Cells involved in photosynthesis. Photosynthesis

Photosynthesis is the process that results in the formation and release of oxygen by plant cells and some types of bacteria.

Basic concept

Photosynthesis is nothing more than a chain of unique physical and chemical reactions. What does it consist of? Green plants, as well as some bacteria, absorb sunlight and convert them into electromagnetic energy. The end result of photosynthesis is the energy of chemical bonds of various organic compounds.

In a plant exposed to sunlight, redox reactions occur in a certain sequence. Water and hydrogen, which are donor-reducing agents, move in the form of electrons to the acceptor-oxidizing agent (carbon dioxide and acetate). As a result, reduced carbohydrate compounds are formed, as well as oxygen, which is released by plants.

History of the study of photosynthesis

For many millennia, man was convinced that a plant’s nutrition occurs through its root system through the soil. At the beginning of the sixteenth century, the Dutch naturalist Jan Van Helmont conducted an experiment with growing the plant in a pot. After weighing the soil before planting and after the plant had reached a certain size, he concluded that all representatives of the flora received nutrients mainly from water. Scientists adhered to this theory for the next two centuries.

An unexpected but correct assumption about plant nutrition was made in 1771 by the English chemist Joseph Priestley. The experiments he carried out convincingly proved that plants are capable of purifying air that was previously unsuitable for human breathing. Somewhat later, it was concluded that these processes are impossible without the participation of sunlight. Scientists have found that green plant leaves do more than simply convert the carbon dioxide they receive into oxygen. Without this process their life is impossible. Together with water and mineral salts, carbon dioxide serves as food for plants. This is the main significance of photosynthesis for all representatives of the flora.

The role of oxygen for life on Earth

The experiments carried out by the English chemist Priestley helped humanity explain why the air on our planet remains breathable. After all, life is maintained despite the existence of a huge number of living organisms and the burning of countless fires.

The emergence of life on Earth billions of years ago was simply impossible. The atmosphere of our planet did not contain free oxygen. Everything changed with the advent of plants. All the oxygen in the atmosphere today is the result of photosynthesis occurring in green leaves. This process changed the appearance of the Earth and gave impetus to the development of life. This invaluable significance of photosynthesis was fully realized by humanity only at the end of the 18th century.

It is not an exaggeration to say that the very existence of people on our planet depends on the state of the plant world. The importance of photosynthesis lies in its leading role for the occurrence of various biosphere processes. On a global scale, this amazing physicochemical reaction leads to the formation of organic substances from inorganic ones.

Classification of photosynthesis processes

Three important reactions occur in a green leaf. They represent photosynthesis. The table in which these reactions are recorded is used in the study of biology. Its lines include:

Photosynthesis;
- gas exchange;
- evaporation of water.

Those physicochemical reactions that occur in the plant during daylight allow green leaves to release carbon dioxide and oxygen. In the dark - only the first of these two components.

The synthesis of chlorophyll in some plants occurs even in low and diffuse lighting.

Main stages

There are two phases of photosynthesis, which are closely related to each other. At the first stage, the energy of light rays is converted into high-energy compounds ATP and universal reducing agents NADPH. These two elements are the primary products of photosynthesis.

At the second (dark) stage, the resulting ATP and NADPH are used to fix carbon dioxide until it is reduced to carbohydrates. The two phases of photosynthesis differ not only in time. They also occur in different spaces. For those who study the topic "photosynthesis" in biology, a table with precise indication characteristics of the two phases will help in a more accurate understanding of the process.

Mechanism of oxygen production

After plants absorb carbon dioxide, nutrients are synthesized. This process occurs in green pigments called chlorophylls when exposed to sunlight. The main components of this amazing reaction are:

Light;
- chloroplasts;
- water;
- carbon dioxide;
- temperature.

Sequence of photosynthesis

Plants produce oxygen in stages. The main stages of photosynthesis are as follows:

Absorption of light by chlorophylls;
- division of water obtained from soil into oxygen and hydrogen by chloroplasts (intracellular organelles of green pigment);
- movement of one part of oxygen into the atmosphere, and the other - for the respiratory process of plants;
- formation of sugar molecules in protein granules (pyrenoids) of plants;
- production of starches, vitamins, fats, etc. as a result of mixing sugar with nitrogen.

Despite the fact that photosynthesis requires sunlight, this reaction can also occur under artificial light.

The role of flora for the Earth

The basic processes occurring in a green leaf have already been studied quite fully by the science of biology. The importance of photosynthesis for the biosphere is enormous. This is the only reaction that leads to an increase in the amount of free energy.

During the process of photosynthesis, one hundred and fifty billion tons of organic substances are formed every year. In addition, during this period, plants release almost 200 million tons of oxygen. In this regard, it can be argued that the role of photosynthesis is enormous for all of humanity, since this process serves as the main source of energy on Earth.

In the process of a unique physical and chemical reaction, the cycle of carbon, oxygen, and many other elements occurs. This implies another important significance of photosynthesis in nature. This reaction maintains a certain composition of the atmosphere at which life on Earth is possible.

A process occurring in plants limits the amount of carbon dioxide, preventing it from accumulating in increased concentrations. This is also an important role for photosynthesis. On Earth thanks green plants the so-called greenhouse effect is not created. Flora reliably protects our planet from overheating.

Flora as the basis of nutrition

The role of photosynthesis for forest and agriculture. Flora is a nutritional base for all heterotrophic organisms. However, the significance of photosynthesis lies not only in the absorption of carbon dioxide by green leaves and the production of such a finished product of a unique reaction as sugar. Plants are capable of converting nitrogen and sulfur compounds into substances that make up their bodies.

How does this happen? What is the importance of photosynthesis in plant life? This process is carried out through the production of nitrate ions by the plant. These elements are found in soil water. They enter the plant through the root system. The cells of a green organism process nitrate ions into amino acids, which make up protein chains. The process of photosynthesis also produces fat components. They are important reserve substances for plants. Thus, the seeds of many fruits contain nutritious oil. This product is also important for humans, as it is used in the food and agricultural industries.

