Every living thing on the planet needs food or energy to survive. Some organisms feed on other creatures, while others can produce their own nutrients. they themselves produce food, glucose, in a process called photosynthesis.
Photosynthesis and respiration are interconnected. The result of photosynthesis is glucose, which is stored as chemical energy in the. This stored chemical energy comes from the conversion of inorganic carbon (carbon dioxide) to organic carbon. The breathing process releases stored chemical energy.
In addition to the foods they produce, plants also need carbon, hydrogen, and oxygen to survive. Water absorbed from the soil provides hydrogen and oxygen. During photosynthesis, carbon and water are used to synthesize food. Plants also need nitrates to make amino acids (an amino acid is an ingredient in protein production). In addition to this, they need magnesium to produce chlorophyll.
The note: Living things that depend on other foods are called. Herbivores such as cows as well as insect-eating plants are examples of heterotrophs. Living things that produce their own food are called. Green plants and algae are examples of autotrophs.
In this article, you will learn more about how photosynthesis occurs in plants and the conditions necessary for this process.
Determination of photosynthesis
Photosynthesis is the chemical process by which plants, some and algae produce glucose and oxygen from carbon dioxide and water, using only light as an energy source.
This process is extremely important for life on Earth, because thanks to it oxygen is released, on which all life depends.
Why do plants need glucose (food)?
Like humans and other living things, plants also need food to keep them alive. The value of glucose for plants is as follows:
- Glucose from photosynthesis is used during respiration to release energy that the plant needs for other vital processes.
- Plant cells also convert some of the glucose into starch, which is used as needed. For this reason, dead plants are used as biomass because they store chemical energy.
- Glucose is also needed to produce other chemicals such as proteins, fats, and plant sugars, which are needed for growth and other important processes.
Phases of photosynthesis
The process of photosynthesis is divided into two phases: light and dark.
Light phase of photosynthesis
As the name suggests, light phases need sunlight. In light-dependent reactions, the energy of sunlight is absorbed by chlorophyll and converted into stored chemical energy in the form of an electron carrier molecule NADPH (nicotinamide adenine dinucleotide phosphate) and an energy molecule ATP (adenosine triphosphate). The light phases occur in thylakoid membranes within the chloroplast.
The dark phase of photosynthesis or the Calvin cycle
In the dark phase, or Calvin cycle, excited electrons from the light phase provide energy for the formation of carbohydrates from carbon dioxide molecules. The light-independent phases are sometimes called the Calvin cycle due to the cyclical nature of the process.
Although the dark phases do not use light as a reagent (and as a result, can occur day or night), they need the products of light dependent reactions to function. Light-independent molecules depend on energy carrier molecules - ATP and NADPH - to create new carbohydrate molecules. After the transfer of energy, the molecules of the energy carriers return to the light phases to obtain more energetic electrons. In addition, several dark phase enzymes are activated by light.
Photosynthesis phase diagram
The note:This means that the dark phases will not continue if the plants are deprived of light for too long, as they are using light phase products.
Plant leaf structure
We cannot fully study photosynthesis without knowing more about leaf structure. The leaf is adapted to play a vital role in the process of photosynthesis.
External structure of leaves
- Square
One of the most important features of plants is their large leaf surface area. Most green plants have wide, flat, and open leaves that can capture as much solar energy (sunlight) as needed for photosynthesis.
- Central vein and petiole
The central vein and petiole are joined together and form the base of the leaf. The petiole positions the leaf so that it receives as much light as possible.
- Leaf blade
Simple leaves have one leaf plate, and complex leaves have several. The leaf blade is one of the most important components of the leaf, which is directly involved in the process of photosynthesis.
- Veins
A network of veins in the leaves carries water from the stems to the leaves. The released glucose is also directed to other parts of the plant from the leaves through the veins. In addition, these portions of the sheet support and keep the sheet metal plate flat for greater sunlight capture. The location of the veins (venation) depends on the type of plant.
- Sheet base
The base of the leaf is its lowest part, which is articulated with the stem. Often, a pair of stipules is located at the base of the leaf.
- Leaf edge
Depending on the type of plant, the leaf edge can have a different shape, including: whole-edged, serrated, serrated, notched, crenate, etc.
- Top of the leaf
Like the edge of a leaf, the tip comes in a variety of shapes, including: sharp, rounded, obtuse, elongated, drawn, etc.
Internal structure of leaves
Below is a similar diagram of the internal structure of leaf tissues:
- Cuticle
The cuticle acts as the main protective layer on the surface of the plant. It is usually thicker at the top of the sheet. The cuticle is coated with a wax-like substance that protects the plant from water.
- Epidermis
The epidermis is the layer of cells that is the integumentary tissue of the leaf. Its main function is to protect the inner tissues of the leaf from dehydration, mechanical damage and infections. It also regulates the process of gas exchange and transpiration.
- Mesophyll
Mesophyll is the main plant tissue. This is where the process of photosynthesis takes place. In most plants, the mesophyll is divided into two layers: the upper one is palisade and the lower one is spongy.
- Protective cells
Defense cells are specialized cells in the leaf epidermis that are used to control gas exchange. They have a protective function for the stomata. The stomatal pores become large when water is freely available, otherwise the defense cells become flaccid.
- Stoma
Photosynthesis depends on the penetration of carbon dioxide (CO2) from the air through the stomata into the mesophyll tissue. Oxygen (O2), produced as a byproduct of photosynthesis, leaves the plant through the stomata. When the stomata are open, water is lost by evaporation and must be replenished through the transpiration stream with water absorbed by the roots. Plants are forced to balance the amount of absorbed CO2 from the air and the loss of water through the stomatal pores.
Conditions for photosynthesis
Below are the conditions that plants need to carry out the process of photosynthesis:
- Carbon dioxide. A colorless, odorless natural gas found in the air and has the scientific designation CO2. It is formed when carbon and organic compounds are burned, and also occurs during respiration.
- Water... A clear liquid chemical, odorless and tasteless (under normal conditions).
