Photosynthesis

Photosynthesis is the process by which plants use sunlight, water, and carbon dioxide to produce glucose and oxygen.

Definition

Photosynthesis is the process by which green plants, algae, and some bacteria convert light energy — usually from the sun — into chemical energy stored as glucose. The process takes place mainly in the chloroplasts of plant cells, which contain the green pigment chlorophyll that absorbs light. Carbon dioxide and water are the raw materials; glucose and oxygen are the products.

The overall equation for photosynthesis is:

$\text{carbon dioxide} + \text{water} \xrightarrow{\text{light energy}} \text{glucose} + \text{oxygen}$

$6\text{CO}_2 + 6\text{H}_2\text{O} \xrightarrow{\text{light}} \text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2$

Key Characteristics

Occurs in chloroplasts, which contain the pigment chlorophyll
Requires light energy, carbon dioxide (from the air), and water (from the soil)
Produces glucose (used for energy and building materials) and oxygen (released as a by-product)
Rate of photosynthesis is affected by light intensity, CO₂ concentration, and temperature
Takes place in two stages: the light-dependent reactions (in the thylakoid membranes) and the light-independent reactions / Calvin cycle (in the stroma)

Real-World Analogy

Photosynthesis is like a solar-powered food factory. The leaf is the factory building, chlorophyll molecules are the solar panels collecting light energy, and CO₂ and water are the raw materials delivered to the factory. The finished product — glucose — is the food that powers all life in the plant.

Note

Photosynthesis is the entry point for almost all energy in Earth's ecosystems. Plants use light energy to build glucose from CO₂ and water, releasing oxygen as a by-product. Every animal ultimately depends on photosynthesis for its food and for the oxygen it breathes.

Worked Examples

Measuring photosynthesis rate

A student places pondweed (Elodea) in a beaker of water under a lamp. As the lamp moves closer, the pondweed produces more oxygen bubbles per minute. This shows that increasing light intensity increases the rate of photosynthesis — up to the point where another factor (CO₂ or temperature) becomes limiting.

Variegated leaves and photosynthesis

A variegated leaf has green and white areas. If tested with iodine solution after a period in light, only the green areas turn blue-black (indicating starch), because only those areas contain chlorophyll and can carry out photosynthesis.

Practice Problems

Write the word equation for photosynthesis.

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Carbon dioxide + water → glucose + oxygen (in the presence of light energy)

Where exactly in a plant cell does photosynthesis occur, and what pigment is responsible for absorbing light?

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Photosynthesis occurs in the chloroplasts. The pigment responsible for absorbing light is chlorophyll.

Name three factors that can limit the rate of photosynthesis.

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Light intensity, carbon dioxide concentration, and temperature are the three main limiting factors.

Why does removing the leaves from a plant cause it to eventually die, even if its roots still have access to water and minerals?

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Leaves are the main sites of photosynthesis. Without leaves, the plant cannot produce glucose, so it has no energy source for growth, reproduction, or cellular respiration. The plant starves.