27-Feb-2014 | One Comment | Lolita Okolnova
Photosynthesis- the process of formation of organic substances from carbon dioxide and water in the light with the participation of photosynthetic pigments.
Chemosynthesis- a method of autotrophic nutrition, in which the oxidation reactions of inorganic compounds serve as an energy source for the synthesis of organic substances from CO 2
Usually, all organisms capable of synthesizing organic substances from inorganic substances, i.e. organisms capable of photosynthesis and chemosynthesis, refer to .
Some are traditionally classified as autotrophs.
We briefly talked about in the course of considering the structure of a plant cell, let's take a look at the whole process in more detail ...
The essence of photosynthesis
(total equation)
The main substance involved in the multi-step process of photosynthesis - chlorophyll. It is it that transforms solar energy into chemical energy.
The figure shows a schematic representation of the chlorophyll molecule, by the way, the molecule is very similar to the hemoglobin molecule ...
Chlorophyll is built into chloroplast grana:
Light phase of photosynthesis:
(carried out on thylakoid membranes)
- Light, hitting the chlorophyll molecule, is absorbed by it and brings it into an excited state - an electron that is part of the molecule, having absorbed the energy of light, goes to a higher energy level and participates in the synthesis processes;
- Under the action of light, the splitting (photolysis) of water also occurs:
At the same time, oxygen is removed to the external environment, and protons accumulate inside the thylakoid in the "proton reservoir"
2H + + 2e - + NADP → NADP H 2
NADP is a specific substance, a coenzyme, i.e. a catalyst, in this case a carrier of hydrogen.
- synthesized (energy)
Dark phase of photosynthesis
(occurs in the stroma of chloroplasts)
actual glucose synthesis
a cycle of reactions occurs in which C 6 H 12 O 6 is formed. These reactions use the energies of ATP and NADP·H 2 formed in the light phase; In addition to glucose, other monomers of complex organic compounds are formed during photosynthesis - amino acids, glycerol and fatty acids, nucleotides
Please note: this phase is dark is called not because it goes at night - glucose synthesis occurs, in general, around the clock, but the dark phase no longer needs light energy.
“Photosynthesis is the process upon which all manifestations of life on our planet ultimately depend.”
K.A. Timiryazev.
As a result of photosynthesis, about 150 billion tons of organic matter are formed on Earth and about 200 billion tons of free oxygen are released per year. In addition, plants involve billions of tons of nitrogen, phosphorus, sulfur, calcium, magnesium, potassium and other elements in the circulation. Although a green leaf uses only 1-2% of the light falling on it, organic matter and oxygen in general are created by the plant.
Chemosynthesis
Chemosynthesis is carried out due to the energy released during the chemical reactions of oxidation of various inorganic compounds: hydrogen, hydrogen sulfide, ammonia, iron oxide (II), etc.
According to the substances included in the metabolism of bacteria, there are:
- sulfur bacteria - microorganisms of water bodies containing H 2 S - sources with a very characteristic odor,
- iron bacteria,
- nitrifying bacteria - oxidize ammonia and nitrous acid,
- nitrogen-fixing bacteria - enrich the soil, extremely increase yields,
- hydrogen-oxidizing bacteria
But the essence remains the same - this is also
Every living thing on the planet needs food or energy to survive. Some organisms feed on other creatures, while others can produce their own nutrients. They 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:
- The glucose obtained from photosynthesis is used during respiration to release the energy the plant needs for other vital processes.
- Plant cells also convert some of the glucose into starch, which is used as needed. For this reason, dead plants are used as biomass because they store chemical energy.
- Glucose is also needed to produce other chemicals such as proteins, fats and vegetable 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
In the dark phase or the Calvin cycle, excited electrons from the light phase provide energy for the formation of carbohydrates from carbon dioxide molecules. The light-independent phases are sometimes called the Calvin cycle because of 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 is 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 leaf blade, while compound leaves have several. The leaf blade is one of the most important components of the leaf, which is directly involved in the process of photosynthesis.