- Shine.Although artificial light is also suitable for plants, natural sunlight tends to create the best conditions for photosynthesis because it contains natural UV radiation that has a positive effect on plants.
- Chlorophyll.It is a green pigment found in plant leaves.
- Nutrients and Minerals.Chemicals and organic compounds that plant roots absorb from the soil.
What is formed as a result of photosynthesis?
- Glucose;
- Oxygen.
(Light energy is shown in parentheses as it is not matter)
The note: Plants get CO2 from the air through their leaves, and water from the soil through their roots. Light energy comes from the sun. The resulting oxygen is released into the air from the leaves. The resulting glucose can be converted into other substances, such as starch, which is used for energy storage.
If factors promoting photosynthesis are absent or present in insufficient quantities, this can negatively affect the plant. For example, less light creates favorable conditions for insects that eat the leaves of the plant, and lack of water slows down.
Where does photosynthesis take place?
Photosynthesis takes place inside plant cells, in small plastids called chloroplasts. Chloroplasts (mostly found in the mesophyll layer) contain a green substance called chlorophyll. Below are the other parts of the cell that work with the chloroplast to carry out photosynthesis.
Plant cell structure
Functions of plant cell parts
- : provides structural and mechanical support, protects cells from, fixes and defines the shape of the cell, controls the rate and direction of growth, and gives shape to plants.
- : provides a platform for most of the enzyme controlled chemical processes.
- : acts as a barrier, controlling the movement of substances into and out of the cell.
- : as described above, they contain chlorophyll, a green substance that absorbs light energy during photosynthesis.
- : a cavity within the cellular cytoplasm that stores water.
- : contains a genetic mark (DNA) that controls cell activity.
Chlorophyll absorbs light energy needed for photosynthesis. It is important to note that not all color wavelengths of light are absorbed. Plants primarily absorb red and blue waves - they do not absorb light in the green range.
Carbon dioxide from photosynthesis
Plants get carbon dioxide from the air through their leaves. Carbon dioxide seeps through a small hole at the bottom of the leaf - the stomata.
The lower part of the leaf has loosely spaced cells so that carbon dioxide reaches other cells in the leaves. It also allows the oxygen generated during photosynthesis to easily leave the leaf.
Carbon dioxide is present in the air we breathe at very low concentrations and is a necessary factor in the dark phase of photosynthesis.
Light in the process of photosynthesis
The sheet usually has a large surface area, so it can absorb a lot of light. Its upper surface is protected from water loss, disease and weather by a wax layer (cuticle). The top of the leaf is where the light falls. This layer of mesophyll is called palisade. It is adapted to absorb a large amount of light, because it contains many chloroplasts.
In light phases, the process of photosynthesis increases with more light. More chlorophyll molecules are ionized, and more ATP and NADPH are generated if the light photons are focused on the green leaf. Although light is extremely important in light phases, it should be noted that excessive amounts of it can damage chlorophyll and reduce photosynthesis.
The light phases are not very dependent on temperature, water or carbon dioxide, although all of them are needed to complete the photosynthesis process.
Water during photosynthesis
Plants get the water they need for photosynthesis through their roots. They have root hairs that grow in the soil. The roots have a large surface area and thin walls that allow water to pass through easily.
The image shows plants and their cells with sufficient water (left) and lack of water (right).
The note: Root cells do not contain chloroplasts because they are usually in the dark and cannot photosynthesize.
If the plant does not absorb enough water, it withers. Without water, the plant will not be able to photosynthesize fast enough and may even die.
How important is water for plants?
- Provides dissolved minerals that support plant health;
- Is a medium for transportation;
- Supports stability and uprightness;
- Cools and moisturizes;
- It makes it possible to carry out various chemical reactions in plant cells.
The importance of photosynthesis in nature
The biochemical process of photosynthesis uses energy from sunlight to convert water and carbon dioxide into oxygen and glucose. Glucose is used as the building blocks in plants for tissue growth. Thus, photosynthesis is the way in which roots, stems, leaves, flowers and fruits are formed. Without the process of photosynthesis, plants cannot grow or reproduce.
- Producers
Because of their photosynthetic ability, plants are known as producers and form the backbone of nearly every food chain on Earth. (Algae are the equivalent of plants in). All the food we eat comes from organisms that are photosynthetic. We eat these plants directly or eat animals such as cows or pigs that consume plant foods.
- The backbone of the food chain
Within aquatic systems, plants and algae also form the backbone of the food chain. Algae serve as food for, which, in turn, act as a food source for larger organisms. Without photosynthesis in the aquatic environment, life would be impossible.
- Removal of carbon dioxide
Photosynthesis converts carbon dioxide into oxygen. During photosynthesis, carbon dioxide from the atmosphere enters the plant and is then released as oxygen. In today's world, where levels of carbon dioxide are rising at an alarming rate, any process that removes carbon dioxide from the atmosphere is environmentally important.
- Nutrient Cycle
Plants and other photosynthetic organisms play a vital role in the nutrient cycle. Nitrogen in the air is fixed in plant tissues and becomes available for making proteins. Trace elements found in the soil can also be incorporated into plant tissue and made available to herbivores further down the food chain.
- Photosynthetic addiction
Photosynthesis depends on the intensity and quality of light. At the equator, where sunlight is abundant throughout the year and water is not a limiting factor, plants grow at high rates and can get quite large. Conversely, photosynthesis in deeper parts of the ocean is less common because light does not penetrate these layers, and as a result this ecosystem is more sterile.
Photosynthesis is a process of synthesis of organic substances from inorganic ones due to the energy of light. In the overwhelming majority of cases, photosynthesis is carried out by plants using such cell organelles as chloroplastscontaining green pigment chlorophyll.
If plants were not capable of synthesizing organic matter, then almost all other organisms on Earth would have nothing to feed on, since animals, fungi and many bacteria cannot synthesize organic substances from inorganic ones. They only absorb the ready-made ones, split them into simpler ones, from which they again assemble complex ones, but already characteristic of their body.
