Intensity and spectral composition of light

On average, leaves absorb 80 - 85% of the energy of the photosynthetically active rays of the solar spectrum (400 - 700 nm) and 25% of the energy of infrared rays, which is about 55% of the total radiation energy. Photosynthesis consumes 1.5 - 2% of the absorbed energy (photosynthetically active radiation - PAR).

The dependence of the rate of photosynthesis of light intensity has the form of a logarithmic curve (Fig. 1). A direct dependence of the process rate on the energy influx is observed only at low light intensities. Photosynthesis starts in very low light; This was first shown by A. S. Famintsyn in 1880 on an installation with artificial lighting. The light of a kerosene lamp was enough to start photosynthesis and the formation of starch in plant cells. In many light-loving plants, the maximum (100%) intensity of photosynthesis is observed at illumination reaching half of the full sun, which, therefore, is saturating. A further increase in illumination does not increase photosynthesis and then reduces it.

Fig.1. The dependence of the rate of photosynthesis on the intensity of light in corn

An analysis of the light curves of photosynthesis makes it possible to obtain information about the nature of the work of photochemical systems and the enzymatic apparatus. The slope of the curve characterizes the rate of photochemical reactions and the content of chlorophyll: the larger it is, the more actively light energy is used. It is usually greater in shade-tolerant plants living under the forest canopy, and in deep-sea algae. In these plants, adapted to low light conditions, a well-developed pigment apparatus makes it possible to make more active use of low light intensities.

Photosynthesis activity in the region of saturating light intensity characterizes the power of CO 2 absorption and reduction systems and is determined by the CO 2 concentration. The higher the curve in the light intensity saturation region, the more powerful the CO 2 absorption and reduction apparatus. In light-loving plants, saturation is achieved at much higher illumination than in shade-tolerant ones. In shade-tolerant marshantia liver moss, light saturation of photosynthesis is achieved at 1000 lux, in light-loving woody plants - at 10 - 40 thousand lux, and in some alpine plants of the Pamirs (where the illumination reaches the maximum values ​​on Earth of about 180 thousand lux) - at 60 thousand .lx and above. The majority of agricultural and woody plants, as well as shallow water algae, are photophilous.

In plants that carry out the C 3 -way of photosynthesis, saturation occurs at a lower light intensity than in plants with the C 4 -way of carbon conversion, whose high photosynthetic activity is manifested only when high level illumination.

In the area of ​​light saturation, the intensity of photosynthesis is much higher than the intensity of respiration. When the illumination decreases to a certain value, the intensities of photosynthesis and respiration become equal. The level of illumination at which the absorption of CO 2 during photosynthesis is balanced by the release of CO 2 during respiration is called the light compensation point. Its value is determined at 0.03% CO 2 and a temperature of 20 °C. The value of the light compensation point is not the same not only for shade-tolerant (about 1% of total light) and light-loving plants (about 3-5% of total sunlight), but also for leaves of different tiers of the same plant, it also depends on the concentration CO 2 in the air. Excessively high illumination dramatically disrupts the process of pigment biosynthesis, photosynthetic reactions and growth processes, which ultimately reduces the overall productivity of plants.

It is significant that even a short-term change in illumination conditions affects the intensity of photosynthesis. This important adaptive property allows plants in phytocenoses to make better use of light. The photosynthetic apparatus is "tuned" to periodic shifts in illumination in the wind, to the frequency of flashing glare in a fraction of a second.

Changes in other factors affect the course of photosynthesis light curves external environment. For example, when low temperatures(12 C) increasing the light intensity becomes ineffective. The temperature optimum in plants with C 3 -type of photosynthesis lies in the range of 25-35 C. An increase in the concentration of CO 2 with an increase in illumination leads to an increase in the rate of photosynthesis (Fig. 2).

Fig.2. Mutual influence of light intensity and carbon dioxide concentration on the rate of photosynthesis in moss

Why are red rays the most efficient for photosynthesis? Firstly, because the energy of 1 quantum of red light (176 kJ/mol = 42 kcal/mol) is quite sufficient for the transition of the chlorophyll molecule to the first singlet excitation level S*. This energy can then be used entirely for photochemical reactions. The energy of 1 quantum of blue light is higher (293 kJ/mol = 70 kcal/mol). Having absorbed a quantum of blue light, the chlorophyll molecule passes to a higher level of singlet excitation S*, and this excess energy is converted into heat when the molecule passes into the S* state. The energy of 1 quantum of red light is approximately equivalent to the energy of the transition of the redox potential of the system from E " 0 \u003d + 0.8 V to E "o \u003d -0.8 V. The energy of 1 quantum of infrared rays is already insufficient for the photooxidation of water, but in photosynthetic sulfur bacteria this energy completely ensures the photooxidation of H 2 S in the process of photoreduction. Therefore, photosynthesis in sulfur bacteria with the participation of bacteriochlorophyll is carried out under the action of infrared light invisible to the human eye.