- veins
A network of veins in 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 leaf, the apex comes in a variety of shapes, including: sharp, round, blunt, elongated, retracted, etc.
The internal structure of the leaves
Below is a close diagram of the internal structure of leaf tissues:
- 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. Its main function is to protect 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 a 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 the 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, odorless and tasteless (under normal conditions).
- Light. Although artificial light is also suitable for plants, natural sunlight generally creates the best conditions for photosynthesis because it contains natural ultraviolet radiation, which has a positive effect on plants.
- Chlorophyll. It is a 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 photosynthesis process.
Water during photosynthesis
Plants get the water they need for photosynthesis through their roots. They have root hairs that grow in the soil. The roots 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;
- It makes it possible to carry out 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 serve as the backbone of almost every food chain on Earth. (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.
- synthesis of organic substances from carbon dioxide and water with the obligatory use of light energy:
6CO 2 + 6H 2 O + Q light → C 6 H 12 O 6 + 6O 2.
In higher plants, the organ of photosynthesis is the leaf, the organelles of photosynthesis are chloroplasts (the structure of chloroplasts is lecture No. 7). The thylakoid membranes of chloroplasts contain photosynthetic pigments: chlorophylls and carotenoids. There are several different types of chlorophyll ( a, b, c, d), the main one being chlorophyll a. In the chlorophyll molecule, a porphyrin “head” with a magnesium atom in the center and a phytol “tail” can be distinguished. The porphyrin “head” is a flat structure, is hydrophilic, and therefore lies on the surface of the membrane that faces the aquatic environment of the stroma. The phytol "tail" is hydrophobic and thus keeps the chlorophyll molecule in the membrane.
Chlorophyll absorbs red and blue-violet light, reflects green and therefore gives plants their characteristic green color. Chlorophyll molecules in thylakoid membranes are organized into photosystems. Plants and blue-green algae have photosystem-1 and photosystem-2; photosynthetic bacteria have photosystem-1. Only photosystem-2 can decompose water with the release of oxygen and take electrons from the hydrogen of water.
Photosynthesis is a complex multi-stage process; photosynthesis reactions are divided into two groups: reactions light phase and reactions dark phase.
light phase
This phase occurs only in the presence of light in thylakoid membranes with the participation of chlorophyll, electron carrier proteins, and the enzyme ATP synthetase. Under the action of a light quantum, the chlorophyll electrons are excited, leave the molecule and enter the outer side of the thylakoid membrane, which eventually becomes negatively charged. Oxidized chlorophyll molecules are restored by taking electrons from the water located in the intrathylakoid space. This leads to the decomposition or photolysis of water:
H 2 O + Q light → H + + OH -.
Hydroxyl ions donate their electrons, turning into reactive radicals. OH:
OH - → .OH + e - .
Radicals.OH combine to form water and free oxygen:
4NO. → 2H 2 O + O 2.
In this case, oxygen is removed to the external environment, and protons accumulate inside the thylakoid in the "proton reservoir". As a result, the thylakoid membrane, on the one hand, is positively charged due to H +, on the other, negatively due to electrons. When the potential difference between the outer and inner sides of the thylakoid membrane reaches 200 mV, protons are pushed through the channels of ATP synthetase and ADP is phosphorylated to ATP; atomic hydrogen is used to restore the specific carrier NADP + (nicotinamide adenine dinucleotide phosphate) to NADP H 2:
2H + + 2e - + NADP → NADP H 2.
Thus, photolysis of water occurs in the light phase, which is accompanied by three major processes: 1) ATP synthesis; 2) the formation of NADP·H 2; 3) the formation of oxygen. Oxygen diffuses into the atmosphere, ATP and NADP·H 2 are transported to the stroma of the chloroplast and participate in the processes of the dark phase.