The role of photosynthesis in crop production

In the world practice of agricultural enterprises, the results of studying the basic patterns of plant development and growth are widely used. As you know, the basis for crop formation is photosynthesis. Its intensity, in turn, depends on the water regime of crops, as well as on their mineral nutrition. How does a person achieve an increase in crop density and leaf size so that the plant makes maximum use of the sun's energy and takes carbon dioxide from the atmosphere? To achieve this, the conditions for mineral nutrition and water supply to agricultural crops are optimized.

It has been scientifically proven that yield depends on the area of ​​green leaves, as well as on the intensity and duration of the processes occurring in them. But at the same time, an increase in crop density leads to shading of the leaves. Sunlight cannot penetrate to them, and due to deteriorating ventilation of air masses, carbon dioxide enters in small volumes. As a result, the activity of the photosynthesis process decreases and plant productivity decreases.

The role of photosynthesis for the biosphere

According to the most rough estimates, only autotrophic plants living in the waters of the World Ocean annually convert from 20 to 155 billion tons of carbon into organic matter. And this despite the fact that the energy of solar rays is used by them only by 0.11%. As for terrestrial plants, they annually absorb from 16 to 24 billion tons of carbon. All these data convincingly indicate how important photosynthesis is in nature. Only as a result of this reaction is the atmosphere replenished with the molecular oxygen necessary for life, which is necessary for combustion, respiration and various industrial activities. Some scientists believe that when carbon dioxide levels in the atmosphere increase, the rate of photosynthesis increases. At the same time, the atmosphere is replenished with missing oxygen.

The cosmic role of photosynthesis

Green plants are intermediaries between our planet and the Sun. They capture the energy of the heavenly body and ensure the existence of life on our planet.

Photosynthesis is a process that can be discussed on a cosmic scale, since it once contributed to the transformation of the image of our planet. Thanks to the reaction taking place in green leaves, the energy of the sun's rays is not dissipated in space. It turns into chemical energy of newly formed organic substances.

Human society needs the products of photosynthesis not only for food, but also for economic activities.

However, not only those rays of the sun that fall on our Earth at the present time are important to humanity. Those products of photosynthesis that were obtained millions of years ago are extremely necessary for life and production activities. They are found in the bowels of the planet in the form of layers of coal, combustible gas and oil, and peat deposits.

Do you know that every green leaf is a miniature “factory” of nutrients and oxygen, which is necessary for normal life not only for animals, but also for humans. Photosynthesis is the process of producing these substances from water and carbon dioxide from the atmosphere. This is a very complex chemical process that occurs with the participation of light. Undoubtedly, everyone is interested in how the process of photosynthesis occurs. The process consists of two stages: the first stage is the absorption of light quanta, and the second stage is the use in various chemical reactions their energy.

How does the process of photosynthesis occur?
The plant absorbs light using a green substance called chlorophyll. Chlorophyll is contained in chloroplasts, which are found in fruits and stems. But especially them large number is located in the leaves, because the leaf, due to its rather simple structure, can attract a large amount of light, and accordingly, receive much more energy for the photosynthesis process.
Chlorophyll, after absorption, is in an excited state and transfers energy to other molecules of the plant body, especially those that directly take part in photosynthesis. The second stage of the photosynthesis process occurs without the mandatory participation of light and consists of obtaining a chemical bond with the participation of carbon dioxide, which is obtained from water and air. At this stage, the synthesis of various very useful substances for life, such as glucose and starch, occurs.

The plants themselves use these organic substances to nourish their various parts, as well as to maintain normal life functions. In addition, these substances are also obtained by animals that feed on plants. A person obtains these substances by eating foods of plant and animal origin.

Conditions for photosynthesis
The process of photosynthesis can occur not only under the influence of artificial light, but also sunlight. In nature, as a rule, plants actively carry out their activities in the spring and summer, that is, at a time when a lot of sunlight is needed. There is less light in autumn, the days are shortened, the leaves turn yellow and then fall off. But as soon as the warm spring sun appears, the green foliage wakes up and the green “factories” resume their work again in order to provide a large amount of nutrients and oxygen, which is so necessary for life.

Where does the process of photosynthesis take place?
Photosynthesis mainly occurs, as we said above, if you remember, in the leaves of plants, for the reason that they have the ability to receive a large amount of light, which is so necessary for the photosynthesis process.

In conclusion, we can summarize and say that a process such as photosynthesis is an integral part of plant life. We hope that our article has helped many people understand what photosynthesis is and why it is necessary.

Plants obtain water and minerals from their roots. The leaves provide organic nutrition to the plants. Unlike roots, they are not in the soil, but in the air, therefore they provide not soil, but air nutrition.

From the history of studying aerial nutrition of plants

Knowledge about plant nutrition accumulated gradually. About 350 years ago, the Dutch scientist Jan Helmont first experimented with the study of plant nutrition. He grew willow in a clay pot filled with soil, adding only water. The scientist carefully weighed the fallen leaves. After five years, the mass of the willow together with fallen leaves increased by 74.5 kg, and the mass of the soil decreased by only 57 g. Based on this, Helmont came to the conclusion that all substances in the plant are formed not from soil, but from water. The opinion that a plant increases in size only due to water persisted until late XVIII century.

In 1771, the English chemist Joseph Priestley studied carbon dioxide, or, as he called it, “spoiled air” and made a remarkable discovery. If you light a candle and cover it with a glass cover, then after burning a little it will go out. A mouse under such a hood begins to suffocate. However, if you place a mint branch under the cap with the mouse, the mouse does not suffocate and continues to live. This means that plants “correct” the air spoiled by the breathing of animals, that is, they convert carbon dioxide into oxygen.

