Use of sunlight in biological processes

Use of sunlight in biological processes

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Sunlight is used by green plants in photosynthesis, but it is also used by animals in the synthesis of Vitamin D. Are there any similarities between the two processes and how is the light energy actually used?

We don't get it from the sun, it's synthesized.

Humans can get it…

  • via nutrition.
  • via synthesis in the skin, which depends on UV radiation. Sun is the major source of it (the radiation, not the vitamin), and synthesis in the skin the major source of the vitamin. However, it needs further modification in the liver or kidney to become bioactive. UV radiation is necessary and directly causing the opening of a cyclic molecule structure, changing a cholesterol to a previtamin D3.

For more details see chapter 'Biosynthesis' at


Despite major overall differences, there are some basic similarities in the two processes:

  • Light energy excites an electron to a higher energy level.
  • A covalent bond is broken as the electron moves elsewhere. This process is called 'photolysis' and is purely chemical.


The difference is in what happens to the excited electron and the extent of involvement of cellular proteins and lipids:

  • In Vitamin D synthesis the electron just moves 'spontaneously' to another bond in the new molecule that is formed. That is the end of the matter and enzymes are not involved in this reaction.
  • In photosynthesis the light is first captured by photoreceptors and funnelled to the site of photolysis. After photolysis, the co-operation of many proteins in a complex process within a membrane-bond system ensures that the excited electron is transferred to NAD+, forming NADH, the reducing power for then subsequent conversion of carbon dioxide to phosphoglycerate. (An electrochemical membrane potential is also generated, allowing ADP to be converted to ATP, the hydrolysis of which is used for formation of the C-C bonds of the phosphoglycerate.)

Vitamin D synthesis

The photolysis step in Vitamin D synthesis is shown below:

The precursor molecule is formed from cholesterol in a series of enzymic reactions and the Vitamin D3 produced (actually resulting from a spontaneous rearrangement of the initial product molecule) may be metabolized further in enzymic processes (see Berg et al. for more detail). However the photolysis of the bond indicated in red is purely chemical.

One may ask why no other electrons are raised to a higher energy level and the corresponding bonds broken. Other electrons may conceivably be excited, but in the absence of a suitable reaction pathway to a compound of lower thermodynamic free energy the electron will fall back to its original energy level, liberating heat. In the case of 7-dehydrocholesterol the co-ordinated double-bond system provides a suitable reaction pathway to a product of lower free energy. One can think of light (of appropriate wavelength) providing the activation energy of the reaction (overcoming the energy barrier resulting from the higher free energy of the reaction intermediate).


The reactions of photosynthesis that utilize light energy (the so-called 'light reactions') involve two complex photosystems that are described in standard texts, e.g. Berg et al.. The simplified diagram deals only with the overall photolytic process.

The molecule undergoing photolysis here is water - the two H-O bonds are broken and oxygen is produced. However in this case the product is incidental to enabling the electron to reduce NAD+. (Chemically oxidation is removal of electrons, and reduction their addition.) As mentioned above, this complex series of reactions also results in an electrochemical gradient within the chloroplast. The movement of hydrogen ions through an ATP synthase in the thylakoid membrane of the chloroplast stroma converts ADP to ATP (in a similar manner to that of oxidative phosphorylation).

The result of the photolysis in this case is the provision of molecular reducing power and 'energy' that will be used to convert 1-C carbon dioxide to the 3-C reduced sugar in a separate series of reactions that do not themselves involve light energy (these are referred to as 'dark reactions', although they obviously do not have to take place in the dark).


Although one often hears loose reference to light energy being used to 'make' Vitamin D or sugars, this can be quite misleading. I would suggest that it is better to express what the light energy is achieving in chemical terms and then place this in the context of the relevant synthetic process.

Use of sunlight in biological processes - Biology

Have you ever noticed that plants need sunlight to live? It seems sort of strange doesn't it? How can sunlight be a type of food? Well, sunlight is energy and photosynthesis is the process plants use to take the energy from sunlight and use it to convert carbon dioxide and water into food.