This is the case when it comes to photosynthesis and its role very briefly. To understand photosynthesis, more needs to be said: what specific inorganic substances are used, how does synthesis take place?
Photosynthesis requires two inorganic substances - carbon dioxide (CO 2) and water (H 2 O). The first is absorbed from the air by the aerial parts of plants mainly through the stomata. Water - from the soil, from where it is delivered to the photosynthetic cells by the plant's conducting system. Also, photosynthesis requires the energy of photons (hν), but they cannot be attributed to matter.
In total, photosynthesis produces organic matter and oxygen (O 2). Usually, organic matter most often means glucose (C 6 H 12 O 6).
Organic compounds are mostly composed of carbon, hydrogen and oxygen atoms. They are the ones found in carbon dioxide and water. However, oxygen is released during photosynthesis. Its atoms are taken from water.
Briefly and generally, the equation for the reaction of photosynthesis is usually written as follows:
6CO 2 + 6H 2 O → C 6 H 12 O 6 + 6O 2
But this equation does not reflect the essence of photosynthesis, does not make it understandable. Look, although the equation is balanced, it has a total of 12 atoms in free oxygen. But we said that they come from water, and there are only 6 of them.
In fact, photosynthesis takes place in two phases. The first is called light, the second is dark... Such names are due to the fact that light is needed only for the light phase, the dark phase is independent of its presence, but this does not mean that it goes in the dark. The light phase occurs on the chloroplast thylakoid membranes, the dark phase in the chloroplast stroma.
In the light phase, no CO 2 binding occurs. There is only the capture of solar energy by chlorophyll complexes, its storage in ATP, the use of energy to reduce NADP to NADP * H 2. The flow of energy from chlorophyll excited by light is provided by electrons, which are transferred along the electron transport chain of enzymes built into the thylakoid membranes.
Hydrogen for NADP is taken from water, which, under the influence of sunlight, decomposes into oxygen atoms, hydrogen protons and electrons. This process is called photolysis... Oxygen from water is not needed for photosynthesis. Oxygen atoms from two water molecules combine to form molecular oxygen. The reaction equation for the light phase of photosynthesis is briefly as follows:
H 2 O + (ADP + F) + NADP → ATP + NADP * H 2 + ½O 2
Thus, oxygen is released during the light phase of photosynthesis. The number of ATP molecules synthesized from ADP and phosphoric acid per photolysis of one water molecule can be different: one or two.
So, ATP and NADP * H 2 come from the light phase to the dark one. Here the energy of the first and the reducing force of the second are spent on binding carbon dioxide. This stage of photosynthesis cannot be explained simply and succinctly, because it does not proceed in the way that six CO 2 molecules combine with hydrogen released from NADP * H 2 molecules to form glucose:
6CO 2 + 6NADP * H 2 → C 6 H 12 O 6 + 6NADP
(the reaction proceeds with the expenditure of energy ATP, which decomposes into ADP and phosphoric acid).
The above reaction is just a simplification to facilitate understanding. In fact, carbon dioxide molecules bind one at a time, attach to a ready-made five-carbon organic substance. An unstable six-carbon organic matter is formed, which decomposes into three-carbon carbohydrate molecules. Some of these molecules are used for the resynthesis of the original five-carbon substance for binding CO 2. Such resynthesis is provided calvin cycle... A minority of the three-carbon carbohydrate molecules leave the cycle. All other organic substances (carbohydrates, fats, proteins) are synthesized from them and other substances.
That is, in fact, three-carbon sugars, and not glucose, are released from the dark phase of photosynthesis.
DEFINITION: Photosynthesis is the process of the formation of organic substances from carbon dioxide and water, in the light, with the release of oxygen.
Brief explanation of photosynthesisThe process of photosynthesis involves:
1) chloroplasts,
3) carbon dioxide,
5) temperature.
In higher plants, photosynthesis occurs in chloroplasts - oval-shaped plastids (semi-autonomous organelles) containing the chlorophyll pigment, due to the green color of which plant parts also have a green color.
In algae, chlorophyll is contained in chromatophores (pigment-containing and light-reflecting cells). Brown and red algae that live at considerable depths, where sunlight does not reach well, have other pigments.
If you look at the food pyramid of all living things, photosynthetic organisms are at the very bottom, in the composition of autotrophs (organisms that synthesize organic substances from inorganic ones). Therefore, they are the source of food for all life on the planet.
During photosynthesis, oxygen is released into the atmosphere. In the upper atmosphere, ozone is formed from it. An ozone shield protects the Earth's surface from harsh ultraviolet radiation, allowing life to escape from the sea to land.
Oxygen is essential for the respiration of plants and animals. When glucose is oxidized with the participation of oxygen, the mitochondria store almost 20 times more energy than without it. This makes the use of food much more efficient, resulting in high metabolic rates in birds and mammals.
A more detailed description of the process of photosynthesis of plants
Photosynthesis progress:
The process of photosynthesis begins with the ingress of light on chloroplasts - intracellular semi-autonomous organelles containing a green pigment. Under the influence of light, chloroplasts begin to consume water from the soil, breaking it down into hydrogen and oxygen.
Part of the oxygen is released into the atmosphere, the other part goes to oxidative processes in the plant.
Sugar combines with nitrogen, sulfur and phosphorus coming from the soil, in this way green plants produce starch, fats, proteins, vitamins and other complex compounds necessary for their life.
Photosynthesis is best done under the influence of sunlight, but some plants can be content with artificial light.
A complex description of the mechanisms of photosynthesis for the advanced reader
Until the 60s of the 20th century, scientists knew only one mechanism for fixing carbon dioxide - by the C3-pentose phosphate pathway. Recently, however, a group of Australian scientists was able to prove that in some plants the reduction of carbon dioxide occurs via the C4-dicarboxylic acid cycle.
In plants with the C3 reaction, photosynthesis occurs most actively under conditions of moderate temperature and light, mainly in forests and in dark places. Such plants include almost all cultivated plants and most of the vegetables. They form the basis of the human diet.
