Photosynthetic Reaction

The equation, the process, and the ten-second strategy for the photosynthesis question that almost every Paper 1 includes.

SPEC 4.4.1.1 • MODULE 4 • PAPER 1

The Equation Question Almost Every Paper Includes

Photosynthesis equation questions appear on almost every Paper 1. Often they're "complete the equation" style — the paper pre-fills some formulae and you supply the rest. They look easy. They aren't.

1

Typical Mark

2

Halves To Get Right

4

Chemical Symbols To Recognise

The Trap

Mark schemes typically award the mark only if both sides are correct. Filling in H₂O and CO₂ on the left but forgetting O₂ on the right — or vice versa — scores zero. By the end of this lesson, you'll never lose that mark.

What You'll Learn

By the end of this lesson, you'll be able to:

Objective 1

Write the word and symbol equations for photosynthesis — both sides, balanced, in the correct mark-scheme order.

Objective 2

Identify photosynthesis as endothermic and explain it using the exact spec wording: "energy is transferred from the environment to the chloroplasts by light".

Objective 3

State where photosynthesis happens in the plant — chloroplasts, palisade mesophyll, chlorophyll — and what each part does.

What Photosynthesis Is

Photosynthesis

The reaction in which plants make their own food (glucose) using carbon dioxide and water, with light energy from the Sun absorbed by chlorophyll.

Photosynthesis powers almost every food chain on Earth. Plants don't take in food — they build it from CO₂ + H₂O using light energy.

Inputs (blue) → reactants enter the leaf. Outputs (green) → products produced by photosynthesis. CO₂ enters via stomata; H₂O travels up the xylem from the roots; O₂ exits via stomata; glucose is made in the leaf and stored or transported around the plant.

INPUTS (REACTANTS)

Carbon dioxide — from the air, enters via stomata

Water — absorbed from soil by roots, transported up xylem

Light energy — absorbed by chlorophyll

OUTPUTS (PRODUCTS)

Glucose (C₆H₁₂O₆) — used for respiration, stored as starch

Oxygen (O₂) — released into the air through the stomata

The Word Equation

Memorise this exactly. The word equation is the safer one to write because spelling errors are forgiven but missing reactants and products are not.

carbon dioxide + water → glucose + oxygen light energy absorbed by chlorophyll (above the arrow)

Three Common Slips

Putting glucose on the wrong side — it's a product, on the right.

Writing "sugar" instead of "glucose" — "glucose" is the preferred mark-scheme term.

Forgetting oxygen is a product, not a reactant.

The Symbol Equation

The spec specifically requires students to recognise the chemical symbols CO₂, H₂O, O₂ and C₆H₁₂O₆.

6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂ Six CO₂ + six H₂O → one glucose + six O₂

Mark-Scheme Pattern

Across past mark schemes, H₂O and CO₂ are accepted in either order on the left. O₂ must appear on the right. The mark is only awarded when both sides are correct.

Memory Hook

"6 carbon dioxide and 6 water make 1 glucose and 6 oxygen." The 6s are everywhere except glucose, which has its own 6 hidden inside (C₆H₁₂O₆).

Photosynthesis is Endothermic

Endothermic means energy is transferred from the environment to the reaction — the reaction takes in energy from its surroundings.

Spec Wording — Memorise Verbatim

"Energy is transferred from the environment to the chloroplasts by light."

Photosynthesis

Endothermic — energy is taken in. Light energy is stored in the chemical bonds of glucose.

Respiration (Lesson 4.04)

Exothermic — energy is released from glucose bonds. The reverse direction.

Energy Verb Trap

Never write "energy is produced" or "energy is made". Neither endothermic nor exothermic reactions create energy. Use the directional verbs: transferred / taken in / absorbed for photosynthesis; released / transferred for respiration.

Where Photosynthesis Happens

Photosynthesis takes place in chloroplasts, found mainly in the leaves of the plant. The spec explicitly links this content to 4.2.3 Plant tissues, the leaf — examiners can test the two together.