Three things plants need to live

Plants need three basic things to live: water, sunlight, and carbon dioxide. Plants breathe carbon dioxide just like we breathe oxygen. When plants breathe carbon dioxide in, they breathe out oxygen. Plants are the major source of oxygen on planet Earth and help keep us alive.

We know now that plants use sunlight as energy, they get water from rain, and they get carbon dioxide from breathing. The process of taking these three key ingredients and making them into food is called photosynthesis.

How do plants capture sunlight?

Plants capture sunlight using a compound called chlorophyll. Chlorophyll is green, which is why so many plants appear green. You might think at first that it's green because it wants to absorb and use green light. However, from our study of light, we know that the color we see is actually the color of light that is reflected. So chlorophyll actually reflects green light and absorbs blue and red light.

More details on Photosynthesis

Inside a plant's cells are structures called chloroplasts. It's in these structures where the chlorophyll resides.

There are two main phases to the process of photosynthesis. In the first phase, sunlight is captured by the chloroplasts and the energy is stored in a chemical called ATP. In the second phase, the ATP is used to create sugar and organic compounds. These are the foods plants use to live and grow.

The first phase of the process must have sunlight, but the second phase can happen without sunlight and even at night. The second phase is called the Calvin Cycle because it was discovered and described by scientist Melvin Calvin.

Even though plants need sunlight and water to live, different plants need different amounts of each. Some plants need just a little water while others need a lot. Some plants like to be in the direct sunlight all day, while others prefer the shade. Learning about the needs of plants can help you learn where to plant them in your yard and how best to water them so they will flourish.

Now we know that plants need sunlight, water, and carbon dioxide to live. They take these three components and use chlorophyll to help convert them into food, which they use for energy, and oxygen, which they breathe out and we use to live. All plants use photosynthesis, so they all need some sunlight.

Use of sunlight in biological processes - Biology

The Biological and Environmental Research (BER) program supports scientific research and facilities to achieve a predictive understanding of complex biological, earth, and environmental systems with the aim of advancing the nation’s energy and infrastructure security. The program seeks to discover the underlying biology of plants and microbes as they respond to and modify their environments. This knowledge enables the reengineering of microbes and plants for energy and other applications. BER research also advances understanding of the dynamic processes needed to model the Earth system, including atmospheric, land masses, ocean, sea ice, and subsurface processes.

Over the last three decades, BER has transformed biological and Earth system science. We helped map the human genome and lay the foundation for modern biotechnology. We pioneered the initial research on atmospheric and ocean circulation that eventually led to climate and Earth system models. In the last decade, BER research has made considerable advances in biology underpinning the production of biofuels and bioproducts from renewable biomass, spearheaded progress in genome sequencing and genomic science, and strengthened the predictive capabilities of ecosystem and global scale models using the world’s fastest computers.

BER supports three DOE Office of Science user facilities, the Atmospheric Radiation Measurement (ARM) user facility, Environmental Molecular Sciences Laboratory (EMSL), and Joint Genome Institute (JGI). These facilities house unique world-class scientific instruments and capabilities that are available to the entire research community on a competitive, peer review basis. Additionally, four DOE Bioenergy Research Centers were established to pursue innovative early-stage research on bio-based products, clean energy, and next-generation bioenergy technologies.

How Do Living Things Obtain and Use Energy?

Plants get energy from the sun and use carbon dioxide and water in the process called photosynthesis to produce energy in the form of sugars. Animals use sugars provided by plants and other organisms to produce energy in the form of adenosine triphosphate (ATP).

All living organisms get their matter and energy from the environment, whether it is from the air, soil, water or food. Scientists devised the Pyramid of Energy to explain how energy moves across the food chain. The pyramid contains producers, primary consumers, secondary consumers and tertiary consumers. Producers, such as plants, are at the bottom of the pyramid. Producers do not consume other organisms for energy. Primary consumers rely on plants for their energy and secondary consumers rely on primary consumers for their energy. At the top of the pyramid are tertiary consumers, or decomposers, which gain energy from secondary consumers. Decomposers break down decaying organisms to obtain their energy. Cells in living organisms require energy to maintain their structures and function, as well as to grow and reproduce. Living organisms also produce electrical energy and can make copies of DNA molecules by using ATP. Energy is also used to move muscles and carry signals from the brain to different nerves.