In plants with the C4 reaction, photosynthesis occurs most actively under high temperature and light conditions. Such plants include, for example, corn, sorghum and sugarcane, which grow in warm and tropical climates.
The metabolism of plants itself was discovered quite recently, when it was possible to find out that in some plants that have special tissues for water storage, carbon dioxide accumulates in the form of organic acids and is fixed in carbohydrates only after a day. This mechanism helps plants conserve water supplies.
How photosynthesis takes place
The plant absorbs light with a green substance called chlorophyll. Chlorophyll is found in chloroplasts, which are found in stems or fruits. They are especially abundant in the leaves, because, due to its very flat structure, the leaf can attract a lot of light, and, accordingly, receive much more energy for the photosynthesis process.
After absorption, chlorophyll is in an excited state and transfers energy to other molecules of the plant's body, especially those that are directly involved in photosynthesis. The second stage of the photosynthesis process takes place without the obligatory participation of light and consists in obtaining a chemical bond with the participation of carbon dioxide obtained from air and water. At this stage, various substances that are very useful for life are synthesized, such as starch and glucose.
These organic substances are used by the plants themselves to feed various parts of it, as well as to maintain normal life. In addition, these substances are also obtained by animals, feeding on plants. People also get these substances by eating foods of animal and plant origin.
Conditions for photosynthesis
Photosynthesis can occur both under the influence of artificial light and sunlight. As a rule, in nature, plants intensively "work" in the spring-summer period, when there is a lot of necessary sunlight. In autumn, there is less light, the day is shortened, the leaves first turn yellow and then fall off. But as soon as the warm spring sun appears, green foliage reappears and green "factories" again resume their work to provide oxygen, which is so necessary for life, as well as many other nutrients.
Alternative definition of photosynthesis
Photosynthesis (from ancient Greek phot - light and synthesis - combination, folding, binding, synthesis) - the process of converting light energy into the energy of chemical bonds of organic substances in the light by photoautotrophs with the participation of photosynthetic pigments (chlorophyll in plants, bacteriochlorophyll and bacteriorhodopsin in bacteria ). In modern plant physiology, photosynthesis is more often understood as a photoautotrophic function - a set of processes of absorption, conversion and use of the energy of light quanta in various endergonic reactions, including the conversion of carbon dioxide into organic substances.
Phases of photosynthesis
Photosynthesis is a rather complex process and includes two phases: light, which always occurs exclusively in light, and dark. All processes take place within chloroplasts on special small organs - tilakodia. During the light phase, a quantum of light is absorbed by chlorophyll, resulting in the formation of ATP and NADPH molecules. In this case, water decomposes, forming hydrogen ions and releasing an oxygen molecule. The question arises, what are these incomprehensible mysterious substances: ATP and NADH?
ATP is a special organic molecule found in all living organisms and is often referred to as "energy" currency. It is these molecules that contain high-energy bonds and are the source of energy for any organic synthesis and chemical processes in the body. Well, NADPH is actually a source of hydrogen, it is used directly in the synthesis of high molecular weight organic substances - carbohydrates, which occurs in the second, dark phase of photosynthesis using carbon dioxide.
Light phase of photosynthesis
Chloroplasts contain a lot of chlorophyll molecules, and they all absorb sunlight. At the same time, light is absorbed by other pigments, but they are not able to carry out photosynthesis. The process itself takes place only in some chlorophyll molecules, of which there are very few. Other molecules of chlorophyll, carotenoids and other substances form special antenna, as well as light harvesting complexes (SSC). They, like antennas, absorb light quanta and transmit excitation to special reaction centers or traps. These centers are located in photosystems, of which plants have two: photosystem II and photosystem I. They contain special chlorophyll molecules: respectively, in photosystem II - P680, and in photosystem I - P700. They absorb light of exactly this wavelength (680 and 700 nm).
The diagram makes it clearer how everything looks and happens during the light phase of photosynthesis.
In the figure, we see two photosystems with chlorophylls P680 and P700. The figure also shows the carriers through which the transport of electrons occurs.
So: both chlorophyll molecules of the two photosystems absorb a quantum of light and are excited. Electron e- (red in the figure) goes to a higher energy level.
Excited electrons have a very high energy, they break off and enter a special carrier chain, which is located in the membranes of thylakoids - the internal structures of chloroplasts. The figure shows that from photosystem II from chlorophyll P680 an electron passes to plastoquinone, and from photosystem I from chlorophyll P700 to ferredoxin. In the chlorophyll molecules themselves, in place of the electrons after their detachment, blue holes with a positive charge are formed. What to do?
To make up for the lack of an electron, the chlorophyll P680 molecule of photosystem II receives electrons from water, while hydrogen ions are formed. In addition, it is due to the decomposition of water that oxygen released into the atmosphere is formed. And the chlorophyll P700 molecule, as can be seen from the figure, makes up for the lack of electrons through the system of carriers from photosystem II.
In general, no matter how difficult it is, this is how the light phase of photosynthesis proceeds, its main essence lies in the transfer of electrons. It can also be seen from the figure that in parallel with the transport of electrons, hydrogen ions H + move across the membrane, and they accumulate inside the thylakoid. Since there are a lot of them there, they move outward with the help of a special coupling factor, which is orange in the figure, shown on the right and looks like a mushroom.
In conclusion, we see the final stage of electron transport, the result of which is the formation of the above-mentioned NADH compound. And due to the transfer of H + ions, an energy currency is synthesized - ATP (seen in the figure on the right).
So, the light phase of photosynthesis is completed, oxygen has been released into the atmosphere, ATP and NADH have been formed. What's next? Where is the promised organic? And then comes the dark stage, which consists mainly of chemical processes.
Dark phase of photosynthesis
For the dark phase of photosynthesis, a mandatory component is carbon dioxide - CO2. Therefore, the plant must constantly absorb it from the atmosphere. For this purpose, there are special structures on the surface of the leaf - stomata. When they open, CO2 enters the inside of the leaf, dissolves in water and enters into the reaction of the light phase of photosynthesis.