In 1862, the German botanist Julius Sachs proved through experiments that green plants not only produce oxygen, but also create organic substances that serve as food for all other organisms.

Photosynthesis

The main difference between green plants and other living organisms is the presence in their cells of chloroplasts containing chlorophyll. Chlorophyll has the property of capturing solar rays, the energy of which is necessary for the creation of organic substances. The process of formation of organic matter from carbon dioxide and water using solar energy is called photosynthesis (Greek pbo1os light). During the process of photosynthesis, not only organic substances - sugars - are formed, but oxygen is also released.

Schematically, the process of photosynthesis can be depicted as follows:

Water is absorbed by the roots and moves through the conductive system of the roots and stem to the leaves. Carbon dioxide - component air. It enters the leaves through open stomata. The absorption of carbon dioxide is facilitated by the structure of the leaf: the flat surface of the leaf blades, increasing the area of ​​contact with air, and the presence large number stomata in the skin.

Sugars formed as a result of photosynthesis are converted into starch. Starch is an organic substance that does not dissolve in water. Kgo can be easily detected using an iodine solution.

Evidence of starch formation in leaves exposed to light

Let us prove that starch is formed from carbon dioxide and water in the green leaves of plants. To do this, consider an experiment that was once carried out by Julius Sachs.

A houseplant (geranium or primrose) is kept in the dark for two days so that all the starch is used up for vital processes. Then several leaves are covered on both sides with black paper so that only part of them is covered. During the day, the plant is exposed to light, and at night it is additionally illuminated using a table lamp.

After a day, the leaves under study are cut off. To find out in which part of the leaf starch is formed, the leaves are boiled in water (to swell the starch grains) and then kept in hot alcohol (the chlorophyll dissolves and the leaf becomes discolored). Then the leaves are washed in water and treated with a weak solution of iodine. Thus, areas of leaves that have been exposed to light acquire a blue color from the action of iodine. This means that starch was formed in the cells of the illuminated part of the leaf. Therefore, photosynthesis occurs only in light.

Evidence for the need for carbon dioxide for photosynthesis

To prove that carbon dioxide is necessary for the formation of starch in leaves, houseplant also pre-conditioned in the dark. One of the leaves is then placed in a flask with a small amount of lime water. The flask is closed with a cotton swab. The plant is exposed to light. Carbon dioxide is absorbed by lime water, so it will not be in the flask. The leaf is cut off and, just as in the previous experiment, examined for the presence of starch. It is kept in hot water and alcohol and treated with iodine solution. However, in this case, the result of the experiment will be different: the sheet is not painted in blue, because it does not contain starch. Therefore, for the formation of starch, in addition to light and water, carbon dioxide is needed.

Thus, we answered the question of what food the plant receives from the air. Experience has shown that it is carbon dioxide. It is necessary for the formation of organic matter.

Organisms that independently create organic substances to build their body are called autotrophamnes (Greek autos - itself, trophe - food).

Evidence of oxygen production during photosynthesis

To prove that during photosynthesis plants release oxygen into the external environment, consider an experiment with aquatic plant Elodea. Elodea shoots are dipped into a vessel with water and covered with a funnel on top. Place a test tube filled with water at the end of the funnel. The plant is exposed to light for two to three days. When exposed to light, elodea produces gas bubbles. They accumulate at the top of the test tube, displacing water. In order to find out what kind of gas it is, the test tube is carefully removed and a smoldering splinter is introduced into it. The splinter flashes brightly. This means that oxygen has accumulated in the flask, which supports combustion.

The cosmic role of plants

Plants containing chlorophyll are able to absorb solar energy. Therefore K.A. Timiryazev called their role on Earth cosmic. Some of the solar energy stored in organic matter can be stored for a long time. Coal, peat, oil are formed by substances that in ancient geological times were created by green plants and absorbed the energy of the Sun. By burning natural combustible materials, a person releases energy stored millions of years ago by green plants.

Photosynthesis occurs in plants (mainly in their leaves) in the light.

This is a process in which the organic substance glucose (one of the types of sugars) is formed from carbon dioxide and water. Next, glucose in the cells is converted into a more complex substance, starch. Both glucose and starch are carbohydrates.

The process of photosynthesis not only produces organic matter, but also produces oxygen as a by-product.

Carbon dioxide and water are inorganic substances, while glucose and starch are organic. Therefore, it is often said that photosynthesis is the process of formation of organic substances from inorganic substances in the light. Only plants, some single-celled eukaryotes, and some bacteria are capable of photosynthesis. There is no such process in the cells of animals and fungi, so they are forced to absorb from environment organic substances. In this regard, plants are called autotrophs, and animals and fungi are called heterotrophs.

The process of photosynthesis in plants occurs in chloroplasts, which contain the green pigment chlorophyll.

So, for photosynthesis to occur, you need:

chlorophyll,

carbon dioxide.

During the process of photosynthesis, the following are formed:

organic matter,

oxygen.

Plants are adapted to capture light. For many herbaceous plants the leaves are collected in a so-called basal rosette, when the leaves do not shade each other. Trees are characterized by a leaf mosaic, in which the leaves grow in such a way as to shade each other as little as possible. In plants, leaf blades can turn towards the light due to the bending of the leaf petioles. With all this, there are shade-loving plants that can only grow in the shade.

Waterfor photosynthesisarrivesinto the leavesfrom the rootsalong the stem. Therefore, it is important that the plant receives enough moisture. With a lack of water and certain minerals, the process of photosynthesis is inhibited.

Carbon dioxidetaken for photosynthesisdirectlyout of thin airleaves. Oxygen, which is produced by the plant during photosynthesis, on the contrary, is released into the air. Gas exchange is facilitated by intercellular spaces (spaces between cells).

Organic substances formed during photosynthesis are partly used in the leaves themselves, but mainly flow into all other organs and are converted into other organic substances, used in energy metabolism, and converted into reserve nutrients.