Negative (harmful) effects of UV

Causes skin cancer – UV is an environmental human carcinogen. It’s the most prominent and universal cancer-causing agent in our environment. There is very strong evidence that each of the three main types of skin cancer (basal cell carcinoma, squamous cell carcinoma and melanoma) is caused by sun exposure. Research shows that as many as 90% of skin cancers are due to UV radiation.

Causes sunburn – UV burns the skin. Sunburn is a burn that occurs when skin cells are damaged. This damage to the skin is caused by the absorption of energy from UV rays. Extra blood flows to the damaged skin in an attempt to repair it, which is why your skin turns red when you are sunburnt.

Damages immune system – Over-exposure to UV radiation has a harmful suppressing effect on the immune system. Scientists believe that sunburn can change the distribution and function of disease-fighting white blood cells in humans for up to 24 hours after exposure to the sun. Repeated over-exposure to UV radiation can cause even more damage to the body's immune system. The immune system defends the body against bacteria, microbes, viruses, toxins and parasites (disease and infection). You can see how effective the immune system is by looking at how quickly something decays when it dies and the immune system stops working.

Damages eyes – Prolonged exposure to UV or high intensities of UV (for example, in sunbeds) damages the tissues of eyes and can cause a ‘burning’ of the eye surface, called ‘snow blindness’ or photokeratitis. The effects usually disappear within a couple of days, but may lead to further complications later in life. In 1998, the Journal of the American Medical Association reported that even low amounts of sunlight can increase the risk of developing eye damage such as cataracts (which, left untreated, will cause blindness), pterygium and pinguecula. UV damage to the eyes is cumulative, so it is never too late to start protecting the eyes.

Ages skin – UV speeds up the aging of skin, since the UV destroys collagen and connective tissue beneath the top layer of the skin. This causes wrinkles, brown ‘liver’ spots and loss of skin elasticity. The difference between skin tone, wrinkles, or pigmentation on the underside of a person's arm and the top side of the same arm illustrate the effects of sun exposure on skin. Usually, the top side of the arm has had more exposure to the sun and shows greater sun damage. Because photo-aging of the skin is cumulative, it is never too late for a person to start a sun protection programme. Otherwise, though a tan may look good now, you could be paying for it with wrinkly leathery skin or skin cancer later.

Weakens plastics – Many polymers used in consumer items (including plastics, nylon and polystyrene) are broken down or lose strength due to exposure to UV light.

Fades colours – Many pigments (used for colouring food, cosmetics, fabric, plastic, paint, ink and other materials) and dyes absorb UV and change colour. Fabrics, furnishings and paintings need protection from UV (fluorescent lamps as well as sunlight) to prevent colour change or loss.

Computational biology turns ideas into hypotheses

Finally, computers reshaped biology by making fuzzy concepts rigorous and testable. Here is one example from my own research: for decades, cancer researchers have discussed the idea that genetic heterogeneity between cells in the same tumour helps to make a cancer resistant to therapy [6]. It is a simple idea: the more diverse the cell population is, the more likely it is that a subset of the cells is resistant to therapy and can regrow the tumour after all other cells were killed.

But how exactly can you measure “genetic heterogeneity,” and how big is its influence on resistance development? To answer these questions, we had to turn the idea into a testable hypothesis. We used genomic approaches to measure changes in cancer genomes at different sites in a patient and then defined quantitative measures of heterogeneity, which could be compared statistically to clinical information on treatment resistance. And indeed, we found evidence supporting the initial idea that heterogeneity determines resistance [7].

This is just one of many examples in which a quantitative computational approach was needed to turn a fuzzy idea into a testable hypothesis. Computational biology excels at distilling huge amounts of complex data into something testable in the wet lab, thus, shaping and directing experimental follow-up.

Why are these topics important?

Understanding the role of solar radiation in the Earth's climate system can help us grasp important concepts such as:

The causes of the seasons.

Seasons are caused by the tilt of Earth's axis. The tilted axis means that the northern and southern parts of Earth do not receive equal amounts of solar radiation (energy per unit area). When the southern hemisphere is tilted toward the sun, it is summer in the southern hemisphere and winter in the northern hemisphere. (Principle 1c)

The reason ice ages occur.

