A Day in the Life of a Plant – Photosynthesis and Phytochemistry

A photograph showing two hands together holding a clod of earth with a small green seedling.Plant Life

Plant life is one of Nature’s miracles.  Imagine being a plant and almost all you will ever need to keep on striving is sheer sunlight.  In green plants, both photosynthesis and aerobic respiration occur.  It’s a lot like the way in which the human body breaks down food into fuel that it can store.  Essentially, using energy from the Sun, a plant can transform carbon dioxide CO2 and water into glucose and oxygen…

Any plant starts life as a seed, which then germinates and goes on to develop into a plant.  The mature plant produces flowers, which are fertilised and produce seeds in a fruit or seedpod.  Eventually, the plant dies, leaving behind seeds which germinate to produce new plants.

Different plants have different life cycles…


Plant Life Cycles

Annuals Plants

A photograph showing a rice plant with seeds.
The rice plant is an annual.

Annuals take one year to complete their life cycle.


Winter annuals germinate during the autumn and mature during the spring or summer of the following calendar year.

One seed-to-seed life cycle for an annual can occur in as little as a month, or stretch to several months for most species.

Among true annuals are corn, wheat, rice, lettuce, peas, watermelon, beans, zinnia and marigold.


Biennials take up to two years to complete their biological life cycle.

The plant germinates, grows leaves, stems, and roots in their first year.  Then, it enters a period of dormancy over the colder months.  The stem usually remains very short and the leaves are low to the ground, forming a rosette.

Many biennials require a cold treatment, or vernalization, before they will flower.  During the following spring or summer, the stem of the biennial plant elongates greatly, or “bolts”.

This latter process renders biennial vegetables, such as spinach, fennel and lettuce, unusable for food.

Ultimately, the biennial plant sets seeds and dies in their second year.

Some biennials are usually grown as annual crops for their edible roots, petioles and leaves.  They include carrot, celery or parsley.


A close-up photograph of a red petunia flower.
The humble petunia is often grown as an annual.

Perennials can live for several years after germination.

Small flowering plants, that grow and bloom over the spring and summer, die back every autumn and winter, and return in the spring from their rootstock.  They are known as herbaceous perennials.

Depending on the rigours of a local climate, a plant that is considered to be a perennial in its native habitat, or in a milder garden, may be treated by a gardener as an annual and planted out every year, from seed, from cuttings or from divisions.

Evergreen,  or non-herbaceous perennials, retain a mantle of leaves throughout the year.

Some ornamental perennials are also commonly grown as annuals, including impatiens, wax begonia, snapdragon, Pelargonium, coleus and petunia.

Monocarpic Plants

Monocarpic plants produce seeds only once, but may take several years before reaching maturity.

The Talipot Palm may live for 60 years or more before it produces flowers and seeds, and it then dies.

From a gardening and food production point of view, a plant’s status as annual, biennial, or perennial often varies based on its location or purpose.  For example, biennials grown for flowers, fruits, or seeds have to be grown for two years, and biennials grown for their edible leaves or roots for just one year.


Photosynthesis and Energy Generation in Plants

A balanced chemical equation for the process of photosynthesis. Using sunlight energy, a plant can transform carbon dioxide and water into glucose and oxygen.
Balanced chemical equation for Photosynthesis Source: photosynthesiseducation.com


Photosynthesis is a natural process whereby plants and other live organisms convert light energy from the Sun, into chemical energy that can be later released to fuel the organisms’ activities.  This chemical energy is stored in carbohydrate molecules, such as sugars, synthesised from carbon dioxide (CO2) and water.

Oxygen is normally released as a by-product of the chemical reaction that takes place.  Photosynthesis maintains atmospheric oxygen levels and supplies all of the organic compounds and most of the energy necessary for life on Earth.

While we inhale oxygen and exhale carbon dioxide, plants inhale carbon dioxide and exhale oxygen.


Some scientists believe that our atmosphere had little to no oxygen before plants evolved and started releasing it.

Without the Sun to feed plants (and plants to release oxygen), we might not have breathable air.

Without plants to feed us and the animals most people use for food, there would be no life.


Plants help control the amount of carbon dioxide, a greenhouse gas, in the atmosphere.  They protect the soil from wind and from water run-off, helping to control erosion.  Additionally, they release water into the air during photosynthesis.