Secondly, red light is always present in direct solar radiation. If the sun is at a 90° angle, then the red rays are about 1/4 of the total sunlight. If the sun is low, the red rays become predominant. When the sun stands at an angle of 5 0, the red light is 2/3 of the full. Plants grown in blue and red light differ significantly in the composition of photosynthesis products. According to N. P. Voskresenskaya (1965), when blue and red light are aligned by quanta, i.e. under the same lighting conditions for the photochemical stage of photosynthesis, blue light activates the incorporation of 14 C into non-carbohydrate products - amino- and organic acids, mainly into alanine, aspartate, malate, citrate, and later (after minutes) into the protein fraction, and red light at short exposures into the soluble carbohydrate fraction and at minute exposures into starch. Thus, under blue light, in comparison with red light, non-carbohydrate products are additionally formed in the leaves.

These differences in carbon metabolism under the action of light different quality found in whole plants with C 3 - and C 4 -ways of CO 2 assimilation, in green and red algae they are stored at different concentrations of CO 2 and unequal light intensity. But in isolated chloroplasts, no differences were found in the formation of starch under blue and red light. Flavins are believed to be the photoreceptor responsible for changes in carbon metabolism under blue light in green plants. The rate of photosynthesis is rapidly and significantly increased by the addition of a large number(20% of red light saturation) blue light to red. Apparently, this is due to the fact that the photochemical step of photosynthesis is regulated by blue light.

Carbon dioxide concentration

Carbon dioxide is the main substrate of photosynthesis; its content determines the intensity of the process. The concentration of CO 2 in the atmosphere is 0.03%. An air layer 100 m high above a hectare of arable land contains 550 kg of CO 2 . Of this amount, plants absorb 120 kg of CO 2 per day. The dependence of photosynthesis on CO 2 is expressed by a logarithmic curve (Fig. 3). At a concentration of 0.03%, the intensity of photosynthesis is only 50% of the maximum, which is achieved at 0.3% CO 2. This indicates that in evolution the process of photosynthesis was formed at a higher concentration of CO 2 in the atmosphere. In addition, such a course of the dependence of the productivity of photosynthesis on the concentration of CO 2 indicates the possibility of feeding plants in enclosed spaces with CO 2 to obtain a larger yield. Such CO 2 feeding has a strong effect on the yield of plants with the C 3 -type of CO 2 assimilation and does not affect the plants with the C 4 -type, which have a special mechanism for concentrating CO 2 .

Fig.3. The dependence of the intensity of photosynthesis on the concentration of carbon dioxide

The intensity of CO 2 assimilation depends on the rate of its entry from the atmosphere into chloroplasts, which is determined by the rate of CO 2 diffusion through stomata, intercellular spaces, and in the cytoplasm of leaf mesophyll cells. In the open state, the stomata occupy only 1-2% of the leaf area, the rest of the surface is covered with a cuticle that is poorly permeable to gases. However, in the presence of a cuticle, CO2 enters the leaf through the stomata per unit of time in the same amount as without it. This is explained by Stefan's law, according to which the speed of movement of gas molecules through small holes is proportional to their circumference, and not to the area. The smaller the hole, the greater the ratio of circumference to area. And at the edge of the hole, the molecules collide with each other to a lesser extent and diffuse faster. Therefore, through a stomata with an aperture (openness) of the order of 10 μm, gas molecules move at high speed. The processes of opening and closing of stomata are affected by CO 2 , tissue saturation with water, light, and phytohormones.

Temperature

Primary photophysical processes of photosynthesis (absorption and migration of energy, excited states) do not depend on temperature. The processes of photosynthetic phosphorylation are very sensitive to temperature. The rate of the complex of enzymatic reactions associated with the reduction of carbon increases by 2–3 times with an increase in temperature by 10 °C (Q 10 = 2–3). The general dependence of photosynthesis on temperature is expressed by a single-peak curve (Fig. 4). The curve has three main (cardinal) temperature points: the minimum at which photosynthesis begins, the optimum and the maximum. The intensity of photosynthesis at superoptimal temperatures depends on the duration of their exposure to plants. The lower temperature limit of photosynthesis in plants of northern latitudes is within -15 ° C (pine, spruce) ... -0.5 ° C, and in tropical plants - in the zone of low positive temperatures of 4 - 8 ° C. In plants of the temperate zone, in the range of 20 - 25 ° C, the maximum intensity of photosynthesis is achieved, and further increase temperatures up to 40 °C leads to a rapid inhibition of the process (at 45 °C the plants die).

Some desert plants are capable of photosynthesis at 58°C. The temperature limits of photosynthesis can be extended by preliminary hardening, adaptation of plants to a temperature gradient. The reactions of carboxylation, the conversion of fructose-6-phosphate to sucrose and starch, as well as the transport of sucrose from leaves to other organs are most sensitive to the action of temperature. It should be noted that the effect of light, CO 2 concentration, and temperature on photosynthesis is carried out in a complex interaction. Particularly closely related are light, which affects the rate of photochemical reactions, and temperature, which controls the rate of enzymatic reactions. In conditions of high intensity! light and low temperatures (5-10 °C), when enzymatic reactions are the main factor limiting the rate of the entire process, Q 10 values ​​controlled by temperature can be > 4. At more high temperatures Q 10 decreases to 2. At low light intensities, Q 10 \u003d 1, i.e., photosynthesis is relatively independent of temperature, since its rate in this case limited to photochemical reactions.