The ice ages were caused by changes in the distribution of solar radiation received over Earth's surface. The path of Earth's orbit is not constant. Variations in Earth's orbital path causes the solar radiation reaching any point on Earth's surface to change. (Principle 1d)

How the amount of energy emitted from the sun (sun's luminosity) changes over time.

The sun's output is not constant. Its luminosity (total energy emitted by the sun) has increased over geologic time, and varies slightly over shorter time scales.

Why recent climate warming has not been caused by increases in the sun's energy output.

The sun's energy output has not changed enough over the last decades to account for the increases in temperatures that have been observed during this same time. (Principle 1e)

Most forms of energy that humans use are derived from solar energy.

Many forms of energy that humans use ultimately derive from solar radiation, such as food, hydrocarbons (such as oil and natural gas), wind energy, hydroelectric power, and of course, solar energy.

Evolution of the biosphere

During Earth's long history, life-forms have drastically altered the chemical composition of the biosphere. At the same time, the biosphere's chemical composition has influenced which life-forms inhabit Earth. In the past, the rate at which nutrients were transformed from one chemical form to another did not always equal their transformation back to their original form. This has resulted in a change in the relative concentrations of chemicals such as carbon dioxide and oxygen in the biosphere. The decrease in carbon dioxide and increase in atmospheric oxygen that occurred over time was due to photosynthesis occurring at a faster rate than respiration. The carbon that was present in the atmosphere as carbon dioxide now lies in fossil fuel deposits and limestone rock.

Scientists believe that the increase in atmospheric oxygen concentration influenced the evolution of life. It was not until oxygen reached high concentrations such as exist on Earth today that multicellular organisms like ourselves could have evolved. We require high oxygen concentrations to accommodate our high respiration rates and would not be able to survive had the biosphere not been altered by the organisms that came before us.

STEMvisions Blog

When you get hungry, you grab a snack from your fridge or pantry. But what can plants do when they get hungry? You are probably aware that plants need sunlight, water, and a home (like soil) to grow, but where do they get their food? They make it themselves!

Plants are called autotrophs because they can use energy from light to synthesize, or make, their own food source. Many people believe they are “feeding” a plant when they put it in soil, water it, or place it outside in the Sun, but none of these things are considered food. Rather, plants use sunlight, water, and the gases in the air to make glucose, which is a form of sugar that plants need to survive. This process is called photosynthesis and is performed by all plants, algae, and even some microorganisms. To perform photosynthesis, plants need three things: carbon dioxide, water, and sunlight.

By taking in water (H2O) through the roots, carbon dioxide (CO2) from the air, and light energy from the Sun, plants can perform photosynthesis to make glucose (sugars) and oxygen (O2). CREDIT: mapichai/

Just like you, plants need to take in gases in order to live. Animals take in gases through a process called respiration. During the respiration process, animals inhale all of the gases in the atmosphere, but the only gas that is retained and not immediately exhaled is oxygen. Plants, however, take in and use carbon dioxide gas
for photosynthesis. Carbon dioxide enters through tiny holes in a plant’s leaves, flowers, branches, stems, and roots. Plants also require water to make their food. Depending on the environment, a plant’s access to water will vary. For example, desert plants, like a cactus, have less available water than a lilypad in a pond, but every photosynthetic organism has some sort of adaptation, or special structure, designed to collect water. For most plants, roots are responsible for absorbing water.

The last requirement for photosynthesis is an important one because it provides the energy to make sugar. How does a plant take carbon dioxide and water molecules and make a food molecule? The Sun! The energy from light causes a chemical reaction that breaks down the molecules of carbon dioxide and water and reorganizes them to make the sugar (glucose) and oxygen gas. After the sugar is produced, it is then broken down by the mitochondria into energy that can be used for growth and repair. The oxygen that is produced is released from the same tiny holes through which the carbon dioxide entered. Even the oxygen that is released serves another purpose. Other organisms, such as animals, use oxygen to aid in their survival.

If we were to write a formula for photosynthesis, it would look like this:

The whole process of photosynthesis is a transfer of energy from the Sun to a plant. In each sugar molecule created, there is a little bit of the energy from the Sun, which the plant can either use or store for later.

