Photosynthesis

Photosynthesis converts carbon dioxide and water into glucose and oxygen using light energy in the chloroplast. Light reactions on the thylakoid make ATP, NADPH, and O₂, and the Calvin cycle fixes CO₂ in the stroma. Rate rises with light intensity until the light saturation point, after which it no longer increases. Slide light intensity and watch how the light reactions and photosynthetic rate change.

Making food from light

🌿 Photosynthesis = solar energy into glucose

①Plants convert light energy into chemical energy (glucose)

②Location: chloroplast (chlorophyll absorbs light)

③Two stages: light reactions (need light) + Calvin cycle (CO₂ fixation)

Visualize photosynthesis

Light intensity5

💡 Try changing light intensity

①Stronger light → more active light reactions → more ATP, NADPH

②But beyond a point, no further increase → "light saturation point"

③After saturation, CO₂ or temperature becomes the limiting factor

Core formula

Photosynthesis overall reaction

6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂

light energy converts CO₂ and water into glucose and O₂

Photosynthesis stages

ChartLight reactions vs Calvin cycle

Item	Light reactions	Calvin cycle
Location	thylakoid membrane	stroma
Light needed	✓ (required)	✗ (not directly)
Inputs	H₂O, light	CO₂, ATP, NADPH
Products	ATP, NADPH, O₂	G3P (→ glucose)
Reaction	photophosphorylation, water split	CO₂ fixation, carbon reduction

Photosynthesis vs Cellular Respiration

Photosynthesis vs Respiration

ChartComparison

Item	Photosynthesis	Respiration
Location	chloroplast	mitochondria
Reaction	CO₂+H₂O → glucose+O₂	glucose+O₂ → CO₂+H₂O
Energy	light → chemical	chemical → ATP
Organisms	plants, algae	almost all organisms

Worked Examples and Exam Practice

Example 1

Inside the chloroplast, where do the light reactions and the Calvin cycle each take place?

The light reactions occur in the thylakoid membrane, where the light-absorbing pigments are packed.

The Calvin cycle occurs in the stroma, the fluid surrounding the thylakoids.

▸ Light reactions: thylakoid membrane / Calvin cycle: stroma

The light reactions run on the membrane (thylakoid); the Calvin cycle in the fluid (stroma). The ATP and NADPH from the light reactions fuel the Calvin cycle.

Example 2

A plant is lit so that its apparent gas exchange is zero (apparent photosynthesis = 0). How do true photosynthesis and respiration compare here?

Apparent photosynthesis = true photosynthesis − respiration.

If apparent photosynthesis is 0, then true photosynthesis = respiration; this light level is the light compensation point.

▸ Photosynthesis = respiration (light compensation point)

At the compensation point, respiration uses up all the O₂ that photosynthesis makes, so the net change is zero. Any darker and respiration dominates.

CSAT-style

Which statement about plant photosynthesis is correct?

①The light reactions occur in the stroma

②Water is split in the light reactions, releasing O₂

③The Calvin cycle proceeds only when light is directly present

④Glucose is a direct product of the light reactions

⑤Above the light saturation point, stronger light keeps raising the rate

▸ ② Water is split in the light reactions, releasing O₂

In the light reactions, water (H₂O) is split, releasing oxygen (O₂) and forming ATP and NADPH.

The light reactions are in the thylakoid membrane (①), the Calvin cycle needs no direct light (③), glucose is a Calvin-cycle product (④), and past saturation more light does not raise the rate (⑤).

Summary

Photosynthesis core

6CO₂ + 6H₂O + light energy → C₆H₁₂O₆ + 6O₂

chloroplast converts light energy to chemical energy

🎯 Exam Points

①Light reactions: thylakoid, water split (H₂O → O₂), produces ATP/NADPH

②Calvin cycle: stroma, CO₂ fixation, makes G3P (glucose precursor)

③Light saturation: more light no longer increases rate

④Compensation point: photosynthesis rate = respiration rate (apparent O₂ change = 0)

⑤Limiting factors: light intensity, CO₂, temperature

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Cellular Respiration

DNA Replication & Gene Expression

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