Plant photosynthesis

Photosynthesis is a unique physical and chemical process carried out on Earth by all green plants and some bacteria and ensures the conversion of the electromagnetic energy of solar rays into the energy of chemical bonds of various organic compounds. The basis of photosynthesis is a sequential chain of redox reactions, during which electrons are transferred from a donor - a reducing agent (water, hydrogen) to an acceptor - an oxidizing agent (CO2, acetate) with the formation of reduced compounds (carbohydrates) and the release of O2 if water is oxidized

Photosynthesis plays a leading role in biosphere processes, leading on a global scale to the formation of organic matter from inorganic matter.

Photosynthetic organisms, using solar energy in photosynthesis reactions, connect life on Earth with the Universe and ultimately determine all its complexity and diversity. Heterotrophic organisms - animals, fungi, most bacteria, as well as non-chlorophyll plants and algae - owe their existence to autotrophic organisms - photosynthetic plants that create organic matter on Earth and replenish the loss of oxygen in the atmosphere. Humanity is increasingly aware of the obvious truth, first scientifically substantiated by K.A. Timiryazev and V.I. Vernadsky: the ecological well-being of the biosphere and the existence of humanity itself depends on the state of the vegetation cover of our planet.

Processes occurring in the sheet

Leaf implements three important process– photosynthesis, water evaporation and gas exchange. During the process of photosynthesis, organic substances are synthesized in leaves from water and carbon dioxide under the influence of sunlight. During the day, as a result of photosynthesis and respiration, the plant releases oxygen and carbon dioxide, and at night - only carbon dioxide produced during respiration.

Most plants are able to synthesize chlorophyll in low light. In direct sunlight, chlorophyll is synthesized faster.
The light energy required for photosynthesis, within certain limits, is absorbed the more, the less the leaf is darkened. Therefore, in the process of evolution, plants have developed the ability to turn the leaf blade towards the light so that more sunlight falls on it. The leaves on the plant are arranged so as not to crowd each other.
Timiryazev proved that the source of energy for photosynthesis is predominantly the red rays of the spectrum. This is indicated by the absorption spectrum of chlorophyll, where the most intense absorption band is observed in the red part, and less intense in the blue-violet part.

Photo: Nat Tarbox

Chloroplasts contain the pigments carotene and xanthophyll along with chlorophyll. Both of these pigments absorb blue and, partly, green rays and transmit red and yellow ones. Some scientists attribute carotene and xanthophyll to the role of screens that protect chlorophyll from the destructive effects of blue rays.
The process of photosynthesis consists of a number of sequential reactions, some of which occur with the absorption of light energy, and some in the dark. The stable final products of photosynthesis are carbohydrates (sugars and then starch), organic acids, amino acids, and proteins.
Photosynthesis occurs at different rates under different conditions.

The intensity of photosynthesis also depends on the phase of plant development. The maximum intensity of photosynthesis is observed in the flowering phase.
The normal carbon dioxide content in the air is 0.03% by volume. Reducing the carbon dioxide content in the air reduces the intensity of photosynthesis. Increasing the carbon dioxide content to 0.5% increases the rate of photosynthesis almost proportionally. However, with a further increase in carbon dioxide content, the intensity of photosynthesis does not increase, and at 1%, the plant suffers.

Plants evaporate or transperate very large amounts of water. Evaporation of water is one of the causes of upward current. Due to the evaporation of water by the plant, minerals accumulate in it, and a beneficial temperature decrease for the plant occurs during solar heating.
The plant regulates the process of water evaporation through the work of stomata. The deposition of cuticle or waxy coating on the epidermis, the formation of its hairs and other adaptations are aimed at reducing unregulated transperation.

The process of photosynthesis and the constant ongoing respiration of living leaf cells require gas exchange between the internal tissues of the leaf and the atmosphere. During photosynthesis, assimilated carbon dioxide is absorbed from the atmosphere and returned to the atmosphere as oxygen.
The use of the isotope analysis method showed that the oxygen returned to the atmosphere 16O belongs to water, and not to carbon dioxide in the air, in which its other isotope, 15O, predominates. During the respiration of living cells (oxidation of organic substances inside the cell by free oxygen to carbon dioxide and water), oxygen must be supplied from the atmosphere and carbon dioxide must be returned. This gas exchange is also mainly carried out through the stomatal apparatus.

The process of photosynthesis consists of two successive and interconnected stages: light (photochemical) and dark (metabolic). At the first stage, the energy of light quanta absorbed by photosynthetic pigments is converted into the energy of chemical bonds of the high-energy compound ATP and the universal reducing agent NADPH - the actual primary products of photosynthesis, or the so-called “assimilation force”. In the dark reactions of photosynthesis, ATP and NADPH formed in the light are used in the cycle of carbon dioxide fixation and its subsequent reduction to carbohydrates.
In all photosynthetic organisms, the photochemical processes of the light stage of photosynthesis occur in special energy-converting membranes called thylakoid membranes and are organized into the so-called electron transport chain. Dark reactions of photosynthesis take place outside the thylakoid membranes (in the cytoplasm in prokaryotes and in the stroma of the chloroplast in plants). Thus, the light and dark stages of photosynthesis are separated in space and time.

Photosynthesis rate woody plants varies widely depending on the interaction of many external and internal factors, and these interactions vary over time and differ among species.

Photosynthetic capacity is sometimes assessed by the net increase in dry mass. Such data are of particular importance because the gain represents the average true increase in mass over a long period of time under environmental conditions that include normal periodic stresses.
Some angiosperm species perform photosynthesis efficiently under both low and high light intensities. Many gymnosperms are much more productive in high light conditions. Comparing these two groups at low and high light intensities often gives a different picture of photosynthetic capacity in terms of nutrient accumulation. In addition, gymnosperms often accumulate some dry mass during dormancy, whereas deciduous angiosperms lose it through respiration. Therefore, a gymnosperm plant with a slightly lower photosynthetic rate than a deciduous angiosperm during the growth period can accumulate as much or more total dry mass during the year due to the much longer period of photosynthetic activity.

