🎓 Prepared by students from Boğaziçi University

What Are Chloroplasts and How Does Photosynthesis Work?

Chloroplasts are the organelles in plant and algal cells that capture light energy and convert it into chemical energy through photosynthesis. This process produces glucose and oxygen from carbon dioxide and water.

Short answer

Photosynthesis is the process by which chloroplasts use light energy to convert carbon dioxide and water into glucose and oxygen, occurring in two stages: the light-dependent reactions and the Calvin cycle.

Stages of Photosynthesis

1
Light Absorption
Chlorophyll in the thylakoid membrane absorbs sunlight
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2
Light Reactions
Water splits (releasing O2), producing ATP and NADPH in the thylakoid
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3
Calvin Cycle
In the stroma, ATP and NADPH power CO2 fixation into G3P/glucose
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4
Glucose Formation
G3P molecules combine to build glucose and other sugars

Try it: interactive calculator

Glucose molecules produced

10molecules

= 60*(1/6)

CO2 molecules fixed60 molecules

Step-by-step worked examples

A plant fixes 60 CO2 molecules during the Calvin cycle. How many glucose molecules can it produce (6 CO2 → 1 glucose)?

Glucose = CO2 ÷ 6
Glucose = 60 ÷ 6 = 10 glucose molecules

For every glucose molecule made, 6 O2 molecules are released. How much O2 is released when a leaf makes 15 glucose molecules?

O2 = glucose × 6
O2 = 15 × 6 = 90 O2 molecules

A greenhouse increases light intensity so a plant's photosynthesis rate doubles from 20 to 40 CO2 molecules fixed per minute. How many more glucose molecules form per minute?

Glucose before = 20 ÷ 6 ≈ 3.33
Glucose after = 40 ÷ 6 ≈ 6.67
Extra glucose ≈ 6.67 − 3.33 ≈ 3.33 more glucose molecules per minute

Flashcards

Quick quiz

Q1.Where do the light-dependent reactions occur?

Correct answer: B. The light reactions occur in the thylakoid membrane, where chlorophyll captures light.

Q2.What is the overall formula for photosynthesis?

Correct answer: B. Photosynthesis converts CO2 and water into glucose and oxygen using light energy.

Q3.Where does the Calvin cycle take place?

Correct answer: B. The Calvin cycle occurs in the stroma of the chloroplast.

Q4.What produces the oxygen released during photosynthesis?

Correct answer: B. Water molecules are split during the light reactions, releasing O2.

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Common mistakes

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Photosynthesis only happens in the leaves' green color. — Correct: Photosynthesis happens specifically in chloroplasts, which contain chlorophyll (giving the green color).

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Plants only do photosynthesis, never respiration. — Correct: Plants perform both photosynthesis (using light) and cellular respiration (in mitochondria) all the time.

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The Calvin cycle requires direct light. — Correct: The Calvin cycle doesn't need light directly, but it depends on ATP and NADPH made during the light reactions.

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Oxygen released comes from carbon dioxide. — Correct: The released oxygen comes from splitting water molecules, not CO2.

FAQ

What is photosynthesis?

The process chloroplasts use to convert light energy, CO2 and water into glucose and oxygen.

What is the formula for photosynthesis?

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

What are examples of photosynthesis?

A leaf converting sunlight into sugar, or algae in a pond producing oxygen in daylight.

How do you calculate glucose produced from CO2 fixed?

Divide the number of CO2 molecules fixed by 6, since 6 CO2 molecules make 1 glucose molecule.