Sunlight in Photosynthesis — Role & Importance

Why Do Plants Need Sunlight?

Sunlight has a very important job in photosynthesis. Plants can't survive without it.

In the leaves of a plant are very important organelles called chloroplasts. Within the chloroplasts are specialized pigment molecules called chlorophylls. Chlorophyll can absorb light and turn it into chemical energy that the plant can use.

Since the sun gives off a mix of mostly red and blue light, these are the colors that chlorophyll absorbs best. On the other hand, green light is reflected by chlorophyll, which is why most plants have green leaves.

The energy captured by chlorophyll can be used in photosynthesis to make sugar. When a plant gets limited sunlight, photosynthesis slows down. This also means that the plant might not be getting enough sugar—its energy source. We can see that the role of sunlight is extremely important.

How Does Sunlight Help Plants?

What Is Sunlight? What Are Photons?

The sun is the source of almost all energy on Earth. It enables plants and other organisms to turn water and carbon dioxide into sugars through a process called photosynthesis. The sun releases light that travels many millions of miles through space. A unit of that light is known as a photon. Photons have properties of both particles and waves.

The energy of a photon determines the color of light it emits. The higher the energy, the smaller the wavelength. Wavelengths are measured in nm, or nanometers. One nanometer is one billionth of a meter.

What Kind of Light Is Visible?

Humans can only see a portion of all emitted light. The visible light spectrum ranges from roughly 380 to 750 nanometer wide wavelengths.

Why Are Leaves Green?

Plants absorb light and store energy from it. They tend to absorb red and blue light, but they do not absorb green light. Green light bounces back off of the leaves. That is part of why we perceive plants as green.

How Do Plants Absorb Sunlight? What Do Chloroplasts Do?

Inside of the plant's leaves are organelles known as chloroplasts. Chloroplasts have parts called thylakoids. The thylakoids stack up inside chloroplasts like how you would stack plates. Chloroplasts also contain chlorophyll. The two main types of chlorophyll are chlorophyll A and B. They are pigments that absorb specific wavelengths of light. Chlorophyll A can absorb blue, violet, red, and orange light. Chlorophyll B mostly absorbs blue light.

What Are P680 and P700?

Chlorophyll A can also have special dimers known as P680 and P700. Both have their peak light absorption at the frequency of red light: 680 nm and 700 nm wavelengths. P700 is found in Photosystem I. P680 is located in Photosystem II, along with chlorophyll B.

How Do P680 and P700 Capture Sunlight Energy? How Is Sunlight Energy Captured During Photosynthesis?

P680 and P700 are very important for photosynthesis. P680 is the strongest natural oxidizer, and P700 is the strongest natural reducer. Remember that oxidizers take away electrons from a molecule and reducers add electrons. When P680 and P700 are hit by a light photon, they become excited.

P680 releases an electron that is taken away by the photosystem. Then, P680 takes an electron away from a water molecule with the help of a manganese molecule. This sets off a chain reaction of electron transports that leads to chemiosmotic synthesis of ATP.

P700 also ends up releasing electrons. This leads to a chemical reaction involving iron molecules that reduces NADP to NADPH. P700 is unstable after losing electrons. Electrons that used to belong to P680 are given to P700 to stabilize it for the next reaction.

That probably sounds very complex. The key thing to remember is that when P680 and P700 are hit by sunlight, it results in the production of 2 ATP, 9 NADPH, and the photolysis of water. These products are then used in the dark reactions, also called the Calvin cycle, to produce even more energy.

$$\color{#8925AE}{\text{Vocabulary Review:}}$$

$\color{#8925AE}{\text{Photosynthesis}}$ : The process of capturing solar energy to produce chemical energy

$\color{#8925AE}{\text{ATP}}$ : A molecule that stores energy in its bonds. The cell is able to break these bonds to release energy

$\color{#8925AE}{\text{Light Reaction}}$ : Light-dependent reactions use solar energy to convert ADP and NAD$^+$ to ATP and NADPH, while also producing oxygen

$\color{#8925AE}{\text{Calvin Cycle}}$ : How ATP and NADPH from the light reactions are used by the cell to create sugars

The Light Spectrum

Sunlight's job in photosynthesis is to provide energy. The sun releases energy in the forms of heat and light. As light travels through space towards Earth, it travels as a little bundle of energy called a photon. There is a lot of complicated physics explaining what a photon is, but that's not really important for biology.

So, if this photon of light finally ends its 93 million mile journey to Earth by entering a plant cell, it might just strike a pigment molecule. A pigment molecule will receive this photon of light and become energetically excited, starting the chain of reactions called photosynthesis.

Light comes in a spectrum of energy, some of which humans perceive as our color spectrum. Low energy lights are on the red end of the spectrum and high energy lights are on the blue end. When sunlight is sent through a prism, the prism breaks it into the complete spectrum of color, some of which we can see and some we cannot. Infrared is too low energy for us to see and ultraviolet (UV) rays are too high-energy for us to see.

Chlorophyll a and b are the main pigment molecules in plant cells. They absorb light at the red and blue ends of the light spectrum. They do not, however, absorb the green spectrum of light. Green light is either reflected out of the cell or allowed to pass through. This is why most plants appear green–the chlorophyll a and b pigments in their cells reflect the green wavelengths of light.

There are other pigments in plants as well, which is why not all plants look green. Carotenoids reflect light in the red-orange and yellow area of the spectrum and are found in high concentrations in plants such as carrots, pumpkins, and fall leaves.

(Video) Leaf Pigments and Light

by Teacher's Pet

This video by Teacher's Pet gives a great explanation about the spectrum of light and how chlorophyll absorbs certain wavelengths of light while reflecting others.