The first experiments on photosynthesis were carried out by Joseph Priestley in the 1770-1780s, when he drew attention to the “spoilage” of air in a sealed vessel with a burning candle (the air was no longer able to support combustion, animals placed in it suffocated) and its “correction” by plants . Priestley concluded that plants produce oxygen, which is necessary for respiration and combustion, but did not notice that plants need light for this. This was soon shown by Jan Ingenhouse. Later it was found that in addition to releasing oxygen, plants absorb carbon dioxide and, with the participation of water, synthesize organic matter in the light. In 1842, Robert Mayer, based on the law of conservation of energy, postulated that plants convert the energy of sunlight into the energy of chemical bonds. In 1877, W. Pfeffer called this process photosynthesis.

N.Yu.FEOKTISTOVA

Night life of plants

Dendrobium speciosum orchid, opening flowers only at night

What do plants “do” at night? I just want to answer this question: “They are resting.” After all, it would seem that all active life» plants occur during the day. During the daytime, flowers open and are pollinated by insects, leaves unfurl, young stems grow and stretch their tops towards the sun. It is during daylight hours that plants use solar energy to convert the carbon dioxide they absorb from atmospheric air, into sugar.

However, the plant not only synthesizes organic substances - it also uses them in the process of respiration, again oxidizing it to carbon dioxide and absorbing oxygen. But the amount of oxygen that plants need for respiration is about 30 times less than what they release during photosynthesis. At night, in the dark, photosynthesis does not occur, but even at this time the plants consume so little oxygen that this does not affect us at all. Therefore, the old tradition of removing plants from the patient’s room at night is completely unfounded.

There are also a number of plant species that consume carbon dioxide at night. Since the energy from sunlight necessary to completely reduce carbon is not available at this time, sugar, of course, is not formed. But carbon dioxide absorbed from the air is stored in the composition of malic or aspartic acids, which then, already in the light, decompose again, releasing CO2. It is these molecules of carbon dioxide that are included in the cycle of basic reactions of photosynthesis - the so-called Calvin cycle. In most plants, this cycle begins with the capture of a CO2 molecule directly from the air. This “simple” method is called the C3 path of photosynthesis, and if carbon dioxide is preliminarily stored in malic acid, it is the C4 path.

It would seem, why do we need additional complications? First of all, in order to save water. After all, a plant can absorb carbon dioxide only through open stomata, through which water evaporates. And during the day, in the heat, much more water is lost through the stomata than at night. And in C4 plants, the stomata are closed during the day, and water does not evaporate. These plants carry out gas exchange during the cool night hours. In addition, the C4 pathway is generally more efficient; it allows the synthesis more organic matter per unit time. But only in conditions of good lighting and at a sufficiently high air temperature.

So C4 photosynthesis is characteristic of “southerners” - plants from hot regions. It is inherent in most cacti, some other succulents, and a number of bromeliads - for example, the well-known pineapple ( Ananas comosus), sugar cane and corn.

Interestingly, back in 1813, long before the biochemical reactions underlying photosynthesis were known, researcher Benjamin Hayne wrote to the Linnean Scientific Society that the leaves of a number of succulent plants tasted especially pungent in the morning, and then, by mid-day, their taste becomes softer.

The ability to use CO2 bound in organic acids is determined genetically, but the implementation of this program is also under the control of the external environment. During heavy rain, when there is no threat of drying out and the light level is low, C4 plants can open their stomata during the day and switch to the usual C3 path.

What else can happen to plants at night?

Some species have adapted to attract their pollinators at night. To do this, they use different means: a smell that intensifies at night, and a color that is pleasant and noticeable to the eyes of night pollinators - white or yellowish-beige. Moths fly to such flowers. They are the ones who pollinate jasmine flowers ( Jasminum), gardenias ( Gardenia), moon flowers ( Ipomea alba), noctule, or night violet ( Hesperis), Lyubka bifolia ( Platanthera bifolia), curly lily ( Lilium martagon) and a number of other plants.

Lilium martagon, vintage drawing

And there are plants (they are called chiropterophilous) that are pollinated at night by bats. Most of these plants are in the tropics of Asia, America and Australia, and less in Africa. These are bananas, agaves, boababs, some representatives of the families of myrtaceae, legumes, begoniaceae, gesneriaceae, and cyanaceae.

The flowers of chiropterophilous plants open only at dusk and are not very bright in color - as a rule, they are greenish-yellow, brown or purple. The smell of such flowers is very specific, often unpleasant for us, but probably attractive for bats. In addition, the flowers of chiropterophilous plants are usually large, have a strong perianth, and are equipped with “landing sites” for their pollinators. Such sites can be thick pedicels and peduncles or leafless areas of branches adjacent to flowers.

Some chiropterophilous plants even “talk” to their pollinators, attracting them. When the vine flower Mucuna holtonii, belonging to the legume family and growing in the tropical forests of Central America, becomes ready for pollination, one of its petals takes on a specific concave shape. This concave lobe concentrates and reflects the signal emitted by bats in search of food, thereby informing them of their location.

But not only chiropteran mammals pollinate flowers. More than 40 species of animals from other orders are known in the tropics, actively participating in the pollination of about 25 plant species. Many of these plants, like those pollinated by bats, have flowers that are large and robust, often foul-smelling, and produce large quantities of pollen and nectar. Usually the number of flowers on such plants or in their inflorescences is small; the flowers are located low above the ground and open only at night to provide maximum convenience for nocturnal animals.