During the light phase, in most plants, CO2 binds to a five-carbon organic compound (which is a chain of five carbon molecules), resulting in two molecules of a three-carbon compound (3-phosphoglyceric acid). Because The primary result is precisely these three-carbon compounds; plants with this type of photosynthesis are called C3 plants.
Further synthesis in chloroplasts is rather difficult. As a result, a six-carbon compound is formed from which glucose, sucrose or starch can be synthesized in the future. The plant stores energy in the form of these organic substances. At the same time, only a small part of them remains in the leaf, which is used for its needs, while the rest of the carbohydrates travel throughout the plant, going to where energy is most needed - for example, to growth points.
Photosynthesis is the conversion of light energy into the energy of chemical bonds organic compounds.
Photosynthesis is characteristic of plants, including all algae, a number of prokaryotes, including cyanobacteria, and some unicellular eukaryotes.
In most cases, photosynthesis produces oxygen (O 2) as a by-product. However, this is not always the case, as there are several different pathways for photosynthesis. In the case of oxygen release, its source is water, from which hydrogen atoms are split off for the needs of photosynthesis.
Photosynthesis consists of a variety of reactions involving various pigments, enzymes, coenzymes, etc. The main pigments are chlorophylls, in addition to them, carotenoids and phycobilins.
In nature, there are two ways of plant photosynthesis: C 3 and C 4. Other organisms have their own specificity of reactions. Everything that unites these different processes under the term "photosynthesis" - in all of them, in total, the energy of photons is converted into a chemical bond. For comparison: during chemosynthesis, the energy of the chemical bond of some compounds (inorganic) is converted into others - organic.
There are two phases of photosynthesis - light and dark. The first depends on the light radiation (hν), which is necessary for the reactions to proceed. The dark phase is light-independent.
In plants, photosynthesis takes place in chloroplasts. As a result of all reactions, primary organic substances are formed, from which carbohydrates, amino acids, fatty acids, etc. are then synthesized. Usually, the total reaction of photosynthesis is written in relation to glucose - the most common product of photosynthesis:
6CO 2 + 6H 2 O → C 6 H 12 O 6 + 6O 2
The oxygen atoms that make up the O 2 molecule are not taken from carbon dioxide, but from water. Carbon dioxide is a carbon sourcemore importantly. Thanks to its binding, plants have the opportunity to synthesize organic matter.
The above chemical reaction is generalized and cumulative. It is far from the essence of the process. This way glucose is not formed from six separate carbon dioxide molecules. CO 2 binding takes place one molecule at a time, which first attaches to the already existing five-carbon sugar.
Prokaryotes are characterized by their own peculiarities of photosynthesis. So in bacteria, the main pigment is bacteriochlorophyll, and oxygen is not released, since hydrogen is not taken from water, but often from hydrogen sulfide or other substances. In blue-green algae, chlorophyll is the main pigment, and oxygen is released during photosynthesis.
Light phase of photosynthesis
In the light phase of photosynthesis, ATP and NADPH 2 are synthesized due to radiant energy. It happens on the thylakoids of chloroplastswhere pigments and enzymes form complex complexes for the functioning of electrochemical circuits, through which electrons and partly hydrogen protons are transferred.
The electrons eventually end up at the coenzyme NADP, which, being charged negatively, attracts some of the protons to itself and turns into NADPH 2. Also, the accumulation of protons on one side of the thylakoid membrane and electrons on the other creates an electrochemical gradient, the potential of which is used by the enzyme ATP synthetase to synthesize ATP from ADP and phosphoric acid.
The main pigments of photosynthesis are various chlorophylls. Their molecules capture the emission of certain, partly different, spectra of light. In this case, some electrons of chlorophyll molecules move to a higher energy level. This is an unstable state, and, in theory, electrons by the same radiation should give into space the energy received from outside and return to the previous level. However, in photosynthetic cells, excited electrons are captured by acceptors and, with a gradual decrease in their energy, are transferred along the carrier chain.
On thylakoid membranes, there are two types of photosystems that emit electrons when exposed to light. Photosystems are a complex complex of mostly chlorophilic pigments with a reaction center, from which electrons are detached. In the photosystem, sunlight catches many molecules, but all the energy is collected in the reaction center.
The electrons of photosystem I, passing through the carrier chain, reduce NADP.
The energy of electrons detached from photosystem II is used for the synthesis of ATP. And the electrons of photosystem II themselves fill the electron holes of photosystem I.
The holes of the second photosystem are filled with electrons resulting from photolysis of water... Photolysis also occurs with the participation of light and consists in the decomposition of H 2 O into protons, electrons and oxygen. It is as a result of water photolysis that free oxygen is formed. Protons are involved in creating an electrochemical gradient and reducing NADP. Chlorophyll of photosystem II receives electrons.
The approximate total equation of the light phase of photosynthesis:
H 2 O + NADP + 2ADP + 2P → ½O 2 + NADP · H 2 + 2ATP
Cyclic transport of electrons
The above is the so-called non-cyclic light phase of photosynthesis... Is there some more cyclic electron transport when NADP reduction does not occur... In this case, electrons from photosystem I go to the carrier chain, where ATP is synthesized. That is, this electron transport chain receives electrons from photosystem I, not II. The first photosystem realizes, as it were, a cycle: the emitted electrons return to it. On the way, they spend part of their energy to synthesize ATP.
Photophosphorylation and oxidative phosphorylation
The light phase of photosynthesis can be compared to the stage of cellular respiration - oxidative phosphorylation, which occurs on the mitochondrial cristae. There, too, ATP synthesis occurs due to the transfer of electrons and protons along the carrier chain. However, in the case of photosynthesis, energy is stored in ATP not for the needs of the cell, but mainly for the needs of the dark phase of photosynthesis. And if during respiration organic substances serve as the primary source of energy, then during photosynthesis it is sunlight. The synthesis of ATP during photosynthesis is called photophosphorylationrather than oxidative phosphorylation.