Leaf cross-section — palisade mesophyll cells (upper) are packed with chloroplasts and are the main site of photosynthesis. Stomata on the lower epidermis let CO₂ in.

Chloroplast

The sub-cellular structure (organelle) where the reaction happens. Contains chlorophyll.

Chlorophyll

The green pigment inside the chloroplast that absorbs light energy.

A chloroplast is an organelle. Chlorophyll is the green pigment inside the chloroplast — chloroplasts contain it; chlorophyll absorbs the light.

Chloroplast vs Chlorophyll

Mark schemes are strict on this distinction. Chloroplasts contain chlorophyll; it's the chlorophyll molecule that absorbs light — not the chloroplast itself. Write "chlorophyll absorbs light", never "chloroplast absorbs light".

Synoptic Link — 4.2.3 Plant Tissues

The spec mandates a link to plant tissues. From Module 2 you should know: palisade mesophyll cells in the upper leaf are packed with chloroplasts and are the main site of photosynthesis. CO₂ enters via stomata (on the underside) and water arrives via the xylem. Questions can synthesise both topics.

What Plants Need

For photosynthesis to happen at a reasonable rate, the plant needs four things. (Lesson 4.02 covers what happens when these run short — limiting factors.)

RequirementSource / Why

Carbon dioxideFrom the air, enters the leaf through stomata on the underside of the leaf. A reactant in the equation.

WaterAbsorbed from soil by root hair cells , transported up the stem through xylem to the leaves. A reactant in the equation.

Light energyFrom the Sun (or any light source), absorbed by chlorophyll . Provides the energy for the endothermic reaction.

Suitable temperaturePhotosynthesis is enzyme-controlled. Too cold → enzymes work too slowly. Too hot → enzymes denature .

Practice — PEE Worked Example

Explain why photosynthesis is described as an endothermic reaction.3 marks · AO1

P

Point: Photosynthesis is endothermic because energy is transferred from the environment to the reaction.

E

Evidence: The energy comes from light, which is absorbed by chlorophyll in the chloroplasts.

E

Explain: The light energy is then transferred to the chemical bonds in the glucose and oxygen produced — so the products store more energy than the reactants. That's the defining feature of an endothermic reaction.

⚠️

Mark-loss check: Writing "the plant takes in energy" or "absorbs energy from the sun" misses the spec verb. Use "transferred" from the environment to the chloroplasts. The bonds-store-more-energy point is what separates 2/3 from 3/3.

The Top 3 Mark-Losers

These three are the recurring reasons students lose marks on photosynthesis questions, according to AQA examiner reports across multiple series.

1. Unbalanced Equation

Writing "CO₂ + H₂O → glucose + O₂" without the 6s. 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂ is the credit-worthy form.

2. Chloroplast vs Chlorophyll

"Chloroplast absorbs light" is wrong. Chloroplasts contain chlorophyll; it's the chlorophyll molecule that absorbs. Mark schemes test this distinction.

3. "Produces Energy"

Photosynthesis is endothermic — energy is taken in from light. Don't write "produces energy" — neither endothermic nor exothermic reactions create energy. Energy is transferred.

Wrong vs Right

Four exact swaps that turn 0 marks into full marks. Memorise the right column.

✗ Wrong

"CO₂ + H₂O → glucose"

Not balanced. Oxygen missing.

✓ Right

"6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂"

Balanced. Both sides correct.

✗ Wrong

"The chloroplast absorbs light."

Wrong subject — chloroplast is the organelle.

✓ Right

"Chlorophyll (in chloroplasts) absorbs light."

The pigment absorbs; the organelle contains it.

✗ Wrong

"Photosynthesis produces energy."

Energy isn't created.

✓ Right

"Photosynthesis is endothermic — energy is taken in / transferred from light."

Directional verb. Spec wording.

✗ Wrong

"Oxygen is a reactant."

Side confusion — basic error.

