Introduction to Photosynthesis in Green Plants

Photosynthesis is the process by which green plants use light energy to make glucose from carbon dioxide and water, releasing oxygen as a by-product. In brief: the word equation is carbon dioxide + water → glucose + oxygen, and it needs light and chlorophyll. A bean plant placed in a dark cupboard cannot photosynthesize, so it soon turns pale and stops growing normally.

Photosynthesis is the starting point of almost every food chain, and it produces the oxygen that most living things use in respiration. This guide gives a basic overview for beginners: what the process does, why plants need light, water and carbon dioxide, how to test for it, how to set up a worked experiment and which mistakes to avoid.

What is photosynthesis?

Photosynthesis is a chemical process in which energy from sunlight is stored in sugar molecules. The name comes from Greek words meaning “putting together with light”. Plants, algae and some bacteria can do it.

Organisms that photosynthesize are called producers, because they make organic compounds from simple inorganic substances. All other organisms in most ecosystems depend directly or indirectly on the compounds that producers make.

What is the equation for photosynthesis?

The word equation summarizes the overall process:

carbon dioxide + water → glucose + oxygen (in the presence of light and chlorophyll)

The balanced chemical equation is 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂. Count the atoms to check it: on each side there are 6 carbon atoms, 12 hydrogen atoms and 18 oxygen atoms. The actual process has many steps, and this equation shows only the starting materials and the final products.

Light is not a substance in the equation, but it supplies the energy. Aerobic respiration runs in roughly the opposite direction, because it uses glucose and oxygen and releases energy, so photosynthesis and respiration are complementary processes.

Why do plants need light, water, and carbon dioxide?

Light

Light provides the energy that drives the process. Chlorophyll, the green pigment in chloroplasts, absorbs mainly red and blue light and reflects green light, which is why leaves look green. More light usually increases the rate of photosynthesis until another factor becomes limiting.

Water

Water is absorbed by the roots and carried to the leaves in xylem. In the light-dependent part of photosynthesis, water molecules are split, and the oxygen released comes from this water.

Carbon dioxide

Carbon dioxide enters the leaf through stomata. Air contains only a small amount of it, about 0.04 percent by volume, so carbon dioxide often limits the rate of photosynthesis in bright conditions.

Where does photosynthesis take place?

Photosynthesis takes place in chloroplasts, which are small structures found in the cells of leaves and other green tissues. Each chloroplast contains stacks of membranes that hold chlorophyll.

Leaves are built to support this process. They are thin and flat, so light reaches the cells and gases diffuse quickly. The palisade layer near the top holds the most chloroplasts, and air spaces in the spongy layer let carbon dioxide reach the cells.

What happens to the glucose?

The glucose made in photosynthesis is used in several ways, and the plant often converts it to other substances.

• Respiration: Glucose is broken down to release energy for the cell.
• Starch: It is stored as insoluble starch in leaves, roots, seeds and tubers.
• Cellulose: It is used to build cell walls.
• Sucrose: It is a soluble sugar that moves through phloem to other parts of the plant.
• Fats, proteins and other compounds: With minerals such as nitrates from the soil, glucose is used to make amino acids and proteins.

Oxygen is released through the stomata and is used by the plant itself and by other organisms.

Worked example: testing a leaf for starch

Because glucose is often turned into starch, a starch test shows whether photosynthesis has taken place. The question is whether light is needed. Use a plant such as geranium with some leaves covered by black paper and others in the light.

1. Keep the plant in the dark for 24 hours so that stored starch is used up.
2. Cover part of one leaf with black paper and place the plant in bright light for several hours.
3. Remove the leaf and put it in boiling water for about one minute to kill the cells and soften them.
4. Heat the leaf in hot ethanol in a water bath, never over a flame, to remove the chlorophyll. The leaf becomes pale.
5. Rinse the leaf in warm water and add iodine solution.

Iodine solution turns blue-black where starch is present. The uncovered part of the leaf turns blue-black, and the covered part stays brown-orange. This result shows that starch was made only where light reached the leaf. It supports the conclusion that light is needed for photosynthesis.

The same method can be used with a variegated leaf, which has green and white areas. Only the green areas form starch, so chlorophyll is also needed.

Inputs and outputs of photosynthesis

How was photosynthesis discovered?

The Flemish scientist Jan van Helmont grew a willow in the seventeenth century and showed that the tree gained far more mass than the soil lost. In 1771 Joseph Priestley found that a plant could restore air in which a candle had burned.

Jan Ingenhousz later reported that plants release this gas only in light and only from their green parts. In the 1800s scientists measured the carbon dioxide taken in and showed that starch is formed. The detailed chemical steps were worked out mainly in the twentieth century.

Why does photosynthesis matter today?

Photosynthesis is the source of the food we eat, the oxygen we breathe and the fossil fuels that formed from ancient plants. Forests and oceans take large quantities of carbon dioxide from the air through photosynthesis, which affects climate.

In agriculture and horticulture, growers increase yields by adjusting light, temperature and carbon dioxide in greenhouses. Scientists also study how to make photosynthesis more efficient to help meet future food needs.

Which mistakes are common?

Mistake 1: Saying plants breathe in carbon dioxide and breathe out oxygen only

Plants also respire all the time and use oxygen. In bright light photosynthesis is faster than respiration, so there is a net release of oxygen.

Mistake 2: Thinking plants get their food from the soil

Plants make their food in leaves. The soil provides water and minerals, but most of the dry mass of a plant comes from carbon dioxide and water.

Mistake 3: Writing that light is a raw material

Light is a source of energy and not a substance that is used to build glucose. The raw materials are carbon dioxide and water.

Mistake 4: Heating ethanol over a flame

Ethanol is highly flammable. In the starch test it should be heated only in a water bath that has been heated away from open flames.