Dark phase of photosynthesis
For the first time, the dark phase of photosynthesis was studied in detail by Calvin, Benson, Bassem. The cycle of reactions discovered by them was later called the Calvin cycle, or C 3 photosynthesis. Certain groups of plants have a modified C4 photosynthetic pathway, also called the Hatch-Slack cycle.
In the dark reactions of photosynthesis, CO 2 is fixed. The dark phase occurs in the chloroplast stroma.
The reduction of CO 2 occurs due to the energy of ATP and the reducing power of NADP · H 2, formed in light reactions. Without them, carbon fixation does not occur. Therefore, although the dark phase does not directly depend on light, it usually also occurs in the light.
Calvin cycle
The first reaction of the dark phase is the addition of CO 2 ( carboxylatione) to 1,5-ribulezobiphosphate ( ribulose-1,5-diphosphate) – RiBF... The latter is doubly phosphorylated ribose. This reaction is catalyzed by the enzyme ribulose-1,5-diphosphate carboxylase, also called rubisco.
As a result of carboxylation, an unstable six-carbon compound is formed, which, as a result of hydrolysis, decomposes into two three-carbon molecules phosphoglyceric acid (FHA) - the first product of photosynthesis. FHA is also called phosphoglycerate.
RuBP + CO 2 + H 2 O → 2FGK
FHA contains three carbon atoms, one of which is part of the acidic carboxyl group (-COOH):
Three-carbon sugar (glyceraldehyde phosphate) is formed from FHA triose phosphate (TF), including already an aldehyde group (-CHO):
FHA (3-acid) → TF (3-sugar)
This reaction requires the energy of ATP and the reducing force of NADP · H 2. TF is the first carbohydrate in photosynthesis.
After that, most of the triose phosphate is spent on the regeneration of ribulose biphosphate (RuBP), which is again used to bind CO 2. Regeneration involves a series of ATP-costly reactions involving sugar phosphates with 3 to 7 carbon atoms.
This cycle of RuBF is the essence of the Calvin cycle.
A smaller part of the TF formed in it leaves the Calvin cycle. In terms of 6 bound carbon dioxide molecules, the yield is 2 triose phosphate molecules. Total reaction of the cycle with input and output products:
6CO 2 + 6H 2 O → 2ТФ
In this case, 6 molecules of RuBP participate in the binding and 12 FHA molecules are formed, which are converted into 12 TF, of which 10 molecules remain in the cycle and are converted into 6 molecules of RuBP. Since TF is a three-carbon sugar, and RuBP is a five-carbon sugar, then in relation to carbon atoms we have: 10 * 3 \u003d 6 * 5. The number of carbon atoms providing the cycle does not change, all the necessary RuBP is regenerated. And six molecules of carbon dioxide included in the cycle are spent on the formation of two molecules of triose phosphate leaving the cycle.
For the Calvin cycle, per 6 bound CO 2 molecules, 18 ATP molecules and 12 NADPH 2 molecules are spent, which were synthesized in the reactions of the light phase of photosynthesis.
The calculation is carried out for two triose phosphate molecules leaving the cycle, since the subsequently formed glucose molecule includes 6 carbon atoms.
Triose phosphate (TF) is the end product of the Calvin cycle, but it can hardly be called the end product of photosynthesis, since it hardly accumulates, but, reacting with other substances, turns into glucose, sucrose, starch, fats, fatty acids, amino acids. In addition to TF, FGK plays an important role. However, such reactions do not occur only in photosynthetic organisms. In this sense, the dark phase of photosynthesis is the same as the Calvin cycle.
Six-carbon sugar is formed from FHA by stepwise enzymatic catalysis fructose-6-phosphatewhich turns into glucose... In plants, glucose can polymerize into starch and cellulose. The synthesis of carbohydrates is similar to the reverse process of glycolysis.
Photorespiration
Oxygen inhibits photosynthesis. The more O 2 in the environment, the less efficient the CO 2 fixation process. The fact is that the enzyme ribulose biphosphate carboxylase (rubisco) can react not only with carbon dioxide, but also with oxygen. In this case, the dark reactions are somewhat different.
Phosphoglycolate is a phosphoglycolic acid. The phosphate group is immediately split off from it, and it turns into glycolic acid (glycolate). Oxygen is again needed to "utilize" it. Therefore, the more oxygen in the atmosphere, the more it will stimulate photorespiration and the more the plant will need oxygen to get rid of the reaction products.
Photorespiration is the consumption of oxygen and the production of carbon dioxide, which is dependent on light. That is, the exchange of gases occurs as during breathing, but occurs in chloroplasts and depends on light radiation. Photorespiration depends on light only because ribulose biphosphate is formed only during photosynthesis.
During photorespiration, carbon atoms are returned from glycolate into the Calvin cycle in the form of phosphoglyceric acid (phosphoglycerate).
2 Glycolate (C 2) → 2 Glyoxylate (C 2) → 2 Glycine (C 2) - CO 2 → Serine (C 3) → Hydroxypyruvate (C 3) → Glycerate (C 3) → FHA (C 3)
As you can see, the return is not complete, since one carbon atom is lost during the conversion of two glycine molecules into one serine amino acid molecule, while carbon dioxide is released.
Oxygen is required in the steps of converting glycolate to glyoxylate and glycine to serine.
Conversion of glycolate to glyoxylate, and then to glycine occurs in peroxisomes, serine synthesis in mitochondria. Serine again enters the peroxisomes, where it first produces hydroxypyruvate and then glycerate. Glycerate already enters the chloroplasts, where FHA is synthesized from it.
Photorespiration is typical mainly for plants with the C 3 -type of photosynthesis. It can be considered harmful as energy is wasted in converting glycolate to FHA. Apparently photorespiration arose due to the fact that ancient plants were not ready for a large amount of oxygen in the atmosphere. Initially, their evolution took place in an atmosphere rich in carbon dioxide, and it was he who mainly captured the reaction center of the Rubisco enzyme.