The night life of flowers is not limited to attracting pollinators. A number of plants close their petals at night, but insects remain overnight inside the flower. Most famous example A similar “hotel” for insects is the Amazon lily ( Victoria amasonica). Europeans saw it for the first time in 1801, and detailed description the plant was made in 1837 by the English botanist Schomburg. The scientist was simply shocked by its giant leaves and wonderful flowers and named the flower “Nymphea Victoria”, in honor Queen of England Victoria.

Amazonian Victoria seeds were first sent to Europe in 1827, but then they did not germinate. In 1846, the seeds were sent to Europe again, this time in water bottles. And they not only withstood the road perfectly, but also developed into full-fledged plants, which bloomed after 3 years. This happened at the Kew Botanical Gardens in England. The news that Victoria was about to bloom quickly spread not only among the employees of the botanical garden, but also among artists and reporters. A huge crowd had gathered in the greenhouse. Everyone eagerly watched the clock, waiting for the flower to open. At 5 o'clock in the evening, the still closed bud rose above the water, its sepals opened and snow-white petals appeared. The wonderful smell of ripe pineapple spread throughout the greenhouse. A few hours later the flower closed and sank under the water. He appeared again only at 7 pm the next day. But, to the surprise of everyone present, the petals of the miracle flower were no longer white, but bright pink. Soon they began to fall off, while their color became more and more intense. After the petals completely fell, the active movement of the stamens began, which, according to the testimony of those present, was even audible.

But in addition to their extraordinary beauty, Victoria flowers also have amazing features associated with attracting insects. On the first day, the temperature in the white Victoria flower increases by about 11°C compared to the surrounding air, and in the evening, with the onset of coolness, a large number of insects accumulate in this “warm place”. In addition, special food bodies are formed on the carpels of the flower, which also attract pollinators. When the flower closes and sinks under the water, insects also sink with it. There they spend the night and the entire next day, until the flower rises to the surface again. Only now it is already cold and not fragrant, and insects, loaded with pollen, fly in search of new warm and fragrant white flowers to pollinate them, and at the same time to spend the night in the next warm and safe “hotel”.

One more, perhaps no less beautiful flower also provides night quarters for its pollinators - this is the lotus. There are two types of lotus. In the Old World, the nut-bearing lotus with pink flowers grows, and in America - the American lotus with yellow flowers. The lotus is able to maintain a relatively constant temperature inside its flowers - much higher than the temperature of the surrounding air. Even if it’s only +10°C outside, inside the flower it’s +30…+35°C!

Lotus flowers are heated 1–2 days before opening, and a constant temperature is maintained in them for 2–4 days. During this time, the anthers ripen, and the stigma of the pistil becomes capable of receiving pollen.

The lotus is pollinated by beetles and bees, whose active flight requires a temperature of just about 30°C. If insects find themselves in a flower after it closes and spend the night in warmth and comfort, actively moving and being covered with pollen, then in the morning, when the flower opens, they are immediately able to fly to other flowers. Thus, the “residents” of the lotus gain an advantage over the numb insects that spent the night in the cold. Thus, the warmth of the flower, transferred to the insect, contributes to the prosperity of the lotus population.

Many members of the aroid family, such as giant amorphophallus ( Amorphophallus titanus), the well-known monstera and philodendrons have flower petioles that produce heat at night, enhancing the smell and helping pollinating insects spend the night with maximum comfort. The unpleasant smell of amorphophallus attracts, for example, a lot of beetles, which find among the petals of the giant inflorescence a warm apartment, food, and marriage partners. Another interesting plant from the aroid family is Typophonium brownii – mimics heaps of animal droppings, attracting dung beetles, which it “captures” at night and forces to carry its pollen on itself.

Photosynthesis is the process of synthesis of organic substances from inorganic ones using light energy. In the vast majority of cases, photosynthesis is carried out by plants using cellular organelles such as chloroplasts containing the green pigment chlorophyll.

If plants were not capable of synthesizing organic matter, then almost all other organisms on Earth would have nothing to eat, since animals, fungi and many bacteria cannot synthesize organic substances from inorganic ones. They only absorb ready-made ones, break them down into simpler ones, from which they again assemble complex ones, but already characteristic of their body.

This is the case if we talk about photosynthesis and its role very briefly. To understand photosynthesis, we need to say more: what specific inorganic substances are used, how does synthesis occur?

Photosynthesis requires two inorganic substances - carbon dioxide (CO2) and water (H2O). The first is absorbed from the air by above-ground parts of plants mainly through stomata. Water comes from the soil, from where it is delivered to 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 (O2). Typically, organic matter most often means glucose (C6H12O6).

Organic compounds are mostly composed of carbon, hydrogen and oxygen atoms. They are found in carbon dioxide and water. However, during photosynthesis, oxygen is released. Its atoms are taken from water.

Briefly and generally, the equation for the reaction of photosynthesis is usually written as follows:

6CO2 + 6H2O → C6H12O6 + 6O2

But this equation does not reflect the essence of photosynthesis and does not make it understandable. Look, although the equation is balanced, in it the total number of atoms in free oxygen is 12. But we said that they come from water, and there are only 6 of them.

In fact, photosynthesis occurs in two phases. The first one is called light, second - 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 occurs in the dark. The light phase occurs on the membranes of the thylakoids of the chloroplast, and the dark phase occurs in the stroma of the chloroplast.

During the light phase, CO2 binding does not occur. All that happens is that solar energy is captured by chlorophyll complexes, stored in ATP, and energy is used to reduce NADP to NADP*H2. The flow of energy from light-excited chlorophyll is provided by electrons transmitted along the electron transport chain of enzymes built into the thylakoid membranes.

The hydrogen for NADP comes from water, which is decomposed by sunlight 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 briefly looks like this:

H2O + (ADP+P) + NADP → ATP + NADP*H2 + ½O2

Thus, the release of oxygen occurs 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*H2 come from the light phase to the dark phase. Here, the energy of the first and the reducing power of the second are spent on the binding of carbon dioxide. This step of photosynthesis cannot be explained simply and concisely because it does not proceed in the way that six CO2 molecules combine with hydrogen released from NADP*H2 molecules to form glucose:

6CO2 + 6NADP*H2 →С6H12O6 + 6NADP
(the reaction occurs with the expenditure of energy ATP, which breaks down into ADP and phosphoric acid).