Rice. Fig. 4. Dependence of the intensity of photosynthesis in spruce on temperature

Water regime

Water is directly involved in photosynthesis as a substrate for oxidation and a source of oxygen. Another aspect of the effect of water content on photosynthesis is that the amount of water content in the leaves determines the degree of stomata opening and, consequently, the influx of CO2 into the leaf. When the leaf is completely saturated with water, the stomata close, which reduces the intensity of photosynthesis. Under drought conditions, excessive leaf water loss also causes stomatal closure due to an increase in the content of abscisic acid in the leaves in response to a lack of moisture. Long-term water deficit in leaf tissues during drought leads to inhibition of non-cyclic and cyclic electron transport and photophosphorylation and to a decrease in the ATP/NADPH ratio due to greater inhibition of ATP formation. Maximum photosynthesis is observed with a slight water deficit of the leaf (about 5–20% of full saturation) with open stomata.

mineral nutrition

For the normal functioning of the photosynthetic apparatus, the plant must be provided with the whole complex of macro- and microelements. Two main processes of nutrition plant organism- air and root - are closely interconnected. The dependence of photosynthesis on mineral nutrition elements is determined by their necessity for the formation of the photosynthetic apparatus (pigments, components of the electron transport chain, catalytic systems of chloroplasts, structural and transport proteins), as well as for its renewal and functioning.

Magnesium is a part of chlorophylls, participates in the activity of conjugating proteins in the synthesis of ATPy, affects the activity of carboxylation reactions and reduction of NADP +. As a result, its deficiency disrupts the process of photosynthesis. Iron in reduced form is necessary for the processes of biosynthesis of chlorophyll and iron-containing compounds of chloroplasts (cytochromes, ferredoxin). Iron deficiency dramatically disrupts the functioning of cyclic and non-cyclic photophosphorylation, pigment synthesis, and changes the structure of chloroplasts.

Need; manganese for green plants is associated with its role in the photooxidation of water. Therefore, malnutrition for manganese adversely affects the intensity of photosynthesis. Chlorine is also required in water photooxidation reactions. Copper is a component of plastocyanin, therefore, in plants, copper deficiency causes a decrease in the intensity of photosynthesis. The lack of nitrogen strongly affects the formation of pigment systems, chloroplast structures and its general activity. Nitrogen concentration determines the amount and activity of RDF-carboxylase.

Under conditions of phosphorus deficiency, photochemical and dark reactions of photosynthesis are disturbed. Phosphorus deficiency is especially pronounced at high light intensity, while dark reactions are more sensitive. However, when the phosphorus content is halved, the intensity of photosynthesis decreases to a lesser extent than growth processes and the overall productivity of plants. An excess of phosphorus also inhibits the rate of photosynthesis, apparently due to changes in membrane permeability.

A decrease in the potassium content in tissues is accompanied by a significant decrease in the intensity of photosynthesis and disturbances in other processes in the plant. In chloroplasts, the gran structure is destroyed, the stomata open weakly in the light and do not close enough in the dark, water regime leaf, all processes of photosynthesis are disturbed. This indicates a polyfunctional role of potassium in the ionic regulation of photosynthesis.

Oxygen

The process of photosynthesis is usually carried out under aerobic conditions and at an oxygen concentration of 21%. An increase in the content or lack of oxygen for photosynthesis is unfavorable. The usual concentration of 0 2 exceeds the optimal value for photosynthesis. In plants with a high level of photorespiration (beans, etc.), a decrease in the oxygen concentration from 21 to 3% increased photosynthesis, while in corn plants (with a low level of photorespiration), this kind of change did not affect the intensity of photosynthesis. High concentrations of 0 2 (25 - 30%) reduce photosynthesis ("Warburg effect"). The following explanations for this phenomenon are proposed. Partial pressure increase 0 2 and a decrease in the concentration of CO 2 activate photorespiration. Oxygen directly reduces the activity of RDF-carboxylase. Finally, O 2 can oxidize the primary reduced products of photosynthesis.

Daily and seasonal rhythms of photosynthesis

Studies of plant photosynthesis in natural terrestrial ecosystems began in the first quarter of the 20th century. the works of V. N. Lyubimenko, S. P. Kostychev and others. The environmental factors discussed earlier act jointly and in various combinations. However, light, temperature and water regime play a decisive role. With sunrise, the intensity of photosynthesis increases along with illumination, reaching maximum values ​​at 9-12 o'clock. The further nature of the process is determined by the degree of watering of the leaves, air temperature and the intensity of sunlight. In the midday hours, the intensity of photosynthesis does not increase: it can remain approximately at the level of the morning maximum (on cool, cloudy days) or decrease slightly, but then by 16-17 hours the process is again intensified. The intensity of photosynthesis falls after 22:00 with sunset.

