 |
| Photosynthesis |
Photosynthesis is the process by which organic sugar molecules are synthesized from an inorganic carbon source (carbon dioxide or bicarbonate), using sunlight as the energy source to drive the process.
Although most often associated with plants (in which the reactions of photosynthesis occur within compartments called chloroplasts), algae and certain types of bacteria are also capable of photosynthesis.
From an ecological perspective, photosynthesis is significant because the conversion of inorganic carbon to organic carbon represents the entry point of carbon atoms into biological systems. Photosynthesis is also significant because it is the means where by oxygen is released into the atmosphere.
    |
The atmospheric concentration of oxygen is approximately 21 percent, and most of this oxygen originates from photosynthesis. In addition, solar energy absorbed during photosynthesis serves as the ultimate source of energy for almost all nonphotosynthetic organisms.
Nature of Light
Light from the sun is composed of various types of radiation. Only a portion of this solar radiation can be used by plants for photosynthesis.
This photosynthetically active radiation (PAR) ranges in wavelength from 400 to 700 nanometers and corresponds approximately to the visible light perceived by the human eye. The energy content of light depends on its wave length, with shorter wave lengths having a higher energy content than longer wavelengths.
Role of Pigments
For light energy to drive photosynthesis, it first must be absorbed. Several types of photosynthetic pigments are found in plants. When these pigments absorb light, some of the pigments’ electrons are elevated to a high energy level. These high-energy electrons are used to drive the reactions of photosynthesis, thus converting light energy into chemical energy.
The most common photosynthetic pigment in plants is the green-colored chlorophyll. Two types of chlorophyll are found in plants, chlorophyll a and chlorophyll b, with other types of chlorophyll found in various types of algae and photosynthetic bacteria.
Additional plant accessory pigments, such as carotenoids, which are yellow or orange, play a minor role in the absorption of wavelengths of light not absorbed by chlorophyll. Carotenoids also help protect chlorophyll from damage that may occur as a result of absorbing excess light energy.
As the most abundant plant pigment, chlorophyll gives plants their green color and usually masks the other colored pigments. In deciduous trees and shrubs, however, chlorophyll is degraded during the autumn, revealing a spectacular display of colors from carotenoids and other pigments.
Reactions of Photosynthesis
The process of photosynthesis is complex, involving many biochemical reactions. Historically, the reactions of photosynthesis have been divided into the light reactions and the dark reactions. The light reactions include the absorption of light and the conversion of light energy to chemical energy.
The dark reactions use the chemical energy produced in the light reactions to incorporate (or fix) carbon dioxide molecules into organic molecules (sugars). Within the chloroplast, the light reactions are localized in the internal network of membranes called thylakoid membranes.
The dark reactions occur in the aqueous region of the chloroplast called the stroma. The term "dark reactions" is some what misleading because several photosynthetic enzymes are not active in the dark, so these reactions will not occur without light.
Although it is common to separate the light and dark reactions when describing photosynthesis, it should be noted that these reactions are tightly coupled and occur simultaneously in the plant.
Light Reactions
Two types of photosystems occur in plants, photosystem I and photosystem II, and both are embedded in the thylakoid membranes. When light is absorbed by photosystems, the energy is transferred to special chlorophyll molecules, called reaction center chlorophylls, where the energy is transferred to electrons.
High-energy electrons are released from the reaction centers and are passed along the thylakoid membranes by a series of electron transport molecules. Energy is extracted from the electrons as they are passed along, and the energy is used to transport protons (H+) across the thylakoid membrane to the thylakoid interior.
This process establishes a proton concentration gradient that is used tomake ATP (adenosine triphosphate) from ADP (adenosine diphosphate) and inorganic phosphate (Pi) in a process called photophosphorylation.
The acceptor molecule NADP+ (nicotinamide adenine dinucleotide phosphate, oxidized form) finally accepts the high-energy electrons and combines them with protons (H+) to form the high-energy molecule NADPH (nicotinamide adenine dinucleotide phosphate, reduced form). This process is called noncyclic electron flow, and the ATP and NADPH produced are forms of chemical energy that will be utilized by the dark reactions.
Many of the functions of photosytems I and II are similar. However, only photosystem II is able to split apart water molecules in the thylakoid interior into electrons, protons, and oxygen in a process called photolysis. The electrons released from water during photolysis replace electrons lost by chlorophyll molecules during the electron transport reactions.
The protons released from water accumulate in the thylakoid interior, adding to the concentration gradient established by noncyclic electron flow. Oxygen produced from the photolysis of water is released as a gas to the atmosphere.
Therefore, oxygen gasmay be considered a by-product of plant photosynthesis. Algae and some bacteria (cyanobacteria) also release oxygen during photosynthesis, but other photosynthetic bacteria do not split water molecules and thus do not release oxygen.
The proton gradient created across the thylakoid membranes represents a source of potential energy used in making ATP. Protons are unable to diffuse across the thylakoid membranes unless permitted to do so by a special enzyme complex called the ATP synthase.
The energy required to generate ATP is provided by protons as they move through the ATP synthase from the thylakoid interior, where there is a high concentration of protons to the stroma, where there is a lower concentration of protons.
The energy associated with the proton gradient is analogous to a reservoir of water held back by a dam. Water may be allowed to pass through the dam by way of a turbine, thus using water to produce electrical power.
The use of a proton gradient across amembrane as the energy source for the synthesis of ATP by ATP synthase is called chemiosmosis, and it occurs in both the chloroplast and the mitochondria. In chloroplasts, during photosynthesis, the process is called photophosphorylation.
In mitochondria, ATP is synthesized during the process of oxidative phosphorylation, a component of cellular respiration. ATP, like NADPH, is a high energy molecule produced by the light reactions that is consumed during the dark reactions.