1 - stroma of the chloroplast; 2 - grana thylakoid.
dark phase
This phase takes place in the stroma of the chloroplast. Its reactions do not require the energy of light, so they occur not only in the light, but also in the dark. The reactions of the dark phase are a chain of successive transformations of carbon dioxide (comes from the air), leading to the formation of glucose and other organic substances.
The first reaction in this chain is carbon dioxide fixation; carbon dioxide acceptor is a five-carbon sugar ribulose bisphosphate(RiBF); enzyme catalyzes the reaction ribulose bisphosphate carboxylase(RiBP-carboxylase). As a result of carboxylation of ribulose bisphosphate, an unstable six-carbon compound is formed, which immediately decomposes into two molecules phosphoglyceric acid(FGK). Then there is a cycle of reactions in which, through a series of intermediate products, phosphoglyceric acid is converted to glucose. These reactions use the energies of ATP and NADP·H 2 formed in the light phase; The cycle of these reactions is called the Calvin cycle:
6CO 2 + 24H + + ATP → C 6 H 12 O 6 + 6H 2 O.
In addition to glucose, other monomers of complex organic compounds are formed during photosynthesis - amino acids, glycerol and fatty acids, nucleotides. Currently, there are two types of photosynthesis: C 3 - and C 4 -photosynthesis.
C 3 -photosynthesis
This is a type of photosynthesis in which three-carbon (C3) compounds are the first product. C 3 -photosynthesis was discovered before C 4 -photosynthesis (M. Calvin). It is C 3 -photosynthesis that is described above, under the heading "Dark phase". Characteristic features of C 3 photosynthesis: 1) RiBP is an acceptor of carbon dioxide, 2) RiBP carboxylase catalyzes the RiBP carboxylation reaction, 3) as a result of RiBP carboxylation, a six-carbon compound is formed, which decomposes into two FHAs. FHA is restored to triose phosphates(TF). Part of TF is used for regeneration of RiBP, part is converted into glucose.
1 - chloroplast; 2 - peroxisome; 3 - mitochondrion.
This is the light-dependent uptake of oxygen and the release of carbon dioxide. Even at the beginning of the last century, it was found that oxygen inhibits photosynthesis. As it turned out, not only carbon dioxide, but also oxygen can be a substrate for RiBP carboxylase:
O 2 + RiBP → phosphoglycolate (2С) + FHA (3С).
The enzyme is called RiBP-oxygenase. Oxygen is a competitive inhibitor of carbon dioxide fixation. The phosphate group is cleaved off and the phosphoglycolate becomes glycolate, which the plant must utilize. It enters the peroxisomes, where it is oxidized to glycine. Glycine enters the mitochondria, where it is oxidized to serine, with the loss of already fixed carbon in the form of CO 2. As a result, two molecules of glycolate (2C + 2C) are converted into one FHA (3C) and CO 2. Photorespiration leads to a decrease in the yield of C 3 -plants by 30-40% ( C 3 -plants- plants that are characterized by C 3 -photosynthesis).
C 4 -photosynthesis - photosynthesis, in which the first product is four-carbon (C 4) compounds. In 1965, it was found that in some plants (sugarcane, corn, sorghum, millet) the first products of photosynthesis are four-carbon acids. Such plants are called With 4 plants. In 1966, the Australian scientists Hatch and Slack showed that C 4 plants have practically no photorespiration and absorb carbon dioxide much more efficiently. The path of carbon transformations in C 4 plants began to be called by Hatch-Slack.
C 4 plants are characterized by a special anatomical structure of the leaf. All conducting bundles are surrounded by a double layer of cells: the outer one is mesophyll cells, the inner one is lining cells. Carbon dioxide is fixed in the cytoplasm of mesophyll cells, the acceptor is phosphoenolpyruvate(PEP, 3C), as a result of PEP carboxylation, oxaloacetate (4C) is formed. The process is catalyzed PEP carboxylase. In contrast to RiBP carboxylase, PEP carboxylase has a high affinity for CO 2 and, most importantly, does not interact with O 2 . In mesophyll chloroplasts, there are many granae, where reactions of the light phase are actively taking place. In the chloroplasts of the sheath cells, reactions of the dark phase take place.