Imagine a pea plant. If that pea plant is forming new pods, it requires a large amount of sugar energy to grow larger. This is similar to how you eat food to grow taller and stronger. But rather than going to the store and buying groceries, the pea plant will use sunlight to obtain the energy to build sugar. When the pea pods
are fully grown, the plant may no longer need as much sugar and will store it in its cells. A hungry rabbit comes along and decides to eat some of the plant, which provides the energy that allows the rabbit to hop back to its home. Where did the rabbit’s energy come from? Consider the process of photosynthesis. With the help of carbon dioxide and water, the pea pod used the energy from sunlight to construct the sugar molecules. When the rabbit ate the pea pod, it indirectly received energy from sunlight, which was stored in the sugar molecules in the plant.

We can thank photosynthesis for bread! Wheat grains, like the ones pictured, are grown in huge fields. When they are harvested, they are ground into a powder that we might recognize as flour. CREDIT: Elena Schweitzer/

Humans, other animals, fungi, and some microorganisms cannot make food in their own bodies like autotrophs, but they still rely on photosynthesis. Through the transfer of energy from the Sun to plants, plants build sugars that humans consume to drive our daily activities. Even when we eat things like chicken or fish, we are transferring energy from the Sun into our bodies because, at some point, one organism consumed a photosynthetic organism (e.g., the fish ate algae). So the next time you grab a snack to replenish your energy, thank the Sun for it!

This is an excerpt from the Structure and Function unit of our curriculum product line, Science and Technology Concepts TM (STC). Please visit our publisher, Carolina Biological, to learn more.

[BONUS FOR TEACHERS] Watch "Photosynthesis: Blinded by the Light" to explore student misconceptions about matter and energy in photosynthesis and strategies for eliciting student ideas to address or build on them.

National Forage & Grasslands Curriculum

All plants, including forage crops, need relatively large amounts of nitrogen (N) for proper growth and development. Biological nitrogen fixation (BNF) is the term used for a process in which nitrogen gas (N2) from the atmosphere is incorporated into the tissue of certain plants. Only a select group of plants is able to obtain N this way, with the help of soil microorganisms. Among forage plants, the group of plants known as legumes (plants in the botanical family Fabaceae) are well known for being able to obtain N from air N2.

In forage production, this process can be very important because it means that the much needed N can be obtained from three sources: the atmosphere via BNF, the soil, and from fertilizers. Forage producers who find ways to maximize the amount of N obtained from the atmosphere via BNF will be able to reduce their fertilizer costs while maintaining soil fertility, high levels of forage protein, and high yields.

The process by which some forage crops can incorporate N2 from the air into their tissues involves a host plant (also known as the macrosymbiont). For example: alfalfa and a microorganism (also known as the microsymbiont) that is associated with the host plant function in what is called a symbiotic relationship or symbiosis. A symbiotic relationship is one in which two organisms form a mutually beneficial relationship. With most forage crops the second organism is a bacteria that occurs naturally in the soil. The bacteria that is most often involved with forage crops is popularly known as rhizobia, because it is classified as part of the bacterial genus known as Rhizobium.

These soil bacteria infect the roots of the plant and form structures known as nodules. The chemical reactions, that is the process known as BNF, take place in the nodules.

Although the process involves a number of complex biochemical reactions, it may be summarized in a relatively simple way by the following equation:

The equation above indicates that one molecule of nitrogen gas (N2) combines with eight hydrogen ions (also known as protons) (8H+) to form two molecules of ammonia (2NH3) and two molecules of hydrogen gas (2H2). This reaction is conducted by an enzyme known as nitrogenase. The 16 molecules of ATP (ATP = Adenosine Triphosphate, an energy storing compound) represent the energy required for the BNF reaction to take place. In biochemical terms 16 ATP represents a relatively large amount of plant energy. Thus, the process of BNF is 'expensive' to the plant in terms of energy usage. What is the ultimate source of this energy needed for BNF? The sun, via the process of photosynthesis. As ammonia (NH3) is formed it is converted to an amino acid such as glutamine. The Nitrogen in amino acids can be used by the plant to synthesize proteins for its growth and development.