What do photons do in photosynthesis

Visible Light : The colors of visible light do not carry the same amount of energy. Violet has the shortest wavelength and, therefore, carries the most energy, whereas red has the longest wavelength and carries the least amount of energy. 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. 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.

Chlorophyll a and b , which are identical except for the part indicated in the red box, are responsible for the green color of leaves. Each pigment has d a unique absorbance spectrum. 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. Pigments in Plants : Plants that commonly grow in the shade have adapted to low levels of light by changing the relative concentrations of their chlorophyll pigments. 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. 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.

Light energy is converted into chemical energy in a multiprotein complex called a photosystem. Each photosystem consists of multiple antenna proteins that contain a mixture of — chlorophyll a and b molecules, as well as other pigments like carotenoids. Pigments in the light-harvesting complex pass light energy to two special chlorophyll a molecules in the reaction center.

The light excites an electron from the chlorophyll a pair, which passes to the primary electron acceptor. Chlorophyll a absorbs wavelengths from either end of the visible spectrum blue and red , but not from green.

Because green is reflected, chlorophyll appears green. Other pigment types include chlorophyll b which absorbs blue and red-orange light and the carotenoids. Each type of pigment can be identified by the specific pattern of wavelengths it absorbs from visible light, which is its absorption spectrum.

Many photosynthetic organisms have a mixture of pigments; between them, the organism can absorb energy from a wider range of visible-light wavelengths. Not all photosynthetic organisms have full access to sunlight.

Some organisms grow underwater where light intensity decreases with depth, and certain wavelengths are absorbed by the water. Other organisms grow in competition for light. Plants on the rainforest floor must be able to absorb any bit of light that comes through, because the taller trees block most of the sunlight Figure 5. The overall purpose of the light-dependent reactions is to convert light energy into chemical energy.

This chemical energy will be used by the Calvin cycle to fuel the assembly of sugar molecules. The light-dependent reactions begin in a grouping of pigment molecules and proteins called a photosystem. Photosystems exist in the membranes of thylakoids. A photon of light energy travels until it reaches a molecule of chlorophyll.

To replace the electron in the chlorophyll, a molecule of water is split. Technically, each breaking of a water molecule releases a pair of electrons, and therefore can replace two donated electrons. The replacing of the electron enables chlorophyll to respond to another photon.

The oxygen molecules produced as byproducts find their way to the surrounding environment. The hydrogen ions play critical roles in the remainder of the light-dependent reactions.

Keep in mind that the purpose of the light-dependent reactions is to convert solar energy into chemical carriers that will be used in the Calvin cycle. In eukaryotes and some prokaryotes, two photosystems exist. The first is called photosystem II, which was named for the order of its discovery rather than for the order of the function. After the photon hits, photosystem II transfers the free electron to the first in a series of proteins inside the thylakoid membrane called the electron transport chain.

As the electron passes along these proteins, energy from the electron fuels membrane pumps that actively move hydrogen ions against their concentration gradient from the stroma into the thylakoid space. This is quite analogous to the process that occurs in the mitochondrion in which an electron transport chain pumps hydrogen ions from the mitochondrial stroma across the inner membrane and into the intermembrane space, creating an electrochemical gradient.

To drive photosynthesis, a photon must be energetic enough to excite photosynthetic pigments. In particular, this excitation refers to the fact that an electron in the pigment needs to get shifted from one molecular energy level to another. These pigments are coupled in turn to the photochemical machinery that converts electromagnetic energy into chemical energy by producing charge separation.

This charge separation takes several forms. One contribution comes from an imbalance of protons across membranes which drive the ATP synthases, the molecular machines that synthesize ATP, resulting in an end product of ATP itself. The second form of charge separation manifests itself in the form of reducing power, the term for transient storage of electrons in carriers such as NADP used later for stable energy storage in the form of sugars produced in the Calvin-Benson cycle.

The redox reactions that drive the production of this reducing power are themselves driven by the light-induced excitation of pigments. Figure 2: The flow of energy in the biosphere. These energy currencies are then used in order to fix inorganic carbon by taking carbon dioxide from the air and transforming it into sugars that are the basis for biomass accumulation and long-term energy storage in the biosphere.

Raven, Functional Plant Biology, , of these photochemical transformations are performed by the most familiar and important pigment of them all, namely, chlorophyll. We thus get an energy scale of 1. This unit of energy is equivalent to the energy that an electron will gain when moving across a potential difference of 1.