Oxaloacetate (4C) is converted to malate, which is transported through plasmodesmata to the lining cells. Here it is decarboxylated and dehydrated to form pyruvate, CO 2 and NADP·H 2 .
Pyruvate returns to mesophyll cells and regenerates at the expense of ATP energy in PEP. CO 2 is again fixed by RiBP carboxylase with the formation of FHA. The regeneration of PEP requires the energy of ATP, so almost twice as much energy is needed as with C 3 photosynthesis.
The Importance of Photosynthesis
Thanks to photosynthesis, billions of tons of carbon dioxide are absorbed from the atmosphere every year, billions of tons of oxygen are released; photosynthesis is the main source of the formation of organic substances. The ozone layer is formed from oxygen, which protects living organisms from short-wave ultraviolet radiation.
During photosynthesis, a green leaf uses only about 1% of the solar energy falling on it, the productivity is about 1 g of organic matter per 1 m 2 of surface per hour.
Chemosynthesis
The synthesis of organic compounds from carbon dioxide and water, carried out not at the expense of light energy, but at the expense of the oxidation energy of inorganic substances, is called chemosynthesis. Chemosynthetic organisms include some types of bacteria.
Nitrifying bacteria oxidize ammonia to nitrous, and then to nitric acid (NH 3 → HNO 2 → HNO 3).
iron bacteria convert ferrous iron to oxide (Fe 2+ → Fe 3+).
Sulfur bacteria oxidize hydrogen sulfide to sulfur or sulfuric acid (H 2 S + ½O 2 → S + H 2 O, H 2 S + 2O 2 → H 2 SO 4).
As a result of the oxidation reactions of inorganic substances, energy is released, which is stored by bacteria in the form of high-energy bonds of ATP. ATP is used for the synthesis of organic substances, which proceeds similarly to the reactions of the dark phase of photosynthesis.
Chemosynthetic bacteria contribute to the accumulation of minerals in the soil, improve soil fertility, promote wastewater treatment, etc.
Go to lectures №11“The concept of metabolism. Biosynthesis of proteins"
Go to lectures №13"Methods of division of eukaryotic cells: mitosis, meiosis, amitosis"
The history of the discovery of an amazing and such a vitally important phenomenon as photosynthesis is rooted deep in the past. More than four centuries ago, in 1600, the Belgian scientist Jan Van - Helmont set up a simple experiment. He placed a willow branch in a bag containing 80 kg of earth. The scientist recorded the initial weight of the willow, and then for five years watered the plant exclusively with rainwater. What was the surprise of Jan Van - Helmont when he re-weighed the willow. The weight of the plant increased by 65 kg, and the mass of the earth decreased by only 50 grams! Where did the plant get 64 kg 950 g of nutrients for the scientist remained a mystery!
The next significant experiment on the path to the discovery of photosynthesis belonged to the English chemist Joseph Priestley. The scientist put a mouse under the cap, and after five hours the rodent died. When Priestley placed a sprig of mint with the mouse and also covered the rodent with a cap, the mouse remained alive. This experiment led the scientist to the idea that there is a process opposite to breathing. Jan Ingenhaus in 1779 established the fact that only the green parts of plants are capable of releasing oxygen. Three years later, the Swiss scientist Jean Senebier proved that carbon dioxide, under the influence of sunlight, decomposes in the green organelles of plants. Just five years later, the French scientist Jacques Bussingault, conducting laboratory research, discovered the fact that the absorption of water by plants also occurs during the synthesis of organic substances. A landmark discovery in 1864 was made by the German botanist Julius Sachs. He was able to prove that the volume of carbon dioxide consumed and oxygen released occurs in a ratio of 1: 1.