The given reaction is just a simplification to make it easier to understand. In fact, carbon dioxide molecules bind one at a time and join the already prepared five-carbon organic substance. An unstable six-carbon organic substance is formed, which breaks down into three-carbon carbohydrate molecules. Some of these molecules are used to resynthesize the original five-carbon substance to bind CO2. This resynthesis is ensured Calvin cycle. A minority of carbohydrate molecules containing three carbon atoms exit the cycle. All other organic substances (carbohydrates, fats, proteins) are synthesized from them and other substances.

That is, in fact, three-carbon sugars, not glucose, come out of the dark phase of photosynthesis.

Photosynthesis- the process of synthesis of organic substances using light energy. Organisms that are capable of synthesizing organic substances from inorganic compounds are called autotrophic. Photosynthesis is characteristic only of cells of autotrophic organisms. Heterotrophic organisms are not capable of synthesizing organic substances from inorganic compounds.
The cells of green plants and some bacteria have special structures and complexes of chemicals that allow them to capture energy from sunlight.

The role of chloroplasts in photosynthesis

Plant cells contain microscopic formations - chloroplasts. These are organelles in which energy and light are absorbed and converted into the energy of ATP and other energy carrier molecules. The grana of chloroplasts contain chlorophyll, a complex organic substance. Chlorophyll captures light energy for use in the biosynthesis of glucose and other organic substances. The enzymes necessary for the synthesis of glucose are also located in chloroplasts.

Light phase of photosynthesis

A quantum of red light absorbed by chlorophyll transfers the electron to an excited state. An electron excited by light acquires a large supply of energy, as a result of which it moves to a higher energy level. An electron excited by light can be compared to a stone raised to a height, which also acquires potential energy. He loses it, falling from a height. The excited electron, as if in steps, moves along a chain of complex organic compounds built into the chloroplast. Moving from one step to another, the electron loses energy, which is used for the synthesis of ATP. The electron that wasted energy returns to chlorophyll. A new portion of light energy again excites the chlorophyll electron. It again follows the same path, spending energy on the formation of ATP molecules.
Hydrogen ions and electrons, necessary for the restoration of energy-carrying molecules, are formed by the splitting of water molecules. The breakdown of water molecules in chloroplasts is carried out by a special protein under the influence of light. This process is called photolysis of water.
Thus, the energy of sunlight is directly used by the plant cell to:
1. excitation of chlorophyll electrons, the energy of which is further spent on the formation of ATP and other energy carrier molecules;
2. photolysis of water, supplying hydrogen ions and electrons to the light phase of photosynthesis.
This releases oxygen as a by-product of photolysis reactions. The stage during which, due to the energy of light, energy-rich compounds are formed - ATP and energy-carrying molecules, called light phase of photosynthesis.

Dark phase of photosynthesis

Chloroplasts contain five-carbon sugars, one of which ribulose diphosphate, is a carbon dioxide acceptor. A special enzyme binds five-carbon sugar with carbon dioxide in the air. In this case, compounds are formed that, using the energy of ATP and other energy carrier molecules, are reduced to a six-carbon glucose molecule. Thus, the light energy converted during the light phase into the energy of ATP and other energy carrier molecules is used for the synthesis of glucose. These processes can take place in the dark.
It was possible to isolate chloroplasts from plant cells, which in a test tube, under the influence of light, carried out photosynthesis - they formed new glucose molecules and absorbed carbon dioxide. If the illumination of the chloroplasts was stopped, the synthesis of glucose also stopped. However, if ATP and reduced energy carrier molecules were added to the chloroplasts, then glucose synthesis resumed and could proceed in the dark. This means that light is really only needed to synthesize ATP and charge energy-carrying molecules. Absorption of carbon dioxide and formation of glucose in plants called dark phase of photosynthesis because she can walk in the dark.
Intense lighting and increased carbon dioxide content in the air lead to increased photosynthesis activity.

1. Is photosynthesis a process of plastic or energy metabolism? Why?

Photosynthesis refers to the processes of plastic metabolism because accompanied by:

● by the synthesis of complex organic compounds from simpler substances, namely: glucose (C 6 H 12 O 6) is synthesized from inorganic substances (H 2 O and CO 2);

● absorption of light energy.

2. In which organelles of a plant cell does photosynthesis occur? What is a photosystem? What function do photosystems perform?

Photosynthesis occurs in green plastids - chloroplasts.

Photosystems are special pigment-protein complexes located in the membranes of chloroplast thylakoids. There are two types of photosystems – photosystem I and photosystem II. Each of them includes a light-harvesting antenna formed by pigment molecules, a reaction center and electron carriers.

The light-harvesting antenna functions like a funnel: pigment molecules absorb light and transfer all the collected energy to the reaction center, where the trap molecule represented by chlorophyll a is located. Having absorbed energy, the trap molecule goes into an excited state and gives one of its electrons to a special carrier, i.e. oxidizes. Thus, photosystems perform the function of absorbing light and converting light energy into chemical energy.

3. What is the importance of photosynthesis on Earth? Why would the existence of the biosphere be impossible without phototrophic organisms?

Photosynthesis is the only process on the planet during which the light energy of the Sun is converted into the energy of chemical bonds of synthesized organic substances. In this case, the starting compounds for the synthesis of organic substances are energy-poor inorganic substances - carbon dioxide and water.

Organic compounds formed during photosynthesis are transferred as part of food from phototrophic organisms to herbivores, then to carnivores, being a source of energy and building material for the synthesis of other substances, for the formation of new cells and structures. Consequently, thanks to the activity of phototrophs, the nutritional needs of heterotrophic organisms are satisfied.

