Photosynthesis is a process by which plants use solar energy to make food molecules from CO2 and water. It is the most important chemical process on earth for it provides food supply for virtually all organisms.
Plant protoplasts capture light energy and convert it to chemical energy that is stored in glucose and other organic molecules.
Photosynthesis is given by the following equation, which is a simplified version of a very complex process.
Photosynthesis consists of two reactions, each with multiple steps. These are the light-dependent and dark-dependent reactions. Light-dependent reactions occur in the presence of light and generate adenosine triphosphate (ATP) and nicotinamide adenine dinucleotide phosphate (NADPH). Dark-dependent reactions do not require light but use ATP and NADPH generated during the light reactions to fix carbon dioxide and make carbohydrates.
Photosynthetic pigments
When light strikes matter, it can be absorbed, radiated or reflected. Substances that absorb visible light are called pigments. Different pigments absorb light of different wavelengths. If a pigment absorbs all the wavelengths of light, we see black. However, if a pigment absorbs all the other colours and reflects yellow or green, we see the colour that is reflected. For example, we see green when we look at a leaf because chlorophyll absorbs red and blue light while reflecting green light.
Absorption spectra of chlorophyll a and b
The absorption spectrum of chlorophyll a (the form of chlorophyll most important in photosynthesis) has two peaks, corresponding to wavelengths for blue and red light (the colours chlorophyll a absorbs best). Chlorophyll a can participate directly in the light reactions that convert solar energy to chemical energy. Other pigments can absorb light and transmit energy to chlorophyll a. One of these is chlorophyll b, which is almost identical to chlorophyll a, except for a small difference. Chlorophyll a is grass-green, whereas chlorophyll b is yellow-green.
Other pigments
The chloroplast has other pigments. These are carotenoids, which are of various shades of yellow and orange. Chlorophyll a, chlorophyll b and the carotenoids are found in the thylakoid membrane. The location of the chlorophyll a in the pigment is called the reaction centre. The other chlorophyll molecules and the molecules of chlorophyll b and the carotenoids collectively function as light gathering antennae that absorb photons (light energy) and pass them to the reaction centre. For example, if a photon of light strikes a carotenoid or chlorophyll b, the energy is conveyed to chlorophyll a.
Photosynthesis takes place entirely within chloroplasts. Like mitochondria, chloroplasts have a double membrane: an inner membrane and an outer membrane. The inner membrane, which is relatively smooth, encloses a fluid matrix called the stroma. In addition, chloroplasts have a third membrane called the thylakoid membrane. This is folded into thin vesicles (the thylakoids), enclosing small spaces called the thylakoid lumen. The thylakoid vesicles are often layered in stacks called grana (single, granum).
Structure of chloroplast
Photosystems
Photosystems are light harnessing units in the thylakoid membrane. A photosystem consists of an antenna complex, a reaction centre and a primary acceptor of energized electrons. Two types of photosystems are found in the thylakoid membrane: photosystem I and photosystem II, in order of their discovery. At the reaction centre of photosystem 1, is a specialized chlorophyll a molecule called P700, so called because the light the pigment absorbs best has a wavelength of 700 nm, which is in the far-red part of the spectrum. At the reaction centre of photosystem II is a specialized chlorophyll a molecule designated P680, with a peak absorption spectrum of 680 nm, also in the red part of the spectrum.
Photosystem I and Photosystem II
Light-dependent reactions
Light reactions take place in the light and in the thylakoid membrane of chloroplasts. The key reactions of the light reactions are (1) the absorption of light energy (2) the excitation of electrons and (3) the formation of ATP and NADPH. There are two possible routes for light reactions: cyclic and noncyclic routes. Both of these reactions are also known as photophosphorylation because they use light energy to phosphorylate (add inorganic phosphate) to a molecule of ADP to obtain ATP.