Photosynthesis is one of the most important biological processes
In scientific terms, photosynthesis (from ancient Greek φῶς - light and σύνθεσις - connection, binding) is a process in which organic substances are formed from carbon dioxide and water in the light. The main role in this process belongs to photosynthetic segments.
Speaking figuratively, the leaf of a plant can be compared with a laboratory, the windows of which face the sunny side. It is in it that the formation of organic substances occurs. This process is the basis for the existence of all life on Earth.
Many will reasonably ask the question: what do people who live in the city breathe, where not only trees, and you can’t find blades of grass during the day with fire. The answer is very simple. The fact is that land plants account for only 20% of the oxygen released by plants. Algae play a major role in the production of oxygen into the atmosphere. They account for 80% of the oxygen produced. In the language of numbers, both plants and algae release 145 billion tons (!) of oxygen into the atmosphere every year! No wonder the world's oceans are called the "lungs of the planet."
The general formula for photosynthesis is as follows:
Water + Carbon Dioxide + Light → Carbohydrates + Oxygen
Why do plants need photosynthesis?
As we have understood, photosynthesis is a necessary condition for the existence of man on Earth. However, this is not the only reason why photosynthetic organisms actively produce oxygen into the atmosphere. The fact is that both algae and plants annually form more than 100 billion organic substances (!), which form the basis of their life activity. Remembering the experiment of Jan Van Helmont, we understand that photosynthesis is the basis of plant nutrition. It has been scientifically proven that 95% of the crop is determined by organic substances obtained by the plant in the process of photosynthesis, and 5% - those mineral fertilizers that the gardener introduces into the soil.
Modern summer residents focus on the soil nutrition of plants, forgetting about its air nutrition. It is not known what kind of harvest gardeners could get if they were attentive to the process of photosynthesis.
However, neither plants nor algae could produce oxygen and carbohydrates so actively if they did not have an amazing green pigment - chlorophyll.
The secret of the green pigment
The main difference between plant cells and cells of other living organisms is the presence of chlorophyll. By the way, it is he who is the culprit of the fact that the leaves of plants are colored precisely in green. This complex organic compound has one amazing property: it can absorb sunlight! Thanks to chlorophyll, the process of photosynthesis becomes possible.
Two stages of photosynthesis
In simple terms, photosynthesis is a process in which water and carbon dioxide absorbed by a plant in the presence of light with the help of chlorophyll form sugar and oxygen. Thus, inorganic substances are miraculously transformed into organic ones. The resulting sugar is the energy source of plants.
Photosynthesis has two stages: light and dark.
Light phase of photosynthesis
Occurs on thylakoid membranes.
Thylakoid are structures bounded by a membrane. They are located in the stroma of the chloroplast.
The order of events of the light stage of photosynthesis:
- Light hits the chlorophyll molecule, which is then absorbed by the green pigment and brings it into an excited state. The electron included in the molecule goes to a higher level, participates in the synthesis process.
- There is a splitting of water, during which protons under the influence of electrons turn into hydrogen atoms. Subsequently, they are spent on the synthesis of carbohydrates.
- At the final stage of the light stage, ATP (adenosine triphosphate) is synthesized. This is an organic substance that plays the role of a universal energy accumulator in biological systems.
Dark phase of photosynthesis
The site of the dark phase is the stroma of chloroplasts. It is during the dark phase that oxygen is released and glucose is synthesized. Many will think that this phase got such a name because the processes taking place within this stage are carried out exclusively at night. Actually, this is not entirely true. Glucose synthesis occurs around the clock. The fact is that it is at this stage that light energy is no longer consumed, which means that it is simply not needed.
Importance of photosynthesis for plants
We have already identified the fact that plants need photosynthesis no less than we do. It is very easy to talk about the scale of photosynthesis in the language of numbers. Scientists have calculated that only land plants store as much solar energy as 100 megacities could use up within 100 years!
