Plants, algae, and certain bacteria, however, don’t need to consume other organisms for their energy. For the sake of simplicity, let’s exclude algae and bacteria from this discussion for the moment and focus on the ability of plants alone to capture energy from sunlight and convert it into the compounds needed to fuel the metabolic processes that support life.
In addition to converting energy from the sun into a form that provides sustenance for living organisms, photosynthesis also releases both oxygen and water into the environment. Nearly all of the oxygen in Earth’s atmosphere comes from photosynthesis. Photosynthesis is truly the basis for all life on the planet.
Nobody would describe the process of photosynthesis as easy to understand. (If you’ve ever studied organic chemistry, you’ve discovered that for yourself.) But learning about it gives us a greater appreciation for the complex interactions that go on continuously in every ecosystem—interactions our very lives depend upon.
DEFINITION
The simplest definition of photosynthesis is:
“The process by which a plant uses the energy from the light of the sun to make its own food.”
Adding another level of detail, photosynthesis is:
“The process in which green plants use energy from the sun to transform water, carbon dioxide, and minerals into oxygen and organic compounds.”
And at yet another level it is:
“The natural chemical-process by which chlorophyll (magnesium-containing pigment in green plants, blue-green algae, phytoplankton, and green and purple bacteria) uses sunlight (radiation) energy to convert (synthesize) water and atmospheric carbon dioxide into life sustaining organic compounds such as glucose.”
Let’s see if we can shed a little light (no pun intended) on how photosynthesis works.
A TWO-STAGE PROCESS
Photosynthesis occurs in two stages:
light-dependent reactions, which are followed by
light-independent reactions known as the Calvin cycle
LIGHT-DEPENDENT REACTIONS
The light-dependent reactions begin when energy from the sun is absorbed by chlorophyll, the green pigment in the leaves of plants. Think of chlorophyll molecules as “light harvesters.”
They are found in the special organelles known as chloroplasts that are found in the cells of plants.
These light-dependent reactions produce two different types of molecules: ATP and NADPH. Through a series of chemical reactions, energy is stored in the bonds that hold each ATP molecule together.
When those bonds are eventually broken, the stored energy is released. NADPH is the carrier molecule for the energized electrons.
THE CALVIN CYCLE
In the second stage of photosynthesis, the plant uses carbon dioxide and water from the atmosphere, together with the energy stored in ATP, to produce glucose and other sugars.
Glucose fuels the plant’s growth through the formation of larger molecules of cellulose and starch that store energy. Cellulose is formed when many glucose molecules are linked together into long chains called polysaccharides, which means “many sugars.”
Most of a plant’s cell walls are made up of cellulose, which is the plant’s primary building material. The more cellulose the plant produces, the larger the plant grows, and the larger the plant grows, the more of the sun’s energy it stores.
So, some of the glucose made by a plant through the conversion of light energy into chemical energy is used by the plant as fuel. The rest of it is turned into cellulose, and the plant grows bigger.
IT’S
A PROCESS
EVERY PROCESS STARTS WITH INPUTS AND PRODUCES ONE OR MORE OUTPUTS. THE NECESSARY INPUTS FOR PHOTOSYNTHESIS ARE:
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light energy![]()
water![]()
carbon dioxide![]()
chlorophyll
light energy
water
carbon dioxide
chlorophyllThe outputs are the sugars that release energy when ingested by a living organism.
INPUTS AND OUTPUTS
HERE’S WHERE EACH OF THE INPUTS TO THE PROCESS OF PHOTOSYNTHESIS COMES FROM:
The light energy comes from the sun.
Water is absorbed from the soil by the roots of the plant and travels through the stem to reach the leaves where photosynthesis takes place.
Carbon dioxide is absorbed from the atmosphere through microscopic openings, called stomata, in the plant’s leaves and stem.
Chlorophyll is found in special structures, called chloroplasts, within the leaves of a plant. Chloroplasts have been described as the “kitchens” of the plant cells, because they are where the plant’s food is produced.
Deep within the chloroplasts there are structures called photosystems, which is where light is harvested and light-dependent reactions occur. The outputs of these light-dependent reactions are the ATP and NADPH that are essential inputs to the Calvin cycle and oxygen, which is released into the atmosphere through the stomata.
The Calvin cycle also takes place within the chloroplasts of plant cells. The main output of the Calvin cycle is the glucose that provides fuel for the plant.
PHOTOSYNTHESIS AND THE FOOD CHAIN
The energy produced by and stored in plants is transferred from one organism to the next in a series of relationships referred to collectively as a food chain. The different levels in the food chain are called trophic levels.
Plants are at the very bottom of the food chain, because they are the only organisms (other than those algae and bacteria already noted as an exception) that make their own food from elements in the non-living environment.
Plants are referred to as autotrophs, from the Greek words for “self” and “nutrition.” In a food chain they are the producers. All of the other organisms in a food chain are consumers.
Immediately above the producers, at the next level of any food chain, are the primary consumers. These are the animals that eat plants—herbivores and omnivores.
