While humans have proudly developed their own power sources using mostly fossil fuels, uranium and rivers, photosynthesis in nature captures approximately 6 times as much energy consumed by modern civilisation. This capture rate is about 100 terawatts, gained from our closest star, the Sun.
How is this done? The leaves of trees and plants have tiny organelles containing chlorophyll which absorb mostly blue and red light, so we see them as green. An idea of this global activity can be observed in the world map showing concentration of chlorophyll in the sea (from phytoplankton), and vegetation concentration on land.
This energy captured promotes, through the production of electrons, the reaction between carbon dioxide from the atmosphere and water to produce sugar and oxygen. In trees, the water comes from the roots transported via the xylem (see previous blog) and reacts with carbon dioxide which enters through tiny holes in the leaf, or stomata (see pic).
The sugars produced are then transported via the phloem to the roots. The basic reaction is 6CO2 + 6H2O –> C6H12O6 (sugar) + 6O2, meaning for each part of carbon dioxide reacting, an equivalent amount of oxygen is produced.
In this way a mature tree can provide enough oxygen for 2 people to live per year, while in total, photosynthetic organisms convert about 100-115 petagrams (15 zero’s!) of carbon into biomass per year.
The competition for light in the forest is intense and sometimes trees are growing too close to each other, thereby undermining each other’s health. Shade tolerance is therefore a key competitive advantage. However, with human activity, competition for light is the least of a tree’s worries. Trees have evolved at different latitudes in different ways to capture this light. At the equator where the sun is overhead all year round, trees have broad canopies. While at higher latitudes trees generally have narrow and extended crowns to capture light at lower angles, e.g. conifers.
There is thus perhaps a simple way of being more conscious of trees and having them in our awareness, if on a sunny day we can occasionally be mindful when we enjoy a deep, luxurious breath of air, and think about that oxygen that sustains us.