Cyclic Photophosphorylation
In cyclic photophosphorylation, light energy is trapped by pigment molecules in the P700 antenna system and the excited state eventually reaches the reaction centre P700. Energized electrons from P700 move from one acceptor molecule to the next releasing free energy at each step and eventually returning to the P700 molecule. Some of the energy released as the electron is eased down the energy gradient is used by the cell to transport H+ ions across the thylakoid membrane creating an H+ ion gradient. The energy of this gradient is used to phosphorylate ADP into ATP. Because the electrons are returned to the chlorophyll molecules from which they originated, this process is termed cyclic photophosphorylation. ATP is the only product of this process.
Noncyclic Photophosphorylation
Noncyclic photophosphorylation involves both photosystem I and photosystem II. When a photon energy is received by P680, electrons become highly excited so that a few actually leave the photosystem II molecule, leaving ‘holes’ in reaction centre P680. These empty spaces (holes) are then filled by electrons coming from two molecules of water split in a process called photolysis. Oxygen produced by photolysis diffuses out of the cell and escapes into the air through the leaf stomata. This is the oxygen that sustains life on earth.
2H20 → 4H+ +4e- + O2
The electrons are received by P700, the reaction centre for photosystem I. However, prior to their arrival, the pigment complex in reaction centre P700 has absorbed a photon of light energizing its electrons some of which are captured by an acceptor molecule. The resulting vacancy in P700 can now be filled by electrons from photosystem II. The excited electron of photosystem I reduces nicotinamide adenine dinucleotide phosphate (NADP+) to NADPH.
Therefore, light reactions are concerned with (a) the production of oxygen from water and (2) formation of energy rich compounds NADPH and ATP needed during the synthesis of carbohydrates from CO2 in the dark reactions.
The Dark Reactions
Dark reactions (also known as the Calvin cycle) are involved in the final stage of photosynthesis, which is the synthesis of sugar, using CO2, ATP, and hydrogen atoms from NADPH. These reactions occur in the stroma of the chloroplast and do not require light. Carbon dioxide from the atmosphere diffuses into the leaf through the stomata and enters the stroma of the chloroplast. It then binds with a 5-carbon sugar called ribulose bisphosphate (RuDP), present in the leaf. The reaction is catalyzed by an enzyme called rubisco carboxylate. This enzyme is present in large quantities in the stroma of chloroplasts.
The product is unstable 6-carbon compound, which immediately splits into two molecules of a 3-carbon compound called phosphoglyceric acid (PGA. Each PGA is phosphorylated by ATP and reduced by hydrogen from NADP. The resulting energy-rich 3-carbon compound is called phosphoglyceraldehyde (PGAL). This is a stable sugar and,
in a way, forms the final product of photosynthesis.
PGAL is a 3-carbon sugar and this is the reason photosynthesis is sometimes referred to as a C3 photosynthesis. Three carbon compounds like PGAL are the simplest carbohydrates that combine to form larger, more complex monosaccharides such as glucose and fructose. Glucose molecules in turn combine to form the storage carbohydrate starch. To form the 6-carbon molecule of glucose, takes six CO2 molecules and six turns of the Calvin cycle.
Leaves as Organs of Photosynthesis
Photosynthesis can occur in all green parts of a plant but in most plants the leaf is the main photosynthetic part. The structure of a dicot leaf consists of a stalk or petiole, which attaches the leaf to the plant and a flat, broad blade. The blade contains a complex system of veins, which serve as transport channels for water and nutrients. Because of the flatness of the blade, the leaf exposes a large surface area to sunlight.
A transverse section of the leaf shows that the outer surface is formed of epidermal layers, usually one but sometimes two or more layers thick. A waxy cuticle covers the outer surfaces of both the upper and lower epidermal layers. The chief function of the epidermis is the protection of the internal tissues of the leaf from excessive loss of water, invasion by fungi and mechanical injury. Most epidermal cells do not contain chloroplasts. The cuticle is transparent and permits light to pass through to the inner cells of the leaf.
Internal structure of a leaf
The region between the upper and lower epidermis is called the mesophyll and is usually divided into an upper palisade mesophyll consisting of cells arranged vertically, and a spongy mesophyll made up of irregularly shaped, loosely packed cells with air spaces between them. The air spaces communicate with the outside via tiny pores on the leaf called stomata (singular, stoma). The palisade cells are filled with chloroplasts and it in palisade cells where most of the photosynthesis occurs. The spongy layer cells contain chloroplasts but it seems their major function is temporary storage of food substances manufactured by the leaves.