In addition, photosynthesis is a source of molecular oxygen necessary for the respiration of most living organisms. The ozone layer is formed and maintained from oxygen, protecting living organisms on the planet from the harmful effects of short-wave ultraviolet radiation. Thanks to photosynthesis, a relatively constant CO 2 content in the atmosphere is maintained.

4. Characterize the light and dark phases of photosynthesis according to the plan:

1) location of the leak; 2) starting materials; 3) ongoing processes; 4) final products.

What products of the light phase of photosynthesis are used in the dark phase?

Light phase of photosynthesis.

1) Place of leakage: thylakoid membranes.

2) Starting substances: H 2 O, oxidized NADP (NADP +), ADP, H 3 PO 4. Photosynthetic pigments (chlorophylls, etc.) are also necessary for the light phase to occur, but they cannot be called the initial substances of the light phase.

3) Occurring processes: absorption of light by photosystems, photolysis of water, transport of electrons to the outside of the thylakoid and accumulation of protons inside the thylakoid (i.e., the appearance of an electrochemical potential on the thylakoid membrane), ATP synthesis, reduction of NADP +.

4) End products: ATP, reduced NADP (NADP H+H +), by-product - molecular oxygen (O 2).

Dark phase of photosynthesis.

1) Place of leakage: chloroplast stroma.

2) Initial substances: CO 2, ATP, reduced NADP (NADP H+H +).

3) Ongoing processes: glucose synthesis (reduction of CO 2 to organic substances), during which ATP hydrolysis and NADP H+H + oxidation occur.

4) End products: glucose (C 6 H 12 O 6), oxidized NADP (NADP +), ADP, H 3 PO 4.

In the dark phase of photosynthesis, light phase products such as NADP H+H + (serves as a source of hydrogen atoms for the synthesis of glucose) and ATP (serves as a source of energy for the synthesis of glucose) are used.

5. Compare photosynthesis and aerobic respiration. Indicate similarities and differences.

Similarities:

● Complex multi-stage processes involving enzymes.

● Photosynthesis and the final (oxygen) stage of aerobic respiration occur in double-membrane organelles (chloroplasts and mitochondria, respectively).

● Redox processes, which are accompanied by the transfer of electrons along the electron transport chains of the internal membranes of the corresponding organelles, the appearance of a potential difference on these membranes, the work of ATP synthetase and ATP synthesis.

Differences:

● The process of photosynthesis refers to plastic metabolism because is accompanied by the synthesis of organic substances from inorganic ones and occurs with the absorption of light energy. The process of aerobic respiration refers to energy metabolism, since complex organic substances are broken down and the energy contained in them is released.

● Photosynthesis occurs only in the cells of phototrophic organisms, and aerobic respiration occurs in the cells of most living organisms (including phototrophs).

● Various starting materials and final products. If we consider the summary equations of photosynthesis and aerobic respiration, we can see that the products of photosynthesis are actually the starting materials for aerobic respiration and vice versa.

● NAD and FAD serve as carriers of hydrogen atoms in the process of respiration, and NADP in photosynthesis.

And (or) other significant features.

6. A person consumes approximately 430 g of oxygen per day. An average-sized tree absorbs about 30 kg of carbon dioxide per year. How many trees are needed to provide one person with oxygen?

● In a year, a person consumes: 430 g × 365 = 156,950 g of oxygen.

● Let's calculate the chemical amount of carbon dioxide absorbed per year by one tree:

M (CO 2) = 12 + 16 × 2 = 44 g/mol. n (CO 2) = m: M = 30,000 g: 44 g/mol ≈ 681.8 mol.

● Summary equation of photosynthesis:

6CO 2 + 6H 2 O → C 6 H 12 O 6 + 6O 2

The absorption of 6 moles of carbon dioxide is accompanied by the release of 6 moles of oxygen. This means that, absorbing 681.8 moles of carbon dioxide per year, the tree releases 681.8 moles of oxygen.

● Let’s find the mass of oxygen released by the tree per year:

M (O 2) = 16 × 2 = 32 g/mol. m (O 2) = n × M = 681.8 mol × 32 g/mol = 21,817.6 g

● Let's determine how many trees are needed to provide one person with oxygen. Number of trees = 156,950 g: 21,817.6 ≈ 7.2 trees.

Answer: To provide one person with oxygen, on average, 7.2 trees will be needed (acceptable answers would be “8 trees” or “7 trees”).

7. Researchers divided wheat plants into two groups and grew them in the laboratory under the same conditions, except that the plants in the first group were illuminated with red light, and the plants in the second group were illuminated with green light. In which group of plants did photosynthesis proceed more intensively? What is this connected with?

Photosynthesis proceeded more intensely in plants illuminated with red light. This is due to the fact that the main photosynthetic pigments - chlorophylls - intensively absorb red light (as well as the blue-violet part of the spectrum), and reflect green, which determines the green color of these pigments.

8*. What experiment can be used to prove that the oxygen released during photosynthesis is formed precisely from water molecules, and not from molecules of carbon dioxide or any other substance?

If water labeled with radioactive oxygen is used to carry out photosynthesis (the molecules contain oxygen radionuclide instead of the stable nuclide 16 O), then the radioactive label can be detected in the released molecular oxygen. If you use any other substance containing oxygen radionuclide for photosynthesis, then the released O2 will not contain a radioactive label. In particular, radioactive oxygen contained in the molecules of absorbed carbon dioxide will be found in the synthesized organic substances, but not in the composition of O 2.

*Tasks marked with an asterisk require students to put forward various hypotheses. Therefore, when marking, the teacher should focus not only on the answer given here, but take into account each hypothesis, assessing the biological thinking of students, the logic of their reasoning, the originality of ideas, etc. After this, it is advisable to familiarize students with the answer given.