Daytime depression of photosynthesis (if any) is associated with disturbances in the activity of the photosynthetic apparatus and the outflow of assimilates during overheating, since the temperature of the leaves during this period can exceed the air temperature by 5-10°C. If the loss of water by the tissues is large and there is an increase in photorespiration, then the stomata close at this time. Seasonal changes in photosynthesis, studied by O.V. Zalensky in desert plants and in the conditions of the Arctic, showed that in desert plants they depend on the characteristics of ontogeny, and in ephemers with a short growing season, the maximum intensity of photosynthesis is observed in late March - mid-April and coincides with beginning of fruiting. In plants that finish active vegetation at the beginning of summer, the seasonal maximum of photosynthesis is noted before the onset of summer dormancy.

In long-vegetating trees and shrubs, the seasonal maximum is recorded at the very beginning of the hot and dry period. By autumn, the intensity of photosynthesis gradually decreases. In arctic plants, seasonal changes in photosynthesis are manifested in a decrease in its intensity at the beginning and at the end of the growing season, when plants are often exposed to frost. The maximum of photosynthesis is noted in the most favorable period of the polar summer.

Each creature on the planet needs food or energy to survive. Some organisms feed on other creatures, while others can produce their own nutrients. They make their own 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 body. This stored chemical energy comes from the conversion of inorganic carbon (carbon dioxide) into organic carbon. The process of breathing releases stored chemical energy.

In addition to the products 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 for making protein). 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.

Definition 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 it releases oxygen, on which all life depends.

Why do plants need glucose (food)?

Just like humans and other living things, plants also need food to stay alive. The value of glucose for plants is as follows:

• Glucose obtained from photosynthesis is used during respiration to release energy, necessary for the plant 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 needed for growth and other essential 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 the electron carrier molecule NADPH (nicotinamide adenine dinucleotide phosphate) and the energy molecule ATP (adenosine triphosphate). Light phases occur in thylakoid membranes within the chloroplast.

Dark phase of photosynthesis or Calvin cycle

AT dark phase or the Calvin cycle, excited electrons from the light phase provide the energy to form carbohydrates from carbon dioxide molecules. The light-independent phases are sometimes called the Calvin cycle due to the cyclic nature of the process.

Although the dark phases do not use light as a reactant (and as a result can occur day or night), they require the products of light-dependent reactions to function. The light-independent molecules depend on the energy carrier molecules ATP and NADPH to create new carbohydrate molecules. After the transfer of energy to the molecules, the energy carriers return to the light phases to obtain more energetic electrons. In addition, several dark phase enzymes are activated by light.

Diagram of the phases of photosynthesis

The note: This means that the dark phases will not continue if the plants are deprived of light for too long, as they use the products of the light phases.

The structure of plant leaves

We cannot fully understand photosynthesis without knowing more about leaf structure. The leaf is adapted to play a vital role in the process of photosynthesis.

The external structure of the leaves

• Square

One of the most important features of plants is the large surface area of ​​the leaves. Most green plants have broad, flat and open leaves that are capable of capturing as much solar energy(sunlight) as needed for photosynthesis.

• Central vein and petiole

The midrib and petiole join together and form the base of the leaf. The petiole positions the leaf in such a way that it receives as much light as possible.

• leaf blade

Simple leaves have one sheet plate, and complex - a few. 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 leaves carries water from the stems to the leaves. The released glucose is also sent to other parts of the plant from the leaves through the veins. In addition, these parts of the leaf support and hold the leaf plate flat for greater sunlight capture. The arrangement of veins (venation) depends on the type of plant.

• leaf base

The base of the leaf is its lowest part, which is articulated with the stem. Often, at the base of the leaf there is a pair of stipules.

• leaf edge

Depending on the type of plant, the leaf edge may have various shapes, including: entire, serrated, serrate, notched, crenate, etc.

• Leaf tip

Like the edge of the sheet, the top is various shapes, including: sharp, round, blunt, elongated, retracted, etc.

The internal structure of the leaves

Below is a close diagram internal structure leaf tissue:

• Cuticle

The cuticle acts as the main, protective layer on the surface of the plant. As a rule, it is thicker on the top of the leaf. The cuticle is covered with a wax-like substance that protects the plant from water.

• Epidermis

The epidermis is a layer of cells that is the integumentary tissue of the leaf. His main function- protection of the internal tissues of the leaf from dehydration, mechanical damage and infections. It also regulates the process of gas exchange and transpiration.

• Mesophyll

The mesophyll is the main tissue of the plant. 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

Guard cells are specialized cells in the leaf epidermis that are used to control gas exchange. They perform protective function for the stomata. The stomatal pores become large when water is freely available, otherwise the protective cells become lethargic.

• Stoma

Photosynthesis depends on the penetration of carbon dioxide (CO2) from the air through the stomata into the mesophyll tissues. Oxygen (O2), obtained as a by-product of photosynthesis, exits the plant through stomata. When the stomata are open, water is lost through evaporation and must be replenished through the flow of transpiration by water taken up by the roots. Plants are forced to balance the amount of CO2 absorbed from the air and the loss of water through the stomatal pores.