A system of veins known as vascular bundles branch from a central vein to the rest of the blade forming a structural framework for the leaf and serving as transport channels. Each vein contains xylem and phloem vascular tissues, and each is usually surrounded by a bundle sheath, made up of very tightly packed cells that leave very little space between them.
Adaptation to Water Conservation
C3, C4 and CAM are three types of photosynthetic pathways that are adapted to water conservation. C3 photosynthesis is the typical photosynthesis that most plants use and that most of us learn about at school. C4 and CAM photosynthesis are adaptations to arid conditions because they result in better water conservation by plants.
C3 plants
Most plants use the C3 pathway to incorporate CO2 into the Krebs cycle during the making of sugar compounds. These plants incorporate CO2 from the air directly into the Calvin cycle. They derive their name from the fact that the first organic compound produced is the 3-carbon compound, PGA. They require moderate sunlight intensity, moderate temperatures, high CO2 concentration, about 200 ppm or more and plenty of ground water. C3 plants open their stomata during the day. However, during a very dry, hot day, they close the stomata to prevent water loss and avoid wilting. Another characteristic of these plants is the fact that photosynthesis takes place throughout the leaf.
One of the problems of growing C3 plants is that dry weather can reduce the rate of photosynthesis and decrease crop productivity. On a dry, hot day, a C3 plant closes its stomata. This adaptation at water conservation prevents CO2 from entering the leaf and O2 from leaving the leaf. This can lead to very low levels of CO2 and high concentrations of O2 in the leaf.
When this happens, the first enzyme of the Calvin cycle, called rubisco, incorporates O2 instead of CO2 and the Calvin cycle produces a 2-C compound called phosphoglycolate instead of its usual 3-C product. The plant cell then breaks the 2-C compound down to CO2 and water. This process is called photorespiration. Unlike photosynthesis, photorespiration is wastage of energy because it produces no sugar molecules. Unlike cellular respiration, it does not produce ATP.
C3 plants include soyabeans, oats, wheat and rice.
C4 Plants
C4 plants represent about 5% of the world’s plant biomass. They are called C4 plants because CO2 is first incorporated into a 4-carbon compound. C4 plants have developed a mechanism that delivers CO2 to the rubisco enzyme efficiently. They use their specific leaf anatomy called Kranz Anatomy, where chloroplasts exist not only in the mesophyll cells in the outer part of their leaves but also in the bundle sheath cells as well. Instead of the direct fixation in the Calvin cycle, CO2 is converted to a 4-carbon organic acid that has the ability to regenerate CO2 in the chloroplasts of the bundle sheath cells. Bundle sheath cells can then utilise this CO2 to generate carbohydrates by the usual C3 pathway.
Examples of C4 plants include corn, sugarcane, sorghum and finger millet.
CAM plants are called after the plant family in which they were first found (Crassulaceae) and because CO2 is stored in form of acid before use in photosynthesis. Characteristics of CAM plants include:
· Stomata open at night when evaporation is low to prevent water loss and remain closed during the day
· CO2 is converted to an acid and stored during the night. During the day, the acid is broken down and CO2 is released to rubisco for photosynthesis.
Significance of CAM metabolism is that the use of water is more efficient than in C3 plants under hot dry conditions. This is because the stomata open only at night when the transpiration is at its lowest and remain closed during the day when transpiration is highest.
However, under very hot, dry conditions, CAM plants keep their stomata closed day and night. Oxygen given off in photosynthesis is used for respiration and CO2 produced by respiration is used for photosynthesis. This CAM idling as it is called allows the plant to survive harsh dry spells and recover quickly once water becomes available.
C4 and CAM plants have competitive advantage over C3 plants under conditions of drought, high temperatures, and limited nitrogen or carbon dioxide. Both have overcome the tendency of rubisco (the first enzyme in the Calvin cycle) to photorespire or waste energy by using O2 to break down carbon compounds to CO2. However, C4 fixation still uses more energy in form of ATP than C3 fixation.
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