# Photosynthesis

> IB Biology SL · IB DP Biology SL
> Source: https://www.owlsprep.com/study/ib-biology-sl-u3-photosynthesis/

This module breaks down the two core stages of photosynthesis, key reactants and products, limiting factor dynamics, and standard experimental protocols tested in IB SL Biology assessments.

**Prerequisites:** [Chloroplast ultra-structure and function](https://www.owlsprep.com/study/ib-biology-sl-u2-chloroplast-structure/); [Properties of light and plant pigments](https://www.owlsprep.com/study/ib-biology-sl-u3-pigment-absorption/)

## Learning objectives

- Define photosynthesis as the conversion of light energy to stored chemical energy in organic molecules
- Distinguish between the locations, inputs and outputs of light-dependent and light-independent reactions
- Explain how limiting factors alter the rate of photosynthesis for C3 plants
- Outline standard experimental methods used to measure photosynthesis rate for IB SL assessments

## Photosynthesis Core Overview

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.

**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

**Check your understanding**

1. What energy transformation occurs during photosynthesis?

   - Chemical to light
   - Light to chemical
   - Heat to electrical
   - Electrical to heat

   *Why:* Photosynthesis captures light photon energy and stores it in the bonds of glucose

## Light-Dependent Reactions

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

> **mnemonic**
>
> Use the phrase 'LDR = Light, Water, Make ATP/NADPH' to remember the core inputs and outputs of this stage without mixing them up with the Calvin cycle.

**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.

## Light-Independent Reactions (Calvin Cycle)

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.

**Carbon fixation** — The attachment of inorganic CO2 molecules to 5-carbon RuBP molecules to form 3-carbon glycerate 3-phosphate

*Notation:* Rubisco enzyme

**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

## Limiting Factors of Photosynthesis

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 |

**Exam command terms**

- **Outline the effect** — Describe the general trend only, no full causal explanation required

- **Evaluate the data** — Identify which factor is limiting, and suggest why the trend deviates from expected values

**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

## Common pitfalls

- **Wrong:** Stating the light independent reaction requires total darkness to proceed
  - Why it fails: 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: Clarify the light independent reaction does not require direct light, not that it cannot occur in light conditions
- **Wrong:** Claiming green light is the most strongly absorbed wavelength by chlorophyll
  - Why it fails: Chlorophyll reflects almost all green light, which is why most plant leaves appear green to human eyes
  - Correct: State red and blue wavelengths are the most efficiently absorbed to drive photosynthesis
- **Wrong:** Writing that increasing temperature will always increase the photosynthesis rate
  - Why it fails: Above 35°C, rubisco denatures rapidly and the reaction rate falls to near zero
  - Correct: Note temperature only increases rate up to the enzyme's optimal value, after which rate declines sharply
- **Wrong:** Using the full 12 water molecule photolysis equation in SL exam responses
  - Why it fails: IB SL mark schemes only award marks for the standard net 6 water molecule balanced equation
  - Correct: Use the simplified net photosynthesis equation unless explicitly asked to describe photolysis details
- **Wrong:** Forgetting to control CO2 levels when measuring photosynthesis rate via oxygen bubble count
  - Why it fails: Dissolved CO2 is a separate limiting factor that can skew results and invalidate experimental conclusions
  - Correct: Standardize all variables other than your independent variable for all photosynthesis practicals

## 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 |

## 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.

---

From [OwlsPrep](https://www.owlsprep.com) — free study guides for A-Level, IB, AP and IGCSE, written against the official syllabus. Canonical page: https://www.owlsprep.com/study/ib-biology-sl-u3-photosynthesis/