Conditions required for photosynthesis

The following 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 during the combustion of carbon and organic compounds, and also occurs during respiration.
• Water. transparent liquid Chemical substance odorless and tasteless (under normal conditions).
• Light. While artificial light is also suitable for plants, natural sunlight tends to create Better conditions for photosynthesis, because it contains natural ultraviolet radiation, which has positive influence on plants.
• Chlorophyll. it green pigment found in the leaves of plants.
• 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 because it is not a substance)

The note: Plants take in 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 as an energy store.

If the factors that promote 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 a plant, while a lack of water slows it 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 other parts of the cell that work with the chloroplast to carry out photosynthesis.

The structure of a plant cell

Functions of plant cell parts

• : provides structural and mechanical support, protects cells from bacteria, 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 chemical processes controlled by enzymes.
• : 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 cell cytoplasm that stores water.
• : contains a genetic mark (DNA) that controls the activity of the cell.

Chlorophyll absorbs the light energy needed for photosynthesis. It is important to note that not all color wavelengths of light are absorbed. Plants mainly absorb red and blue wavelengths - they do not absorb light in the green range.

Carbon dioxide during photosynthesis

Plants take in carbon dioxide from the air through their leaves. Carbon dioxide seeps through a small hole at the bottom of the leaf - the stomata.

The underside of the leaf has loosely spaced cells to allow carbon dioxide to reach other cells in the leaf. It also allows the oxygen produced by photosynthesis to easily leave the leaf.

Carbon dioxide is present in the air we breathe in 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 waxy layer (cuticle). The top of the sheet is where the light falls. This layer of mesophyll is called the palisade. It is adapted to absorb a large amount of light, because it contains many chloroplasts.

In the light phases, the process of photosynthesis increases with more light. More chlorophyll molecules are ionized and more ATP and NADPH are generated if light photons are focused on a green leaf. Although light is extremely important in the light phases, it should be noted that too much of it can damage chlorophyll and reduce the process of photosynthesis.

Light phases are not too dependent on temperature, water or carbon dioxide, although they are all needed to complete the process of photosynthesis.

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 are characterized by a large surface area and thin walls, which allows water to easily pass through them.

The image shows plants and their cells with enough water (left) and its lack (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 will wilt. Without water, the plant will not be able to photosynthesize fast enough, and may even die.

What is the importance of water for plants?

• Provides dissolved minerals that support plant health;
• Is the medium for transportation;
• Supports stability and uprightness;
• Cools and saturates with moisture;
• Allows for various chemical reactions in plant cells.

Importance of photosynthesis in nature

The biochemical process of photosynthesis uses the energy of sunlight to convert water and carbon dioxide into oxygen and glucose. Glucose is used as 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 basis of almost every the food chain on the ground. (Algae are the plant's equivalent). All the food we eat comes from organisms that are photosynthetic. We eat these plants directly, or we eat animals such as cows or pigs that consume plant foods.

• Basis of the food chain

Within aquatic systems, plants and algae also form the basis 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 carbon dioxide levels are rising at an alarming rate, any process that removes carbon dioxide from the atmosphere is environmentally important.

• Nutrient cycling

Plants and other photosynthetic organisms play a vital role in nutrient cycling. 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 up the food chain.

• photosynthetic addiction

Photosynthesis depends on the intensity and quality of light. At the equator, where sunlight is plentiful all year round and water is not the limiting factor, plants have high growth rates and can become quite large. Conversely, photosynthesis is less common in the deeper parts of the ocean, because light does not penetrate these layers, and as a result, this ecosystem is more barren.

Of all the factors simultaneously affecting the process of photosynthesis limiting will be the one that is closer to the minimum level. It installed Blackman in 1905. Different factors can be limiting, but one of them is the main one.

1. In low light, the rate of photosynthesis is directly proportional to the light intensity. Light is the limiting factor in low light conditions. At high light intensity, chlorophyll becomes discolored and photosynthesis slows down. Under such conditions in nature, plants are usually protected (thick cuticle, pubescent leaves, scales).