C 4 -photosynthesis, or the Hatch-Slack cycle
If at C 3 -photosynthesis the first product of the dark phase is phosphoglyceric acid, which includes three carbon atoms, then at the C 4 -way the first products are acids containing four carbon atoms: malic, oxaloacetic, aspartic.
C 4-photosynthesis is observed in many tropical plants, for example, sugar cane, corn.
C 4 -plants absorb carbon monoxide more efficiently, they have almost no expressed photorespiration.
Plants in which the dark phase of photosynthesis proceeds along the C 4 pathway have a special leaf structure. In it, the conducting bundles are surrounded by a double layer of cells. The inner layer is the covering of the conducting beam. The outer layer is mesophyll cells. Chloroplast cell layers differ from each other.
Mesophilic chloroplasts are characterized by large granules, high activity of photosystems, and the absence of the enzyme RiBP carboxylase (rubisco) and starch. That is, the chloroplasts of these cells are adapted mainly for the light phase of photosynthesis.
In the chloroplasts of the cells of the conducting bundle, grana are almost undeveloped, but the concentration of RuBP carboxylase is high. These chloroplasts are adapted for the dark phase of photosynthesis.
Carbon dioxide first enters the mesophyll cells, binds to organic acids, in this form is transported to the sheath cells, is released and then binds in the same way as in C 3 plants. That is, the C 4 -path complements rather than replaces the C 3.
In the mesophyll, CO 2 is added to phosphoenolpyruvate (PEP) to form oxaloacetate (acid), which contains four carbon atoms:
The reaction takes place with the participation of the enzyme PEP-carboxylase, which has a higher affinity for CO 2 than rubisco. In addition, PEP-carboxylase does not interact with oxygen, which means that it is not spent on photorespiration. Thus, the advantage of C 4 photosynthesis lies in a more efficient fixation of carbon dioxide, an increase in its concentration in the sheath cells, and, consequently, a more efficient operation of RiBP carboxylase, which is almost not consumed for photorespiration.
Oxaloacetate is converted to 4-carbon dicarboxylic acid (malate or aspartate), which is transported to the chloroplasts of the sheathing cells of the conductive bundles. Here the acid is decarboxylated (CO 2 removal), oxidized (hydrogen removal) and converted to pyruvate. Hydrogen reduces NADP. Pyruvate returns to the mesophyll, where PEP is regenerated from it with the consumption of ATP.
Torn off CO 2 in the chloroplasts of the sheath cells goes to the usual C 3 pathway of the dark phase of photosynthesis, that is, to the Calvin cycle.
Photosynthesis along the Hatch-Slack path requires more energy.
It is believed that the C 4 pathway evolved later than the C 3 pathway and is in many ways an adaptation against photorespiration.
- synthesis of organic substances from carbon dioxide and water with the obligatory use of light energy:
6CO 2 + 6H 2 O + Q light → C 6 H 12 O 6 + 6O 2.
In higher plants, the organ of photosynthesis is a leaf, organelles of photosynthesis are chloroplasts (the structure of chloroplasts - lecture No. 7). Photosynthetic pigments are built into the chloroplast thylakoid membranes: chlorophylls and carotenoids. There are several different types of chlorophyll ( a, b, c, d), the main one is chlorophyll a... In the chlorophyll molecule, one can distinguish a porphyrin “head” with a magnesium atom in the center and a phytol “tail”. The porphyrin "head" is a flat structure, is hydrophilic and therefore lies on the membrane surface that faces the aqueous medium of the stroma. The phytol "tail" is hydrophobic and, due to this, keeps the chlorophyll molecule in the membrane.
Chlorophylls absorb red and blue-violet light, reflect green and therefore give plants their characteristic green color. Chlorophyll molecules in thylakoid membranes are organized in photo systems... Plants and blue-green algae have photosystem-1 and photosystem-2, while photosynthetic bacteria have photosystem-1. Only photosystem-2 can decompose water with the release of oxygen and take electrons from the hydrogen of the water.
Photosynthesis is a complex multistage process; photosynthetic reactions are divided into two groups: reactions light phase and reactions dark phase.
Light phase
This phase occurs only in the presence of light in the membranes of thylakoids with the participation of chlorophyll, electron transfer proteins and an enzyme - ATP synthetase. Under the influence of a quantum of light, the electrons of chlorophyll are excited, leave the molecule and enter the outer side of the thylakoid membrane, which ultimately becomes negatively charged. Oxidized chlorophyll molecules are reduced by taking electrons from water, which is in the intratilakoid space. This leads to the breakdown or photolysis of water:
H 2 O + Q light → H + + OH -.
Hydroxyl ions donate their electrons, turning into reactive radicals.
OH - → .OH + e -.
Radicals OH combine to form water and free oxygen:
4NO. → 2H 2 O + O 2.
In this case, oxygen is removed into the external environment, and protons accumulate inside the thylakoid in the "proton reservoir". As a result, the thylakoid membrane on the one hand is charged positively due to H +, on the other hand due to electrons it is negatively charged. When the potential difference between the outer and inner sides of the thylakoid membrane reaches 200 mV, protons are pushed through the channels of ATP synthetase and phosphorylation of ADP to ATP occurs; atomic hydrogen is used to reduce the specific carrier NADP + (nicotinamide adenine dinucleotide phosphate) to NADPH 2:
2Н + + 2е - + NADP → NADPH 2.
Thus, photolysis of water occurs during the light phase, which is accompanied by three most important processes: 1) ATP synthesis; 2) the formation of NADP · H 2; 3) the formation of oxygen. Oxygen diffuses into the atmosphere, ATP and NADP · H 2 are transported to the chloroplast stroma and participate in the dark phase processes.
1 - chloroplast stroma; 2 - grana thylakoid.
Dark phase
This phase takes place in the chloroplast stroma. For its reactions, the energy of light is not needed, so they occur not only in the light, but also in the dark. The dark phase reactions are a chain of sequential transformations of carbon dioxide (coming from the air), leading to the formation of glucose and other organic substances.
