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Light and photosynthesis

  Photosynthesis and respiration: a simple carbon cycle
  Harvesting light          

forest canopy forest canopy forest canopy forest canopy

Forests are sometimes called the Earth's lungs because they produce much of the oxygen we breathe. The ocean should rightly share this title.

sea grass kelp forest coral reef diatoms

Photosynthesis in the ocean. From left to right: Seagrass bed, kelp forest, pillar coral with photosynthetic algae, and diatoms (a type of phytoplankton) under the microscope.

Oceans and forests:
Earth's lungs

All plants and some bacteria trap energy from sunlight and use it to build sugars, which they need to live and grow. This process is called photosynthesis.

Nearly all life on Earth depends on photo- synthesis. If plants stopped photosynthesising, animals would have no food, and the world would run out of oxygen in less than 2000 years.

Plants on land produce about half the world's oxygen. The other half is produced by plants in the ocean - mostly by the microscopic plants known as phytoplankton.

A simple carbon cycle

The carbon cycle.

Oranic carbon includes all carbon-based chemicals produced by living organisms. which include carbon (C) and hydrogen (H) in combination with each other and other elements. During respiration by plants and animals organic carbon is broken down into water and carbon dioxide - a form of inorganic carbon.


The basic ingredients needed for photosynthesis are carbon dioxide and water. Using energy from light, plants build this into the sugar glucose. The oxygen we breathe is a byproduct of photosynthesis.

Water + Carbon dioxide + Light arrow with chlorophyll as catalyst Glucose + Oxygen

From the glucose plant cells can build a myriad different molecules of organic* carbon, which they need to live and grow.


But life and growth requires energy. Both plants and animals get this by 'burning' organic molecules during respiration.

Chemically respiration is the opposite of photosynthesis. During respiration cells release energy by converting glucose and oxygen into water an carbon dioxide.

Glucose + Oxygen arrow Water + Carbon dioxide + Energy

Together photosynthesis and respiration make up the simplest form of the carbon cycle.

More about photosynthesis.

Harvesting light

chlorophyll absorption spectra

Chlorophylls are green pigments that absorb blue and red light. The most important is chlorophyll-a, which is found in all plants. Many many marine plants also contain other chlorophylls (b, c or d).

other plant pigment absorption spectra

Carotenoids are orange or brown pigments that absorb green and blue-green light. Examples are:
1. Beta-carotene (in carrots and many other plants).
2. Peridinin (in dinoflagellates, a type phytoplankton).
3. Fucoxanthin (in kelp and many phytoplankton).
Compare these to chlorophyll (green line) and you see that carotenoids trap light from other parts of the spectrum. This helps the plants harvest more light than with chlorophyll alone.

To capture light for photosynthesis, plants use special chemicals known as photosynthetic pigments.

Pigments are chemicals that absorb some wavelengths of light and reflect others. This makes them coloured. The most important of the photosynthetic pigments is chlorophyll-a which gives plants their green colour.

Chlorophyll-a is found in all plants, but it can only capture light at some wavelengths. To trap as much light as possible, plants also have other pigments, each with a different absorption spectrum and colour.

These pigments do not carry out photosynthesis directly, but pass their energy to chlorophyll-a. For this reason they are known as accessory (helper) pigments.

There are three basic pigment types:

Different plant types contain different pigments. This means we can use the pigments from a sample of seawater to help us identify the phytoplankton in the water.

Scientists are still working on ways to identify different types of phytoplankton from measurements of water colour.

Link to the main Euro-Argo project website.