To understand how this works, we will consider a number of factors affecting the rate of photosynthesis in a plant:

  • the compensation point for light

  • the compensation point for CO2.


The Compensation Point for Light

A graph illustration showing the compensation point of light in plant growth.
The Light Intensity Compensation Point Source: tomatosphere.org

One simple way to get an estimate of the level of photosynthetic activity in a green plant is to place the plant in a sealed container and measure the rate at which oxygen is produced.  We looked at this in a previous article regarding scientific effort to grow plant life in the challenging environment of the International Space Station.

When such an experiment is actually performed, it is found that increasing the brightness (intensity) of the light increases the rate of photosynthesis.  This is true only up to a certain point, beyond which increasing the brightness of light has little or no effect on the rate of photosynthesis.

Conversely, reducing the intensity of the light causes a decrease in photosynthetic activity.

Light intensity at which the net amount of oxygen produced is exactly zero is the compensation point for light.  At this point, the consumption of oxygen by the plant due to cellular respiration is equal to the rate at which oxygen is produced by photosynthesis.

The compensation point for light intensity varies according to the type of plant.  Typically, it falls in the range 40 to 60 W/m2 for sunlight.  The compensation point for light can be reduced (somewhat) by increasing the amount of carbon dioxide available to the plant, allowing the plant to grow under conditions of lower illumination.


The Compensation Point for CO2

A graph illustration showing the compensation point of carbon dioxide in plant growth.
The Carbon Dioxide CO2 Compensation Point Source: tomatosphere.org

Under conditions of constant and uniform illumination, the rate of photosynthesis can be increased by simply increasing the amount of carbon dioxide (i.e. increasing the atmospheric partial pressure) available to plants.

As before, one can measure the rate of photosynthesis as a function of carbon dioxide pressure by placing a green plant in a sealed container and measuring the rate at which oxygen is produced.  As the partial pressure of carbon dioxide increases, there is an almost linear increase in the rate of oxygen production, which implies an identical increase in the rate of photosynthesis.

This increase eventually levels off, and further increases in the concentration of carbon dioxide have no further effect.  Conversely, a reduction in the carbon dioxide concentration reduces the rate of photosynthetic activity.

The level at which the oxygen production rate drops to zero is called the compensation point for carbon dioxide.


What the diddly is a “Chromophore”?

A diagram showing the lower transition levels of an electron in a caesium atom.
Transitions of an electron to the lower or higher atomic energy levels of an atom

According to Wikipedia, a chromophore is the part of a molecule responsible for its colour.  The colour arises when a molecule absorbs certain wavelengths of visible light, and either transmits it or reflects others.

The chromophore is a region in the molecule where the energy difference between two different molecular orbitals falls within the range of the visible spectrum.  Visible light that hits the chromophore can thus be absorbed by exciting an electron from its ground state into an excited state.

In biological molecules that serve to capture or detect light energy, the chromophore causes a conformational change of the molecule when it is hit by the light.



Some of these are metal complex chromophores, which contain a metal in a coordination complex with ligands – ions or molecules that binds to a central metal atom.  The bonding between metal and ligand generally involves the formal donation of one or more of the ligand’s electron pairs.  The nature of metal-ligand bonding can range from covalent to ionic.

Familiar examples of metal-complex chromophores include:

  • chlorophyll, which is used by plants in photosynthesis

  • hemoglobin, which serves as the oxygen carrier in the blood of vertebrate animals.

In these two examples, a metal is complexed at the centre of a tetrapyrrole macrocyclering.  In the case of hemoglobin, the metal is iron from the heme group (in a porphyrin-ring) of hemoglobin.

Whereas in chlorophyll, the metal in question is magnesium, complexed in a chlorin-type ring.

A stick equation of phytochemicals in a molecule of Beta carotene.
The chemical structure of Beta-Carotene, with its eleven conjugated double bonds that form the chromophore of the molecule.  Carotenoids, such as beta-carotene, are responsible for the orange-red colours of popular foods like carrots, oranges and tomatoes.  Beta-carotene is converted in the body to vitamin A by a specific enzyme present in the gut. Source: sciencelearn.org.nz

And the reason this is all so interesting, even from a physicist’s point of view, is because with photosynthesis, we are entering the Quantum World of plant energy generation.

Bear with me…