The first reaction in this chain is carbon dioxide fixation; carbon dioxide scavenger is five-carbon sugar ribulose biphosphate (RiBF); enzyme catalyzes the reaction ribulose biphosphate carboxylase (RuBP carboxylase). As a result of carboxylation of ribulose bisphosphate, an unstable six-carbon compound is formed, which immediately decomposes into two molecules phosphoglyceric acid (FGK). Then a cycle of reactions takes place in which phosphoglyceric acid is converted into glucose through a series of intermediate products. These reactions use the energies of ATP and NADP · H 2 formed in the light phase; the cycle of these reactions is called the "Calvin cycle":
6CO 2 + 24H + + ATP → C 6 H 12 O 6 + 6H 2 O.
In addition to glucose, in the process of photosynthesis, other monomers of complex organic compounds are formed - amino acids, glycerol and fatty acids, nucleotides. Currently, two types of photosynthesis are distinguished: C 3 and C 4 photosynthesis.
C 3 photosynthesis
This is a type of photosynthesis in which the first product is three-carbon (C 3) compounds. C 3 photosynthesis was discovered earlier than C 4 photosynthesis (M. Calvin). It is C 3 photosynthesis that is described above, under the heading "Dark phase". Characteristic features of C 3 photosynthesis: 1) the carbon dioxide acceptor is RuBP, 2) the carboxylation of RuBP is catalyzed by RuBP carboxylase, 3) as a result of carboxylation of RuBP, a six-carbon compound is formed, which decomposes into two FHA. FGK is restored to triose phosphates (TF). Part of TF goes to the regeneration of RiBP, part is converted into glucose.
1 - chloroplast; 2 - peroxisome; 3 - mitochondria.
It is a light-dependent absorption of oxygen and the release of carbon dioxide. At the beginning of the last century, it was found that oxygen suppresses photosynthesis. As it turned out, for RiBP carboxylase, the substrate can be not only carbon dioxide, but also oxygen:
О 2 + RuBP → phosphoglycolate (2C) + FHA (3C).
The enzyme is called RiBP-oxygenase. Oxygen is a competitive inhibitor of carbon dioxide fixation. The phosphate group is cleaved off and the phosphoglycolate becomes the glycolate for the plant to utilize. It enters the peroxisomes, where it is oxidized to glycine. Glycine enters the mitochondria, where it is oxidized to serine, while the already fixed carbon is lost in the form of CO 2. As a result, two molecules of glycolate (2C + 2C) are converted into one FHA (3C) and CO 2. Photorespiration leads to a decrease in the yield of C 3 plants by 30-40% ( C 3 -plants - plants that are characterized by C 3 photosynthesis).
С 4 -photosynthesis - photosynthesis, in which the first product is four-carbon (С 4) compounds. In 1965, it was found that in some plants (sugarcane, corn, sorghum, millet), the first products of photosynthesis are four-carbon acids. Such plants were named With 4 plants... In 1966, Australian scientists Hatch and Slack showed that C 4 plants have virtually no photorespiration and are much more efficient at absorbing carbon dioxide. The path of carbon transformation in C 4 -plants began to be called by Hatch-Slack.
For C 4 plants, a special anatomical structure of the leaf is characteristic. All vascular bundles are surrounded by a double layer of cells: the outer one is mesophyll cells, the inner one is the sheath cells. Carbon dioxide is fixed in the cytoplasm of mesophyll cells, the acceptor is phosphoenolpyruvate (FEP, 3C), as a result of carboxylation of PEP, oxaloacetate (4C) is formed. The process is catalyzed PEP-carboxylase... Unlike RuBP carboxylase, PEP carboxylase has a high affinity for CO 2 and, most importantly, does not interact with O 2. In the chloroplasts of the mesophyll there are many grains, where the reactions of the light phase are active. In the chloroplasts of the sheath cells, reactions of the dark phase take place.
Oxaloacetate (4C) is converted into malate, which is transported through the plasmodesmata into the sheath cells. Here it is decarboxylated and dehydrated to form pyruvate, CO 2 and NADPH 2.
Pyruvate returns to the mesophyll cells and is regenerated at the expense of ATP energy in PEP. CO 2 is again fixed by RiBP carboxylase with the formation of FHA. Regeneration of PEP requires ATP energy; therefore, almost twice as much energy is needed as with C 3 photosynthesis.
The importance of photosynthesis
Thanks to photosynthesis, billions of tons of carbon dioxide are absorbed from the atmosphere every year, billions of tons of oxygen are released; photosynthesis is the main source of organic matter formation. Oxygen forms the ozone layer, which protects living organisms from short-wave ultraviolet radiation.
During photosynthesis, a green leaf uses only about 1% of the solar energy falling on it, the productivity is about 1 g of organic matter per 1 m 2 of surface per hour.
Chemosynthesis
The synthesis of organic compounds from carbon dioxide and water, carried out not due to the energy of light, but due to the energy of oxidation of inorganic substances, is called chemosynthesis... Chemosynthetic organisms include some types of bacteria.
Nitrifying bacteria ammonia is oxidized to nitrous and then to nitric acid (NH 3 → HNO 2 → HNO 3).
Iron bacteria converting ferrous iron into oxide (Fe 2+ → Fe 3+).
Sulfur bacteria oxidize hydrogen sulfide to sulfur or sulfuric acid (H 2 S + ½O 2 → S + H 2 O, H 2 S + 2O 2 → H 2 SO 4).
As a result of the oxidation reactions of inorganic substances, energy is released, which is stored by bacteria in the form of high-energy ATP bonds. ATP is used for the synthesis of organic substances, which proceeds in a similar way to the reactions of the dark phase of photosynthesis.
Chemosynthetic bacteria contribute to the accumulation of mineral substances in the soil, improve soil fertility, promote wastewater treatment, etc.
Go to lectures number 11 “The concept of metabolism. Protein biosynthesis "
Go to lectures No. 13 "Methods of division of eukaryotic cells: mitosis, meiosis, amitosis"