1. The dark reactions of photosynthesis require carbon dioxide, which is included in organic matter, is a limiting factor in the field. The concentration of CO 2 in the atmosphere varies from 0.03-0.04%, but if you increase it, you can increase the rate of photosynthesis. Some greenhouse crops are now grown under elevated content CO2.
2. temperature factor. Dark and some light reactions of photosynthesis are controlled by enzymes, and their action depends on temperature. Optimum temperature for temperate plants is 25 °C. With each increase in temperature by 10 °C (up to 35 °C), the reaction rate doubles, but due to the influence of a number of other factors, plants grow better at 25 °C.
3. Water- source material for photosynthesis. Lack of water affects many processes in cells. But even temporary wilting leads to serious crop losses. Reasons: when withering, the stomata of plants close, and this interferes with the free access of CO 2 for photosynthesis; with a lack of water in the leaves of some plants accumulates abscisic acid. It is a plant hormone - a growth inhibitor. In laboratory conditions, it is used to study the inhibition of the growth process.
4. Chlorophyll concentration. The amount of chlorophyll can decrease with powdery mildew, rust, viral diseases, lack of minerals and age (with normal aging). When the leaves turn yellow, chlorotic phenomena or chlorosis. The reason may be a lack of minerals. For the synthesis of chlorophyll, Fe, Mg, N and K are needed.
5. Oxygen. A high concentration of oxygen in the atmosphere (21%) inhibits photosynthesis. Oxygen competes with carbon dioxide for the active site of the enzyme involved in CO 2 fixation, which reduces the rate of photosynthesis.
6. Specific inhibitors. The best way to kill a plant is to suppress photosynthesis. To do this, scientists have developed inhibitors - herbicides- dioxins. For example: DHMM - dichlorophenyldimethylurea- inhibits the light reactions of photosynthesis. Successfully used to study the light reactions of photosynthesis.
7. Pollution environment . Gases of industrial origin, ozone and sulfur dioxide, even in small concentrations, severely damage the leaves of a number of plants. Lichens are very sensitive to sulfur dioxide. Therefore, there is a method lichen indications– determination of environmental pollution by lichens. Soot clogs the stomata and reduces the transparency of the leaf epidermis, which reduces the rate of photosynthesis.

6. Plant life factors, heat, light, air, water- Plants throughout their lives are constantly in interaction with the external environment. Plant requirements for life factors are determined by the heredity of plants, and they are different not only for each species, but also for each variety of a particular crop. That is why a deep knowledge of these requirements makes it possible to correctly establish the structure of sown areas, the rotation of crops, the placement crop rotations.
Plants need light, heat, water, nutrients including carbon dioxide and air.
The main source of light for plants is solar radiation. Although this source is beyond human influence, the degree of use of the light energy of the sun for photosynthesis depends on the level of agricultural technology: sowing methods (rows directed from north to south or from east to west), differentiated seeding rates, tillage, etc.
Timely thinning of plants and the destruction of weeds improve the illumination of plants.
Heat in plant life, along with light, is the main factor in plant life and necessary condition for biological, chemical and physical processes in the soil. Each plant at various phases and stages of development makes certain, but unequal requirements for heat, the study of which is one of the tasks of plant physiology and scientific agriculture. heat in plant life affects the rate of development in each stage of growth. The task of agriculture also includes the study of the thermal regime of the soil and methods of its regulation.
Water in plant life and nutrients, with the exception of carbon dioxide coming from both the soil and the atmosphere, are the soil factors of plant life. Therefore, water and nutrients are called elements of soil fertility.
Air in plant life(atmospheric and soil) is necessary as a source of oxygen for the respiration of plants and soil microorganisms, as well as a source of carbon that the plant absorbs during photosynthesis. In addition, Air in the life of plants is necessary for microbiological processes in the soil, as a result of which the organic matter of the soil is decomposed by aerobic microorganisms with the formation of soluble mineral compounds of nitrogen, phosphorus, potassium and other plant nutrients.

7 . Indicators of photosynthetic productivity of crops

A crop is created in the process of photosynthesis, when organic matter is formed in green plants from carbon dioxide, water and minerals. Energy sunbeam converted into plant biomass energy. The efficiency of this process and ultimately the yield depend on the functioning of the crop as a photosynthetic system. In field conditions, sowing (cenosis) as a set of plants per unit area is a complex dynamic self-regulating photosynthetic system. This system includes many components that can be considered as subsystems; it is dynamic, as it constantly changes its parameters over time; self-regulating, since, despite various influences, sowing changes its parameters in a certain way, maintaining homeostasis.

Indicators of photosynthetic activity of crops. Seeding is an optical system in which leaves absorb PAR. In the initial period of plant development, the assimilation surface is small and a significant part of the PAR passes by the leaves and is not captured by them. With an increase in the area of ​​leaves, their absorption of solar energy also increases. When the leaf surface index* is 4...5, i.e. the area of ​​leaves in the crop is 40...50 thousand m 2 /ha, the absorption of PAR by the leaves of the crop reaches a maximum value - 75...80% of the visible, 40% of total radiation. With a further increase in leaf area, PAR absorption does not increase. In crops where the course of formation of the leaf area is optimal, the absorption of PAR can be on average 50...60% of the incident radiation during the growing season. PAR absorbed by the plant cover is the energy basis for photosynthesis. However, only part of this energy is accumulated in the crop. The PAR utilization factor is usually determined in relation to the PAR incident on the vegetation cover. If in the biomass crop in middle lane Russia has accumulated 2...3% of PAR sowing, then the dry weight of all plant organs will be 10...15 t/ha, and the possible yield will be 4...6 t of grain per 1 ha. In sparse crops, the PAR utilization factor is only 0.5...1.0%.

Considering a crop as a photosynthetic system, the dry biomass yield generated during a growing season, or its growth over a given period, depends on the average leaf area, the length of the period, and the net photosynthesis productivity for that period.