In a process called cyclic electron flow, the electrons can travel within the electron transport system as described above but are diverted to an acceptor in the electron transport chain between photosystems I and II.
Passing through the chain back to photosystem I, the electrons enable the transport of protons across the thykaloid membrane, thus supplying power for the generation of ATP.
Dark Reactions
Carbon dioxide gas is a normal, butminor, component of the atmosphere and enters leaves when air diffuses through stomata, small pores on the plant surfaces.
The first reaction in converting carbon dioxide to sugarmolecules occurswhen the enzyme Rubisco (also known as ribulose bisphosphate carboxylase/oxygenase, and reportedly the most abundant protein on earth) combines ribulose bisphosphate (RuBP) containing five carbon atoms with a carbon dioxide molecule to produce two identical molecules of a simple sugar, each containing three carbon atoms.
These three-carbon sugar molecules are then subsequently metabolized through a series of reactions leading to the production of a three-carbon sugar called glyceraldehyde 3-phosphate (G3P).
Some of the G3P is used to make glucose and other organic molecules, and the remaining G3P is used to regenerate RuBP so the process can continue. This cyclic pathway is known as the Calvin cycle (or the Calvin-Benson cycle).
Sugar products may be stored within the chloroplast as starch, or they may be transported as sucrose to other parts of the plant as needed. ATP and NADPH from the light reactions are required for several of the reactions of the Calvin cycle.
Because the first molecule produced by this pathway is a simple sugar with three carbon atoms, the pathway is known as the C3 pathway. Plants using this pathway are known asC3 plants, and common examples include many trees and the majority of agricultural crops.
The Rubisco enzyme, in addition to catalyzing the uptake of carbon dioxide during the Calvin cycle, can take up oxygen, initiating another metabolic pathway called photorespiration.
In photorespiration, when oxygen is attached to RuBP instead of carbon dioxide, a product results that cannot be used in the Calvin cycle, and that product must go through a different set of complex reactions.
For this reason photorespiration is often described as a wasteful process that competes with photosynthesis. The relative amounts of carbon dioxide and oxygen gases inside the chloroplast determine the relative rates of photosynthesis and photorespiration.
Experiments in which the carbon dioxide concentration of air has been altered have demonstrated that the rate of photosynthesis increases and the rate of photorespiration decreases when the concentration of carbon dioxide is increased.
In some agricultural and horticultural greenhouse operations, carbon dioxide amounts in the atmosphere are elevated to stimulate photosynthesis, leading to increases in plant production and yield.
Some plants have an adaptation where by carbon dioxide is initially fixed by a pathway other than the Calvin cycle. This adaptation involves the enzyme phosphoenolpyruvate carboxylase (PEP carboxylase), an enzyme that lacks the oxygenase activity of Rubisco.
PEP carboxylase actually attaches bicarbonate to phosphoenolpyruvate (PEP) in mesophyll cells that are in contactwith air spaces in the leaf. PEP is then converted into a series of organic acids and, in the process, is transported into a specialized set of cells called bundle sheath cells that are separated from the air spaces in the leaf.
In the bundle sheath cells carbon dioxide is released from the last organic acid in the series, which raises the carbon dioxide level in these cells where the carbon dioxide is used in the C3 cycle. Raising the carbon dioxide concentration within the chloroplast increases photosynthesis while reducing photorespiration.
The initial product of the pathway in the mesophyll cells is an organic acid with four carbon atoms, and thus the pathway is called the C4 pathway. Plants possessing this pathway are known as C4 plants. Examples includemost grasses and a few crops, including corn and sugarcane.
A second adaptation that circumvents photorespiration is the CAM (crassulacean acid metabolism) pathway, named after the family of plants in it was first observed, Crassulaceae, or the stonecrop family. CAM photosynthesis is similar to C4 photosynthesis in that it is another adaptation for raising the concentration of carbon dioxide inside the chloroplast.
CAM plants accomplish photosynthesis using a biochemical process essentially the same as that of C4 plants, but instead of carrying out these reactions in separate cells, they carry out certain reactions at night. CAM plants open their stomata only at night, and the carbon dioxide is transferred to PEP, which is converted into another organic acid that is stored throughout the night.
During the day, the stomata remain closed, and the carbon dioxide needed for the C3 cycle is supplied by releasing carbon dioxide from the last organic acid in the CAM cycle. Examples of CAM plants include cactus and pineapple. Both C4 and CAM plants typically require less water than do C3 plants and may be found in warmer and drier environments.
C4 plants tend to have high rates of photosynthesis, whereas CAM plants have low photosynthetic rates because the CAM cycle is less efficient than the C4 cycle. C3 plants typically have intermediate photosynthetic rates under optimal conditions.
Photosynthesis and the Environment
Several environmental factors affect the rate of photosynthesis. For example, temperature extremes and water stress inhibit photosynthesis. As light intensity increases, so do photosynthetic rates.
However, when photosynthesis becomes light-saturated, further increases in light intensity will not result in greater rates of photosynthesis. Leaves of plants that grow in full-sun conditions are smaller and thicker, with more extensive vascular systems than those found in shade plants.
Although so-called sun leaves and shade leaves have similar photosynthetic rates in low light, shade leaves have much lower rates of photosynthesis at high light intensities and can be damaged when exposed to such conditions.
As mentioned above, atmospheric carbon dioxide concentrations can also regulate photosynthesis. At the present time the concentration of carbon dioxide in the atmosphere is less than 0.04 percent, but scientific data show that this concentration is increasing.
Higher concentrations of atmospheric carbon dioxide may stimulate plant photosynthesis and plant growth but may have other undesirable climatic effects.