On the next higher level are the secondary consumers, with third order consumers one level higher, then fourth order, and so on.
As each successive level of consumers feeds on those below them, energy is transferred up the food chain.
The eagle at the top of the food chain in its ecosystem is the recipient of energy that was produced by plants through photosynthesis and then transferred up the food chain through the grasshopper that ate the grass seeds to the frog that ate the grasshopper to the eagle that ate the frog.
ENERGY LOSS
The higher up the food chain, the less energy is available. Only about 10% of the energy produced by plants is available to primary consumers—a 90% energy loss.
That 90% loss occurs again at each trophic level, so secondary consumers only get about 1% of the original amount produced through photosynthesis and third order consumers get only 90% of that, or about 0.1%.
This energy loss occurs because organisms burn it to support their metabolic processes. They burn energy in moving, maintaining a stable body temperature, reproducing, and so on.
What’s left over to pass on is the amount of energy that the organism stored during growth.
It makes perfect sense if you think of it in terms of size:
the bigger the meal you eat, the more calories (a measure of potential energy) you take in. The bigger the frog, the more of the energy produced through photosynthesis the eagle ends up with.
Fortunately, plants (the producers in every food chain) don’t stop growing when the sun goes down. In fact, that’s when they do most of their growing.
WHAT HAPPENS IN THE DARK?
Plants don’t photosynthesize at night. Without sunlight, there can be no light-dependent reactions, and thus—no photosynthesis. But plants still need energy at night.
In the absence of light, plants switch over from photosynthesis to respiration as the process through which they obtain energy.
Respiration is the process that uses oxygen to convert glucose into energy. Respiration occurs continuously, during the day and at night, but at night photosynthesis ceases and respiration alone sustains the plant. Burning glucose, day or night, provides energy that enables a plant to grow, repair itself, and reproduce.
Think of it this way. During the day, plants have one job to do: convert as much sunlight into energy as possible and store it. (That is, unless the sunlight is too bright and hot. Photosynthesis typically shuts down temporarily during the hottest hours of the day under such circumstances to prevent damage to the biological structures that make photosynthesis possible.)
While the sun is shining, plants are busy making food. When the sun goes down, their mission shifts to growing.
EARTH’S
LUNGS
Respiration and photosynthesis work together to store energy and pass it up the food chain. They also are essential to maintaining the proper levels of oxygen and carbon dioxide in Earth’s atmosphere.
Animals, including us, take in oxygen and release carbon dioxide back into the environment. Plants do just the opposite: they take in carbon dioxide and give off oxygen.
Earth’s rain forests, especially in the Amazon region, are sometimes referred to as the lungs of our planet because they “inhale” carbon dioxide and “exhale” oxygen.
It would be more accurate to apply that term to all of the world’s vegetation collectively, because all plants contribute to regulating the balance of oxygen and carbon dioxide in the atmosphere.
One of the main concerns environmentalists have about the deforestation of the Amazon and other tropical regions is the possible adverse effects that decreased photosynthetic activity would have on the atmosphere.
WHAT WOULD THE END OF PHOTOSYNTHESIS MEAN?
There are a few plausible doomsday scenarios that could lead to a drastic reduction or cessation of photosynthesis around the world. A number of Hollywood disaster movies have depicted the Earth in the grips of a global, endless winter—an ice age unlike any the world has already survived. What you don’t see is what happens after the end credits roll.
Photosynthesis has been chugging along quite nicely for billions of years, despite drastic shifts in climate conditions. But just suppose that photosynthesis were to end abruptly. What would happen?
Most of the plants on Earth would die within a matter of days or at most weeks, though the giant sequoias might hang on for a few years because of the large amount of glucose stored in their cells.
- It wouldn’t take long for the herbivores to die of starvation, and the carnivores and omnivores, including human beings, wouldn’t be far behind.
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- The vultures and other carrion eaters, would be next, and the fungi and other detritivores would be the last to go.
ARTIFICIAL PHOTOSYNTHESIS
Scientists have been working on developing an artificial photosynthesis-like process that does not require living plants in the role of producers, not because they fear that one of those doomsday scenarios will come to pass but rather in search of a new energy source.
Today’s solar technology is expensive and inefficient, and the biggest problem is the limited storage capacity of even trailer-sized batteries. But there is no doubt that we need to tap into the energy available in sunlight.
Bill Gates, a staunch advocate, has founded a coalition of private investors to help fund research into artificial photosynthesis and other potential clean energy sources.
The goal of artificial photosynthesis research is to use the same inputs and similar chemical interactions to produce clean, renewable energy instead of glucose.
Artificial photosynthesis is based on an artificial “leaf” in the form of a silicon solar cell. Scientists have already proved the feasibility of using it to collect sunlight and split water molecules in oxygen and hydrogen.
Photosynthesis is one of those things that, as vital as it is to our continued existence, we rarely think about. It’s like an old, reliable friend that we all too often take for granted.
Perhaps the next time you’re out in nature, you’ll look at the green things around you with a new sense of wonder at the activity going on within them.