Y \u003d FP NPF,

where Y is the yield of dry biomass, t/ha;

FP - photosynthetic potential, thousand m 2 - days / ha;

NPP - net productivity of photosynthesis, g/(m2 - days).

Photosynthetic potential is calculated by the formula

where Sc is the average leaf area for the period, thousand m 2 /ha;

T is the duration of the period, days.

The main indicators for the cenosis, as well as the yield, are determined per unit area - 1 m 2 or 1 ha. So, the leaf area is measured in thousand m 2 / ha. In addition, they use such an indicator as the leaf surface index. The main part of the assimilation surface is made up of leaves, it is in them that photosynthesis takes place. Photosynthesis can also occur in other green parts of plants - stems, awns, green fruits, etc., but the contribution of these organs to total photosynthesis is usually small. It is customary to compare crops with each other, as well as various states of one sowing in dynamics by leaf area, identifying it with the concept of "assimilation surface". The dynamics of the area of ​​leaves in the crop follows a certain regularity. After germination, the leaf area slowly increases, then the growth rate increases. By the time the formation of lateral shoots stops and the plants grow in height, the leaf area reaches its maximum value during the growing season, then it begins to gradually decrease due to yellowing and dying off. lower leaves. By the end of the growing season in the crops of many crops (cereals, legumes), green leaves on the plants are absent. The leaf area of ​​various agricultural plants can vary greatly during the growing season depending on the conditions of water supply, nutrition, and agricultural practices. The maximum leaf area in arid conditions reaches only 5...10 thousand m 2 /ha, and with excessive moisture and nitrogen nutrition, it can exceed 70 thousand m 2 /ha. It is believed that with a leaf surface index of 4...5, sowing as an optical photosynthesizing system works in optimal mode, absorbing the greatest amount of PAR. With a smaller area of ​​leaves, a part of the PAR is not captured by the leaves. If the leaf area is more than 50000 m2/ha, then the upper leaves shade the lower ones, and their share in photosynthesis sharply decreases. Moreover, the upper leaves "feed" the lower ones, which is unfavorable for the formation of fruits, seeds, tubers, etc. The dynamics of the leaf area shows that different stages during the growing season, sowing as a photosynthetic system functions differently (Fig. 3). During the first 20...30 days of vegetation, when the average leaf area is 3...7 thousand m 2 /ha, most of the PAR is not captured by the leaves, and therefore the PAR utilization factor cannot be high. Further, the area of ​​leaves begins to increase rapidly, reaching a maximum. As a rule, this occurs in bluegrasses in the phase of the milky state of the grain, in cereal legumes in the phase of full seed filling in the middle layer, and in perennial grasses in the flowering phase. Then the leaf area begins to decrease rapidly. At this time, the redistribution and outflow of substances from vegetative organs into generative ones. The duration of these periods and their ratio is influenced by various factors, including agrotechnical ones. With their help, it is possible to regulate the process of increasing the area of ​​leaves and the duration of periods. In arid conditions, the density of plants, and hence the area of ​​leaves, is deliberately reduced, since with a large area of ​​leaves, transpiration increases, plants suffer more from a lack of moisture, and yields decrease.

How is the energy of sunlight in the light and dark phases of photosynthesis converted into the energy of chemical bonds of glucose? Explain the answer.

Answer

In the light phase of photosynthesis, the energy of sunlight is converted into the energy of excited electrons, and then the energy of excited electrons is converted into the energy of ATP and NADP-H2. In the dark phase of photosynthesis, the energy of ATP and NADP-H2 is converted into the energy of glucose chemical bonds.

What's going on in light phase photosynthesis?

Answer

The electrons of chlorophyll, excited by the energy of light, go along the electron transport chains, their energy is stored in ATP and NADP-H2. Photolysis of water occurs, oxygen is released.

What are the main processes that take place during the dark phase of photosynthesis?

Answer

From carbon dioxide obtained from the atmosphere and hydrogen obtained in the light phase, glucose is formed due to the energy of ATP obtained in the light phase.

What is the function of chlorophyll in a plant cell?

Answer

Chlorophyll is involved in the process of photosynthesis: in the light phase, chlorophyll absorbs light, the chlorophyll electron receives light energy, breaks off and goes along the electron transport chain.

What role do chlorophyll electrons play in photosynthesis?

Answer

Chlorophyll electrons, excited by sunlight, pass through electron transport chains and give up their energy to the formation of ATP and NADP-H2.

At what stage of photosynthesis is free oxygen produced?

Answer

In the light phase, during the photolysis of water.

During what phase of photosynthesis does ATP synthesis occur?

Answer

light phase.

What is the source of oxygen during photosynthesis?

Answer

Water (oxygen is released during the photolysis of water).

The rate of photosynthesis depends on limiting (limiting) factors, among which are light, carbon dioxide concentration, temperature. Why are these factors limiting for photosynthesis reactions?

Answer

Light is necessary for the excitation of chlorophyll, it supplies energy for the process of photosynthesis. Carbon dioxide is needed in the dark phase of photosynthesis; glucose is synthesized from it. A change in temperature leads to the denaturation of enzymes, the reactions of photosynthesis slow down.