Plant respiration is a process opposite to photosynthesis. The meaning of plant respiration is to release energy in the process of photosynthesis and direct it to the needs of plants. In simple terms, harvest is the difference between photosynthesis and respiration. The more photosynthesis and the lower respiration, the greater the harvest, and vice versa!
Photosynthesis is the amazing process that makes life possible on Earth!
Photosynthesis is the process of synthesizing organic substances from inorganic substances using light energy. In the vast majority of cases, photosynthesis is carried out by plants using cell organelles such as chloroplasts containing 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, split them 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, you need to say more: what specific inorganic substances are used, how does synthesis occur?
Photosynthesis requires two inorganic substances - carbon dioxide (CO 2) and water (H 2 O). The first is absorbed from the air by the aerial parts of plants mainly through the stomata. Water - from the soil, from where it is delivered to the photosynthetic cells by the conducting system of plants. Photosynthesis also requires the energy of photons (hν), but they cannot be attributed to matter.
In total, as a result of photosynthesis, organic matter and oxygen (O 2) are formed. Usually, under organic matter, glucose (C 6 H 12 O 6) is most often meant.
Organic compounds are mostly made up of carbon, hydrogen and oxygen atoms. They are found in carbon dioxide and water. However, photosynthesis releases oxygen. Its atoms come from water.
Briefly and generally, the equation for the reaction of photosynthesis is usually written as follows:
6CO 2 + 6H 2 O → C 6 H 12 O 6 + 6O 2
But this equation does not reflect the essence of photosynthesis, does not make it understandable. Look, although the equation is balanced, it has a total of 12 atoms in free oxygen. But we said that they come from water, and there are only 6 of them.
In fact, photosynthesis occurs in two phases. The first 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 goes in the dark. The light phase flows on the membranes of the thylakoids of the chloroplast, the dark phase - in the stroma of the chloroplast.
In the light phase, CO 2 binding does not occur. There is only the capture of solar energy by chlorophyll complexes, its storage in ATP, the use of energy for the reduction of NADP to NADP * H 2. The flow of energy from chlorophyll excited by light is provided by electrons transmitted through the electron transport chain of enzymes built into thylakoid membranes.
Hydrogen for NADP is taken from water, which, under the action of sunlight, decomposes into oxygen atoms, hydrogen protons and electrons. This process is called photolysis. Oxygen from water is not needed for photosynthesis. The oxygen atoms from two water molecules combine to form molecular oxygen. The reaction equation for the light phase of photosynthesis briefly looks like this:
H 2 O + (ADP + F) + NADP → ATP + NADP * H 2 + ½O 2
Thus, the release of oxygen occurs in the light phase of photosynthesis. The number of ATP molecules synthesized from ADP and phosphoric acid per photolysis of one water molecule can be different: one or two.
So, ATP and NADP * H 2 enter the dark phase from the light phase. Here, the energy of the first and the restorative force of the second are spent on the binding of carbon dioxide. This step of photosynthesis cannot be explained simply and briefly, because it does not proceed in such a way that six CO 2 molecules combine with hydrogen released from NADP * H 2 molecules and glucose is formed:
6CO 2 + 6NADP * H 2 → C 6 H 12 O 6 + 6NADP
(the reaction takes place with the expenditure of energy from ATP, which breaks down into ADP and phosphoric acid).
The above reaction is just a simplification for ease of understanding. In fact, carbon dioxide molecules bind one at a time, joining the already prepared five-carbon organic matter. An unstable six-carbon organic substance is formed, which breaks down into three-carbon carbohydrate molecules. Some of these molecules are used for the resynthesis of the initial five-carbon substance for CO 2 binding. This resynthesis is provided Calvin cycle. A smaller part of the carbohydrate molecules, which includes three carbon atoms, leaves the cycle. Already from them and other substances, all other organic substances (carbohydrates, fats, proteins) are synthesized.
That is, in fact, three-carbon sugars, and not glucose, come out of the dark phase of photosynthesis.