The Light-Dependent Reactions of Photosynthesis

Introduction to Light Energy

All electromagnetic radiation, or light energy, travels at a particular wavelength and carries a certain amount of energy.

What Is Light Energy?

The sun emits an enormous amount of electromagnetic radiation (solar or light energy). Humans can see only a fraction of this energy, which is referred to as "visible light." The manner in which solar energy travels is described as waves. Scientists can determine the amount of energy of a wave by measuring its wavelength, the distance between consecutive points of a wave, such as from crest to crest or from trough to trough.

Visible light constitutes only one of many types of electromagnetic radiation emitted from the sun and other stars. The electromagnetic spectrum is the range of all possible frequencies of radiation. The electromagnetic spectrum shows several types of electromagnetic radiation originating from the sun, including X-rays and ultraviolet (UV) rays. The higher-energy waves can penetrate tissues and damage cells and DNA, which explains why both X-rays and UV rays can be harmful to living organisms. Scientists differentiate the various types of radiant energy from the sun within the electromagnetic spectrum.The difference between wavelengths relates to the amount of energy carried by them.

Each type of electromagnetic radiation travels at a particular wavelength. The longer the wavelength, the less energy is carried. Short, tight waves carry the most energy. This may seem illogical, but think of it in terms of a person moving a heavy rope. It takes little effort by a person to move a rope in long, wide waves. To make a rope move in short, tight waves, a person would need to apply significantly more energy.

Absorption of Light

Pigments, like chlorophyll and carotenoids, absorb and reflect light at a certain region of the electromagnetic spectrum.

Absorption of Light

Light energy initiates the process of photosynthesis when pigments absorb the light. Organic pigments have a narrow range of energy levels that they can absorb. Energy levels lower than those represented by red light are insufficient to raise an orbital electron to an excited, or quantum, state. Energy levels higher than those in blue light will physically tear the molecules apart, a process called bleaching. For example, retinal pigments can only "see" (absorb) 700 nm to 400 nm light; this is visible light. For the same reasons, plant pigment molecules absorb only light in the wavelength range of 700 nm to 400 nm; plant physiologists refer to this range for plants as photosynthetically-active radiation.

The visible light seen by humans as the color white light actually exists in a rainbow of colors in the electromagnetic spectrum, with violet and blue having shorter wavelengths and, thus, higher energy. At the other end of the spectrum, toward red, the wavelengths are longer and have lower energy.

Understanding Pigments

Different kinds of pigments exist, each of which has evolved to absorb only certain wavelengths or colors of visible light. Pigments reflect or transmit the wavelengths they cannot absorb, making them appear in the corresponding color.

Chlorophylls and carotenoids are the two major classes of photosynthetic pigments found in plants and algae; each class has multiple types of pigment molecules. There are five major chlorophylls: a, b, c and d, along with a related molecule found in prokaryotes called bacteriochlorophyll.

With dozens of different forms, carotenoids are a much larger group of pigments. The carotenoids found in fruit, such as the red of tomato (lycopene), the yellow of corn seeds (zeaxanthin), or the orange of an orange peel (β-carotene), are used to attract seed-dispersing organisms. In photosynthesis, carotenoids function as photosynthetic pigments that are very efficient molecules for the disposal of excess energy. When a leaf is exposed to full sun, the light-dependent reactions are required to process an enormous amount of energy; if that energy is not handled properly, it can do significant damage. Therefore, many carotenoids are stored in the thylakoid membrane to absorb excess energy and safely release that energy as heat.

Each type of pigment can be identified by the specific pattern of wavelengths it absorbs from visible light, which is the absorption spectrum. Chlorophyll a absorbs light in the blue-violet region, while chlorophyll b absorbs red-blue light. Neither a or b absorb green light; because green is reflected or transmitted, chlorophyll appears green. Carotenoids absorb light in the blue-green and violet region and reflect the longer yellow, red, and orange wavelengths.

Many photosynthetic organisms have a mixture of pigments. In this way organisms can absorb energy from a wider range of wavelengths. Not all photosynthetic organisms have full access to sunlight. Some organisms grow underwater where light intensity and quality decrease and change with depth. Other organisms grow in competition for light. Plants on the rainforest floor must be able to absorb any light that comes through because the taller trees absorb most of the sunlight and scatter the remaining solar radiation

When studying a photosynthetic organism, scientists can determine the types of pigments present by using a spectrophotometer. These instruments can differentiate which wavelengths of light a substance can absorb. Spectrophotometers measure transmitted light and compute its absorption. By extracting pigments from leaves and placing these samples into a spectrophotometer, scientists can identify which wavelengths of light an organism can absorb.

Processes of the Light-Dependent Reactions

Light-dependent reactions, which take place in photosystem I and II, convert solar energy into NADPH and ATP.

How Light-Dependent Reactions Work

The overall function of light-dependent reactions, the first stage of photosynthesis, is to convert solar energy into chemical energy in the form of NADPH and ATP, which are used in light-independent reactions and fuel the assembly of sugar molecules. Protein complexes and pigment molecules work together to produce NADPH and ATP.

Producing Chemical Energy

Light energy is converted into chemical energy in a multiprotein complex called a photosystem. Two types of photosystems, photosystem I (PSI) and photosystem II (PSII), are found in the thylakoid membrane inside the chloroplast. Each photosystem consists of multiple antenna proteins that contain a mixture of 300–400 chlorophyll a and b molecules, as well as other pigments like carotenoids. Cytochrome b6f complex and ATP synthase are also major protein complexes in the thylakoid membrane that work with the photosystems to create ATP and NADPH.

The two photosystems absorb light energy through proteins containing pigments, such as chlorophyll. The light-dependent reactions begin in photosystem II. In PSII, energy from sunlight is used to split water, which releases two electrons, two hydrogen atoms, and one oxygen atom. When a chlorophyll a molecule within the reaction center of PSII absorbs a photon, the electron in this molecule attains a higher energy level. Because this state of an electron is very unstable, the electron is transferred to another molecule creating a chain of redox reactions called an electron transport chain (ETC). The electron flow goes from PSII to cytochrome b6f to PSI; as electrons move between these two photosystems, they lose energy. Because the electrons have lost energy prior to their arrival at PSI, they must be re-energized by PSI. Therefore, another photon is absorbed by the PSI antenna. That energy is transmitted to the PSI reaction center. This reaction center, known as P700, is oxidized and sends a high-energy electron to reduce NADP+ to NADPH. This process illustrates oxygenic photosynthesis, wherein the first electron donor is water and oxygen is created as a waste product.

Cytochrome b6f and ATP synthase work together to create ATP. This process, called photophosphorylation, occurs in two different ways. In non-cyclic photophosphorylation, cytochrome b6f uses the energy of electrons from PSII to pump hydrogen ions from the lumen (an area of high concentration) to the stroma (an area of low concentration). The energy released by the hydrogen ion stream allows ATP synthase to attach a third phosphate group to ADP, which forms ATP. This flow of hydrogen ions through ATP synthase is called chemiosmosis because the ions move from an area of high to an area of low concentration through a semi-permeable structure. In cyclic photophosphorylation, cytochrome b6f uses the energy of electrons from both PSII and PSI to create more ATP and to stop the production of NADPH. Cyclic phosphorylation is important to maintain the right proportions of NADPH and ATP, which will carry out light-independent reactions later on.

The net-reaction of all light-dependent reactions in oxygenic photosynthesis is: 2H₂O + 2NADP+ + 3ADP + 3Pi → O₂ + 2NADPH + 3ATP