In what metabolic reactions in plants is carbon dioxide the initial substance for the synthesis of carbohydrates?

Answer

in the reactions of photosynthesis.

In the leaves of plants, the process of photosynthesis proceeds intensively. Does it occur in mature and unripe fruits? Explain the answer.

Answer

Photosynthesis takes place in the green parts of plants exposed to light. Thus, photosynthesis occurs in the skin of green fruits. Inside the fruit and in the skin of ripe (not green) fruits, photosynthesis does not occur.

Main external factors that affect the intensity of photosynthesis are light, carbon dioxide concentration and temperature. If the change in any of the listed factors is plotted along the horizontal axis, then the curves of the dependence of the intensity of photosynthesis on these factors will have the form shown in the figure. First, with an increase in the value of any of the limiting factors, a linear increase in the intensity of photosynthesis is observed. Then, as another factor or factors become limiting, the intensity of the reaction slows down and stabilizes.

In the following, we will assume that only one, discussed, changes factor, and the rest have optimal values.

Light and photosynthesis

At low light intensity of photosynthesis increases in proportion to the increase in the amount of incident light. Gradually, under the influence of other factors, the intensity of photosynthesis decreases. The illuminance on a clear summer day is about 100,000 lux (10,000 fc), while only 10,000 lux is required for normal photosynthesis. Therefore, for most plants (except plants in the shade), light is not the main limiting factor in photosynthesis. Highly high values light intensity can lead to the discoloration of chlorophyll and slow down the reactions of photosynthesis. At the same time, plants constantly under such conditions are usually well adapted to them; for example, their leaves are covered with a thick cuticle or densely pubescent.

Carbon dioxide concentration and photosynthesis

Carbon dioxide used in dark reactions to produce sugar. Under normal conditions, carbon dioxide is the main limiting factor in photosynthesis. The atmosphere contains from 0.03 to 0.04% carbon dioxide. If you increase its content in the air, you can achieve an increase in the intensity of photosynthesis. For a short period, the optimum concentration of 0.5% can be maintained, however, with prolonged exposure, this concentration becomes dangerous for the plant. Therefore, a carbon dioxide concentration of about 0.1% is considered to be the most favorable. Some greenhouse crops, such as tomatoes, are grown in an atmosphere enriched with carbon dioxide. Currently, plants that can effectively remove carbon dioxide from the atmosphere and at the same time give increased yields. Such plants, called C4 plants, are discussed in the appropriate section.

Temperature and photosynthesis

Dark and, to some extent, light reactions are controlled enzymes, so the air temperature is great importance. For temperate plants, the most favorable temperature is about 25 °C. For every 10°C increase in temperature, the reaction rate doubles (up to 35°C), but other evidence suggests that the plant develops better at 25°C.

Chlorophyll concentration and photosynthesis

By her own chlorophyll concentration is not a factor limiting photosynthesis. The reasons for the decrease in the level of chlorophyll may be important: diseases ( powdery mildew, rust, viral diseases), lack of trace elements, normal aging processes. When a leaf turns yellow, it is said to have become chlorotic, and the process by which a yellowish leaf color develops is called chlorosis. Chlorotic spots are often a symptom of disease or mineral deficiency. Some elements, such as iron, magnesium and nitrogen (the last two are directly included in the chlorophyll molecule), are necessary for the formation of chlorophyll, so these elements are especially important. In addition, the plant needs potassium. Another reason for the occurrence of chlorosis is the lack of light, since light is necessary at the final stage of chlorophyll synthesis.

Specific inhibitors and photosynthesis

If suppress photosynthesis the plant will inevitably die. This was the basis for the development of various herbicides, such as DHMM (dichlorophenyldimethylurea). This drug starts a bypass of the non-cyclic electron flow in chloroplasts, thus inhibiting light reactions. DXMM has played an important role in the study of light reactions in photosynthesis.

There are two more factors render big influence on crop growth and have more general meaning for plant growth and photosynthesis is the presence of water and environmental pollution.

Water and photosynthesis

Water is the starting material for photosynthesis. However, since water affects a huge number of cellular processes, it is impossible to assess its direct impact on photosynthesis. However, by studying the amount of synthesized organic matter in plants suffering from a lack of water, it can be seen that temporary wilting leads to a sharp decrease in yield. Even if the plants show no visible changes, a slight water deficit leads to a significant drop in yield. The reasons for this are complex and not fully understood. One of the obvious reasons can be considered the closing of stomata during wilting, which prevents the flow of carbon dioxide for photosynthesis. In addition, it was shown that with a lack of water, abscisic acid, which is a growth inhibitor, accumulates in the leaves of some plants.

Environmental pollution and photosynthesis

Some gases of industrial origin, for example ozone and sulfur dioxide, even in small amounts are very dangerous for plant leaves, although the exact reasons for this have not yet been established. Thus, grain crops in polluted areas lose up to 15% of their mass, especially during dry summers. It turned out that lichens are very sensitive to sulfur dioxide. Soot clogs the stomata and reduces the transparency of the leaf epidermis.