I remember some biology lessons from school. If you are like me, you can’t remember all that you learnt.
I know that Photosynthesis is how trees create energy from light. I also know it is the process that makes trees so important to our environment.
But as I walk around my local forest, I realized I wasn't aware of exactly how it worked and why some trees were evergreen and others were not
There was a lot that I didn’t know. How much do you know?
Are There Different Types of Photosynthesis?
Photosynthesis is the process used by all plants, algae and some bacteria to create energy from light.
There are two types of photosynthesis processes, anoxygenic and oxygenic, which are very similar in their basic principles. Oxygenic is the most commonly used by plants.
Oxygenic photosynthesis is the process that brings so much benefit to our environment. It takes light and carbon dioxide and creates carbohydrate and oxygen.
Technically, the process of oxygenic photosynthesis uses light energy to transfer electrons from water (H2O) to carbon dioxide (CO2) to produce the carbohydrates. In this transfer, the CO2 receives electrons which the water loses electrons (oxidisation). Ultimately, this leaves at the end of the process carbohydrates and oxygen.
Anoxygenic photosynthesis doesn’t produce any oxygen, hence the name. This is because the substance that the organicism uses as the electron donor is different. If, for example, a bacterium uses hydrogen sulphide, then a solid sulphur will be the output instead of oxygen.
Both types of photosynthesis are complex, with many process steps, we can articulate the process in an equation
What is the Photosynthesis Equation?
First a quick chemistry refresh, as they made the equation up of the symbols for atoms. H = hydrogen, O= Oxygen, C= carbon.
The equation for oxygenic photosynthesis, the type used by trees, is;
6CO2 + 12H2O + Light Energy → C6H12O6 + 6O2 + 6H2O
The process is using 6 carbon dioxide molecules and combining them with 12 water molecules using the energy it gets from light.
The result is the creation of a single carbohydrate molecule (glucose – C6H12O6) along with 6 breathable oxygen molecules and 6 water molecules.
With anoxygenic photosynthesis as we have already mentioned, the difference is that a different molecule can be used, replacing the water in the formula. As a result, the x in the following formula acts as a variable;
CO2 + 2H2 x + Light Energy → [CH2O] + 2 x + H2O
So as an example, if the bacteria use hydrogen sulphide (H2S) as the electron donor would replace 2H2 x in the formula.
So now we understand what photosynthesis does, now we need to look where photosynthesis happens.
Where does Photosynthesis Occur?
In trees, like other plants, photosynthesis takes place is a structure called chloroplasts. The Chloroplasts contain the chlorophyl, the substance that makes most leaves look green.
A double membrane surrounds the chloroplasts and contains a third inner membrane called the thylakoid membrane.
Under the microscope, the thylakoid membranes look like stacks of coins. However, the compartments that they form are connected in a maze of chambers.
The Photosynthetic cells have special pigments that absorb the light energy, different pigments use different wavelengths of light. Chlorophyll, the main pigment used in photosynthesis reflects green light giving leaves of trees their green colour. It absorbs red and blue light
We find chlorophyl within the thylakoid membrane.
What are the Products of Photosynthesis?
Photosynthesis uses solar energy to create glucose, which it needs to grow and survive. The by-products are Oxygen and Water.
This process is great to offset carbon emissions, as the tree is using carbon dioxide and releasing oxygen.
However, when other tree cells use the glucose they will as a by-product create carbon dioxide, like we do as we breathe, we know this process as respiration.
Fortunately, some carbon is retained in the tree cells as they grow, so the net process is usually that there more oxygen is released than carbon dioxide.
When do Trees Photosynthesise?
In simple terms, if a tree has green leaves and it daylight the tree will be photosynthesising.
There are exceptions, of course.
Some trees have a different chlorififly pigment which might mean that the tree has reddish or yellow leaves not just in fall.
Some trees can photosynthesise through young stems and branches if it is warm enough without leaves.
Remember that at night there is no solar energy so the tree will not photosynthesise, but it will still continue to grow so the respiration process is active.
How do Deciduous Trees Stay a Live in Winter?
Deciduous trees stay alive in winter by storing food during the spring and summer. As the tree is photosynthesising, creating carbohydrates, it stores some to prepare for the dormant winter period as starches in structures underground like roots where it is unlikely to freeze.
As the days become shorter during fall, this triggers the tree to lose its leaves. The tree will become more and more dormant, stopping growth, and the process that the tree uses for water movement are stopped.
This is to protect the tree from damage from being frozen. Water expands when it is frozen and this would destroy the vascular tissues of the tree's water transport system and would cause the tree dying.
Some trees can use the stored starch to create a type of sugary antifreeze which increase it chances of survival.
Do Evergreen Trees Photosynthesise All Year?
The straightforward answer is no.
Evergreens have evolved a slightly different process to their deciduous cousins, that has enabled them to survive in more northerly latitudes and higher elevations.
In spring deciduous trees have to use a lot of energy to re-grow its canopy and then require a relatively lengthy growing period to ensure that it can not only grow, but to restock the reserves to survive the winter and to kick start the growing period next spring.
Evergreens have evolved a compromise, by retaining their needles (their form of leaves) they don’t have to make that huge spring investment and so they can survive in climates where there is a considerably shorter growing period.
With this basic blueprint in mind, not all evergreens keep their needles for the same length of time.
White pine will only keep needles for 2 growing seasons, meaning that a particular needle will only endure one winter. Whereas a Bristlecone pine will keep its needles for almost 50 years.
To overcome the threat of freezing in winter, conifers accumulate high concentrations of cryoprotectant that lowers the freezing point of water that allows their cells and important structures to survive.
Moreover, the shape of the tree and the layout of the branches stops high amounts of snow accumulation which would otherwise result in broken branches.
First the needles offer a small surface area for snow to accumulate, secondly the branches generally sweep outwards and downwards from the trunk. Any snow that collects will weight down the flexible branch soon reaching an angle where it simply slides off.
Photosynthesis is one of the miracles of life,.
It is one reason trees are so valuable to us by being traps for carbon dioxide.
It is amazing the strategies that trees have evolved to survive in different climates
What would the world be without trees?