✓ Right

"Oxygen is a product — released by the plant through stomata."

Right side of the arrow.

✗ Wrong

"Photosynthesis happens at night."

Common misconception — light is a requirement.

✓ Right

"Photosynthesis requires light — it happens during the day only."

No light → no photosynthesis.

✗ Wrong

"Plants don't respire."

Confuses respiration with breathing.

✓ Right

"Plants respire continuously AND photosynthesise during the day."

Two separate processes, both happening.

Two Examiner Alerts

Two pieces of mark-scheme intelligence that protect easy marks on this lesson's content.

Alert 1 — Use The Spec Verb "Transferred"

The spec describes photosynthesis as an endothermic reaction in which "energy is transferred from the environment to the chloroplasts by light". That's the credit-worthy wording. Loose phrasing like "the plant takes in energy" or "absorbs energy from the sun" may score partial credit but doesn't guarantee the mark. Transferred is the spec's directional verb of choice — used consistently across the energy-stores topics.

Alert 2 — Write CO₂ With The Subscript

Examiner reports flag students losing credit for writing "CO2", "Co2", or "CO2" (no subscript) on the first instance — these are penalised. Two fixes: (1) type/write with the proper subscript 2, or (2) write "carbon dioxide" in full and avoid the issue. The same logic applies to H₂O and O₂.

Carry This Forward

In Lesson 4.04 you'll meet the matching verb for respiration: "energy is released by aerobic respiration in the mitochondria". Same verb family — transferred / released — never produced / made / created.

Three Takeaways

Recap. Three things to lock in before Lesson 4.02.

The Equation

Word: carbon dioxide + water → glucose + oxygen (light + chlorophyll above arrow). Symbol: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂.

Endothermic

Energy transferred from the environment to the chloroplasts by light. Stored in glucose bonds. Use "taken in / absorbed / transferred" — never "produced".

Location

Chloroplasts contain chlorophyll — the green pigment that absorbs the light. Mainly in palisade mesophyll cells of leaves (spec links to 4.2.3).

You're Now Set For

Any "complete the equation" question on Paper 1. Any "explain endothermic" question. Any "where does photosynthesis happen" question. That's three Module 4 mark-scheme points covered in 10 minutes.

Lesson 4.01 Summary

Key Facts

Photosynthesis = the reaction in which plants make glucose using CO₂ and H₂O, with light energy absorbed by chlorophyll.

Word equation: carbon dioxide + water → glucose + oxygen.

Symbol equation: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂.

Endothermic — energy is transferred from the environment to the chloroplasts by light .

Location: chloroplasts in palisade mesophyll cells; chlorophyll is the green pigment that absorbs light.

Plants need: CO₂ (stomata) + water (roots/xylem) + light (chlorophyll) + suitable temperature (for enzymes).

The Three Mark-Losers (Avoid These)

Unbalanced equation — always include the 6 s.

Chlorophyll absorbs light, not chloroplast.

Endothermic reactions take in energy — never "produce" it.

Spec Reference & Synoptic Link

Spec 4.4.1.1 Photosynthetic reaction — Paper 1. The spec explicitly links this content to 4.2.3 Plant tissues, the leaf — expect synoptic questions combining the equation with where in the leaf it happens (palisade mesophyll, stomata, xylem). Required practical RP6 builds on this in Lesson 4.02.

Next: Lesson 4.02

You've nailed what photosynthesis is. Next lesson, we look at what makes it go faster or slower.

Coming Up — Rate of Photosynthesis & Limiting Factors

Light intensity, CO₂ concentration, temperature, chlorophyll amount — the four factors that limit how fast plants photosynthesise. Plus the "explain the graph" question type that's a known high-failure category on Paper 1 — we'll cover exactly how to attack it.

Before Lesson 4.02

Make sure you can write the symbol equation from memory. Take a piece of paper, hide this slide, and write it out. If both halves are right, you've banked the 4.01 mark for life.