	Raises rate until all chlorophyll is saturated	500-1000 μmol m⁻² s⁻¹
CO2 concentration	Raises rate until rubisco is fully saturated	0.04-0.1%
Temperature	Raises rate to enzyme optimum, then drops	20-25 °C

📐 Worked Example

A plant is in a growth chamber set to 20°C, 0.04% CO2, and 2000 μmol m⁻² s⁻¹ light intensity. Identify the limiting factor and justify your answer

1
Check the values against the optimum ranges for C3 plants
2
Light intensity is far above the saturation threshold, so it cannot be limiting
3
Temperature is at the lower end of the optimum range, but CO2 concentration is at normal atmospheric levels far below the 0.1% optimum
4
Increasing CO2 concentration will raise the photosynthesis rate, so CO2 is the limiting factor

5. Common Pitfalls

Wrong move:

Stating the light independent reaction requires total darkness to proceed

Why:

The LIR does not use light directly, but can run in full light as long as ATP and NADPH are supplied from the LDR

Correct move:

Clarify the light independent reaction does not require direct light, not that it cannot occur in light conditions

Wrong move:

Claiming green light is the most strongly absorbed wavelength by chlorophyll

Why:

Chlorophyll reflects almost all green light, which is why most plant leaves appear green to human eyes

Correct move:

State red and blue wavelengths are the most efficiently absorbed to drive photosynthesis

Wrong move:

Writing that increasing temperature will always increase the photosynthesis rate

Why:

Above 35°C, rubisco denatures rapidly and the reaction rate falls to near zero

Correct move:

Note temperature only increases rate up to the enzyme's optimal value, after which rate declines sharply

Wrong move:

Using the full 12 water molecule photolysis equation in SL exam responses

Why:

IB SL mark schemes only award marks for the standard net 6 water molecule balanced equation

Correct move:

Use the simplified net photosynthesis equation unless explicitly asked to describe photolysis details

Wrong move:

Forgetting to control CO2 levels when measuring photosynthesis rate via oxygen bubble count

Why:

Dissolved CO2 is a separate limiting factor that can skew results and invalidate experimental conclusions

Correct move:

Standardize all variables other than your independent variable for all photosynthesis practicals

6. Quick Reference Cheatsheet

Stage	Location	Key Inputs	Key Outputs
Light Dependent Reaction	Thylakoid membrane	Light, H2O, ADP, NADP+	O2, ATP, NADPH
Light Independent Reaction	Stroma	CO2, ATP, NADPH	Glucose, ADP, NADP+
Limiting Factor	Optimum C3 value	Saturation threshold	Inhibitory threshold
Light intensity	500-1000 μmol m⁻² s⁻¹	1000 μmol m⁻² s⁻¹	2000 μmol m⁻² s⁻¹
CO2 concentration	0.04-0.1%	0.1%	0.15%
Temperature	20-25 °C	25 °C	35 °C

When this came up on past exams

AI-estimated based on syllabus patterns — cross-check with official past papers for accuracy. Use only as revision-focus signals.

2023 · Paper 2
Limiting factors extended response
2022 · Paper 1
Light reaction multiple choice
2021 · Paper 3
Photosynthesis practical analysis

What's Next

Mastering photosynthesis is critical to connecting plant energy production to broader ecosystem interactions, including carbon cycling, food web dynamics, and the impacts of climate change on global primary productivity. This content is a required foundation for understanding cellular respiration, the complementary catabolic pathway that releases stored chemical energy for use by all living organisms. You will frequently see photosynthesis paired with respiration in extended response exam questions that ask you to compare inputs, outputs, and energy transformations across both pathways, or analyze net gas exchange in plants under different light conditions.